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).