Fog may be an important source of water for forest vascular epiphytes on trees, because they lack direct access to sources of soil water, but little is known about the water use proportions from various sources and potential water uptake pathways in epiphytes. Here, we analyzed leaf carbon isotope ratios as a measure of water use efficiency (WUE), proportions of fog, rain, and soil water use, and foliar water uptake (FWU) in species of epiphyte and their host trees in a tropical karst dwarf forest in China during the dry season. We found that the WUE, as represented by leaf δ¹³C, was generally enriched in the epiphyte species compared to their host trees. Epiphytes used substantial proportions of fog water, whereas water use in the host trees was dominated by soil water. The leaves of epiphytes and host trees absorbed water following immersion in water for 3 h and FWU possibly related to foliar epicuticular structures, such as fungal endophytes. Our results show a divergence of water use strategies between epiphytes and their hosts and highlight the importance of fog water for epiphytes during the dry season and under a climate change scenario with a reduced occurrence of fog events.

Understanding the stoichiometric traits of plants is critical for studying their ecological adaptation strategies. Facultative epiphytes (which can also live on the ground) are an important component of epiphytic flora of montane forest ecosystems. However, a key gap persists in our understanding how facultative epiphytes can adapt different nutritional conditions of ground and canopy habitats? To study adaptive strategies of facultative epiphytes and the characteristics of the content and stoichiometric homeostasis of C, N, and P elements, we conducted a field experiment and a greenhouse N and P additions cultivation experiment. We found that epiphytic individuals of facultative epiphytes showed lower C:N and C:P ratios, higher variation in elemental composition, and more pronounced N limitation than terrestrial individuals. Moreover, facultative epiphytes showed strong control over the elemental composition of leaves, and their stoichiometric homeostasis of leaves and stems were stronger than roots. Furthermore, the homeostasis of facultative epiphytes decreased in the order N > P. Our results indicated that epiphytic and terrestrial individuals of facultative epiphytes have difference in nutrient limitation, and they use plastic strategies in different habitats. Epiphytic individuals survive in the intermittent habitat through luxury consumption of nutrient while terrestrial individuals were relatively conservative nutrient users. Furthermore, our results implied that facultative epiphytes maintain stable metabolic leaf activity via variable element concentrations of roots to adapt to highly heterogeneous forest habitats.

Fog water is generally considered to be an important water source for epiphytes in cloud forests because they cannot directly access ground-level water sources. However, the water use proportions of potential water sources and water use efficiency of epiphytes in the subtropical montane cloud forests (MCF) remain to be further explored. In this study, we investigated the water use pattern in the dry season and the intrinsic water use efficiency (${\mathrm{WUE}}_{\mathrm{i}}$) of four epiphyte groups (i.e., epiphytic lichens, epiphytic bryophytes, epiphytic ferns, and epiphytic seed plants) using stable isotope (δ²H, δ¹⁸O, and δ¹³C) techniques. Our results indicated that the water sources of epiphytes were significantly different among groups and species. The contribution proportions of fog water to epiphytic lichens, epiphytic bryophytes, epiphytic ferns, and epiphytic seed plants were 83.2%, 32.7%, 38.8% and 63.7%, respectively. Epiphytic lichens and epiphytic seed plants mainly depended on fog water whereas the epiphytic bryophytes and epiphytic ferns relied on both fog water and humus. This may be due to their differences in morphological and structural traits (e.g., thallus or leaves, rhizoid or roots). Additionally, the difference in ${\mathrm{WUE}}_{\mathrm{i}}$ was also significant among epiphyte groups and species, which could be related to their different water acquisition patterns. In conclusion, our study reveals the differentiation of water utilization in epiphytes and confirms the importance of fog water for epiphytes during the dry season.