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African Journal of Range & Forage Science Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tarf20 A century of woody plant encroachment in the dry Kimberley savanna of South Africa David Warda, M Timm Hoffmanb & Sarah J Collocotta a School of Life Sciences, University of KwaZulu-Natal, Pietermaritzburg, South Africa b Plant Conservation Unit, Botany Department, University of Cape Town, Cape Town, South Africa Published online: 06 Jun 2014.

To cite this article: David Ward, M Timm Hoffman & Sarah J Collocott (2014) A century of woody plant encroachment in the dry Kimberley savanna of South Africa, African Journal of Range & Forage Science, 31:2, 107-121, DOI: 10.2989/10220119.2014.914974 To link to this article: http://dx.doi.org/10.2989/10220119.2014.914974

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A century of woody plant encroachment in the dry Kimberley savanna of South Africa

David Ward1*, M Timm Hoffman2 and Sarah J Collocott1

1 School of Life Sciences, University of KwaZulu-Natal, Pietermaritzburg, South Africa 2 Plant Conservation Unit, Botany Department, University of Cape Town, Cape Town, South Africa * Corresponding author, e-mail: [email protected]

Woody plant encroachment is frequent in dry savannas. Grazing is often considered to be a major cause of encroachment in dry savannas because grasses are removed by livestock, leaving bare areas for trees to colonise in wetter years. Earlier experiments conducted in the Kimberley area of the Northern Cape showed that neither fire nor grazing was important for woody plant encroachment. We used aerial and fixed-point repeat ground photographs, including historical photographs taken at the time of the Second Anglo-Boer War of 1899–1902, to assess the scale and timing of woody plant encroachment in the dry savannas near Kimberley in South Africa (mean annual rainfall  300–400 mm). There were large increases in woody plant encroachment in most areas. Even at the battlefield of Magersfontein, where grazing has been virtually absent since its protection in 1960, we found that encroachment by trees and shrubs has occurred. Using aerial photographs, we found that the rate of encroachment has increased substantially since 1993. However, repeated photographs at certain sites indicate that encroachment produced cohorts of trees. We show that global drivers are perhaps of greater importance than local drivers such as heavy grazing and absence of fire.

Keywords: aerial photography, fixed-point photographs, global climate change, semi-arid, shrub encroachment

Introduction

Long-term data sets are needed to differentiate between trees may access topsoil water and are able to germinate local causes of woody plant encroachment in savannas, en masse. This would then lead to woody plant encroach- such as grazing (Skarpe 1990a, 1990b; Moleele and ment (Walter 1939). A wide range of studies have claimed Perkins 1998), fire (Higgins et al. 2000; Bond et al. 2003; that heavy grazing (predominantly by cattle) has resulted Higgins et al. 2007) or physical disturbance (e.g. Ward in woody plant encroachment (Walker and Noy-Meir 1982; 2009, Puttick et al. 2011) from effects of global causes such Hobbs and Mooney 1986; Scholes and Archer 1997;

as CO2 increases (Ward 2010; Wigley et al. 2010), nitrogen Roques et al. 2001). Downloaded by [196.215.105.61] at 01:10 14 July 2014 deposition, temperature or rainfall (see e.g. Archer et al. Much of the central area of South Africa is consid- 1988; Wiegand et al. 2005; Kraaij and Ward 2006; Wiegand ered semi-arid to arid (Hoffman and Ashwell 2001). The et al. 2006; Bowman et al. 2010; Petty and Werner 2010). area may be described as a semi-arid savanna, albeit Despite the importance of woody plant encroachment in dry considerably drier (mean annual precipitation [MAP]  regions due to greater dependence on ranching than other c. 300–400 mm) than those studied by most other agricultural activities (see e.g. Rappole et al. 1986; Ellis researchers in southern Africa (e.g. O’Connor 1995; Bond et and Swift 1988; Skarpe 1990a, 1990b; Idso 1992; Jeltsch al. 2003; Higgins et al. 2007; Buitenwerf et al. 2012; but see et al. 1997; Moleele and Perkins 1998; Cabral et al. 2003), Rohde and Hoffman 2012). Most of the Kimberley region is there have been surprisingly few studies of encroachment only suitable for grazing (primarily by cattle and sheep). It in these ecosystems (Archer et al. 1988; Kraaij and Ward is generally recognised that fire is an uncommon factor in 2006; Joubert et al. 2008). dry savannas owing to diminished fuel loads provided by There is considerable debate over the role of grazing grasses (Teague and Smit 1992; Meyer et al. 2005; Kraaij in woody plant encroachment, particularly in arid and and Ward 2006). Indeed, Bond (2008) and Higgins et al. semi-arid environments (e.g. Kraaij and Ward 2006; (2010) consider fire in African savannas to be of importance Eamus and Palmer 2007; Ward 2005, 2010; Bond and only above about 750 and 820 mm MAP, respectively. Midgley 2012). One of the earliest studies to consider There is considerable debate regarding the role of global the possible effects of grazing was that of Walter (1939) climate change in woody plant encroachment, especially

(reviewed in Ward et al. 2013). Walter (1939) suggested with regard to increases in CO2 (e.g. Archer et al. 1995; that trees had deeper roots than grasses and that an Bond and Midgley 2000; Körner 2006; Ward 2010; Bond equilibrium existed between trees and grasses in their and Midgley 2012; Leakey and Lau 2012). Regarding

access to water. When grasses are removed by grazing, potential effects of global increases in CO2, Eamus and

African Journal of Range & Forage Science is co-published by NISC (Pty) Ltd and Taylor & Francis 108 Ward, Hoffman and Collocott

