Interuniversity Programme in Physical Land Resources

INTERUNIVERSITY I C E PROGRAMME

IN PHYSICAL LAND RESOURCES

Ghent University Vrije Universiteit Brussel Belgium

Relating tree growth to climate using tree ring analysis for the Congo basin forest

Promoters : Master dissertation submitted in Dr. ir. Hans Verbeeck partial fulfillment of the requirements Prof. dr. ir. Kathy Steppe for the degree of Master of Science in Physical Land Resources Tutors : ir. Marjolein De Weirdt by Promise Simwinde Muleya Dr. ir. Hans Beeckman (Zimbabwe)

Academic Year 2011-2012

This is an unpublished M.Sc dissertation and is not prepared for further distribution. The author and the promoter give the permission to use this Master dissertation for consultation and to copy parts of it for personal use. Every other use is subject to the copyright laws, more specifically the source must be extensively specified when using results from this Master dissertation.

Gent,

The Promoter(s), The Author, Dr. ir. H. Verbeeck (LA09)

Prof. dr. ir. K. Steppe (LA09) Promise Simwinde Muleya

Tutor: ir.Marjolein De Weirdt

Dr. ir. Hans Beeckman

Acknowledgements I express my sincere and profound gratitude to all the people that helped me in this work. To my committed and kind tutor, ir. Marjolein De Weirdt, thank you a million times. Your patience, strength and encouragement will forever be the etched in my memories for as long as l live. Your tireless work and corrections made this thesis a success. To my promoter, Hans Verbeeck. It all started in the course of Meteorology and Climatology when l heard about climate reconstruction and the tiny seed of passion and interest in the subject grew. Thank you so very much for your comments and pointers. Your guidance has made me the richer. To Prof. Dr. ir. Kathy Steppe, thank you for affording me the opportunity to do this thesis in the lab of Plant Ecology, l must say, I am more ecological in my thinking now. To Dr. Hans Beeckman, thank you for introducing me to the world of wood! Never did it dawn upon me all those years growing up in rural Africa that a simple piece of wood could be a great archive and library of information. Thank you so very much. To ir. Maaike, thank you very much for your immense contribution to this work and the sacrifice of your time. You clarified the analysis with the wisdom of a seasoned dendrochronologist! To all the people at Tervuren, the likes of Lore, Sarah, Jöelle and many more, thank you very much. To all my classmates and friends in the Master of Physical Land Resources, you were a great company and will keep the memories alive. I would also like to extend heartfelt thanks to the VLIR UOS program for the financial assistance. And lastly Bedankt voor alles, merci, Belgium!

Abstract Climate change is a real challenge that humankind has to face head on in order to prepare beforehand for many major effects that are yet to be fully known and understood. Rising global temperatures is expected to have an impact to our atmosphere and ocean circulation trends and hence rainfall patterns. Future scenarios predict marked precipitation decrease during the rainy season and also the annual precipitation totals. Forest ecosystems act as the key sources for sequestering carbon and also contain vast amounts of biodiversity. However the general lack of reliable data on the carbon stocks in Congo basin and the knowledge on how these forest ecosystems will respond to changing environmental factors still remain largely unknown and segregated. Tree ring analyses were carried out on 20 Pericopsis elata stems discs from Biaro, DRC at the Laboratory of Wood Biology and Xylarium, Tervuren. Growth was compared to rainfall and the above ground woody biomass from ORCHIDEE. The tree show annual rings marked by variations in vessel distribution and size, repeated pattern of alternating fibre and parenchyma bands, marginal parenchyma bands and density variations all with varying degrees of distinctiveness. Low threshold values were used in this study to improve cross-dating procedure (t ≥ 2 (p < 0.05) and GLK ≥ 50 %.). Incidence of wedging rings and missing rings, unclear distinction of rings in the sapwood and problems with the stitching process of images with Fiji ImageJ were encountered and may have given an underestimation of the actual tree ages. Raw ring counts ranged between 89 to 195 years and showed a relationship with mean annual radial growth. The highest mean annual radial growth of 3.7 mm year-1 was recorded and corresponded to the youngest stem disc (89 years) and lowest radial growth rate was 2.5 mm year -1corresponding to the oldest stem disc. The tree width index which formed a chronology (1912 to 2003) showed a marginal correlation with annual rainfall (r = 0.11). There was a significant correlation (r = 0.29; P < 0.01) between the ring width chronology and the previous rainfall sums of September and October. Comparison of the ring width index to the AGWB did not show explicit correlation (r = 0.08) but some synchronous peaks were observed. Pericopsis elata shows potential for dendrochronological analysis. More research can be done in relation to AGWB ORCHIDEE for longer time series.

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Table of Contents Copyright ...... i Acknowledgements ...... ii Abstract ...... iii Table of Contents ...... iv List of figures ...... vi List of tables ...... viii List of abbreviations ...... ix 1 Introduction ...... 1 2 Literature review ...... 3 2.1 Tropical rainforests ...... 3 2.1.1 A general view ...... 3 2.1.2 The Congo Basin ...... 4 2.1.3 Pericopsis elata ...... 6 2.2. Global carbon dynamics ...... 6 2.2.1 Carbon cycle ...... 6 2.2.2 Carbon stocks in tropical rainforest ...... 7 2.3 Tree growth ...... 8 2.3.1 Wood anatomical features ...... 8 2.3.2. Tree rings ...... 8 2.3.3. Factors influencing tree ring growth ...... 9 2.3.4 Dendrochronology: an overview ...... 10 2.3.5 Dendrochronology in the tropical areas ...... 11 2.3.6 Development of tropical ring chronologies ...... 12 2.3.7 Wedging rings ...... 12 2.3.8 Relationship between tree growth and rainfall ...... 14 3 Material and methods ...... 16 3.1 Study site ...... 16 3.2 Data collection ...... 17 3.2.1 Sampling and preparation of samples ...... 17 3.3.2. Tree Ring imaging ...... 19 3.3.3 Stitching the images using Fiji ImageJ ...... 19 3.3.4 Measurement of ring width ...... 20

3.4 Data treatment and analysis ...... 20

3.4.1 Climatic effects analysis on radial tree growth ...... 21 3.4.2 Average ring width and above ground woody biomass increments from ORCHIDEE ...... 21 4. Results and discussion ...... 22 4.1 Annual rainfall and mean seasonal distribution ...... 22 4.2 Distinctiveness of tree growth rings and other measured parameters ...... 23 4.3 Checking quality and inter-series correlation ...... 25 4.4 Correlation between ring widths and rainfall ...... 27 4.5 Growth rates ...... 29 4.6 Comparing above ground woody biomass to the ORCHIDEE Model ...... 30 5 Conclusions ...... 33 6 References ...... 35 7 Appendices ...... 46 Appendix 1. Above ground woody biomass ORCHIDEE...... 46 Appendix 2. Climate correlations with ring width of Pericosis elata...... 47

List of figures Figure 1. The global distribution of tropical rainforests Source: Mongabay.com ...... 4

Figure 2. Congo Basin land cover map derived from 300 m satellite ENVISAT MERIS full resolution and 1km SPOT-Vegetation time series. Source: Verhegghen & Defourny (2010) .. 6

Figure 3. Comparison of average growth levels of three understorey species (Aidia ochroleuca, Corynanthe paniculata and Xylopia wilwerthii) and two canopy species (Terminalia superba and Prioria balsamifera) in Luki forest, DRC (Couralet, 2010)...... 10

