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You are here: Home Forests and Old Growth Old Growth Protection and Restoration Forests & Global Warming Global Warming Report Part IV. How will climate change affect ecosystems, forests, and trees?
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Part IV. How will climate change affect ecosystems, forests, and trees?

Part IV of “The Straight Facts on Forests, Carbon, and Global Warming,” an Oregon Wild report.

Some biological effects of climate change can already be seen. There is evidence that some trees are leafing out earlier and forbs are flowering earlier. Also, some birds are migrating earlier, and seasonal peaks in some insect populations are occurring earlier1. “[C]limate change is not something that will happen in the future but is already in progress”2. 

We should expect shifting “isoclimes” (zones of similar climate). Forest communities will shift toward the poles and toward higher elevations, but the climate may change faster than species’ natural capacity to migrate. Species are not expected to shift together as intact communities because of differing capacities for dispersal, migration, establishment, and tolerance of climate change. As a result, forest community composition will likely change. Climate change will disrupt co-evolved relationships between predators and their prey, plants and their pollinators, migration timing and flowering3, etc. During the tumultuous period of shifting biomes, opportunistic “weedy” species will readily replace native species that are displaced by climate change4.

Expected decreases in streamflow and increases in stream temperatures will place additional stress on cold-water fish such as salmon and trout. Forests may consequently be deprived of large quantities of marine-derived nutrients that for millennia have been conveyed by salmon from the ocean to continental ecosystems5

The following trends in forest ecosystems should be expected as a result of climate change. Forest disturbances such as fire and defoliating insects will likely increase, causing a reduction in the average age of trees (although old-growth forests will persist because of natural refugia, ecological inertia, and stochastic variation). Forests will likely become simplified due to the ascendancy of weedy species. The movement of existing forest types northward and toward higher elevations will likely cause extirpation of species where natural or human-induced habitat bottlenecks are encountered6.

There are significant feedbacks between climate and forests. Increasing temperatures can lead to longer growing seasons and more plant growth which can store more carbon or become fuel for fires. Longer fire seasons will likely occur due to earlier drying of fuels. Milder winters (more frost-free days) and warmer summers will allow insect populations to increase7. Warmer temperatures will also increase rates of respiration and decomposition which release CO2 to the atmosphere, yet this effect might be partially countered by drying of soil surface layers which limits respiration8.

Changes in forest disturbance regimes will likely be tightly coupled with the changes described above and may overshadow the direct physiological effects of climate change on plants and trees9.  It is reasonable to anticipate increased disturbances from wildfire, flooding, wind and storm damage, insect damage, and invasive species. Disturbance typically disrupts photosynthesis and favors respiration/decomposition processes thereby liberating CO2

Plants will likely face increased seasonal drought stress. Higher temperatures will increase evaporative losses from soils and increase transpiration from plants. “Forests at upper (cold) and lower (dry and/or hot) timberlines are most likely to show strong direct effects of climatic variation on tree growth, since they are closer to their physiological limits and, therefore, more prone to stress at these locations”10.  Interestingly, “[s]hade-tolerant trees show greater growth responses to CO2 than do shade-intolerant species because of more efficient use of light, water, and nutrients”11.  This could account in part for the proliferation of shade tolerant ladder fuels in our forests.

Trees “breathe” both in and out. During the day plants engage in photosynthesis that captures CO2 to build sugars and releases oxygen, but plants also engage in respiration (like animals), a process that uses some of the sugars produced during photosynthesis, consumes oxygen, and returns CO2 to the atmosphere. Plant growth is a result of a net imbalance between photosynthesis and respiration. In trees the extra carbon is turned into wood. Experiments reveal significant variability in plants’ response to elevated CO2 concentrations, but studies show several consistent results including: increased rates of photosynthesis, increased concentration of non-structural carbohydrates, enhanced efficiency of water use and nitrogen use, and decreased plant nutrient concentration12.  Elevated CO2 may increase growth at the expense of other aspects of plant health and could degrade the quality of the resulting plant material as food and fiber. 

Plants grow better when night-time temperatures are about 5 degrees C cooler than day-time temperatures, because lower night time respiration reduces the use of carbohydrates and allows more carbohydrates to be stored or used for growth. If climate change reduces the temperature difference between day and night then plants may suffer because respiration will increase relative to photosynthesis. 

