Timing and Magnitude of C Partitioning Through a Young Loblolly Pine (Pinus taeda L.) Stand Using 13C Labeling and Shade Treatments
Tree Physiology Advance Access http://dx.doi.org/10.1093/treephys/tpr129
The dynamics of rapid changes in carbon (C) partitioning within forest ecosystems are not well understood, which limitsimprovement of mechanistic models of C cycling. Our objective was to inform model processes by describing relationshipsbetween C partitioning and accessible environmental or physiological measurements, with a special emphasis on short-termC flux through a forest ecosystem. We exposed eight 7-year-old loblolly pine (Pinus taeda L.) trees to air enriched with 13CO2and 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 effluxrate (CER) were assessed for each tree over a 3-week period. The progression of the 13C label was concurrently tracked fromthe 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. Soil CERwas strongly correlated with sap flow on the previous day, but not the current day, with no apparent treatment effect on therelationship. Although there were apparent reductions in new C flux belowground, the HS treatment did not noticeablyreduce the magnitude of belowground autotrophic and heterotrophic respiration based on surface soil CER, which was overwhelminglydriven by soil temperature and moisture. The 13C label was immediately detected in foliage on label day (halflife= 0.5 day), progressed through phloem by Day 2 (half-life = 4.7 days), roots by Days 2–4, and subsequently was evidentas respiratory release from soil which peaked between Days 3 and 6. The δ13C of soil CO2 efflux was strongly correlated withphloem δ13C on the previous day, or 2 days earlier. While the 13C label was readily tracked through the ecosystem, the fate ofroot C through respiratory, mycorrhizal or exudative release pathways was not assessed. These data detail the timing andrelative magnitude of C flux through various components of a young pine stand in relation to environmental conditions.