Thomas Andrew Black
Relevant Degree Programs
Measurement of greenhouse gas fluxes in agricultural systems
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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - Nov 2020)
There is a lack of knowledge of the processes controlling the net ecosystem production (NEP) of forest stands after disturbance (e.g., logging and nitrogen (N) fertilization) in the Pacific Northwest. To answer these questions, long-term eddy-covariance (EC) measurements of carbon-dioxide (CO₂) above a Douglas-fir stand on Vancouver Island, Canada were used in this study. Additionally, answering these questions also requires that NEP can be accurately partitioned into gross primary production (GPP) and ecosystem respiration (Re) to understand the biophysical controls of their C dynamics. Therefore, an intensive stable C isotope campaign was conducted and various partitioning techniques were deployed in this study. The Physiological Principles Predicting Growth (3-PG) model was modified to compare its predictions with EC-measured data to better understand the effects of N fertilization on CO₂ and water vapour fluxes. Application of N fertilizer to this stand led to a short-term (first two years) increase in GPP followed by little change over the long term. Re increased over the short-term (first year), while it was appreciably suppressed over the long term. N fertilization resulted in an average increase in NEP by 170 g C m-² year-¹ on average and led to an average increase in annual water use of 15%. The light-inhibition effect of daily Re for the stand during the peak growing season was estimated to be 37% by comparing the nighttime relationship and the stable C isotope methods. The daytime intercept partitioning method only partly accounted for light inhibition effects. By applying four gap-filling models using different partitioning techniques (nighttime relationship, daytime intercept, ANN–nighttime, and FVS–ANN models) on the 18-years of measured data, it was estimated that the mean annual NEP totals ranged from 195 to 238 g C m-² year-¹, hence the choice of the gap-filling model would have a great impact on long-term C budgets. There were even larger differences in Re and GPP estimates. Our findings have implications for the interpretation of EC measurements, a widely-used data source for understanding terrestrial C cycling. I further argue that the consideration of light inhibition of daytime Re in terrestrial ecosystem models is critical.
Plastic film soil mulches (i.e., protective soil covers) and plastic covered low tunnels (i.e., enclosure) have the potential to alter crop microclimate, lengthen the growing-season, and increase plant productivity. A plastic film’s ability to alter microclimate is related to its shortwave (S) and longwave (L) radiative properties (reflectivity (ρ), transmissivity (τ) and absorptivity (α)). This thesis examines the effect of plastic films with different radiative properties on 1) surface energy balance, 2) crop microclimate, and 3) crop productivity.A study of nine plastic film mulches with various radiative properties (Chapter 2) showed that all films increased daily soil heat flux density, including a high ρs value film (0.45), due to its insulating effect at night. A comparison of three black plastic films with high shortwave absorptivity (αs ≈ 0.95) but different αL values showed that low and high αL value films achieved the highest and lowest daytime soil temperatures, respectively.A study of vegetation-free plastic film low tunnels with similar τs but different αL values (Chapter 3) showed that a high αL value cover (i.e., glass-like) increased net longwave radiation inside the low tunnel compared to a low αL value cover, and increased inside air temperature (Tain) by 5 and 2°C during the daytime and nighttime, respectively. A model to predict daytime and nighttime Tain is presented and validated. A study of Padrón peppers (Capsicum annuum) grown inside and outside low tunnels (Chapter 4) showed that low tunnels increase pepper growth, productivity (10%) and growing-season length (~2 weeks), but CO₂ depletion and high water vapour density occurs when leaf area index is high. A study of summer squash (Cucurbita pepo) grown within black plastic mulch showed that the addition of a perforated low tunnel increased yield 27%. A study of broccoli (Brassica oleracea) showed that low tunnels increased yield due to wind protection in spring, which conserved soil moisture and increased Tain during low temperatures. This thesis shows that plastic film covered low tunnels and soil mulches are an effective tool for altering crop microclimate and increasing yield, but trade-offs regarding microclimate exist that crop producers should consider.
Nearly one tenth of the world’s forest is located in Canada, with one third being boreal forest. Understanding how this biome is responding to climate change is important to global carbon (C) and water balances. Long-term climate and eddy-covariance (EC) measurements of C and water vapour fluxes were made on a mature deciduous aspen stand (old aspen, OA) and a mature coniferous black spruce stand (old black spruce, OBS) to determine the impact of climate variations and disturbances on the C and water fluxes, and test if commonly utilized models can successfully model these fluxes. The impact of a defoliation event at OA in the summer of 2016, due to forest tent caterpillar infestation, was investigated and annual GEP was found to be reduced by ~20% that year, leading to the most negative annual NEP (-72 g C m⁻² year⁻¹) over the observation period. Long-term trends in annual climate variables, along with trends in gross ecosystem production (GEP), ecosystem respiration (Re), net ecosystem production (NEP), evapotranspiration (E), P – E, and water use efficiency (WUE = GEP/E) were investigated using 22 and 19 years of continuous data at OA and OBS, respectively. The impacts of growing season (GS) metrics on GS and annual NEP, GEP and E were investigated. Trends were also investigated at GS and monthly scales for climate variables and E, along with the impact of GS metrics on E at GS scale. The two stands showed varied responses to climatic variability, including responses to a multi-year drought that affected both sites but had a larger impact on the C and water fluxes at OA than OBS. The dependence of the canopy conductance (Gc) and E on their controlling variables was investigated at GS and monthly scales for both sites. The Jarvis-Stewart (JS) and modified Ball-Woodrow-Berry (MBWB) models were tested to estimate half-hourly Gc at each site and compute both Gc and E at seasonal to annual scales. This thesis demonstrates the importance of long-term observations in capturing variations in climate and disturbances forests experience, that need to be further studied and modelled for improved understanding of their impacts.
