Forest plantations have been long-employed to reverse land degradation and support biodiversity, and are now recognized to both take in atmospheric carbon dioxide, reducing the intensity of the greenhouse effect, and moderate local weather. It is important to consider the impact forest aging and management will have on provisioning of these services under climate change and extreme weather events, such as drought. This study encompasses a chronosequence of three Eastern White Pine stands planted in 1939, 1974 and 2002, situated in Turkey Point, Ontario, Canada. The oldest forest received two selective thinning treatments, removing 30% of trees, in 1983 and 2012. Forest water use efficiency (WUE), which represents the amount of gross ecosystem productivity (GEP) per unit of water released through evapotranspiration (E), was compared among the three sites over 2008-2013. The youngest forest’s annual WUE increased over the study period, surpassing that of the older sites by 2013. When bulk surface conductance (Gs), representing gas exchange, was compared across the sites for the same years, the youngest site had the lowest Gs, particularly during drought. Gs at the oldest forest was highest and the most variable. Statistical analysis showed that across all the sites, E was more responsive to air temperature than atmospheric demand, soil moisture, and incident radiation. This study indicated that younger plantations may be more water-conservative during drought, and that air temperature is important to consider in projections of temperate coniferous forests’ carbon and water exchange. To assess the impact of the 2012 selective thinning on tree-level and ecosystem-level water use at the oldest forest, sapflow velocity (Js), transpiration (Et) and E were compared between the two stands planted in 1939 and 1974, from 2011 to 2013. A relatively severe drought over the 2012 growing season led to a decline in Et at the unthinned site for that year, however the Et decline was more pronounced at the older, thinned site. From 2011 to 2012, Js increased at the thinned site, converse to the unthinned site – wherein Js was low as expected during drought. Hydraulic redistribution and lag time from sapflow at 1.3 m height to canopy evapotranspiration were seemingly unaffected by the thinning, indicating that low-level selective harvesting was not detrimental to the hydrological functionality of the stand, and may have been beneficial in allowing more soil moisture access per tree. As such, the stand may be better positioned to withstand recurrent dry spells resulting from precipitation variability, as predicted with climate change. / Thesis / Master of Science (MSc)

<p> Forest ecosystems play an important role in the global carbon (C) cycle by exchanging large amounts of carbon dioxide (CO₂) with the atmosphere. Their potential to act as significant sink for atmospheric CO₂ has been recognized and is relevant to current efforts in reducing atmospheric CO₂ concentrations. Besides the most important greenhouse gas CO₂, forests also emit and consume methane (CH₄) and nitrous oxide (N₂O) as the two other important atmospheric greenhouse gases (GHGs). To date, few attempts have been made to quantify the net effect of forest GHG exchange on the global warming potential. Furthermore, a better understanding of successional and environmental effects on forest processes is required to improve large scale estimates of forest C and GHG exchange. </p> <p> This thesis examines C dynamics and the exchange of the three major greenhouse gases (CO₂, CH₄, and N₂O) in an age-sequence (7-, 20-, 35-, and 70-years-old as of 2009) of afforested pine forests, in southern Ontario, Canada. The impacts of environmental controls on these GHG exchanges were also evaluated. Forest C exchange was determined for 2003 to 2008 using the eddy-covariance (EC) technique and inventory-based biometric measurements. Soil CH₄ and N₂O measurements were conducted from 2006 to 2007 using the static closed-chamber method. In addition, concentrations and fluxes of dissolved organic carbon (DOC) throughout the vertical profile in forest canopy and soil were determined from 2004 to 2005 using throughfall buckets and lysimeters. </p> <p> During periods without climatic constraints, monthly gross ecosystem productivity (GEP) and ecosystem respiration (RE) corrected for differences in site index increased with stand age, whereas monthly net ecosystem productivity (NEP) peaked at the 35-year-old site. In contrast, during constrained periods (e.g. seasonal drought events), monthly GEP and NEP at the 20-year-old site were higher compared to the 35-year-old site because trees may have benefited from sustained availability of soil water in deeper layers. This study further demonstrates that differences in site quality may affect the interpretation of age-related C flux dynamics in chronosequence and synthesis studies (Chapter 2). </p> <p> The temperature-RE relationship was an important control on daily NEP anomalies under optimum growing conditions, whereas constrains on GEP primarily determined NEP during environmentally constrained periods. Furthermore, effects from single environmental variable constrains on NEP anomalies were enhanced as well as outbalanced under multiple environmental variable constrains. The results further indicate that future changes in temperature and precipitation patterns towards drier and warmer conditions as well as greater cloud cover may result in reduced C sequestration potentials in these temperate pine forests (Chapter 3). </p> <p> Early summer drought and heat events in 2005 caused NEP to decrease by approximately 100 g C m⁻² y⁻¹ at each site compared to the other years. This