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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]

Forest plantations are commonly used to restore the ecological and hydrological functionality of landscapes. In this study, we investigated the hydrological response of a 74-year-old pine plantation forest in southern Ontario, Canada to a selective thinning, wherein 30% of trees were harvested in winter of 2012. Tree-level and ecosystem-level water fluxes were monitored from 2011 to 2013 using sapflow and eddy-covariance techniques, and were compared with an adjacent 39-year-old pine plantation forest that was not thinned. In the 74-year-old forest, transpiration declined after thinning in the 2012 growing season, while tree-level water transport increased despite a severe drought. Time lag between sapflow and ecosystem evapotranspiration showed that the older stand had a significantly shorter time lag when compared to the younger stand, particularly in the thinning year. Linear regression modeling indicated that this was likely due to higher soil moisture status in the older stand versus the younger forest. Hydraulic redistribution of soil water was apparent during drought periods in both forests and did not appear to be negatively affected by the thinning treatment in the older forest. We conclude that selective low density thinning did not negatively impact the forest's response to seasonal drought, and that the availability of more soil moisture as a result of thinning may improve the resilience of the forest to future climatic extreme events such as drought. [ABSTRACT FROM AUTHOR]

This study analysed age-related water-use dynamics across three temperate conifer forest plantations (aged 11, 39 and 74 years, as of 2013, henceforth referred to as TP02, TP74 and TP39, respectively, where the last two digits represent the year of planting) in southern Ontario, Canada, from 2008 to 2013. Eddy covariance-measured mean evapotranspiration over the growing season (April-October) was 438 ± 19, 392 ± 19 and 323 ± 25 mm at TP39, TP74 and TP02, respectively. Daytime bulk surface conductance was highest and most variable at the TP39 site (8.5 ± 4.0 mm s−1), followed by the TP74 (7.0 ± 2.8 mm s−1) and TP02 (5.4 ± 2.5 mm s−1) sites. Evapotranspiration at all the forests was sensitive to air temperature and tended to decrease with increasing atmospheric dryness. The youngest forest's evapotranspiration was most conservative, which led to an increase in water-use efficiency throughout the study period, in particular during drought events. The oldest forest was the least restrictive in its water use during drought, which led to lower water-use efficiency during such events as compared with the younger forests. The oldest forest was thinned in early 2012, when about one third of trees were commercially harvested. No significant change in evapotranspiration or water-use efficiency was observed at this site following thinning; however, daytime bulk surface conductance declined. Our results suggested that changes in stand structure with forest ageing, such as reduction in stem density and increase in sapwood area, were responsible for differences in soil water demand during drought and non-drought periods, leading to differences in forest water use. Hence, forest age, because of its structural implications, is an important control on stand-level water-use efficiency and forests' response to drought events. Our study suggested that younger forests may be best suited to maximize growth and carbon uptake efficiency under rising air temperatures and increasing precipitation variability as predicted by climate models for eastern North America. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The effects of early growing season droughts on water and carbon balances in conifer forests are poorly understood. In this study, the response of canopy transpiration ( Ec) and growth rates to reduced precipitation input during the early growing season was evaluated in a 70-year old temperate white pine ( Pinus strobus L.) plantation forest, in Southern Ontario, Canada. In order to induce the drought, a 20 × 20 m throughfall exclusion setup was established. Throughfall was excluded from 1 April to 3 July 2009. During this period, 270 mm of rainfall occurred (27% of annual precipitation), of which more than 90% was excluded. Sapflow, stem growth, soil moisture and soil temperature were measured in both drought and reference plots. Prior to the induced drought, both plots showed similar soil water content, transpiration rates and tree diameters. The primary control on forest water loss was vapour pressure deficit, whereas soil moisture had an effect when it reached below 0.068 m3 m−3 during the growing season. The rainfall exclusion did not negatively affect Ec until early June, approximately 54 days after drought initiation. Ec was 27% less in the drought plot compared to the reference plot when evaluated at the end of the growing season in November. Tree growth estimates at the end of the growing season indicated a 17% decrease in growth in the drought plot as compared to the reference plot. Because climate predictions foresee changes in precipitation pattern, drought spells - similar to this artificial short-term rainfall manipulation - may be more frequent in the future. Hence, although overall precipitation may remain the same, the short-term deficit in water supply may have important implications for forest ecosystems. The findings of this rainfall manipulation will help quantify the impacts of spring and early summer water deficit on forest ecosystems and evaluate their potential responses to future climate regimes. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]