Search

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]

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]

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]