Point-pattern analysis reveals density-dependent processes that influence structure ofJuniperus virginiana L. stands

Abstract

This research aims to identify the processes that influence the structure of eastern redcedar (Juniperus virginiana L.)stands across a range of size classes and in stands that are co-dominated by deciduous species. The trend of eastern redcedar encroachment into prairies and old fields in North America is well-documented; but the mechanisms that shape the distribution of con- and heterospecific trees within stands are poorly understood. Eastern redcedar stands representing a variety of size classes were sampled in the Midwest to examine how stand-shaping processes vary with stand age. Additionally, mature eastern redcedar stands where Quercus spp. are codominant were sampled to evaluate how competitive interactions influence stand structure. Point-pattern analyses were conducted to detect signals of underlying point-processes. We found several signals of density-dependent growth including evidence of self-thinning and regular-spacing between mature individuals. We found segregation within and between mature eastern redcedar and Quercus spp., indicating competitive partitioning of space. These findings indicate density-dependent intra- and interspecific competition are the most important process influencing the structure of eastern redcedar stands in its encroaching range.

Keywords: eastern redcedar; density-dependence; woody encroachment; point-pattern analysis

Introduction

The structure of plant communities is largely determined by the interacting effects of resource patch availability, intra- and interspecific competition, the demography of constituent species, the growth rates and habits of individuals, and dispersal ability (Getzinet al. 2008; Pommerening et al. 2011). The relative influence of these processes can be identified by examining the spatial distribution of individuals in a community and drawing inference from observed patterns (Wiegand and Moloney 2004; Velázquez et al.2016). One process with a distinct spatial signature is density-dependent growth, where there is an increased likelihood of mortality among neighboring individuals over time (e.g. He and Duncan, 2000). The spatial distribution of trees follows typical patterns during succession (Pickett et al. 1987). For example, mature stands self-thin as they age due to competition for light, resulting in a regularly-spaced (overdispersed) spatial pattern (Kenkel 1988). In mature stands where canopy gaps are opened by disturbance, aggregation of younger trees frequently occurs (Condit 2000). Clumping of plants can occur when individuals have a facultative effect on others or where patchy resources occur (e.g. Alados et al., 2006; Pillay and Ward, 2012; Ward et al., 2022). In the absence of defined processes influencing establishment, stands should be randomly distributed (Wiegand and Moloney 2004; Perry et al. 2006).
A non-manipulative method of examining the spatial distribution of plants is to record the location of individuals and their attributes in a defined study area. Analyses of these point-pattern data are useful for examining interactions between points (Perry et al.2006; Wiegand et al. 2013). Point-pattern data can be integrated with continuous quantitative data (e.g. soil water availability) and categorical data (e.g. soil type) in point-pattern analyses that elucidate the processes that are responsible for the observed patterns (Dray et al. 2012). In plant communities, point-pattern analyses can be useful in detecting processes such as conspecific density dependence that can limit the recruitment of dominant species and lead to increases in subdominant and understory diversity (Ramage et al. 2017).
This study examines spatial structure of eastern redcedarJuniperus virginiana woodlands ranging from the Great Lakes to the Great Plains of the United States. J. virginiana (hereafter referred to as redcedar ) is a North American tree that has expanded its range significantly over the past 200 years (Briggset al. 2005; Barger et al. 2011; Streit Krug et al.2017). Redcedar encroachment into old fields and grassy openings in the Midwest and eastern United States is an increasing problem (Copenheaveret al. 2005; Hamati 2022). Samples of redcedar point-pattern data from differing size classes will be analyzed to determine which processes are important and at which stage in stand development (Guet al. 2019; Yang et al. 2019). Interspecific interactions between encroaching redcedar and previously established deciduous trees will be examined in the transition zone between the eastern deciduous forest and the central Plains. Spatial point-pattern analyses of these mixed stands, including checks for density-dependent competition and size-distance correlation, can provide information on the importance of intra- and interspecific competitors on their respective growth (Gray and He 2009; Pillay and Ward 2012; Mureva and Ward 2016; Ward et al. 2022). Assuming that density-dependent competition is an important process in shaping the distribution of redcedar within a stand, and that the strength of this interactive effect will vary depending on the size of redcedar and the size of their neighbors, the following predictions will be tested: (1) stands comprised of younger and/or smaller redcedars will have a random distribution, and (2) stands comprised of adult redcedars will display overdispersion (i.e. competition). (3) Strong competitive interactions between neighboring larger trees will be detectable. (4) The mean nearest-neighbor distance between among redcedar will be greater than between redcedar and deciduous trees due to intraspecific competitive interactions being stronger than interspecific interactions.

Methods