Response of stomatal conductance to termination of long-term CO2 enrichment

Background/Question/Methods  

Stomata are the common pathways through which carbon uptake and water vapor loss take place in a plant. Therefore, the response of stomatal conductance (G_S) to environmental factors, including the rise of atmospheric CO₂ concentration (eCO₂), affect tree and forest water use and photosynthesis. eCO₂ may affect G_S by increasing intercellular CO₂ concentration (direct) and by adjusting allometrics or hydraulics (indirect). Direct and indirect processes may take different exposure time to play out, inducing differing responses among species. Separating direct from indirect responses of different functional types (e.g., coniferous v. broadleaved deciduous species), is necessary for proper rendering in models from leaf to regional scales.

We investigated the G_S responses of Pinus taeda and Liquidambar styraciflua during and following free-air CO₂ enrichment (FACE). The study was conducted at the Duke FACE experiment site, NC, USA from 2009 to 2012. Four plots in both ambient CO₂ (aCO₂) and eCO₂ were established in 1996, and the CO₂ enrichment terminated in 2010. Half of each plot received nitrogen (N) fertilization since 2005. Using the hierarchical Bayesian state-space framework, we modeled G_S of each species as a multiplicative function of reference G_S, vapor pressure deficit, photosynthetic photon flux density and soil moisture.

Results/Conclusions

Total canopy conductance and transpiration in eCO₂ was not significantly different from that in aCO₂ both before and after the termination of FACE regardless of N fertility. Ratio of monthly daytime mean G_S of P. taeda (eCO₂/aCO₂) before termination of FACE (0.69 and 0.74 under native soil and N-fertilized condition, respectively) increased to 1.12 and 0.97 after the termination of FACE. The mean ratio of L. styraciflua was slightly higher after termination than before (0.94 and 0.84, respectively) under native soil condition; the ratio increased from 0.64 to 1.11 under N-fertilized condition. Leaf area index (L) of P. taeda was greater in eCO₂, but the enhancement of L declined quickly after FACE termination. L enhancement of L. styraciflua was only significant under N-fertilized condition under eCO₂, but the enhancement disappeared after termination of FACE. Because direct response of G_S to short-term manipulation of CO₂ was not observed, the decline of L likely has led to the increase in G_S indirectly. Because c_i/c_a was unaffected by eCO₂, we conclude that, at the canopy scale, trees operating under eCO₂ for 17 years, assimilated similar amount of carbohydrates as those in aCO₂ soon after enrichment termination.