Variability of radiation use efficiency in mixed pastures under varying resource availability, defoliation and time scale

Abstract

Forage productivity is the product between incident radiation, the fraction of radiation absorbed by the canopy (fAPAR) and radiation use efficiency (RUE). The extent to which forage productivity may be estimated through remote sensing depends on the relative effects of stress and defoliation on fAPAR and RUE. In a mixed pasture, we determined the relative impact of resource availability and defoliation on forage productivity, fAPAR and RUE, and we evaluated the impact of time scale on fAPAR and RUE under stress conditions. We conducted a one-year experiment in a temperate-mixed pasture subjected to different resource availability treatments and defoliation management. We measured RUE and fAPAR with different temporal windows. RUE was less responsive than fAPAR to changes in resource availability and defoliation intensity. However, RUE slightly decreased under water stress and increased under severe defoliation. Additionally, RUE variability during regrowth and among treatments depended on the temporal scale of observation: RUE was more variable for 12-day periods than 45- or 90-day periods. Our results reinforce the value of fAPAR as an explanatory variable of the variations in forage productivity due to changes in resource availability and management. In addition, the temporal scale of observation affects RUE variability. Thus, most variations of forage productivity may be captured by monitoring systems based on remote sensing of fAPAR provided that the time scale is coarse enough. However, the contrasting response of RUE and fAPAR to defoliation indicates a potential weakness of such a system when situations with contrasting defoliation regime are compared.