Abstract
In recent decades the global average of temperature, precipitation, and humidity patterns, are changing due to natural dynamics but above all due to anthropogenic activities. The rate of global warming has accelerated, with each of the last four decades being successively warmer than any decade that preceded it since 1850. The Himalayas as a whole is very sensitive to global climate change. The progressive increases in regional rate of warming in Himalaya, with some variation, has been observed in line with global climate warming. Furthermore, in recent decades, opposite trends in precipitation have also been recorded in the Himalayas. In this context, mountain forests are subject to ongoing and increasing changes in climate that impact on the growth of tree species. Dendrochronology is a powerful tool to determine the climatic influence on growth performance of tree species. The history of tree ring studies in the Nepal Himalaya dates back to late 1970s. The conifer Abies spectabilis is one of the key species occurring from the treeline at the limit of tree growth down to the Himalayan valleys, making it an ideal study object to understand climate impacts on tree growth across elevation. In order to better understand how this tree species reacts on climate and its changes, we first conducted a literature review on dendrochronological and -ecological studies of this species, focusing on the Central and Western Himalaya. This review summarized the current stage of research on this matter and revealed major research gaps, that led us to, secondly, conduct a case study in order to contribute to filling these gaps. Most previous studies on this species have focused on climate reconstructions and growth performance. The principal parameter used to estimate growth performance of this species was ring widths and basal area increment using tree ring-based analyses. The findings of our review showed that studies generally reported an increased radial tree growth linked to the temperature of the current and previous season and a tree growth limitation either by winter temperature or, linked to precipitation, by moisture in pre-monsoon season. A research gap was evident in case of growth performance and climate response of A. spectabilis at its elevational range limits. Specifically, also scientific studies about the wood anatomy and its variation in this species were lacking. Our subsequent case study investigated these topics in A. spectabilis trees at its range limits in Langtang National Park in Nepal. From our dendrochronological studies based on some series analyses of 20 trees across the entire chronology of the trees lifespans, we were able to show a positive impact of some mid to late spring temperatures on tree growth at both elevation range limits, while a positive impact in summer (July) was observed only for the upper range limit. We also could show that the climate had experienced a significant warming in the past three decades (1990-2019) compared to the three decades at the beginning of the investigation period (1901-1930). These findings were generally in line with the outcome of the studies in the previous literature review. Moreover, our findings indicated that there were generally more relationships with temperature in recent decades in the tree growth at the upper elevation. By contrast, at the lower elevation, there were also significant growth-temperature relationships in the most distant past decades. Our findings about the impact of precipitation on tree growth showed a more complex pattern. Precipitation exerted positive or negative impacts depending on season and elevation. Besides direct effects of precipitation on growth, also indirect effects such as the cloud cover impact on soil temperature might play a role. Regarding the temporal stability, we found diverse significant relationships across the whole timespan of the study suggesting that rainfall impact mostly did not strongly change in the recent decades. On the other hand, some specific months had impact of precipitation on growth in the past decades and different months had impact in the recent decades. Changing the focus on results about wood anatomical investigation, differences in the physiological and anatomical growth adjustment of trees became visible. One of the main findings was that tracheids were smaller in lumen area and denser towards higher elevation. The larger lumen radial diameter and lumen area in the lower elevation across all tree ring sectors compared to the upper elevation suggested an elevation-dependent anatomical variation. However, more research is needed to cover better the entire elevational gradient and the different ages of trees and relate them to climatic variation.