城市与区域生态国家重点实验室知识库

      在黄土高原地区，自然条件的制约和人类活动的影响导致该地区 生态系统退
化 、 水土流失等生态问题 日益 突出。为改善黄土高原地区的生态 环境 我国政府实施了一系列的 生态恢复 工程 ，其中植树造林是生态恢复的主要措施之一。虽然这些 恢复 措施在改善区域生态环境方面取得了一定的成效， 但是也存在很多问题 。由于没有考虑到植物 的 蒸腾 耗水情况和对环境条件的适应性 从而在一定程度上导致 了土壤干层 的形成和植物提前衰老 等 现象 。 因此 研究 退耕还林物种 的耗水特征 及其 对 干旱 环境 的响应与适应机制， 理解 植被恢复对生态水文过程的影响， 可为 恢复生态系统的可持续发展提供科学依据。 灌木物种 是黄土高原地区 主要的退耕物种 之一 ，但是 目前 在 该地区 对 典型 灌木 植物 的 耗水特征及生态水文效应 研究还不够深入 对 水分 的响应还 缺乏定量估算和机制的解释 。

本研究地点在黄土高原典型丘陵沟壑区 延安羊圈沟小流域 ，在小流域内相同的地形条件下 设置三个 5 × 5 典型灌木 植物 的观测样地 ，包括 荆条（ Vitex negundo L.L.）、沙棘 Hippophae rhamnoides L.L.）和绣线菊 Spiraea pubescens Turcz 在2015 2016 年 主要 生长季 6 月 9 月） 进行定位观测。研究 采用 热平衡 原理，用包裹式 树干液流的 方法 测定三种灌木 植物 单株 的液流 速率 sap flow den s i ty Fd并采用 叶面积的方法 进行尺度上推 ，推算出样地的蒸腾量 Transpiration Tr 。同 时 对 太阳辐射 solar radiation Rs ）），空气温度 air temperature Ta 、相对湿度 relative humidity RH 、降雨量 precipitation P 等气象因子 和 0 200 cm的土壤含水量 soil water content SWC 等环境因子进行连续的自动监测 ，并计算出了水汽压亏缺（ V PD ）。 与此同时，每月测定 了叶片的生理生态指标，包括叶片尺度的蒸腾作用 leaf transpiration Ei 与 气孔导度 、 叶片水势 predawnleaf water potential 的 日动态 等 指标 。主要研究结果包括以下几个方面：
（1） 在不同 尺度上，影响灌木蒸腾作用 的因 素 不同 。在小时尺度 和日尺度
上，三 种 灌木 植物 的 蒸腾作用 与 水汽压亏缺 、 太阳辐射 和 空气温度 呈极显著正相关关系 P 0.05 与 空气相对湿度呈现显著的负相关关系 P 0.05 ），但 是蒸
腾作用 对 水汽压亏缺 和 太阳辐射 的响应呈现非线性 规律 不同深度土壤水分对蒸腾影响不同，在相对 干旱年份（ 2015 ），蒸腾与深层土壤水分 120 200 cm ）呈现显著的负相关（ P < 0.05 ），而在 相对湿润的 2016 年，荆条与表层土壤水分（ 0 40cm ）相关 P < 0.05 ），而沙棘和绣线菊与土壤水分无显著相关性 P >0.05 ）；在月尺度上，影响植物蒸腾作用的主要是叶面积指数（荆条）和土壤水分（沙棘和绣线菊）。
（2 ）降雨是黄土高原土壤水分的主要来源，不同降雨量对植物生理 的 影响不同。 在小降雨事件之后（ 0.2 2 mm ），由于冠层截留作用，土壤水分没有 明显变化 但是 气象因子 显著提高，促使三种灌木的液流速率增加显著； 在降雨量为2 10 mm 区间内 ，由于降雨之后空气温度的降低和湿度的增加，液流速率增加不显著（荆条和绣线菊），甚至 出现了 液流速率 的降低 的情况 （沙棘）；在降雨量 >10 mm 时，三个灌木样地土壤水分显著增加，进而促进了液流 速率的增加 。
（3） 植物在适应环境的过程，除调节白天的蒸腾来适应干旱之外，也可以
调节夜间液流速率来适应干旱。 夜间液流占全天液流的比 例 较少（ 5 %-7%），但是它 在植物的适应性方面有重要的生理意义。 夜间液流占全天液流比例随土壤水分的降低而升高， 夜间液流提高了黎明前 茎干和叶片水势，同时 增加 了黎明前气孔导度 的张开程度 。 同时又 由于 夜间 液流过程中的茎干储水作用， 使得 植物在第二天上午的蒸腾 作用 可以 优先利用茎干 中储存 的水分，使植物避免因土壤水分的干旱而影响到蒸腾作用和 光合作用 。
（4 ）水量平衡的各组分（ 包括 植物蒸腾 、 冠层截留 、 地表 径流 、 土壤蒸发
均受到降雨量的影响 。 在 2015 年（相对干旱年份） 2016 年 （相对湿润年份
的 主要生长季（ 6 月 9 月），荆条、沙棘和绣线菊灌木样地 两年 主要 生长季总的蒸腾量分别为 107.21 mm 和 180.60 mm （荆条 122.33 mm 和 180.20 mm （沙棘） 84.03 mm 和 137.02 mm （绣线菊 在湿润年份 蒸腾相比于其它组分占 蒸散发 的比例最大，其次为土壤蒸发和冠层截留；在干旱年份，绣线菊样地水分的主要损失是土壤蒸发，其次为植物蒸腾 和冠层截留 。 地表径流 R 在两年里均较少 ，两年的径流量共为 6.37 mm （沙棘）和 3.05 mm （绣线菊）。 在降雨较多的2016 年灌木样地的蒸散发多于 2015 年。 在两年里，三个灌木样地的土壤储水量D SW 均处于下降趋势， 而且在干旱年份下降更多。
