Herbivory on temperate rainforest seedlings in sun and shade: resistance, tolerance and habitat distribution

Abstract

Differential herbivory and/or differential plant resistance or tolerance in sun and shade environments may influence plant distribution along the light gradient. Embothrium coccineum is one of the few light-demanding tree species in the temperate rainforest of southern South America, and seedlings are frequently attacked by insects and snails. Herbivory may contribute to the exclusion of E. coccineum from the shade if 1) herbivory pressure is greater in the shade, which in turn can result from shade plants being less resistant or from habitat preferences of herbivores, and/or 2) consequences of damage are more detrimental in the shade, i.e., shade plants are less tolerant. We tested this in a field study with naturally established seedlings in treefall gaps (sun) and forest understory (shade) in a temperate rainforest of southern Chile. Seedlings growing in the sun sustained nearly 40% more herbivore damage and displayed half of the specific leaf area than those growing in the shade. A palatability test showed that a generalist snail consumed ten times more leaf area when fed on shade leaves compared to sun leaves, i.e., plant resistance was greater in sun-grown seedlings. Herbivore abundance (total biomass) was two-fold greater in treefall gaps compared to the forest understory. Undamaged seedlings survived better and showed a slightly higher growth rate in the sun. Whereas simulated herbivory in the shade decreased seedling survival and growth by 34% and 19%, respectively, damaged and undamaged seedlings showed similar survival and growth in the sun. Leaf tissue lost to herbivores in the shade appears to be too expensive to replace under the limiting light conditions of forest understory. Following evaluations of herbivore abundance and plant resistance and tolerance in contrasting light environments, we have shown how herbivory on a light-demanding tree species may contribute to its exclusion from shade sites. Thus, in the shaded forest understory, where the seedlings of some tree species are close to their physiological tolerance limit, herbivory could play an important role in plant establishment.

Figure 1. Herbivory pressure and plant resistance in Embothrium coccineum seedlings from contrasting light environments.

A) Index of herbivory, IH (± SE) on seedlings in sun (white bar) and shade environments (gray bar) in a southern temperate rainforest. Means were significantly different (P<0.001; t-test). B) Leaf area consumed (% ± SE) by the generalist snail Helix aspersa in a 48 h no-choice palatability test with leaves from seedlings grown in sun (white bar) and shade (black bar) sites. Means were significantly different (P<0.005; Mann-Whitney U test).

Figure 2. Effects of simulated herbivory on Embothrium coccineum seedlings in sun and shade sites.

Open bars: undamaged seedlings; hatched bars: seedlings subjected to 50% leaf damage. Bars show results 10 months after inflicting damage. A) Seedling survival (proportions, ± SE). B) Seedling relative growth rate, RGR (cm cm⁻¹ day⁻¹, ± SE). Bars sharing a letter are not significantly different (Tukey HSD test).