Death comes for the drought-stressed forest

[originally posted at www.foreststeward.com on Jan 7, 2011]

-> This post graciously provided by the Battles lab at UC Berkeley <-

Article Reviewed: Tree mortality in drought-stressed mixed-conifer and ponderosa pine forests, Arizona, USA

By J.L. Ganey and S.C. Vojta. Published in Forest Ecology and Management 261 (2011) 162–168

Plot line: This short paper documents tree death in northern Arizona between 1997 and 2007, a period of record drought in the southwest. The focus is on ponderosa pine and mixed conifer forest sites randomly distributed throughout National Forest lands along an elevation gradient from 1800 to 2800 m. Observations are of all dead trees over 20-cm diameter (at 1.37 m height) in an 1-ha plot; live trees were measured in nested 0.09ha plot located in the center of the larger plot. Mortality was characterized in two ways. Mortality per time period was calculated from tree counts at the beginning and end of a five year interval. The cumulative mortality, a measure of snag residency time, was defined as the ratio of standing snags to live trees at a single given point.

On the whole, the observed mortality patterns were more distinct in mixed conifer forests than in ponderosa pine. In 2002-2007, mixed conifer forests recorded a 200% increase over the previous five years, and ponderosa pine forest mortality increased 74% over the earlier period. Mortality during 2002-2007 was positively correlated with the proportion of live white firs in mixed conifer forests, but tree death was not related to elevation or stand density in either forest type. White fir and aspen, the least drought tolerant species in these communities, had the highest relative mortality, at 28% and 85% of individuals. However, Douglas fir, another among the less drought-tolerant species, showed very low mortality. The greatest number of tree deaths in all species occurred in the smallest size classes, but a larger proportion of the largest size classes on these sites died during the study. Cumulative mortality was also assessed with an eye toward structures for wildlife habitat. Mixed-conifer forests had median cumulative mortality rates of 21.1%, compared to 11.3% in 1995-2002. These mortality rates are quite high, but not unprecedented for comparable forests in the region.

Most of the snags showed signs of damage by forest insects, particularly bark beetles and western tent caterpillar. These pests probably acted in concert with a prolonged drought and extreme climate events, including the third driest year in the last 1400 years, 2002. The findings suggest that as forests experience more frequent and severe drought (as is predicted under most climate change scenarios), tree mortality may follow patterns similar to those observed here, with mortality disproportionately affecting species that are less drought tolerant than others the forest species mix.

Relevant Quote: “Mixed-conifer and ponderosa pine forests in northern Arizona have experienced high and accelerating drought-mediated tree mortality between 1997 and 2007. This mortality is altering species composition and size-class distributions in these forests rapidly.”

Relevance to landowners/stakeholders

Many people have a vested interest in our forests’ readiness for the new challenges that come with a changing climate. Studies like this one point to the ways in which our existing forests are unsuited to thrive in expected future conditions. It is also important to note that there is enormous variation in mortality for these forests. Some locations experienced almost total dieoff, while others were virtually unaffected. This serves as reminder that many of the factors that contribute to tree death are particularly difficult to predict at the individual and stand level.

These findings also may prompt some concern about habitat loss, since large trees died slightly more than other size classes. If aged and complex forests die, habitat for the Mexican Spotted Owl and other structure-dependent species might become scarcer.

Relevance to managers

Throughout the Southwest, management practices in the past century have contributed to inflated stand densities and an increase in white fir compared to historic norms. Because mortality was not density or elevation dependent in this study, it provides evidence that thinning cannot be expected to completely protect stands from the stresses of extreme drought and forest insects. The authors note that a number of other studies have also not found a relationship between density and mortality (including van Mantgem et al 2009 – reviewed on Forest Steward Feb 27, 2009), and that a better understanding of this interplay is needed, particularly with regard to mortality of small trees. Rather than focus solely on thinning as an insurance policy against climatic changes, they suggest a broader goal of resilience. Paths to resilience might include thinning treatments aimed at restoring more open stands, but should also consider management that favors species tolerant of drought and frequent fires. Such an approach might help alleviate competition for moisture in an increasingly arid future.

Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:

The analysis of whether mortality is related to stand density uses a density measurement that excludes trees less than 20cm in diameter. Trees under this size were not included in mortality counts. A more complete record of density and mortality dynamics in the smaller size classes would add an important component to the authors’ findings. Smaller tree mortality is quite likely the most density sensitive, so inclusion of these trees may help riddle out some density questions important for directing management of drought affected forests. It might also be helpful to see the total mortality broken down by tree species. This would illustrate how much of the overall mortality is comprised of drought intolerants like, white fir and aspen, as compared to other species.

It is also important to note that the definition of tree mortality used in this paper is based on changes in the number of live trees and the accumulation of standing dead trees in plots over the census interval. They do not track the fate of a cohort of marked trees through time as is typical in population ecology. Thus care must be taken when comparing mortality rates across studies that use different definitions of tree mortality.

The connection of drought to this mortality pattern might be improved by incorporating other mortality factors in the analysis. Comparisons with slope and aspect, for instance, have been used in other work, and the record of bark beetle and caterpillar irruptions might be useful as evidence this incident was not merely insect-caused.