Distinct epiphyte responses to drought in tropical mountain cloud forests

Abstract

Epiphytes are often considered to be significantly impacted by precipitation changes because their lack of direct access to soil water. However, few in situ studies have examined how reduction in water availability may affect epiphyte growth. Using a unique stemflow collection and diversion device, we examined the effects of stemflow reduction of 25% and 50% on the growth and leaf traits of two common but distinct epiphyte species in a subtropical forest. One species (Asplenium nidus) has a large substrate, and another (Haplopteris zosterifolia) without known water storage structure. The 25% stemflow reduction had limited effects on the growth and leaf traits of the two epiphytes, indicating that 25% stemflow reduction did not reach the water stress threshold for the two epiphytes. It is also possible that the actual reduction in water availability was less than 25% since epiphytes do not use all available stemflow and there could be other sources of water such as fog and throughfall. The 50% stemflow reduction reduced leaf number and leaf area of H. zosterifolia, but not A. nidus, likely because water stored in the large substrate of A. nidus mitigated the impact of stemflow reduction. The thinner leaves, smaller leaf dry matter content, and lower δ13C of A. nidus than H. zosterifolia support the role of water storage of the substrate of A. nidus on mitigating water stress. The 50% stemflow reduction increased leaf thickness, leaf dry matter content of H. zosterifolia, and thickness of abaxial cuticle layer of both epiphytes but had no effect on δ13C. Stemflow reduction had no effects on nutrient concentration and nutrient ratios of both epiphytes suggesting that the epiphytes were able to maintain stoichiometry. Our results indicate that the epiphytes minimized nonstomatal water loss when environmental dryness increased but maintained stomata conductance, which could be important in minimizing the impacts of drought on plant growth and quickly resuming growth once drought ends. Our study highlights that not all epiphytes are similarly vulnerable to drought and precipitation reduction may change the relative abundance of epiphytes with and without water storage structure.