Tropical epiphytes in a CO2-rich atmosphere

We tested the effect on epiphyte growth of a doubling of pre-industrial CO2 concentration (280 vs. 560 ppm) combined with two light (three fold) and two nutrition (ten fold) treatments under close to natural humid conditions in daylight growth cabinets over 6 months. Across co-treatments and six species, elevated CO2 increased relative growth rates by only 6% (p = 0.03). Although the three C3 species, on average, grew 60% faster than the three CAM species, the two groups did not significantly differ in their CO2 response. The two Orchidaceae, Bulbophyllum (CAM) and Oncidium (C3) showed no CO2 response, and three out of four Bromeliaceae showed a positive one: Aechmea (CAM, +32% p = 0.08), Catopsis (C3, +11% p = 0.01) and Vriesea (C3, +4% p = 0.02). In contrast, the representative of the species-rich genus Tillandsia (CAM), which grew very well under experimental conditions, showed no stimulation. On average, high light increased growth by 21% and high nutrients by 10%. Interactions between CO2, light and nutrient treatments (low vs. high) were inconsistent across species. CO2 responsive taxa such as Catopsis, could accelerate tropical forest dynamics and increase branch breakage, but overall, the responses to doubling CO2 of these epiphytes was relatively small and the responses were taxa specific.