Spatial Heterogeneity of the Forest Canopy Scales with the Heterogeneity of an Understory Shrub Based on Fractal Analysis

Abstract

Spatial heterogeneity of vegetation is an important landscape characteristic, but is difficult to assess due to scale-dependence. Here we examine how spatial patterns in the forest canopy affect those of understory plants, using the shrub Canada buffaloberry (Shepherdia canadensis (L.) Nutt.) as a focal species. Evergreen and deciduous forest canopy and buffaloberry shrub presence were measured with line-intercept sampling along ten 2-km transects in the Rocky Mountain foothills of west-central Alberta, Canada. Relationships between overstory canopy and understory buffaloberry presence were assessed for scales ranging from 2 m to 502 m. Fractal dimensions of both canopy and buffaloberry were estimated and then related using box-counting methods to evaluate spatial heterogeneity based on patch distribution and abundance. Effects of canopy presence on buffaloberry were scale-dependent, with shrub presence negatively related to evergreen canopy cover and positively related to deciduous cover. The effect of evergreen canopy was significant at a local scale between 2 m and 42 m, while that of deciduous canopy was significant at a meso-scale between 150 m and 358 m. Fractal analysis indicated that buffaloberry heterogeneity positively scaled with evergreen canopy heterogeneity, but was unrelated to that of deciduous canopy. This study demonstrates that evergreen canopy cover is a determinant of buffaloberry heterogeneity, highlighting the importance of spatial scale and canopy composition in understanding canopy-understory relationships.

Direct and Indirect Effects of Overstory Canopy and Sex-Biased Density Dependence on Reproduction in the Dioecious Shrub Shepherdia canadensis (Elaeagnaceae)

“…As an early-flowering dioecious shrub, pollinated by generalist flies [7,40], buffaloberry is susceptible to pollen limitation as a result of conspecific sex-biased density dependence. Buffaloberry presence and fruit production, and brown bear feeding on buffaloberry fruit, has been associated with open forests such as immature evergreen stands and deciduous stands [8,17,18,70]. Our study lends support to recommendations of increasing the extent of young, regenerating forests with minimal scarification of soils [8] to increase the availability of buffaloberry fruit and animal species that depend on them.…”

“…Despite the effects of niche and other interacting environmental conditions, in dioecious and other fruiting shrubs, light availability has been frequently linked to increased flower number [58,62] and increased fruit production [13,19,37,58,62]. Buffaloberry has shown a consistent association between light availability and flower and fruit production [8,17], demonstrating the centrality of light for fitness of the shrub. Similar to other actinorhizal plants that depend on light availability to drive nitrogen fixation [21,69], buffaloberry is shade-intolerant, and thus depends on forest disturbances [8,18,70].…”

“…Buffaloberry should not, however, simply be considered a pioneer species, as the dichotomous pioneer-climax framework is better conceptualized as a continuum [20,63]. The shrub does, however, display traits favoring disturbances that lead to canopy gaps, crown-replacing fires, and tree canopies with higher rates of understory irradiance [17,18], placing it more toward the pioneer side of life history responses to disturbance. Our results, demonstrating both reduced flower and fruit production as direct effects of higher canopy cover, support this classification.…”

“…Optimal light requirements for plant species vary [14,15], but canopy closure limits woody plant biomass [15,16] and understory diversity [1] across forest types. Buffaloberry occurs less frequently and produces less fruit as canopy cover increases [8,17,18], as in other dioecious shrubs [19]. Logging, fires, and other disturbances that decrease canopy cover and reduce competition with canopy trees for light favor growth and higher rates of reproductive success for buffaloberry and other understory species [16,18,20].…”

Endemicity and prevalence of multipartite viruses under heterogeneous between-host transmission

“…Its topology may be diverse, depending on plant distribution and vector behavior, with two limit cases being the distribution of plant species in the wild (see, e.g. [30] and references therein) and huge modern agriculturally homogeneous regions [31]. These different architectures are implemented by tuning the distribution of contact rates among hosts, with limiting cases being fixed contact rate, and power law-distributed contact rate.…”