Foliage Senescence As A Key Parameter For Modeling Gross Primary Productivity In A Mediterranean Shrubland

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); r(2) = 0.53, RMSE = 0.77). However, including the foliage senescence parameter significantly improved model performance (r(2) = 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 r(2) = 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.