Acclimation in Leaf Morphological and Eco-physiological Characteristics of Different Canopy-dwelling Epiphytes in a Lower Subtropical Evergreen Broad-leaved Forest

Epiphytes have been well characterized in terms of the morphological and eco-physiological traits that permit them to thrive in the complex forest canopy.Our aim was to characterize and analyze the morphological and eco-physiological traits of different canopy-dwelling epiphytes in a lower subtropical evergreen broad-leaved forest in Southern China.Results showed that the differences in morphological and eco-physiological characteristics between the upper and lower canopy-dwelling epiphytes were largely explained by changes in environmental factors such as photosynthetic active radiation (PAR),temperature,and humidity within the complex forest canopy.Two epiphytes,Dischidia chinensis and Psychotria serpens located in the upper canopy had low Tr (0.17±0.02 mmol H₂O and 0.34±0.05 mmol H₂O,respectively) and low P_max (2.2±0.1 μmol CO₂·m^-2·s^-1 and 3.2±0.4 μmol CO₂·m^-2·s^-1,respectively) associated with thick leaf (558±63 μm and 217.1±33.1 μm,respectively) and small stomata size (185.7±3.7 μm² and 225.4±5.2 μm²,respectively) to adapt to their upper dwelling environments (high temperature,low air humidity and high PAR).At the same time,the special structures led to high WUE (11.35±0.87 μmol CO₂/mmol H₂O and 7.88±1.31 μmol CO₂/mmol H₂O,respectively).However,the lower canopy-dwelling epiphytes Fissistigma glaucescens and Piper hancei had thin leaf (90.8±9.9 μm and 114.9±18.2 μm,respectively) and large stomata size (260.6±6.3 μm² and 362.5±8.7 μm²,respectively).The ratios of palisade to spongy tissues thickness (P/S),the thickness of leaf epidermis thickness and other structures also changed with various canopy-dwelling heights.In comparison with Dischidia chinensis and Psychotria serpens located at upper canopy-dwelling conditions, Fissistigma glaucescens and Piper hancei showed high P_max (9.5±1.3 μmol CO₂·m^-2·s^-1 and 7.1±0.8 μmol CO₂·m^-2·s^-1,respectively),high Tr (0.67±0.10 mmol H₂O and 0.74±0.13 mmol H₂O,respectively),and low WUE (4.4±1.0 μmol CO₂/mmol H₂O and 3.4±0.9 μmol CO₂/mmol H₂O,respectively),which was 1.2-3.3 times (P_max), 1.2-2.3 times (Tr),and 30%-48% higher (WUE).