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In eastern North America, many deciduous forest ecosystems grow at the northernmost extent of their geographical ranges, where climate change could aid or impede their growth. This region experiences frequent extreme weather conditions, allowing us to study the response of these forests to environmental conditions, reflective of future climates. Here we determined the impact of seasonal and annual climate variations and extreme weather events on the carbon (C) uptake capacity of an oak‐dominated forest in southern Ontario, Canada, from 2012 to 2016. We found that changes in meteorology during late May to mid‐July were key in determining the C sink strength of the forest, impacting the seasonal and annual variability of net ecosystem productivity (NEP). Overall, higher temperatures and dry conditions reduced ecosystem respiration (RE) much more than gross ecosystem productivity (GEP), leading to higher NEP. Variability in NEP was primarily driven by changes in RE, rather than GEP. The mean annual GEP, RE, and NEP values at our site during the study were 1,343 ± 85, 1,171 ± 139, and 206 ± 92 g C m−2 yr−1, respectively. The forest was a C sink even in years that experienced heat and water stresses. Mean annual NEP at our site was within the range of NEP (69–459 g C m−2 yr−1) observed in similar North American forests from 2012 to 2016. The growth and C sequestration capabilities of our oak‐dominated forest were not adversely impacted by changes in environmental conditions and extreme weather events experienced over the study period. Plain Language Summary: Globally, forests are an important carbon sink, removing carbon dioxide from the atmosphere on an annual basis. Midlatitude temperate deciduous forests are an important component of the global forest carbon sink, representing ~11% of the global carbon stock. The strength of carbon uptake by these forests remains uncertain under future climate projections. With increasing temperatures and changes in precipitation patterns expected in many regions, tree species may advance north, changing the carbon uptake capacities of local forest ecosystems. In addition, extreme weather events such as heat waves and droughts may place these forests under added stress and reduce their carbon sink capacities. Our study found that despite frequently changing environmental conditions, an oak‐dominated forest, growing at the northernmost extent of temperate deciduous forests within the Great Lakes region of North America, was a strong sink of carbon with mean annual net ecosystem productivity of 2 t of C per hectare per year. Key Points: Seasonal and annual carbon fluxes were studied from 2012 to 2016 at a mature temperate deciduous forest in southern Ontario, CanadaMeteorological conditions over a short midsummer period greatly impacted annual NEPOn average, the forest was an annual carbon sink of 206 ± 92 g C m−2 yr−1 [ABSTRACT FROM AUTHOR]

This study presents the energy, water, and carbon (C) flux dynamics of a young afforested temperate white pine ( Pinus strobus L.) forest in southern Ontario, Canada during the initial fourteen years (2003–2016) of establishment. Energy fluxes, namely, net radiation (Rn), latent heat (LE), and sensible heat (H) flux increased over time, due to canopy development. Annual values of ground heat flux (G) peaked in 2007 and then gradually declined in response to canopy closure. The forest became a consistent C-sink only 5 years after establishment owing in part to low respiratory fluxes from the former agricultural, sandy soils with low residual soil organic matter. Mean annual values of gross ecosystem productivity (GEP), ecosystem respiration (RE), and net ecosystem productivity (NEP) ranged from 494 to 1913, 515 to 1774 and −126 to 216 g C m −2  year −1 respectively, over the study period. Annual evapotranspiration (ET) values ranged from 328 to 429 mm year −1 over the same period. Water use efficiency (WUE) increased with stand age with a mean WUE value of 3.92 g C kg −1 H 2 O from 2008 to 2016. Multivariable linear regression analysis conducted using observed data suggested that the overall, C and water dynamics of the stand were primarily driven by radiation and temperature, both of which explained 77%, 48%, 28%, and 76% of the variability in GEP, RE, NEP, and ET, respectively. However, late summer droughts, which were prevalent in the region, reduced NEP. The reduction in NEP was enhanced when summer drought events were accompanied by increased heat such as those in 2005, 2012 and 2016. This study contributes to our understanding of the energy, water and C dynamics of afforested temperate conifer plantations and how these forests may respond to changing climate conditions during the crucial initial stage of their life cycle. Our findings also demonstrate the potential of pine plantation stands to sequester atmospheric CO 2 in eastern North America. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]