Number of large scale niches in single plant community

Background/Question/Methods The word of “niche” is used equivocally. Some authors have described niches as a relatively large group of species that uses similar resources (e.g. carnivores or epiphytes; large scale niche). Other author defined niches based on relatively small differences in the resource requirements of individual species, and limiting similarity of niche have been studied (small scale niche). When we consider the spatial scale in plant trait space, the large scale niche corresponds to the whole pattern of under-dispersion (e.g. species with higher shade tolerance have higher occurrence probability in a climax forest community), and the small scale niche corresponds to the local over-dispersion (e.g. among shade tolerant trees, adjacent two species have quite different occurrence probability). In large scale niches, plant communities have been considered to be the mixture of various niches (e.g. herbs, shrubs, trees). However, the neutral theory of community, the maximum entropy formalism, and logistic regression analysis to predict species composition are all assuming that a plant community has only a single large scale niche. We enumerated large scale niches in plant communities (old-growth forests, secondary forests, meadows, and arable weed communities) in rural landscape in cool-temperate, warm-temperate, and subtropical zones of Japan. We set 1 km x 1 km research area for assessing species pool, and plant traits were measured. Occurrence and absence of species in each plant communities were analyzed based on plant traits. We identified key traits for community assembly, and examined the species distribution pattern within the trait space composed of these key traits by non-parametric analysis as decision-tree.

Results/Conclusions In all types of communities, the occurrence probability to a given community showed unimodal distribution in the trait space. In contrast with the traditional understanding, this suggests that each of the communities (communities in a rural landscape with forests, meadows and arable lands) represents only a single large scale niche. All plants in old-growth forest including herbs, small trees and canopy trees, belonged to a single niche: “long-lived shade-tolerant or tall plants”. Herbaceous communities also had a single niche: "clonally spreading or short-lived shade-intolerant plants”.