Warming and drying climate, bark beetle outbreaks, and wildfire all pose challenges to western US conifer forests.  Widespread bark beetle outbreaks have been impacting large swaths of western North America.  In the Lake Tahoe Basin, there is a strong link between severe drought and beetle outbreaks.  Beetle outbreaks can also be affected by the density of their hosts because many beetle species use specific tree genera.  As an example, mountain pine beetle only uses pine trees for hosts. 

In drier, more fire-prone forests, fire-exclusion has increased tree density and in some cases host density.  This change in structure has also increased the risk of stand-replacing fire.  Since management activities to reduce the risk of stand-replacing fire typically involve reducing tree density and restoring surface fire, we sought to determine if these treatments might also reduce beetle outbreak potential.

In a study of Lake Tahoe Basin forests, led by Rob Scheller, we used the LANDIS-II simulation model to determine if beetle outbreaks would increase with climate change and if management activities to reduce wildfire hazard would reduce the impacts of beetle outbreaks.  We hypothesized that climate change would increase beetle outbreaks and reduce carbon uptake by the forest and that management activities would reduce beetle-caused mortality and increase carbon uptake by the forest.

We found that climate change without beetles caused a reduction in carbon stored in trees and that beetles without climate change caused a reduction in carbon stored in trees.  However, the combined effects of climate change and bark beetles caused a large reduction in the amount of carbon stored in trees across the study area (see Figure 1).  

White fir and Jeffrey pine are two of the more common species in these forests.  Previous research has shown that prior to fire-exclusion, Jeffrey pine was more common than white fir and with fire exclusion, white fir has become more common.  In areas around communities, managers often focus harvesting efforts on white fir to reduce fire hazard.  When we looked at these individual species in the areas treated around communities, we found that without management bark beetles caused both species to decline (see Figure 2).  When we simulated management and no bark beetles, white fir decreased and Jeffrey pine increased.  The combined effects of beetles and management really reduce the amount of white fir and resulted in a similar amount of Jeffrey pine as the simulation that only included beetles.

​Our management simulations did not include treating all forests within the Basin.  Treatments focused on communities and roadways.  Because climate projections for the Basin get warmer and drier later in the 21st century, we looked at the chance that these forests become a source of carbon to the atmosphere with continued climate change and bark beetles.  We found that the chance of these forests become a source of carbon to the atmosphere increases later this century with both beetles alone and beetles and management combined.  Reducing the impacts of beetles may require more thinning to further reduce host density.  These results show that we have to consider the full suite of disturbance agents when trying to determine the best path forward for managing forests under climate change.

The carbon carrying capacity of an ecosystem is the maximum amount of carbon that can be sustained in a given location on the planet under prevailing climate and natural disturbance conditions.  The idea is that given the temperature range and amount of precipitation at a given location, there are only certain types of plant species that can grow there.  This is exactly why we don’t see giant sequoia trees growing next to Joshua trees in the Mojave desert.

rests in the Sierra are heavily dependent on the winter snowpack for providing growing season moisture.  Under a moderate-high emission scenario, the snowline elevation is projected to increase by about 200 meters (650 feet) of elevation over the next 100 years.  Now, as any geographer will tell you when you go up in elevation there is less land area.  This increasing snowline elevation means there will be considerably less area that maintains a snowpack to provide water during the growing season.    

In a recent paper led by Shuang Liang, we ran simulations of forests of the Sierra Nevada under projected climate change and wildfire.  We used climate projects from three climate models and area burned projections from Leroy Westerling.  We ran our simulations out over several hundred years and assumed that the climate beyond the year 2100 would be similar to the climate projected for 2090-2100.  These long simulation times allowed us to evaluate the effects of altered climate on the carbon carrying capacity of currently forested areas in the Sierra Nevada. 

Our simulations show that with increased warming and drying and more area burned by wildfire that the amount of carbon these ecosystems can sustain will decrease by as much as 73% (top panel).  A big part of that reduction is due to as much as a 65% reduction in forested area (middle panel).  The loss of forest cover is driven by fewer species of tree seedlings being able to establish in these future conditions, especially after more area is burned by wildfire.  

As the figure shows, there is quite a bit of variability between the different climate model projections.  The GFDL (red) and CNRM (green) models show a large decline in both carbon and forested area, whereas the CCSM3 (blue) climate model shows both carbon and forested area holding pretty steady.  The loss of forest cover leads to the carbon flux results in the bottom panel.  The bottom panel shows the net ecosystem carbon balance.  When the values are positive, the forests are removing carbon from the atmosphere and when the values are negative, the forests are a source of carbon to the atmosphere.  Under two of the climate model projections, Sierran forests become sources of carbon to the atmosphere and will be another thing we have to consider when thinking about mitigating climate change. 
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The big question is – what can we do about the prospect of having 50% of the forested area converting to something other than forest?  Reducing our greenhouse gas emissions to the atmosphere is the logical place to start, but since this blog is about forest ecology research you will have to stay tuned for the results of our current work that is looking at the role of management in reducing the risk of forest cover loss to climate change and wildfire.

In a previous study we looked at the effects of forest treatments to restore surface fire and how they influence forest carbon storage.  That study used historical climate to grow the forests, which left me wondering if how carbon dynamics and the influence of restoring surface fires might be altered by changing climate.  To answer this question, I used the same forest landscape model as the previous study and obtained projected climate data from the climate models that were run in support of the IPCC’s Fifth Assessment Report.

Using all of these climate projections, I developed monthly climate distributions for early, middle, and late 21st century.  As the figure on the right shows, each period is progressively warmer and the amount of precipitation falling each month is quite variable.

The figure below shows the influence of climate change and wildfire without treatments to restore surface fire (solid lines) and with treatments to restore surface fire (dotted lines).  Without treatments, the combination of tree-killing wildfire and warmer temperatures cause the amount of carbon stored in ponderosa pine trees to decline over time.  With treatments, the amount of carbon initially decreases because of the removal of trees by thinning and then increases over time.  This is because fewer ponderosa pine trees are killed when wildfire occurs.  The treatment simulations (dashed lines) also show how climate affects tree growth.  The blue line and shaded area represents carbon under late-century climate (2090-2099) and it is significantly lower than under early-century climate (2010-2019). 

Interestingly, the results from simulations that don’t include treatments are heavily influenced by a large increase in Gambel oak.  This species can resprout after fire and provide a fuel source to carry a subsequent fire before the pine trees are large enough to withstand fire.  As shown in the photos below, this is an outcome that we are already seeing in the southwestern US following large, hot wildfires.

The results from this study suggest that by restoring natural fires to this ponderosa pine forest, we can maintain a pine forest for a longer period of time under changing climate.