桂花树冠层气孔导度模型的优化及其参数分析

Optimization of canopy stomatal conductance models for Osmanthus fragrans and analysis of its parameters

冠层气孔导度(g_c)是许多陆面过程模型中的重要参数,提高对冠层气孔导度的模拟精度非常重要。以环境因子阶乘的Jarvis形式的模型是气孔导度模型中的典型代表,但研究中不同的环境因子有不同的响应方程和参数。研究认为不同的响应方程有不同的模拟效果,并通过比较各环境因子的不同响应方程组合的模型的模拟效果来确定最优的g_c模型。以桂花树为例,测定了树干液流、茎水势和微气象环境,用Penman-Monteith(PM)方程反推计算冠层气孔导度并检验不同方程组合的16种模型。模型的参数用DiffeRential Evolution Adaptive Metropolis(DREAM)模型优化。结果表明这种方法能够有效地找到各环境因子最优的响应方程,从而最优化g_c模型。优化的g_c模型很好地模拟了桂花树冠层气孔导度的变化,尤其是对干旱的响应,模拟值与PM计算值的相关系数和均方根误差分别为0.803和0.000623 m/s。同时也证明了模型中温度函数f(T)>1的现象并非个例,由于温度(T)和水汽压亏缺(D)常是高度相关的,建议在以后的g_c模型研究中应把T和D看成一个影响因子,但f(T)>1的这种现象是否具有全球性还有待进一步研究证实。

Canopy stomatal conductance （go） controls transpiration and photosynthesis processes. Thus, the simulation of gc and its environmental variation forms a significant component of many land surface models. A Jarvis-type model, which calculates gc from a reference value multiplied by scaling （or response） functions of influencing environmental variables, is a typical representation of Src in land surface modeling. Influential environmental factors often include solar radiation, vapor pressure deficit, and temperature and soil water conditions. Studies have applied different response functions to each individual environmental factor, often without rigorous evaluation. Thus, there is a need to determine which combination of response functions is most appropriate for a specific vegetation cover. In this study, an optimization model of g~ was determined for O. fragrans, an evergreen tree species in the southern China, based on field measurements. Sapflow, stem water potential, and microchmatic variables were recorded at an O. fragrans plantation site in 2013, where a severe drought occurred in July and August of that same year. Sap flow data were used to calculate transpiration, from which go was estimated from the inversed Penman-Monteith （PM） equation, based on micrometeorological data. Predawn stem water potential data were used to estimate root zone water potential, one of the environmental variables influencing go. Other environmental variables were available or could be derived from the micrometeorological measurements. A total of sixteen gc models composed of different response functions were examined. Parameters of each candidate model were optimized using the DiffeRential Evolution Adaptive Metropolis（DREAM） model. DREAM runs multiple different chains simultaneously for global exploration and automatically tunes the scale and orientation of the distribution in randomized subspaces during the search for the optimized parameters. The measurement data points were separated to form two sets of data, one for parameter optimization using DREAM, and the other for model testing. The best model was determined based on the statistics of model testing results. The results indicate that this method is useful in determining the appropriate response function for each environmental factor in order to optimize the gc model. For O. fragrans, an exponential function of vapor pressure deficit and root zone water potential, and a parabolic function of air temperature are the most appropriate response functions, whereas no significant difference is observed between different functions of solar radiation. The optimized model shows a significantly improved estimation of the gc of O. fragrans, especially for the drought period. The correlation coefficient and root-mean- square error based on the model testing result were 0.803 and 0.000623 m/s, respectively. The results also suggest that the temperature stress function can be larger than one, a finding that is inconsistent with the conceptual definition of a stress function. Similar findings have been reported in previous studies. This discrepancy is likely attributed to the fact that air temperature and vapor pressure deficit are often strongly interdependent. Thus, to be conceptually consistent, the function of temperature and that of vapor pressure deficit should be combined into one single stress function. Further studies are required to examine if this result applies to other vegetation types globally.