An instrument design and sample strategy for measuring soil respiration in the coastal temperate rain forest

The coastal temperate rainforest (CTR) along the northwest coast of North America is a large and complex mosaic of forests and wetlands located on an undulating terrain ranging from sea level to thousands of meters in elevation. This biome stores a dynamic portion of the total carbon stock of North America. The fate of the terrestrial carbon stock is of concern due to the potential for mobilization and export of this store to both the atmosphere as carbon respiration flux and ocean as dissolved organic and inorganic carbon flux. Soil respiration is the largest export vector in the system and must be accurately measured to gain any comprehensive understanding of how carbon moves though this system. Suitable monitoring tools capable of measuring carbon fluxes at small spatial scales are essential for our understanding of carbon dynamics at larger spatial scales within this complex assemblage of ecosystems. We have adapted instrumentation and developed a sampling strategy for optimizing replication of soil respiration measurements to quantify differences among spatially complex landscape units of the CTR. We start with the design of the instrument to ease the technological, ergonomic and financial barriers that technicians encounter in monitoring the efflux of CO2 from the soil. Our sampling strategy optimizes the physical efforts of the field work and manages for the high variation of flux measurements encountered in this difficult environment of rough terrain, dense vegetation and wet climate. Our soil respirometer incorporates an infra-red gas analyzer (LiCor Inc. LI-820) and an 8300 cm3 soil respiration chamber; the device is durable, lightweight, easy to operate and can be built for under $5000 per unit. The modest unit price allows for a multiple unit fleet to be deployed and operated in an intensive field monitoring campaign. We use a large 346 cm2 collar to accommodate as much micro spatial variation as feasible and to facilitate repeated measures for tracking temporal trends. Our collar design minimizes root interference yet provides a highly stable platform for coupling with the respirometer. Meso-scale variability characterized by large down woody debris, wind throw pits and mounds and surface roots is negotiated with by a hexagonal array of seven collars at two meter spacing (sample pod). Landscape scale variability is managed through stratification and replication amongst ecosystem types arrayed across a hydrologic gradient from bogs to forested wetlands to upland forests. Our strategy has allowed us to gather data sets consisting of approximately 1800 total observations with approximately 600 measurements per replication per year. Mean coefficients of variation (CV) at the collar (micro-scale) were approximately 0.67. The pod level mean CV was reduced to approximately 0.29 at the pod (meso-scale). The CV at the vegetation strata were 0.43, 0.18 and 0.21 for bog, forested wetland and upland forest respectively. With temperature and hydrological data we are able to measure and model carbon dynamics in this large and complex environment. The analysis of variability at the three spatial scales has confirmed that our approach is capturing and constraining the variability.