Introduction
Plant population fitness is determined by multiple vital rate responses
to abiotic and biotic factors and their interactions. Vital rates
describe discrete components of the lifecycle and include rates such as
emergence, survival, and seed production which collectively describe the
fitness of a population, often measured as per capita population growth
rate (Caswell 2001). Understanding how population growth varies with
abiotic conditions and biotic interactions allows us to explain patterns
of diversity and predict the response of populations to changes in the
environment (Adler et al. 2009; Dahlgren & Ehrlen 2009).
However, quantifying population growth is an empirical challenge in many
natural systems, where measuring multiple vital rates may not be
feasible (Laughlin et al. 2020). Consequently, many studies
measure a single vital rate as a proxy for lifetime fitness, even though
it is well appreciated that this approach may be misleading if vital
rates trade-off in their effects on population growth (Laughlin et
al. 2020; Klimeš et al. 2022).
Demographic trade-offs have long been described in life-history theory,
for example between reproductive effort and survival (Stearns 1989).
Demographic compensation has more recently been coined to describe
opposing vital rate trends across environments (Doak & Morris 2010),
and may be a common phenomenon explaining population growth rates across
species’ geographical ranges (Villellas et al. 2015). Yet at the
local scale, few studies have examined the way that different vital
rates respond to both abiotic and biotic factors to influence population
growth (Dahlgren & Ehrlen 2009).
Understanding how vital rates vary in response to plant-plant
interactions is crucial for forecasting how plant communities will
respond to future conditions, as species are likely to encounter new
interaction neighbourhoods as they track preferred climates at different
rates (Alexander et al. 2015). Lyu and Alexander (2023)
recently revealed evidence of widespread demographic compensation in
response to competition among herbaceous species and highlighted the
potential for variation in vital rate responses to influence population
dynamics. Hence, a challenge of accurately forecasting the impacts of
climate change is predicting the outcomes of existing and novel
plant-plant interactions under changed abiotic conditions (Parmesan
2006; HilleRisLambers et al. 2013; Alexander et al. 2015;
Ettinger & HilleRisLambers 2017). Despite a historical focus on abiotic
conditions, both abiotic and biotic factors have important effects on
vital rates (Ettinger & HilleRisLambers 2013; Morris et al.2020; Paquette & Hargreaves 2021), but isolating their effects is
difficult due to interactions among them (Callaway et al. 2002;
Kraft et al. 2015; Germain et al. 2018; Funk 2021).
Annual plants provide an ideal system for studying the interactive
effects of abiotic and biotic factors on vital rates and population
growth (e.g. Angert et al. 2009; Alexander & Levine 2019; Jameset al. 2020), as it is possible to measure vital rates across the
entire life cycle over relatively short timeframes (Ge et al.2019; Laughlin et al. 2020). In addition, their small size is
amenable to manipulative experiments which allow us to assess responses
to local-scale environmental heterogeneity, such as variation in shade
(Towers et al. 2020) and neighbourhood composition (Bowleret al. 2022). To date, little is known about how vital rates vary
with local-scale variation in abiotic conditions and plant-plant
interactions simultaneously to determine population growth rates.
To address this knowledge gap, we assessed how abiotic conditions and
plant-plant interactions influenced emergence, survival, and seed
production within a guild of Australian winter annual species. We
manipulated water availability and interaction neighbourhoods across a
natural gradient of shade (cast by trees) and soil to answer the
following questions:
How do fitness-environment relationships vary among species?
Within species, how do vital rates and population growth rate differ
in response to the separate and combined effects of abiotic conditions
and plant-plant interactions?