Interactions with soil fungi alter density dependence and neighborhood effects in a locally abundant dipterocarp species

Abstract

Seedling recruitment can be strongly affected by the composition of nearby plant species. At the neighborhood scale (on the order of tens of meters), adult conspecifics can modify soil chemistry and the presence of host microbes (pathogens and mutualists) across their combined canopy area or rooting zones. At local or small spatial scales (on the order of one to few meters), conspecific seed or seedling density can influence the strength of intraspecific light and resource competition and also modify the density-dependent spread of natural enemies such as pathogens or invertebrate predators. Intrinsic correlation between proximity to adult conspecifics (i.e., recruitment neighborhood) and local seedling density, arising from dispersal, makes it difficult to separate the independent and interactive factors that contribute to recruitment success. Here, we present a field experiment in which we manipulated both the recruitment neighborhood and seedling density to explore how they interact to influence the growth and survival of Dryobalanops aromatica, a dominant ectomycorrhizal tree species in a Bornean tropical rainforest. First, we found that both local seedling density and recruitment neighborhood had effects on performance of D. aromatica seedlings, though the nature of these impacts varied between growth and survival. Second, we did not find strong evidence that the effect of density on seedling survival is dependent on the presence of conspecific adult trees. However, accumulation of mutualistic fungi beneath conspecifics adults does facilitate establishment of D. aromatica seedlings. In total, our results suggest that recruitment near adult conspecifics was not associated with a performance cost and may have weakly benefitted recruiting seedlings. Positive effects of conspecifics may be a factor facilitating the regional hyperabundance of this species. Synthesis: Our results provide support for the idea that dominant species in diverse forests may escape the localized recruitment suppression that limits abundance in rarer species.

Keywords: Borneo; Janzen–Connell; Lambir Hills; coexistence; ectomycorrhiza; feedbacks; fungi; natural enemies; oomycete; pathogen; population.

FIGURE 1

Kaplan–Meier (KM) estimated survivorship curves across treatment groups over the course of the experiment. KM curves are a univariate analysis and do not reflect interactions among main treatments. Nonparametric log‐rank tests indicate significant differences between or among the plotted curves in each panel. The main treatment groups presented are (a) seedling cohorts at six different initial densities, (b) seedling cohorts in conspecific or heterospecific recruitment neighborhoods, and (c) seedling cohorts in control or fungicide groups. Dashed horizontal line indicates 50% seedling mortality

FIGURE 2

Fungicide moderates the effect of initial seedling density on survival but not growth. (a) Mortality risk increases with initial seedling density in control but not fungicide plots. Trend lines show model prediction estimated from beta‐binomial GLMM model. Point size indicates the number of seedlings per plot (4–25). The Initial density × Fungicide interaction is significant at p < .01, and we present estimated p values for individual trends within the interaction derived from simple slopes analysis indicating whether or not the slopes differ significantly from zero. (b) Initial seedling density reduces seedling growth response. The main effect of Initial density is marginally significant (p = .062). Because the Initial density × Fungicide interaction was not significant (p = .94), no further testing of individual slopes was applied

FIGURE 3

Effects of recruitment neighborhood and fungicide application on seedling mortality and growth response. Boxes indicate the median and interquartile ranges (IQR) of percent survival among plots (a) and growth response among individual seedlings (b), with whiskers extending to data points within 1.5*IQR. White triangles indicate group means. (a) The effect of fungicide effect on seedling mortality depends on recruitment neighborhood. Fungicide × Neighborhood interaction p < .01 in beta‐binomial GLMM. (b) Fungicide consistently reduces growth across neighborhoods. Main effect of Fungicide is marginally significant in mixed model ANCOVA (p = .051), while effects of Neighborhood and the Neighborhood × Fungicide interaction are not (p = .43, p > .99, respectively)