Publication

Processes of natural regeneration in highly fragmented landscapes are strongly influenced by edge environments due to the effects of vegetation cover on microclimate conditions. These effects may be diminished or exacerbated through synergistic interactions with extreme climatic events such as drought. This study assessed the growth and physiological responses of five native tree and shrub species (Quercus acutifolia Nee, Q. castanea Nee, Pinus oaxacana (Mart.) Mirov. Dodonea viscosa (L.) Jacq., Rhus virens Adams) to microsites along edge gradients occurring between forest remnants and openings of different sizes (<0.2 and >1 ha) in both a highly fragmented and a relatively undisturbed adjacent landscape over a 2-year period (1 year with normal rainfall and one with extreme drought). Of the species studied, the shrub seedlings had significantly lower mortality (3-13%) than the oak (37-53%) and pine (35%). Seedling survival for all species was greatest in the edge and forest understory environments, suggesting that vegetation cover may have a facilitative effect on seedling regeneration. Greater survivorship and biomass of the shrub seedlings, especially in the open microsites, was attributed to their root system having a sufficient biomass and depth to more effectively access deeper water supplies. Interactions between edge effects and an extreme drought event were found to have a more pronounced impact on the performance of oak and pine seedlings compared to shrub seedlings. Patterns of pre-dawn water potential (psi(pd) < -3.8) and leaf phenology (slow turnover) indicated that oak species experienced high water stress that prevented their successful establishment in open environments, especially during drought years. In contrast, shrub seedlings, and to a lesser extent pine seedlings, maintained higher pre-dawn moisture potentials (psi(pd) > -2.9) and greater phenological plasticity (fast turnover) in response to drought, contributing to greater growth and survival success. To effectively manage fragmented tropical highland oak forest landscapes for reestablishment of the native oak ecosystem, there is a need to understand the complex and often synergistic interactions between different plant species, the edge environment, and climatic variability that strongly influence regeneration processes. (C) 2004 Elsevier B.V. All rights reserved.