Strong topographic sheltering effects lead to spatially complex treeline advance and increased forest density in a subtropical mountain region

Abstract

Altitudinal treelines are typically temperature limited such that increasing temperatures linked to global climate change are causing upslope shifts of treelines worldwide. While such elevational increases are readily predicted based on shifting isotherms, at the regional level the realized response is often much more complex, with topography and local environmental conditions playing an important modifying role. Here, we used repeated aerial photographs in combination with forest inventory data to investigate changes in treeline position in the Central Mountain Range of Taiwan over the last 60 years. A highly spatially variable upslope advance of treeline was identified in which topography is a major driver of both treeline form and advance. The changes in treeline position that we observed occurred alongside substantial increases in forest density, and lead to a large increase in overall forest area. These changes will have a significant impact on carbon stocking in the high altitude zone, while the concomitant decrease in alpine grassland area is likely to have negative implications for alpine species. The complex and spatially variable changes that we report highlight the necessity for considering local factors such as topography when attempting to predict species distributional responses to warming climate.

Temperature and sheltering determine patterns of seedling establishment in an advancing subtropical treeline

“…This result agrees with a larger‐scale study of patterns of adult trees in the same region (Greenwood et al. ), and also with other studies of establishment patterns and treeline dynamics (Resler et al. ; Holtmeier & Broll , ).…”

“…Remote sensing analyses have demonstrated the importance of shelter for treeline form and advance on a large scale (Greenwood et al. ), and here we have identified the microsite‐scale processes that influence these regional patterns; seedlings will establish more successfully, and treeline will thus advance further, in sites that offer some degree of topographic shelter.…”

Community change and species richness reductions in rapidly advancing tree lines

“…Following Harsch & Bader () and Greenwood et al . (), local patterns of tree line advance in the Central Mountain Range of Taiwan can be classified as abrupt advancing (tree lines advancing at high density over short distances), diffuse advancing (tree lines advancing at low density over longer distances) and static (tree lines that are not advancing), and of these, the authors have shown that the diffuse form is advancing most rapidly in the Hehuanshan area (Greenwood et al ., ).…”

“…The A. kawakamii forest is advancing upward in altitude on Hehuanshan in a complex and topographically mediated way, with considerable spatial variation (Greenwood et al ., , ). Following Harsch & Bader () and Greenwood et al .…”

“…Spatially variable changes in tree line position and structure associated with rising temperatures have been observed in the Central Mountain Range of Taiwan (Greenwood et al ., , ). We sought to determine if tree migration in this area is leading to changes in the lichen community.…”