B4WarmED forest warming experiment: Increased temperature effects on herbaceous plant phenology

Background/Question/Methods

Climate change may alter forest biodiversity and function including herbaceous plant dynamics such as phenology. Herbaceous phenology is an important indicator of herbaceous plant response to climate but also may influence community interactions. Climate change may alter the timing of events such as leaf emergence, flowering and fruiting, which are crucial to plants’ overall success. We hypothesize that warming will increase the growing season length of herbaceous plants by causing earlier leaf out and delayed senescence. To test for this we monitored phenology of numerous individuals of 25 species of herbaceous plants in the Boreal Forest Warming at an Ecotone in Danger (B4WarmED) experiment. The project has two forested sites in northern Minnesota with 96 7.1m²  plots established in open and closed canopy conditions. Treatments include simultaneous above (via infrared lamps) and below-ground (via buried cables) warming to elevate the temperature to three target levels (ambient, + 1.7 °C, + 3.4 °C) and an undisturbed ambient treatment. For each species, we recorded emergence, leaf out, fruiting, flowering and senescence weekly during the growing season from 2009 through 2011. 

Results/Conclusions

Both the direction and magnitude of phenological responses varied across species and among treatments. Some species initiated their annual growth and reproductive cycle earlier in the spring, others extended their growing season later into the fall and some did not respond at all. Differences in response to warming may alter competitive interactions of co-occurring species. For instance, tall growing Eurybia macrophylla emerged earlier and senesced later when warmed. In contrast, its smaller neighbor, Maianthemum canadense, did not shift phenology with warming. A positive response to warming allows E. macrophylla to usurp limited resources then not available to species that emerge at the same time as under ambient conditions. Maianthemum canadense, which did not respond to warming and emerged later than E. macrophylla, may be harmed by increased shading from its competitor. Results from this study reveal the complex nature of phenological responses to a warmer climate and call for broader examination of impacts at the community level.