Overshoot in leaf area development in wet years leads to bark beetle outbreaks on low infiltration soils

Background/Question/Methods

As warmer temperatures move to more northern latitudes, combinations of drought stress and more hospitable conditions lead to increased bark beetle attacks in western North American conifer forests. Environmental conditions such as precipitation and temperature play a role in where and when beetles can overcome tree defenses. Soils govern the reception, storage and redistribution of water to plants and the atmosphere. A big question is: Why do bark beetles attack ponderosa pines in uneven/patchy patterns across the landscape? I hypothesize that soil characteristics affect the trees' water stress responses to changes in precipitation and temperature.

I used a combined approach of GIS analysis, remote sensing and a process-based forest growth model. The GIS analysis used SSURGO soils data and US Forest Service forest mortality data to find correlations between forest dieback and soil characteristics across the landscape. I used Landsat imagery to track seasonal NDVI and estimate a maximum Leaf Area Index (LAI) value. Using the 3-PG process-based model for forest growth, I tested the sensitivity of two sites in Montana to soil fertility, available water storage capacity and soil texture. Both sites had similar climate, topography and fire histories, but one had more beetle activity early in the time period (1998 to 2010). 

Results/Conclusions

The greatest difference in the sites was the clay content of the soils. The site with more beetle attacks early in the time period (site 1) had high clay soils. Conversely, the site with less beetle activity early in the time period (site 2) had high sand soils. Both sites showed increasing trends in temperatures, but an average of seven SNOTEL sites across the area showed an increasing trend in snow water equivalent over the period, with dips in the years preceding the two major beetle outbreaks (2001 and 2007). Modeling in 3-PG showed that the growth efficiency on site 1 shows significant responses to the peaks and dips in snow water equivalent, while the growth efficiency on site 2 shows little fluctuation around the mean for the time period.

I conclude that trees on clayey soils are more stressed during lower precipitation periods due to their limited infiltration capacity. Also, in regions where snowpack is highly variable, trees may add too much foliage in wet years and experience stress during drier years, making them more susceptible to beetle attacks.