Response of two epiphyte species to nitrogen and phosphorus fertilization at a humid subtropical rainforest in northeastern Taiwan

Several studies indicate that epiphytes share similar nutrient limitation with soil-rooted plants in the same ecosystem. Compared to trees, epiphytes have a much shorter life span so that epiphytes should respond more rapidly to fertilization than trees and can be used as an indicator for ecosystem nutrient limitation. We conducted a fertilization experiment on two epiphyte species Haplopteris zosterifolia and Asplenium antiquum, a basket-shaped epiphyte that collects litterfall and forms a nutrient rich substrate, at the Fushan Experimental Forest of northeastern Taiwan. The co-occurrence of the two species is common but they can also grow alone. We compared nutrient concentrations and nutrient ratios nine months after fertilization. The results show that adding the nutrients, nitrogen (N) or phosphorus (P), did not lead to a greater biomass increment compared to the control group for both species. P fertilization increased P concentration and decreased the N:P ratio in both species, while N fertilization increased the foliar N concentration and decreased the C:N ratio only in A. antiquum. The positive response to P fertilization but not N fertilization for both species fits the general assumption of greater P than N limitation at low elevation forests in Taiwan. The biomass change of A. antiquum over nine months was not different between individuals growing alone and individuals co-occurring with H. zosterifolia. In contrast, biomass of H. zosterifolia increased in co-occurring individuals but decreased in individuals growing alone possibly due to drought mitigation from the substrate of A. antiquum. In summary, our study suggests that epiphyte growth is probably more limited by water than by nutrients. Although high atmospheric nitrogen deposition at the Fushan Experimental Forest and elevated atmospheric CO₂ concentrations have the potential to enhance plant growth, the effects may not be realized for epiphytes if water supply is not increased. Our results also indicates that although nutrient limitation may be similar between epiphytes and trees, the ultimate limiting factor may be different because water shortage is less a problem for trees than epiphytes in moist forests.