Microclimate and seeding predation as drivers of tree recruitment in human-disturbed oak forests

Abstract

Forest conservation and restoration are global priorities because of the wide variety of environmental services that these ecosystems provide, but achieving these goals depends on our knowledge about the factors driving tree recruitment. Tree establishment within forests is promoted by favorable microclimatic conditions, but seedlings in this habitat can be subjected to elevated consumer pressure by vertebrates. Conversely, while predation is usually reduced in neighboring human matrices, severe microclimatic conditions can reduce seedling survival in these habitats. Unfavorable microclimatic conditions can be ameliorated at forest edges and this may increase tree establishment rates, as compared to those in human matrices. However, seedlings at forest edges are exposed to predators from both habitats and consumer pressure may be even higher than within the forest. Thus, tree establishment in human-disturbed forests seems to depend on tradeoffs between microclimate and predation. This study focused on these issues in an oak forest from Mexico where extensive forested areas have been replaced by plantations of the exotic tree Eucalyptus camaldulensis. For this, two experiments were conducted by transplanting oak seedlings within the forest, at the forest edge and in a neighboring plantation. Temperature, air humidity and light intensity were measured in all habitats during the experiments. The first experiment assessed the effect of microclimate on seedling survival and, therefore, transplants were protected with wire cages to prevent predation by vertebrates. Survival of seedlings was monitored during a year and their physiological responses to microclimate were assessed by measuring their photosynthetic efficiency. In the second experiment, seedlings were not protected from herbivores and these survival data were used to discriminate between the effects of microclimate and predation. Temperature and light intensity decreased from the Eucalyptus plantation toward the forest interior, while relative humidity increased in that direction. The physiological performance and survival rates of protected seedlings increased from the plantation toward the forest, suggesting that local microclimate largely influences oak recruitment across these habitat types. Nevertheless, predation rates also increased from the plantation to the forest, suggesting that seedling consumers strongly reduce oak recruitment both within the forest and its edge. These results indicate that the relative importance of physical stress and seedling predation vary across the environmental gradients that compose human-disturbed forests. Thus, restoration and conservation actions should implement specific protection mechanisms addressed to reduce the impact of the main sources of seedling mortality at each particular habitat of these landscapes.