Palmer (2007) and Ward (2010) have suggested that Hoffman 2012). This requires that the observer relocate increased photosynthetic and water-use efficiency could and rephotograph the historical image from as close to the

lead to increases in the density of C3 plants (trees) relative original position as possible and then interpret the changes

to the current dominance of C4 grasses in semi-arid and visible in the matched images in terms of the environ- arid environments. Eamus and Ceulemans (2001) note mental and cultural influences on the landscape. A number that the proportional increase in tree growth is larger under of British and Boer photographers accompanied the military xeric than mesic conditions. Körner (2006) and Leakey columns during the Second Anglo-Boer War of 1899–1902,

and Lau (2012) also note that effects of increased CO2 are allowing us an insight into the environments where the often greater in dry years than in wetter years. Wigley et al. soldiers fought during the Relief of Kimberley by the British (2010) and Buitenwerf et al. (2012) recognise that claims on 15 February 1900. In many cases, these photographs

regarding the effects of increasing CO2 on woody cover are of adequate quality to warrant comparison of the require long periods during which other major drivers such environment with the current day (Hoffman and O’Connor as fire and grazing need to be held constant. 1999; Hoffman and Rohde 2011; Rohde and Hoffman 2012). Other global factors such as rainfall and temperature We were also fortunate that Homer Shantz, from the have also changed over time as a consequence of global University of Arizona, USA, took photographs of Carter’s climate change, as is the case with nitrogen pollution. Ridge and a site nearby (on the road from Kimberley to

Besides the predicted increase in CO2 described above, Schmidtsdrift; hereafter called Schmidtsdrift Road) in 1919, we can detail some of the expected trends in the remaining and then returned in 1956 to repeat these photographs. global drivers: One of us (MTH) returned to Shantz’s sites in 1989 and (1) Some studies have indicated that, in dry regions, there 1995 to repeat the photographs, prior to taking subsequent should be greater variance in rainfall (e.g. Karl and photographs in 2010. Knight 1998; Kraaij and Ward 2006; Bates et al. 2008; To ensure that the changes observed in the photographs Volder et al. 2010), with an increase in very large precip- were not merely due to local (small-scale) changes, we itation events (Karl and Knight 1998) and more frequent also accessed aerial photographs, which are effective in droughts (Volder et al. 2010). determining landscape-level changes in the environment (2) Wakeling et al. (2012) have suggested that there is a (Goslee et al. 2003; Laliberte et al. 2004; Rango et al. 2005; ‘treeline’ between savannas and highland grasslands Britz and Ward 2007; Morgan et al. 2010). We obtained that may be caused by the greater frequency of aerial photographs from Magersfontein dating back to 1940, freezing nights in grasslands or at least a greater which were repeated irregularly (see Methods below). The frequency of low temperature days below minimum battleground at Magersfontein was declared a national temperatures required for growth. Increases in monument in 1927. The McGregor Museum took control minimum temperatures may result in a change from over the battlefield in 1960 and their botanist, R Gubb, grasslands to savannas. recorded that there was heavy grazing that occurred prior (3) Wenig et al. (2003) have shown that South Africa to this time. There has been little grazing since that time. has some of the highest levels of nitrogen oxides in Thus, we could separate the effects of grazing from other the industrial Highveld (Gauteng and Mpumalanga sources of woody plant encroachment. provinces). This nitrogen may be redeposited in We addressed the following questions: savanna environments and may affect tree:grass ratios. (1) Has there been a change in vegetation, particularly We used repeat photography (both fixed-point and aerial) woody plant cover, since 1899–1900?

Downloaded by [196.215.105.61] at 01:10 14 July 2014 to assess the scale and timing of changes in vegetation (2) Is the effect dependent on spatial scale? In other words, since the Second Anglo-Boer War of 1899–1902 (fixed- do local-scale fixed photographs show different results point photography) and since 1940 (aerial photography) from large-scale aerial photographs? near Kimberley, South Africa. We fully recognise that (3) If there was encroachment post-1899 and pre-1960, is linking changes in vegetation to particular environmental there any evidence of encroachment post-1960 when drivers, be they local or global, is considered to be weak the level of grazing was very low? inference (sensu Platt 1964). Nonetheless, through a (4) Which local (fire and grazing) and global drivers (rainfall,

process of elimination (what Platt (1964) calls analytical nitrogen deposition, temperature and atmospheric CO2 inductive inference or ‘Baconian exclusion’ — after Francis concentration) best account for the observed changes? Bacon), we believe that we can eliminate drivers that are unlikely to be important and suggest some possible Methods causes of vegetation change. Furthermore, by using evidence from experimental studies published elsewhere, Study area we can strengthen the inference gained. In essence, this The current vegetation structure of Magersfontein, is the method of multiple working hypotheses, as originally Paardeberg and Carter’s Ridge is classified as Kimberley framed by Chamberlin (1897). Thorn Bushveld (van Rooyen and Bredenkamp 1996; Mucina and Rutherford 2006). This savanna consists of Photography and the Relief of Kimberley during the plains vegetation with a continuous grassy layer and a Second Anglo-Boer War scattered layer of woody vegetation. On the flatter and A useful approach to the analysis of environmental changes gently undulating plains, the majority of woody plants is through the use of historical photographs (Hoffman and are Acacia tortilis (Vachellia tortilis) trees with scattered O’Connor 1999; Hoffman and Rohde 2011; Rohde and A. mellifera (Senegalia mellifera) shrubs and occasional African Journal of Range & Forage Science 2014, 31(2): 107–121 109