decrease was primarily driven by a decrease in photosynthesis, while the effect of these events on ecosystem respiration was small. Overall, for the years 2003-2007, annual NEP was 219, 155, 36, 148, and 120 g C m⁻² y⁻¹ at the 68-year-old site, 666, 318, 346, 511 and 366 g C m⁻² y⁻¹ at the 33-year-old site, 768, 885, 684, 708 and 826 g C m⁻² y⁻¹ at the 18-year-old site, and-18, 145, 125, 34 and 164 g C m⁻² y⁻¹ at the 5-year-old seedling site, respectively (negative numbers indicating net C source (Chapter 4). </p> <p> Four-year mean values of biometric NEP_(B) and EC-based NEP_(EC) were similar at the 7-year-old seedling (77 and 66 g C m⁻² y⁻¹) and the 70-year-old mature site (135 and 124 g C m⁻² y⁻¹), but differed considerably at the 20-year-old (439 and 736 g C m⁻² y⁻¹) and the 35-year-old sites (170 and 392 g C m⁻² y⁻¹). Integrating NEP across the age-sequence resulted in a total net C sequestration of 137 and 229 t C ha⁻¹ over the initial 70 years as estimated by the biometric and EC method, respectively. The total ecosystem C pool at the 70-year-old site suggested an accumulation of 160 t C ha⁻¹. These three estimates resulted in a mean C sequestration of 175 ± 48 t C ha⁻¹ (Chapter 5). </p> <p> For both CH₄ and N₂O, we observed uptake and emission ranging from -160 to 245 μg CH₄ m⁻² hour⁻¹ and -52 to 21 μg N₂O m⁻² hour⁻¹, respectively (negative values indicate net uptake). Mean N₂O fluxes from mid-April to mid-December across the 7-, 20-, 35-, 70-years old stands were -3.7, 1.5, -2.2, and-7.6 μg N₂O m⁻² hour⁻¹, without age-related pattern, whereas the uptake rates of CH₄ increased with stand age from 6.4 to -7.9, -10.8, and-23.3 μg CH₄ m⁻² hour⁻¹, respectively. For the same period, the combined contribution of CH₄ and N₂O exchanges to the global warming potential (GWP) calculated from net ecosystem exchange of CO₂ and aggregated forest floor exchanges of CH₄ and N₂O was on average <4% (Chapter 6). </p> <p> DOC concentration in forest floor leachates was positively correlated to stand age, aboveground biomass and forest floor carbon pools. From the period of Mid-April to December, DOC fluxes via precipitation, throughfall, and leaching through forest floor and Ah-horizon were in the range of ~1 to 2, 2 to 4, 0.5 to 3.5, and 0.1 to 2 g DOC m⁻², respectively. DOC export from the forest ecosystem during that period through infiltration and groundwater discharge decreased with increasing stand age from ~7 to 4, 3, and 2 g DOC m⁻² (Chapter 7). </p> <p> This thesis improved our understanding of C and GHG exchange dynamics and their environmental, physical, and physiological controls in forest ecosystems. This study will also contribute to efforts being made to better predict future forest C and GHG dynamics and their feedbacks on climate under changing environmental conditions. <p> / Thesis / Doctor of Philosophy (PhD)

This study presents the energy, carbon (C), and water exchange dynamics of a recently afforested temperate white pine (Pinus strobus L.) forest, established on former agricultural land in 2002, in southern Ontario, Canada during the initial thirteen years (2003–2015). Our observations show that the forest became a consistent sink of C after only 5 years of its establishment (ranging from 105 g C m–2 to 216 g C m–2 between 2008 to 2015), owing to sandy soils and low residual soil organic matter from prior agricultural activities. This region frequently experiences low precipitation (P) and soil moisture (VWC) limitations and/or heat stress in late summer, causing a reduction in net ecosystem productivity (NEP). Seasonal and annual dynamics of NEP showed reduced C uptake during years with heat and/or drought events (i.e. 2007 and 2012). In 2007, the impact of a seasonal drought was much more exacerbated when combined with a heatwave, resulting in a strong C source. Similarly, the inter-annual variability of evapotranspiration (ET) gradually increased with stand age (mean 370 mm yr–1) and water use efficiency (WUE) consistently increased (mean 2.65 g C kg–1 H2O). Quantum yield, α (0.019 to 0.045) and maximum photosynthetic capacity, Amax (4.37 to 33.6 µmol m–2s–1) increased steadily as the size and density of the canopy increased with stand age. Energy fluxes were influenced by canopy development as net radiation (Rn), latent heat (LE), and sensible heat (H) flux increased, while ground heat flux (G) peaked in 2007 and then gradually declined. Our analysis showed that daily C fluxes are primarily driven by Rn and temperature (Ts, Ta) which explained 47%, 61%, 52%, and 68% of the variability in gross ecosystem productivity (GEP), ecosystem respiration (RE), NEP, and ET. This study is a significant contribution to our understanding of the energy, C, and water dynamics of young planted conifer forests and controls on their growth and C uptake. Our findings demonstrate the potential of utilizing white pine as a means to sequester atmospheric CO2 in southern Ontario and other regions of North America with similar climate and site history. / Thesis / Master of Science (MSc)

