Abstract
The proximity of adult neighbors often increases the performance of woody seedlings under harsh environmental conditions but this nurse plant effect becomes less intense when abiotic stress is alleviated, as predicted by the stress gradient hypothesis (SGH). Although some studies have tested how the net nurse effect is changed by stress, few studies have tested how the mechanism that drives the facilitative effect of nurse responds to changes in stress. We conducted field experiments in a subtropical coastal dune to test if shading drives the known nurse effect of adults of the tree Guapira opposita on seedling performance of another tree species, Ternstroemia brasiliensis. We transplanted T. brasiliensis seedlings to three neighbor environments: under a G. opposita crown, under artificial shade and without neighbor as a control. Furthermore, assuming that proximity to the seashore correlates with stress intensity, we tested if the potential shade-driven facilitation became less intense as stress decreased. Regardless of the proximity to the seashore, after a year, the survival of T. brasiliensis seedlings was twice as high when the seedlings were under G. opposita or under artificial shade compared to the control, indicating that the nurse effect is driven by shade and that this facilitation mechanism is constant along the stress gradient. However, G. opposita and artificial shade had a negative effect on seedlings growth. Overall, our results showed that the facilitation mechanism behind the nurse effect did not wane as the stress was reduced. Furthermore, in spite of the potential costs in terms of biomass production, our study highlights the potential of nurse plants and artificial shade as techniques to improve the survival of transplanted seedlings used in the restoration of degraded shrubland coastal dunes.
Highlights
Facilitation occurs when the fitness of an individual increases due to the presence of another individual of the same trophic level [1]
The absence of the interaction term between neighbor and proximity in the selected model indicates that the effect of neighbor presence did not change along the environmental gradient, which does not support the predictions of the stress gradient hypothesis
We found that G. opposita trees have a positive net effect on the survival of T. brasiliensis seedlings, which was equivalent to the effect of artificial shade
Summary
Facilitation occurs when the fitness of an individual increases due to the presence of another individual of the same trophic level [1]. The emblematic type of facilitative interaction in plant communities is the nurse plant effect, in which adults improve the performance of young individuals of the same species or other species that grow in their vicinity [4,5]. Some of the potential mechanisms behind the nurse plant effect are amelioration of the microclimatic conditions, increases in nutrients and soil moisture and protection from herbivores [6,7,8,9]. The establishment of stress-sensitive species facilitated by nurse plants has the potential to affect species diversity [10,11] and in some cases the succession of plant communities [8,12,13]. The nurse plant interaction has practical applications; for example, it may be used as a tool to improve the survival of transplanted seedlings in the restoration of degraded areas [5,14]
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