The most recent mountain pine beetle (MPB) (Dendroctonus ponderosae) outbreak in British Columbia (BC), which began in the late 1990s, killed ~54% of the mature merchantable lodgepole pine volume and was expected to impact gross primary productivity (GPP), ecosystem respiration (R) and thus net ecosystem productivity (NEP), as well as evapotranspiration (E), snow accumulation and melt in infested stands due to tree mortality. To quantify these effects, eddy-covariance (EC) measurements of carbon (C) and water vapour fluxes have been made above two not-salvage-harvested MPB-attacked pine stands, one with little understory (MPB-06) and another with considerable understory for ten and six years, respectively, and for three years in a partial-salvage-harvested stand, complemented with short-term EC measurements in nearby clearcuts. To determine long-term recovery of the C and water balances following attack, I modified the 3-PG (Physiological Principles Predicting Growth) model to simulate the effects of MPB attack on MPB-06. Modifications included a 2-layer canopy with a partly-dying overstory and growing understory, water availability from snowmelt, and a heterotrophic respiration sub-model. Modelled monthly and annual fluxes at MPB-06 agreed well with the respective EC-estimated values during the decade following attack. Modelled annual GPP, R, NEP and E decreased by about 52%, 35%, 126% and 62%, respectively, in the first year following attack compared to pre-attack values in 2005. While modelled GPP and R, as well as EC-estimated GPP, showed a relatively steady increase over the following decade, EC-estimated R changed little in the first eight years after attack and then increased in the last two years. Both modelled and measured NEP increased significantly over the decade with MPB-06 becoming C neutral within three to four years following attack. EC-measured annual E remained remarkably stable for five years after the attack, and then increased in the last five years, whereas the model indicated a relatively steady increase over the decade. Model projections for five climate change scenarios show 2026 average GPP, R, NEP and E being 14%, 1%, 65% and 5%, respectively, lower than pre-attack values. The quick recovery suggests that not-salvage-harvesting can be a beneficial management practice for C sequestration and hydrology.
Over the past decade British Columbia (BC) has experienced the largest mountain pine beetle (MPB) outbreak on record. This study used the eddy covariance (EC) technique to examine the impact of the MPB outbreak on the net ecosystem production (NEP) and evapotranspiration (E) of two lodgepole pine stands in the central interior of BC from 2007 to 2010. MPB-06, an 85-year-old stand, and MPB-03, a 110-year-old stand, were first attacked by the beetle in 2006 and 2003, respectively. EC measurements were also made in two harvested stands, one in 2005 and one in 1997 (CC-05 and CC-97, respectively) during the 2007 growing season. Annual NEP increased from -81 to 64 g carbon (C) m-² from 2007 to 2010 at MPB-06 due to an increase in gross ecosystem photosynthesis (Pg). At MPB-03, annual NEP also varied with Pg, ranging from -57 g C m-² in 2007 to 6 g C m-² in 2009. Annual ecosystem respiration (Re) did not vary greatly over the four years at both sites. At MPB-03, Pg was reduced by drought in 2009 and 2010. The increase in Pg at both sites was due to an increase in the photosynthetic capacity of the surviving trees and vegetation, as shown by foliar net-assimilation measurements. Light response analysis indicated that daytime Re values derived using nighttime NEP data were likely realistic estimates of the actual respiratory fluxes. NEP measurements at CC-97 and CC-05, showed that these stands are likely to remain C sources for as many as 10 years following harvesting. There was little interannual variation in E at both sites as the surviving trees and vegetation compensated for reductions in E due to the death of the overstory. Root-zone drainage was much greater at MPB-03 than at MPB-06, due to larger P at MPB-03. Growing season water deficit showed both stands to be water limited in spite of the high proportion of dead pine trees. Results from this study showed the importance of the remaining healthy trees and vegetation in the recovery of these stands from MPB attack.
