Influence of repeated canopy scorching on soil CO2 efflux
Forest ecosystems experience various disturbances that can affect belowground carbon cycling to different degrees. Here, we investigate if successive annual foliar scorching events will result in a large and rapid decline in soil CO2 efflux, similar to that observed in girdling studies. Using the fire-adapted longleaf pine (Pinus palustris Mill.) tree species, we experimentally manipulated foliar leaf area and thus, canopy photosynthesis, via foliar scorching over two consecutive growing seasons. We monitored the effect of scorching on soil CO2 efflux and fine root production, mortality, standing crop, and nitrogen (N) and non-structural carbohydrate (i.e. sugar and starch) concentrations. Despite an immediate 80% reduction in foliar leaf area and sap flow rates from the scorch treatment, there was no effect on soil CO2 efflux in either year. Likewise, the cumulative soil CO2 flux after two scorch treatments remained comparable to that of the control treatment, even after assuming a 100% decline in the autotrophic component for the month following the two scorching events. Fine root standing crop was not diminished by scorching because both fine root production and mortality increased commensurately in the scorch treatment. Fine root N and sugar concentrations were not diminished by scorching, but starch concentrations of 5th order roots decreased after the second scorching treatment, presumably because starch was mobilized from larger roots to maintain more metabolically active 1st order roots. The lack of response observed in soil CO2 efflux following successive canopy scorches differs from the response often observed after girdling and suggests that the carbohydrate reserves of longleaf pine trees are sufficient to maintain root metabolism for extended periods even after an extreme canopy perturbation. We propose that tree species in ecosystems that experience frequent disturbance may allocate more carbon to storage than those in less disturbed ecosystems, and as a result are more resilient to disturbances that affect photosynthate supply. Such species should be capable of maintaining belowground autotrophic respiration during periods of minimal or nonexistent carbon assimilation.
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