Figure 4. Pictures showing stem cross sections of Tabebuia barbata, Bignoniaceae from the Amazon floodplains growing under (a) changing light conditions (b) in the open. (c) Terminalia guianensis, Combretaceae from the Western Llanos, Venezuela with concentric growth in the inner part and wedging rings in the buttresses part of the stem disc (d) Annona sp, Annonaceae from the Amazon floodplains with wedging rings and (e) Tectona grandis (Verbenaceae) from the Royal Botanical Gardens in Calcutta, India. Source: Worbes (2002)...... 14

Figure 5. Location of the Biaro forest reserve near Kisangani, DRC ...... 16

Figure 6. Sanded surface after the superfinish of a stem cross-section of Pericopsis elata from the Biaro Forest Reserve...... 18

Figure 7. Image taken along the radius of the TW62220 stem section, showing six tree rings. The black arrows point to the ring boundaries along the radius. The growth is from left to right...... 19

Figure 8. A mosaic image of the reconstructed images along the radius (TW62220, radius 1) ...... 20

Figure 9. Mean annual rainfall distribution from 1912 – 2007 for Yangambi. Data source: (climexp.knmi.nl ) ...... 22

Figure 10. Annual precipitation from 1912–2007 for Yangambi. Data source: climexp.knmi.nl ...... 23

Figure 11. Time series of tree ring widths for four radii in Pericopsis elata (TW60947)...... 27

Figure 12. The chronology (1833-2007) of Pericopsis elata represented by four trees and contemporaneous annual rainfall. (Pearson’s correlation = 0.11)...... 28

Figure 13. Correlation between tree ring index and rainfall for Pericopsis elata in DRC. Significant correlations (Pearson; P < 0.05) are shown by the solid columns (blue) ...... 29

Figure 14. The relationship between mean radial growth and the number of rings denoting age...... 30

Figure 15. ORCHIDEE above ground woody biomass (AGWB) increment (1989- 2008) plotted along with ring width index (1989-2003)...... 31

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List of tables Table 1: Classification of ring structures according to: distinctiveness of the growth zones, mean increments (calculated by dividing the diameter by the number of rings) of the analyzed individual % and growth zone types (1 = density variations; 2 = marginal parenchyma bands; 3 = patterns of parenchyma bands; 4 = variation in vessel distribution or vessel size) of the studied species (Frichtler et al., 2003) ...... 9

Table 2: Stem cross-section ascension numbers for the Pericopsis elata used in this study, showing the dates they were felled and the number of radii selected for analysis, (Tervuren Wood)...... 17

Table 3: Summary of tree-ring measurement results for the twenty stem cross sections of Pericopsis elata from Biaro forest, DRC...... 25

Table 4: Descriptive statistics of 14 tree ring series showing correlation in Pericopsis elata, from Biaro forest, DRC...... 26

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List of abbreviations

AC Auto Correlation AGWB Above Ground Woody Biomass (increment CAR Central Africa Republic CITES Convention on International Trade in Endangered Species of Wild Fauna and Flora DRC Democratic Republic of the Congo ENVISAT Environmental Satellite FAO Food and Agriculture Organisation FRA Forest Resources Assessment GLK Gleichäufigkeitskoeffizient IAWA International Association of Wood Anatomist IPCC Intergovernmental Panel on Climate Change ITTO International Tropical Timber Organisation IUCN International Union for Conservation of Nature MERIS Medium Resolution Imaging Spectrometer MS Mean Series NPP Net Primary Production ORCHIDEE Organising Carbon and Hydrology in Dynamic Ecosystems TVBP T value Baille Pilchard UN United Nations UNEP United Nations Environment Programme

1 Introduction Nowadays climate change is topical news the world over. Every day we are reminded in various news outlets and forums of the sheer consequences of a changing world climate. Rising global temperatures will likely impact the atmosphere and ocean circulation trends and hence to a greater extent the rainfall patterns. Climate models predict that precipitation will decrease markedly by about 5 to 15 percent during the rainy season and decrease between 3 to 4 percent in yearly rainfall totals per decade in the African tropics as reported by several authors (Malhi and Wright, 2004; Boisvenue and Running 2006). Amidst all these sad but realistic predictions, world population is expected to increase from about 6 billion in 1998 to 8 billion by year 2025 and 9.4 billion by 2050 (Hulse, 1995; Fisher and Helig, 1997; Litvin, 1998).

Estimates show that global forests store 652 Gt of carbon in their biomass, deadwood, litter and the soil as reported by the Global Forest Resources Assessment (FRA, 2010). Congo basin, carbon stocks (carbon stored in biomass) are estimated at 119.3t ha-1 (FAO, 2010). The vulnerability of forests in the face of warmer and drier conditions will likely increase by the influx of the human population (Koenig, 2008). Deforestation attributed to search of new arable lands for cultivation; harvesting forest products and the industrial logging by concession companies are major common news accompanying the debate of climate change, and also a main human activity. According to Malhi et al. (2005), deforestation is expected to enforce more the effects of warming and droughts if modelling results are anything to go by. Congo basin has several challenges and one of the most challenges being faced is the lack of sufficient knowledge of the forest ecosystem. This challenge is due to insufficient and inadequate research efforts hence the need for fundamental research in this area.

Worryingly is the fact that the puzzle on how these forests and other types will develop and thrive with the changing climate, largely remain unknown owing to lack of long term records showing the responses to changing environmental factors within the tropical forest species (Clark 2004, Phillips et al. 2009). Moreover the crucial role of regulating climate largely remains poorly understood and there is great gap in reliable data about the Congo basin. In this context, dendrochronology is used to help further advance knowledge on the tropical tree species. Tree rings of Pericopsis elata from Biaro area in DRC, were measured and analysed on 20 stem discs.

Therefore the objective of this study was to found out if the growth of Pericopsis elata, a tropical rainforest tree correlates to rainfall and to annual above ground woody biomass increment. Furthermore, specific objectives included to investigate whether Pericopsis elata produced annually distinct growth boundaries, determining the ages of stem discs of Pericopsis elata by ring counting, checking if tree ring growth were synchronised to the rainfall pattern, to see how radial growth changed over time and lastly to have a comparison between the tree ring growth and the above ground woody biomass increments from the ORCHIDEE model.

It was found that Pericopsis elata showed annual growth rings, however, with varying distinctiveness but clear enough for dendrochronological analysis. The age of the stem discs investigated ranged from 89 years to 195 years although this range could be underestimated due to missing rings and uncertainty of sapwood rings. The mean radial growth showed a negative linear relationship with the number of tree rings. Pericopsis elata rings showed synchronicity with the annual rainfall pattern and a marginal correlation. On comparing the ring width index and the above ground woody biomass (AGWB) no explicit relationship could be determined, however synchronicity was seen between AGWB and ring width index peaks.

Pericopsis elata offers a great potential as a suitable species for dendrochronological analysis for the future. However more research on the tree rings in Pericopsis elata can be done and related to the above ground wood biomass in ORCHIDEE for longer time series.

2 Literature review There is a great need to explore the growth of tree species in the tropics and their response to external factors especially at a time of renewed debate on climate change. This review aims to inspire the reader to have a general view of the tropical rainforest environment. It critically explores the carbon stocks and the inconsistencies within the global carbon budget. Tree ring research in the tropical areas is reviewed. Furthermore the relationship between tree growth and climate reconstruction potential of tree rings is explored.