Master's Student Supervision (2010 - 2018)
Hybrid poplar (HP) plantations established on former agricultural land in the aspen parkland of Canada have the potential to provide fibre, bio-energy and ecosystem services. The low precipitation and large summertime vapor pressure deficits in the aspen parkland raise questions about HP plantation water use and its effects on regional water supplies. In 2010 and 2011, I began using the eddy-covariance (EC) technique to measure CO₂, water vapor and sensible heat fluxes above two young HP plantations planted in 2009 (HP09) and 2011 (HP11) on clay loam Chernozemic soil located near Edmonton, AB and Winnipeg, MB, respectively. Measurements showed that both HP09 and HP11 shifted from carbon (C) sources to C sinks in the 3rd year of growth. EC measured evapotranspiration (E) and climate data were used to calculate bulk surface conductance (Gs) using the inverted Penman-Monteith (PM) equation and were compared to Gs estimates derived from a biophysical model that permits the partitioning of E into canopy transpiration (Ec) and evaporation from the soil (Es). Es was estimated using the equilibrium evaporation rate modified to account for soil moisture effects on Es using a soil water content based multiplier (f), and Ec was estimated using a canopy conductance (Gc) sub-model and the PM equation. Modelled half-hourly values of Gs showed excellent diurnal and seasonal agreement with EC-calculated Gs. Measured and modelled E also had excellent agreement, and using the Gs model, I was able to show the relative contribution of Ec and Es to E as the plantation grew. For example, in the 5th year of growth at HP09, measured and modelled E was 400 and 428 mm, respectively, of which 138 and 290 mm occurred as Es and Ec, respectively. Values of water use efficiency calculated as gross primary productivity divided by E, increased every year of growth and were similar at both sites. Results show Es dominates E during the first 2 years of HP growth and as Ec becomes increasingly dominant in the following years, E can exceed P, suggesting HP planted on highly productive agricultural soils in Canada’s aspen parkland can become water limited.
Stand-replacing disturbances, such as harvesting, have a major impact on the exchanges of carbon (C) and energy between forested land and the atmosphere. The former forest CO₂ sinks become net CO₂ sources due to the continued respiratory losses and to the significantly reduced photosynthetic uptake. Chronosequence studies, where current different-aged stands are used to reconstruct the development of an older stand, have been widely used to quantify the influence of harvesting on C and energy exchanges of forested stands. Almost no replicated measurements have been made within the Fluxnet community for same-age stands within an ecozone. Chronosequence studies assume that all sites differ only in age, and have had the same history in their abiotic and biotic components; this main assumption has been shown to be invalid in several ecological studies using chronosequences and replications are needed to explain these differences. This study used data from the well-studied Fluxnet-Canada Douglas-fir chronosequence on Vancouver Island, where the most mature site recently reached harvesting age and was commercially harvested. C and energy balances were measured using the eddy-covariance technique and other micrometeorological instruments at the recently harvested site (HDF11) for two years following the harvest. These measurements were then compared to pre-harvest measurements at the same site (DF49) and to post-harvest measurements from another previously harvested stand (HDF00) 3 km away in the chronosequence. The results from this study showed that the net radiation decreased from pre- to post-harvest due to the increase in albedo and surface temperature. The average annual Bowen ratio increased slightly due to the reduction in evapotranspiration following the harvest. From pre- to post-harvest, the site transitioned from being a moderate sink to being a strong source of CO₂. In comparison, the previously harvested stand (HDF00) was a weaker source of CO₂ due to lower respiratory rates and faster vegetation recovery. The results show the importance of replicated measurements to characterize the C and energy exchanges for an ecosystem-specific stand age following a stand-replacing disturbance.
The recent mountain pine beetle (MPB) outbreak has had a major impact on the carbon (C) cycling of lodgepole pine forests in British Columbia. Mitigation efforts to control the insect outbreak have led to increased harvesting rates in the province. This study determines whether partial harvesting as an alternative forest management response to clearcutting can increase the net ecosystem production (NEP) of a mixed conifer forest (MPB-09) in Interior BC. Using the eddy-covariance (EC) technique, the C dynamics of the 70-year old stand were studied over the two years after partial harvest following MPB attack and also compared to an adjacent clearcut (MPB-09C) over the growing season. The annual NEP at MPB-09 increased from -107 g C m⁻² in 2010 to -57g C m⁻² in 2011. The increase of NEP was because the associated increase in annual gross ecosystem photosynthesis (GEP) from 812 g C m⁻² in 2010 to 954 g C m⁻² in 2011 exceeded the increase in annual respiration (Re) from 920 g C m⁻² to 1011 g C m⁻² in the two years of study. During the growing season of 2010, NEP at MPB-09C was -132 g C m⁻² indicating high C losses in the clearcut. MPB-09 was a C sink during the growing season of both years, increasing from 9 g C m⁻² in 2010 to 47 g C m⁻² in 2011. The increase of NEP in the partially harvested forest suggests stand recovery following harvest, which corresponds to a 25% increase in the maximum assimilation rate in the second year. This study shows that retaining the healthy residual forest can greatly enhance the C sequestration of MPB-attacked stands and has important implications for forest management.