Seasonal respiration of foliage, fine roots, and woody tissues in relation to growth, tissue N, and photosynthesis
Autotrophic respiration may regulate how ecosystem productivity responds to changes in temperature, atmospheric [CO2], and N deposition. Estimates of autotrophic respiration are difficult for forest ecosystems, because of the large amount of biomass, different metabolic rates among tissues, and seasonal variation in respiration rates. We examined spatial and seasonal patterns in autotrophic respiration in a Pinus strobus ecosystem, and hypothesized that seasonal patterns in respiration rates at a common temperature would vary with [N] for fully expanded foliage and fine roots, with photosynthesis for foliage, and with growth for woody tissues (stems, branches, and coarse roots). We also hypothesized that differences in [N] would largely explain differences in maintenance or dormant-season respiration among tissues.
For April–November, mean respiration at 15°C varied from 1.5 to 2.8 m>mol kg-1 s-1 for fully expanded foliage, 1.7–3.0 for growing foliage, 0.8–1.6 for fine roots, 0.6–1.1 (sapwood) for stems, 0.5–1.8 (sapwood) for branches, and 0.2–1.5 (sapwood) for coarse roots. Growing season variation in respiration for foliage produced the prior year was strongly related to [N] (r2 = 0.94), but fine root respiration was not related to [N]. For current-year needles, respiration did not covary with [N]. Night-time foliar respiration did not vary in concert with previous-day photosynthesis for either growing or fully expanded needles. Stem growth explained about one-third of the seasonal variation in stem respiration (r2 = 0.38), and also variation among trees (r2 = 0.43). We did not determine the cause of seasonal variation in branch and coarse root respiration, but it is unlikely to be directly related to growth, as the pattern of respiration in coarse roots and branches was not synchronized with stem growth. Seasonal variations in temperature-corrected respiration rates were not synchronized among tissues, except foliage and branches. Spatial variability in dormant-season respiration rates was significantly related to tissue N content in foliage (r2 = 0.67), stems (r2 = 0.45), coarse roots (r2 = 0.36), and all tissues combined (r2=0.83), but not for fine roots and branches. Per unit N, rates for P. strobus varied from 0.22 to 3.4 mmol mo1N-1s-1 at 15°C, comparable to those found for other conifers. Accurate estimates of annual autotrophic respiration should reflect seasonal and spatial variation in respiration rates of individual tissues.