山西忻州地区1900-2012年典型森林的健康历史

健康的森林对维持其区域生态系统服务起着至关重要的作用，了解树木的生长历史对评估气候变化背景下森林的健康状况具有促进作用。选择山西高原中北部忻州地区保存较好的4个森林为研究对象，利用树木年轮学方法分析其生长变化特征，获得了该地区森林健康的时间和空间特征。结果表明：该地区森林在过去1个世纪中，存在3次不健康事件（1910-1940、1970-1987和1990-2012年）且在空间上表现出明显的同步性，不同时期的不健康事件持续时间和强度不尽相同，1930s的不健康事件持续时间最长也最为显着。树木径向生长与气象观测资料的相关分析显示，该地区森林生长主要与当年5-6月份温度呈负相关，与7月份降水和5-9月PDSI指数显着正相关。生长季持续的高温或降水减少造成的极端干旱事件是不健康事件空间一致性的主要驱动力，各采样点树种以及林分组成差异是影响时间特征不一致的可能原因。研究结果提供的森林健康历史数据对评估极端气候条件对森林健康生长的影响及制定合理的森林保护措施具有积极的现实意义。

History of forest health from 1900 to 2012 in Xinzhou Prefecture, Shanxi Province, China

Forest health is a serious problem worldwide. Knowledge of the intensity and causes of rapidly declining forests is important for managing various forest types under the scenario of climate change. In the present study, we report on the health conditions of four young conifer forests by applying dendrochronological techniques to measure unexpected growth decline in Xinzhou District, northern Shanxi Plateau. A total of 86 samples in the form of increment cores were obtained from the study sites. The widths of tree-rings from all the samples were measured and cross-dated, then each individual tree series was standardized to identify health conditions using spatial characteristics of temporal growth. We defined unhealthy tree growth intervals as being a decrease in the growth of the individual (tree-ring index < 0.9) for at least five years when the mean tree-ring index was less than 0.7 and at least one year of tree-ring index values reached a minimum of 0.5. We calculated the percentage of trees that were unhealthy in each year, and defined an unhealthy event for the site if greater than 30% of the trees were in decline. We found three major unexpected decline growth events in the majority of individual trees in the four forests, these being 1910-1940, 1970-1987, and 1990-2012. However, the growth declines in each forest showed deviations in intensity and duration such as declines were more vigorous in the KeLan forest during the 1930s than in the other forests. To better understand the causes of the unhealthy events, a correlation analysis was performed for the three tree ring width chronologies and the meteorological records. The correlation coefficients showed that forest growth was negatively associated with May-June temperatures. However, precipitation revealed positive correlation in July and the Palmer Drought Severity Index showed positive correlation over the growing season of May-September. To validate these results, we compared our study with historical records and local paleoclimate studies. The drought events recorded in the historical documents and other paleoclimate reconstructions reasonably matched our results. These results suggest that extreme drought events caused by persistent high temperature or reduction of precipitation in the growing season are the main driving factors for radial growth decreases in trees, which was the main reason for the spatial coherence of unhealthy events. The differentiation of local stand characteristics or tree species might play a role in the inconsistency of the time characteristics. Therefore, the present study provides useful information for forest health, which has positive and realistic significance for assessing the impact of extreme climatic conditions on the healthy growth of forests and for developing reasonable forest protection measures.