Pawpaws prevent predictability: A locally-dominant tree alters understory beta-diversity and community assembly

Description

Abstract

Abstract

Methods

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.