Facilitation of Conspecific Seedling Recruitment and Shifts in Temperate Savanna Ecotones

Abstract

In contrast to documented increases in woody plant dominance of savannas and grasslands of North America, oak (Quercus L.) savannas that form lower tree lines in the southwestern United States and northwestern Mexico have been stable over the last several centuries. We sought to identify potential biotic and abiotic constraints on seedling recruitment of Quercus emoryi within the context of potential shifts in lower tree line. We used field surveys to describe seedling distribution at and below lower tree line, and to determine the potential for acorn dispersal from lower tree line into adjacent grassland. Field and greenhouse experiments were used to test explanatory hypotheses generated by descriptive surveys. Q. emoryi seedlings were located almost exclusively beneath mature, conspecific tree canopies within the woodland and savanna and were absent from adjacent semidesert grassland in 1993 and 1995. Seed bank surveys indicated that acorns were concentrated beneath tree canopies and were dispersed into adjacent grassland in low numbers. Although soil N, C, and P were about two times greater beneath trees than in adjacent grassland, experimental nutrient amendments to subcanopy and grassland soils indicated that soil nutrients did not limit Q. emoryi growth. Reciprocal transfers of subcanopy and grassland soil to subcanopy and grassland microsites indicated that microsite was more important than soil source for seedling growth. Overstory shade was important at all stages of seedling development investigated: the provision of artificial or natural shade increased rates of seedling emergence and subsequent survival as much as 19-fold and increased recruitment rates between 30- and 60-fold. We conclude that rates of Q. emoryi recruitment within grasslands below tree line are relatively low and are constrained by low rates of seed dispersal coupled with a low probability of seedling emergence. In contrast, large numbers of acorns are dispersed directly beneath Q. emoryi trees, where they have a higher probability of emergence than in adjacent grassland. Survival rates of emerged seedlings were low, regardless of landscape position. Thus, observed patterns of seedling distribution on the landscape resulted from interactions between seed dispersal and habitat-specific response of seedlings to environmental variation. Results of this and complementary research suggest that the lower tree line in southern Arizona is stabilized by self-enhancing feedback mechanisms of overstory shade, seed dispersal, and seedling establishment, coupled with strong abiotic constraints beyond the current ecotone. These processes stabilize the woodland–grassland ecotone both spatially and temporally, consistent with Wilson and Agnew's one-sided positive feedback switch. Although this switch would not produce an indefinitely stable vegetation mosaic, upslope or downslope shifts in lower tree line are apparently resistant to decadal or even century-scale climatic perturbation. The observed shift in tree line in the last millennium was less likely the result of slow, spatial progression of autogenic safe sites than the result of episodic and infrequent allogenic processes that simulated or negated the importance of conspecific, biogenic safe sites.