Contrasting fine-root production, survival and soil CO2 efflux in pine andpoplar plantations

Tree root activity, including fine-root production, turnover and metabolic activity are significant components of forest productivity and nutrient cyc ling. Differences in root activity among forest types are not well known. A 3-year study was undertaken in red pine (Pinus resinosa Ait.) and hybrid p oplar (Populus tristis X P. balsamifera cv `Tristis no. 1') plantations to compare belowground root dynamics. We measured fine-root production, mortal ity and standing crop, as well as soil CO2 efflux. Pine fine-root productio n was only 2.9% of that of poplar during three years; 85 pine roots were ob served in minirhizotron tubes compared with 4088 poplar roots. Live-root de nsity oscillated seasonally for both species with late winter minimum and a utumn maximum. Poplar reached constant maximum live-root length within the first growing season, but pine continued to increase observed fine-root len gth for three growing seasons. Within the first 100 days following initial appearance, 22% of the pine roots disappeared and 38% of the poplar roots d isappeared. Median fine-root longevity of pine was 291 days compared with 1 49 days for poplar roots. Fine-root longevity increased with depth in the s oil, and was greater for roots with initial diameter >0.5 mm. The probabili ty of poplar root death from late February to May was more than three times that in any other season, regardless of root age. Despite the greater popl ar root production and live-root length, fine-root biomass and soil CO2 eff lux was greater in pine. Greater metabolic activity in the pine stand may b e due to greater fine-root biomass or greater heterotrophic respiration.