Growth-climate relationships of Himalayan conifers along elevational and latitudinal gradients

摘要

High mountains are some of the most vulnerable regions to climate change and therefore a matter of global concern. Here, the climatic growth factors of conifers and their course in time and space along an elevational gradient in the northwestern Himalayan part of India were studied. Increment cores of Juniperus semiglobosa and Cedrus deodara (xeric species), and Abies pindrow and Picea smithiana (mesic species) were collected from thirteen sites. Tree-ring width and maximum latewood density were measured and cross-dated. The time-series were standardized and site- and species-level chronologies were built. Static and moving bootstrap correlation and response functions between the tree-ring chronologies and monthly climatic variables were computed. The largest climate changes in the region were increasing winter and early-spring temperatures and decreasing monsoon precipitation. The growth of all species was negatively correlated with pre-monsoon temperature, as the higher temperatures probably increased evapotranspiration and caused water deficit. The phenomenon was most pronounced in May, but also in June for the Juniperus at the northernmost, highest, and driest sites. The pre-monsoon temperature signal of the drought-prone Juniperus and Cedrus endured, while the signal of the climate-susceptible Abies and Picea at the mesic sites was unstable. Namely, the May temperature signal of Picea became significant since the second half of the last century whereas the signal of Abies shifted from May to April. This apparently related to the earlier onset of spring due to the accelerated warming in the region. Besides, maximum latewood density of Picea and Abies negatively correlated with May and June temperature, respectively. Additionally, the Cedrus benefitted from winter precipitation and the Abies and Picea from pre-monsoon rainfall. Counterintuitively, we detected no direct effect of monsoon precipitation decrease on the conifers because their growth was driven by pre-monsoon conditions, which changed only slightly.