Trees obtain CO2 from the atmosphere by opening stomatal pores on their leaves, but they unavoidably lose water in the process. Some plant species may react to CO2 enrichment by actively constricting their stomata (and by reducing the density of stomata on new leaves) which will reduce water loss, thereby increasing water use efficiency and partly mitigating drought stress13.  Constricted stomata may also reduce plants’ exposure to damaging ozone pollution. These intriguing plant responses to warming and CO2 enrichment are likely species-specific and more research is needed. These mitigating benefits of CO2 appear to manifest themselves more during times of stress than during periods of peak plant growth14

Furthermore, complex interactions among all the geophysical and biological responses to climate change will certainly lead to non-linear dynamics, threshold behavior, and rapid phase transitions that are difficult to model15.  “Many disturbances are cascading. … [W]hen ecosystems experience more than one disturbance, the compounded effects can lead to new domains or surprises”16.  For instance, increased herbivory of above-ground vegetation by insects could shift the normally favorable below-ground relationship between fungi and tree roots. Mutualistic mycorrhizal fungi could be replaced by competitive or parasitic organisms, thereby harming trees and increasing liberation of CO217.  Also, the migration of species toward the poles will likely be facilitated by disturbance because (relative to intact forests) disturbed sites will be more readily colonized by new arrivals from the south18.

It gets even more complex. Since forests are dark green, they tend to absorb rather than reflect sunlight, so the local albedo19 effect of forests tends to counteract forests’ carbon sequestration effects. Loss of forest cover tends to increase albedo thereby reflecting more of the sun’s energy back into space (the effect can be temporary or long-term depending on how snowy the region is and how quickly forests regrow). On the other hand, new forests growing on formerly treeless landscapes will lower albedo, thereby absorbing more of the sun’s energy. As the northern treeline moves north into the tundra the value of the carbon stored in the new forest may be more than off-set by the loss of albedo20.  Another complexity — evapotranspiration from forests, combined with forests’ natural release of organic aerosols that act as “cloud condensation nuclei” are credited with enhancing cloud formation, as well as the reflectance and longevity of clouds, potentially increasing albedo, and further highlighting forests’ significant and varied influence on our global climate21.

[1] Walther, G.R., Post, E., Convey, P., Menzel, A., Parmesan, C., Beebee, T.J.C., Fromentin, J.R., Hoegh-Guldberg, O., Bairlein, O., 2002. Ecological responses to recent climate change. Nature 416, 389–395.

[2] An Interview with Dr. Gian-Reto Walther. ESI Special Topics: October 2006.

[3] Sherry, A., X. Zhou, S. Gu, J. A. Amone III, D. S. Schimel, P. S. Verburg, L. L. Wallace, and Y. Luo. 2007. Divergence of reproductive phenology under climate warming. PNAS, 104: 198-202.

[4] Hansen, Neilson, Dale, Flather, Iverson, Currie, Shafer, Cook, and Bartlein. 2001. Global Change in Forests: Responses of Species, Communities, and Biomes. BioScience vol 51, no. 9, pp 765-779.

[5] Naiman, R.J., R.E. Bilby, D.E. Schindler, and J.M. Helfield. 2002. Pacific salmon, nutrients, and the dynamics of freshwater and riparian ecosystems. Ecosystems. 5:399–417. Helfield, J.M., and R.J. Naiman. 2001. Effects of salmon-derived nitrogen on riparian forest growth and implications for stream productivity. Ecology 82(9) : 2403-2409.

[6] Nigel Dudley. 1998. Forests And Climate Change. Forest Innovations – a joint project of IUCN, GTZ and  WWF.

[7] Insects’ “short life cycles, mobility, reproductive potential, and physiological sensitivity to temperature” lead to a conclusion that small changes in climate can lead to large changes in the distribution and abundance of insects. Ayers & Lombardero. 2000. Assessing the Consequences for Global Change for Forest Disturbance from Herbivores and Pathogens. The Science of the Total Environment 262 (2000) 263-286.

