Gas exchange and stand-level estimates of water use and gross primary productivity in an experimental pine and switchgrass intercrop forestry system on the Lower Coastal Plain of North Carolina, U.S.A
Despite growing interest in using switchgrass (Panicum virgatum L.) as a biofuel, there are limiteddata on the physiology of this species and its effect on stand water use and carbon (C) assimilationwhen grown as a forest intercrop for bioenergy. Therefore, we quantified gas exchange rates of switch-grass within intercropped plots and in pure switchgrass plots during its second growing season in anintensively managed loblolly pine (Pinus taeda L.) plantation in North Carolina. Switchgrass physiol-ogy was characterized over the growing season from June to October 2010 in terms of photosynthesis(mol m−2s−1), stomatal conductance (mmol m−2s−1), and assimilation responses to photosyntheticphoton flux density and intercellular carbon dioxide concentration (CO2). We then used a process-based model of the soil–plant–atmosphere continuum to scale leaf-level gas exchange data to provideestimates of pine and switchgrass stand-level water use (mm) and carbon exchange (g C m−2) over athree-year period. Peak switchgrass photosynthesis (32.7 ± 0.9 mol m−2s−1) and stomatal conductance(252 ± 12 mmol m−2s−1) rates were measured in July, with minimum values (18.7 ± 1.4 mol m−2s−1and104 ± 6 mmol m−2s−1, respectively) recorded at the end of the growing season (October). Switchgrassgas exchange and parameter estimates from the light- and CO2response curves did not vary betweentreatments. However, gas exchange values differed significantly between measurement dates. Modelpredictions of stand-level transpiration ranged from 287 to 431 mm year−1for pine and from 245 to296 mm year−1for switchgrass. Annual C exchange for loblolly pine ranged from 1165 to 1903 g m−2compared to 1386 to 1594 g m−2for switchgrass. At this stage of stand development, no effect of inter-cropping was evident and there was no effect of distance from the nearest pine row on any switchgrassgas exchange variable measured. However, we anticipate that as this intercropped system develops overtime, competition for resources such as light, water or nitrogen may change, with the potential to impactswitchgrass physiology and biomass production.