Some like it hot? Growth responses to warming in a wintergreen understory orchid, Tipularia discolor

Background/Question/Methods

Most climate predictions agree that the southern US will be 2-4°C warmer by the end of the 21^st century, whereas the average temperature increase in North Carolina over the last century was 0.7°C. Studies of the effects of warming on native plants have focused on plants that are photosynthetically active in summer; much less is known about the effects on plants that acquire most of their carbon in winter. We measured changes in reproductive phenology, seasonal photosynthesis, and growth of a wintergreen understory orchid species, Tipularia discolor, to understand how future climate change may impact this species. Previous work has indicated that the optimum temperature for photosynthesis in T. discoloris about 10°C higher than air temperature throughout most of its growing season, indicating that growth should increase with future warming. The positive effects of warming, however, might be offset by increased vapor pressure deficit (VPD). This work was performed in an experimental warming site in the piedmont of NC. Warming was achieved by actively heating open-top chambers from +1.5 to +5.5°C above ambient temperature, with 0.5°C steps between treatments. As a consequence of heating, mean VPD also varied among chambers, ranging from +0.3 to +1.5 kPa.

Results/Conclusions

This terrestrial orchid flowers in late summer, and a +2°C warming treatment delayed flowering by 10 days in 2011 (P<0.0001). Only individuals in +0°C and +2°C chambers produced flowers that developed into fruits, whereas flowering stalks aborted before any flowers were produced for orchids growing in +4°C, +4.5°C, and +5.5°C chambers in 2010-2011. In agreement with previous work, light-saturated photosynthetic rates were higher for orchids growing in hot chambers than in control chambers (11.0 vs. 7.6 μmol m^-2 s^-1, P=0.0002, respectively). Photosynthetic rates declined significantly with increasing chamber temperature, however, in early autumn (R²=0.78, P=0.004, November; R²=0.52, P=0.045, December) when photosynthetic capacity of T. discolor is the highest, but when shading from dense overstory foliage strongly limits photosynthesis. We hypothesize that VPD is more important than temperature in determining photosynthetic rate in this species, because high VPD (>1.4 kPa) strongly limited stomatal conductance under both high and low light intensities (R²=0.19, P<0.0001; R²=0.53, P<0.0001, respectively). Leaf area increased in all chambers from 2010-2012, and there was significantly less leaf area growth in the hottest chambers (R²=0.41, P=0.047). These results emphasize the importance of explicitly accounting for changes in VPD when estimating temperature responses of plant species under future warming scenarios.