Effects Of Ultraviolet-B Irradiance On Plant Intraspecific Competition And Facilitation

In terms of plant interactions, competition known as negative interaction has been considered as the dominant biotic factor shaping plant populations and communities, whilstfacilitation, which is defined asthe positive effect from neighbour plants via environmental amelioration to not only against the impacts of herbivores, potential competitors or extremes of climate but also provide additional resources through canopy leaching, microbial enhancement, mycorrhizal networks and hydraulic lift, has been proved to play a key part in shaping plant system by a multitude of researches. Moreover, the’stress-gradient hypothesis’(SGH)predicting the shifts between facilitation and competition along environment gradients has stimulated the bulk of studies about facilitation. In terms of abiotic stress, owing to our plenty of previous studies, we mainly focused on UV-B radiation differing from resource-related stress such as water, light and nutrients.Althoughthe effects of enhanced UV-B radiation involve not only stress but also facilitation among individuals in population, but it is difficult to separate them in reality.As a consequence, in order to investigate the influences of both stress and facilitation on plant populations, we set density and UV-B radiation as two fixed factor with three levels for each factor (totally9treatments), then proposed and tested three predictions about self-thinning, size inequality, and phenotypic plasticity respectively by applying the’stress-gradient hypothesis’(SGH)with the field experiment.Our results from the field experiment totally supported the predictions:(1)mainly owing to the stress without positive interactions rather than facilitation, enhanced UV-B radiation could increase survival rateduring self-thinning obviously.(2) Exactly as competition,facilitation was also asymmetric during the development of plants in the population, specifically, larger individuals benefited from positive interactions more greatly than smaller ones. In addition,positive interactions increased size inequality of populations especially at high density,even when density-dependent mortality occurred, while stress decreased the size inequality.(3) Plants changed the strategy for acquiring light resource and avoiding damage from UV-B radiation in different growing stages because the trade-off between interactions (including both competition and positive interaction) and stress could be varied.In conclusion, positive interactions should not be neglected in research about thepattern of size structure of populations. As a result, future study should integrate facilitation in current allometric theory which mainly based upon competition to explicitly explain the allometric relationship of plants within populations and more work in conceptual fieldis needed.