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?