Physiological responses of Populus euphratica Oliv. to groundwater table variations in the lower reaches of Heihe River, Northwest China

Riparian vegetation in the lower reaches of Heihe River serves important ecological functions. However, the riparian ecosystems have been constantly deteriorating in the past 30 years simply due to water interception for oasis agricultural irrigation in the middle reaches of the river. This study pays a particular attention to Populus eu- phratica Oily. forest because it is a dominant component of the riparian ecosystem in the lower reaches of Heihe River where the depth of groundwater table is the controlling factor in sustaining riparian ecosystems. To reveal leaf-related physiological responses of Populus euphratica Oliv. forest to groundwater table variations, we analyzed the relationships between the depth of groundwater table （DG） and three leaf-related parameters, i.e. leaf stomatal density （SD）, specific leaf area （SLA）, and stable carbon isotopic composition （6~SC）. Our results show that the relationship between DG and leaf SD is a bi-mode one shaped by both salt stress and water stress. That is, salt stress appeared in shallow groundwater conditions and water stress happened in deep groundwater conditions, and the thin layer around 2.7 m of DG is a stress-free layer. Leaf SD fluctuated according to the DG variation, first de- creased with increasing DG, then increased at depths ranging 2.7-3.7 m, and after a relatively stable plateau of SD at depths ranging 3.7-5.2 m, decreased again with increasing DG. Our results also show that SLA decreased ex- ponentially with increasing DG and foliar 6130 values are also strongly dependent on DG, further demonstrating that these two parameters are sensitive indicators of water stress. The exponential curve suggests that SLA is more sensitive to DG when groundwater table is shallow and 3 m seems to be a threshold beyond which SLA becomes less sensitive to DG. Foliar 613C becomes more sensitive when the groundwater table is deep and 7 m seems to be a threshold below which the 6130 signature becomes more sensitive to DG. These findings should be helpful in monitoring the growth and development of Populus euphratica Oliv. forests and also in providing protection measures （i.e. DG related） for Heihe River riparian forests.

Mapping Daily Temperature and Precipitation in the Qilian Mountains of Northwest China

Daily meteorological data are the critical inputs for distributed hydrological and ecological models. This study modified mountain microclimate simulation model(MTCLIM) with the data from 19 weather stations, and compared and validated two methods(the MTCLIM and the modified MTCLIM) in the Qilian Mountains of Northwest China to estimate daily temperature(i.e., maximum temperature,minimum temperature) and precipitation at six weather stations from 1 January 2000 to 31 December2009. The algorithm of temperature in modified MTCLIM was improved by constructing the daily linear regression relationship between temperature and elevation, aspect and location information. There are two steps to modify the MTCLIM to predict daily precipitation: firstly, the linear regression relationship was built between annual average precipitation and elevation, location, and vegetation index; secondly, the distance weight for measuring the contribution of each weather station on target point was improved by average wind direction during the rainy season. Several regression analysis and goodness-of-fit indices(i.e., Pearson's correlation coefficient, coefficient of determination, mean absolute error, root-mean-square error and modeling efficiency) were used to validate these estimated values. The result showed that the modified MTCLIM had a better performance than the MTCLIM.Therefore, the modified MTCLIM was used to map daily meteorological data in the study area from 2000 to 2009. These results were validated using weather stations with short time data and the predicted accuracy was acceptable. The meteorological data mapped could become inputs for distributed hydrological and ecological models applied in the Qilian Mountains.