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.

中文翻译：

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资源变化，落叶和时间尺度变化的混合牧场中辐射利用效率的变化

牧草生产力是入射辐射，冠层吸收的辐射比例（fAPAR）和辐射利用效率（RUE）之间的乘积。可以通过遥感估算草料生产力的程度取决于应力和落叶对fAPAR和RUE的相对影响。在混合牧场中，我们确定了资源可用性和落叶对牧草生产力，fAPAR和RUE的相对影响，并评估了时间尺度对胁迫条件下fAPAR和RUE的影响。我们在温带混合牧场中进行了为期一年的实验，该牧场经历了不同的资源可利用性处理和脱叶管理。我们用不同的时间窗口测量了RUE和fAPAR。与fAPAR相比，RUE对资源可用性和脱叶强度的变化反应较慢。然而，在水分胁迫下，RUE略有降低，而在严重的落叶下，RUE升高。此外，再生期间和治疗之间的RUE变异性取决于观察的时间尺度：12天期间RUE的变异性大于45天或90天时期。我们的结果加强了fAPAR的价值，作为由于资源可用性和管理方面的变化而导致的牧草生产力变化的解释变量。此外，观察的时间尺度会影响RUE的可变性。因此，前提是时间尺度足够粗，基于fAPAR遥感的监测系统可以捕获大多数草料生产力的变化。但是，RUE和fAPAR对脱叶的对比响应表明，当比较具有对比脱叶机制的情况时，这种系统可能存在弱点。