2.1 Tropical rainforests 2.1.1 A general view Tropical rainforests (Figure 1) are important to the global ecosystem. They provide a pivotal habitat for animals and vast species of plant life, and sustain as much as 50% of all species on earth, but their immense biological diversity is yet to be fully explored.

Forests play a major role in regulating climate and in the carbon cycle. The products from forest such as timber and non-timber forest product play a key role to the livelihoods of the people. Studies done by Ndoye et al. (2007), Aveling (2009), Eba’a Atyi and Bayol (2009) show that forest products have various uses in the day to day aspects of people.

Tropical rainforests occupy a rather relatively small portion of the world, restricted between the Tropic of Capricorn and the Tropic of Cancer (Butler, 2011). According to Cramer et al. (2004), the tropical forests encompass the lowland evergreen forest along the equator, moist deciduous and highlands forests, in total having an approximate area of about 17 million km², situated in South America, Southeast Asia and Central Africa. On average the areas have high precipitation totals with half of that attributed from their evapotranspiration.

Figure 1. The global distribution of tropical rainforests Source: Mongabay.com

2.1.2 The Congo Basin The Congo Basin, otherwise known as the Central African tropical rainforest, is one of the largest stretches of contiguous tropical rainforest in the world, second only to the Amazon rainforest and covers about 2 million km² (Mayaux et al., 1998). This forest contiguity spans over the countries of Cameroon, Congo (Brazzaville), the Central African Republic (CAR), the Democratic Republic of Congo (DRC), Gabon and Equatorial Guinea. Findings on the state of the forests of Central Africa by Atyi et al. (2008) however showed that six countries have a combined area of about 4 million km². In 2005, these countries had a population influx of approximately 86 million people, a figure expected to grow rapidly in the next decades due to a very strong population growth rate (United Nations, 2007). The world’s largest tropical swamp lies in the central part of the Congo Basin and the two mountainous areas in Cameroon and to the eastern part of DRC are the most distinct features (Ernst et al., 1998).

Congo basin is constituted of 35 % primary forest, with the remainder being naturally regenerated forest and an insignificant amount of planted forest (FAO & ITTO, 2011). Deforestation and forest degradation, armed conflicts, lack of adequate knowledge of the forest ecosystem and the better use of trees and forest products and services are some of the issues and challenges facing the Congo basin (FAO & ITTO, 2011). Karsenty and Gourlet- Fleury (2006) report harvesting rates of 1 to 2 trees per hectare in the Congo Basin, a logging intensity that remains modest compared to the Amazon basin. Earlier studies by Bawa and

Seidler (1998) reported that selective logging has an effect on the physical structure of forests, positively by creating some space between the canopy and the understorey.

Figure 2. Congo Basin land cover map derived from 300 m satellite ENVISAT MERIS full resolution and 1km SPOT-Vegetation time series. Source: Verhegghen & Defourny (2010) 2.1.3 Pericopsis elata Pericopsis elata is a member of the Faboideae subfamily belonging to the flowering plant family Fabaceae or Leguminosae. It is commonly known as African teak, or Afrormosia is a pioneer species that indicates extensive human disturbances in the African rainforest. It is one of the flagship species of the Congo basin and its distribution covers West and Central Africa, the Soudano-Zambesian region (White, 1983).

Due to its desired characteristics like the hard and durable wood, it makes an excellent substitute for teak in the use of ship rails and decks, shop fittings and first class joinery (Irvine, 1961). Afrormosia is threatened by the illegal logging and habitat loss (ITTO, 2005) and currently listed in the Convention on International Trade in Endangered Species of Wild

Fauna and Flora (CITES), IUCN 2012). According to Betti (2007), the timber volumes for Afrormosia remain in illegality for most of Central Africa due to insufficient knowledge pertaining to the anatomic and physiognomic characteristics of Pericopsis elata.

Research by Toirambe et al. (unpublished) on samples from the semi-deciduous rainforests from Cameroon and DRC of this species shows that Pericopsis elata wood has anatomically distinct growth rings, delineated by marginal parenchyma bands like those from the Leguminosae family.

2.2. Global carbon dynamics 2.2.1 Carbon cycle

The Keeling curve shows the continuous rise of the atmospheric CO2 concentration as observed at the Mauna Loa Observatory in Hawaii since 1950 (Keeling & Whorf, 1999; IPCC, 1995).

Quantification of the global carbon cycle, especially the amount of carbon that is stored in or released from tropical forest is poor and often conflicting and most often lacks consistency (Achard et al., 2004; DeFries et al., 2002). For example, Lewis et al. (2006) reported the contradiction in results due to lack of atmospheric observation points in the tropics. Verbeeck et al. (2011) noted that there is a poor representation of carbon stock inventory sites in the Congo Basin.

2.2.2 Carbon stocks in tropical rainforest According to Denman et al. (2007) and Friedlingstein et al. (2006), predictions for future levels of atmospheric CO2 concentration depend on the interaction of terrestrial vegetation, with the changing world climate. This interaction usually shows strong positive and negative effects involving climate and vegetation. The interaction of tropical forests especially is important, since these forests are carbon dense and highly productive (Malhi and Grace, 2000; Lewis et al., 2006). A global vegetation modelling study by Cramer et al. (2004) shows that the available carbon stocks in the world’s wet tropics are rapidly becoming endangered due to human deforestation. Estimates from the UN Tropical Assessment 1990 report (FAO, 1993) indicate that about 0.5% of the Congo Basin dense forest is lost due to deforestation. However these estimates were done through sampling approaches combined with Landsat Satellite imagery, that are hampered by consistent cloud cover and absence of a Landsat ground receiving station in Central Africa which means an incomplete Landsat record, likely to affect the accuracy of these estimates. According to a recent report by FAO and ITTO (2011) carbon stocks showed a decrease of 1.2 Gt from 2000 until 2010 for the rainforest globally with the greater part attributed to deforestation.

According to Justice et al. (2001), there are typically two methods to estimate carbon emissions from deforested areas. The first one is by subtracting carbon stocks between two periods and the second one is by direct calculation of emissions from an estimation of the rate of deforestation and land use change. Overall, Central African carbon emissions from deforestation are estimated to range from 0.02 to 0.41 Pg year-1 for the period 1980–1990 (Perlack et al., 1994; Gaston et al., 1998). The carbon stocks for this region are estimated at ± 7.23 Pg for 1980, a figure derived from land cover area estimates of FAO/UNEP (1981) and then adjusted by FAO (1993) and Hall & Uhlig (1991). Using a similar methodology, carbon stocks for 1990 are estimated at 6.20 Pg (FAO, 1993) and this almost corresponds to the 6.06 Pg estimates of Mayaux et al. (1998). From these studies, it is clear that the carbon stocks decreased by almost 15% although the figures are subject to debate due to errors arising from some of the inputs (Justice et al., 2001).

2.3 Tree growth 2.3.1 Wood anatomical features Wood anatomical features depend upon the cells produced in the cambium and the differentiation that follows thereof. The International Association of Wood Anatomist (IAWA committee, 1964) defines cambium as “the actively dividing layer of cells that lies between and gives rise to the secondary phloem and the secondary xylem”. The cambium activity is continuous throughout the life of the plant and therefore can give us a great clue on the plants’ entire life history. The wood density for a given tree species contains valuable information about the life strategy for that very tree. Swaine and Whitmore (1988) reported that pioneer trees generally have softwood while those from established mature forests exhibit hard wood. Studies by Toirambe et al. (unpublished data) on Pericopsis elata in DRC found anatomically distinct growth rings marked by continual marginal parenchyma band with average two cells thickness that are more close to the ring borders, flattened thick walled fibres and a widening of the rays.

