Shrestha, K. B. 2013. Treeline and vegetation dynamics in response to environmental changes in Nepal, the central Himalaya. Ph.D. Thesis, Faculty of Science, University of Bergen, Norway. Aims: To describe and evaluate patterns of vegetation response to ongoing environmental changes across climate-limited (alpine treeline ecotone) and humanmodified (temperate Himalayan oak forests) ecosystems in Nepal, central Himalaya. Methods: I used dendroclimatological techniques to examine spatial and temporal changes in tree growth responses (paper I) and recruitment patterns (paper II) to climatic variability across a dry Pinus wallichiana and a mesic Abies spectabilis treeline ecotone. Trees from various ecological elevations (forest line, treeline and krummholz line) were cored, annual growth was measured and site chronology was developed for analysing climate-growth relationships. Transects were laid out between the forest line and tree species line, crossing the treeline. Seedlings, saplings and trees were sampled in each transect for age analysis. Alpha, beta and gamma diversity were studied across a forest border from a forested to open landscape in a subalpine-alpine region (paper III) and an anthropogenic disturbance gradient (paper IV). Vascular plant species richness and environmental variables were recorded in each plot (10 m × 10 m). Alpha diversity was estimated as average species richness per plot. Beta diversity was based on gradient length estimated by Detrended Correspondense Analysis (DCA). Gamma diversity was estimated as total species number present in the landscape. Main results: At the dry locality, tree growth at the forest line responded positively to warm summers and after cold winters: possibly a response to early onset of growing season. At the mesic locality, growth at lower altitudes (forest line) showed signals of drought limitation, whereas at higher altitudes, decreased growth was associated with an early onset of the monsoon. Reduced growth at the treeline was related to the high winter snow fall and delayed onset of growing season. The current treelines in both areas have remained stationary over the decades. Climate and land-use are both important factors for treeline structuring processes. vii List of papers My thesis is based on the following four scientific research papers; hereafter referred to by their roman numerals (Papers I-IV). Papers III & IV are reproduced with the kind permission of Folia Geobotanica and International Journal of Biodiversity Science, Ecosystem Services & Management respectively.

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Climate warming is expected to positively alter upward and poleward treelines which are controlled by low temperature and a short growing season. Despite the importance of treelines as a bioassay of climate change, a global field assessment and posterior forecasting of tree growth at annual scales is lacking. Using annually resolved tree‐ring data located across Eurasia and the Americas, we quantified and modeled the relationship between temperature and radial growth at treeline during the 20th century. We then tested whether this temperature–growth association will remain stable during the 21st century using a forward model under two climate scenarios (RCP 4.5 and 8.5). During the 20th century, growth enhancements were common in most sites, and temperature and growth showed positive trends. Interestingly, the relationship between temperature and growth trends was contingent on tree age suggesting biogeographic patterns in treeline growth are contingent on local factors besides climate warming. Simulations forecast temperature–growth decoupling during the 21st century. The growing season at treeline is projected to lengthen and growth rates would increase and become less dependent on temperature rise. These forecasts illustrate how growth may decouple from climate warming in cold regions and near the margins of tree existence. Such projected temperature–growth decoupling could impact ecosystem processes in mountain and polar biomes, with feedbacks on climate warming.

1Tree growth at the treeline ecotone is known to be sensitive to climate variability, and is thus 2 considered a world-wide bio-monitor of climate change. However, our understanding of 3 within-region variation in growth responses through space and time is limited. A dry south-4 facing slope dominated by Pinus wallichiana and a wet north-facing slope dominated by 5Abies spectabilis in Nepal, central Himalaya, were used to analyze the inter-site (i.e., dry vs. 6 wet sites) and intra-site (i.e. treeline vs. forest line elevations) tree-growth relationships, as 7 well as response to monthly and seasonal temperature and precipitation at annual and bi-8 decadel time scales. At both study sites, and at two elevations within each site, growth can be 9 strongly affected by growing-season and non-growing-season factors, however, there are 10 inconsistencies in terms of the climate-growth relationship across space and over time. At the 11 dry site, only a weak positive growth response to summer temperature is observed. At both 12 sites, there is a negative growth response to winter precipitation at both high and low 13 elevations, and this response is markedly independent of the summer and winter temperature 14 trends of the respective site. At the wet site, growth at the higher elevation is negatively 15 correlated to early summer temperature, whereas a positive growth response to spring 16 precipitation is observed at the lower elevation, indicating a possible drought effect. The 17 results illustrate how different climatic drivers may govern tree-growth responses both 18 between sites with contrasting climates within a region, and along elevational gradients within 19 the treeline ecotone. This underlines the need for multi-scale studies, and a focus on multiple 20 climate variables, when analyzing treeline ecotone responses to climate change. 21 22

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