In the last few decades it has become clear that positive interactions among plants (facilitation) can play important roles in structuring communities and potentially maintaining populations. This is often particularly the case in stressful environments where the relative importance of positive interactions can outweigh negative interactions such as competition. Although this area of research has made significant progress, many of the experimental results regarding facilitation rely on specific measures of individual plant performance (e.g. aboveground biomass, survival, seed production) and do not capture demographic processes throughout the lifecycle.
This study investigates the population dynamics of the perennial forb, Smelowskia calycina(Brassicaceae), at two high elevation sites in Glacier National Park, Montana, USA. More than 200 individuals have been monitored in annual censuses since 2013, with individuals categorized based on whether they have a close spatial association with neighboring plants or are growing alone. Demographic vital rates were used to parameterize integral projection models (IPMs) of population dynamics. The IPMs were then used to drive spatially-explicit individual-based models (IBMs) in order to estimate rates of seedling establishment required to match observed spatial patterns.
Results/Conclusions
Observed spatial patterns indicate a high degree of clustering, with more than 80% of S. calycina individuals exhibiting a close spatial association with one or more neighboring plants (of multiple species). However, size-dependent vital rates for established S. calycinaindividuals generally did not strongly differ based on the presence/absence of neighboring plants. Between 2013 and 2014, the population growth rate (assuming equal seedling establishment) was higher for plants growing alone, suggesting an overall negative influence of neighboring plants that year. Thus, the demography of established plants by itself fails to explain the pattern of far more individuals growing with neighbors than alone.
Simulations using the IBM suggest that large differences in seedling establishment among microhabitats with and without neighbors are likely driving the observed spatial patterns. Simulations that resulted in spatial patterns reasonably matching those observed required seedling establishment rates in microhabitats without neighbors to be 2 orders of magnitude below those with neighbors (or roughly 1 seedling per 10,000 seeds). Thus, the potential for plant-plant facilitation appears to be largely limited to seedling establishment, but with strong implications for populations of S. calycina at high elevation.