Defoliation intensity and leaf area index recovery in defoliated swards: implications for forage accumulation

ABSTRACT: This study evaluated the leaf area index (LAI) recovery mechanisms and forage accumulation rates on the regrowth of different grass species subjected to different defoliation intensities. For that purpose, plots of Pennisetum clandestinum (kikuyugrass), Lolium multiflorum (annual ryegrass), and Avena strigosa (black oats) were defoliated from 20 to 80 % of their initial heights (25, 20, and 25 cm, respectively). At different increments in height, forage samples were collected to ground level and used to estimate tiller population density (TPD), leaf area per tiller (LA), and forage mass. From these data, we calculated the leaf area index (LAI), average leaf area index (aLAI), and average and instantaneous forage accumulation rate (FAR and IFAR, respectively). Data were plotted over time (days) to describe LAI recovery and forage accumulation rates. As the defoliation intensity increased, greater canopy heights were needed for pastures to achieve their maximum forage accumulation rates, which required longer regrowth intervals. The need for high tiller recruitment after defoliation, which delayed canopy LAI recovery, seemed to be one of the main cause. Thus, grazing management strategies that involve costly tiller recruitment could decrease both overall forage production and sward persistence over time. However, the plant ability to recover LAI after successive intense defoliations seems to be species-dependent and related to their phenotypic plasticity.

摘要：本研究针对不同刈割强度下多种禾本科植物的再生过程，评估了其叶面积指数 (leaf area index) 恢复机制与牧草累积速率。为此，我们对铺地狼尾草（Pennisetum clandestinum，kikuyugrass）、多花黑麦草（Lolium multiflorum，annual ryegrass）以及野燕麦（Avena strigosa，black oats）的样地开展刈割处理，刈割强度为植株初始高度的20%至80%（三者初始高度分别为25 cm、20 cm与25 cm）。在植株高度增长的不同梯度阶段，我们采集齐地面的牧草样本，以此估算分蘖种群密度 (tiller population density, TPD)、单分蘖叶面积 (leaf area per tiller, LA) 与牧草生物量。基于上述数据，我们计算得到叶面积指数 (LAI)、平均叶面积指数 (aLAI)，以及平均牧草累积速率 (FAR) 与瞬时牧草累积速率 (IFAR)。我们将数据按时间（天）维度绘图，以描述LAI恢复过程与牧草累积速率变化规律。随着刈割强度提升，牧草群落达到最大牧草累积速率所需的冠层高度更高，对应需要更长的再生间隔期。刈割后需大量萌生新分蘖，这一过程延缓了冠层LAI的恢复，似乎是核心成因之一。因此，依赖高成本分蘖萌生的放牧管理策略，会随时间推移降低整体牧草产量与草地持久性。不过，植物在连续高强度刈割后恢复LAI的能力具有物种特异性，且与其表型可塑性 (phenotypic plasticity) 相关。