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