Facilitation of conspecific seedling recruitment and shifts in temperate savanna ecotones

In contrast to documented increases in woody plant dominance of savannas an d grasslands of North America, oak (Quercus L.) savannas that form lower tr ee lines in the southwestern United States and northwestern Mexico have bee n stable over the last several centuries. We sought to identify potential b iotic and abiotic constraints on seedling recruitment of Quercus emoryi wit hin the context of potential shifts in lower tree line. We used held survey s to describe seedling distribution at and below lower tree line, and to de termine the potential. for acorn dispersal from lower tree line into adjace nt grassland. Field and greenhouse experiments were used to test explanator y hypotheses generated by descriptive surveys. Q. emoryi seedlings were located almost exclusively beneath mature, conspec ific tree canopies within the woodland and savanna and were absent from adj acent semidesert grassland in 1993 and 1995. Seed bank surveys indicated th at acorns were concentrated beneath tree canopies and were dispersed into a djacent grassland in low numbers. Although soil N, C, and P were about two times greater beneath trees than in adjacent grassland, experimental nutrie nt amendments to subcanopy and grassland soils indicated that soil nutrient s did not limit Q. emoryi growth. Reciprocal transfers of subcanopy and gra ssland soil to subcanopy and grassland microsites indicated that microsite was more important than soil source for seedling growth. Overstory shade wa s important at all stages of seedling development investigated: the provisi on of artificial or natural shade increased rates of seedling emergence and subsequent survival as much as 19-fold and increased recruitment rates bet ween 30- and 60-fold. We conclude that rates of Q. emoryi recruitment within grasslands below tre e line are relatively low and are constrained by low rates of seed dispersa l coupled with a low probability of seedling emergence. In contrast, large numbers of acorns are dispersed directly beneath Q. emoryi trees, where the y have a higher probability of emergence than in adjacent grassland. Surviv al rates of emerged seedlings were low, regardless of landscape position. T hus, observed patterns of seedling distribution on the landscape resulted f rom interactions between seed dispersal and habitat-specific response of se edlings 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 stab ilize the woodland-grassland ecotone both spatially and temporally, consist ent with Wilson and Agnew's one-sided positive feedback switch. Although th is switch would not produce an indefinitely stable vegetation mosaic, upslo pe or downslope shifts in lower tree line are apparently resistant to decad al or even century-scale climatic perturbation. The observed shift in tree line in the last millennium was less likely the result of slow, spatial pro gression of autogenic safe sites than the result of episodic and infrequent allogenic processes that simulated or negated the importance of conspecifi c, biogenic safe sites.