Atuahene (1996) reasons that the longitudinal growth in trees is a result of the elongation of the derivatives from the apical meristem while the growth of the girth is due to the radial expansion of derivatives from the cambium and interfascicular cambium. While this explanation relating to the critical responses in the growth of the girth to defoliation observed in Pericopsis elata has scientific basis, it does not elaborate comprehensively how these mechanisms work in relation to other external factors that come into play in as far the growth of plants is concerned. Perhaps such shortcomings could be explained by bearing in mind that during stress caused by leaf losses, the apical meristem still absorbs nutrients due to apical dominance. These cells then help to contribute to the height growth. Cambial activity is significantly reduced owing to lower nutrients and therefore fewer derivatives are produced.

2.3.2. Tree rings Basically, tree rings are well defined growth increments encircling the entire stem, and growth zones are partially faint increments which do not encircle the entire stem (Schweingruber, 1996). The basic structure of a tree ring is determined by genetic factors, which in rare cases are specific to particular species. Later on, environmental factors modify the size and cell wall thickness of cells, however some characteristics generally remain constant e.g. the qualitative composition of the tissue (parenchyma), the cell arrangement (ring porous or diffused porous) and the structure of the cell wall (nodular tertiary walls or 8 pitted types). Fichtler et al. (2003) classified the ring structures for species in a Costa Rican wet forest, (see Table 1) according to the growth zone types (Coster 1927, 1928; Worbes 1995) into density variations, marginal parenchyma bands, patterns of parenchyma bands and the variations in vessel distribution or the size of the vessel. Larger canopy trees tend to have larger and clearer rings than those of trees growing in the understorey and hence are better suited for dendrochronological studies (Brienen and Zuidema, 2005).

Table 1: Classification of ring structures according to: distinctiveness of the growth zones, mean increments (calculated by dividing the diameter by the number of rings) of the analyzed individual % and growth zone types (1 = density variations; 2 = marginal parenchyma bands; 3 = patterns of parenchyma bands; 4 = variation in vessel distribution or vessel size) of the studied species (Frichtler et al., 2003)

2.3.3. Factors influencing tree ring growth Couralet (2010) reported that the inter- and intra-specific tree growth variability seems to be higher in the upper canopy layer than in the understorey (Figure 3). They argued that the canopy acts as a buffer that softens the effect of climate variations for the understorey trees. The direct and abrupt changes such as those of weather variables like temperature or humidity first impact the upper forest storey and then progressively reach the lower layers and the environmental conditions are different in the understorey with more constant humidity (Couralet, 2010). Furthermore, findings from a Cameroon forest stand (Worbes et al. 2003) suggests that tall emergent trees, which are more exposed to light than the smaller trees, show higher growth rates and attain larger stem diameters. Stem size, however does not seem to be related to age as young trees can also have large stems provided that their growth rates are high. Worbes et al. (2003) also found that measured tree curves had high year to year variation, in strong contrast to the work of Swaine and Putz (1987) who reported that the tree growth trends are moderate over time.

Tropical climates show periods with a distinct dry season rainfall. Borchert (1999) reported this to be the key climatic driving factor for tree development and cambial activity in these regions. But one would argue that these studies do not as yet give comprehensive analysis on the relationship between soil science and tree growth as also alluded by Schweingruber (1996).

The need to know the growth dynamics and the precise growth rates of tropical trees in their natural environments is ever increasing since this information is very poor and usually the estimates used are full of uncertainties and differ greatly with methodologies used. For instance the work of Bruenig (1996) reported wood production results of up to 18 t ha-1 per year deduced from the net primary production (NPP) which is thrice as much as those reported by Ellenberg (1986).

2.3.4 Dendrochronology: an overview Dendrochronology is a method of dating based on the analysis of patterns of tree ring growth, and can be used to study historical climatic changes. Andrew Ellicot Douglass noted that wider tree rings were produced during wet years and narrower tree rings during dry years on Sequoiadendron giganteum and Pinus ponderosa trees (Webb, 1983). The focus of 10 dendrochronological research to conditions of the temperate areas resulted in low scientific acceptance of existence of tree rings in the tropics or seasonal tree growth (Worbes, 2002). Most authors agree that temperature is rarely a limiting growth factor in the tropical regions as it is in temperate regions, except in the tropical highlands. Most often, moisture deficiencies and seasonality of precipitation cause seasonal changes captured in tree ring growth (Walter and Leith, 1967; Lauer, 1989; Worbes, 1995; Détienne 1989; Mariaux 1995 and Worbes 2002). Most literature reports the inherent error or rather the generalisation in classification of tropical climate as being ever wet only after taking into account the long- term annual precipitation averages (Worbes, 2002). According to Whitmore (1990), Lang and Knight (1983) and Lieberman et al. (1985), dendrochronology is not widely applied in the tropics due to the assumption that tropical trees grow continuously and do not form tree rings.

2.3.5 Dendrochronology in the tropical areas Most researches and predictions are in general convergence as to the immense knowledge gaps and little being known about the tree species from the tropical rainforests, most importantly lacking is the knowledge relating tree growth and climatic interactions (Koenig

2008 and Clark 2004). Considering the role tropical rainforest play in the global climate change agenda and yet little is known how trees will respond given the predictions that precipitation would decrease considerable in the African rainforest (Malhi and Wright 2004, Boisvenue and Running 2006). Dendrochronology is showing positive signs in bridging research gaps, where trees show annual rings (Stahle 1999; Worbes et al. 2003). However tree ring analysis in the tropics is still met with great controversy and analysis of these annual growth structures is a challenging task. “The high phanerophyte biodiversity of tropical forests and woodlands, the vast variety of wood anatomical structures and the occasionally faint growth boundaries and tangential growth inhomogeneity of tropical trees” are some of the difficulties that dendrochronologists face while studying tropical trees (Bräuning, 2010). However, various studies from the Amazonian and Central African tropical rainforest do report the existence of annual rings in tropical tree species (Worbes, 1989; 1995; Détienne, 1989; Gourlay, 1995; Devall et al., 1995; Fitchler et al., 2003; Worbes et al., 2003; Rozendaal, et al., 2005; Couralet et al., 2010). How interesting these results may show, however, it still needs to be investigated for a lot of species within the usually diverse tropical rainforest so that we are able to comprehensively understand the dynamics of tropical forests, and hence adapt to the best possible management strategies.

2.3.6 Development of tropical ring chronologies Stahle (1999) outlined in his paper some useful strategies for the development of tropical ring chronologies which include and not limited to effective sampling strategies for the tree species transcending naturally into tropics, sampling species from already sampled similar botanical families and also cross-dating. To develop a tropical ring chronology, a method known as cross-dating is applied. According to Stokes and Smiley (1968) and Schweingruber (1996) cross-dating is ring width growth pattern matching to minimise and to get rid of counting errors arising from false rings and wedging rings. In tropical region of Central African forest, dated chronologies do not exist yet. Hence cross-dating is done by comparing, growth curves of different trees visually and statistically to bring ring-width series in a synchronous position (Cook and Kairiukstis 1992; Worbes 1995). According to Eckstein et al. (1981) a desirable cross-dating procedure is important and as such shows the influence of an external factor influencing the growth of a tree at a particular place. Another important principle in dendrochronology is the standardisation and characterisation of the tree ring chronologies. Standardisation is done to stabilise the growth variance, since tree growth during early years is characterised by large variances and increments which impact the age of a tree. Schweingruber (1996) reports that standardisation is trend removal by applying a moving average or differentiation technique.

