Although drylands cover >40% of the land surface, models of ecosystem gross primary productivity (GPP) generally have been designed for mesic temperate ecosystems. Arguably, GPP models often lack a good representation of vegetation phenology, particularly not estimating the ecosystem effects of the prolonged foliage senescence which may be common in drylands. To estimate daily GPP for a water‐limited Mediterranean shrubland, we propose a simple framework (GPP_mod) using light use efficiency, a spectral vegetation index derived from digital cameras, and five meteorological variables, including an index of functional senescence of foliage (i.e., heat degree‐days). We tested the model with different combinations of meteorological variables but without senescence, using 1 year's data. The best formulation showed good agreement with GPP derived from eddy covariance (GPP_EC; r² = 0.53, RMSE = 0.77). However, including the foliage senescence parameter significantly improved model performance (r² = 0.74, RMSE = 0.49), especially during the fall season. In the following year, we validated the parameters: The overall GPP_mod and GPP_EC comparison yielded an r² = 0.78. We postulate that models that mainly rely on meteorological variables or greenness indices could yield an overestimation of annual GPP between 24% and 90%, while models including the foliage senescence parameter reduced that bias by 10% to 34%. Our results highlight the importance of incorporating the phenology of foliage senescence in models regarding productivity in drylands or dry sclerophyll ecosystems.

中文翻译：

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叶子衰老作为建模地中海灌木丛总初级生产力的关键参数

尽管旱地覆盖了40％以上的陆地表面，但生态系统总初级生产力（GPP）模型通常是为中等温带生态系统设计的。可以说，GPP模型通常缺乏对植物物候的良好表述，尤其是不能估计干旱地区常见的延长叶片衰老对生态系统的影响。为了估算一个缺水的地中海灌丛的每日GPP，我们提出了一个简单的框架（GPP _mod）利用光的利用效率，从数码相机获得的光谱植被指数以及五个气象变量，包括树叶的功能衰老指数（即高温天数）。我们使用1年的数据对具有不同气象变量组合但没有衰老的模型进行了测试。最佳公式显示出与涡流协方差衍生的GPP良好的一致性（GPP _EC；r ²  = 0.53，RMSE = 0.77）。但是，包括树叶衰老参数可以显着改善模型性能（r ²  = 0.74，RMSE = 0.49），尤其是在秋季。在第二年，我们验证了参数：总体GPP_模式与GPP _EC比较得出r ²  = 0.78。我们假设主要依赖于气象变量或绿色指数的模型可能会导致年度GPP的高估在24％至90％之间，而包括叶子衰老参数的模型将这一偏差减少了10％至34％。我们的结果凸显了在有关干旱地区或干燥硬叶生态系统生产力的模型中纳入叶子衰老物候的重要性。