[Originally posted on PLOS Ecology Field Reports, August 15, 2015]
By Jeff Atkins
Mountain ecosystems cover nearly a quarter of the earth’s landscape, offering habitats to diverse plant and animal species, and supplying us with water and timber along with winter and summer recreation opportunities while also reducing carbon dioxide in our atmosphere and slowing the buildup of greenhouse gas.
But mountain ecosystems are also especially vulnerable to climate change.
In the western United States, Rocky Mountain National Park alone has experienced a 1.8° C increase in temperature since 1900, almost 2.5 times the rate of the US as a whole. In the eastern US, once expansive forests of red spruce and balsam fir have been relegated to isolated ridgelines and solitary mountain tops.
And perhaps nowhere serves as better exemplar of susceptibility to climate change as Glacier National Park, in Montana. In 1850, it was estimated there were approximately 150 glaciers in the park. That number is around 25 presently. It is possible that in the next few decades that number may drop to zero.
Several articles featuring how our changing climate affects mountain ecosystems are included in the 2015 PLOS Ecological Impacts of Climate Change Collection. While this collection spans a dizzying array of species and ecosystems, mountain ecosystems offer a fragile and unique perspective on the impacts of climate change. Here I highlight three articles within this collection related to mountain and forest ecosystems.
HOME ON THE FRONT RANGE
Since 2002, federal wildfire protection and suppression efforts in the US have averaged 3 billion dollars annually. However, in the 1990’s, protection and suppression efforts cost less than 1 billion. The common line of thinking has been to attribute this dramatic rise in cost to a perceived increase in the severity of wildfires driven by higher fuel loads that are attributed to a combination of nearly a century of fire exclusion efforts, warming temperatures, changing precipitation regimes, and increased insect outbreaks. Large wildfires often attract wide media attention, potentially inspiring and propagating these ideas.
But are wildfires really any worse than they have ever been?
A 2014 paper in PLoS ONE by Sherriff et al. seeks to answer this question by testing historical fire severity against observed fire severity. The authors find that in the 564,613 ha of Colorado Front Range montane forests, only 16% of the area has shifted from a low-severity fire regime towards a regime marked by higher risk of fire severity and potential of stand replacing crown fires.
Figure 5 from Sherriff et al., showing the 232 sites included in the analysis with historical (pre-1920) evidence of low severity and mixed-severity fires. Low severity fires are defined as non-canopy fires that leave fire scares and have a return interval of less than 30 years. Mixed-severity includes low-severity and extreme canopy fires as well.
Of concern, is that much of the area showing increases in fire severity potential is within lowland areas (below 2200 m) and lowland areas are where the people tend to live. Lowland areas do demonstrate greater evidence of increased fuel load from exclusion measures.
Despite the widely accepted notion that across the west there are recent increases in wildfire severity, the data show precedent for large, severe wildfires in this area in the past and indicate that present conditions are not radically different. This study highlights the complexity of fire regimes and calls on ecologists and forest managers to reconsider current conceptualizations and practices.
THE GOOD, THE BAD, AND THE BUDWORM
Gypsy moth. Hemlock woolly adelgid. Bark Beetle. Insect outbreaks in forested ecosystems appear to be on the rise and are a major research and management concern. But aside from leaving less than picturesque forests in their wake, insect outbreaks call into question forest health and elicit concerns about increased likelihood of fire. In isolation, increases in forest insect outbreaks cannot solely be attributed to climate change, as forest insect population dynamics are driven by a complex set of interactions.Flower et al. describe the relationship between western spruce budworms and fire frequency, severity, and synchrony over three centuries that influenced the development (pre-1890) of the Douglas-fir forests spanning from Oregon to Montana.
The majestic western spruce budworm that carries the moniker of “the most destructive defoliator of coniferous trees in the west.”
Geographic distribution of the western spruce budworm in Western United States and Canada. (Fellin and Dewey, 1982)
Flower et al. contend that studies showing negative correlations between western spruce budworm outbreaks and wildfires are shaded by a recency bias from limiting data to the late 20th century, during times of heavy fire exclusion. By comparing paired disturbance histories that include dendrochronological records, western spruce budworm outbreak history, and fire chronologies within 10 stands, the authors find no direct relationship before 1890 linking western spruce budworm outbreaks to increased frequency or severity of fires during their three century historical reconstruction. The authors note that after suppression efforts were implemented, outbreaks became more inveterate, revealing “ . . .a subtle synergistic relationship between the two disturbance types that influences their severity, but not probability of occurrence, over long time scales.”
TO SEE THE FOREST, WE MUST SEE THE TREES
Shifting subtly from strictly speaking about mountains, there is the question of how trees, and specifically their habitat range, will response to rising temperatures. In the past 20,000 years, plant and animal ranges have shifted by as much as 1000 km in response to the 7° C warming over that time period. Current climate projections indicate that species may have to move as much as an additional 500 km over the next century. In the 2014 PLoS ONE paper Pushing the pace of tree species migration, Lazarus and McGill make the exceedingly valid point that paleo-range shifts, estimated as high as 1 km a year for some tree species, did not occur in the presence of anthropologic landscape fragmentation.
Lazarus and McGill present a numerical model of the dispersal dynamics of a “generic tree species” to evaluate this environmental question. The numerical model, which is more specifically a probabilistic spatial model, is paired with a configuration of cellular landscapes (where cells are either “suitable” or “unsuitable”) of increasing fragmentation — fragmentation is increased by the presence of more “unsuitable” cells in each landscape. By forgoing the inclusion of ecological traits impacted by climate change, or any other biophysical variable for that matter, the authors present an elegantly logical model focused simply on landscape disturbance. The harrowing truth, however, is that even in the best-case-scenario, tree migration rates lack the ability to keep up with the velocity of climate change.
To combat this, an intentional planting program is suggested as a means to abate species loss. Model output suggests this as a possibility, but as the authors highlight, this would have to be a massive undertaking, potentially at the continental scale.
Figure one from Lazarus and McGill. The top panel shows the initial model state with direction of spread indicating the direction of migration. The second panel indicates an environment where the entire landscape is suitable. Each successive landscape has increasing fragmentation as indicated by the d number on the left ranging from 10–80%. There is a noticeable decrease in migration spread with increasing fragmentation.
Sherriff RL, Platt RV, Veblen TT, Schoennagel TL, Gartner MH (2014) Historical, Observed, and Modeled Wildfire Severity in Montane Forests of the Colorado Front Range. PLoS ONE 9(9): e106971. doi:10.1371/journal.pone.0106971
Flower A, G. Gavin D, Heyerdahl EK, Parsons RA, Cohn GM (2014) Western Spruce Budworm Outbreaks Did Not Increase Fire Risk over the Last Three Centuries: A Dendrochronological Analysis of Inter-Disturbance Synergism. PLoS ONE 9(12): e114282. doi:10.1371/journal.pone.0114282
Lazarus ED, McGill BJ (2014) Pushing the Pace of Tree Species Migration. PLoS ONE 9(8): e105380. doi:10.1371/journal.pone.0105380
JEFF ATKINS is a Ph.D. candidate at the University of Virginia and Field Scientist for the Shenandoah Watershed Study. His research focuses on the interaction of vegetation and landscape position to influence biogeochemical cycles within complex terrain and the effects of inter-annual climate variability on ecosystems. You can reach him via Twitter (@atkinsjeff).