半干旱区城市环境下油松林分蒸腾特征及其影响因子

The stand transpiration characteristics of Pinus tabulaeformis and its influential factors in a semi-arid urban environment

在城市环境下,由于不透水地面面积的增加,土壤-植物-大气之间水汽循环减弱,水汽调节能力差,因而研究城市树木蒸腾对环境因子的响应对于城市进行合理的水汽调节具有重要意义。于2017年生长季,在内蒙古呼和浩特市区树木园内选择58年生油松(Pinus tabulaeformis Carr.)作为研究树种,采用热扩散法测定其树干液流,并同步监测气象因子和土壤含水量变化,利用彭曼公式计算冠层气孔导度。结果表明:(1)生长季内,油松林分蒸腾存在明显日、月变化,晴天天气下林分蒸腾日变化呈单峰曲线,月林分蒸腾量5月最大,其次是7月、8月、6月和9月,分别为20.96、19.89、18.09、17.25 mm和7.49 mm。(2)油松林分蒸腾与饱和水汽压差、太阳总辐射、土壤含水量和风速均存在极显著相关关系(P<0.01),太阳总辐射、饱和水汽压差和土壤含水量是影响林分蒸腾的主要环境因子(R^2=0.47、R^2=0.31和R^2=0.16),风速对林分蒸腾的影响程度最小(R^2=0.12);不同降雨量对林分蒸腾的影响作用不同,10 mm以上的日降雨量对油松林分蒸腾作用明显。(3)除环境因子外,油松叶片气孔通过响应环境变化控制蒸腾作用,当饱和水汽压差<1.5 kPa时,叶片气孔对饱和水汽压差的响应更敏感;当太阳总辐射<250 W/m^2时,叶片气孔对蒸腾起促进作用,超过该阈值,叶片气孔关闭从而抑制树木蒸腾。

The water vapor cycle between soil-plant-atmosphere is weakened and its ability for regulation is poor due to the increase in impervious surface areas in urban environments. Therefore, it is of great significance to study the transpiration response of urban trees to environmental factors amidst reasonable water vapor regulation in urban areas. In the growing season of 2017, 58-year-old Pinus tabulaeformis Carr. was selected as a research tree species for sap flow measurement, by thermal diffusion, at the Hohhot arboretum, Inner Mongolia. We measured meteorological factors and soil water content simultaneously and estimated canopy stomatal conductance using the Penman Formula. The results showed that(1) stand transpiration of P. tabulaeformis had obvious daily and monthly changes in the growing season. The diurnal stand transpiration variation graph showed a single peak curve on sunny days. Monthly stand transpiration reached the maximum in May, followed by July, August, June and September, which was 20.96, 19.89, 18.09, 17.25 mm and 7.49 mm, respectively.(2) There were extreme significant correlations between stand transpiration and vapor pressure deficits, global radiation, soil water content, and wind speed(P<0.01). Global radiation, vapor pressure deficit, and soil water content were the main environmental factors affecting forest transpiration(R^2=0.47, R^2=0.31 and R^2=0.16), and wind speed had the smallest impact on forest transpiration(R^2=0.12). Different rainfall events had varied effects on stand transpiration, but a significant effect when daily rainfall exceeded 10 mm.(3) In addition to environmental factors, leaf stomata of P. tabulaeformis controlled transpiration in response to environmental factors. Leaf stomata were more sensitive to vapor pressure deficits when it was < 1.5 kPa, whereas, global radiation, when < 250 W/m^2, promoted transpiration. When the threshold was exceeded, leaf stomata would close to suppress tree transpiration.