Benefit of shading by nurse plant does not change along a stress gradient in a coastal dune

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.

Figure 1. Three levels of the neighbor factor implemented to investigate if shading is the mechanism that causes adults of Guapira opposita to provide a nurse effect on survival of Ternstroemia brasiliensis seedlings in coastal dune vegetation in São Paulo, Brazil.

A) beneath a G. opposita canopy; B) beneath artificial shade provided by a mimic of G. opposita shade; and C) control with no neighbor and no shade. In the three levels of neighbor, it is possible to see the seedlings of Ternstroemia brasiliensis with their metallic tags. All other plants in a 0.5 m radius were regularly clipped.

Figure 2. Odds ratios (95% confidence intervals) of seedling survival of Ternstroemia brasiliensis after 358 days.

The odds of seedling survival beneath Guapira opposita, artificial shade and seashore proximity II and III was compared to seedling survival in the reference condition (control with no neighbor in the seashore proximity I, the closest proximity to the seashore). An odds ratio of 1 indicated no difference between odds of seedling survival in the treatment and in the reference; >1 indicated a greater survival odds in the treatment than in the reference. The estimates are based in the selected model (model M2.S in Table 1).

Figure 3. Growth of Ternstroemia brasiliensis seedlings after 358 days in the three levels of the neighbor treatment (mean±95% confidence interval).

The estimates are based in the selected model (model M3.G in Table 1).

Figure 4. A graphical model of the stress gradient hypothesis regarding facilitation mechanism, which shows how a constant facilitation effect can occur along an environmental gradient.

A) A gradient is defined by the monotonic variation in the availability of a resource (gray bars), which increases by a constant amount due to a plant facilitation mechanism (black bars). The dashed line represents the resource limitation threshold: below this line, the resource is limiting, and above this line, the resource is not limiting. B) The relationship between the environmental gradient (as described for graph A) and the benefits provided by the additive facilitation effect. Note that in the region of the gradient where the resource is limiting (levels I and II in graph A), the benefits provided by the facilitator are constant along the environmental gradient (levels I and II in the graph B). However, from the point where the total availability of the resource (from the environment and the facilitator) reaches the resource limitation threshold (gradient level III), further increases in the resource availability along the gradient will reduce the portion of the resource used by the plant that is provided by the facilitator. As a consequence, in this region of the gradient (levels III–V), the benefits provided by the facilitator wane as the stress is reduced.