A. erioloba (Vachellia erioloba) trees, and on the slopes photographs of Magersfontein were taken in 1940, 1964, of the small sandstone hills Tarchonanthus camphoratus 1973 and 1993. The images were monochromatic and shrubs are common. Scattered A. tortilis trees also occur on were not georeferenced. Georeferenced satellite imagery in the hillsides. At Paardeberg, the area is more grassy but is colour was also available for the years 2008/2009. still classified as Kimberley Thorn Bushveld. At Belmont, the vegetation is shrubland and consists largely of dwarf shrubs Pre-processing of aerial photographs (e.g. Pentzia incana and Chrysocoma ciliata), shrubs Images were projected using Transverse Mercator (Rhigozum trichotomum and A. mellifera) and grasses WGS 84 to match the projections of the satellite images (e.g. Eragrostis curvula). It is classified as Northern Upper and to correct error caused by the transformation onto a Karoo by Mucina and Rutherford (2006). two-dimensional plane (Kennedy 2000). Georeferencing The area experiences summer rainfall with a highly was performed to account for error. Points on each aerial variable mean annual precipitation (MAP) of 414 mm at photograph were matched to their corresponding points on Kimberley (range: 185–980 mm; CV  0.326) and as little as the georeferenced 2008/09 satellite image. Spline transfor- 290 mm at Belmont. The majority of rain falls in the form of mations were performed on each aerial photograph to best thunderstorms from November to March, with little or (often) match the subsequent points. A minimum of 40 points was no rain from May to October. The mean annual temperature assigned per aerial photograph using ArcGIS 10. of Kimberley is 18.5 °C, with the daily maximum summer We followed Hudak and Wessman (1998) who used temperature often being over 30 °C and can reach as high a travelling window of 3  3 pixels that is considered as 44 °C. Winter nights are relatively cold (mean minimum adequate for determining woody plant encroachment. We temperature is 10.8 °C), with frost a common occurrence used the mean probability of grey level co-occurrence in (Bezuidenhout 1994; Kraaij and Ward 2006). The soils ENVI 4.3 (following Haralick et al. 1973). found in this area are from the Dwyka and Ecca groups (which are within the Karoo supergroup) with dolerite Change detection intrusions (Vorster 2003). The texturally analysed images were clipped to the area within the Magersfontein boundary (2.7 km2) that was Fixed-point photography covered by all the years that photographs were available We were able to match sites at Belmont (one photograph), (determined by the 1940 image). The images of two Magersfontein (six photographs), Carter’s Ridge (one consecutive years were laid over each other and a grid photograph), Schmidtsdrift Road (one photograph) and of 50 m by 50 m blocks was placed over the top. Each Paardeberg (two photographs). All of these sites were block in the grid was observed and compared between rephotographed by us in 2010, with the exception of a the two time steps and given a rank of vegetation change single site that we rephotographed at Magersfontein in between the years. The ranks allocated were zero for no 2003. The sites at Carter’s Ridge and Schmidtsdrift Road observed change, one for increased size of existing trees, were not photographed during the Second Anglo-Boer War two for the appearance of between one and three new (1899–1902) but were photographed in 1919, 1956, 1989, trees, three for an increase of four or more trees but not 1995 and 2010. entirely encroached, and four for complete encroach- We matched the season of photography (our photographs ment of the block. This was performed for each time step. were taken in December and that of the battle of A percentage for each rank was calculated and plotted Magersfontein was taken on 11 December 1899). At each against the time steps 1940 to 1964, 1964 to 1973, 1973

Downloaded by [196.215.105.61] at 01:10 14 July 2014 of the sites, field notes were taken of the dominant plant to 1993, and 1993 to 2008 (see Scanlan and Archer 1991; species present in the landscape as well as the major Wigley et al. 2009). We did not attempt to classify the trees changes that have occurred since 1899/1900 (see also to the species level because there are potential errors Hoffman and O’Connor 1999). We obtained the large involved in identification (Browning et al. 2009). (landscape) scale fixed-point photographs from the Second Anglo-Boer War from a variety of sources. Rainfall We used the Braun–Blanquet method for assessing Data acquisition overall cover at each site, based on a 10% increment We acquired monthly rainfall data from the South African (Londo 1976). We used the step-point method (Mentis Weather Service. Rainfall data were supplied from several 1981) at Schmidtsdrift Road to assess vegetation density in weather stations in the area surrounding Magersfontein. 1989, 1995 and 2010 (n  200 points). This is an effective Years with missing monthly recordings or suspected method for comparing the density of species (Mentis inadequate readings were eliminated from the data sets 1981; Etchberger and Krausman 1997). Etchberger and of each station. Also, no station had complete rainfall Krausman (1997) have shown that 200 points is sufficient data for the entire time period studied. As a result, each for an asymptotic relationship between species richness year was individually examined and the station closest to and number of steps to be reached. Magersfontein that had an accurate reading was brought into the final rainfall data set, resulting in total annual rainfall Aerial photography for the area from 1940 to 2008 (see also Schulze and Data acquisition Maharaj 2003). The stations and their coordinates are as Our analysis of aerial photographs lies within the follows, in order of distance from Magersfontein: Kimberley Magersfontein war memorial site (28.96° S, 24.69° E; (28.8° S, 24.77° E), Banksdrift (28.92° S, 25.23° E) and 1 197 m above sea level) and covers roughly 2.7 km2. Aerial Slangsfontein (29.71° S, 25.55° E). 110 Ward, Hoffman and Collocott