     综上所述，气象因素和土壤水分可在不同时间 尺度上影响 植物 个体 的蒸腾作用 ，进而影响样地尺度 的水量平衡过程 。 同时植物通过对小降雨事件的响应和调节夜间液流速率来加强植物对环境的适应性。在本研究时间 段 内，不同物种在不同 水分 条件下对环境因子的响应 存在差异。虽然不同植物通过不同的策略来适应干旱和半干旱的环境，但是也对当地的生态水文过程产生影响。 长期的生态水文效应还需要做更长的时间 和空间 尺度和更深入的研究。

      In the hilly region of Chinese Loess Plateau,the rest riction of the nature conditions and the effect of human activities lead to ecological degradation and ecological problems, such as soil and water loss. To improve the ecological fragility of the Loess Plateau, our government took a lot of measures of ecological restoration, and afforestation is one of the most important measures for ecological restoration. Although these measures had made certain achievements in regional ecological
environment, there also existed problems. Due to not consider the plant adaptation to the environment, the restoration in the Loess Plateau caused soil dry layer and low growth rate for plants. So to study the soil water consumption of restoration vegetation and the response of vegetations to soil moisture is helpful to understand the sustainable development of restoration ecosystem. So the ai m of this study was to
invest igated the adaptation of physiolo gical process of restoration vegetations to environmental factors, analyze the its water consumption in water balance model, and understand the ecological hydrolo gical process es of restoration plantation. Shrub species are one of the most important restoration species on the Loess Plateau. However, the study of water consumption and ecological hydrological effect of typical shrub species were not insufficient, lack ing the quantitatively estimates of
response to soil moisture and mechanism explanation.
      We conducted our research in Yangjuangou catchment, Yanan, with three typical shrub plantation, which is Vitex negundo L., Hippophae rhamnoides L., and Spirea pubesce ns Turcz. The experiments were conducted in the main growing seasons during 2015 and 2016 (from June to September). Sap flow density ( was measured using heat balance method. The s tand transpiration was cal culated basing on the stand
sap flow density an d leaf area. In addition, environmental variables, including solar radiation (Rs), air temperature (Ta), precipitation (P), relative humidity (RH) and soil water content (SWC) above 200 cm. We also calculated the vapor pressure deficit (VPD), and potential evapotranspiration (ET 0 ) using micro meteorological variables.
