25

Biodiversity, thresholds, resilience and forest degradation

I. Thompson

Following certain ecological orests comprise multiple ecosys- goods and services that are expected principles in management tems associated with variance in (e.g. FAO, 2009), the loss of 4 Fedaphic and microclimatic con- is a key criterion for measuring forest ditions across broad landscapes. The degradation. Conserving biodiversity adaptation to . composition and nature of forest eco- is a cornerstone of sustainable forest systems vary over time, depending on management (e.g. Montreal Process, natural disturbances and changes to the 2009) and a key to maintaining forest climate regime. However, they remain ecosystem functioning. more or less the same within the bounds This article explores the ways in which of natural variation (see Figure), referred forests maintain their stable states over to as a stable state. In a stable state, a time and outlines what happens when forest can produce a range of associated disturbances overwhelm the natural goods and services that humans value. mechanisms of recovery. It describes Biodiversity underpins most forest eco- how sustainable management of forests, system goods and services, and many including the conservation of biodi- tropical forests, in particular, maintain versity, is key to supporting a forest’s high levels of biodiversity. Loss of bio- recovery mechanisms, and presents eco- diversity may have considerable negative logical principles that can be applied to consequences for the productive capac- forest management. ity of forests (e.g. Thompson et al., 2009; Bridgeland et al., 2010; Cardinale et al., RESILIENCE AND RESISTANCE 2011) and for the provision of goods ] and services. Therefore, because forest An important characteristic of forests degradation can be defined as the loss is their resilience, which is the capacity of the ability of a forest to produce the to recover following major disturbances

Illustration of tipping points, or thresholds, in Pressures Tipping point

Existing biodiverstity CHANGED STATE

Actions to increase resilience

SAFE Less diverse Fewer ecosystem OPERATING services Degradation of SPACE human well being

Changed Ian Thompson is a research scientist, working biodiverstity for the Canadian Forest Service, in Sault Ste. Marie, Ontario, Canada. Source: Secretariat of the Convention on Biological Diversity, 2010.

Unasylva 238, Vol. 62, 2011/2 26

(e.g. Gunderson, 2000). Under most caused by the loss of functional groups in large part, on these key species and natural regimes, forests (see Mechanisms and Tipping points) the functions that they perform rede- maintain their resilience over time. For- resulting from environmental changes veloping as a forest recovers following est resilience is an emergent ecosystem such as large-scale climate change, poor disturbances, including forest manage- property resulting from biodiversity at forest management or a sufficiently large ment interventions. multiple scales, from genetic to land- or continual alteration of natural dis- At a genetic level, the capacity for scape diversity (Thompson et al., 2009). turbance regimes (Folke et al., 2004). resilience comes from the ability of To sustain the goods and services that a species to persist over a range of humans derive from forests, forest eco- Mechanisms environmental variability, such as by systems must recover after disturbances There is strong evidence that forest resil- tolerating a range of temperatures or a and not become degraded over time. ience is tied to the biodiversity that certain level of drought. At the species Related to the concept of resilience normally occurs in the ecosystem (e.g. level, there are various behavioural and is resistance, which is the capacity of Folke et al., 2004; Thompson et al., functional responses that can assist a a forest to resist minor disturbances 2009). In particular, certain species and species to repopulate a disturbed area over time, such as the death of a few groups of species perform key functions or respond to environmental changes. trees or a chronic level of herbivory in forests and so are essential for the Further, ecosystem assembly processes by insects. Forests are generally stable forest to maintain all of its functional very much reflect the landscape pool of and change little as a result of non- processes (Díaz and Cabido, 2001). For available species (e.g. Tylianakis et al., catastrophic disturbances. Minor example, bird can maintain 2008), as well as landscape connectiv- changes are mitigated, such as when low abundances of insects in a forest, ity. At the landscape scale, heterogene- canopy gaps created by the death of reducing the possibility of catastrophic ity among forest patches can provide a individual or small groups of trees are levels of insect herbivory of trees, and measure of redundancy among species quickly filled by new young trees. For- thus increasing tree (e.g. and a source for colonizers that, as a ests may also be resistant to certain Bridgeland et al., 2010). Pollinators, forest begins to redevelop or recover environmental changes, such as weather including some insects, bats and birds, after disturbance, should enable com- patterns over time, owing to redundancy are also excellent examples of highly munities to converge on the original among the functional species (redun- functional species in ecosystems, and forest types. Hence, the consideration of dancy refers to the overlap or duplica- without them, many plants could not resilience necessarily involves thinking tion in ecological functions performed reproduce. Forest resilience depends, from small to large scales. by a group of species; see Mechanisms) (e.g. Díaz and Cabido, 2001). Ecosystems may be highly resilient but have low resistance to a given dis- turbance. For example, many boreal forests are not especially resistant to fire, but they are highly resilient to it and usually recover fully over a number of years. Generally, most natural forests, especially primary old forests, are both resilient and resistant to various kinds of changes. Loss of resilience may be FAO/A. YANCHUK FAO/A. Hardwood forest composed primarily of trembling aspen in the boreal forest of northern Canada. Forests may not be particularly resistant to certain disturbances to which they are resilient

