Short-Term Effects of Passive Warming on Two Invaded Plant Communities Using a Novel Open-Top Chamber Design

Climate change is expected to directly favor exotic plant species over native species because they tend to have wider climatic tolerances and greater phenological plasticity. Warming is also likely to indirectly favor exotic species by herbivore behaviors and/or preference and intensifying the enemy release phenomenon. To examine these direct and indirect effects, I initiated a field experiment in two heavily invaded plant communities in northern and southern Michigan, USA. Passive warming methods such as open-top chambers are often used to warm aboveground systems; however, the limited height of their design restricts their use to low-stature (~0.4 m) plant communities. I introduced a new chamber design for year-round warming in taller stature (~1.5 m) plant communities and analyzed its effects on the abiotic environment. During the second year of warming, I collected data for species cover, phenology (green-up, flowering time, timing of seed set), and cumulative leaf herbivory. The new chamber design achieved reliable levels of warming (+1.8 ºC), consistent with all four Representative Concentration Pathways (RCPs) scenarios for 2046-2065 from the Intergovernmental Panel on Climate Change's 5 th Assessment Report. The effects of warming were largely consistent with theoretical expectations at the northern site and favored exotic plant abundance. Warming did not favor exotic species dominance at the southern site, but rather responses indicated plants at this site experienced physiological stress, likely because ambient temperatures were already close to plant species' thermal maxima.