Why do tree death rates decrease with elevation in the Sierra Nevada?

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Because of the great spatial and temporal scales involved, most climatic change research relies on computer models to project possible changes in forest structure, composition, and dynamics. However, another approach is possible: analysis of forest characteristics along natural climatic gradients. Here we examine elevational gradients in the Sierra Nevada, California, and determine whether tree death rates are driven by elevational changes in (1) forest structure, (2) composition, or (3) causes of death.

Permanent study plots

Permanent study plots were established in the coniferous forest belts of Sequoia National Park and Yosemite National Park. The study plots ranged in elevation from lower treeline (1500 m) to upper treeline (3100 m) and ranged in size from 0.9- to 2.5 hectares. Plot locations were selected to be representative of major forest types along the elevational gradient -- namely, the ponderosa pine-mixed conifer, white fir-mixed conifer, Jeffrey pine, red fir, and western white pine forest types.

In each plot, trees greater than or equal to 1.4 m in height were tagged, mapped, measured for diameter, and identified by species. Every five years the tagged trees were checked for mortality, diameters were re-measured, and new trees (reaching 1.4 m height) were incorporated. For each tree that died, we attempted to determine the possible causes of death. Overall, we have monitored more than 18,000 trees and have recorded 1,813 tree deaths.

* Plots in Yosemite National Park. All other plots are in Sequoia National Park.
** Mortality checks were completed annually for these plots; mortality cause data were taken 1995-1997

What are the project's results?

Overall tree death rate decreased significantly with elevation (r² = 0.55; p < 0.001)

Death rate relative to species composition

Linear regression techniques were used to examine elevational trends in death rates within individual species (species inclusion required 50 individuals in at least five plots). Species meeting these criteria were Abies concolor, A. magnifica, Calocedrus decurrens, and Pinus lambertiana.

Death rates decreased with elevation for three of the four species examined, though none of the slopes were significant at p < 0.05. When fit with a common slope, however, death rate decreases in these three species became significant (p < 0.05). Within two study plots P. lambertiana experienced a recent outbreak of white pine blister rust, an introduced pathogen. When outbreak years were excluded from analysis, death rates for P. lambertiana also decreased with elevation.

Tree death rates relative to elevation and species.
ABCO = Abies concolor, ABMA = Abies magnifica, CADE = Calocedrus decurrens, and PILA = Pinus lambertiana.

Death rate relative to population size structure

Linear regression techniques were used to examine elevational trends in death rates relative to stem size (low elevation plots have proportionally more small trees, which tend to have higher death rates, than high elevation plots). Six diameter size classes of trees were established: 0-10, 10-20, 20-40, 40-60, 60-100, and >100 cm.

Death rates decreased significantly with elevation in four of six size classes (p < 0.01 to p < 0.05). One high-elevation plot had only six trees in the 20-40 cm size class, and three of these were killed in a single avalanche. When this plot is removed from analysis, death rate decreases with elevation in the 20-40 cm size class, though non-significantly.

Tree death rates relative to elevation and tree size class.
The size classes, in cm dbh, are a = 0-10, b = 10-20, c = 20-40, d = 40-60, e = 60-100, and f > or = 100.
* p < 0.05; ** p < 0.01.

Death rate relative to stand density

To determine whether elevational changes in stand density drive death rates (low elevation stands being more dense than high elevation stands), we compared regressions of death rate on elevation alone and on stand density alone, and performed multiple regressions using both independent variables.

Elevation and stand density were negatively correlated (r² = 0.50; p < 0.001) suggesting that declining death rate with elevation might be due to declining stand densities. However, elevation alone explains more of the variance in tree death rate (r² = 0.55; p < 0.001) than stand density alone (r² = 0.18; p = 0.064). In multiple regression analysis, elevation was the stronger correlate of death rate (p=0.001 for elevation versus p=0.38 for stand density).

Death rate relative to factors associated with death

To determine whether factors associated with tree death drive changes in death rates with elevation, we regressed death rate on elevation for three broad death factor categories: physical (uprooting, breaking, or being crushed), biotic (insects and pathogens), and unknown (most likely biotic).

Death by physical factors increased slightly and non-significantly with elevation. In contrast, death associated with biotic factors decreased dramatically and significantly with elevation (r² = 0.35; p < 0.01), as did death by unknown causes (r² = 0.56; p < 0.001).

Tree death rates relative to elevation and factors associated with death.
Physical = tree uprooted, broken or was crushed.
Biotic = tree was killed by insects or pathogens.
Uncertain = factors are most likely biotic, but not known with certainty.

What are the implications for global warming?

Declines in tree death rate with elevation are independent of changes in stand structure and composition and appear to be related to biotic (insects and pathogens) and unknown (most likely biotic) factors. Death rate declines may be driven by (1) reduced insect and pathogen activity with declining temperature at higher elevations, or (2) decreased length/severity of summer drought stress at higher elevations, hence lower susceptibility of trees to biotic causes of death. If these findings hold in other forest types and regions, they suggest a potential tree death rate increase in the face of global warming.

Also involved were Linda S. Mutch, Adrian J. Das, Veronica G. Pile and Crystal I. Dickard of the Sequoia and Kings Canyon Field Station, and Peggy E. Moore of the USGS-BRD Yosemite Field Station.

Selected Publications:

Climate and Tree Growth
Conifer Physiology | Forest Demography | Species-Environment Relationships

Tree Growth Model | Forest Dynamics Model | Fire Behavior and Spread Model
Fuel Dynamics Model

Paleo-Fire (tree-ring data) | Paleo-Climate | Paleo-Vegetation
Paleo-Fire (charcoal data)

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Last Updated 17 August 1998