[Responses of transpiration to variation in evaporative demands and soil water in a larch plantation at the south side of Liupan Mountains, China]. / å ­ç›˜å±±å—ä¾§åŽåŒ—è½å¶æ¾äººå·¥æž—è’¸è ¾å¯¹åœŸå£¤æ°´åˆ†å’Œæ½œåœ¨è’¸æ•£çš„å“åº”.

Quantifying the response of tree transpiration (T) to the variation of soil water supply capability and atmospheric evaporative demand is beneficial for a better prediction of water use and hydrological cycles in forests and deepen the understanding of the relationship between forest and water. Larix principis-rupprechtii in the Xiangshuihe watershed at the south side of Liupan Mountains was used as the research object. We simultaneously monitored sap flow density by thermal diffusion probe and the environmental factors. The response of the T to the soil volumetric water content (VWC) and potential evapotranspiration (PET) was analyzed. The results showed the response curve of T to VWC was quite similar under any different PET levels. With increasing VWC, T increased rapidly and then slowly, and began to be stable when VWC reached a threshold. This process could be well fitted by the saturated exponential function. However, the VWC threshold was different, and its value increased with rising PET. The relationship of daily T to PET was a quadra-tic equation, and PET also had a threshold effect. A stand transpiration model considering the effect of soil water supply capacity and atmospheric evaporative potential was founded which coupled the response relationship of T to PET and VWC in the rapid growth season. This model could well estimate the diurnal variation of transpiration, and provide guidance for the management of plantation water control.

Response of the daily transpiration of a larch plantation to variation in potential evaporation, leaf area index and soil moisture.

Tree transpiration (T) is a major water budget component and varies widely due to the integrated effects of many environmental and vegetation factors. This study aimed to separate, quantify, and then integrate the effects of the main individual factors, to improve water use estimation and manage the hydrological impacts of forests. A field study was conducted at 3 plots of larch (Larix principis-rupprechtii) plantation in the semi-humid area of the Liupan Mountains, northwest China. The main influencing factors were the atmospheric evaporative demand expressed by potential evapotranspiration (PET), the soil water availability expressed by volumetric soil moisture (VSM) within the 0-100 cm layer, and the canopy transpiration capacity expressed by forest canopy leaf area index (LAI). The daily stand T was estimated through the up-scaling of sap-flow data from sampled trees. It displayed a high degree of scattering in response to PET, VSM and LAI, with an average of 0.76 mm·day-1 and range of 0.01-1.71 mm·day-1 in the growing season of 2014. Using upper boundary lines of measured data, the response tendency of T to each factor and corresponding function type were determined. The T increases firstly rapidly with rising PET, VSM and LAI, then gradually and tends to be stable when the threshold of PET (3.80 mm·day-1), VSM (0.28 m3·m-3) and LAI (3.7) is reached. The T response follows a quadratic equation for PET and saturated exponential function for VSM and LAI. These individual factor functions were coupled to form a general daily T model which was then fitted using measured data as: T = (0.793PET - 0.078PET2)·(1 - exp(-0.272LAI))·(1 - exp(-9.965VSM)). It can well explain the daily T variation of all 3 plots (R2 = 0.86-0.91), and thus can be used to predict the response of daily T of larch stands to changes in both environmental and canopy conditions.

Heterogeneity of competition at decameter scale: patches of high canopy leaf area in a shade-intolerant larch stand transpire less yet are more sensitive to drought.

Small differences in the sensitivity of stomatal conductance to light intensity on leaf surfaces may lead to large differences in total canopy transpiration (EC) with increasing canopy leaf area (L). Typically, the increase of L would more than compensate for the decrease of transpiration per unit of leaf area (EL), resulting in concurrent increase of EC. However, highly shade-intolerant species, such as Larix principis-rupprechtii Mayr., may be so sensitive to increased shading that such compensation is not complete. We hypothesized that in such a stand, windfall-induced spatial variation at a decameter scale would result in greatly reduced EL in patches of high L leading to lower EC than low competition patches of sparse canopy. We further hypothesized that quicker extraction of soil moisture in patches of lower competition will result in earlier onset of drought symptoms in these patches. Thus, patches of low L will transition from light to soil moisture as the factor dominating EL. This process should progressively homogenize EC in the stand even as the variation of soil moisture is increasing. We tested the hypotheses utilizing sap flux of nine trees, and associated environmental and stand variables. The results were consistent with only some of the expectations. Under non-limiting soil moisture, EL was very sensitive to the spatial variation of L, decreasing sharply with increasing L and associated decrease of mean light intensity on leaf surfaces. Thus, under the conditions of ample soil moisture maximum EC decreased with increasing patch-scale L. Annual EC and biomass production also decreased with L, albeit more weakly. Furthermore, variation of EC among patches decreased as average stand soil moisture declined between rain events. However, contrary to expectation, high L plots which transpired less showed a greater EL sensitivity to decreasing stand-scale soil moisture, suggesting a different mechanism than simple control by decreasing soil moisture. We offer potential explanations to the observed phenomenon. Our results demonstrate that spatial variation of L at decameter scale, even within relatively homogeneous, single-species, even-aged stands, can produce large variation of transpiration, soil moisture and biomass production and should be considered in 1-D soil-plant-atmosphere models.

[Evapotranspiration characteristics of artificial and natural forests in Liupan Mountains of Ningxia, China during growth season].

In order to understand the effects of the structure of forest ecosystem on the hydrological processes, a comparative study by using thermal dissipation technique and hydrological methodology was made on the evapotranspiration (ET) and its components of Larix principis-rupprechtii plantation and Pinus armandi natural forest in two adjacent stands in a small catchment Xiangshuihe of Liupan Mountains during the growth season (May-October) in 2009. Throughout the growth season, the total ET from the plantation was 518.2 mm, which accounted for 104.6% of the precipitation and was much higher than that (420.5 mm) of the natural forest. The allocation of ET in the vertical layers performed similarly between the two stands, with the order of canopy layer > herb and soil layer > shrub layer, but the ratio of each component to total ET differed significantly. The plantation consumed 0.2 and 0.9 times more water for canopy interception (19.6 mm per month) and tree transpiration (25.2 mm per month) than the natural forest, respectively. However, the transpiration from the plantation was 4.4 mm per month, and took up 23.4% of the natural forest. In contrast, the sum of soil evaporation and herbage evapotranspiration consumed 37.1 mm water per month in the plantation, which was 0.8 times higher than that in the natural forest. The ET was calculated by Penman-Monteith equation to compare the results estimated by sap flow measurements, and the values estimated by the two methods were similar.