Forest Resilience, Biodiversity, and Climate Change

Forest Resilience, Biodiversity, and Climate ChangeThere is considerable evidence that climate changemay lead to large losses in biodiversity in alltropical forests (e.g. Bazzaz 1998, Miles et al. 2004,Possingham and Wilson 2005, Rull and Vegas-Vilarrubia 2006, Fitzherbert et al. 2008, Malhi etal. 2008), with consequent effects on the flow ofgoods and services from these forests. This will beespecially true for montane and cloud forests, owingto a lack of surrogate habitats for species, and whereevidence of biodiversity loss has already accumulated(Bunker et al. 2005, Rull and Vegas-Vilarrubia 2006,Colwell 2008). Wilson and Agnew (1992) providedan example of permanent regime shift in tropicalcloud forests following unsustainable harvestingthat resulted in a negative feedback involving theneeded condensation moisture for remaining treesto survive; climate drying would have an identicaleffect. Tropical forests are at a substantial risk forbiodiversity loss under climate change for severalreasons including disruptions to complex ecosystemdynamics, the high degree of specialization andnarrow niches for many tropical species, and becauseclimate change will exacerbate an already high rateof deforestation (Bazzaz 1998). Large-scale loss ofbiodiversity will have dramatic negative effects oncarbon sequestration capacity by tropical forests(Cramer et al. 2004, Fischlin et al. 2009).5.3.2 Case-study: Amazon rain forestThe Amazon rain forest is an extensive forest systemabout as large as the United States occurring in eightSouth American countries. It contains many foresttypes, depending on soils, topography, and climate,but there is a large area of evergreen forest with littleseasonality where 200-900 cm of rain falls annually.These forests are highly resilient to the chronicdisturbances of herbivory and blowdown typicalof the region (table 9). However, land-clearingand logging had reduced the original extent of theAmazon forest by 15% by 2003 (Soares-Filho et al.2006). Recent occasional drought episodes haveexacerbated the human impacts by increasing forestfires (Malhi et al. 2008). Climate change is predictedto have long-term effects on forest structure andfunction by changing the mortality and growth ratesof trees and increase the frequency of disturbances,especially an increasing fire frequency under adrier climate regime (Malhi et al. 2008, Phillips etal. 2008). Increased carbon dioxide concentrationsseem to be having a direct positive impact on theproductivity and relative competitive success amongtropical plant species (Baker et al. 2004, Malhi et al.2009).Modelling global warming of >3°C, as expected intropical areas, reduces the tropical forest sink by midcentury,and results in a net carbon source towardsthe end of this century (Scholze et al. 2006, Fischlinet al. 2009). The most likely impact of climatechange on Amazon forests will be drought and thedevelopment of seasonality in the rainforest (Malhiet al. 2009, Phillips et al. 2009), although models arefar from certain in their prediction of rainfall. Thepredicted decreased rainfall and ground moisturewill increase the likelihood of fire and shift therainforest into drier seasonal forest. This process hasa positive feedback owing to the loss the rainforestcanopy that otherwise tends to maintain regionalmoisture levels (Laurance and Williamson 2001).As a result, much of the rainforest will change statesto drier and possibly more open forests, reducinghabitats, lowering regional water supplies, andbecoming a far less productive forest (Malhi et al.2009, Cochrane and Barber 2009). Climate changewill exacerbate the many negative effects of ongoingdeforestation and forest loss (Laurance 1998, Cookand Vizy 2008, Cochrane and Barber 2009), and theforests will be considerably different than at present.5.4 Summary among forest biomesAll forest types will undergo some change as a resultof altered climate conditions; some of these changesare already occurring but widespread change is expectedover the next 50-100 years (e.g., Alcamo etal. 2007, Fischlin et al. 2009). From the case-studies,it is clear that some forests are considerably morevulnerable (less resilient) than others as a result ofaltered disturbance regimes that are predicted underclimate change. This is especially the case forforests where previously rarely-seen disturbanceswill become more common, such as fire in rainforests.In some cases, even ecological resilience will beovercome and forests are expected to change statesto non-forest or savannah (IPCC 2007), as has happenedin many areas previously, such as the northernSahara area of Africa (Kröpelin et al. 2008). Inmany cases, forests will change states, however, atleast among most boreal and some temperate forests,ecological resilience is expected. In many tropicalforests, however, many rainforests may become drytropical forests with reduced carbon storage capacity(case-studies, Fischlin et al. 2009). The diversityin these tropical regions suggests that some form offorest will continue to exist even with severe disturbance,but that many of the functions will changeowing to the lack of resilience and new states, ingeneral, will produce considerably less goods andservices while supporting less biodiversity than atpresent.41