If this trend is not treated, unwanted trends can affect the final analysis. The flexible cubic spline method is widely used to detect and remove this trend for trees with closed-canopy forests (Cook and Kairiukstis, 1992). When the cubic spline curve is fitted to a tree ring series the residuals are computed by dividing each raw value by the value of the fitted spline (Brienen and Zuidema, 2005). For the characterisation and comparison of tree ring series, parameters including mean sensitivity (MS) and autocorrelation (AC) are widely used (Schweingruber, 1996). The calculated average values of the MS give an inference on the sensitivity of the climatic variability and also environmental factors in general. The AC aids in checking the quality of the de-trending method that would have been applied by comparing the AC at the beginning and after the standardisation.

2.3.7 Wedging rings The large complexity of tropical environments leads to a generally greater spatial and temporal variability of growth patterns of trees. This high spatio-temporal variability of moisture regimes makes tropical trees such interesting recorders of the environmental history 12

(Bräuning, 2010). However, the analysis of tropical tree rings poses specific questions that require answers outside of the classical dendrochronology. Worbes et al. (1995) and Worbes et al. (1999) report the presence of distinct rings and other rings that show frequently wedging rings. Several reasons were suggested as being the contributing factors for the formation of wedging rings. Worbes (2002) reported that wedging rings are formed under poor light conditions and also when trees are subjected to competition as shown by Figure 4.).This behaviour takes place with changing light saturation in the life of a tree. For instance with changing competition pressure that occurs when a tree falls or canopy gaps are formed or simply neighbouring trees are logged which results in the imbalances in local supply of carbohydrates, water, mineral substances and phytohormones, and hence wedging rings are formed (Dünisch et al. (1991). LaMarche et al. (1982) report it to be a result dictated by the genotype of the plant.

2.3.8 Relationship between tree growth and rainfall The phenomenon of how tree rings grow is difficult, tree ring formation can be seen as an equation with numerous variables, and solving such an equation is difficult (Schwiengruber, 1996). From the theoretical perspective of Fritts (1976), tree ring formation is influenced by the interaction between weather factors, soil properties, and physiological reactions that occur in trees. Further, he stated that the tree ring width is the result of complex chain reactions. For 14 example low precipitation results in reduced soil moisture levels and increased drought in the tree which consequently leads to a reduced net photosynthesis and directly reduced rates of cell division. This scenario will have greatly reduced xylem cell being differentiated and finally a narrow ring is formed. His explanation seems logical but from a soil science point of view it requires comprehensive studies to ascertain the levels of soil moisture that have a significant impact on tree growth in the tropical rainforest, since an argument can be brought forward questioning the depletion of soil moisture levels, given the significantly high precipitation levels in the tropics and a very brief period without rainfall.

Numerous studies attempted to link the growth of tree rings to precipitation, mostly in and around the South American rainforests (Worbes, 1999 for Venezuela; Brienen et al., 2005 for Bolivia; Hayden et al., 2010 for Mexico). Few studies have been done on this subject. Schöngart et al. (2006), Wils et al. (2010), and Worbes et al. (2003) reported on this in Northeast and West African forests, Ethiopia and Cameroon respectively. Most of this research, points to a greater correlation with climate variable explained by precipitation. As mentioned earlier, precipitation is the most limiting factor for growth in the tropics. The discrimination of growth rings on tropical stem discs is generally more difficult as compared to the temperate areas (Détienne 1989, Sass et al. 1995). However more work can still be done as many species in tropical regions show great potential.

3 Material and methods 3.1 Study site Biaro forest is situated south of Kisangani, the river port city of DRC. It lies on the eastern side of the road between 21 and 41 km in the direction of Ubundu together with another protected forest called Yoko National Reserve as shown in Figure 5. The total area for the reserve is 6975 ha, with predominately young and old secondary and primary tropical rainforest. According to Mukinzi et al. (2005), the forest in this region is dense and diverse. To the west side of the road, the forest is used for agricultural purposes.

This part of DRC lies at about 447 m above sea level. It receives high average annual rainfall totals throughout the year, with annual mean precipitation of 1728 mm. Two short dry seasons occur from December to February and from June to August, with a mean precipitation of 60mm for the driest month. The mean annual temperature is 24.5°C and remains relatively constant during the year. A high mean annual humidity of about 82% characterises this region (Nshimba, 2008). The climatic data from the nearby area of Yangambi from 1961 to 2009 was also provided.

Figure 5. Location of the Biaro forest reserve near Kisangani, DRC

3.2 Data collection 3.2.1 Sampling and preparation of samples The tree-ring widths measurements were done on 20 Pericopsis elata stem cross-sections from the Tervuren Wood reference collection (Table 2). These stem discs originate from trees of the Biaro forest in the DRC and were felled between 2007 and 2008 by logging companies requiring the useful timber from Afrormosia.

Table 2: Stem cross-section ascension numbers for the Pericopsis elata used in this study, showing the dates they were felled and the number of radii selected for analysis, (Tervuren Wood). Sample ascension number Date felled Number of radii selected (Tervuren Wood) TW60932 11/03/2007 3 TW60933 11/03/2007 5 TW60934 11/08/2007 4 TW60935 11/04/2007 4 TW60937 03/19/2008 4 TW60938 02/27/2008 4 TW60941 11/16/2007 4 TW60942 11/10/2007 2 TW60944 2007 4 TW60945 06/05/2008 2 TW60947 11/09/2007 5 TW60950 11/10/2007 4 TW60951 04/13/2007 3 TW60952 04/24/2008 4 TW60953 10/15/2007 4 TW60959 05/07/2007 4 TW60960 11/01/2007 4 TW62214 11/20/2007 2 TW62220 10/29/2007 4 TW62221 2007 4

A Rotex RO 150 FEQ- Plus geared eccentric sander with sandpaper of grit sizes P500, P800 and P1200 was used progressively (Festool, 2012). This tool is useful for coarse sanding, fine sanding and polishing. After sanding with a grit-size of 1200, the wood structure of the stem disc became visible with some tree-rings being clearly visible to the naked eye.

Figure 6. Sanded surface after the superfinish of a stem cross-section of Pericopsis elata from the Biaro Forest Reserve.

After polishing, two to six radii were selected per tree disc to identify, count and measure the tree ring width. The annual diameter growth rates were calculated by averaging the ring- widths from those radii. The radii were selected in such a way that they corresponded to the measured average diameter of the stem disc and the number of the radii chosen was further influenced by the regularity of the tree stem discs. The number of radii was increased for stem discs with increased eccentricity and difficulties with the ring borders.

The tree ring boundaries were examined macroscopically with the naked eye and microscopically. After the super finish polishing of the stem discs, some of the tree rings could be seen easily without the use of a microscope. For the others, an Olympus SZ51 stereo microscope was used with the lowest magnification being preferred due to clarity of the rings. Where rings were not all that visible, especially in the sapwood, safranin, that stains the xylem vessels black, was added to improve the identification of the rings. The incidence of ring wedging was identified by marking every tree ring and interconnecting every tenth ring from radius to radius. This was done as a quality check considering the difficult work and the errors involved in dendrochronology.

The International Association for Wood Anatomists, (IAWA) hardwood features list (Wheeler and Gasson, 1989) helps on the examination of the tree rings. During ring identification, false rings and wedged rings encountered during every stem disc were noted.

