Abstract
The dynamics of rapid changes in carbon (C) partitioning within forest ecosystems are not well understood, which limits improvement of mechanistic models of C cycling. Our objective was to inform model processes by describing relationships between C partitioning and accessible environmental or physiological measurements, with a special emphasis on short-term C flux through a forest ecosystem. We exposed eight 7-year-old loblolly pine (Pinus taeda L.) trees to air enriched with 13CO2 and then implemented adjacent light shade (LS) and heavy shade (HS) treatments in order to manipulate C uptake and flux. The impacts of shading on photosynthesis, plant water potential, sap flow, basal area growth, root growth, and soil CO2 efflux rate (CER) were assessed for each tree over a three-week period. The progression of the 13C label was concurrently tracked from the atmosphere through foliage, phloem, roots, and surface soil CO2 efflux. The HS treatment significantly reduced C uptake, sap flow, stem growth and fine root standing crop, and resulted in greater residual soil water content to 1 m depth. Sap flow was strongly correlated with CER on the previous day, but not the current day, with no apparent treatment effect on the relationship. Although there were apparent reductions in new C flux belowground, the heavy shade treatment did not noticeably reduce the magnitude of belowground autotrophic and heterotrophic respiration based on surface soil CO2 efflux rate (CER), which was overwhelmingly driven by soil temperature and moisture. The 13C label was immediately detected in foliage on label day (half-life = 0.5 d), progressed through phloem by day 2 (half-life = 4.7 d), roots by day 2-4, and subsequently was evident as respiratory release from soil which peaked between days 3-6. The δ13C of soil CO2 efflux was strongly correlated with phloem δ13C on the previous day, or two days earlier. While the 13C label was readily tracked through the ecosystem, the fate of root C through respiratory, mycorrhizal or exudative release pathways were not assessed. These data detail the timing and relative magnitude of C flux through various components of a young pine stand in relation to environmental conditions.
