Inconsistent Responses of Transpiration of Different Canopy Layers to Simulated Canopy and Understory N Depositions in a Low‐Subtropical Evergreen Broadleaf Forest

Abstract

Quantifying transpiration and its response to meteorological factors at a stand scale and different canopy layers of forest is important for understanding the hydrological impact of nitrogen (N) deposition on local water balance. For this purpose, sap flows were measured in three upper (Schima superba, Castanea henryi, and Machilus chinensis) and three lower (Symplocos ramosissima, Ilex ficoidea, and Schefflera octophylla) canopy species based on a manipulative experimental platform with canopy (CN) and understory (UN) N additions in a low-subtropical evergreen broadleaf forest. Each application included concentrations of 0, 25, and 50 kg·ha −1 ·a −1 N. Results showed that N addition obviously influenced the sap flux density and such influence varied among different species and between canopy layers. Upper canopy species contributed the most transpiration for the whole forest, and CN25 greatly affected the transpiration in both upper and lower canopy layers. Daily water use (Q d ) increased logarithmically with daytime average photosynthetically active radiation and daily average vapor pressure deficit. An appropriate canopy N addition (CN25) promoted the Q d and its sensitivity to micrometeorological factors, whereas UN had a minimal effect. These results suggest a substantial difference in response of transpiration to the N addition ways. CN had more effective influence on the canopy transpiration. Thus, UN could not fully reflect the effects of increased N deposition on the canopy-associated transpiration, and the response of forest transpiration to N deposition needs to be analyzed beyond species and upscaled to the canopy level in favor of meaningful assessments of N deposition.

Multi‐ecosystem services differently affected by over‐canopy and understory nitrogen additions in a typical subtropical forest

“…The service of water yield (runoff and soil water) showed opposite trends under the over-canopy and understory N additions, which indicates different effects of two N addition methods (Figure 2). Li et al (2020) found that over-canopy N addition produces greater effects on annual transpiration than understory N addition at our study site, demonstrating differences between over-canopy and understory N additions on water yield service. Another study conducted in a tropical forest about 200 km away from our study site found that plants increase transpiration to keep stable foliar nutrient while soil cations, especially Ca and Mg, are depleted under N addition, which finally caused the decrease of water yield, mainly based on the results of N addition rates of 100 and 150 kg ha −1 year −1 (Lu et al, 2018).…”

Effects of Simulated Nitrogen Deposition and Micro-Environment on the Functional Traits of Two Rare and Endangered Fern Species in a Subtropical Forest

“…Among those studies, canopy N addition can induce increased survival stresses for some understory plants [ 5 , 17 ]; it can also promote the diversity of soil biota and macrofauna but shows no impact on plant diversity [ 15 , 31 ]. Canopy N addition promoted or showed no impact on the growth of dominant canopy tree species, depending on the specific tree species [ 18 , 38 ]. Moreover, compared with CAN, UAN always showed greater effects on the dominant understory species but weaker effects on the dominant tree species and soil fauna [ 17 , 18 , 38 , 39 ].…”

“…Canopy N addition promoted or showed no impact on the growth of dominant canopy tree species, depending on the specific tree species [ 18 , 38 ]. Moreover, compared with CAN, UAN always showed greater effects on the dominant understory species but weaker effects on the dominant tree species and soil fauna [ 17 , 18 , 38 , 39 ]. However, the effects of N addition on REFs and whether these ferns have similar responses to dominant species are still unknown.…”