3.3.2. Tree Ring imaging Tree ring images were taken along the predetermined radii using the Olympus UC30 camera mounted on the Olympus SZX 120 stereo microscope and coupled with the Cell B life science basic imaging software. About one to five rings were taken per image with the following image starting from the last ring of the previous image. This produced an overlap, required for the stitching process.

Figure 7. Image taken along the radius of the TW62220 stem section, showing six tree rings. The black arrows point to the ring boundaries along the radius. The growth is from left to right.

3.3.3 Stitching the images using Fiji ImageJ In a next step, the images were stitched to each other, reconstructing the whole predetermined radius with marked tree rings using the open source Fiji ImageJ software. Images were converted to TIFF format before it can be read fine as advised by Preibisch et al. (2009). After the reconstruction of the whole radius, a mosaic image similar like that shown in Figure

9 is produced. A quality check was performed to cross check the number of rings visible on the acquired mosaic image to the initial counted rings along the preselected radius.

Figure 8. A mosaic image of the reconstructed images along the radius (TW62220, radius 1)

3.3.4 Measurement of ring width The Fiji ImageJ software was used to measure the ring width. Prior to the measurement, a scale was set to determine the correct measurements. This was done by calibrating the 1 millimetre scale. For this, an image of the calibration ruler was taken under similar resolution and image dimension as the other tree ring images. On this image, the number of pixels corresponding to 1 mm was determined. For this study the global scale was calculated and set to 166 pixels per millimetre. Tree ring measurements were done from the shortest possible distance between two ring boundaries and at right angle to the preceding ring to the nearest 0.01 mm.

3.4 Data treatment and analysis Synchronicity in growth rates between different radii within the same stem section was visually checked and statistically analysed with TSAP win software (Rinn, 2003. The TSAP- win software was used for the cross-dating analysis to match the ring widths series, which is an important procedure to factor out the presence of wedged rings. The incidence of false rings was also observed and reported in this study. Average tree ring growths are also important for checking synchronicity between trees of similar species. When synchronicity is observed between trees of a similar species, this might indicate a similar influencing outside factor on tree radial growth as reported by Worbes, (1995) and Cherubini et al. (1998).

The mean sensitivity is a measure of ring variability within the measured series and it mirrors the response of the radial stem growth to external factors (Fritts, 1976; Schweingruber 1996), while the autocorrelation mirrors the year to year dependence of the measured ring width values to the influence of a growing season on the following (Holmes 1983; Grissmo-Mayer

2001). Under these circumstances, appropriate auto regression and de-trending models are performed on the data so as to purely highlight the influence of climatic variations on the radial tree growth.

3.4.1 Climatic effects analysis on radial tree growth The results from standardisation and the auto correlation (master chronologies) are then compared to the available rainfall data of 1961 to 2009 from Yangambi, DRC. These data are analysed using appropriate regression statistics to find correlations between the radial tree growth and the annual rainfall values.

Two statistical parameters namely the Pearson’s correlation (r) and the t-value of Baillie- Pilcher were taken into account to evaluate the success of cross-dating (Baillie and Pilcher 1973). Also a non-statistical parameter called Gleichläufigkeit (GLK) (Eckstein and Bauch 1969) or percentage of parallel run (ppr) which reflects the percentage of oscillations in the same direction within the overlapping interval was used still, especially for tropical trees, visual control and expert knowledge of Pericopsis elata wood anatomy and tree rings are indispensable. The work of Trouet et al. (2010) gives lower thresholds for ppr (≥ 60%) and t- values (≥ 2) in the tropics compared to the temperate regions.

3.4.2 Average ring width and above ground woody biomass increments from ORCHIDEE The average ring width for the Pericopsis elata chronology was compared with above ground woody biomass increments from the global process-based model Organising Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE). According to Krinner et al. (2005), ORCHIDEE describes the water, energy and carbon dioxide flux changes affected by climate, vegetation and soil dynamics.

4. Results and discussion 4.1 Annual rainfall and mean seasonal distribution The mean annual rainfall distribution for Yangambi, a city located along the Congo River 85 km northwest of Kisangani, DRC is shown in Figure 9. The graph shows a bimodal seasonal rainfall pattern with higher rainfall from March to May and from September to October. The wettest season coincides with the months of September, October and November with 176 ± 26 mm, 211 ± 30 mm and 180 ± 27 mm rainfall respectively and a drier season in December, January and February with 121 ± 23 mm, 80 ± 22 mm and 98 ± 19 mm rainfall respectively.

Figure 9. Mean annual rainfall distribution from 1912 – 2007 for Yangambi. Data source: (climexp.knmi.nl )

Figure 10 shows higher annual precipitation totals with the mean rainfall total being 1723 ± 8 mm from the years 1912 to 2007. At first sight it would seem that there are no long term trends. The year 1966 shows the highest mean annual rainfall of 2241 ± 35 mm and the lowest mean annual rainfall is 1275 ± 21 mm for the year 1921. The rainfall values are typical of the tropical rainforest which are generally characterised by high rainfall totals. However, a closer look at the figure does not clearly capture this seasonality in rainfall distribution, According to Worbes (2003), this generalisation of most tropical rainforest climate as being over wet is erroneous if more emphasis is put on annual precipitation. This is true considering that annual precipitation does not explain seasonality.

Figure 10. Annual precipitation from 1912–2007 for Yangambi. Data source: climexp.knmi.nl

4.2 Distinctiveness of tree growth rings and other measured parameters The distinctiveness of the growth rings varied between the radii within one stem cross section and also between the different stem cross sections studied. Tree rings around the pith and in the outer rings of the sapwood areas were difficult to identify and demarcate. The wider rings were not as difficult to distinguish as the narrower rings. The narrower rings showed a tendency to form wedging rings or to eventually diffuse out into the wood and become missing rings. However, with the cross-dating procedure, errors resulting from false or missing rings were reduced. The tree ring growth types that were found during this study can be broadly classified into four ring types, according to Coster (1927), Worbes (2005) and Fritchler et al. (2003). The first ring type shows variations in vessel distribution and size and has a clear to rather clear distinction. The second type shows a repeated pattern of alternating fibre and parenchyma bands, also having a clear to rather clear distinction. The third ring type showing marginal parenchyma bands gave a very clear distinction and the last ring type showing density variations that were more prominent in rings from the sapwood. In most cases the analysed tree stems showed two or more of these ring classes being found within each stem disc .The first ring type, the one that showed variations in vessel distribution and size was encountered the most than the other ring classes during the ring identification. These findings are not surprising as the other species from the main Fabaceae family showed similar ring types, for instance Amburana cearensis (Zuidema, 2005) and Dipterex panamensis

(Fritchler et al., 2003). Tropical tree growth rings are more difficult to distinguish as compared to those from temperate areas (Détienne, 1989; Sass et al., 1995).