Unasylva 238, Vol. 62, 2011/2 27

Degraded juniper (Juniperus thurifera) forest in the High Atlas, Morocco

Loss of resilience and forest degradation An ecosystem state is defined by the dominant floristic (tree) composition and stand structure expected for a given stand. A change in forest state results from a loss of resilience, with a partial or complete shift to a different ecosys- tem type from what is expected for that area. Such changes in state result in a reduction in the production of goods and PERLIS FAO/A. services. Therefore, “change in ecosys- tem state” can be used as an indicator of degradation. For example, if a forest is expected to be of mixed species but is has been degraded can be determined Tipping points can be reached rap- instead dominated by only a few species, through remote sensing. Using satel- idly or as a result of chronic change or if it should be a closed canopy forest lite data, Souza et al. (2003) mapped that wears away the capacity of an eco- but is actually open or savannah, then forest in the Amazon region of Brazil system to recover, such as through the the state has changed. These would be that had been excessively burned or gradual attrition of species over time. considered negative changes in state, as heavily logged and burned, and Strand For example, forest fragmentation is a they degrade the forest, from a biodiver- et al. (2007) reported on several cases process that opens up continuous forests sity perspective and from a production in which remote sensing was used for through multiple disturbances. A forest perspective, and would generally affect monitoring forests affected by invasive can readily tolerate some loss of spatial the level of goods and services available. tree species and insects from several continuity and still maintain its species Often, the degradation of forests results regions of the world. and functions, but studies suggest that from the use of poor harvesting tech- certain levels of fragmentation are actu- niques over a period of time. However, TIPPING POINTS ally tipping points, with a resulting loss forests can also become degraded for Forests may not always recover after of forest biodiversity and f unction and a many reasons not involving logging. For severe and protracted disturbances. reduced capacity to produce goods and example, forests may appear intact but Thresholds exist for populations of indi- services (e.g. Andrén, 1994; Arroyo- be missing most large animal species as vidual species and for individual pro- Rodríguez et al., 2007). a result of over-hunting (e.g. Redford, cesses within ecosystems, and ultimately Ecosystems can be used and harvested 1992). As a result, there could be long- for the ecosystems themselves. The point for services, but the derivation of those term consequences for forest health at which the ecosystem loses its capacity services cannot exceed sustainable levels, because of increased insect herbivory to recover, or at which its resilience and nor can goods be removed in a manner that resulting from a lack of control by preda- integrity are lost, is referred to as a tip- destroys ecosystem processes (Figure). tors, or reduced seed dispersal, functions ping point, or an ecological threshold. If Once a tipping point is reached, changes that the missing animals might have per- there is too much disturbance, a cascade to the ecosystem are large and nonlinear, formed. Another example of degradation of effects with marked changes to the often unpredictable, and usually dramatic might be the successful establishment of forest ecosystem will result, ultimately (e.g. Scheffer and Carpenter, 2003). For an that out-competed moving the forest to a new state. For example, parts of northern Africa under- endemic species, thereby constraining example, severe drought and fire can went a rather spectacular change from the goods available from the ecosystem. convert a dry forest type to a savannah dry forest to desert as a result of past In any of the cases described, if changes or even into grassland. Most often, the climate change (Kröpelin et al., 2008). are severe enough to cause a change new state will provide a lower level of Unfortunately, we often only recognize in state, the extent to which the forest products and services to humans. a tipping point once it has already been