Data analysis refuse dump that is adjacent to the site, and many trees A linear regression was performed on the data using SPSS (especially A. tortilis) have been cut by the refuse workers version 15 (SPSS, Inc., Chicago, USA) to detect any overall in recent years, as evidenced by the number of cut stumps increase or decrease in rainfall during the time period. observed in the field. So there too, it is likely that woody Rainfall events that consisted of an above-average rainfall plants would have increased in density. While there are no year following a below-average rainfall year were identi- woody plants in the foreground at the Paardeberg hill site fied because this could lead to woody plant encroach- there has been a substantial increase in tree density on the ment (Dean et al. 1995; O’Connor 1995; O’Connor and lower slopes. This is a common situation in the Northern Crow 1999; Kraaij and Ward 2006; Joubert et al. 2008). Cape and adjacent parts of the Free State province where The monthly data for Magersfontein for the above-average woody plant cover is common on rocky outcrops and on rainfall was more closely examined to better identify any the deeper pediments at the base of such outcrops but not possible triggering events. A year of above-average rainfall on the flat, open plains. At the two sites where sequential that consisted of an unusually high occurrence of months of photographs were taken (Carter’s Ridge and Schmidtsdrift above-average rainfall is more likely to be a triggering event Road, Figures 2 and 3), there was an incremental increase because post-germination rainfall has been seen to aid in woody plant cover that can be seen as far back as 1956 establishment of woody species (Condon 1986). and between 1956 and 1989/1995. Encroachment of woody species usually occurs in the form of cohorts of trees that germinate together following a Aerial photographs period of spatial and temporal overlap of several (localised) The pre-processed, textured and clipped images of the rainfall events (Archer et al. 1988; O’Connor 1995; study site in Magersfontein for 1940, 1964, 1973, 1993 and Wiegand et al. 2005; Joubert et al. 2008; Ward 2009). 2008 showed enhanced vegetation structures that allowed Woody plants are inconspicuous in aerial photographs accurate vegetation analysis. The results of the 50 m by unless they are at least 2 m tall (Buitenwerf et al. 2012) or 50 m grid analysis of these images (Figure 4) showed an have a canopy size of 4–5 m in diameter (see e.g. Ward increase in woody vegetation. The majority of the woody and Rohner 1997; Hudak and Wessman 2001; Browning plant encroachment occurred between 1993 and 2008. et al. 2009). Therefore there is an expected lag phase Between 1940 and 1993, a low percentage of the area between the triggering event and the detection of woody contained trees that grew in size. Between 1993 and 2008, plant encroachment as the trees are allowed time to grow when woody plant encroachment was greatest, little of and reach a detectable size. Chi-square statistical analyses the area experienced growth of individual trees. Between were performed to test differences in frequency of rainfall 1940 and 1973, a low percentage of the site experienced events during each of the four time steps. The four types of encroachment of between one and three trees (contra events that may have occurred are (1) an above-average expectations based on observations of heavy grazing by rainfall year following a below-average rainfall year, R Gubb – see Introduction), while the highest percentage (2) below-average following above-average, (3) above- occurred between 1973 and 1993. However, after 1993, average following above-average, and (4) below-average more than double the percentage of land experienced following below-average. encroachment by between one and three trees. Between 1940 and 1993, each time step experienced less than Temperature analyses 2.5% of the land being encroached by more than three We obtained temperature data for Kimberley from the South trees, which was the highest rank of encroachment type

Downloaded by [196.215.105.61] at 01:10 14 July 2014 African Weather Service for the period 1932–2011. We that occurred after 1993. In the 1993–2008/09 time step counted the number of days where temperature was below there was full encroachment of 50 m by 50 m blocks in 13 °C (the minimum temperature for plant growth of fruit the grid. This vegetation change was particularly clear trees – we assumed that a similar value was true of trees on the hill slopes, which were mostly encroached by in our study) and the number of freezing days from the T. camphoratus and A. tortilis. information on minimum temperatures, following Wakeling et al. (2012). We tested for the effects of changes in the Rainfall data number of days below 13 °C and number of freezing days The linear regression showed no significant increase or for the time period of 1932–2011 by best-fit linear regres- decrease in average rainfall for the time period (Figure 5). sions. Similarly, we examined changes in mean maximum The rainfall is extremely variable. We examined the relation- and minimum daily temperatures per annum over the same ship between CV in rainfall vs time in 10-year intervals; time period by best-fit linear regressions. there was no significant correlation (r  0.03, p  0.938), indicating that this variability was constant throughout Results the time period. There were several periods when rainfall exceeded 500 mm for several years, consistent with predic- Fixed-point photographs tions of Joubert et al.’s (2008) model (late 1930s and In all of the fixed-point repeat photographs, it is clear 1974–1976). that there has been an increase in woody plant density A breakdown of those rainfall events that consisted of (Figures 1–3), with the exception of the site at the Carter’s an above-average rainfall year following a below-average Ridge memorial (see difference between 1995 and rainfall year and above-average rainfall per month for the 2010 photographs, Figure 2) and in the foreground at area revealed a potential year for causing woody plant Paardeberg hill. However, at Carter’s Ridge, there is a encroachment at Magersfontein is the year 1988 where African Journal of Range & Forage Science 2014, 31(2): 107–121 111

(a)1900 (b) 2010

Figure 1a and b: Fixed-point photographs taken of the Highland Brigade memorial at Magersfontein. (a) Taken in 1900 by an unknown photographer. (b) Taken in 2010 (8 December) by Hoffman and Ward. They show the change from pure grassland to savanna, with Acacia tortilis in the foreground and Tarchonanthus camphoratus in the background on the hillside. There was about 30% cover of the shrub T. camphoratus, 10% cover of A. tortilis trees, 10% cover of the grass Eragrostis curvula and about 40% rock cover. Vegetation cover values less than 10% are excluded (as in all the other photographs). Some vegetation cover values (in 10% increments) exceed 100% because of the two-layered nature of savannas (trees and herbaceous plants)

Downloaded by [196.215.105.61] at 01:10 14 July 2014 (c)1900 (d) 2010

Figure 1c and d: Magersfontein ‘bird’s eye view’. (c) Photograph taken by FH Hancox in about 1900. The shrubs in the foreground are possibly Diospyros lycioides and those in the background on the plains are Acacia tortilis trees. (d) Repeat photograph by Hoffman and Ward in 2010 (8 December). On the plains, in 2010, there was 10% cover of A. tortilis trees, 20% cover of Schmidtia pappophoroides grass, 40% Eragrostis lehmanniana grass and about 10% bare ground. On the hillsides, there was 30% cover of Tarchonanthus camphoratus shrubs, 20% Eragrostis lehmanniana grass and 10% Schmidtia pappophoroides grass 112 Ward, Hoffman and Collocott