Table 3 shows a summary of the ring measurements done for all the studied stem discs. The average diameter of the trees measured is 81 ± 9 cm, the average annual radial growth is 2.9 mm year-1. Highest values are recorded for the tree stems TW60932, TW60937, TW60938 and TW60950 with values of 3.7 mm year-1, 3.7 mm year-1, 3.8 mm year-1 and 3.4 mm year-1, respectively. These are very young trees with number of tree rings ranging from 89 to 116 years. The older trees TW60935, TW60953 and TW60947 with ages of 175, 195 and 173 years respectively, show lower average annual radial growths, being 2.3 mm year-1 on average. The ages found could be an underestimation of the real age for the trees in which the discs were cut from due to missing and not very distinctive rings in the sapwood and also at the pith. The mean growth rates are lower compared to other species from the Congo River basin, for instance studies by Couralet (2010) on T. superba (7.9 mm year-1) and Détienne -1 (1998) reported 9 to 11 mm year for tree ages between 18 and 55 years. However these findings are in tandem with the low mean annual radial increment of 2.2 mm year-1 reported for P. balsamifera (Couralet, 2010).

Table 3: Summary of tree-ring measurement results for the twenty stem cross sections of Pericopsis elata from Biaro forest, DRC. Stem disc number Mean Selected Age Growth Mean annual (Tervuren Wood) diameter number (rings span radial growth (cm) ± SD of radii counted) (Years) (mm) TW60932 92± 1 3 116 1892 - 2007 3.7 TW60933 83± 3 5 131 1877 - 2007 3.3 TW60934 87 ± 15 4 154 1854 – 2007 2.7 TW60935 72 ± 11 3 175 1833 – 2007 2.1 TW60937 73 ± 11 4 89 1920 – 2008 3.7 TW60938 84 ± 4 4 115 1894 – 2008 3.8 TW60941 77 ± 6 2 120 1888 – 2007 2.9 TW60942 86 ± 8 2 134 1874 - 2007 2.5 TW60944 84 ± 4 4 117 1891 - 2007 3.6 TW60945 91 ± 5 2 148 1861 – 2008 2.8 TW60947 95 ± 30 4 173 1835 – 2007 2.2 TW60950 ** 4 107 1901 - 2007 3.4 TW60951 51 ± 17 3 149 1859 – 2007 2.3 TW60952 83 ± 2 4 131 1878 – 2008 2.9 TW60953 89 ± 13 3 195 1813 – 2007 2.5 TW60959 92 ± 13 4 166 1842 – 2007 2.5 TW60960 83 ± 6 4 130 1878 – 2007 2.7 TW62214 92 ± 3 2 153 1855 - 2007 2.7 TW62220 65 ± 14 4 116 1892 - 2007 3.3 TW62221 59 ± 4 4 100 1908 - 2007 2.9 ** no measurements taken

4.3 Checking quality and inter-series correlation Table 4 shows the results from the statistical analysis done to cross date intra tree series. Out of the 20 stem discs about thirty percent of them were rejected, with the remaining 14 stem discs being used in the eventual analysis and further for the computing of the tree ring chronology. Those that were rejected did not show any synchronicity whatsoever with other ring series. Incidence of wedging rings or partially missing rings were factored out during the cross-dating and hence the rejection of other series. The stem disc TW60947 showed a strong

25 intra tree correlation with a t-value of 3.6. Parameter threshold values for boreal and temperate regions that indicate successful cross-dating are t ≤ 3.5 and the GLK ≥ 70 %. The parameter threshold values to estimate the signal of strength of the mean chronologies were adapted for this study to t ≥ 2 (P < 0.05) and GLK ≥ 60%. The threshold values for the tropical areas were lowered since no ring chronologies exist yet for this study area and also considering that, although the exact felling dates were known, the exact age as determined by the number of rings cannot be exactly ascertained, due to missing and false rings (Trouet et al., 2010; Schöngart et al., 2004). According to Baillie and Pilcher (1973), the t-value shows the extent of affinity of time series with each other, while taking into account the number of observations. However the tropical tree ring studies by Schöngart et al. (2005) showed significantly higher correlations (0.32< r <0.59) and (2.9 < t 7.5).

Table 4: Descriptive statistics of 14 tree ring series showing correlation in Pericopsis elata, from Biaro forest, DRC. Stem disc number Gleichäufigkeitsko Intra-tree t-value Baillie and (Tervuren Wood) effizient correlation Pilcher (TVBP) (GLK) (%) TW60932 55 0.17 2.0 TW60933 63 0.31 1.9 TW60934 55 0.17 2.2 TW60938 62 0.23 2.8 TW60942 58 0.31 2.4 TW60947 55 0.25 3.6 TW60950 58 0.46 1.5 TW60951 58 0.59 3.4 TW60953 60 0.23 2.2 TW60959 59 0.26 3.2 TW60960 59 0.30 2.2 TW60214 70 0.54 5.9 TW60220 60 0.14 2.1 TW60221 62 0.27 2.6

The correlation for the four radii of stem disc TW60947 produced the most successful cross- dating procedure (Figure 11). Out of the fourteen stem discs, an average chronology could be formed consisting of nine stem discs. Problems were encountered with dissimilar lengths and differences in ages or number of rings and hence cross-dating was further performed. The cross-dating parameters for the nine Pericopsis elata ring series were GLK ≥ 53%, t ≥ 1.7 from the student’s t-test (p < 0.05) and the Pearson correlation = 0.11 after Baillie-Pilcher standardisation of the series. These values are generally low from the standard thresholds but the visualisation and knowledge of the tree stems helped for a successful cross-dating. The average chronology shows that every year is represented by four trees. The ring curves show a high variation, which is expected for a typical tropical tree, however this shows great difference when compared to, for instance, Swaine and Putz (1987) who reported that the growth rates show a rather uniform pattern over time.

Figure 11. Time series of tree ring widths for four radii in Pericopsis elata (TW60947).

4.4 Correlation between ring widths and rainfall The average growth curve of the nine Pericopsis elata trees was compared to the rainfall records as shown in Figure 12. The graph shows a clear average tree chronology trend. The correlation between this age trend (1912 to 2003) and the contemporaneous rainfall data showed to be marginal. Highest correlation value between the average chronology and rainfall was 0.11 and this was not improved by standardisation of the ring widths series. 27

Positive synchronous peaks of rainfall and tree ring index were seen in the years 1935, 1954, 1971, 1973, 1974, 1981, 1990, 1991, 2005 and 2007. Also synchronous negative peaks during the years 1936, 1964, 1967, 1970, 1971, 1972, 1975, 1976, 1982, 1983, 1989, 1993, 1996, 1998 and 2006 were seen. The correlation is such that the rainfall of the end of the last year in a particular season has a positive influence on tree ring growth. The low correlation value found here is comparable to other studies from the tropics (Worbes, 1999; Stahle et al., 1999; Couralet, 2010).

Figure 12. The chronology (1833-2007) of Pericopsis elata represented by four trees and contemporaneous annual rainfall. (Pearson’s correlation = 0.11).

Correlations of the ring widths and precipitation showed that the ring width index of Pericopsis elata reacts to different rainfall variables and to running two monthly rainfall sums throughout the year. Figure 13 shows the monthly correlations between ring width indices and rainfall for Pericopsis elata. From the graph, the highest positive synchronicity between the chronology and rainfall is seen in the previous month of September (r = 0.27 p < 0.05) and also negative synchronicity in the previous month of November (r = - 0.21 p < 0.05).

There was a rather significant correlation (r = 0.29; P < 0.01) between ring width chronology and rainfall sums of September and October of the previous year. However there was no

28 correlation reported for the rainfall combinations of June and July (r = -0.07 p < 0.01); January and February (-0.13 P < 0.01); May and June(r = 0 P< 0.01); July and August (r = - 0.12 P < 0.01) of the previous year. There was no synchronicity seen between ring width and current rainfall throughout the whole year and during the certain seasons. The absence of correlation in the current year and the ring width index was not surprising given that the sapwood rings were poorly recorded in most stem discs and hence the ages could be underestimated. The other reason could be that the ring width growth was not controlled by the variations in rainfall during the different times of the year as also reported by Couralet et al. (2010). More so, the poor visibility of the parenchyma bands in the sapwood could have led to underestimation of the real age as also reported by Brienen and Zuidema (2005).

