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Vegetation Distribution (2008)

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Ron Neilson, Managing Disturbance Regimes Program, Pacific Northwest Research Station

The current versions of this paper (2014) are available at


Rapid climate change is expected to dramatically alter the distribution, function, and disturbance regimes of natural ecosystems. During the 21st century, the global temperature is anticipated to increases 1.5 to 6°C. The low end of that range matches the maximum temperature 5,000 to 9,000 years ago during the middle Holocene. At the high end, the temperature would be nearly as great as periods when the Earth has shifted out of a full ice age to warmer conditions similar to today. During the middle Holocene thermal maximum, the northern treeline was about 90 miles north of the present location; upper elevational treeline was about 100 to 150 m above the present location (Carrara et al. 1991, Tinner and Theurillat 2003), and plants in the Great Basin were as far as 250 miles north of their present distribution (Cottam et al. 1959, Neilson and Wullstein 1983).

The biosphere can certainly survive under vastly different climates than we have today. The primary issue is the rate of change and how that would manifest in terms of species migration or potential extinction and ecosystem function, such as carbon balance, floods and droughts, infestations and diseases, and possible catastrophic fires. It is increasingly clear that the rate of climate change may far exceed the potential rates of response of many important processes of the biosphere such as plant migration and the sequestration of carbon in ecosystems. Although ecosystems contain the full spectrum of process rates, from fast to slow, there are enormous lags in many processes (Davis et al. 2005, García-Ramos and Rodríguez 2002, Neilson 1993, Neilson et al. 2005). Other processes, however, such as drought-induced forest dieback, insect infestation, disease, and catastrophic fire can happen quite rapidly if an ecosystem is pushed beyond critical thresholds or key constraints.

Tools to analyze change

Global shifts in vegetation distribution were initially simulated by process-based vegetation distribution (biogeography) models. The USDA Forest Service MAPSS Team built the only such model for North America. Biogeography models were coupled with biogeochemical cycling models and fire models to simulate the dynamics of ecosystems as they transition from the present into future ecosystems. These new models, Dynamic General Vegetation Models (DGVM), are the terrestrial ecosystem analog of the General Circulation Models (GCM) that simulate global climate dynamics. Earth System Models (ESM) are being constructed by coupling GCMs with DGVMs, dynamic ocean models, and dynamic ice models, thus coupling earth, atmosphere, ocean, and ice.

Likely changes

The treeline will rapidly extend to the north, sequestering carbon from the atmosphere into the biosphere. Such gains in sequestered northern carbon also will be accompanied by enhanced forest growth over much of the temperate to higher latitudes over the early part of the 21st century; increases in high-latitude precipitation increases, longer growing seasons, and elevated CO2 concentration will facilitate this growth (Neilson et al. 1998, Scholze et al. 2006). However, with further warming, rapidly increasing evaporative demands will likely cause widespread drought stress in boreal and temperate forested and nonforested ecosystems (Neilson et al. 1998, Scholze et al. 2006). This widespread temperature-induced drought stress is expected to cause dramatic increases in the amount of biomass consumed by fire throughout much of the boreal forest, especially in continental interior regions. The drought-insect infestation processes currently underway are expected to continue. Drought and fire are expected to increase in both the western and eastern forests of the United States (Bachelet et al. 2001, in press; Lenihan et al., in press.). Parts of the interior West could experience increased precipitation, causing both enhanced woody expansion and increased fire, as a consequence of more fuel (Bachelet et al. 2001; Lenihan et al., in press).

Options for Management

The challenges for management are significant, largely owing to the rapid rate of change, coupled with the inherent uncertainty of what the future will bring to ecosystems. We are used to a sense of certainty of the future, that the future will echo the past. This view results, in part, from the long timeframes over which natural ecosystem processes can occur, such as successional processes following a disturbance. There are two major risks associated with climate change uncertainty: (1) the risk of loss of key species that are required to maintain important ecosystem services, such as the production of clean water, wildlife habitat and forest products (Neilson et al. 2005); and (2) the risk of catastrophic disturbance owing to temperature-induced drought stress, that is, a potentially rapid reduction in the carrying capacity of the ecosystem driven by extremely rapid increases in evaporative demand from higher temperatures. Managers have three broad choices: (1) do nothing, (2) respond to a disturbance after the fact, or (3) try to prepare the ecosystem for disturbance by increasing its resistance to disturbance (minimize the impact) or enhance its responsive capability via increased resilience (Millar et al. 2007). Under the third category, for example, managers might enhance diversity of all sorts, from genetic (Wang et al. 2006) to structural, so that the ecosystem can find its own route into the future. Another, functional, approach would be to reduce the leaf area via thinning to reduce transpiration and increase the resistance to drought stress. The two approaches can also be combined, for example, by thinning strategically to enhance wildlife habitat and also to reduce the intensity or spread rate of fire.

