Overstory and niche attributes drive understory biomass production in three types of subtropical plantations

Abstract Understory vegetation plays a very important role in maintaining biodiversity, soil nutrient cycling and carbon stocks, yet biomass productivity at the understory layer has been largely overlooked in previous studies. Our study provides one of the most comprehensive tests of how niche attributes, soil fertility and canopy attributes shape understory biomass in a forest restoration context. We investigated the relative effects on understory biomass of five categories of ecological variables related to five hypotheses – i) niche complementarity: functional diversity indices of understory plants, ii) mass ratio: community weighted mean (CWM) traits of understory plants, iii) overstory attributes: planted tree diameter, basal area, height, canopy cover and stem density, iv) soil nutrients: soil pH and soil C, N, P, and v) plantation type (an exotic broad-leaved monoculture, a native broad-leaved monoculture and a native conifer mix). Here, understory biomass was studied at two understory layers: high (2 to 7 m above ground) and low (less than 2 m above ground) in the plantations of subtropical area of Southern China. We surveyed vegetation, measured soil nutrients and determined five important leaf functional traits for each species. Multiple linear regression models were performed to select the best models and to assess the statistical significance of the effects of each variable. Our results showed that the best models for predicting biomass in the high (mean = 603.29 g m−2) and low (mean = 596.81 g m−2) layers were, respectively, the models for canopy cover and functional richness (FRic) interacting with plantation type. Model analyses showed a positive relationship between canopy cover and high-layer biomass (P