Figure 13. Correlation between tree ring index and rainfall for Pericopsis elata in DRC. Significant correlations (Pearson; P < 0.05) are shown by the solid columns (blue)

4.5 Growth rates Figure 15 shows the mean radial growth rate of the raw data from the 20 stem discs of Pericopsis elata plotted against the number of measured tree rings. Figure 14 shows that there is a negative linear relationship between radial growth and age with R2 for the fitted regression line being 0.62. A higher mean annual radial growth rate (3.7 mm year-1) corresponds to a tree stem disc with seemingly less number of rings (89 rings). On the other hand, high numbers of tree rings (195) have lower mean annual radial growth rates of about (2.5 mm year -1). Betti (2007) reported mean growth rates of 0.4 cm year-1 in Pericopsis elata, comparable to the average growth rates found in this study. Several factors influence the

29 radial growth in trees, among them the local conditions in which the tree is growing. Younger trees are assumed to have higher growth rates, which is expected but is not always the general trend as great caution is required when analysing and interpreting the rings from the juvenile wood (Brienen and Zuidema, 2005).

Figure 14. The relationship between mean radial growth and the number of rings denoting age.

4.6 Comparing above ground woody biomass to the ORCHIDEE Model In this section description of a first exploratory test to compare the tree ring data with simulated above ground woody biomass increment from a global vegetation model. In Figure 15, the ring width index is plotted along with above ground woody biomass (AGWB) increments from the global terrestrial ecosystem model ORCHIDEE driven by ERA-Interim six hourly meteorological data (Berrisford et al., 2009) and compared against each other from the years 1989 until 2003. There is no significant correlation between AGWB increment and the ring width index (r = 0.08). However, remarkable synchronous positive peaks of tree ring width indices were seen during the years 1995, 1996, 2001 with their corresponding above ground woody biomass increments of 84.1 g C m-2 year-1 , 93.5 g C m-2 year-1, and 92.3 g C m-2 year-1 , respectively. Also synchronous negative peaks are visible for the years 1991, 1997 and 2002 with corresponding above ground woody biomass increments of 86.5 g C m-2 year-1, 90.8 g C m-2 year-1 and 80.2 g C m-2 year-1 respectively.

Above ground woody biomass increments values correspond to those of tropical rainforest areas. The above ground woody biomass increments found in this study 0.84 Mg C ha-1 yr-1,

0.94 Mg C ha-1 yr-1, 0.92 Mg C ha-1 yr-1 and 0.91 Mg C ha-1 yr-1 being the positive peaks ( values converted from g C m-2 yr-1 to Mg C ha-1 yr-1) are little lower but comparable to, for instance Djomo et al. (2011) reported annual increment of carbon biomass values in the National Park in Cameroon of 1.98 Mg C ha-1 yr-1 in the overstorey of the forest.

Burrows et al. (2002) reported a net above ground annual carbon increment of 0.53 t C ha−1 year−1 in an Australian savannah despite the below average rainfall during their study period. In the tropical forest of India, carbon accumulations of about 0.13 t C ha -1 year-1 of which 0.58 ± 1.18 from surviving trees and 0.55 ± 0.33 added by recruits were reported by Bhat and Ravindranath (2011).

Figure 15. ORCHIDEE above ground woody biomass (AGWB) increment (1989- 2008) plotted along with ring width index (1989-2003).

Numerous reasons could help explain the absence of a rather strong correlation between the above ground woody biomass increments and the ring width index. The sapwood rings as reported earlier in this research were problematic to distinguish which would imply that most of the rings formed later in the tree life, rings lying in the sapwood, numbering about 10 to 20 rings could have been missed. This resulting in the underestimation of overall tree ages hence

31 the lack of correlation. Perhaps with a comparison between a longer time series of above ground woody biomass increments (with driver data back to 1954) and more tree rings measured outside of the sapwood area in the chronology, a stronger correlation could be possible. Also lack of data and uncertainty associated with models in biomass estimates could have actually underestimated the AGWB increments. For instance uncertainty of models resulting from inaccurate accounting for the carbon estimates (Henry et al., 2010). Ebuy et al. (2011) states that the accurate estimation of biomass in the Congo basin tends to be the missing part in the future scenarios about above ground biomass.

5 Conclusions Pericopsis elata forms annual tree growth rings that vary from very clear to rather clear in their distinctiveness, with the wider rings usually being more easily identified than the smaller ones. The smaller rings showed a tendency to develop wedging rings or otherwise missing rings in nearly all of the 20 stem discs investigated. The sapwood area rings which had about 10 to 20 rings are difficult to discriminate since the wood is diffuse. Hence the tree age from the direct ring counts cannot be very accurate.

Growth rates for Afrormosia were generally low, compared to other studied tropical rainforest species. A near linear relationship was found between tree ring width and the number of rings, with younger trees showing higher annual radial growth increments and older trees had lower annual radial increments.

Pericopsis elata showed a generally good intra-tree series correlation and a successful cross- dating of tree radii. However the issue of the last ring not being exactly known affected the accuracy of the average chronology.

The average ring width chronology dating from 1933 to 2007 showed a marginal correlation with the precipitation of the region. Tree growth is influenced by the rainfall of the preceding season. The development of accurate tree ring chronologies in the Congo basin will improve knowledge of how tree species respond to climate parameters. However the major limitation to accurate ring chronologies is the accuracy of the last rings which were not very visible.

The Fiji ImageJ showed good potential as an imaging software helping greatly in the identification of tree rings. However it showed problems during the uploading of images and stitching process hence great deal of time was used.

The correlation between above ground woody biomass increment and ring width index was not very strong. The soil water relations for the Pericopsis elata could be further studied to strengthen and build on the tree ring chronology found. It could be further explored how the wood structure responds to climate parameters as opposed to only checking the ring width measurements. It will be interesting to extend this research further by including other

33 important climatic parameters like solar radiation since they equally play an important function in tree growth.

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7 Appendices

Appendix 1. Above ground woody biomass ORCHIDEE Year AGWB (gC m-2 yr-1) Mean9 1989 109.25 90 1990 90.61 106 1991 86.46 95 1992 85.71 84 1993 83.22 118 1994 83.07 104 1995 84.11 92 1996 93.46 102 1997 90.81 88 1998 85.24 110 1999 86.57 98 2000 90.01 85 2001 92.31 113 2002 80.21 73 2003 75.35 87 2004 69.54 2005 61.34 2006 66.51 2007 64.4 2008 66.66 R 0.08

Appendix 2. Climate correlations with ring width of Pericosis elata. Months of Year r previous year r current year J -0.18 0.04 F -0.01 0.09 M -0.03 0.06 A 0.08 -0.10 M 0.00 -0.02 J 0.01 -0.14 J -0.11 0.15 A -0.06 0.06 S 0.27 -0.10 O 0.15 -0.13 N -0.21 0.15 D -0.08 -0.04 Mean9 JJ -0.07 0.01 JF -0.13 0.08 MJ 0.00 -0.09 Yearly rainfall 0.03 0.02 SO 0.29 -0.13 JA -0.12 0.12

** (p< 0.05)