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Competition for light in heterogeneous canopies: Application of MAESTRA to a coffee (Coffea arabica L.) agroforestry system

Charbonnier F., Le Maire G., Dreyer E., Casanoves F., Christina M., Dauzat J., Eitel J.U.H., Vaast P., Vierling L.A., Roupsard O.. 2013. Agricultural and Forest Meteorology, 181 : p. 152-169.

DOI: 10.1016/j.agrformet.2013.07.010

In agroforestry systems (AFS), quantifying the competition for light is a prerequisite toward understand-ing the impact of shade trees on the productivity of the under-crop. Models for homogeneous canopiesand shade/full-sun approaches do not address the intra-plot heterogeneity, typical of AFS. For the firsttime, MAESTRA, a 3D light absorption model, was fully parameterized in a heterogeneous 2-canopy layersAFS. We quantified competition for photosynthetic photon flux density (Q) between shade trees (Eryth-rina poepiggiana) and coffee (Coffea arabica), with a spatial resolution from the plant to the plot (2.7 ha)and a temporal resolution from half-hour to one full year. The predicted transmittance through the 2-canopy layers was verified against field measurements. The goodness of fit (R2> 0.75, RRMSE < 26%) wascomparable to the predictions from 10 other studies using 3D light models and mostly verified in one-layered systems (mean R2= 0.89 and mean RRMSE = 17%). Maps of absorbed Q showed that despite theirlow density in the plot (5.2 trees ha?1), the tall Erythrina trees reduced Q available for the coffee layer by14% annually. Annual pruning of the oldest unproductive coffee resprouts maintained a large horizontalheterogeneity in coffee LAI, with direct impact on the Q absorption map. This management practice hada strong impact on seasonal variations of absorbed Q by the coffee canopy. We proposed also a simpleapproach to estimate Q absorbed yearly by the coffee plants in AFS of variable tree density, requiring onlyfew measurements in the field. An extrapolation indicated that the amount of Q absorbed by the coffeecanopy would display a negative exponential relationship (k = ?0.34) when increasing shade tree density(from nil to 29 trees ha?1). The estimated k was similar to the shade tree extinction coefficient of diffuseradiation measured with a plant canopy analyzer. We showed that the presence of shade trees tends toreverse the diurnal time course of the fraction of Qawhen compared to a plantation in the open.Overall, MAESTRA proved to successfully unlock the question of intra-plot heterogeneity for lightabsorption and to provide defensible light budgets as a continuous and mapped covariable, a crucialstep for many field experimentations.

Mots-clés : coffea arabica; erythrina poeppigiana; plante d'ombrage; agroforesterie; compétition biologique; variation saisonnière; densité de population; température; lumière; modèle mathématique; costa rica

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