Coarse and fine root respiration in aspen (Populus tremuloides)

Coarse and fine root respiration rates of aspen (Populus tremuloides Michx.) were measured at 5, 15 and 25 degreesC. Coarse roots ranged from 0.65 to 4.45 cm in diameter, whereas fine roots were less than 5 mm in diameter. To discriminate between maintenance and growth respiration, root respiration rates were measured during aboveground growing periods and dormant periods. An additional measurement of coarse root respiration was made during spring leaf flush, to evaluate the effect of mobilization of resources for leaf expansion on root respiration. Fine roots respired at much higher rates than coarse roots, with a mean rate at 15 degreesC of 1290 mumol CO2 m(-3) s(-1) during the growing period, and 660 mumol CO2 m(-3) s(-1) during the dormant period. The temperature response of fine root respiration rate was nonlinear: mean Q(10) was 3.90 for measurements made at 5-15 degreesC and 2.19 for measurements made at 15-25 degreesC. Coarse root respiration rates measured at 15 degreesC in late fall (dormant season) were higher (370 mumol CO2 m(-3) s(-1)) than rates from roots collected at leaf flush and early summer (200 mumol CO2 m(-3) s(-1)). The higher respiration rates in late fall, which were accompanied by decreased total nonstructural carbohydrate (TNC) concentrations, suggest that respiration rates in late fall included growth expenditures, reflecting recent radial growth. Neither bud flush nor shoot growth of the trees caused an increase in coarse root respiration or a decrease in TNC concentrations, suggesting a limited role of coarse roots as reserve storage organs for spring shoot growth, and a lack of synchronization between above- and below-ground growth. Pooling the data from the coarse and fine roots showed a positive correlation between nitrogen concentration and respiration rate.