不同林龄人工梭梭林对降雨的滞后响应

Lag response of artificial Haloxylon ammodendron forests at different ages to rainfall

降雨波动是荒漠植被生长发育最主要制约因子,直接影响到植被组成、结构和功能的变化。梭梭作为干旱区植被的优势种之一,在防风固沙、涵养水源方面扮演重要角色,本文的目的是理解不同林龄人工梭梭林植被生产力对降雨事件的响应过程。本文以黑河中游荒漠-绿洲过渡带不同林龄人工梭梭为研究对象,以2017-2018年5月1日至9月31日时序MODIS-NDVI(近似于年生产力)日数据和对应日降雨数据为基础,采用阈值模型和统计学方法分析了其NDVI对不同降雨等级的滞后响应过程。结果表明:①降雨小于2 mm时,不同林龄梭梭NDVI增长率均在5%以上,大于40 a生的反应最迅速,增长率为10%~56%;2~5 mm时,10~20 a生的变幅最大,增长率为11%~83%;5~10 mm时,20~40 a的最为敏感,增长幅度最高达到170%;大于20 mm时,大于40 a的NDVI增长率大于其他3个林龄,增幅最高达76%。②降雨后,不同林龄人工梭梭NDVI响应时间不同,0~10 a的为7.4(±2.8) d,10~20 a的为8.3(±3.1) d,20~40 a的为8.1(±2.7) d,大于40 a的为8.2(±3.2) d。③降雨脉动是促成荒漠-绿洲过渡带植被生产力迅速变化的关键因素。不同林龄梭梭对不同级别降雨响应模式不同,2~10 mm降雨条件下,人工梭梭林NDVI增加幅度最大,高于其他2个降雨量量级NDVI响应变化幅度。依据上述分析可以推断,在荒漠-绿洲过渡带以10 mm以下降雨为主导的条件下,10~20 a生及20~40 a生人工梭梭林在防护体系的作用是最重要的。

Rainfall pulses are the most important constraints for the growth and succession of desert vegetation and directly affect the changes in vegetation composition,structure,and function.As one of the dominant species of artificial vegetation in the arid zones,Haloxylon ammodendron is a crucial factor in wind breaking,sand fixation,and water conservation.The objective of this study is to understand the response of the vegetation productivity of the artificial Haloxylon ammodendron woodlands to rainfall events at different stand ages.We studied the Haloxylon ammodendron woodlands exhibiting different stand ages in a desert-oasis transition zone in the middle reaches of the Heihe River using the MODIS-NDVI (the approximate annual productivity) time series from May 1,2017 to September 31,2017.Based on the corresponding daily rainfall data,a threshold model and statistical methods were used to analyze the lag response process of the NDVI to different rainfall levels.The results showed that (1) When the rainfall was less than 2 mm,the NDVI growth rates in case of different stand ages were more than 5% and stands with ages of more than 40 year reacted the fastest;the growth rates fluctuated from 10% to 56%.When the rainfall was 2-5 mm,the stands that exhibited ages of 10-20 year showed the largest growth rate,i.e.,from approximately 11% to 83%.When the rainfall was 5-10 mm,the most sensitive group had stand ages of 20-40 year,with the highest growth rate being 170%.When the rainfall was more than 20 mm,the NDVI growth rate was greater for stand ages greater than 40 year than those for the remaining three stand ages,with an increase of up to 76%.(2)After the rainfall,the NDVI response time was observed to be different for different stand ages: 7.4 (± 2.8) days for the 0-10 year group,8.3 (± 3.1) days for the 10-20 year group,8.1 (± 2.7) days for the 20-40 year group,and 8.2 (± 3.2) days for the > 40 year group.(3) Rainfall pulses contribute to rapid changes in vegetation productivity in the desert-oasis transition zone.Different stand age groups showed different response patterns to different rainfall levels,and the NDVI increase for the artificial Haloxylon ammodendron woodland was the largest when the rainfall was 2-10 mm.According to these results,when the desert-oasis transition zone is dominated by less than 10 mm of rainfall,the roles of 10-20 year and 20-40 year artificial Haloxylon ammodendron woodlands in the protection system are likely to be the most important factors.