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Impacts, Causes and Effects of North America's Forest Dieback
by Brent Herbert Friday, Oct. 01, 2004 at 3:09 PM

The following is a brief discussion of the extent and potential impact of America's forest dieback in the immediate future, as well as a discussion of the various causes and scenarios which describe the impact of forest loss on human civilization and the planetary ecosystem.

The following is a continuation of the discussion begun on the page America's Forest Dieback

Map of Forest Dieback 1997-2003




Cumulative Graph of yearly dieback


The graphic above and the graph below show the forest dieback trends (pdf) in the United States, and the graph, which displays the yearly trend, shows a strongly nonlinear increase in the rate of forest dieback, which rose to 14 million acres in 2003. The North American continent has about 700 million forested acres, and if the trend continues, as well as the increasing rates of dieoff, this could result in significant damage occuring rapidly over a very short time span (this being (possibly) a climate transient which has a very damaging impact over a decade and not over a span of 50 or 100 years - the time span people usually use when referring to the impacts of climate change, this sort of time span being the result of the equilibrium modeling of climate impacts, and it assumes slow and gradual shifts, rather than rapid climate changes and damaging transients).





The damage from dieback is even more extensive in British Columbia (10 million acres in 2003, as compared to 14 million acres for the entire United States west) and this is an illustration of the potential effects of a nonlinear rate of increase in forest dieoff, a type of climate transient of which the impact occurs with rapid suddenness.


According to the USDA Forest Service report on Forest Health Climate Interactions (pdf) drought is the major influence in forest dieback events. According to the scientific literature, "depth of snow-pack was the only parameter of 10 regional climate stresses that showed a consistent link to forest dieback...snow depth was statistically significant for both the onset and recovery of dieback."





In the graph above you can see that as snow pack increases (the blue line) forest dieback decreases (the red line) and similarly as the snow pack drops the forest dieback peaks.





As far as global climate stresses go, forest dieback is shown to be strongly linked to the ENSO cycle (oscillations between El Nino and La Nina). Once again the reason for the dieback is reduced precipitation.




Drought Monitor


In the graphic above darker the color the more extreme the drought, with the blackish areas being 'esceptional' and the red areas being 'extreme' and the orange areas being 'severe'. Such severe conditions have persisted since the late 1990s. It goes without saying that trees and forests can only take so much of this type of extreme abuse, and are currently enduring the one of the worst droughts in recorded history. In order to see a forest dieback event that might be equal to what could be about to take place in North America it would be required to look back at least 13,000 years, and perhaps even tens of thousands of years.

IN terms of both regional and global climate influences, drought is the major influence over forest dieback. IN the case of the North American west the dieback is occuring over an area stretching from British Columbia in the North to the American South West, and drought is the common cause for the wide spread dieback in these various diverse ecosystems. The graphic above shows the extreme drought conditions being endured by the forest ecosystems of the West, a condition that has prevailed since the late 1990s, and it was inevitable that eventually forest ecosystems would be pushed over the mortality threshold, due to the year over year exposure to such extreme conditions. This long lasting drought is not related to the ENSO cycle. It is also worth noting that the ENSO cycle has also collapsed and now there is almost no cycle to speak of as La Nina has pretty much disappeared and now rather than the cycle which existed in the past there are only repeating El Ninos (so then the Southern Oscillation is no longer an oscillation).

Lack of water is the main cause of forest dieback, but not the only cause. Forest dieback in places like Haiti, parts of Africa, or India are caused by economic conditions, which lead to deforestation, which leads to drought, which then leads to desertification (for example in Haiti and parts of Africa, forests are clear cut over the years in order to make a little money selling charcoal). A forest dieback in Croatia is linked to "the anthropogenetic impacts presented by the pollution of air, water and soil, all causing stress and finally, death of forest trees; in addition, various engineering operations cause severe change in their habitats... the forest dieback is the result of industrial, urban, traffic and agricultural pollution, partly though, by the technologies badly adapted to forest ecosystems. The micro-habitat methods helped to assess great changes in the forest soils, dry and moist sulphate deposits, nitrates and other poisons being present in Croatian forests for a considerable time now. Though different tree species react differently, sooner or later all will be destroyed."

Insects and diseases are also causes of dieback, but are usually secondary effects, as these agents target drought weakened forests, often launching mass attacks, and drought weakened forests are also more susceptible to environmental toxins and engineering operations that damage the forest ecosystem.

The IPCC discusses scenarios for climate induced forest impacts

The impact of forest loss goes beyond just simply losing jobs or losing vacation spots, and the IPCC attempted to summarize a large number of papers in the field to prepare a summary of available scenarios which attempt to forecast the impact of climate change on the forests and then the impact of forest change on human society and larger ecosystem. It is worth nothing here that it would seem, if you read through the report, that many of its assumptions and conclusions seem to be both confirmed and outstripped by rapidly occuring events (for example, predictions are made of forest dieoff which occurs decades in the future, making the prediction of a dieoff accurate although it would appear the time scale was to conservative, since that has been taking place only a few years after the report was released in 2001). Many computer simulations predicted increasing and extreme rains in the American West, and it remains to be seen whether the destruction of the forests is a climate transient, and the eventual climate equilibrium is a wet climate, or if those climate simulations were themselves all wet (the report notes that in the field there are two competing scenarios, wetter or dryer).

According to the Intergovernmental Panel on Climate Change ...

