Plant-plant interactions of Aristida stricta individuals across a complex environmental resource gradient

Background/Question/Methods:

Plant-plant interactions influence individual plant performance and shape communities. The stress-gradient hypothesis (SGH) predicts that the nature of neighbor interactions should shift from competitive to facilitative across gradients of increasing resource stress. To explore how neighbor interactions might change across a complex resource gradient, we conducted neighbor removal experiments around focal individuals of Aristida strictaMichx. (wiregrass; n=96) in xeric, nutrient-limited sandhill and mesic seepage slope habitats across local topographical gradients in longleaf pine habitat of Eglin Air Force Base, Florida, USA. We experimentally removed neighbors of focal individuals at four levels (heterospecific, conspecific, all, and no neighbors) following a June 2013 prescribed fire. Over two subsequent growing seasons, we monitored each focal individual’s performance in terms of growth (Δ basal diameter, total aboveground biomass) and potential reproductive output (PRO = no. flowering culms×average no. of flowers/culm). We used performance parameters to calculate the influence of heterospecific, conspecific, and all neighbors on focal individuals. We used univariate analyses to assess directionality of plant-plant interactions, and mixed effect models to analyze relationships among performance variables, removal treatments, and various environmental covariates (average photosynthetically active radiation, average soil moisture content, soil texture, and soil nutrient content) collected throughout the experiment.

Results/Conclusions:

Over the course of the study, neighbor influence in sandhills was facilitative for growth in both basal diameter (P ≤ 0.020) and biomass (P = 0.002). In contrast, neighbor influence in seepage slopes remained neutral for biomass (P = 0.506) and changed across growing seasons from neutral (P > 0.566) to competitive (P < 0.020) for basal diameter. Interactions were facilitative for PRO in seepage slopes (P = 0.007) and neutral in sandhill habitats (P = 0.142).

Habitat influenced biomass (P = 0.006), while presence of neighbors influenced PRO (P = 0.032). Influences of habitat and neighbors interacted for basal diameter (P ≤ 0.045). Distance to nearest pine negatively influenced both biomass (P = 0.029) and PRO (P = 0.012).

Our results demonstrate how plant-plant interactions can shift both in nature and importance across a resource gradient common in longleaf ecosystems. Our findings support the predictions of the SGH in that we observed shifts from facilitation or neutrality to competition between resource-limited sandhills and less-limited seepage slope habitats. The variation observed in the nature and importance of interactions across growing seasons and performance parameters underscores the need for spatially and temporally diverse approaches to studying plant-plant interactions across resource gradients.