Relationships between belowground plant carbon and canopy structure in a temperate forest ecosystem

Background/Question/Methods

Accurate estimation of terrestrial carbon pools is a key focus in an age of concern over global carbon budgets. In the near future a change in climate could alter global CO₂ uptake and decomposition rates of forests, changing how carbon is stored both above- and belowground in these ecosystems. Currently, relationships between aboveground forest structure and belowground plant carbon are poorly understood, thus a better understanding of relationships between aboveground forest structure and belowground forest carbon dynamics is drastically needed, largely because forests store a vast majority of forests respond so heavily to elevated levels of atmospheric CO₂. This study addresses variability in fine root mass and soil CO₂ efflux as they are influenced by the complexity of the upper forest canopy. Here we use a network of permanent plots for estimations of aboveground canopy structure and belowground carbon in a Pacific Northwest, USA forest ecosystem. Forty clear plastic tubes installed below the soil surface are used for measurement of fine root biomass using a minirhizotron optical scanning device. Soil respiration is measured on the forest floor using a differential open system infrared gas analyzer with soil chamber attachment adjacent to all 40 minirhizotron tubes. All measurements were taken monthly from April 2010 to April 2011.  Models of the upper canopy surface obtained from aerial LIDAR flown in 2008, are used for calculation of the standard deviation of canopy height values, hereafter canopy roughness, above all 40 minirhizotron-soil respiration plots.

Results/Conclusions

We found significant positive relationships between soil respiration and canopy roughness where canopy roughness explained 42% of the variation in soil respiration in positive linear relationships (r² = .42; P < 0.05). Similarly, we found significant positive relationships between canopy roughness and fine root mass where canopy roughness explained 35% of the variation in annual fine root biomass (r² = .35; P < 0.05). These data demonstrate that forest canopy structure has the potential to explain a significant portion of the variation in belowground carbon dynamics. Additionally, this study provides a baseline for similar studies in need of remotely predicting belowground plant carbon and has the potential for the creation of repeatable allometric equation for the prediction of belowground carbon using a remote sensing tool.