Decay of ecosystem differences and decoupling of tree community–soil environment relationships at ecotones

Abstract

Ecotones are important landscape features where there is a transition between adjoining ecosystems. However, there are few generalized hypotheses about the response of communities to ecotones, except for a proposed increase in species richness that receives varying empirical support. Based on the assumption that transport of abiotic material and dispersal of organism propagules across ecotones are independent processes, we propose the new hypothesis that ecotones decouple community-environment relationships, increasing the importance of spatial structure that is independent of the environment. We tested this hypothesis by examining the effects of ecotones on relationships between trees and soil properties in a temperate deciduous forest. The study area included different landforms defined by topography, hydrology, and geomorphology, which we designated upland, bottomland, and riparian forests. The site also included a mowed herbaceous corridor. We found that soil properties and tree community composition significantly differed among landforms, and thus they could be treated as differing ecosystem types. However, inclusion of plots near ecotones significantly reduced the variance explained by landform due to introduction of increased noise, increased similarity of ecotone plots in different landforms, or both. To examine tree community-soil environment relationships, factorial kriging analysis was used to decompose variation in soil properties into structures associated with differing spatial scales, which were then used as predictors of tree composition using redundancy analysis. In agreement with the ecotone-decoupling hypothesis, we found that ecotones introduced significant unexplained variation into correlations between tree community composition and soil properties. In addition, spatial variation in tree community composition that was independent of soil properties was only detected when ecotones were included in the analysis, and little variation in tree community composition was attributed to small-scale soil property structures. Together, these results indicate that dispersal limitation and mass effects in the tree community take on increased importance near ecotones. We found no consistent changes in tree species richness associated with ecotones, and we suggest that the ecotonedecoupling hypothesis may correspond with a more general community-level pattern that warrants further testing. Decoupling of community-environment relationships near ecotones also has important implications for accuracy of models predicting community distributions from abiotic information.

Aggregated and complementary: symmetric proliferation, overyielding, and mass effects explain fine‐root biomass in soil patches in a diverse temperate deciduous forest landscape

“…Acer saccharum Marsh., Acer rubrum L., Fagus grandifolia Ehrh., and Quercus rubra L. encompass nearly 60% of the forest tree cover, with 11 other species accounting for most of the remaining basal area (Blackwood et al ., ). The property includes three main forest ecosystems and additional smaller scale variation in soil characteristics and tree communities (Blackwood et al ., ). A riparian forest with fine‐loamy Inceptisols dominates areas within 80 m of the West Branch of the Mahoning River.…”

“…To test for decoupling between biotic and abiotic factors across ecosystem transitions (H2), we used the randomization test described in Blackwood et al . (). In brief, we classified plots and soil samples as within the ecosystem ‘core’ area if they were located > 30 m from a topographically defined ecosystem boundary, and within an ‘ecotone’ area if they were located within 30 m of the boundary.…”

“…Measurement of aboveground species abundance and soil properties are described in detail elsewhere (Blackwood et al ., ; Valverde‐Barrantes et al ., ). Briefly, five soil cores (1.5 cm diameter; 10 cm depth) were collected and pooled from each of the 93 plots.…”

“…Finally, the influence of tree diversity and soil conditions on FRB may vary among ecosystems, as well as due to other features of natural landscapes that can have an effect on mechanisms of community assembly. In particular, transitional areas between ecosystems, called ecotones (Cadenasso et al ., ), have been found to decouple community composition from the environment as a result of independent transport of propagules and abiotic material (Blackwood et al ., ). Dispersal of propagules from an optimal environment across an ecotone can result in ‘mass effects’, in which propagules establish under nonoptimal conditions, but reduced reproductive output limits further spread in the nonoptimal environment.…”

Is β‐diversity of Amazonian ant and dung beetles communities elevated at rainforest edges?

“…This arises as edge responses, and the distances they penetrate, are typically species (Didham et al, 1998) and edge specific (Feer, 2008), meaning that the community-level diversity response is determined by the relative distribution of edgesensitive to edge-tolerant species. For example, if no change in species richness is observed at an edge, this may be that there has been no loss of interior species, but the same pattern may also arise as the loss of an edge-avoiding species is offset by the arrival of an edge specialist (Blackwood, Smemo, Kershner, Feinstein, & Valverde-Barrantes, 2013).…”