Few direct responses to water availability
Our results did not support the expectation that water availability
plays a direct role in survival, seed production, and population growth
rates of herbaceous plants (Mu et al. 2021). We found the impact
of water availability depended on other abiotic conditions and was more
frequently important in models of survival than seed production or
population growth rate. For three species, the wet treatment appeared to
buffer the negative effects of increasing light and decreasing nutrients
on survival, whilst the fourth species only experienced reduced survival
along PC1 in the dry treatment. This result supports existing knowledge
that interactions among shade, soil nutrients, and moisture influence
plant survival, particularly in arid environments (Valladares 2003). In
contrast, water availability rarely modified the effect of neighbours on
vital or population growth rates. In perennial grasses, Adler et
al. (2009) similarly reported little evidence of interactive effects
between precipitation and the presence of neighbours, however unlike our
results they found strong support for direct effects of precipitation.
One reason why the watering treatment had few direct impacts on vital
and population growth rates, and indirect impacts via interactions with
competition, may simply be that the watering treatment did not
significantly alter soil moisture compared to ambient conditions. In the
year we conducted the study, conditions were drier than average at 110
mm of rainfall over the growing season and as such our watering
treatments represented 30-90% of average rainfall. Based on our study
and results from other watering trials in this system (Wainwrightet al. 2018; Towers et al. 2020), it seems likely that in
many years water is not limiting performance during the growing season.
The timing of rainfall events is an important driver of variation in
vital rates, plant-plant interactions, and population dynamics (Levineet al. 2008; Compagnoni et al. 2016; Conquet et al.2023). Even in the single year studied here, unseasonably heavy summer
rainfall cued emergence outside the growing season for the two exotic
species almost exclusively. In the future, it would be interesting to
implement the watering treatment before the onset of winter rainfall to
assess emergence responses, which can have large contributions to
population growth rates (James et al. 2020). It would also be
valuable to track soil moisture in each plot after rainfall events to
measure the magnitude and duration of the effect of the watering
treatment on soil conditions. Since annual plants can employ different
ecological strategies to buffer performance over time in water-limited
environments, the effect of interannual variation in water availability
on vital rates is another important avenue of research (Angert et
al. 2007).