Temperate forests are an important global carbon sink. However, various environmental disturbances can impact carbon sequestration capabilities of these forests. In 2021, a record-breaking defoliation, caused by the spongy moth (Lymantria dispar L., formerly knows as the gypsy moth) occurred in eastern North America. In this study, we assess the impact of this spongy moth defoliation on carbon uptake in a mature oak-dominated temperate forest in the Great Lakes region in Canada, using eddy covariance flux data from 2012 to 2022. The forest is more than 90 years old and known as CA-TPD site in the AmeriFlux and global FLUXNET networks. Study results showed that prior to spongy moth defoliation the forest was a carbon sink with mean annual gross ecosystem productivity (GEP) of 1,367 ± 104, ecosystem respiration (RE) of 1,201 ± 145 and, net ecosystem productivity (NEP) of 197 ± 74 g C m−2 yr−1 over the 2012–2020 period. However, due the defoliation in the early growing season in 2021, GEP declined to 959 g C m-2 yr-1 and RE increased to 1,345 g C m-2 yr-1 causing the forest to became a large source of carbon with annual NEP of -351 g C m-2 yr−1. This large decline in annual NEP was a result of both reduced GEP (30%) and elevated RE (12%). However, in 2022, forest carbon fluxes recovered to pre-infestation levels, with a GEP value of 1,671 g C m-2 yr-1, an RE value of 1,287 g C m-2 yr-1, and an NEP value of 298 g C m-2 yr-1, indicating that the forest was once again a large carbon sink. This research demonstrates that major transient natural disturbances such as the 2021 spongy moth defoliation can have a significant impact on forest carbon dynamics in a future warmer climate. The extent to which North American temperate forests will remain a major carbon sink will depend on the severity and intensity of these disturbance events and rate of recovery of forests following the disturbance. / Thesis / Master of Science (MSc) / Temperate deciduous forests play an important role in carbon sequestration from the atmosphere. However, the impact of climate change, extreme weather, and disturbance events can alter the extent to which these forests sequester carbon, in some cases shifting their role from being a carbon sink to becoming a carbon source to the atmosphere. In 2021, a spongy moth infestation severely defoliated a mature oak-dominated temperate forest north of Lake Erie, Ontario, Canada, turning the forest from a carbon sink to a carbon source. Our analysis indicates that meteorological conditions during the early spring might have influenced the severity of this infestation. Specifically, the prevalence of dry and warm weather conditions enabled the moth to survive and thrive longer. This study shows the significant influence of natural disturbances on forest carbon dynamics as temperatures continue to rise due to climate change. The future role forests play in carbon sequestration will be determined by the severity of disturbance events and the effectiveness of forests to recover in the aftermath of these events.

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<p>Understanding the response of forest carbon uptake and growth to interannual climate variability and forest management practices is important, given the large quantity of carbon stored in forests, and their significant role in the global carbon cycle. Since 2004, biometric and micrometeorological measurements were taken in an age-sequence (10-, 38- and 73-years-old as of 2012) of white pine (<em>Pinus strobes</em> L.) plantation forests in southern Ontario, Canada, providing an 8 year record of carbon sequestration, growth and climate. The 73-year old conifer site was thinned in early 2012, where 25% of trees were removed to improve light and water dynamics of this stand, providing an opportunity to study the impacts of thinning on its carbon cycle. Additionally, in 2012, similar biometric and micrometeorological measurements were initiated in a naturally-regenerated, managed 80-year-old deciduous (Carolinian) forest, located in close proximity to the pine stands. Similar to the conifer sites, the deciduous site is also a managed forest. The objectives of this study were to determine differences in carbon pools and carbon sequestration capacity: (a) across an age-sequence of afforested, managed conifer stands; (b) between similarly-aged managed coniferous and deciduous stands; and (c) in a mature conifer plantation before and after a thinning event. Results show that carbon assimilated in the stem of mature white pine trees follows a linear growth trend, while that of young white pines shows an exponential increase in carbon assimilation over the course of this study. Overall, carbon sequestration increased with stand age across the age-sequence, except when disturbed by an event such as thinning. Thinning substantially reduced the live aboveground carbon pool (by 14%), while increasing woody debris (by 122%) due to logging residue left on-site. Comparison between the mature coniferous and deciduous stands, showed that total aboveground carbon storage within the pine stand (144 t C/ha) was generally higher than in the oak-dominated deciduous stand (83 t C/ha), despite both growing in similar soil and climate. While monthly tree growth exhibited a positive correlation with mean monthly temperature across all sites, tree growth negatively correlated with precipitation at the 10-year old white pine and 80-year old deciduous sites and no apparent correlation existed at the 73- and 38-year old sites. At the three coniferous stands, total annual net primary productivity (NPP) exhibited no correlation with mean growing season temperature or precipitation. This suggested that tree growth in young coniferous stands could be as sensitive as that of mature deciduous stands to precipitation. However, overall NPP seemed to be less sensitive to climatic variables across these stands, irrespective of their age and NPP may be driven more by stand physiology. Finally, eddy covariance and biometric estimations of NPP and NEP were compared, and results showed that although some growth trends do compare between the two techniques, magnitude discrepancies do exist and should be studied further. Results from this study will be informative to forest managers, forest conservationists and those interested in forest carbon sequestration.</p> / Master of Science (MSc)