Abstract
BackgroundAnthropogenic disturbance of old-growth tropical forests increases the abundance of early successional tree species at the cost of late successional ones. Quantifying differences in terms of carbon allocation and the proportion of recently fixed carbon in soil CO2 efflux is crucial for addressing the carbon footprint of creeping degradation.MethodologyWe compared the carbon allocation pattern of the late successional gymnosperm Podocarpus falcatus (Thunb.) Mirb. and the early successional (gap filling) angiosperm Croton macrostachyus Hochst. es Del. in an Ethiopian Afromontane forest by whole tree 13CO2 pulse labeling. Over a one-year period we monitored the temporal resolution of the label in the foliage, the phloem sap, the arbuscular mycorrhiza, and in soil-derived CO2. Further, we quantified the overall losses of assimilated 13C with soil CO2 efflux.Principal Findings 13C in leaves of C. macrostachyus declined more rapidly with a larger size of a fast pool (64% vs. 50% of the assimilated carbon), having a shorter mean residence time (14 h vs. 55 h) as in leaves of P. falcatus. Phloem sap velocity was about 4 times higher for C. macrostachyus. Likewise, the label appeared earlier in the arbuscular mycorrhiza of C. macrostachyus and in the soil CO2 efflux as in case of P. falcatus (24 h vs. 72 h). Within one year soil CO2 efflux amounted to a loss of 32% of assimilated carbon for the gap filling tree and to 15% for the late successional one.ConclusionsOur results showed clear differences in carbon allocation patterns between tree species, although we caution that this experiment was unreplicated. A shift in tree species composition of tropical montane forests (e.g., by degradation) accelerates carbon allocation belowground and increases respiratory carbon losses by the autotrophic community. If ongoing disturbance keeps early successional species in dominance, the larger allocation to fast cycling compartments may deplete soil organic carbon in the long run.
Highlights
The residence time of the assimilated carbon in an ecosystem is a function of its allocation in the plant-soil system [1]
Our results showed clear differences in carbon allocation patterns between tree species, we caution that this experiment was unreplicated
After the chambers were opened, the d13C of the foliage of both labeled trees was strongly elevated as compared to the control trees, with a d13C value of 15576871% for C. macrostachyus and of 248650.5% for P. falcatus
Summary
The residence time of the assimilated carbon in an ecosystem is a function of its allocation in the plant-soil system [1]. During the last few years good progress has been made with 13C and 14C pulse labeling experiments of whole trees to measure the carbon allocation along with the contribution of recent photosynthates to soil CO2 efflux, e.g., see reviews of Bruggemann et al [7] and Epron et al [8]. Studies in boreal [9,10,11] and temperate [12,13,14] forests provided convincing evidence that soil respiration is closely linked to photosynthesis and that the contribution of recently assimilated carbon to soil CO2 efflux is large. Gained carbon can be rapidly transferred belowground, influencing soil CO2 efflux rates on short-time scales (from hours to days) [1,3,18,19]. Quantifying differences in terms of carbon allocation and the proportion of recently fixed carbon in soil CO2 efflux is crucial for addressing the carbon footprint of creeping degradation
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