Unasylva 238, Vol. 62, 2011/2 28

As the global climate changes, for- forest resilience even if the ecosystem est ecosystems will change because the type may change. If ecosystems do physiological tolerances of some spe- change, there must be an understanding cies may be exceeded and the rates of of how to respond through forest man- many biophysical forest processes will agement. In most cases, some forms of be altered (e.g. Scholze et al., 2006). active management will be necessary to Most studies suggest that many tropical enable forests to adapt to climate change. forests may not be resilient to climate Maintaining forest resilience can be an change over the long term, if the cur- important mechanism both to mitigate rent and predicted trend continues, with and to adapt to climate change. reduced rainfall and increased drought (e.g. Betts, Sanderson and Woodward, MANAGING FORESTS TO AVOID 2008; Malhi et al., 2008). TIPPING POINTS Forest ecosystems are composed of Sustainable forest management is eco- distinct assemblages of species. Across system management of forests that, in regions, the ranges of individual species large part, has an underlying objective to reflect their physiological and ecologi- enable the natural resilience to continue. cal niches, which, in turn, reflect where One of a forest manager’s main tasks is environmental conditions are advanta- to help forests recover after harvesting geous. Species with broad physiological of timber or other products, through tolerances may be highly resilient to even sustaining the properties of the ecosys- significant global climate change. Like- tem over the long term. In recent years wise, species with apparently narrow this task has become more complicated ecological niches might be more resilient through the additional stress of climate FAO/T. HOFER than they appear, if changed conditions change on terrestrial ecosystems. While Eastern Himalaya mountains, India. Biodiversity underpins forest provide them with an advantage at the proper, biologically sound sustainable resilience and is a key consideration exp ense of c omp et it or s. I n eit her of t hese forest management is a major part of for forest managers two potential situations, this capacity maintaining forest resilience, response would apply to species that have large to climate change requires additional reached and the generally negative con- and variable enough gene pools to adapt planning and actions. If we understand sequences to the ecosystem have become and the ability to migrate. However, for ecosystems better and can accurately obvious. Therefore, to manage a forest many species this is not the case. Where predict at what level of use thresholds sustainably requires learning to identify and/or genetic diver- might exist, the management of forest possible tipping points in advance. sity have been reduced, or the mobility goods and services can be more benign. of species is restricted through Climate change considerations loss and fragmentation or is naturally Maintaining biodiversity Superimposed on many other human- low, successful autonomous adaptation Maintaining biodiversity is a key to main- caused impacts on forest ecosystems to environmental change becomes less taining forest resilience and avoiding is global climate change, which adds likely. Populations may be doomed to tipping points. The biological diversity uncertainty to the identification of extinction if exposed to a rate of envi- of a forest is linked to and underpins tipping points. Climate has a major ronmental change exceeding the rate at the ecosystem’s productivity, resilience, influence on rates of respiration, produc- which they can adapt, or the rate at which resistance, and its stability over time and tion and other forest processes, acting individuals can disperse (e.g. Schwartz space. A reduction in biodiversity in forest through temperature, radiative forcing et al., 2006). systems has clear, often negative, implica- (increase in energy remaining in the Most of the emphasis in negotiations on tions for the functioning of the systems atmosphere) and moisture regimes, over global climate change concerning forests and the amounts of goods and services medium and long time periods. Climate has been on how to manage forests to that these systems are able to produce. and weather conditions also directly mitigate climate change. Adaptation to Understanding how biodiversity sup- influence shorter-term processes in for- climate change has received less atten- ports local forest resilience and resist- ests, such as wildfires, herbivory and tion. Adaptation of forests to climate ance provides important clues to improve species migration. change is primarily about maintaining forest management. For example, while