During the study, the eco ph ysiological indicators of plants were also measured for three shrub plants, incl uding tem poral dynamics of leaf transpiration, stomatal conductance and leaf water p otential. The main results are as follows
（1 ) The influencing factors of Fd were different at different time scales. On hourly and daily time scale s , the main influencing factors for three shrub species were meteoro lo gical factors . Fd was significantly p ositi vely correlated with VPD, Rs and Ta P < 0.05), while Fd was significantly negative correlated with RH P < 0.05). On daily time s cale , the response of Fd to VPD and Rs showed exponential regression.The SWC in different soil layer had different effec t on Tr. The Tr in three shrub plantations were correlated with deep soil layer (120 200 cm) ( P < 0.05). Yet there is
no significant correlations between Tr and SWC in H. rhamnoides and S.pubescens P > except V. negundo plantation, which is correlated with SWC in topsoil layer (0 40 cm) P < 0.05). Monthly Tr in three plantations were correlated with leaf area index ( V. negundo ) and soil moisture H. rhamnoides and S.pubescens

( 2 ) Precipitation was the main source of s oil moisture. The responses of plant physiological processes to different precipitation amount were different. In this study, the response of sap flow density (Fd) to rainfall events depended on species and the rainfall patterns. The increase in daily Fd w as large with small rainfall events (0.2--2 mm) mainly ascribed to enhanced meteorological conditions on days after rain , although the soil moisture did not change significantly. When rainfall events were
2-- 10 mm, the response percentage of Fd reduced. This behavior mainly because rainfall would increase the air's relative humidity and decrease Ta. When rainfall amount exceeded 10 mm, SWC was the main factor for the response percentage of Fd. In addition, the Fd related hydraulic conductance increased in res ponse to rainfall event for H. rhamnoides , while leaf water potential were F or V. negundo and S.pubescens , the water potential at the soil root interface and leaf water potential
increased , while hydraulic conductance was stable .
(3) In addition to daytime transpiration, plants could adjust the nighttime sap flow density to adapt the drought conditions. Although the low fraction of nighttime Fd to daily Fd of three shrubs, the importance of nighttime sap flow for plant drought adaptation was signi ficant. T he ratios of nighttime Fd to daily Fd of three shrubs were increased with decreasing SWC. Nighttime Fd was importance in increasing predawn
leaf and xylem water potential, opening predawn stomatal conductance earlier . In addition, because of stem refilling during the night, transpiration in the morning used the refilling stem water.
（4 ) The components of water balance (Tr, canopy interception, runoff, soil evaporation) were all influenced by precipitation. During the main growing seasons in 2015 i.e., a relatively dry year, representing 41.47% of the long term meanprecipitation (P) and 2016 (i.e., a relatively wet year, with almost the same level of P as the long term mean rainfall) the total Tr values were 107.21 and 180.60 mm, respectively, fo r V. negundo ; 122.33 and 180.28 mm, respectively, for H. rhamnoides ;84.03 and 137.02 mm, respectively, for S.pubescens . Tr of three plantation w ere the main component of evapotranspiration in 2015 and in 2016 for V. negundo and H.rhamnoides , followed by soil evaporation and canopy interception . Soil evaporation was the main component in S. pubescens plantation in 2015. The observed surface runoff values were very low in both years, suggesting that the restored shrubs effectively reduced soil erosion. Duri ng the study period, water loss exceeded P during most months of the main growing seasons, and the discrepancy between water loss and P was greater in 2015. Thus, the change in net soil water storage was
negative, especially in the HR stand.
      In summary, meteorological variables and SWC were the main factors for the transpiration, and then influencing the ecological hydrological processes. Plant could adjust their response of small rainfall events and nighttime sap flow density to enhance their adaptability to environment. However, mu l ti year observations should be conducted to further investi gate the long term hydrol ogical effects of the plantations on water cycling at stand, watershed and regional scales.