LB 9-106 - Warming influences phenology-demography trade-offs in tundra plants

Background/Question/Methods:

The Arctic is warming four times the global average rate creating dramatic shifts in the seasonality of tundra ecosystems, including changes in plant phenology. While some phenological events such as leaf senescence are shifting later, the majority of tundra plant phenology is becoming earlier with warming, particularly spring leaf emergence and flowering time. There are inherent life-history trade-offs between plant flowering time and size at reproductive maturity and size related traits such as flower, fruit and seed production. Therefore, earlier phenology in response to warming may come at a cost of reduced fitness including smaller size and lower reproductive output. However, warming also has direct positive effects on plant fitness related traits including growth, seed production and height. Here we use long-term (20+ years) of field monitoring data on tagged individual plants from two experimental warming sites in the Canadian Arctic in the International Tundra Experiment (ITEX) to understand the net effects of warming on plant demographic rates including vegetative growth, reproductive height, and flower, fruit and seed production. We ask: 1) Are there life history trade-offs between plant phenology and demographic vital rates in common tundra plant species? 2) If so, how does warming alter these phenology-demography relationships?

Results/Conclusions:

We observed strong trade-offs between plant phenology and key demographic rates of tundra plant species, however often not in the direction predicted by life history theory. Reproductive height was taller in later flowering individuals overall and reproductive structures were taller in warming than control plots, indicating a direct effect of warming on reproductive height. Flower and fruit production followed the opposite pattern, whereby individuals who flowered later had fewer flowers and fruit, which goes against hypothesized life-history tradeoffs. In addition, there was no difference in flower or fruit number between warming and control plots. Finally, there was a negative relationship between vegetative growth (number of leaves) and leaf emergence timing, which was much stronger in warming than control plots. Thus, warmed individuals with later leaf emergence had lower growth than those with early leaf emergence, suggesting a fitness advantage for plants that advance their leaf emergence under warming. In summary, we find that warming influences key demography-phenology relationships in tundra plants, while also directly altering traits such as reproductive height. Understanding the net mechanisms by which warming influences demographic vital rates is critical for predicting changes in population dynamics, plant community composition, and ecosystem function in a rapidly changing Arctic.