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