Effects Of Positive Interactions Among Plants On Population Dynamics And Community Structures

The study of biotic interactions among organisms has been one of the most critical issues in community ecology during the last decades. The majority of current ecological theories on plant community structure and dynamics are permeated with negative interactions (i.e. competition). However, many experiments conducted in the last 20 years, especially conducted in physically harsh environments, have demonstrated that positive interactions (i.e. facilitation) also play an essential role in natural systems. How to incorporate facilitation into mainstream ecological theories mainly based on competition has been an emerging task for ecologists. By combining field experiments, model simulations and statistical inference, the present dissertation explores the potential effects of positive interactions among plants on population dynamics and community structures, and attempts to construct a holistic theoretical framework accounting for both facilitation and competition.Using two constrasted sites, six species, and five plant traits, we exploited the effects of the various factors on the outcome of plant interactions in an alpine meadow of the Qinghai-Tibetan Plateau. The results demonstrated that the responses to the removal of neighbors were species-and trait-specific, as well as the local environmental conditions. These experiments shed light upon the assumptions and prediction tests of population and community models we constructed.By using an extended version of an individual-based'Zone-Of-Influence'model, we constructed grow-rate equations for individual plants in monoculture to test the hypothesis that the balance between facilitative and competitive interactions determined biomass-density relationships (i.e. constant final yield), size inequality and self-thinning trajectories. Our results showed that positive interactions profoundly affect biomass-density relationships, and increase the size inequality of model populations with and without density-dependent mortality. The modelling results were supported by field experiments, in which the greatest individual and the lowest size inequality of E. nutans populations were found at intermediate densities in a high-stress alpine habitat. Additionally, facilitation ameliorated the impact of the abiotic environment per se on. the allometric slopes, which significantly sharpened the self-thinning trajectory for survivors. All these results emphasized that positive interactions among individuals could be another key factor to determine plant population dynamics.With the recognition of species difference, niche-neutrality continuum and species-specific facilitation in mind, based on recently developed tradeoff-based neutral models of Lin et al. (2009), we elucidated the potential effects of facilitative interactions on the spatial distribution of species richness along an elevation gradient. The results illustrated that facilitation could improve the time of coexistence in two-species systems, and greatly increase the number of species in multiple-species systems. The results from field experiments conducted in alpine and subalpine meadows agreed with those from simulation patterns:the number of species in an alpine meadow was higher than that found in a subalpine meadow. Results from warming experiments, such as that of Klein et al. (2004), are in line with those from our simulations, implying that the alleviation of temperature limitation expected under global warming could destroy the interaction webs among plants, resulting in a sharp decline of species richness, especially in fragile ecosystems such as alpine meadows.Taken together, we conclude that facilitation among individuals could be an important factor in shaping population dynamics and community structures. In the context of global climate change, it is imperative to include positive interactions into mainstream ecological theories. This will facilitate the development of new theories and conceptual models, as well as the establishment of sound management actions.