Birch (Betula) trees and forests are found across much of the temperate and boreal zones of the Northern Hemisphere. Yet, despite being an ecologically significant genus, it is not well studied compared to other genera like Pinus, Picea, Larix, Juniperus, Quercus, or Fagus. In the Himalayas, Himalayan birch (Betula utilis) is a widespread broadleaf timberline species that survives in mountain rain shadows via access to water from snowmelt. Because precipitation in the Nepalese Himalayas decreases with increasing elevation, we hypothesized that the growth of birch at the upper timberlines between 3900 and 4150 m above sea level is primarily limited by moisture availability rather than by low temperature. To examine this assumption, a total of 292 increment cores from 211 birch trees at nine timberline sites were taken for dendroecological analysis. The synchronous occurrence of narrow rings and the high interseries correlations within and among sites evidenced a reliable cross‐dating and a common climatic signal in the tree‐ring width variations. From March to May, all nine tree‐ring‐width site chronologies showed a strong positive response to total precipitation and a less‐strong negative response to temperature. During the instrumental meteorological record (from 1960 to the present), years with a high percentage of locally missing rings coincided with dry and warm pre‐monsoon seasons. Moreover, periods of below‐average growth are in phase with well‐known drought events all over monsoon Asia, showing additional evidence that Himalayan birch growth at the upper timberlines is persistently limited by moisture availability. Our study describes the rare case of a drought‐induced alpine timberline that is comprised of a broadleaf tree species.

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Drought influences agriculture, hydrology, ecology and socioeconomic systems globally. As agriculture is the primary source for livelihoods and contributes to $27% of Nepalʼs total gross domestic product, it is essential to understand the impact of drought on maize and wheat crop yields to minimize the droughtrelated risks. This study presents insights about agricultural drought across Nepal during 1987-2017 using the Standardized Precipitation Evapotranspiration Index (SPEI). The temporal evolution of SPEI time series has revealed frequent occurrences of drought episodes during the cropping cycle of summer maize and winter wheat crops. Moreover, the turning point of the drought was detected in 2000 (1987-2000, 2001-2017) in different regions. The averaged frequency for the SPEIs (1, 3, 6 and 12) of drought years for summer maize (winter wheat) in the western, central and eastern regions increased by 13% (12.5%), 6% (7.5%) and 7% (8%), respectively, from 1987-2000 to 2001-2017. The relationship between Standardized Yield Residual Series, the detrended SPEI at 1-12 lags and soil moisture was observed for both crops. The most correlated crop growth period for summer maize and winter wheat was the sowing and growing period, respectively, indicating the sensitive period of water deficit. Besides, the correlation performed in the two sub-periods (1987-2000 and 2001-2017) shows that drought impacts increased in the western and central regions, whereas they substantially decreased in the eastern region during the cropping period of summer maize. However, the drought sensitivity for winter wheat was decreased in the western region but significantly increased in the central and eastern regions of Nepal. The results of this study provide important information useful for policymakers in monitoring and mitigating the drought-related risks on maize and wheat crops in Nepal.

AimThe spatial patterns of tree populations reflect multiple ecological processes. However, little is known on whether these patterns mediate responses to climate in marginal tree populations such as those forming alpine tree lines. We examined the influence of tree‐to‐tree interactions on the responsiveness of tree lines to climate warming.LocationCentral Himalayas.TaxonBetula utilis; Abies spectabilis.MethodsWe analysed a network of 17 tree line sites located across the central Himalayas, encompassing a wide longitudinal gradient characterized by increasing precipitation eastwards. We quantified the changes in density and the spatial patterns of three 50‐year age classes of the two main tree species found at the tree line (Betula utilis and Abies spectabilis), and related them to reconstructed shifts in tree line elevation.ResultsYounger trees showed clustering near the tree line, while older trees tended to show random spatial distribution. Clustering decreased as climate conditions ameliorated, that is, in the wetter eastern sites. Lower rate of tree line elevation change was observed at the sites with higher clustering intensity.Main ConclusionsOur study indicates that tree aggregation weakens tree line responsiveness to climate warming, and thus warming‐induced drought stress tends to lower tree line shift rates by enhancing clustering. It also highlights the complexity and contingency of site‐dependent tree line responses to climate. Hence, to advance our understanding of tree line processes, we should consider both direct and indirect influences of relevant biotic (tree‐to‐tree interactions) and abiotic (climate) drivers of tree line dynamics.

Precipitation is the most important variable in the climate system and the dominant driver of land surface hydrologic conditions. Rain gauge measurement provides precipitation estimates on the ground surface; however, these measurements are sparse, especially in the high-elevation areas of Nepal. Reanalysis datasets are the potential alternative for precipitation measurement, although it must be evaluated and validated before use. This study evaluates the performance of second-generation Modern-ERA Retrospective analysis for Research and Applications (MERRA-2) datasets with the 141-gauge observations from Nepal between 2000 and 2018 on monthly, seasonal, and annual timescales. Different statistical measures based on the Correlation Coefficient (R), Mean Bias (MB), Root-Mean-Square Error (RMSE), and Nash–Sutcliffe efficiency (NSE) were adopted to determine the performance of both MERRA-2 datasets. The results revealed that gauge calibrated (MERRA-C) underestimated, whereas model-only (MERRA-NC) overestimated the observed seasonal cycle of precipitation. However, both datasets were able to reproduce seasonal precipitation cycle with a high correlation (R ≥ 0.95), as revealed by observation. MERRA-C datasets showed a more consistent spatial performance (higher R-value) to the observed datasets than MERRA-NC, while MERRA-NC is more reasonable to estimate precipitation amount (lower MB) across the country. Both MERRA-2 datasets performed better in winter, post-monsoon, and pre-monsoon than in summer monsoon. Moreover, MERRA-NC overestimated the observed precipitation in mid and high-elevation areas, whereas MERRA-C severely underestimated at most of the stations throughout all seasons. Among both datasets, MERRA-C was only able to reproduce the observed elevation dependency pattern. Furthermore, uncertainties in MERRA-2 precipitation products mentioned above are still worthy of attention by data developers and users.

Chir pine (Pinus roxburghii Sarg.) is a common tree species with ecological and economic importance across the subtropical forests of the central Himalayas. However, little is known about its growth response to the recent warming and drying trends observed in this region. Here, we developed a 268-year-long ring-width chronology (1743-2010) from western Nepal to investigate its growth response to climate. Based on nearby available meteorological records, growth was positively correlated with winter (November to February; r = 0.39, p < 0.05) as well as March to April (r = 0.67, p < 0.001) precipitation. Growth also showed a strong positive correlation with the sum of precipitation from November of the previous year to April of the current year (r = 0.65, p < 0.001). In contrast, a negative relationship with the mean temperature in March to April (r = −0.48, p < 0.05) suggests the influence of warming-induced evapotranspiration on tree growth. Spring droughts lasting 4-6 months constrain Chir pine growth. These results are supported by the synchronization between droughts and very narrow or locally missing rings. Warming and drying tendencies during winter and spring will reduce forest growth and resilience and make Chir pine forests more vulnerable and at higher risk of growth decline and dieback.

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