NOAA/WDS Paleoclimatology - Thapa - Salle - PIRO - ITRDB NEPA060

Pine forests provide goods and services crucial to more than ten million people living in the middle-mountains (600-4000m) of Nepal. These critically important forests are already often overexploited and could be at risk from future climate change. In order to investigate the combined effects of climate and human disturbances on the growth of pine forests, we established a new network of tree-ring sites in six Pinus wallichiana and four P. roxburghii forests across the dry inner valleys of eastern Nepal's Koshi River watershed. We produced measurements of total annual ring widths, and detrended individual tree-ring series with 67% cubic splines to produce site-level chronologies. The Koshi tree-ring chronologies were compared against local records of mean monthly temperature and total monthly rainfall to identify the main climatic factor(s) limiting pine growth. We also employed a relative growth change method to estimate growth releases and suppressions in ring-width series as indicators of disturbances. At all sites, trees are relatively young (median age was 102 years) and variations in ring-width provide estimates of tree growth over only the past century. Ring-width chronologies from the Koshi have a weak common signal strength in comparison to trees from the same species obtained from sites in the central Himalaya, and the climate-growth response of Koshi pines appears to be governed primarily by moisture balance during winter. Disturbance events evident in pine ring-width data are largely asynchronous, which suggests these forest have been historically perturbed by human influences rather than large-scale climatic or ecological influences. The sacred forest at Sikri contained the oldest living trees (118 years), had the lowest number of disturbance events, and preserved a stronger common signal, which provided additional evidence of the effects of humans on other pine forests in the Koshi basin. Based on our findings, we suggest that modeling the future growth and distribution of pine trees in eastern Nepal should consider winter moisture. Furthermore, management strategies to better conserve pine forests in eastern Nepal should incorporate the two competing influences of climate and human activities on tree growth.

松林为栖息于尼泊尔中山地带（海拔600-4000米）的逾千万人口提供了至关重要的物资与服务。这些举足轻重的森林本已普遍面临过度开发的压力，且未来还可能受到气候变化的威胁。为探究气候与人类干扰对松林生长的复合影响，我们在尼泊尔东部科西河流域的干旱内陆河谷中，于6处西藏白皮松（Pinus wallichiana）林与4处长叶松（Pinus roxburghii）林内新建了树轮样地（tree-ring sites）网络。我们测定了所有样木的年轮总宽度，并采用67%三次样条法对单棵树木的年轮序列进行去趋势分析，以生成样地水平的树轮年表。我们将科西河流域的树轮年表与当地月均气温、月总降水量记录进行比对，以识别限制松树生长的核心气候因子。此外，我们还采用相对生长变化法，对年轮宽度序列中的生长释放与生长抑制事件进行估算，以此作为干扰事件的指示指标。所有样地的树木均相对年轻（中位树龄为102年），仅能通过年轮宽度的变化反映近一个世纪以来的树木生长情况。相较于喜马拉雅中部地区同树种的树轮年表，科西河流域松树木轮年表的共同信号强度较弱，且科西河松树的气候-生长响应主要受冬季水分平衡调控。松树木轮数据中显现的干扰事件大多呈异步分布，这表明该区域松林历史上主要受人类活动干扰，而非大规模气候或生态干扰。锡克里（Sikri）的圣林拥有该区域现存最古老的树木（树龄118年），干扰事件数量最少，且保留了更强的共同信号，这进一步佐证了人类活动对科西河流域其他松林的影响。基于本研究结果，我们建议在模拟尼泊尔东部松树的未来生长与分布时，应将冬季水分条件纳入考量范畴。此外，为更好地保护尼泊尔东部的松林，相关管理策略应兼顾气候与人类活动对树木生长的双重竞争性影响。