Nurse plant size and biotic stress determine quantity and quality of plant facilitation in oak savannas

Abstract

The nurse plant phenomenon is an important form of facilitative interaction where a “nurse-plant” provides shelter from abiotic or biotic stress to a “beneficiary” plant. However, plant facilitation strongly depends on nurse-plant traits such as size or age. This effect has been mostly attributed to the amelioration of abiotic conditions under larger nurse-plants. However, the effect of nurse-plant size on the overall facilitative process (quantitative and qualitative components) remains largely unexplored, particularly for increasing levels of biotic stress. Here, we investigated the effect of nurse-plant size on the quantitative (recruitment density) and qualitative (recruit shape and growth) components of plant facilitation along two contrasting herbivore-stressed environments. We measured natural oak recruits located under and outside nurse-plants of different size and age.Results show that both components of plant facilitation increased with nurse-plant size but were more pronounced at high biotic stress. Quantitatively, at high biotic stress, facilitation occurred in nurse plants of approximately half the size of those subject to low biotic stress. Interestingly, the qualitative component revealed different results depending on the ontogenetic stage of recruits, with a significant effect of nurse-plant size on large recruits (saplings) but not on small recruits (seedlings). Additionally, at higher biotic stress, more beneficiaries were found further inside the nurse-plant. Similarly, oak recruits located further inside the nurse plant showed greater plant quality. Although nurse-plant age and size were highly correlated, the spatial distribution and quality gradient of recruits suggests that nurse-plant size rather than age enhances plant facilitation in herbivore-dominated environments.We conclude that nurse-plant size plays a crucial role in plant facilitation but its net facilitative effect is strongly mediated by the level of biotic stress and the ontogeny of beneficiaries. We highlight the importance of considering both components (quantity and quality) of plant facilitation to fully understand how plant-plant interactions change at increasing levels of stress.