"Forests hold about 62–78% of the world’s terrestrial biospheric carbon (Perruchoud and Fischlin, 1995), about 14–17% of which is in the forests of North America; about 86% of that is in the boreal forest (Apps et al., 1993; Heath et al., 1993; Sampson et al., 1993). Forests play a large role in global water and energy feedbacks (Bonan et al., 1995) and account for most of the world’s terrestrial evapotranspiration, which is about 64% of the precipitation (Peixoto and Oort, 1992; Neilson and Marks, 1994). Most of the world’s freshwater resources originate in forested regions, where water quality is directly related to forest health."

http://www.grida.no/climate/ipcc/regional/192.htm
Annual tree mortality losses from insect outbreaks in Canada are about 1.5 times the losses from wildfire and amount to about one-third of the annual harvest volume (Fleming and Volney, 1995). Annual losses from insects and fire in the United States also are about one-third of the annual harvest (Powell et al., 1993). Warming-induced changes in the timing of spring frosts may be important in ending or prolonging outbreaks. Increased drought stress also may enhance insect outbreaks, and changes in climate could extend the ranges of some insects and diseases.

http://www.grida.no/climate/ipcc/regional/193.htm
North American forests also are being subjected to numerous other stresses, including deposition of nitrogen and sulfur compounds and tropospheric ozone, primarily in eastern North America (Lovett, 1994). The interactions of these multiple stresses with elevated CO2 and climate change and with large pest infestations (of, for example, the balsam wooly adelgid, gypsy moth, spruce budworm, and others) are very difficult to predict; however, many efforts are under way to address these questions (Mattson and Haack, 1987; Loehle, 1988; Fajer et al., 1989; Taylor et al., 1994; Winner, 1994; Williams and Liebhold, 1995). Anthropogenic nitrogen fixation, for example, now far exceeds natural nitrogen fixation (Vitousek, 1994). Atmospheric nitrogen deposition has likely caused considerable accumulation of carbon in the biosphere since the last century (Vitousek, 1994; Townsend et al., 1996). However, nitrogen saturation in soils also can be deleterious, possibly causing forest dieback in some systems (Foster et al., 1997). Tropospheric ozone also can damage trees, causing improper stomatal function, root death, membrane leakage, and altered susceptibility to diseases (Manning and Tiedemann, 1995). Such ozone-induced changes can render trees more sensitive to warming-induced drought stress (McLaughlin and Downing, 1995). There are many other stress interactions, and researchers think that, in general, multiple stresses will act synergistically, accelerating change due to other stresses (Oppenheimer, 1989).

It is interesting to note the IPCC report was released in 2001 and the strongly nonlinear rate of forest dieback did not appear until 2003, which is something to keep in mind when considering the scenarios being discussed in the scientific papers being summarized in the report.

Writing of computer simulations of the climate they describe

http://www.grida.no/climate/ipcc/regional/196.htm
"two contrasting scenarios of the North American forest future must be considered: one with considerable forest dieback, another with much enhanced forest growth. These contrasting scenarios represent endpoints on a spectrum of possible responses...These results suggest the possibility that early forest responses to global warming could exhibit enhanced growth; later stages could produce widespread decline or dieback. Most combinations of scenarios and CO2 effects produce intermediate scenarios, with a regional mosaic of forest dieback and enhanced forest growth...Forests cannot move across the land surface as rapidly as the climate can. The faster the rate of climate change, the greater the probability of ecosystem disruption and species extinction. Were temperature-induced drought dieback to occur, it likely would begin shortly after observable warming; if accompanied by short-term precipitation deficits, it could occur very rapidly."

One the main human impacts of deforestation is the effect on the water supply.

http://www.grida.no/climate/ipcc/regional/200.htm
Important vulnerabilities of water resources to potential climate change scenarios involve changes in runoff and streamflow regimes, reductions in water quality associated with changes in runoff, and human demands for water supplies. Seasonal and annual runoff may change over large regions as a result of changes in precipitation or evapotranspiration.

http://www.grida.no/climate/ipcc/regional/200.htm
Runoff is simply the area-normalized difference between precipitation and evapotranspiration; as such, it is a function of watershed characteristics, the physical structure of the watershed, vegetation, and climate.

The world's forests are responsible for the majority of the evapotranspiration (plants exhaling water into the atmosphere, which then forms clouds and precipitation).

http://www.grida.no/climate/ipcc/regional/201.htm
Higher air temperatures could strongly influence the processes of evapotranspiration, precipitation as rain or snow, snow and ice accumulation, and melt—which, in turn, could affect soil moisture and groundwater conditions and the amount and timing of runoff in the mid- and high-latitude regions of North America. Higher winter temperatures in snow-covered regions of North America could shorten the duration of the snow-cover season. In mountainous regions, particularly at mid-elevations, warming could lead to a long-term reduction in peak snow-water equivalent, with the snowpack building later and melting sooner (Cooley, 1990). Glacial meltwater also is a significant source of water for streams and rivers in some mountainous regions, with the highest flows occurring in early or midsummer (depending on latitude). For example, glacial meltwater contributes an average of 85% of the August flow in the Mistaya River near Banff, Alberta (Prowse, 1997). Accelerated glacier melt caused by temperature increases means more runoff in the short term, but loss of glaciers could result in streams without significant summer flow in the future (IPCC 1996, WG II, Sections 7.4.2 and 10.3.7). Late-summer stream discharge could decrease suddenly within only a few years. A steady pattern of glacial retreat is apparent in the southern Rocky Mountains below central British Columbia and Alberta. Water supplies in small communities, irrigation, hydroelectric generation, tourism, and fish habitat could be negatively impacted (IPCC 1996, WG II, Chapter 7; Brugman et al., 1997; Prowse, 1997).

portland.indymedia.org/en/2004/09/298439.shtml

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