Tree die-off in western North America

In a Science paper published in 2009, van Mantgem et al. discussed widespread tree mortality in the US, and linked it to both drought stress and increased air temperatures. In the past decade we’ve seen catastrophic death of piñon pine, sudden aspen decline, decimation of large swaths of various pine species, and death of yellow cedar. How are these changes a function of climate, and what are the effects on hydrology?

Piñon Pine

Piñon pine die-off began in 2002, but it was bark beetles (Ips) that delivered the ‘knockout punch’, with more than 90% of piñons dead by the end of 2003. As initially suggested in that Science article, this massive die-off was attributed to a combination of both high air temperatures and moisture deficit.

Piñon pine distribution (by USDA/USGS; from Pinyon Juniper Woodlands Information Network) 

Aspen

Sudden aspen decline (SAD) was first observed in 2004, and reached a peak in 2008. Studies suggest that the same culprits responsible for piñon pine death were also critical in SAD: drought and warm air temperatures. In this case, however, it was the root systems that were destroyed. Aspen are clonal species that use their root systems to draw water from downslope to upslope stands. In the absence of available downslope water, the upslope trees dry out – specifically the root systems. This then leaves them susceptible to invasion by aspen bark beetles.

Extent of SAD in the USA (from USDA) 

Pine (lodgepole, ponderosa, Scotch, limber)

Everyone knows about the mountain pine beetle, which has decimated British Columbia’s lodgepole pine forests and is now eating its way through the pine forests of the American Cordillera. Justin Gillis did a great piece on this for the NY Times. In this case it isn’t only drought that’s weakened tree resistance to the beetle, but increased winter temperatures have allowed the beetle to overwinter successfully – and double its reproduction rate – thus reaching epidemic proportions. 

Range of mountain pine beetle in North America (from USDA info leaflet)

Yellow cedar

Just when you thought these species declines were limited to the Cordillera region, consider the case of yellow cedar, which is dying throughout Alaska and British Columbia. In this case, it’s cold rather than warm weather that’s causing the problem: tree roots are freezing during periods when snow is not available to insulate the soil.

Yellow cedar distribution (from USDA webpage)

The large-scale death of such a wide range of tree species has the potential to completely alter the forest landscape across western North America, with cascading effects on hydrologic processes. Much of my work focuses on the effect of forest change on interception processes – particularly of snow. We see that beetle infestation, wildfire and forest harvesting all affect sub-canopy snow accumulation and the subsequent snowmelt energy balance, with beetle killed stands behaving similarly to undisturbed stands, and burned stands behaving more similarly to clearcut stands. More recent work at the watershed scale suggests that snow processes at lower elevations are more sensitive to changes in forest cover caused by disturbance, which sets up a double whammy given that these locations are also strongly affected by climate change.

Recent studies have also shown how important trees are for the atmospheric moisture budget of a region given their role in evapotranspiration (ET). In the absence of trees, the ET component of the water cycle is significantly less intense, and can reduce regional precipitation.

The soil moisture component of the water cycle is also affected, as dead trees no longer use soil moisture for ET. Even though evaporation from the soil surface increases due to increased radiation exposure, because water removal from the soil profile is reduced overall then soil moisture will likely increase. This will have subsequent effects on infiltration of rainfall and snow melt into the soil profile, and also on runoff generation patterns. While tree die off often occurs during dry years (i.e., low soil moisture), soil moisture in subsequent years may increase significantly above pre-die off levels.

Trees also have a little-discussed connection to groundwater: species with deep root systems draw water for ET from groundwater systems. In these cases the soil moisture is largely unaffected by tree die-off, but groundwater storage may increase. This would affect the hydraulic head of GW systems, and their subsequent linkages to surface water systems.

Of concern to most ecohydrologists is that both research and anecdotal evidence suggest that forest decline is not just a regional issue, but more likely a global phenomenon affecting multiple species. Given the regional hydrologic effects noted above - which don’t even begin to address the major ecological consequences - how might hydrology change on a global scale?