Montane forests and the species that live in them are vulnerable to climate change. This vulnerability presents a challenge to land managers who must prioritize protected areas for conservation under changing environmental conditions. Montane spruce-fir forests in New England illustrate this challenge. Forests of Abies balsamea and Picea rubra are predicted to shrink in size and recede to higher elevations as temperatures increase. Warm-adapted species from lower elevations are predicted to expand upward in the montane forests’ wake. Yet recent changes detected in satellite imagery diverge from these predictions, showing that the boundary between montane forests and their warmer neighbors has remained stable or shifted downward over the past 30 years. In spite of this apparent rebound in montane spruce-fir extent, the future of this ecosystem remains uncertain.
We forecast future spruce-fir forest extent using the landscape model LANDIS-II and the ecosystem models PnET-II and Linkages under a combination of four global circulation model (GCM) projections and two representative concentration pathways (RCP 4.5 and 8.5) for the Green Mountains National Forest (GMNF) in Vermont, USA. We test whether spruce-fir forest extent shrinks as expected and identify where spruce-fir persists as potential climate refugia for conservation.
Results/Conclusions
We found that growth and establishment of A. balsamea and P. rubra were sensitive to the range of climate change projections; increasing, staying stable or decreasing depending on soil type and topographic position. In spite of this variation in vital rates, total spruce-fir extent proved resistant to projected changes in climate and did not decrease significantly between 2010 and 2100. Spruce-fir extent did decline locally on some mountains under RCP 8.5 projections. Local declines were most common in the southern GMNF, where beech (Fagus grandifolia) came to dominate spruce-fir transition zones, more-so than sugar maple (Acer saccharum). These local changes were driven by disturbance regimes, including defoliation outbreaks, that differentially affected growth and mortality of spruce-fir competitors more than climate change alone. Our ability to identify variation in these trends depended strongly on the use of climate data that was downscaled to fine spatial resolutions (800 m cells), as montane forests are locally rare and the environmental conditions that drive their distribution are averaged out at coarser resolutions. Fine-scale landscape simulations successfully identified potential climate refugia for montane spruce-fir forests providing an opportunity to direct conservation efforts towards areas with the highest likelihood of success.