Intra-annual variation of soil respiration across four heterogeneous longleaf pine forests in the southeastern United States

Soil respiration (RS) is one of the largest fluxes of net ecosystem exchange and is related to both soil climate and vegetatively driven substrate supply at various spatial and temporal scales. Relationships between the intra-annual variation in RS and abiotic and biotic variables were examined across diverse longleaf pine (Pinus palustris Mill.) forests to better understand factors related to RS in these low density, spatially heterogeneous forests. Soil respiration, soil temperature, soil moisture, litter mass, size and proximity of nearby trees, understory cover, and root biomass were measured over 13 months in four longleaf pine forests varying in age from 5 to 87 years. The exponential relationship between RS and soil temperature accounted for the majority of the intra-annual variation in RS with a corresponding temperature sensitivity (Q10) of 2.18. Soil moisture affected the RS-temperature relationship by dampening RS and Q10 during times of extremely dry soil conditions, as defined by soil moisture ⩽50% of the texture-derived wilting point, but volumetric soil moisture did not directly correlate with RS. The intra-annual variation in temperature-normalized RS was negatively related to the distance to nearest tree and positively related to pine root biomass, but not related to litter mass, understory cover, or stand structural variables such as stand age, basal area, or tree density. Annual RS estimates ranged from 12.0 Mg ha-1 of C in the 5-year-old stand with mostly grass stage seedlings to 13.9 Mg ha-1 of C in the dense 21-year-old stand. This study contributes to our understanding of carbon fluxes across diverse longleaf pine ecosystems and indicates the importance of climate in determining the carbon sink potential of southeastern longleaf pine forests.