DOI: 10.5281/zenodo.14183437
title: Pawpaws prevent predictability: A locally-dominant tree alters understory beta-diversity and community assembly

description: Data used in "Pawpaws Prevent Predictability: A locally-dominant tree alters understory beta-diversity and community assembly" (Wassel and Myers) accepted for publication in Ecosphere.Metadata for Zenodo.pdf contains more information on the following data files including descriptions of the columns. 

The file understory_abundance_data2021.csv contains all species abundances in 1x1m plots. This data was used for analyses in publication. Each row is a plot, each column is a speceis or plot descriptor, values for columns 5 and higher are species abundances. Data was collected July-August 2021 by Anna Wassel in Missouri, USA. 

The file understory_species_list2021.csv contains a list of the species codes used in the first file with their scientific names and their status as herbs or woody. This was used to filter out herbaceous species from the data set for herbaceous-only analyses. 

 
description: Abstract



While dominant species are known to be important in ecosystem functioning and community assembly, biodiversity responses to the presence of dominant species can be highly variable. Dominant species can increase the importance of deterministic community assembly by competitively excluding species in a consistent way across local communities, resulting in low site-to-site variation in community composition (beta-diversity) and non-random community structure. In contrast, dominant species could increase the importance of stochastic community assembly by reducing the total number of individuals in local communities (community size), resulting in high beta-diversity and more random community structure. We tested these hypotheses in a large, temperate oak-hickory forest plot containing a locally-dominant tree species, pawpaw (Asimina triloba; Annonaceae), an understory tree species that occurs in dense, clonal patches in forests throughout the east-central United States. We determined how the presence of pawpaw influences local species diversity, community size, and beta-diversity by measuring the abundance of all vascular plant species in 1x1-m plots both inside and outside pawpaw patches. To test whether the presence of pawpaw influences local assembly processes, we compared observed patterns of beta-diversity inside and outside patches to a null model in which communities were assembled at random with respect to species identity. We found lower local species diversity, lower community size, and higher observed beta-diversity inside pawpaw patches than outside pawpaw patches. Moreover, standardized effect sizes of beta-diversity from the null model were lower inside pawpaw patches than outside pawpaw patches, indicating more random species composition inside pawpaw patches. Together these results suggest that pawpaw increases the importance of stochastic relative to deterministic community assembly at local scales, likely by decreasing overall numbers of individuals, and increasing random local extinctions inside patches. Our findings provide insights into the ecological processes by which locally-dominant tree species shape the assembly and diversity of understory plant communities at different spatial scales.
description: Study site and focal species

We conducted this study at Washington University in St. Louis’ environmental field station, Tyson Research Center, located 25 miles from St. Louis, Missouri. The 800-ha site is located on the edge of the Ozark highlands, dominated by late-successional, deciduous oak-hickory forest, and contains a topographically heterogeneous landscape characterized by silty loam and silty clay soils that develop from shale and cherty limestone (Zimmerman and Wagner 1979). In the oak-hickory forests at our study site, local plant communities are strongly delineated by three general habitat types: 1) valleys with wet-to-mesic soils and high soil pH and nutrient availability; 2) east- and north-facing slopes with mesic soils and intermediate soil pH and nutrient availability; and 3) west- and south-facing slopes with drier soils and low soil pH and nutrient availability (Fig. 2a,c; Zimmerman and Wagner 1979; Myers et al. 2013; Spasojevic et al. 2014; Reu et al. 2022). Our study was conducted in one of these habitat types (valleys) in the Tyson Research Center Forest Dynamics Plot, a large (20.16 ha; 480 ´ 420 m), stem-mapped forest plot that is part of the Forest Global Earth Observatory (ForestGEO) network (Anderson-Teixeira et al. 2015). The 20-ha plot includes more than 1,600 stems of pawpaw at least 1 cm in diameter at breast height (DBH), most of which occur in 18 patches ranging in area from 5-1,028 m2.

Sampling Design

We selected five blocks located in the same general habitat type (valleys) to contain a pawpaw patch and an adjacent area without pawpaws, referred to as “inside” and “outside” patches, respectively (Fig. 1, Fig. 2). The five inside (pawpaw) patches ranged from 58-435 m2 in size (mean = 189 m2). The paired outside patches were selected to have abiotic (soil and topographic) conditions similar to those inside the pawpaw patch and were 10 to 20 m from the edge of the pawpaw patch (Fig. 2a, 2c). We determined the similarity of soil and topographic conditions between the inside and outside patches through a Principal Component Analysis (PCA) on 17 soil and topographic variables (Appendix S1: Fig. S2). The values were estimated for each 10´10-m subplot in the 20-ha ForestGEO plot based on measurements taken in 2013 (detailed in Spasojevic et al. 2014, LaManna et al. 2016). The outside patches were chosen to have a similar PC1 score as the pawpaw patches. Thus, our sampling design minimized differences in abiotic conditions (soils and topography) among blocks and among plots located inside and outside pawpaw patches (Fig. 2a,c; mean elevation: 195.6 m ± 11.1 m [± 1 standard deviation]; mean PC1 score of soil and topographic variables: -3.1 ± 0.6).  

For each patch type in each of the five blocks, five 1´1-m plots were sampled for plant community composition (n=25 inside plots, n=25 outside plots, n=50 plots total; Fig. 2b). Within each pawpaw patch, we placed the first plot in the center of the patch. We then placed the other four plots at least 1 m away from the center plot with the additional constraints that they could not be within two m of the edge of the pawpaw patch and could not contain larger woody stems over 1 m tall. Given the high density of pawpaw stems, this greatly limited the availability of potential locations for plot placement. Therefore, plot locations were stratified within the central area of each pawpaw patch to minimize edge effects, maintain similar inter-plot distances within and among blocks, and avoid locations physically dominated by large trees to standardize the amount of available ground area available for understory plants.

We recorded the identity and estimated the abundance of all understory vascular plant species, i.e. herbaceous plants including ferns, and woody plants and vines. In each 1´1-m plot, we estimated abundance (number of stems per species) as the number of 10´10-cm cells that contained rooted stems of the given species. For most clonal species, it is not possible to determine whether a rooted stem is a ramet or genet in the field. While this metric may overestimate abundances of clonal species and local community size, it is less likely to confound abundance with individual plant size compared to other metrics such as percent cover. In addition, for species that had more than one rooted stem (ramet or genet) in a given 10´10-cm cell, this method conservatively assigns an abundance value of one for that cell. In cases when individuals could not be identified to species in the field, they were identified to genus or assigned a morphospecies and photos were uploaded to iNaturalist for future assistance with identification; 8% of stems were considered morphospecies for analyses. We estimated local community size by summing the abundances of all species in each plot. Sampling was conducted during the peak growing season from July to September of 2021. Due to the different life stages and biology of young woody seedlings compared to the adult herbaceous plants, we conducted two separate analyses for: 1) herbaceous and woody plant species combined (hereafter total understory community); and 2) herbaceous species only.