(e) 1900(f) 2010

Figure 1 e and f: Black Watch memorial. (e) 1900 photograph by an unknown photographer. (f) 2010 (8 December) photograph by Hoffman and Ward. Note that the same two Acacia tortilis trees appear in the foreground of both photographs. There was 20% cover by Acacia tortilis trees, 10% cover by Rhigozum trichotomum shrubs, 10% cover by Pentzia incana dwarf shrubs, 10% cover by Eragrostis lehmanniana grass and 20% bare ground in 2010

(g)1900 (h) 2010

Figure 1 g and h: Magersfontein at the site where Major-General A Wauchope (leader of the British Black Watch regiment) was mortally wounded. (g) 1900 photograph by an unknown photographer. (h) 2010 (8 December) photograph taken by Hoffman and Ward. There was 40% cover of Pentzia incana dwarf shrubs, 10% cover of Acacia tortilis trees and 10% of Acacia mellifera trees in 2010 Downloaded by [196.215.105.61] at 01:10 14 July 2014

(i) 1900(j) 2010

Figure 1 i and j: Magersfontein three hills. (i) 1900 photograph by an unknown photographer. (j) 2010 (8 December) photograph taken by Hoffman and Ward. In 2010, there was 20% cover of the dwarf shrub Pentzia incana, 20% cover by Acacia mellifera trees and 10% cover of Acacia tortilis trees African Journal of Range & Forage Science 2014, 31(2): 107–121 113

(k)1900 (l) 2010

Figure 1 k and l: Magersfontein gun site. (k) In 1900, the trenches are clearly visible, with a few scattered Acacia tortilis trees visible. (l) By 2003 (26 September), a number of the A. tortilis trees have grown larger and many more are present. The previously bare hillsides are covered mostly by Tarchonanthus camphoratus shrubs and some A. tortilis trees (photographers Hoffman and Ward). Grasses are mostly Eragrostis lehmanniana (40%) and Pogonarthria squarrosa (10%)

(m)1899/1900 (n) 2010

Figure 1 m and n: Belmont station. (m) In 1899 or 1900 (photographer unknown), there was considerable cover (behind the soldiers) by a dwarf shrub such as Pentzia incana or Chrysocoma ciliata. (n) In 2010 (7 December), there was 10% cover by Acacia tortilis trees, 10% cover by P. incana

Downloaded by [196.215.105.61] at 01:10 14 July 2014 dwarf shrubs, 20% cover by the annual Tribulus terrestris and 10% cover by Eragrostis curvula on the plains (photographers Hoffman and Ward)

(o)1900 (p) 2010

Figure 1 o and p: Paardeberg Hill. Comparison of photographs taken (o) in 1900 where there was a reasonable cover of dwarf shrubs (perhaps Pentzia spp. or Chrysocoma ciliata) and (p) on 9 December 2010. In the latter photograph, there was 30% cover of Acacia tortilis and 20% cover of A. erioloba on the hillslopes. On the plains, there was a dominance of grasses: Aristida congesta (10%), Eragrostis lehmanniana (20%), Sporobolus ioclados (30%), and some dwarf shrubs – Chrysocoma ciliata (20%) and Pentzia incana (10%) 114 Ward, Hoffman and Collocott

(a) 1919 (b) 1956

(c)1995 (d) 2010

Figure 2: Carter’s Ridge memorial. Photographs were taken by (a) Shantz and Turner in 1919 and (b) 1956 (Shantz and Turner 1958), (c) by Hoffman in 1995 (25 August) and (d) by Hoffman and Ward on 7 December 2010. In (d), there was 50% cover by Themeda triandra grass, 10% cover by Pogonarthria squarrosa grass, 20% cover by Eragrostis lehmanniana grass, 10% cover by Acacia tortilis trees, 10% cover by Acacia mellifera trees and 10% bare ground. The decrease in tree density in (d) was due to considerable chopping of trees, particularly A. tortilis, in the background

seven of the months throughout the year experienced (viz. that there was environmental change and that it was above-average rainfall. We note that we cannot be particu- shown regardless of scale) are answered. In another study larly specific about the timing of increased encroachment near Barkly West (35 km north-west of Kimberley), Britz because of the absence of annual tree rings in dry years and Ward (2007) used aerial photographs and obtained and the appearance of multiple tree rings in wetter years similar evidence of woody plant encroachment. in non-temperate regions (see e.g. Waisel and Fahn 1965; The effects of local drivers of woody plant encroach- DW pers. obs.). ment, specifically grazing and fire, were not supported in The 2 analysis showed no significant overall differences this study although we recognise that some areas show in frequencies of rainfall events between the time steps effects of both grazing and fire. In the current study,

Downloaded by [196.215.105.61] at 01:10 14 July 2014 observed. The only significant difference in rainfall events from aerial photographs, we observed most woody plant (p  0.01) was found between 1973 and 1993 where a encroachment between the years 1993 and 2008/09 at significantly high number of below-average rainfall years Magersfontein, with little increase in the time steps from followed a below-average rainfall year. 1940 until 1993. The absence of differences between Magersfontein and areas outside of this national monument Temperature data suggest that heavy grazing could not account for woody We found no significant change in mean maximum and plant encroachment (cf. question 3). Similarly, Kraaij minimum temperatures (p  0.05) nor a significant correlation and Ward (2006) have shown in a controlled experiment between the number of days below 13 °C (relative to number conducted near Barkly West that neither grazing nor fire of days available) and years between 1932 and 2011 had a significant effect on woody plant encroachment. (F  0.534, p  0.467, error df  78). However, there was a However, Ward and Esler (2011) found that there was a weak but significant positive correlation between the relative significant effect of grass clipping on recruitment of Acacia number of freezing nights and years between 1932 and mellifera seedlings near Barkly West. 2011 (r  0.17, F  11.089, p  0.001, error df  78), which is We did find some evidence of the effects of grazing, from inconsistent with Wakeling et al.’s (2012) hypothesis. fenceline contrasts between Kimberley and Paardberg (DW and MTH pers. obs.) that a difference exists which can only Discussion be ascribed to grazing (soil types do not differ). However, the duration of their effects is unknown. Similarly, other Woody plant encroachment observed in the fixed-point studies have also used fenceline contrasts to show differ- photographs (Figures 1–3) was also found at a larger ences in grazing effects. For example, Roques et al. (2001) spatial scale through the analysis of the aerial photographs noted that shrub encroachment varied across land-use in this study (Figure 4). As a result, our first two questions fence lines in mesic Swaziland. The key determinants of African Journal of Range & Forage Science 2014, 31(2): 107–121 115

