Effects of biotic factors on emergence and survival of Quercus emoryi at lower treeline,Arizona, U.S.A.

Abstract

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 grassland. We established a large-scale field experiment at this boundary in southern Arizona, U.S.A. Emergence of Q. emoryi seedlings occurred only during years in which supplemental water was added to experimental plots (1993, 1995), and emergence was lower in grassland than within adjacent intact woodland or cleared woodland. Seedling survival of both cohorts was reduced by herbaceous interference; survival of the 1993 cohort was greater in cleared woodland than in intact woodland or grassland, and survival of the 1995 cohort was reduced by vertebrate herbivory. Seedling mortality was almost always attributed to desiccation. Biotic factors (e.g., herbaceous interference, vertebrate herbivory) can reduce seedling emergence and survival; however, their effects become important only after abiotic constraints are overcome. Thus, we conclude that abiotic factors exert primary control in structuring the boundary of these communities. In addition, our research supports the interpretation that the lower treeline in this system is currently static.