Microclimate heterogeneity modulates fine-scale edaphic and vegetation patterns on the Himalayan treelines: Implications under climate change

In an era of global environmental change, the treeline shift triggered by recent climate warming has been reported worldwide. However, it is still unknown how site-specific microclimatic conditions regulate the soil-vegetation relationship at treelines, which constrains our capacity to down-scale broad global trends in the treeline shift at regional scale. In this study, we aimed to unravel fine-scale edaphic and vegetation patterns at two treeline sites in Kashmir Himalaya with in situ-measured microclimate using mini-loggers. At each site, we conducted sampling at the treeline leading edge, 100 and 300 m downslope and upslope. We employed boosted regression trees to find best predictors of the treeline vegetation among the microclimate and edaphic factors. We found intermediate microclimatic conditions at the leading edge but contrasting microclimates at the downslope and upslope. Our results demonstrate that the heterogeneity in microclimate driven by elevation strongly modulates the microsite edaphic conditions at the treelines, which in turn determines the fine-scale vegetation patterns. The soil temperature, moisture, pH, Cu, Zn, electrical conductivity and N were the best predictors of vegetation patterns at the treelines. The length of growing season measured at the treelines matched with the predicted values for the Himalaya, but growing season mean soil temperature of 7.31 – 8.15 °C recorded at the treelines is slightly higher than the postulated 6.4 °C global treeline isotherm. Our findings reveal that the soil microclimate strongly modulates edaphic and vegetation patterns at the treelines, thereby implying that even minor change in microclimate, like ongoing climate warming, can shift the current treeline position. However, this may be partially offset by the higher temperature isotherm in Kashmir Himalaya, thereby suggesting a slow-paced future treeline shift. Overall, the study advances our understanding about the crucial role of microclimate in determining soil-vegetation relationships at treelines with wide implications under ongoing and projected climate change.