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

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 < 0.001 and P < 0.05 respectively in native mixed-conifer and broad-leaved plantations) and between FRic and low-layer biomass (P < 0.001 in both native mixed-conifer and broad-leaved plantations) in the two native plantations. A positive relationship was also found between the CWM of leaf dry matter content and biomass in the high understory layer in the native mixed-conifer plantation (P < 0.001). Meanwhile, soil N was negatively correlated with low layer biomass (P < 0.01). Our study highlighted the dominant role of overstory attributes and niche complementarity in explaining forest understory productivity compared to other hypotheses. The effects of ecological factors on understory biomass varied in direction and magnitude, depending on understory stratum and plantation type. Indeed, the mechanisms behind biomass production in the high and low understory layers appeared to be decoupled. The typical biodiversity-ecosystem functioning relationship only drove low-layer biomass, while high-layer biomass was influenced by canopy cover.