Global Warming Impact on Forests & Woodlands
Forests have been identified as being very vulnerable to climate change in the long run but may be more vulnerable in the short term if other disturbances such as drought, insects, and fires cross key thresholds (IPCC, 2007). Key ecosystems that forests provide include:
- habitats for an increasing fraction of biodiversity
- carbon sequestration
- climate regulation
- soil and water protection or purification (>75% of globally usable freshwater supplies come from forested catchments
- recreational, cultural and spiritual benefits
Forests cover about 42 million km2 in tropical, temperate, and boreal lands, which is about 30% of the global land surface as shown in the figure below (Bonan, et al., 2008).
Forests store about 45% of the terrestrial carbon (see figure below) and contribute about 50% of terrestrial net primary production (Ibid).
Carbon uptake by forests removes about 33% of human carbon emissions from fossil fuel and land use change so forests are critical for tempering climate change and mechanisms related to forests are included in climate model projections.
As shown in the figure below (Ibid), the current generation of climate models treats the biosphere and atmosphere as a coupled system. These models include (A) surface energy fluxes and (B) the hydrologic cycle. Many models also include (C) the carbon cycle and (D) vegetation dynamics so that plant ecosystems respond to climate change. Some models also include (E) land use and (F) urbanization to represent human alteration of the biosphere.
As the climate warms, tree species are expected to migrate. The figures below (USGRP, 2009) show how tree species are projected to migrate northward in the Unites States and how trees migrate to higher altitudes, respectively.
Estimates for migration rates of tree species from palaeoecological records are on average 200-300 m/yr, which is a rate significantly below that required in response to anticipated future climate change (IPCC, 2007).
By 2100 only 18% to 45% of the plants and animals making up ecosystems in global, humid tropical forests may remain as we know them today, according to a study led by Greg Asner at the Carnegie Institution’s Department of Global Ecology. Tropical forests hold more than half of all the plants and animal species on Earth but the combined effect of climate change, forest clear cutting, and logging may force them to adapt, move, or die (e! Science News, 2010).
Tropical forests are vulnerable to a warmer, drier climate, which may exacerbate global warming through a positive feedback that decreases evaporative cooling, releases CO2, and initiates forest dieback (IPCC, 2007). When land clearance (deforestation) crosses a critical threshold, these tropical forests may move to a permanent dry landscape (Bonan et al., 2008).
Boreal forests have a lower surface albedo (lower reflectivity) compared to the surrounding snow and ice or areas with an absence of trees. As forests begin to encroach on these areas, global warming will be exacerbated due to a greater absorption of incoming solar radiation as evidenced by the forest expansion during the mid-Holocene 6000 years ago which amplified warming during that period (Ibid).
Boreal forests are vulnerable to global warming. Trees may expand into tundra, but die back in more southern regions. There may be loss of evergreen trees and a shift toward deciduous trees. Siberian forests may collapse in some areas and become more evergreen in the north (Ibid).
The figure below (Ibid) is a good summary of how forests provide positive and negative climate change feedbacks:
Tree mortality caused by warmer and drier conditions has the potential to rapidly alter forest ecosystems which will subsequently affect climate feedbacks (Allen et al., 2010; Adams et al., 2010) The figure below (Adams, et al., 2010) summarizes these feedbacks:
Reports of tree morality have been increasing in the past few decades and appear to be greater than normal mortality rates. It is strongly suspected that increased temperatures and more severe drought are the culprits. In the southwestern US in the early 2000s, drought and insect-driven mortality of the piñon pine affected more than 12,000 km2 in less than three years killing 40-97% of the trees at some sites (Ibid).
In western Canada, drought and warmer temperatures accelerated mountain pine beetle population growth and range and these insects killed millions of trees across 130,000 km2 of pine forests in six years.
Extreme cold temperatures during winter can reduce pine beetle populations. For freezing temperatures to affect a large number of larvae during the middle of winter, temperatures of at least 30 degrees below zero (Fahrenheit) must be sustained for at least five days (Leatherman, Aguayo, & Mehall, 2010). As climate warms, there will be fewer periods of this killing cold. Other insect outbreaks have been documented in North America from Alaska to Mexico with drought and warming the common drivers of these outbreaks (Adams et al., 2010).
Allen et al. (2010) have documented extensive tree mortality in Arica, Asia, Australia, Europe, North America, and South America as shown in the figures below:
Tree mortality has already switched a carbon sink to a carbon source in British Columbia where mortality associated with beetle outbreaks reduced carbon sinks by 270 megatons over the past 20 years. Increased tree mortality also has the potential to offset the carbon sequestration of increased forest growth caused by CO2 fertilization (Adams et al., 2010).
Nitschke and Innes (2008) developed a model to investigate tree species resilience and vulnerability to climate change within its fundamental regeneration niche. The tree and climate assessment (TACA) model was tested within the interior Douglas-fir ecosystem in south-central British Columbia. TACA modeled the current potential tree species composition of the ecosystem with high accuracy and modeled significant responses amongst tree species to climate change. The response of individual species suggests that the studied ecosystem could transition to a new ecosystem over the next 100 years. The chart below summarizes the model results and shows that many species will no longer be present in this region by the year 2085.
Minimum and maximum GDD thresholds are used to determine the lower and upper relationship limits between temperature and growth. If the maximum and minimum requirements are not met, minimum growth rates occur that can result in species mortality. Species-specific chilling requirements are measured to determine if frost hardiness is induced. Some species are very sensitive to late spring frosts, resulting in very high chilling requirements being required before bud break will occur. Climate change may interfere with the ability of trees to meet their chilling requirements for bud break, flowering and germination. The inability of a species to obtain its chilling requirement can seriously affect the species’ ability to re-establish after disturbance, facilitating changes in community composition. Drought conditions have and will prevent establishment of species on a site and cause mortality of established seedlings. In TACA, If the threshold is exceeded then a species is assumed to be absent from the site (Ibid).
In a few exceptions, climate change may increase diversity locally or regionally but in most cases extinction risks are projected to increase (IPCC, 2007).