DOI: 10.5061/dryad.4b8gthtcn
title: Ericoid mycorrhizal shrubs alter the relationship between tree mycorrhizal dominance and soil carbon and nitrogen

description: 1. Plant-fungal associations strongly influence forest carbon and nitrogen
 cycling. The prevailing framework for understanding these relationships is
 through the relative abundance of arbuscular (AM) versus ectomycorrhizal
 (EcM) trees. Ericoid mycorrhizal (ErM) shrubs are also common in forests
 and interactions between co-occurring ErM shrubs and AM and EcM trees
 could shift soil biogeochemical responses. Here we test hypotheses that
 the effects of ErM shrubs on soil carbon and nitrogen either extend or are
 redundant with those of EcM trees. 2. Using regional vegetation inventory
 data (&gt;3,500 plot observations) we evaluated the frequency,
 richness, and relative abundance of ErM plants in temperate forests in the
 eastern United States and examined their relationship with EcM plant
 cover. We then used surface soil (7 cm) data from 414 plots within a
 single forest to analyze relationships between ErM plant cover, relative
 EcM tree basal area, and soil carbon and nitrogen concentrations while
 accounting for other biogeochemical controls, such as soil moisture. 3. At
 both scales, we found a positive relationship between ErM and EcM plants,
 and the majority of ErM plants were in the shrub layer. Within the forest
 site, ErM plants strongly modulated tree mycorrhizal dominance effects. We
 found negative relationships between EcM relative basal area and soil
 carbon and nitrogen concentrations, but these relationships were weak to
 negligible in the absence of ErM plants. Both EcM relative basal area and
 ErM plant cover were positively associated with the soil
 carbon-to-nitrogen ratio. However, this relationship was driven by
 relatively lower nitrogen for EcM trees and higher carbon for ErM plants.
 As such, the functional effects of ErM plants on soil biogeochemistry
 neither extended nor were redundant with those of EcM trees. 4. Synthesis.
 We found that ErM shrubs strongly influenced the relationship between tree
 mycorrhizal associations and soil biogeochemistry, and the effects of ErM
 shrubs and EcM trees on carbon and nitrogen were functionally distinct.
 Our findings suggest that ErM shrubs could confound interpretation of AM
 versus EcM tree effects in ecosystems where they co-occur but also bolster
 growing calls to consider mycorrhizal functional types as variables that
 strongly influence forest biogeochemistry.
description: We collected soil and vegetation data from 420 plots arrayed
 across a 3,213-ha second-growth, mixed-hardwood forest in Connecticut, USA
 (41°57’ N, 72°07’ W) to test the effects of ericoid mycorrhizal (ErM)
 plant cover on surface carbon and nitrogen concentrations across a tree
 mycorrhizal dominance gradient. The plot arrangement consisted of 42
 transects established in parallel pairs located 50 m apart. Each transect
 included ten 4-m radius plots located every 20 m. In May­­–July 2013, we
 estimated percent cover of all understorey ErM plant species within each
 4-m radius plot and estimated total basal area of trees ≥1.37 m in height
 from the plot center using variable radius plot sampling with a 2.3
 m<sup>2</sup> ha<sup>-1</sup> basal area factor
 angle gauge. For each tree tallied during VRP sampling, we also identified
 the species and measured DBH at 1.3 m. In 2014, we collected and pooled 20
 2-cm diameter, 7-cm deep soil cores within each plot after removing the
 litter layer. We also measured soil temperature and moisture (volumetric
 water content) in the field on three occasions during the 2014 growing
 season (May, July, and September) and recorded five measurements per plot
 at each sampling occasion. Soils were processed at Yale University and
 analyzed at the Yale Analytical and Stable Isotope Center. 
 The U.S. National Park Service (USNPS) data used in this
 manuscript were collated by Robert Warren II and are publicly available
 from the USNPS Vegetation Inventory and Monitoring Program website: https://www.nps.gov/im/vmi-products.htm. Plots collated from this larger dataset met the following criteria: (1) occurred in the eastern U.S., (2) were deciduous forest types, and (3) had a plot size of 20 x 20m (400 m<sup>2</sup>). Although this dataset is publicly available, and our co-author group did not collect it, we included the subset of USNPS plots used here given these selection criteria.
description: This dataset is comprised of three tabs in a single excel file.
 See the "metadata" tab for information pertaining to the
 variables measured and analyzed. The "CT_data" tab includes
 values for all the soil variables analyzed and reported on from the
 Connecticut site. The "USNPS_data" tab includes the percent
 vegetation cover for each plant taxon by stratum/vegetation layer for each
 of the U.S. National Park Service plots analyzed and reported on in the
 manuscript.