Conifer cover increase in the Greater Yellowstone Ecosystem : rates, extent, and consequences for carbon

Abstract

Increases in the extent and density of woody vegetation have been observed in many locations worldwide. Conifer cover increase in the Greater Yellowstone Ecosystem (GYE) has been documented by historical photos, but the rate and extent remain unquantified. Elevated atmospheric CO2 levels have focused research attention on carbon budgeting. Carbon sinks associated with conifer cover increase are believed to account for a fraction of the βmissing carbon sink,γ although estimates of the fraction are highly uncertain. I examined changes in conifer cover and aboveground carbon across biophysical gradients in the GYE using a combination of aerial photos, satellite imagery, field data, allometric equations, and statistical techniques. I quantified the percent conifer cover for samples in 1971 and 1999 to determine the frequency and rate of conifer cover increase. I used satellite image change detection to map the extent of conifer cover increase and aerial photo interpretation to quantify the rates of conifer cover increase. I then estimated aboveground carbon stocks for 1985 and 1999 and quantified the source/sink dynamics associated with conifer cover increase and other trajectories of forest change. I determined that the area of conifer forest increased by 7% during the period 1971-1999, at highly variable rates depending upon elevation, aspect, vegetation type, and proximity to conifer forest. Much of the variation in the rates of change was associated with gradients of soil moisture. Conifer cover increased across 685,075 ha between 1985-1999 and was responsible for the aboveground sequestration of 369 Gg C yr-1, offsetting 34% of the carbon source associated with widespread fire and logging during that time period. Climate variability, fire suppression, grazing dynamics, and elevated atmospheric CO2 levels are the hypothesized determinants of conifer cover increase. Although it is likely that no single factor is singularly responsible, fire frequency has been sufficiently reduced throughout the GYE, and the majority of carbon uptake occurred in forest types adapted to frequent fire. The temporal duration of a carbon sink associated with conifer cover increase therefore remains in question.