III. Forest Range Shift



The main consequences of Global Warming, which recent climate models have predicted of the future of the Pacific Northwest regarding precipitation, are increased seasonal variability in terms of intensity of extreme weathers and occurrence of fire regimes. Although model analysis have shown that annual  mean precipitation is projected to remain fairly similar, seasonal variation will intensify (Mote et al. 2010). Warmer temperature is projected to result in hotter summers with less precipitation, further increasing water stress in vegetation. On the other hand, winters will grow to have more intense storms, more precipitation as rain than snow and shorter season. Changes in such climate pattern can influence different levels of ecosystem. However, according to cses.washington.edu, the forests at upper elevation, where temperature is low, and lower timberlines, where it is dry and/or hot, are likely to show direct effects of climatic impacts on tree growth because they are already living in conditions that are near their physiological limits. On this page, we will discuss about how “range shifts” will occur in the Pacific Northwest forests in response to climate variability.


Mote et. al

Range shift is a common phenological or adaptive response organisms, that are otherwise met with lowered fitness, are adopting as a result of rapidly changing climate. Many terrestrial organisms have been altering distribution in two ways: higher latitudinal or elevation shifts. In an experiment done in Chen et al. 2011, the climatologists have estimated that “the distributions of species have recently shifted to higher elevations at a median rate of 11.0 meters per decade, and to higher latitudes at a median rate of 16.9 kilometers per decade. These rates are approximately two and three times faster than previously reported.” They compiled a meta-analysis of available studies done on range shifts occurring by increasing latitudes in Europe, North America, and Chile and elevation in Europe, North America, Malaysia, and Marion Island for a range of taxonomic groups. This confirmed that the greatest range shift occurred during highest levels of warming. Range shifts, however, for every individual species vary depending on their internal traits and their responses to varying external environmental conditions.

Turning our focus to a specific example of range shift that is projected to occur in the Pacific Northwest, we will examine how the Douglas Fir distribution responds to the changing climate. Summer with less precipitation and higher temperature, which was the predicted future climate of this region mentioned on the previous tab, is most likely going to bring severe water limitation to forests. Climate models have predicted that there will be at least 32% increase of areas with such condition by 2020s and up to 44% in 2049s and 2080s. Douglas Fir, which is one of the most abundant trees living in Pacific Northwest, was used to predict its distribution in the future. 

Littel et al. 2009

The first half of the 21^st century is predicted with an increase of distribution in wetter areas and decrease in drier areas of its range, where the net productivity will initially increase. However, as the temperature continues to increase and amplify evapotranspiration, it will exceed the Douglas Fir species’ thermal window. The result will be devastating, where we will need to face decrease in productivity due to drought stress in low elevations (Littel et al. 2009). The trees will shift its range towards higher elevation, where the temperature used to be too cold to sustain life becomes the optimal temperature for higher fitness. One thing to note is there are limited amount of space to live for trees, if their ranges are shifting to higher elevations. Some implications due to range shifts and lower productivity of Douglas Fir could be decreased carbon sequestration and community composition shifts. (See figure above to visualize the future expansion of drought-stressed forest area over time)

 

As a side note, the tree species that end up at peaks of elevation will become vulnerable from immobilization. Now, the story moves on to concentrate of the species' survival  that will depend climate's ultimate persistence. The speed at which climate changes vary by topography of each region. Loarie et al. elaborated on deriving the velocity of temperature and precipitation change from spatial gradients. The paper claimed that the topographic effects of mountainous biomes result in the lowest velocity of precipitation change due to the influence of rain shadows. This is a fortunate news for the Pacific Northwest region. According to an article written by J.S Clark, trees have a short average dispersal distance but a rather rapid dispersal rate. Combining the effects of low velocity of climate change and fast dispersal rate, the Pacific Northwest can potentially become a relatively less vulnerable region in comparison to regions with high velocity of climate change. However, this information doesn't imply that humans should stop worrying about the forest ecosystem because there are still high uncertainty about the effects of direction and magnitude of precipitation change. Further research is required before making any immediate decisions about PNW conservation value.