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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δ~(13)C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δ~(13)C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δ~(13)C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(ET_0)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.
Three-North Shelterbelt is an important ecological barrier in Beijing, Tianjin and Hebei Province. However, about 4/5 poplar shelterbelts in Zhangbei County have been degraded in recent ten years. Nearly one-third of the poplar trees are dying or dead, resulting in a decrease in the ecological functions of poplar shelterbelts In this study, we studied the difference of δ ~ (13) C and internal water use efficiency (WUE_i) in the annual rings of degraded and non-degenerated poplars using the stable carbon isotope method, and analyzed the causes and sources of poplar degeneration and mortality.Results The results showed that: the diameter of poplar at the same age decreased with the increase of the degree of degeneration, the δ ~ (13) C value of degenerated poplar tree ring ranged from -25.26 ‰ to -22.97 ‰, and the un-poplar was -26.15 ‰ ~ -23.50 ‰ , Δ ~ (13) C values of degraded poplars were higher than those of non-degraded poplar trees since 1997. WUE_i values of poplar and degraded poplar had difference but no significant difference since 1997. Their difference reached significant level after 2002, Difference between degenerate and non-degenerated WUE_i A continuous positive value of ΔWUE_i may be one of the important reasons for the degeneration and degeneration of poplar.The correlation between WUE_i and precipitation, relative humidity and potential evapotranspiration (ET_0) The relationship is not significant, but buried with temperature and groundwater There was a significant linear relationship. In 1997 extreme drought events is the starting point poplar degradation, followed by changes in land use patterns lead to overuse of groundwater, increasing the duration and intensity of drought, thereby accelerating poplar protection forest degradation and death.