Unasylva 238, Vol. 62, 2011/2 29

it is relatively simple to plant trees and may change in response to changes in entry pathways), and reduce reliance produce a short-term wood crop, it is climate. The following suggested actions on non-native tree crop species for much more difficult to recover a forest were developed from ecological princi- plantation, afforestation, or refor- ecosystem. The lack of diversity at all ples that can be employed to maintain estation projects. levels (gene, species of flora and fauna, and enhance long-term forest resilience, 8. Reduce the possibility of negative and landscape) in simple plantation for- and especially to aid adaptation of forests o u t c o m e s b y a p p o r t i o n i n g s o m e a r e a s ests reduces resilience and resistance to to climate change: of assisted regeneration with trees disturbances, degrades the provision of 1. Plan ahead to maintain biodiversity from provenances and from climates services and many goods that the system at all forest scales (stand, landscape, of the same region that approximate can provide and renders it vulnerable to region) and of all elements (genes, expected conditions in the future. For catastrophic disturbance. Through the species, communities) based on an e x a m p l e , i n a r e a s p r o j e c t e d t o b e c o m e application of ecological forest manage- understanding of thresholds and of more dry, consider also planting tree ment principles, forest plantations can expected future climate conditions. species or provenances that may be provide much more than just a wood crop, This means basing actions on ecolog- more drought-resistant than local spe- and forest ecosystems can be restored at ical principles and expert knowledge cies and provenances, with special the same time that the productive capaci- to conserve biodiversity during and consideration to regional species. ties of the forest for the chosen product after forest harvesting. 9. Protect isolated or disjunct popula- are improved (e.g. Parrotta and Knowles, 2. Maintain genetic diversity in forests tions of species, such as populations 1999; Brockerhoff et al., 2008). through management practices that at the margins of their natural dis- do not select only certain trees for tribution ranges, as possible future Understanding thresholds harvesting based on site type, growth source . These populations Forest ecosystems change continuously rate and superior form. may represent pre-adapted gene in response to short- and long-term 3. Do not reduce the landscape-scale pools for responding to climate environmental pressures, resulting in populations of any tree species to change and could form core popula- inherent variance over time. As a result, the extent that self-replacement is tions as conditions change. measures of function, such as produc- not possible. 10. Ensure that there are national and tion of given goods, also fluctuate over 4. Maintain stand and landscape struc- regional networks of comprehensive time. Therefore, thresholds should be tural complexity using natural forests and representative protected areas perceived as a range in values to accom- as models and benchmarks. When that have been established based modate both this fluctuation and the managing forests, managers should on scientifically sound principles. statistical uncertainty associated with try to emulate the processes and com- Incorporate these networks into insufficient understanding of ecosystem position in natural stands, in terms of national and regional planning for functioning. To avoid forest ecosystem species composition and stand struc- large-scale landscape connectivity. degradation, forest managers require ture, by using silvicultural methods 11. Develop an effectiveness monitor- some basic understanding of how local that relate to the major natural dis- ing plan that provides data on natural biodiversity is related to productivity turbance types. disturbances, climate conditions and the levels of disturbance that their 5. Maintain connectivity across for- and consequences of post-harvest ecosystems can tolerate. est landscapes by reducing frag- silvicultural and forest manage- mentation, recovering lost habitats ment actions. Adapt future plan- Suggested actions (forest types), and expanding pro- ning and implementation practices A s for est s cha nge a f t er logg i ng or i nse ct tected area networks. Intact forests as necessary. attack, or because of climate change or are more resilient than fragmented The capacity to conserve, sustainably extreme weather events, managers need forests to disturbances including use, and restore forests rests on our to be concerned with bringing the forest climate change. understanding and interpretation of pat- back to a condition that will supply the 6. Maintain functional diversity (and terns and processes at several scales, the goods and services that were desired species redundancy) and minimize recognition of thresholds, and the ability from that forest. A key aspect of any plan the conversion of diverse natural to translate knowledge into appropriate to maintain a flow of forest goods and forests to monotypic or reduced- forest management actions in an adap- services is an understanding of local for- species plantations. tive manner.  est on which to base sustainable 7. Reduce non-natural by forest management, and how the forest controlling invasive species (and

