Growth variable-specific moisture and temperature limitations in co-occurring alpine tree and shrub species, central Himalayas, Nepal

Trees and shrubs found in the forest-tundra ecotone (FTE) are considered to be highly sensitive to climate change, but their response to climatic drivers is only partially understood. We use dendrochronological techniques to develop growth chronologies for co-occurring tree (Abies spectabilis, radial growth and height) and dwarf shrub (Cassiope fastigiata, stem elongation and leaf number) species in central Himalaya, Nepal, in order to identify and compare climate drivers of growth (temperature, precipitation, moisture (SPEI)). Our results reveal growth variable-specific responses characterized by a predominant response to previous year and non-growing season climate, and to length of the monsoon season. Tree radial growth was significantly correlated with temperature during previous summer and non-growing season months, and moisture in the late monsoon (September). Tree height increment correlated with late-monsoon temperature and moisture in the previous post-monsoon. Shrub stem elongation mainly correlated with temperature and moisture conditions in previous-year pre-monsoon (May), while leaf production correlated with moisture in previous pre-monsoon and monsoon periods, and precipitation in the late-monsoon. These results contribute new evidence that mid-latitude FTE tree and shrub species and individual growth variables are limited by unique climate drivers operational at different periods during and outside the monsoon season. Within the context of rising temperatures and increased precipitation variability in the Himalayas, moisture may become a more frequent stressor on tree and shrub growth. Consideration of climate and site variable interactions at alpine sites is important to detecting subtleties of growth response. Dendroecological studies of co-occurring tree and shrub species help to identify concomitant and disparate growth responses to climate drivers and in turn, provide information and insight into FTE changes in the Himalayas and elsewhere.