Consistent vital rate-environment relationships among species
We found vital rate-environment relationships among species were largely consistent in the direction of response. As values of PC1 increased (becoming more open and less fertile), species tended to have higher emergence, neutral survival, neutral or negative seed production and neutral population growth rates. Although emergence rates tend to increase with light availability, this pattern can be highly variable among systems (Carta et al. 2017) and species (Grime et al. 1981; Baskin & Baskin 1988). Indeed, one of the core tenants of niche theories is that species differ in their responses to environmental variation (Grubb 1977; Chesson 2000). Previous annual plant studies have reported spatial variation in species’ emergence (James et al. 2020) and temporal variation in species’ responses to precipitation (Angert et al. 2009) as mechanisms promoting local diversity. Hence, the high consistency observed among species’ responses to environmental variation in our study was surprising, although in line with another study in this system that found limited evidence of species-specific fecundity responses to the environment (Towers et al. 2020).
By sowing seeds across habitats and removing neighbours from half of the focal plants, we were able to test species’ demographic responses along gradients of abiotic conditions alone and in combination with plant-plant interactions (Chesson 2000; Adler et al. 2013; Bimleret al. 2018). We found a surprising, near ubiquitous lack of responses to main effects of neighbour abundance among species for survival and seed production. However, weak negative effects of neighbours on both survival and seed production resulted in clear competitive effects on population growth for two of the eight species in this study. We expected survival and seed production to be negatively related to PC1, with lower survival and seed production in more open and less fertile environments. Although infrequent among species, for three species these relationships were only observed where neighbour abundance was high, which could reflect responses to competition for water and nutrients in the higher light and lower nutrient environment (Maestreet al. 2005).