The role of heterogeneity in spatial plant population dynamics

Ecological theory names interacting mechanisms that allow competing species to coexist in limited available space, some of them are perceive as antagonistic. Most prominent are niche differentiation, heterogeneity and neutrality (ecological equivalence). Species similarity is also influenced by two mechanisms: Habitat filtering selects for ecologically similar species, while niche differentiation reduces competitive pressure and thus prefers ecologically different species. The spatial arrangement of abiotic resources can determine the spatial pattern and competition framework for a pre-selected tree species ensemble. Spatial occurrence patterns of trees are formed by dispersal, growth and mortality which are influenced by the interacting abiotic and abiotic conditions. The relative impact of these mechanisms are underresearched in temperate forest trees, especially in Europe. We analysed a data set of a temperate old-growth forest with spatially explicit information about more than 15 000 individual trees of six tree species (90 % beech admixed with Ash, Hornbeam, Sycamore, Norway Maple, and Wych Elm) located in the central region of the Hainich National Park in central Germany. We tested space-related coexistence mechanisms under heterogeneous conditions. For this, we employed Point Pattern Analysis for testing several ecological hypotheses on inter- and intraspecific interactions of the species, varying from randomness to strict ecological niche. In order to identify the critical components of possible niches, we collected field data on the abiotic conditions such as the availability of water and light, and considered topography using a Digital Elevation Model. These field data were used for fitting suitability surfaces depending on tree species identity using spatial interpolation methods such as Kriging and Generalised Additive Models. We used Spatial Point Process Models to reconstruct the spatial distribution processes composed of purely biotic, abiotic or mixed covariates of the tree species. We found that spatial heterogeneity was important in all aspects we studied. Both, tree density and the distribution of the abiotic habitat components varied in space. Especially when species interacted with beech, abiotic heterogeneity played an important role: beech outcompeted the admixed species under most prevailing abiotic conditions. This way, beech influenced the spatial pattern of the six studied species by limiting available (niche) space via inter- and intraspecific competition. Here, Beech proved to be the superior competitor with no pronounced abiotic niche, but is mostly excluded from slopes. The remaining available niche space was often occupied by ecologically similar species, which formed typical associations in subregions of the study area less suitable for beech. We found spatial segregation between the three most abundant species Beech, Ash, and Hornbeam, coexistence by niches seem to be rather trait based rather than based on abiotic preferences. Habitat suitability and spatial distribution of Ash, Sycamore, and Norway Maple were more affected by the abiotic environmental condition than Beech, Hornbeam, and Elm. This indicates that the coexistence of rare species seems to be mediated by heterogeneity. Our study revealed that the difference in abiotic conditions, such as soil depth and plant-available water were relevant for habitat suitability at small spatial and temporal scales. When simulating the distribution pattern of the surveyed species, it became apparent that biotic interactions play an important part in shaping the scales at which aggregation or segregation happen in the abiotic environment. Beech and Sycamore both showed endogenous heterogeneity. For both species, point processes models incorporated several different interaction scales of intraspecific interaction. The interspecific interaction played only a minor role compared to the intraspecific one. All results together seem to underline that niche differentiation happens at the level of the individual allowing ecologically similar species to interact de facto neutrally within their niche space and thus, to coexist in presence of a strong competitor.