热带人工橡胶林林冠截留雾水的边缘效应研究

Edge effect of intercepted fog water by forest canopy at a rubber plantation in Xishuangbanna, Southwest China

利用 2 0 0 1年 1 1月～ 2 0 0 2年 1 0月的林缘和林内雾水截留及相关环境因子观测资料 ,对西双版纳热带人工橡胶林林冠截留雾水的边缘效应进行了研究。西双版纳热带人工橡胶林内的雾日数及雾日频率呈现明显的季节变化 ,全年雾日数可达 1 72 d,其中雾季 (1 1～翌年 2月份 )的雾日数及雾日频率最多 (高 ) ,月均雾日数可达 2 3d(76 .7% )。由林缘至林内 ,年雾水截留量在迎风面和背风面均呈现指数形式急剧减小 ,二者均是在林缘最边行外测的林冠下 (0 m处 )达到最大 ,分别为 4 1 .1 mm和 2 4 .3mm。迎风面和背风面林缘 0 m处的雾水截留量分别是林内雾水截留量 (1 6 .2 mm )的 2 .5倍和 1 .5倍。迎风面雾水截留在林内约 2 5 m处趋于稳定 ,而背风面在林内约 1 5 m处趋于稳定。林缘处的雾水截留量与日 0 :0 0～ 1 0 :0 0平均风速呈显著的正相关(P<0 .0 1 )。风速大 ,则雾水截留的边缘效应向林内越深入 ,且空间变异性减小。相对于迎风面林缘 0 m处的雾水截留而言 ,大的风速将削弱林内和林缘雾水截留量的差别 。

Xishuangbanna is located at the northern edge of the distribution of tropical forest in Southeast Asia, and it has a very high frequency of radiation fog, especially during the dry season (November～April). Radiation fog events in this site are generally associated with low wind speeds and region-wide air mass stagnation resulting from strong nighttime radiative cooling. Intercepted fog water by forest canopy from both the windward edge and leeward edge to the interior of the forest and related microclimatic factors were measured during November 2001 and October 2002 at a rubber (Hevea brasiliensis) plantation in Xishuangbanna, Southwest China. The object of the study is to determine whether the windward edge and leeward edge of forest floor receives greater deposition of fog water than the interior of the forest. Bottle-funnel collectors were used to determine daily amount of intercepted fog water during fog-only events along windward and leeward transects in the rubber plantation. Related microclimatic variables including air temperature, relative humidity, wind speed, wind direction, and rainfall were also recorded by a meteorological observation system (MAOS-1) mounted on a 31 m meteorological tower in the study stand. The number of fog days was extremely higher inside the rubber plantation, with 172 days per year. The fog frequency was up to 76.7% in the foggy season (November～February). During the study period, an absolute amount of annual intercepted fog water in the rubber plantation was up to 16.2 mm. Annual intercepted fog water varied dramatically and decreased exponentially from the windward and leeward edge to the interior of the forest. Intercepted fog water at the edge of the forest was on average 1.5 to 2.5 times, and up to 9 times, greater than that in the interior of the forest. The intercepted fog water “stable-distance”, i.e. the point at which the fog water interception is generally no change within the forest edge farther, was found to be 25 m for the windward edge and, 15 m for the leeward edge. We also found that the intercepted fog water at the edge was positively correlated with average wind speed during 0:00~10:00 of the day (P<0.01), demonstrating that high wind speed resulted to further extension of the edge effect and lower spatial heterogeneity. Our intercepted fog water data were compared to other studies, which showed similar result. Although our data are not extensive enough to allow broad generalizations, they provide further evidence that the amount of intercepted fog water from the forest canopy to the forest floor in edge zones can be very different from that in the forest interiors. In regions of high winds and significant intercepted fog water, the edge effect is likely to be even greater than we have shown here. We attribute this difference to the mechanism of fog formation in different sites. The fog in montane forest close to coasts with high wind is mainly caused by the cooling effect of rising air plus long-wave radiation loss. However, the fog in Xishuangbanna, which is far from the coast, is mainly a result of long-wave radiation at relatively low altitude. Meanwhile, it is reasonable to believe that converting multi-layer tropical rain forest with single-layer rubber plantation will reduce intercepted fog water in adjacent tropical rain forest. Furthermore, the method used to collect fog drip water is also different from other studies that could contribute to the relatively low value reported in this study. As the data obtained from canopy drip are net inputs to the forest floor, the estimate is considered to be conservative compared to fog water via impaction by fog gauges. We could further hypothesize that the edge of the rubber plantations would generally intercept less fog drip than that of the tropical rain forest during the dry season. The edge effect phenomenon we have described could have important effects on many ecological processes, biodiversity and forest regeneration. These results also demonstrate the importance of understanding the impacts of climate facto