基于树轮稳定碳同位素的张北杨树防护林退化原因解析

三北防护林是京津冀地区重要的生态屏障,但近十年来张北县4/5的杨树防护林出现退化现象,接近1/3的杨树濒临死亡或枯死,导致杨树防护林生态功能下降.本研究用稳定碳同位素方法追溯研究了退化与未退化杨树年轮中δ¹³C值和内在水分利用效率(WUE_i)的差异,分析导致杨树退化和死亡的原因及其来源.结果表明:相同年龄杨树的直径随着退化程度增加而下降,退化杨树树轮的δ¹³C值变化范围为-25.26‰～-22.97‰,未退化杨树为-26.15‰～-23.50‰,从1997年开始,退化杨树δ¹³C值高于未退化杨树.退化与未退化杨树WUE_i值从1997年开始出现差异但不显著,2002年后其差异达到显著水平,退化与未退化WUE_i的差值ΔWUE_i连续正值可能是退化与未退化杨树产生分化的重要原因之一.退化和未退化杨树WUE_i与降雨量、相对湿度和潜在蒸散量(ET₀)的相关关系均不显著,但与气温和地下水埋深呈极显著的线性关系.1997年极端干旱事件是杨树林退化的起点,随后土地利用方式的改变导致地下水的过度使用,加剧了干旱持续时间和强度,进而加速杨树防护林的退化和死亡.

Retrospective analysis of the poplar plantation degradation based on stable carbon isotope of tree rings in Zhangbei County,Hebei, China

The Three-North Shelter Forest is an important ecological defense of Beijing-Tianjin-Hebei area. About four-fifths of poplar plantations declined and about a third of them were dying or already dead in the last ten years. The mortality of trees resulted in the decline of ecological function of poplar plantation. In this study, we investigated the differences of δ¹³C and intrinsic water-use efficiency (WUE_i) values in the tree-ring between dieback and non-dieback trees with stable carbon isotope method. The cause of poplar plantation degradation and mortality was retrospectively ana-lyzed. The results showed that the diameter of poplar trees of nearly the same age decreased with the increase of degradation degree. The δ¹³C value of rings from dieback trees varied between -25.26‰ and -22.97‰, whereas that of non-dieback ones was -26.15‰ to -23.50‰. The δ¹³C values of dieback trees were higher than that of non-dieback ones from 1997. There was a nonsignificant difference of the WUE_i between dieback and non-dieback tree-ring from 1997 to 2001. And the difference of WUE_i between dieback and non-dieback tree was significant since 2002. The continuous occurrence of positive ΔWUE_i(WUE_idieback-WUE_inon-dieback) values might be one of important factors for subsequent divergence of the dieback and non-dieback poplar trees. The WUE_i of both dieback and non-dieback trees had no significant relationship to precipitation, relative humidity and ET₀, while significantly related with air temperature and ground water depth. The retrospective ana-lysis results showed that extreme drought in 1997 was a threshold when poplar trees began to decline. The underground water was overused because of land use change, which increased the intensity and duration of the drought, thus accelerated the degradation and mortality of poplar trees.