(a) 1919 (b) 1956

(c) 1989 (d) 1995

(e) 2010 Downloaded by [196.215.105.61] at 01:10 14 July 2014

Figure 3: Schmidtsdrift Road: Photographs were taken by (a) Shantz and Turner in 1919 and (b) 1956 (Shantz and Turner 1958), (c) by Hoffman in 1989 (25 August) and (d) 1995 (21 September), and (e) by Hoffman and Ward on 7 December 2010. Note the large increase in size of the Acacia erioloba (on the right) until 1989. Note also the increase in growth of the Acacia tortilis trees on the left from 1989. Details of changes in vegetation cover are listed in Table 1

shrub dynamics were grazing, through its negative effect albeit not at Magersfontein. Regarding fire, Meyer et on fire frequency and an interaction between drought al. (2005) recorded a fire near Barkly West, which had frequency and high shrub cover (see also van Langevelde considerable effects on the regrowth of two woody plants, et al. 2003). Browsing pressure (i.e. woody plant herbivory) A. mellifera and T. camphoratus. However, fires in such in the Roques et al. (2001) study had a significant but minor a dry area are infrequent because of the lack of grass- impact on vegetation dynamics. based fuel load (Teague and Smit 1992; Meyer et al. 2005; We also acknowledge that fires do occur infrequently, Higgins et al. 2010). A rancher in the area, Rudi Bigalke 116 Ward, Hoffman and Collocott

Table 1: Changes in cover of vegetation (%) at Schmidtsdrift R. trichotomum and T. camphoratus. While A. tortilis could Road in 1989, 1995 and 2010, calculated using a step-point have been used for fuelwood (or for mine supports), it is transect. There were also some Acacia erioloba trees present extremely unlikely that A. mellifera, R. trichotomum and at Schmidtsdrift Road (Figure 2) but were not included in the T. camphoratus would have been used in this way because transects. G  grass, DS  dwarf shrub, T  tree they are far smaller and burn too rapidly. Thus, we must consider the potential effects of global Species 1989 1995 2010 drivers and how they might interact with each other, on Bare ground 16.6 48.5 26.9 woody plant encroachment. The major global drivers are Eragrostis lehmanniana (G) 19.7 increases in rainfall, nitrogen oxides, minimum tempera- Eragrostis curvula (G) 29.6 tures and carbon dioxide levels. Pogonarthria squarrosa (G) 29.7 Schmidtia pappophoroides (G) 15.2 31.4 1.1 Themeda triandra (G) 1.3 0.5 32.8 Effects of extreme rainfall events on woody plant Pentzia incana (DS) 3.8 encroachment Acacia tortilis (T) 1.8 3.5 0.5 A number of authors working in arid and semi-arid regions of South Africa have noted that episodic high rainfall events may lead to woody plant encroachment (e.g. Archer et al. 1988; O’Connor 1995; O’Connor and Crow 1999; Wiegand 1 et al. 2005; Kraaij and Ward 2006; Wiegand et al. 2006; 35 2 Meyer et al. 2007a, 2007b; Joubert et al. 2008; Moustakas 30 3 et al. 2009; Joubert et al. 2012). The observed increase in 4 woody plant encroachment detected in aerial photographs 25 at Magersfontein between 1993 and 2008/09 could be the 20 result of an event earlier in the time period (between about 1973 and 1993) because trees are not visible on aerial 15 photographs when they are small (see above). A possible

INCREASE (%) cause for the observed woody plant encroachment is the 10 above-average rainfall year of 1988 where seven months 5 of the year experienced above-average rainfall and/or the period 1974–1976 when rainfall exceeded 500 mm for three 1940–1964 1964–1973 1973–1993 1993–2008 successive years. Kraaij and Ward (2006) found significant TIME STEPS effects on woody plant recruitment only after (simulated) unusually high rainfall events. Below-average rainfall years Figure 4: Percentage of observed grid blocks that experienced preceding above-average rainfall ensure that the grass’ increases in woody plant encroachment within each time step. The competitive ability is decreased, which aids germination of increase was divided into four categories of increase: increased size woody species (Booth 1986; O’Connor 1995; Wiegand et al. of existing trees (1), appearance of between one and three trees (2), 2005, 2006; Ward and Esler 2011). an increase of four or more trees but not entirely encroached (3), Meyer et al. (2007a, 2007b) and Joubert et al. (2012) and complete encroachment (4) found that, for A. mellifera at least, there is no persistent seed bank and that seed production, and recruitment, is

