Geography & Biodiversity Research Centre, University of British Columbia, Canada
Abstract: Climate change is expected to influence the abiotic conditions under which plants allocate investment in survival, growth, and reproduction. Investment in reproduction is expected to trade-off with future demographic rates; however, despite theoretical predictions that reproductive costs should decrease under high resource availability, empirical evidence is sparse. How changes in rainfall will ameliorate or exacerbate costs of reproduction under ambient and simulated future conditions is a key step in assessing climate change’s impacts on life histories and population persistence, especially for cryptic trade-offs like costs of reproduction, which are notoriously difficult to detect in plant populations. Here, we asked how reproduction influences future survival, vegetative growth, flowering, and flower production in a long-lived perennial, Primula hendersonii, across an ongoing rainfall manipulation experiment (part of the International Drought Experiment) in an oak savanna ecosystem in SW Canada. We examined costs of reproduction using both long-term demographic data (6 years) and a one-year clipping experiment that manipulated reproductive effort across simulated drought, increased precipitation, and ambient conditions. In this ecosystem, where water availability can be limited, we predicted that costs would decrease under higher rainfall, and increase under drier conditions.
We found in both the clipping experiment and demographic study that plants that flowered experienced a significant cost of reproduction, measured as a reduced probability of flowering the following year relative to plants that did not flower, regardless of the number of flowers or seeds produced. When we artificially reduced individual plants’ investment in flowering (by clipping flower stalks or individual flowers), demographic rates were not affected in the following year, nor was there an effect of precipitation treatment on the clipping experiment. Across the last six years, plants had higher growth under wetter conditions, and successful flowering and seed set was dramatically decreased by drought treatments. This suggests that reproductive investment in P. hendersonii is determined prior to flower and seed production, and that costs are expressed for whether or not individuals flower, rather than reproductive effort. However, although precipitation influences growth and successful seed set, the cost of flowering is not directly affected by rainfall. Here we show that despite costs not being detected in a short-term experiment, costs can be detected across long-term demographic studies, building upon our understanding of life history trade-offs that are difficult to quantify.
