Aboveground autotrophic respiration in a Spanish black pine forest: Comparison of scaling methods to improve component partitioning

Total wood CO₂ efflux (R_w) varies vertically within individual trees, and leaves experience large variations in foliar respiration (R_f) rates over their life spans and during daily periods. Therefore, accurate sampling approaches are required to improve aboveground autotrophic respiration (R_Aa) estimations in stand-scale carbon cycling studies. We scaled-up R_w (comprising stem and branch CO₂ efflux; ES and EB, respectively) and R_f from biometric and flux-chamber measurements taken between 2011 and 2013 in a Spanish black pine (Pinus nigra Arn. ssp. salzmannii) forest at an unburnt (UB) site and a low burn-severity (LS) site. We measured seasonal ES at breast height (1.30m) on 9 trees at each site, which was also vertically examined on 5 of those trees. We also measured seasonal R_f in current- and previous-year needles on 3 trees at each site, and quantified R_f variations in darkness and light. Finally, we compared complex and simple scale-up methods which did or did not account for the vertical variation in R_w and the effects of leaf ageing and light inhibition on R_f, respectively. The simple methods underestimated the annual stand-level stem, branch, and total wood respiration ≈35%, 55%, and 41%, respectively, and overestimated annual stand-level whole-canopy foliage respiration ≈43% at both sites. Both methods provided similar annual stand-level R_Aa estimates, although the complex methods improved estimations of the relative contribution of R_Aa components. Thus, based on the complex methods the mean annual R_Aa at the stand-level was 4.53±0.25 and 4.45±0.12MgCha^-1year^-1 at the UB and LS sites, respectively. Our data also confirmed that the low-severity fire did not alter the R_Aa rates. Collectively, this study reveals that complex approaches, applicable in other forest ecosystems, enhance the accuracy of partitioning R_Aa sources by reducing the error in scaling-up in chamber-based measurements.