Modeling effects of climate change on spruce-fir forest ecosystems: Changes in the montane ecotone between boreal and temperate forests in the Green Mountains, U.S.A, from forest edge detection in Landsat TM imagery,1989 to 2011

Abstract:

Climate change is projected to affect the integrity of forested ecosystems worldwide. One forest type expected to be severely impacted is the eastern spruce-fir forest, because it is already at the extreme elevational and latitudinal limits of its range within the northern United States. Large-scale bioclimactic models predict declining habitat suitability for spruce and fir species, while causing drought and thermal stress on remnant trees. As rising temperatures reduce or eliminate habitat throughout much of the current spruce-fir range, growth and regeneration of hardwood forests or more southerly conifers will be favored.

The ecotone between northern hardwood forests and montane boreal forests was recently reported to have shifted approximately 100 m upslope over the last 20-40 years in the Green Mountains of Vermont, U.S.A. The research behind this finding relied on long-term forest plot data and change analysis of narrow transects (6 m width) on aerial photos and SPOT imagery. In the White Mountains of New Hampshire, U.S.A., research using vegetation indices from Landsat data reported a conflicting finding; that coniferous vegetation was increasing downslope of the existing ecotone. We carefully matched and topographically corrected Landsat images from 1989 through 2011 to comprehensively map the boreal-temperate forest ecotone throughout the Green Mountains in Vermont, U.S.A. We used edge detection and linear mixed models to evaluate whether the ecotone changed in elevation over 20 years, and whether rates of change varied with Latitude or aspect.

We found that the elevation of the boreal-temperate forest ecotone, and changes in its location over 20 years, were more variable than reported in recent studies. While the ecotone moved to higher elevations in some locations at reported rates, these rates were at the tales of the distribution of elevational change. Other locations showed downward movement of the ecotone, while for the majority of sites, no change was detected. The large variability in both the rate and direction of elevational change of the boreal-temperate forest ecotone suggests that current changes cannot be attributed clearly to climate change, but may equally reflect forest dynamics of older forests senescing and being replaced locally by competing species.