Downloaded by [196.215.105.61] at 01:10 14 July 2014 strongly linked to above-average rainfall events (Joubert (pers. comm.), told us that the last major fire near Kimberley et al. 2008). Joubert et al. (2008) proposed a state-and- occurred in 1934, a year of high rainfall (Figure 5) that led to transition model for encroachment by A. mellifera in the considerable grass production. arid highland savanna of Namibia (MAP  300–400 mm). Another local driver that may be important is woodcut- In this model, they proposed that three years of above- ting, which in the past was widespread. The return of average rainfall were needed for successful recruitment to woody plants might simply reflect a response to the occur. When grazing is maintained at high levels throughout cessation of woodcutting (except see Figure 2). In an the year and fire is excluded, recruitment can occur. If this editorial in the Cape Times of 21 November 1892 [our is the case, then we believe that the 1974–1976 period comments]: ‘Additional information with regard to De Beer’s is critical in this regard because this was one of the few [Kimberley’s largest diamond company] and the destruc- times during the course of the twentieth century when tion of forests in Bechuanaland [now Botswana] is to the annual rainfall was 500 mm for all three years (Figure 5). effect that the management of the Mining Company are However, other encroaching species such as A. tortilis preparing to use wood as fuel on a much larger scale...The and R. trichotomum have a persistent seed bank and work of destruction appears to be carried out in a ruthless T. camphoratus mostly has clonal reproduction; thus, the manner, the consignments not infrequently including connection between rainfall and recruitment might not be as some grand old camelthorn trees [Acacia erioloba] of tightly coupled for all species as it is for A. mellifera. great girth, hundreds of years old. Everything that comes The rainfall in this region is highly variable (Figure 5), is sawn up and consigned to the furnaces...’ In our study with frequent occurrences of below-average rainfall years area, the camelthorn A. erioloba occurs, but is relatively followed by an above-average rainfall year. A number of rare and is restricted to deep sands (see Figure 3). Most these years have had six months of above-average rainfall of the encroaching species are A. tortilis, A. mellifera, but woody plant encroachment did not seem to occur African Journal of Range & Forage Science 2014, 31(2): 107–121 117

900 y = 0.0356x + 437.3 >500 mm <500 mm r = 0.006, F = 0.051, p = 0.96 800

700

600

500

400 RAINFALL (mm) 300

200

100

1933 1940 1947 1954 1961 1968 1975 1982 1989 1996 2003 2010 YEAR

Figure 5: Total annual rainfall at Kimberley for the years 1932 until 2011, running from October until the subsequent September to indicate the main rainfall season in the Southern Hemisphere. There has been no significant change in annual rainfall over time since 1931 (MAP  437.3 mm; see horizontal trendline). We have included values 500 mm MAP as clear bars because Joubert et al. (2008) predicted that three successive years of 500 mm MAP are required for successful encroachment by Acacia mellifera (one of the encroaching species in our study) in arid savannas

following these events. This makes the 1988 event (drought evidence of a rapid increase in tree abundances from aerial followed by high rainfall) unusual in this environment. The photographs from 1993 to 2008 is largely due to an increase 2 analyses of the rainfall data showed that the only unique in the densities of T. camphoratus and A. tortilis on the hills, rainfall events occurred between 1974 and 1993 in the although we were unable to test this because of the similari- form of a high frequency of below-average rainfall years ties in shades on the black-and-white aerial photographs. following below-average rainfall years. However, distur- This further displays the benefits of using fixed-point and bances such as drought periods only cause woody plant aerial photographs in combination. Nonetheless, the question encroachment if followed by an unusually high rainfall year still arises as to why Magersfontein (and the other sites we (O’Connor 1995; Meyer et al. 2007b; Moustakas et al. photographed, for that matter) were not encroached in 1940 2008; Ward and Esler 2011). O’Connor and Crow (1999) (first aerial photographs) or during the Second Anglo-Boer

Downloaded by [196.215.105.61] at 01:10 14 July 2014 also found that encroachment was consistent with high War (first fixed-point photographs)? rainfall years immediately after drought years in the Eastern Cape province (South Africa). In another study, Gordijn Possible effects of increased nitrogen oxides on woody et al. (2012) found that long-term increases in rainfall in plant encroachment KwaZulu-Natal province (South Africa) were consistent with Atmospheric nitrogen deposition is known to reduce plant increased woody cover. diversity in natural ecosystems (Phoenix et al. 2006). The One might expect that cohorts of trees would appear if industrial Highveld region (Gauteng and Mpumalanga episodic high-rainfall years subsequent to drought years provinces) accounts for approximately 90% of South initiated woody plant recruitment, as was found in arid Africa’s emissions of industrial dust, sulphur dioxide, parts of Namibia (Wiegand et al. 2005; Ward 2009) and the and nitrogen oxides (Wells et al. 1996). South Africa has south-western USA (Archer et al. 1988). Thus, the question some of the heaviest levels of pollution by nitrogen oxides

arises as to why there were no earlier incidences of woody (NOx  NO  NO2) outside of western Europe and North plant encroachment prior to 1993–2008 at Magersfontein America (Wenig et al. 2003). Much of this comes from that are detectable from aerial photographs. However, as coal-fired power plants and from producing oil from coal

noted above, when sequential fixed-point photographs were in Mpumalanga province. NOx also contributes to acid taken at Carter’s Ridge (Figure 2) and Schmidtsdrift Road deposition from the atmosphere (Stoddard et al. 1999).

(Figure 3) there was some recruitment between 1919–1956 Wenig et al. (2003) have shown that NOx may spread as and, in particular, between 1956–1989/1995. There is far as Kimberley, even though it is not part of the industrial also evidence of cohorts at Magersfontein from fixed-point Highveld region of South Africa. However, Kraaij and photographs (see several large adult A. tortilis trees in the Ward (2006) have shown experimentally that woody plants Magersfontein photographs in Figure 1, perhaps due to the do not benefit from nitrogen fertilisation, but rather that high rainfall years in the late 1930s). We suspect that the grasses become more competitive when fertilised and 118 Ward, Hoffman and Collocott

reduce the number of surviving tree seedlings. Thus, we temperatures in C3 plants. At current CO2 concentrations,

consider it unlikely that NOx has resulted in a significant C3 trees are only more efficient than C4 plants below 20 °C. increase in woody plant encroachment. The ambient temperatures in the Kimberley region are high (see Methods), indicating that the benefits of increased

