Allocation of carbon to growth and defences in plants is affected by resource availability and herbivory, two determinants of woody plant abundance in savannas. Both factors are critical at the sapling stage of woody plants, yet their combined effects on sapling growth and defence have been poorly studied in savannas. Sclerocarya birrea is an iconic keystone species in southern African savannas, widely used by herbivores and humans. We hypothesised that low resource availability or high herbivory intensity would reduce growth and nutritional value for herbivores, but high resource availability or low herbivory intensity would increase growth and nutritional value. Support for our hypothesis would imply that any level of browsing intensity (relative to unbrowsed plants), regardless of resource availability, would potentially hold saplings in a browser trap because of reduced growth or improved nutritional value. We explored responses of S. birrea saplings to five levels of nutrient fertiliser and six levels of simulated browsing intensity. Low fertiliser application rates did not affect growth, but the highest fertiliser rate increased canopy growth. The most severe intensities of simulated browsing negatively affected growth and nutritional value. Our results suggested that either resource limitation throughout the growing season, or extremely intense browsing early in the growing season, is potentially able to retard long‐term growth of saplings and delay recruitment of adults. Although saplings that are severely browsed early in the growing season might not be re‐browsed later in the same season because of their reduced nutritional value, their weakened nitrogen status would be an additional limitation to regrowth. No significant interactions between resource availability and herbivory intensity were detected, which we suggest resulted from responses to resource availability and herbivory operating at different temporal scales. We recommend that future experiments of this type include carefully timed measurements to detect transient responses.

中文翻译：

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严格的模拟食草植物限制了Sclerocarya birrea幼树，无论资源是否充足

碳在植物生长和防御中的分配受资源可利用性和草食性的影响，草食是稀树草原中木本植物丰度的两个决定因素。在木本植物的幼树期，这两个因素都是至关重要的，但是在热带稀树草原上，它们对幼树生长和防御的综合影响研究很少。硬核是南部非洲稀树草原的标志性基石种，被食草动物和人类广泛使用。我们假设资源利用率低或草食强度高会降低草食动物的生长和营养价值，但是资源利用率高或草食强度低会增加草食动物的生长和营养价值。支持我们的假设将意味着，无论资源的可用性如何，任何水平的浏览强度（相对于未浏览植物的浏览强度）都可能由于生长减少或营养价值提高而使树苗陷入浏览器陷阱。我们探索了S. birrea的反应幼树可达到5种营养肥料水平和6种模拟浏览强度水平。施肥量低不会影响生长，但施肥量最高会促进冠层生长。模拟浏览的最剧烈强度会对生长和营养价值产生负面影响。我们的结果表明，要么在整个生长期限制资源使用，要么在生长期早期进行非常密集的浏览，都有可能阻碍幼树的长期生长并延迟成年的招募。尽管由于营养价值降低，在生长季节初期经过严格浏览的幼树可能不会在同一季节后期重新繁殖，但其氮素水平下降将是对再生长的另一种限制。没有发现资源可用性与草食强度之间的显着相互作用，我们建议这是由于对在不同时间尺度上运作的资源可用性和草食的反应所致。我们建议以后的此类实验包括仔细定时的测量，以检测瞬态响应。