Unasylva 238, Vol. 62, 2011/2 30

FAO. 2009. ! Proc. National Acad. Sciences, 103: " 13116–13120. #, by M. Simula. Forest Scheffer, M. & Carpenter, S.R. 2003. Resources Assessment Working Paper Catastrophic regime shifts in ecosystems: References No. 154. Rome (also available at: ftp.fao. linking theory to observation. Trends in org/docrep/fao/012/k6217e/k6217e00.pdf). Ecology and Evolution, 18(12): 648–656. Andrén, H. 1994. Effects of habitat Folke, C., Carpenter, S., Walker, B., DOI: 10.1016/j.tree.2003.09.002. fragmentation on birds and mammals in Scheffer, M., Elmqvist, T., Gunderson, L. Schwartz, M.W., Iverson, L.R., Prasad, landscapes with different proportions of & Holling, C.S. 2004. Regime shifts, A.M., Matthews, S.N. & O’Connor, suitable habitat: a review. Oikos, 71(3): resilience, and biodiversity in ecosystem R.J. 2006. Predicting extinctions as a 355–366. DOI: 10.2307/3545823. management. 7! result of climate change. Ecology, 87(7): Arroyo-Rodríguez, V., Aguirre, A., Ecology, Evolution, and Systematics, 1611–1615. DOI: 10.1890/0012-9658 Benítez-Malvido, J. & Mandujano, S. 35: 557–581. DOI: 10.1146/annurev. (2006)87[1611:PEAARO]2.0.CO;2. 2007. Impact of rain forest fragmentation ecolsys.35.021103.105711. Secretariat of the Convention on on the population size of a structurally Gunderson, L.H. 2000. : Biological Diversity. 2010. Global important palm species: Astrocaryum in theory and application. Annual Review Biodiversity Outlook 3. Montreal, Canada. mexicanum at Los Tuxtlas, Mexico. 6=6=8, Available at: www.cbd.int/gbo3. Biological Conservation, 138(1–2): 198– 31: 425–439. DOI: 10.1146/annurev. Souza, C. Jr., Firestone, L., Silva, L.M. 206. DOI: 10.1016/j.biocon.2007.04.016. ecolsys.31.1.425. & Roberts, D. 2003. Mapping forest Betts, R., Sanderson, M. & Woodward, S. Kröpelin, S., Verschuren, D., Lézine, A.-M., degradation in the Eastern Amazon from 2008. Effects of large-scale Amazon Eggermont, H., Cocquyt, C., Francus, P., SPOT4 through spectral mixture models. forest degradation on climate and air Cazet, J.-P., Fagot, M., Rumes, B., 786, 87(4): ']$ Russell, J.M., Darius, F., Conley, D.J., 494–506. DOI: 10.1016/j.rse.2002.08.002. water, energy, mineral dust and isoprene. Schuster, M., von Suchodoletz, H. & Strand, H., Höft, R., Strittholt, J., ;7 Engstrom, D. R. 2008. Climate-driven Miles, L., Horning, N., Fosnight, E., Society B: Biological Sciences, 363: ecosystem succession in the Sahara: the & Turner, W., eds. 2007. Sourcebook 1873–1880. DOI: 10.1098/rstb.2007.0027. past 6000 years. Science: 320(5877): on remote sensing and biodiversity Bridgeland, W.T., Beier, P., Kolb, T. & 765–768. DOI: 10.1126/science.1154913. indicators. Technical Series No. 32. Whitham, T.G. 2010. A conditional Malhi, Y., Roberts, J.T., Betts, R.A., Montreal, Canada, Secretariat of the : birds benefit faster Kileen, T.J., Li, W. & Nobre, C.A. 2008. Convention on Biological Diversity. growing trees with strong links between Climate change, deforestation, and the Thompson, I., Mackey, B., McNulty, S., predators and plants. Ecology, 91: 73–84. fate of the Amazon. Science, 319(5680): & Mosseler, A. 2009. Forest resilience, DOI: 10.1890/08-1821.1. 169–172. DOI: 10.1126/science.1146961. biodiversity, and climate change: a Brockerhoff, E.G., Jactel, H., Montreal Process. 2009. Criteria and  Parrotta, J.A., Quine, C.P. & Sayer, J. . 2008. Plantation forests and biodiversity: Technical Series no. 43. Montreal, oxymoron or opportunity? Biodiversity , fourth edition. Canada, Secretariat of the Convention on and Conservation, 17(5): 925–951. DOI: Available at: www.rinya.maff.go.jp/mpci/ Biological Diversity. 10.1007/s10531-008-9380-x. 2009p_4.pdf. Tylianakis, J.M., Rand, T.A., Kahmen, A., Cardinale, B.J., Matulich, K.L., Parrotta, J.A. & Knowles, O.H. 1999. Klein, A.-M., Buchmann, N., Perner, J. Hooper, D.U., Byrnes, J.E., Duffy, E., Restoration of tropical moist forest on & Tscharntke, T. 2008. Gamfeldt, L., Balvanera, P., bauxite-mined lands in the Brazilian heterogeneity moderates the biodiversity- O’Connor, M.I. & Gonzalez, A. 2011. Amazon. , function relationship in real world The functional role of producer diversity in 7(2): 103–116. DOI: 10.1046/j.1526- ecosystems. PLoS Biology, 6(5): e122. ecosystems. >, 100X.1999.72001.x. DOI: 10.1371/journal.pbio.0060122.  98(3): 572–592. DOI: 10.3732/ajb.1000364. Redford, K.H. 1992. The empty forest. Díaz, S. & Cabido, M. 2001. Vive la BioScience, 42(6): 412–422. DOI: différence: plant functional diversity 10.2307/1311860. matters to ecosystem processes. Trends Scholze, M., Knorr, W., Arnell, N.W. & in Ecology & Evolution, 16(11): 646–655. Prentice, L.C. 2006. A climate-change DOI: 10.1016/S0169-5347(01)02283-2. risk analysis for world ecosystems.

Unasylva 238, Vol. 62, 2011/2