南亚热带森林演替过程中小气候的改变及对气候变化的响应

区域水热格局变化和系统演替深刻影响森林内部小气候,不同演替阶段森林内部水热环境对气候变化的响应和反馈作用有待进一步认识和评估。以南亚热带地区的3种不同演替阶段代表性森林生态系统统(人工恢复的马尾松针叶林(Pinus massoniana coniferous forest,PF)、马尾松针阔叶混交林(mixed Pinus massoniana/broad-leaved forest,MF)和季风常绿阔叶林(monsoon evergreen broad-leaved forest,MEBF))为研究对象,通过分析其林内小气候林型间差异以及时间序列上的动态变化,探讨森林系统内部水热环境的改变机理。结果表明:演替驱动下,随着PF→MF→MEBF的正向发展,林内温度条件如气温、土壤温度逐渐降低,林内相对湿度、土壤层及凋落物含水量等水分状况逐步升高。不同林型在"雨热同期"的南亚热带地区其"降温效应"有差,演替初期的PF干、湿季"降温效应"分别为7.9%和3.6%,中期MF分别为11.6%和6.4%,顶级群落MEBF干、湿季"降温效应"可达15.7%和10.5%。总体上,随演替"降温增湿"效应越来越显著,且"降温"表现为干季更明显,而"增湿"表现为湿季明显。此外,演替驱动下后期森林对高温及土壤温度的调节作用更为突出。时间序列上,区域降水趋于"极端化"的格局影响下,森林生态系统的水分固持能力下降。主要表现为:自1984年以来,3种林型0—50cm土壤含水量均呈显著降低的趋势(P0.001),且湿季土壤含水量下降速率高于干季,林型间在全年及湿季均为MFMEBFPF,干季为MEBFMFPF。虽然研究期间3种林型林内气温、土壤温度无明显趋势性变化,但顶级群落MEBF林内相对湿度(P=0.021)、凋落物自然状态下含水量(P=0.003)在年际尺度上均呈现显著下降的趋势。与土壤含水量干、湿季下降速率的格局一致,二者也均为湿季大于干季。研究认为,成熟森林可能在当前南亚热带区域气候变化及水热格局改变背景影响下更为敏感和脆弱。

Forest microclimate change along with the succession and response to climate change in south subtropical region

Forest microclimate is deeply affected by the regional water-thermal pattern change and ecosystem succession. So, it needs to be further studied and assessed about the responses and feedbacks of different successional forests to the current climate change. In this study, we focused on three undisturbed forests in the Dinghushan biosphere reserve Pinus massonniana forest (PF), Mixed Pinus massonniana/broad-leaved forest (MF), Monsoon evergreen broad-leaved forest (MEBF), which forming a natural successional sequence in the south subtropical region of China. By analyzing the microclimates in the three different successional forests and their time-series dynamic trends, the changing mechanisms of forest water-thermal environment were explored. Analysis results indicated that: Driven by the succession from artificial planted PF to transitional forest MF to climax forest MEBF, forest air temperature and surface soil temperature both reduced significantly. Contrarily, the relative air humidity and water content of surface soil and litter increased gradually. In the south subtropical region, where rain and heat appears at the same period, the "cooling effects" of the three forests were different. The air temperatures within PF were 7.9% and 3.6% less than the CK in dry and wet season, respectively; Transitional period MF were 11.6% and 6.4% lower, and climax forest MEBF were 15.7% and 10.5%, respectively. On the whole, the forest "cooling and damping effects" had been strengthened over succession. In particular, the "cooling effect" was more prominent in dry season than in wet season, while the "damping effect" was more obvious in wet season. What is more, the later period forest was more advantageous in regulation of maximum air temperature and soil temperature. Seen from the time-series data, the moisture holding ability of forest also declined due to the global warming and regional precipitation pattern change, especially the intensification of rainfall storms and the increasing number of annual no-rain days. Mainly shows that the 0-50cm depth soil moisture of three forests all declined significantly from the year 1984 (P < 0.001), and the average decreasing rate in wet season was faster than that of dry season. Besides, the average decreasing rate of three forest soil moisture presented the follow order in both annual and wet seasons: MF>MEBF>PF, while in dry seasons it was MEBF>MF>PF order, similarly. Finally, although the air temperature, soil temperature of the three forests changed little during the study period, the relative air humidity (P=0.021) and natural water content of litter (P=0.003) of climax community MEBF both decreased significantly. Consistent with the variation trend of soil moisture, the average decreasing rate of relative air humidity and litter water content also presented to be faster in wet seasons than in dry seasons. We concluded that the mature forest tends to be more sensitive and vulnerable to the regional climate and water-thermal pattern change in south subtropical region of China.