“Shortened winters, increasing summer temperatures, and fewer late-spring frosts correlate to increased insect feeding, faster growth rates, and rapid reproduction. … Drought creates many conditions that are favorable to increased insect reproduction. … Attempts at intervention [to control insects] are proving mostly negligible. ” Dunn, David, Crutchfield, James. 2006. Insects, Trees, and Climate: The Bioacoustic Ecology of Deforestation and Entomogenic Climate Change. Santa Fe Institute Working Paper. However, reduced snow cover might lead to increased winter mortality for some insects that rely on a blanket of snow for winter cover.

[8] Hanson & Weltzin. 2000. Drought Disturbance from Climate Change: Response of United States Forests. The Science of the Total Environment 262 (2000) 205-220.

[9] Flannigan, Stocks & Wotton. 2000. Climate Change and Forest Fires. The Science of the Total Environment 262 (2000) 221-229.

[10] Climate Impacts Group. Climate Impacts on Pacific Northwest Forests. University of Washington.

[11] John Aber, Ronald P. Neilson, Steve Mcnulty, James M. Lenihan, Dominique Bachelet, And Raymond J. Drapek. 2001. Forest Processes and Global Environmental Change: Predicting the Effects of Individual and Multiple Stressors. BioScience vol 51, no. 9, pp735-751.

[12] Luo YQ, Reynolds J,Wang YP. 1999. A search for predictive understanding of plant responses to elevated [CO2]. Global Change Biol. 5:143–56

[13] Since less than 1 percent of the water taken up by plants is used in photosynthesis (the remainder being lost to transpiration), stomatal control could have an enhanced effect on soil moisture during times of water limitation. However, the reduced transpiration could also adversely affect cloud formation, potentially reducing the albedo effect of clouds and increasing warming.

[14] R.A. Houghton. 2007. Balancing the Global Carbon Budget. Annu. Rev. Earth Planet. Sci. 2007. 35:313–47.

[15] Burkett, V.R.; Wilcox, D.A.; Stottlemyer, R.; Barrow, W.; Fagre, D.; Baron, J.; Price, J.; Nielsen, J.L.; Allen, C.D.; Peterson, D.L.; Ruggerone, G.; Doyle, T. 2005. Nonlinear dynamics in ecosystem response to climatic change: case studies and policy implications. Ecological Complexity. 2: 357–394.

[16] Virginia H. Dale, Linda A. Joyce, Steve Mcnulty, Ronald P. Neilson, Matthew P. Ayres, Michael D. Flannigan, Paul J. Hanson, Lloyd C. Irland, Ariel E. Lugo, Chris J. Peterson, Daniel Simberloff, Frederick J. Swanson, Brian J. Stocks, And B. Michael Wotton. 2001. Climate Change and Forest Disturbances. BioScience vol 51, no. 9, pp723-734.

[17] Ayers & Lombardero (2000).

[18] Neilson, Ronald P.; Pitelka, Louis F.; Solomon, Allen M.; Nathan, Ran; Midgley, Guy F.; Fragoso, Jóse M.; Lischke, Heike; Thompson, Ken  2005.  Forecasting regional to global plant migration in response to climate change. Bioscience, Vol. 55(9): 749-759.

[19] “Albedo” is a measure of the reflectivity of surfaces. Light colored surfaces (e.g. snow and deserts) reflect more sunlight back to space, while dark surfaces (e.g. forests and oceans) tend to absorb more of the sun’s energy and contribute to global warming. Large-scale changes in the extent of arctic ice and the composition of vegetation play a significant role in the climate models.

[20] Catherine Brahic. 2006. Location is key for trees to fight global warming. 15 December 2006. G. Bala, K. Caldeira, M. Wickett, T. J. Phillips, D. B. Lobell, C. Delire, & A. Mirin. Combined Climate and Carbon-Cycle Effects of Large-Scale Deforestation. [pre-publication draft]

[21] Gregory C. Roberts, and Meinrat O. Andreae, Jingchuan Zhou, Paulo Artaxo. 2001. Cloud condensation nuclei in the Amazon Basin: "Marine" conditions over a continent? Geophysical Research Letters, Vol. 28, No. 14, Pages 2807-2810, July 15, 2001.

   Tunved, P., Hansson, H.-C., Kerminen, V.-M., Strom, J., Dal Maso, M., Lihavainen, H., Viisanen, Y., Aalto, P.P., Komppula, M. and Kulmala, M. 2006. High natural aerosol loading over boreal forests. Science 312: 261-263. Summarized here: 

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