Conspecific density and reproductive trade‐offs govern population response to climate in a clonal wildflower

Making predictions about the persistence of plant populations under climate change requires explicitly incorporating environmental drivers into population models. However, local density and demographic processes, including reproductive trade‐offs, that may influence how populations respond to environmental drivers are rarely incorporated into these models. For plant species that reproduce both clonally and sexually, trade‐offs between reproductive modes can have important demographic consequences, affect the strength of density dependence and ultimately influence population‐level responses to climate.

We used a manipulated rainfall experiment (experimentally imposed drought and irrigation) across 4 years on an in situ population of a long‐lived forb, Primula hendersonii , to examine the influence of rainfall, seasonal variation in ambient temperature and local conspecific density on: (1) demographic rates (survival, growth, sexual and clonal reproduction), (2) trade‐offs between clonal and sexual reproduction and (3) density‐dependent population growth. In addition to observed density in the field experiment, we paired a manipulative density and watering experiment in pots to confirm the observational effects of density on demographic rates.

We detected a reproductive trade‐off where prior flowering reduced the probability of making clonal offspring, but prior clonality did not affect future flowering. Clonality was also lowest for plants growing at high density. When these effects were scaled to the population level using integral projection models, population growth rates were reduced when flowering did not reduce clonal reproduction (no trade‐off), at high density and under simulated drought.

Manipulated density in a separate pot experiment resulted in a similar negative effect of local conspecific density on clonal reproduction compared to field observations, as well as for plant growth under both ambient and reduced water availability, supporting that density effects can be detected in observational demographic datasets where environmental drivers are explicitly examined.

Synthesis . Our results demonstrate that interactive effects of clonal reproduction and intraspecific density are important contributors to plant demography and that high local density increases the negative impact of drought on population growth rates. We show that quantifying density‐dependent responses to climate while considering the role of clonal reproduction is necessary to understand the consequences of future climate conditions.