山东蒙山森林冠层绿度与树干径向生长的关系

Relationships between forest canopy greenness and tree radial growth in the Mengshan Mountains of Shandong Province

森林冠层绿度和树木年轮宽度是描述森林生长过程的重要指标,它们之间存在怎样的关系以及这种关系的稳定性如何目前还没有清晰的回答。森林冠层绿度通过遥感影像计算,在空间上连续,而树木年轮宽度是树木健康的综合指标,样点上具有代表性。森林冠层绿度和树木年轮宽度的关系的研究能增进对森林生长的多角度理解和森林生长状况的尺度转换。在山东蒙山地区采集了4个赤松(Pinus densiflora)林样点的树木年轮样本,获得了树木年轮宽度数据,分析了增强型植被指数(Enhanced Vegetation Index,EVI)与树木年轮宽度的关系。结果显示:1)对于健康森林,4月和6月的冠层绿度与树木年轮宽度存在因果关系;森林不健康时,两者关系较为复杂;2)其他月份冠层绿度与树干径向生长不存在因果关系,而是共同受其他环境因子,如气候因子的驱动;3)弱冠层绿度降低后5年内有显著的径向生长恢复,但是恢复年份少;强冠层绿度降低之前,树干径向生长已经开始降低,之后的5年内有着持续的径向生长降低。这些结果表明森林冠层绿度的降低并不能反映树干径向生长降低的开始,只有健康的森林冠层绿度和年轮宽度有相关关系。冠层绿度的降低对森林健康有强烈的影响,冠层绿度降低导致的径向生长的降低很难恢复。

Forest canopy greenness and tree-ring widths are important indices describing the processes of forest growth. Questions about the relationships between the two indices and their temporal stability are still not completely answered. The canopy greenness index calculated from remote sensing maps is continuous in space, whereas tree-ring width is a comprehensive index that represents the health of the sample sites. Research concerning the relationship between canopy greenness and tree-ring width could enhance the understanding of growth conditions from multi-perspectives and help transform growth conditions of forests at different spatial scales. We collected increment cores from a Pinus densiflora forest at four sample sites with different health conditions in the Mengshan Mountains of Shandong Province and analyzed the relationships between enhanced vegetation indices and tree-ring widths. We found that 1 ） the variation in tree-ring width was significantly caused by the variation in forest canopy greenness in April and June for healthy forests as calculated using the Granger causality analysis, whereas the relationship was complicated for unhealthy forests. April and June represent the green-up period of the growth season, and according to the results, this green-up period was closely related to radical growth; 2） canopy greenness in other months and radial growth did not have a causality relationship, rather they were both driven by other environmental factors, especially climatic factors; and 3） we used superposed epoch analysis to determine the influence of canopy greenness decrease events on radical growth, and radical growth recovery following the canopy greenness decrease events. Moderate canopy greenness decreases were not always accompanied by a decrease in radial growth; however, if radial growth had already started to decline before the severe greenness loss, then the decrease would continue for five years after canopy greenness decrease event. Radial growth experienced difficult recovery after moderate canopy greenness decrease events. We identified two or three severe canopy greenness decrease events during 2000 to 2014. After these events, radial growth significantly decreased and continued to decrease for years. Radial growth was not completely recovered five years after severe canopy greenness decrease events. The results showed that changes in forest canopy greenness could not represent the beginning of forest stress or the process of forest restoration. Correlations between canopy greenness and tree radial growth existed only in healthy forests. Declines in canopy greenness strongly influenced radical growth and was difficult to recover after losses in greenness