Susceptibility of tree seedlings to herbivores and pathogens is a mechanism of the Janzen-Connell (J-C) effect, which predicts spatial over-dispersion of seedlings around established conspecifics. Counteracting the effects of pathogens, mutualistic soil fungi are similarly expected to have spatially explicit effects on seedling recruitment. It has been proposed that trees exhibit a life-history trade-off such that high growth rate is correlated with low investment in tissue defense against plant enemies, and perhaps lower responsiveness to soil mutualists. Thus, early successional species should be more vulnerable to herbivores and pathogens than late successional species. Because there exists both 1) spatial heterogeneity of plant enemies and mutualists and 2) differing tissue defense strategies among tree species, I expect there to be species-specific differences in spatial patterns of tree recruitment. I hypothesize that early successional species will exhibit over-dispersion of seedlings around conspecific adults, whereas mid or late successional species will reveal random or aggregated patterns of recruitment. I use spatial point pattern analysis (PPA) of mapped forest plots from the Duke Forest (Orange County, North Carolina) to test these hypotheses.
Results/Conclusions
Using global simulation envelopes of the null hypothesis of random labelling, PPA reveals differences in spatial patterns of tree recruitment indicative of the successional status of the tree: Early successional trees exhibit spatial over-dispersion, whereas mid to late successional trees reveal random or aggregated recruitment patterns. Over-dispersion of tree recruitment by certain species may be a mechanism of the maintenance of woody species diversity in temperate forests. But these results, and those of similar research endeavors, have shown that many woody species exhibit random recruitment patterns as well. This poster discusses the possibility that plant mutualists, by counteracting (or overcompensating for) the J-C effect, may explain random or aggregated spatial patterns of recruitment.