Category: Ecological Research
Title: Abiotic and biotic factors explain independent gradients of plant community composition in ponderosa pine forests
Author: * Laughlin, D.C. , * Abella, S.R.
Subject: Modeling, Nutrient cycling, Disturbance
Abstract: General multivariate models have appeal when analyzing ecological systems because they have the potential to shed light on the relative importance of multifaceted factors. We hypothesized that plant community composition is a function of the interactions among threegeneralconstructs:abioticand bioticfactors anddisturbance history. We sampled vegetation and soils on 75 randomly located 0.05 ha plots across a broad soil gradient within a 110,000 ha ponderosapineforest landscapein northernArizonato evaluatethis hypothesis. The variation in abundances of 271 understory plant species was reduced to two orthogonal axes using non-metric multidimensional scaling. We evaluated the relationships between ordination results and environmental conditions using two types of structural equation models. 'Specific' models provided a detailed perspective of hypothesized relationshipsamongobservedvariables.A'general' modelwasevaluated to address thegeneral hypothesis stated above. The general model employed composite variables, which specify the combined effects of multiple factors on a response, in order to evaluate the relative importance of the three theoretical constructs. The results of the final specific model suggested that organic mailer and nitrogen explained unique variation in composition, but the 'total effects' of organic matter were not significant due to offsetting direct and indirect pathways. Soil texture and pH also constrained where m any plant species occurred. Elevation was a strong determinant of species distributions, and its effects could be propagated through a network of processes involving soil mineral and organic properties. Liller depth explained unique variation in community composition,suggesting thatplantspeciesdifferin theirabilitytocolonizesiteswithdifferent litter depths independent of the density of pine trees. Composition also differed under varying densities of pine trees since species vary in their tolerance of a competitive overstory. Finally, heavy grazing tended to favor the abundance of non-palatable species in the community. The results of the general model provided new insight into the underlying structu re of independent plant community gradients in ponderosa pine forests. The model suggested that abiotic factors generated one gradient in community composition, whereas biotic fac tors, and to a lesser degree, disturbance history, generated a second independent gradient in communitycomposition.Futureevaluationsofthiscomposition model across a diversity of ecosystems could determine the generality and utility of this multivariate hypothesis.
Source: Ecological Modelling
Publisher: Elsevier Science B.V