Seed Dispersal, Microsites or Competition—What Drives Gap Regeneration in an Old-Growth Forest? An Application of Spatial Point Process Modelling

Abstract

The spatial structure of trees is a template for forest dynamics and the outcome of a variety of processes in ecosystems. Identifying the contribution and magnitude of the different drivers is an age-old task in plant ecology. Recently, the modelling of a spatial point process was used to identify factors driving the spatial distribution of trees at stand scales. Processes driving the coexistence of trees, however, frequently unfold within gaps and questions on the role of resource heterogeneity within-gaps have become central issues in community ecology. We tested the applicability of a spatial point process modelling approach for quantifying the effects of seed dispersal, within gap light environment, microsite heterogeneity, and competition on the generation of within gap spatial structure of small tree seedlings in a temperate, old growth, mixed-species forest. By fitting a non-homogeneous Neyman-Scott point process model, we could disentangle the role of seed dispersal from niche partitioning for within gap tree establishment and did not detect seed densities as a factor explaining the clustering of small trees. We found only a very weak indication for partitioning of within gap light among the three species and detected a clear niche segregation of Picea abies (L.) Karst. on nurse logs. The other two dominating species, Abies alba Mill. and Fagus sylvatica L., did not show signs of within gap segregation.

Analyzing long-term impacts of ungulate herbivory on forest-recruitment dynamics at community and species level contrasting tree densities versus maximum heights

“…The percentages of variance explained by the PRCs appear to be rather low. It is thus evident that, despite considerable population densities of ungulates, other factors than browsing distinctly drove the dynamics of the largest individuals per species (and plot) in our study such as responses to (micro-)sites and light conditions, intra-and interspecific competition and pathogens 29,35,[85][86][87][88] . We attribute this to the following causes: (1) In the montane forests of the study region, the natural species composition has a high ratio of Norway spruce 49 , and the ratio has been further increased by forestry since the mid of the nineteenth century 94,95 .…”

“…diversity indices; see [82][83][84] ). As regeneration dynamics and survivorship of tree species unfold as a result of neighbourhood processes of individuals and their species-specific life-history traits in connection with environmental filters 29,35,[85][86][87][88] , a synchronous analysis of both the entire regenerating tree community and all tree species (vs. subjectively focusing on certain species only) seems to be essential. However, many standard statistical approaches like GL(M)Ms are not applicable in case of largely multiple response variables.…”

“…Trees in these life stages, however, are frequently under-dispersed 34 and show high densities. This leads to density-dependent mortality and is typically reflected in exponentially decreasing stem density distributions, caused by stage specific mortality factors (like adverse microsite conditions 35 , pathogens 36 , competition, or herbivory 37 ). Consequently, only a comparatively small number of progenies survive 38 and only a small portion of seedlings finally grows into the forest canopy and can thus reproduce.…”

Role of environmental filtering and seed source availability in natural regeneration processes following large-scale disturbances in mountain forests

“…The diversity of habitat types is usually strongly associated with the spatial scale, making niche-based and stochastic effects difficult to disentangle (Chase and Myers 2011). Therefore, environmental filtering can be easier to detect in strongly heterogeneous landscapes, where the environmental factors vary strongly along relatively short distances, which are within the dispersal ranges of the local tree species (Gratzer and Waagepetersen 2018). Mountain ranges are good places to conduct such analysis.…”

Major disturbances test resilience at a long‐term boreal forest monitoring site

“…It seems likely that the survival of areas able to function as refuges and the patchy nature of the disturbance impact have been essential in allowing rapid recolonization at Aneboda by the previously dominant tree species, P. abies. The growth of spruce seedlings is also strongly facilitated by dead wood (Gratzer & Waagepetersen, 2018), while postdisturbance clearance of this dead organic matter can result in the emergence of a birch-dominated pioneer woodland instead (Fischer, Lindner, Abs, & Lasch, 2002). Spruce seedlings are shallow-rooted and relatively slow-growing, making them poor competitors against ground vegetation postdisturbance unless there is coarse woody debris available to provide a seedbed (Jonášová & Prach, 2004;Rozman et al, 2015).…”