Root respiration in North American forests: effects of nitrogen concentration and temperature across biomes.

Root respiration rates have been shown to be correlated with temperature and root N concentration in studies of individual forest types or species, but it is not known how universal these relationships are across forest species adapted to widely different climatic and edaphic conditions. In order to test for broad, cross-species relationships, we measured fine root respiration, as O consumption, over a range of temperatures on excised root samples from ten forested study sites across North America in 1997. Significant differences existed among study sites in root respiration rates, with patterns among sites in respiration rate at a given temperature corresponding to differences among sites in fine root N concentrations. Root respiration rates were highly correlated with root N concentrations at all measurement temperatures (r >0.81, P<0.001, for 6, 18 and 24°C). Lower root respiration rates in gymnosperms than in angiosperms were largely explained by lower fine root N concentrations in gymnosperms, and root N concentrations and respiration rates (at a given temperature) tended to be lower at warm sites (New Mexico, Florida, and Georgia) than at cool sites with short growing seasons (Michigan and Alaska). Root respiration rates increased exponentially with temperature at all sites. The Q for root respiration ranged from 2.4 to 3.1, but there were no significant differences among the forest types. The average Q s for gymnosperms (Q =2.7) and angiosperms (Q =2.6) were almost identical, as were the average Q s for roots of ectomycorrhizal species (Q =2.7) and arbuscular mycorrhizal species (Q =2.6). In 1998, fine root respiration at the study sites was measured in the field as CO production at ambient soil temperature. Respiration rates under field conditions were dependent on both ambient soil temperature and root N concentration. Relationships between respiration (adjusted for temperature) and root N concentration for the field measurements were similar to those observed in the 1997 laboratory experiments. For root respiration in tree species, it appears that basic relationships with temperature and nitrogen exist across species and biomes.