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