西藏东部主要建群树种径向生长对极端干旱的响应差异

不同坡向、不同海拔树木生长对极端干旱事件的响应可能不同,然而这方面的认识不足。为此,选取西藏东部珠角拉山阴、阳坡的建群树种川西云杉 (Picea likiangensis var. rubescens) 和大果圆柏 (Juniperus tibetica),在不同海拔高度建立了树轮宽度年表,分析了径向生长的气候响应,以及对极端干旱事件的抵抗力和恢复力。结果表明:阳坡大果圆柏和阴坡川西云杉的树木生长对气候的响应存在相似性,均与前一年3-6月、11-12月气温显著正相关,与当年4-5月气温显著负相关,与当年4-5月降水和帕尔默干旱指数 (PDSI, Palmer Drought Severity Index) 显著正相关 (P <0.05)。阳坡大果圆柏的抵抗力显著低于阴坡川西云杉。随着海拔升高,阴坡川西云杉树木个体的抵抗力和恢复力均显著提高,而阳坡大果圆柏树木个体抵抗力、恢复力在不同海拔无显著区别。结合混合效应模型表明树木抵抗力主要受当年4-5月平均最高气温限制,树木恢复力主要受干旱事件后四年4-5月平均最高气温限制 (P <0.01),说明生长季高温引起的极端干旱是树木径向生长下降的主要原因。

Differences in response of radial growth to extreme droughts for the main constructive tree species on sunny and shady slopes in eastern Tibet

Tree radial growth may respond differently to extreme drought events on different slopes and at different altitudes, but this is not well understood. In order to reveal the differences in growth response to climate warming, we selected the constructive tree species Picea likiangensis var. rubescens and Juniperus tibetica of the forests from the shady and sunny slopes of Zhujiaola Mountain in eastern Tibet. Tree-ring width chronologies were established for the two investigated tree species at different altitudes, and then used to analyze the climate response of tree radial growth. Furthermore, the characteristics of resistance and resilience to extreme drought events were then explored for the studied trees based on tree-ring width data. The results showed that there were strong similarities in the responses of junipers on the sunny slope and spruces on the shady slope to the climate variations, both of which were significantly and positively correlated with the temperatures in March-June and November-December of the previous year, negatively correlated with the temperature and positively correlated with the precipitation and Palmer Drought Severity Index (PDSI) in April-May of the ring-formation year (P < 0.05). Besides, temperatures in the previous growing season and winter, temperatures and moisture conditions in the current growing season were the main limiting factors for the two tree species but the growth variations of junipers on the sunny slope were more sensitive to drought fluctuations in the growing season as compared with spruce trees. In addition to the climate response analysis we also investigated the differences in the resistance and resilience of the two constructive tree species in response to extreme drought events, which were defined by PDSI according to the growth-climate response results. We found that the resistance of junipers on the sunny slope was significantly lower than that of spruces on the shady slope, indicating that the junipers on sunny slope would be more sensitive to drought extremes, and thus facing greater risk of growth decline when severe drought events occurred. Our results also uncovered altitude-specific differences in resistance and resilience to drought events for the two tree species. With the increase of altitude, the resistance and resilience of individual spruce trees on shady slopes increased significantly, while there was no significant difference in resistance and resilience for junipers on sunny slopes at different altitudes. Therefore, the spruces on the shady slope at low altitudes would be more sensitive to drought extremes. The mixed-effects model further revealed that tree growth resistance was significantly limited by the average maximum temperature in April-May in the drought year. Tree growth resilience was significantly limited by the average April-May maximum temperatures in four years after the drought event (P < 0.01), suggesting that climate warming would lead to a decrease in tree radial growth to extreme drought events and inhibiting growth recovery afterwards.