Canopy conductance and gas exchange of a Japanese cypress forest after rainfall‐induced wetness

Abstract

Increased stomatal conductance of the dry leaves right after wetness ended was reported before, while few studies paid attention to this phenomenon in canopy scale and its contribution to the forest carbon sink function. Therefore, we observed the continuous change of canopy wetness and ecosystem fluxes after wetness ended by using wetness sensors and eddy covariance method for a Japanese cypress forest (Chamaecyparis obtuse Sieb. Et Zucc.) in the Asian monsoon area. Larger g c (canopy conductance) were found in the relationships with both vapor pressure deficit and R n (net radiation) during the first several hours after wetness ended. The results for λE (latent heat flux) were similar to that of g c in spring and summer. Net ecosystem exchange (NEE) in the relationships with R n at different stages after wetness ended did not show clear difference except in spring. Nevertheless, annual summed-up NEE showed that the average CO 2 uptake at the first several hours after wetness ended was larger than the usual dry period. These results suggest that the improved stomatal opening at the first several hours after wetness ended plays a significant role in the canopy-scale gas exchange processes.

Does interception evaporation occur from both sides of leaves at the wet Japanese cypress canopy during and after rainfall?

“…In a previous study, the average daytime net ecosystem exchange (NEE) was −2.1 and −6.2 μmol m −2 s −1 for wet and dry time in this forest (Jiao et al, 2021), which suggests that the forests can maintain gas exchange capacity during the wet canopy period. The carbon uptake by the wet canopy throughout the seasons around 76 g C m −2 in this forest (Jiao et al, 2021) is reckoned as a consequence of the frequently happened Model 1 situation. On the one hand, Model 1 is easier to occur than Model 2 after small rainfall, and the small rainfall events happened more frequently and led to more wet time (Figure 1) than middle and heavy rainfall.…”

“…Leaf wetness is defined as the free water on the leaf surface (Magarey et al, 2005; Park et al, 2019) and measured by a handmade resistance‐based wetness sensor (Jiao et al, 2021; Takanashi et al, 2003). Leaf wetness will lead to the change in sensor resistance and result in the variation of voltage, which is reflected by the output signal.…”

“…For example, Chu et al (2014) found a higher canopy conductance of a yellow cypress forest by 2‐year intensive observation of latent heat flux ( λE ) and sap flow on periodical wet days than on the dry days. Jiao et al (2021) also found increased canopy conductance and λE in the first several hours after wetness ended. The increased gas exchange after wetting can be attributed to the decreasing leaf temperature and vapour pressure deficit (VPD) caused by wetting (Dawson & Goldsmith, 2018; Johnson & Smith, 2008; Smith & McClean, 1989).…”

“…Leaf wetness is defined as the free water on the leaf surface (Magarey et al, 2005;Park et al, 2019) and measured by a handmade resistance-based wetness sensor (Jiao et al, 2021;Takanashi et al, 2003). Secondly, the simulated leaf wetness degree (WD m ) is used to evaluate the model wetness duration, and this wetness degree derived by the model-simulated leaf water storage is calculated as follows:…”

“…in this forest (Jiao et al, 2021) is reckoned as a consequence of the frequently happened Model 1 situation. On the one hand, Model 1 is easier to occur than Model 2 after small rainfall, and the small rainfall events happened more frequently and led to more wet time (Figure 1) than middle and heavy rainfall.…”