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Biometric and eddy-covariance based estimates of carbon fluxes in an age-sequence of temperate pine forests

Peichl, Matthias, Brodeur, Jason J., Khomik, Myroslava, Arain, M. Altaf
Agricultural and forest meteorology 2010 v.150 no.7-8 pp. 952-965
coniferous forests, Pinus, temperate forests, carbon dioxide, gas exchange, chronosequences, tree age, measurement, biometry, primary productivity, oxygen, accuracy, cell respiration, carbon sequestration, dry matter accumulation, seedlings, temporal variation, soil respiration, Ontario
We present four years (2005-2008) of biometric (B) and eddy-covariance (EC) measurements of carbon (C) fluxes to constrain estimates of gross primary production (GPP), net primary production (NPP), ecosystem respiration (RE) and net ecosystem production (NEP) in an age-sequence (6-, 19-, 34-, and 69-years-old in 2008) of pine forests in southern Ontario, Canada. The contribution of individual NPP and respiration component fluxes varied considerably across the age-sequence, introducing different levels of uncertainty. Biometric and EC-based estimates both suggested that annual NPP, GPP, RE, and NEP were greatest at the 19-year-old site. Four-year mean values of NEP₍B₎ and NEP₍EC₎ were similar at the 6-year-old seedling (77 and 66gCm⁻² y⁻¹) and the 69-year-old mature site (135 and 124gCm⁻² y⁻¹), but differed considerably at the 19-year-old (439 and 736gCm⁻² y⁻¹) and the 34-year-old sites (170 and 392gCm⁻² y⁻¹). Both methods suggested similar patterns for inter-annual variability in GPP and NEP. Multi-year convergence of NEP₍B₎ and NEP₍EC₎ was not observed over the study period. Ecosystem C use efficiency was correlated to both forest NEP₍EC₎ and NPP₍B₎ suggesting that high productive forests (e.g. middle-age stands) were more efficient in sequestering C compared to low productive forests (e.g. seedling and mature stands). Similarly, negative and positive relationships of forest productivity with the total belowground C flux (TBCF) to GPP ratio and with the ratio of autotrophic to heterotrophic respiration (RA:RH), respectively, determined inter-annual and inter-site differences in C allocation. Integrating NEP across the age-sequence resulted in a total net C sequestration of 137 and 229tCha⁻¹ over the initial 70 years as estimated by the biometric and EC method, respectively. Total ecosystem C sequestered in biomass at the 69-year-old site suggested an accumulation of 160tCha⁻¹. These three estimates resulted in a mean C sequestration of 175±48tCha⁻¹. This study demonstrates that comparing estimates from independent methods is imperative to constrain C budgets and C dynamics in forest ecosystems.