Hysteresis loops between canopy conductance of grapevines and meteorological variables in an oasis ecosystem

Canopy conductance (gc) is a key parameter that determines plant transpiration rate, and it is sensitive to multiple environmental variables. The relationships between gc and micrometeorological variables still need to be investigated across a wide range of biomes and climatic conditions. In this study, sap flow, micrometeorological and biological factors for a vineyard at an oasis in Northwest China were measured from July to September in 2013 and 2014. Values of gc were calculated from the simplified Penman-Monteith equation, and diurnal patterns of gc were investigated in two study years. The relationships between gc and main micrometeorological variables (global radiation R, vapor pressure deficit D, air temperature T) during the day exhibited hysteresis loops. By dividing daytime into three time periods, it was found that in the first period (7:00-11:00), stomatal opened quickly as R, D and T increase, so gc increased positively to increasing R, D and T. In the second period (11:00-17:00), followed by stomatal closure, gc decreased negatively to increasing R, D and T. In the third period (17:00-21:00), as stomatal aperture kept decreasing until sunset, gc decreased positively to decreasing R, D and T. A simple linear model was used to simulate variations in gc for the first period and the third period, and a negative logarithmic model was used for the second period. These models were consistent in predicting daytime variations in gc. The results are useful in improving our current understanding of relationships between plant physiological and environmental processes in the oasis ecosystem.