Effects of temperature changes on woody plant CO2 to C3 plants have probably not yet been reached (see encroachment also Bond and Midgley 2012). Higgins and Scheiter (2012) A number of studies have shown that freezing temper- modeled woody plant encroachment in African savannas

atures result in decreased tree survival and growth in due to predicted global CO2 increases and suggested savannas and deserts (e.g. Silberbauer-Gottsberger that there should be localised effects of encroachment, et al. 1977; Loik and Nobel 1993; Brando and Durigan assuming rainfall remains constant (as in our study). 2004; Holdo 2006). Wakeling et al. (2012) were mostly Several studies have shown that elevated atmospheric

interested in why highland grasslands contain no trees yet CO2 can also increase the water-use efficiency of plants,

have regular burning and similar suites of C4 grasses to thereby increasing levels of soil moisture, and reducing savannas. In their study, they found that the species they water stress (Centritto et al. 2002; Widodo et al. 2003;

tested all grew more slowly under colder conditions. This Nelson et al. 2004; Leakey et al. 2012). Increased CO2 led them to hypothesise that reduced growth would keep might cause woody plant encroachment by causing woody highland grassland trees under the flame zone where they vegetation to be favoured in drought periods (Eamus and were susceptible to fire. As indicated earlier, fires are very Palmer 2007; Leakey and Lau 2012). However, Körner rare in the Kimberley area because of low grass biomass. (2006) and Leakey and Lau (2012) have pointed out Regardless, if Wakeling et al.’s (2012) hypothesis was that immediate increases in the efficiency of plants due

correct, we could assume that global minimum temper- to increased CO2 have not always been sustained in the atures have increased, resulting in improved tempera- longer term. Additionally, Zak et al. (2003) point out that tures for tree survival and growth. We found no support plants may ultimately be limited by soil nutrients, especially for this hypothesis. Indeed, in the only significant relation- nitrogen, which will constrain the benefits accrued by

ship, there was an increase in the number of freezing days growing in a CO2-richer environment. not a decrease. In conclusion, through a process of elimination, the exogenous driver (i.e. not under human control) of episodic Possible effects of increased carbon dioxide on woody high rainfall events and perhaps increased atmospheric

plant encroachment CO2, which may function interactively, may be considered

Atmospheric CO2 concentrations are currently (April 2014) as causes of large-scale woody plant encroachment near

at 401.30 ppm (measured at Mauna Loa Observatory in Kimberley, South Africa. Should atmospheric CO2 levels Hawai’i; Tans 2014), having increased from c. 350 ppm continue to increase, similar woody plant encroachment

in the last 50 years. This increase in CO2 concentra- may be observed in other dry savanna regions of the world tion is expected to double in this century. Increased (Eamus and Palmer 2007; Bond 2008; Higgins and Scheiter

CO2 has been observed to affect plant photosynthetic 2012). We do not believe that we have provided evidence

efficiency differently in C3 and C4 plants, with a more rapid for savanna as an alternative stable state to the former

increase observed in C3 plants (Ehleringer and Monson state of grassland because tree densities may continue to 1993; Wolfe and Erickson 1993; Vitousek 1994). Ward increase. However, given the low rainfall and high ambient (2010) has predicted that the ecological consequence temperatures, it is clear that forests will not develop from this

Downloaded by [196.215.105.61] at 01:10 14 July 2014 of this C3/C4 difference will be increased growth for C3 savanna, although tree densities may reach canopy closure. plants and reduced carbon losses. The reduced losses Woody plant encroachment in areas receiving 400 mm of may be caused by more carbon being available for the annual rainfall is generally thought to lead to an increase in production of polyphenols, which is a common defence the amount of carbon stored in these ecosystems (Jackson

secondary metabolite in C3 trees likely to encroach, et al. 2002; Hibbard et al. 2003; Knapp et al. 2008), although such as A. mellifera and A. tortilis (Lawler et al. 1997; a decrease is noted above 400 mm of annual rainfall (Ward Kanowski 2001; Mattson et al. 2004). Some smaller- 2009). Given the extensive nature of woody plant encroach-

scale experiments in South Africa using increased CO2 in ment in dry grasslands and savannas in southern Africa and

bins (e.g. Kgope et al. 2010) or from natural CO2 springs worldwide, this potential increase in carbon sequestration (Stock et al. 2005) have had mixed results, with the former may significantly alter the terrestrial carbon sink. study finding results that were consistent with increased

efficiency of C3 plants and the latter not supporting these Acknowledgements — We thank the South African Weather results. There is some debate as to whether the increased Service for the rainfall data, Robert Hart of the McGregor Museum for the provision of some Second Anglo-Boer War photographs, photosynthetic efficiencies of C3 plants is now greater Steve Lunderstedt for advice regarding repeat photographs and than those of C4 plants as a result of an increase in CO2 from c. 350 to c. 400 ppm (Wolfe and Erickson 1993; the Chief Directorate of National Geo-spatial Information in the Department of Rural Development and Land Reform in Cape Bond and Midgley 2000; Ward 2010). If C plants are now 3 Town, South Africa for providing the aerial photography and more efficient (Bond and Midgley 2000), then increased satellite imagery. Clement Adjorlolo, Brice Gijsbertsen and Victor CO2 may indeed function as a global driver for increased Bangamwabo are thanked for assistance with the remote sensing woody plant encroachment. However, Zhu et al. (2008) and GIS analyses, and Jacob Hoffman and Megan Griffiths have shown that increased photosynthetic efficiency for assistance in the field. DW thanks the National Research

with increasing CO2 concentrations is linked to ambient Foundation of South Africa for funding. MTH acknowledges the African Journal of Range & Forage Science 2014, 31(2): 107–121 119

support of the African Climate and Development Initiative at the Eamus D, Ceulemans R. 2001. Effects of greenhouse gases on University of Cape Town and the Mazda Wildlife Vehicle Fund for the gas exchange of forest trees. In: Karnosky D, Ceulemans

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Received 24 June 2013, revised 4 April 2014, accepted 9 April 2014 Associate Editor: Clement Cupido