Effects of biotic factors on emergence and survival of Quercus emoryi at lower treeline, Arizona, USA

Woody plants have increased in abundance in many of the world's grasslands during the last three centuries. In contrast, Quercus emoryi Torr. has not encroached into adjacent grasslands for several centuries. The objective of this study was to identify potential constraints on emergence and survival of Quercus emoryi at the boundary between oak woodland and semi-desert gra ssland. We established a large-scale field experiment at this boundary in s outhern Arizona, U.S.A. Emergence of Q. emoryi seedlings occurred only duri ng years in which supplemental water was added to experimental plots (1993, 1995), and emergence was lower in grassland than within adjacent intact wo odland or cleared woodland. Seedling survival of both cohorts was reduced b y herbaceous interference; survival of the 1993 cohort was greater in clear ed woodland than in intact woodland or grassland, and survival of the 1995 cohort was reduced by vertebrate herbivory. Seedling mortality was almost a lways attributed to desiccation. Biotic factors (e.g., herbaceous interfere nce, vertebrate herbivory) can reduce seedling emergence and survival; howe ver, their effects become important only after abiotic constraints are over come. Thus, we conclude that abiotic factors exert primary control in struc turing the boundary of these communities. In addition, our research support s the interpretation that the lower treeline in this system is currently st atic.