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Tree canopy cover constrains the fertility–diversity relationship in plant communities of the southeastern USA

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The goal of elucidating the primary mechanisms constraining the assembly and distribution of biodiversity remains among the central unresolved challenges facing the field of ecology. Simulation studies and experimental manipulations have focused on how patterns in community assembly result from bivariate relationships along productivity or environmental gradients. However, the joint influence of multiple resource gradients on the distribution of species richness in natural communities remains under‐studied. Using data from a large network of multi‐scale vegetation plots across forests and woodlands of the southeastern US, we find significant evidence for the scale dependent, joint constraints of forest structure and soil resources on the distribution of vascular plant species richness. In addition to their significant partial effects on species richness, understory light levels and soil fertility positively interact, suggesting a trade‐off between the two limiting resources with species richness peaking both in high‐light, low‐fertility conditions as well as low‐light, high‐fertility settings. This finding provides a novel perspective on the biodiversity‐productivity relationship that suggests a transition in limiting resources from soil nutrients to light availability when enhanced productivity results in reduced light resources for subordinate individuals. Results likewise have meaningful implications for our understanding of scale dependent community assembly processes as size‐asymmetric competition replaces environmental filtering as the primary assembly mechanism structuring temperate forest communities along an increasing soil fertility gradient.
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... Along an elevation gradient, closely representing the beech productivity gradient, conditions sub-optimal for beech are characterised by higher species richness than those close to the optimum. Indeed, the mechanisms driving plant richness have been linked to ecosystem productivity in this way (Hakkenberg et al., 2020). Significantly, this pattern could be driven by not only above-ground mechanisms such as canopy closure and litter mass, but also below-ground competition for soil resources (Mölder et al., 2008). ...
... In addition, C/N and N were also related to species richness in our study. Similarly to other studies, our species richness is higher at lower C/N values, typical for locations with higher soil nutrient availability (Hakkenberg et al., 2020). Poor understorey species richness is usually found at both ends of the fertility gradient; more acidic soil reaction may hamper litter decomposition and nutrient release (Staude et al., 2020). ...
... Poor understorey species richness is usually found at both ends of the fertility gradient; more acidic soil reaction may hamper litter decomposition and nutrient release (Staude et al., 2020). Our results reveal an increasingly negative impact of high C/ N ratio on species richness, where low nutrient availability is exacerbated by competitive pressure of fine roots and symbionts associated with beech (Hakkenberg et al., 2020). We found that fine root mass best predicted the understorey species richness in mature stands. ...
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... Piedmont forest composition and structure have been greatly affected by a century of chronic fire suppression and exclusion, and therefore even modern forests with relatively few human impacts do not necessarily reflect historical vegetation patterns. In areas least altered by humans, most of the present vegetation is closed-canopy forest with limited shrub and herb layers owing to the infertility of the soils (Hakkenberg et al. 2020). The canopy is dominated by several oak species and a scattering of representatives of other genera that vary in abundance with edaphic and moisture characteristics. ...
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The Piedmont (PDMT) ecoregion of the USA stretches from New Jersey to Alabama, nestled between the Coastal Plain and Blue Ridge Mountain physiographic provinces. Many of the notable Piedmont plant communities, including the dominant oak-hickory forests of the region, are reliant upon fire to some degree. Before human settlement, most Piedmont vegetation burned relatively frequently and at low intensities, resulting in extensive closed canopy oak-hickory forests, studded with patches of open woodland and savanna largely defined by unusual soil conditions. Indigenous peoples of the Piedmont used fire as a land management tool for both agriculture and game production. Historical changes in land use throughout the region have altered fire regimes and changed forest dynamics dramatically over the past 400 years. Euro-American settlement led to widespread clearing of land for agriculture and logging; by the early twentieth century, very little old-growth forest remained in the Piedmont. During the mid-twentieth century, the decline of agriculture and the aggressive suppression and exclusion of wildfires brought about the growth of successional forests in the place of older, fire-mediated communities. The Piedmont region is currently experiencing a rapid expansion of the human population and land development, making restoration of the historical fire regime a challenge. However, land managers frequently do use prescribed fire to enhance timberland and restore rare plant communities.
... Although structure exerts primary controls on understorey light levels, canopy density and spacing likewise affect root competition for spatially heterogeneous water and nutrient resources (Hakkenberg et al., 2020) and indirectly affect microclimate and soil water availability by constraining ground-level solar radiation (Chen et al., 1999). ...
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