Crucial Questions

What are the most important questions to drive inquiry in this climate change topic? If you could boil these down to a single major question that all the other questions feed into, what would that focusing question be?

What is the range of possible future natural resource dynamics in regional domains, and how can we manage those resources (water, timber, carbon, food, etc.) so we optimize their sustainability and minimize the risk of catastrophic change or collapse of regional, continental, and global natural resources, given the uncertainties of future climate trends and variability?

Response: We now know that the future will not echo the past; yet, we must nevertheless manage the natural environment not only for its sustainability, but also for our own sustainability, while accepting the uncertainty of forecasting, but defining the potential risk to natural resources.


Bachelet, D.; Lenihan, J.: Drapek R.; Neilson, R. [In press]. VEMAP vs VINCERA: a DGVM sensitivity to differences in climate scenarios. Global and Planetary Change.

Bachelet, D.; Neilson, R.P.; Lenihan, J.M.; Drapek, R.J. 2001. Climate change effects on vegetation distribution and carbon budget in the U.S. Ecosystems. 4: 164-185.

Carrara, P.E.; Trimble, D.A.; Rubin, M. 1991. Holocene treeline fluctuations in the northern San Juan Mountains, Colorado, U.S.A., as indicated by radiocarbon-dated conifer wood. Arctic and Alpine Research. 23: 233-246.

Cottam, W.P.; Tucker, J.M.; Drobnick, R. 1959. Some clues to Great Basin postpluvial climates provided by oak distributions. Ecology. 40: 361-377.

Davis, M.B.; Shaw, R.G.; Etterson, J.R. 2005. Evolutionary responses to changing climate. Ecology. 86: 1704-1714.

García-Ramos, G.; Rodríguez, D. 2002. Evolutionary speed of species invasions. Evolution. 56: 661-668.

Lenihan, J.M.; Bachelet, D.; Neilson, R.P.; Drapek, R. [In press]. Simulated response of conterminous United States ecosystems to climate change at different levels of fire suppression, CO2, and growth response to CO2. Global and Planetary Change.

Millar, C.I.; Stephenson, N.L.; Stephens, S.L. 2007. Climate change and forests of the future: managing in the face of uncertainty. Ecological Applications. 17: 2145-2151.

Neilson, R.P. 1993. Vegetation redistribution: a possible biosphere source of CO2 during climatic change. Water Air and Soil Pollution. 70: 659-673.

Neilson, R.P.; Pitelka, L.F.; Solomon, A.; Nathan, R.; Midgley, G.F.; Fragoso, J.; Lischke, H.; Thompson, K. 2005. Forecasting regional to global plant migration in response to climate change: challenges and directions. BioScience. 55: 749-759.

Neilson, R.P.; Prentice, I.C.; Smith, B.; Kittel, T.G.F.; Viner, D. 1998. Simulated changes in vegetation distribution under global warming. In: Watson, R.T.; Zinyowera, M.C.; Moss, R.H.; Dokken, D.J., eds. The regional impacts of climate change: an assessment of vulnerability. Cambridge, United Kingdom: Cambridge University Press: 439-456.

Neilson, R.P.; Wullstein, L.H. 1983. Biogeography of two southwest American oaks in relation to atmospheric dynamics. Journal of Biogeography. 10: 275-297.

Scholze, M.; Knorr, W.; Arnell, N.W.; Prentice, I.C. 2006. A climate-change risk analysis for world ecosystems. Proceedings of the National Academy of Science. 103: 13116-13120.

Tinner, W.; Theurillat, J.P. 2003. Uppermost limit, extent, and fluctuations of the timberline and treeline ecocline in the Swiss Central Alps during the past 11,500 years. Arctic, Antarctic, and Alpine Research. 35: 158-169.

Wang, T.; Hamann, A.; Yanchuk, A.; O'Neill, G.A.; Aitken, S.N. 2006. Use of response functions in selecting lodgepole pine populations for future climates. Global Change Biology. 12: 2404-2416.

How to cite

Neilson, Ron. 2008. Vegetation Distribution and Climate Change. (June 20, 2008). U.S. Department of Agriculture, Forest Service, Climate Change Resource Center.



The MAPSS Team is simulating the spatial and temporal dynamics of all upland terrestrial vegetation over past and potential future climate scenarios. By using Dynamic General Vegetation Model (DGVM) technology, developed by the MAPSS team, the team simulates changing vegetation distribution, its growth and dieback, carbon dynamics, and drought and fire disturbance over the past 100 years and under a set of future climate scenarios over the next 100 years. Recognizing that there are great uncertainties in ecosystem simulations across scales, the team is building a group set of simulations that span the range of scales from landscape to regional, continental, and global. Seasonal forecasts, updated monthly, of potential fire occurrence and intensity with an outlook of 7 months, can be found on the team's Web site.