Effects of canopy phenology on deciduous overstory and evergreen understory carbon budgets in a cool-temperate forest ecosystem under ongoing climate change

Abstract

Canopy phenology is a key regulator of carbon cycling in forest ecosystems. To clarify its possible effects on carbon budgets of forest ecosystems under ongoing climate change, we developed a canopy-phenology model for a forest with deciduous overstory and evergreen understory based on in situ observations, and used it to improve an ecosystem carbon budget model. Under future conditions (2068–2073) based on the IPCC SRES A1B scenario, leaf expansion began 12.5 ± 1.9 days earlier and leaf-fall ended 11.3 ± 2.7 days later than under current conditions (2002–2007). We also estimated the possible influence of altered light availability on understory vegetation. Even though the photosynthetically active period in the understory (i.e., from the end of spring snowmelt to the beginning of late-autumn snow cover) expanded by 15.7 ± 15.7 days, the total downward photosynthetic photon flux density above this vegetation during the snow-free period decreased by 11.8 % because of changing overstory canopy phenology. The net effect of these changes increased ecosystem-level annual gross primary production (GPP) by 12.5 %, net primary production (NPP) by 12.0 %, and net ecosystem production by 12.1 %, especially in late spring (when the highest solar radiation occurred). The increased GPP and NPP were mostly attributable to changes in overstory vegetation. Our analysis indicates that understanding the temporal variation of canopy phenology dynamics and snow cover is important and that the effects of vegetation phenology on the carbon cycle should be evaluated in future climate change studies.

Appendix: photosynthetic and respiration components for the overstory vegetation

NCAR LSM is a sun and shade model. The sunlit fraction (f _sun) of the canopy is based on the fractional area of sunflecks on a horizontal plane below leaf and stem areas and on scattering within the canopy. The shaded fraction (f _shade) equals 1.0 minus the sunlit fraction, and the sunlit and shaded LAIs are [L _sun = f _sun L] and [L _shade = f _shade L], respectively, where L represents LAI. Bonan (1996) provides details of the calculation of incident solar radiation at canopy level and of the energy budget resulting from the radiation and water vapor balances. The photosynthetic part of LSM (Bonan 1996) is based on the parameterizations of Farquhar et al. (1980) and Collatz et al. (1991). Single-leaf photosynthesis of C₃ plants is controlled by the RuBP carboxylase-limited rate of carboxylation, maximum rate of carboxylation allowed by the capacity to regenerate RuBP, and the export-limited rate of carboxylation. The photosynthetic rate is coupled to the parameterization of stomatal resistance (Collatz et al. 1991) and is hence an integral part of the surface energy fluxes. These calculations are made for both the sunlit and shaded parts of the canopy (A _sun and A _shade) and are summed for the entire overstory canopy as $\left[ {{\text{GPP}}_{\text{o}} = A_{\text{sun}} \cdot L_{\text{sun}} + A_{\text{shade}} \cdot L_{\text{shade}} } \right]$.

Overstory deciduous plant respiration is separated into maintenance and growth respiration. Total maintenance respiration in the LSM is determined by the sum of foliar, stem, and root respiration as functions of leaf area index (m² m^–2) and temperature. Parameters are defined for foliar respiration at 25 °C (μmol CO₂ m⁻² s⁻¹), stem biomass (kg m⁻²), stem respiration at 25 °C (μmol CO₂ kg^–1 s^–1), root biomass (kg m⁻²), root respiration at 25 °C (μmol CO₂ kg⁻¹ s⁻¹), and temperature sensitivity. Growth respiration is proportional to overstory net primary production (NPP_o).