Still Watching Trees Grow: A Multispecies and Cross-Scale Examination of the Radial Growth, Climate, and Carbon Interface
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Climate change is progressing at a rapid pace, especially in northern regions, where the most dramatic land surface warming is occurring. Boreal ecosystems are therefore situated in an area which is particularly susceptible to the impacts of climate change. This dissertation represents a comprehensive assessment of the interactions between tree growth, climate, and carbon at multiple temporal scales in the southern boreal forest. The goal of this work is to better understand the potential impacts of climate change on boreal forest growth and carbon dynamics in this region. The collection of research contained herein took place at the Boreal Ecosystem Research and Monitoring Sites (BERMS), Old Jack Pine (OJP), Old Black Spruce (OBS), and Old Aspen (OA). Located in the southern boreal forest of Saskatchewan, these sites represent one of the most comprehensive collections of long-term high-resolution carbon (C) flux data, alongside of which comes an equally impressive suite of meteorological data. To supplement these data, I collected high-resolution stem size data of the dominant and co-dominant tree species at these sites: jack pine (Pinus banksiana), black spruce (Picea mariana), eastern larch (Larix laricina), and trembling aspen (Populus tremuloides) between 2015 and 2018 (the short-term observation period). I also collected tree cores from jack pine, black spruce, and trembling aspen, to extend the radial growth record backwards to stand establishment (~ 100 years; the long-term observation period), and contributed the latest in a series of repeated inventory style measurements which make up a two-decade long record (1994 – 2016) of forest C stocks and fluxes at OJP and OA (the medium-term observation period). Chapter 2 presents a multiscale dendroclimatological assessment of jack pine, black spruce, eastern larch, and trembling aspen. I find shifting growth/climate relationships at annual-scale resolution over the long term (~100 years), including a weakening of the relationship between radial growth and precipitation and an enhanced positive relationship between radial growth and spring and summer air temperature over time. Like the divergence problem, which highlights issues of non-stationarity in the growth/climate relationship in trees further north, I attribute the cause of this dynamic relationship to shifting limitations. In this case, the change likely signals a decrease in moisture limitations and a positive response to recent warming. Over intra-annual scales, during the short-term observation period (2015 – 2018), I find evidence of a positive relationship between daily stem radius change (∆R) and air temperature within the growing season. However, this relationship was only significant when moisture requirements were met, calling reference to the importance of moisture and its role in supporting the relationship between radial growth, or tracheid cell production, and temperature. Changes in moisture conditions in the Canadian boreal forest, both historically and moving forward, are spatially variable across the landscape. In Prince Albert, Saskatchewan, where the nearest and most complete long-term record of climate is recorded, conditions have been warming and wetting since 1890 (Appendix D). It is likely that the BERMS sites are situated in a region where an increase in evaporative demand in response to recent warming has, to date, been successfully offset by a co-occurring increase in precipitation, resulting in a net increase in available moisture. This is likely having a positive impact on tree growth in this region, but one which is unlikely to persist as rates of evapotranspiration are expected to overshadow any gains in moisture related to an increase in precipitation over the long term. In Chapter 3, I apply analytical techniques analogous to those from the dendroclimatological assessment completed in Chapter 2, in this case they are applied to the study of radial growth and stem radius change, and its relationship with ecosystem C-flux over the medium- and short-terms. Overall my findings are similar to those from other recent studies. At annual scale resolution, only the radial growth of jack pine was significantly and positively correlated with ecosystem production (net ecosystem production; NEP) over the medium term (~20 years). Comparatively, black spruce and trembling aspen are more likely to rely situationally on stored carbohydrates, introducing the potential for inconsistency in the relationship between ecosystem production (uptake) and radial growth (allocation). During the observation period (2000 – 2018), the annual radial growth of jack pine benefited from non-structural carbohydrate storage from the previous fall and from elevated levels of NEP during the current spring. Over intra-annual scales, there was effectively no evidence of a relationship between stem size and measures of ecosystem C-flux during the short-term observation period (2015 – 2018). Perceived relationships between these variables over fine temporal scales were likely spurious, driven by a combination of factors, including but not limited to precipitation and soil temperature. In Chapter 4, I identify large-scale changes in the distribution of carbon across the landscape at OJP and OA over the medium term (between 1994 – 2016). The most notable stock changes occurred during periods of enhanced tree mortality, likely resulting from moisture stress at OA, and windthrow at OJP. There is evidence that ecosystem level production was also impacted during periods of enhanced tree mortality, however this differed between the two sites. Tree level net primary production (NPPTree) decreased at OJP in response to enhanced mortality, yet it was maintained at a relatively stable level at OA regardless of the mortality rate. The deciduous trees here were likely more capable of taking advantage of holes in the canopy by increasing their production efficiency. As for eddy covariance based net ecosystem production (NEPEC), the opposite pattern was observed. NEPEC was maintained at a stable level at OJP, and was notably depressed during a period of increased mortality at OA. In summary, over the near term, warming and wetting may continue to benefit the radial growth of several tree species in the southern boreal forest of Saskatchewan, where moisture has long represented the main limiting factor. However, in mature trembling aspen dominated stands, an increase in available moisture may contribute to moderate disturbance, resulting in enhanced tree mortality and a significant flux of carbon from biomass to necromass. Moving forward, rates of evapotranspiration are expected to overshadow any gains in moisture related to an increase in precipitation. Under these circumstances, species in the southern boreal forest will need to rely more heavily on effective precipitation and root-zone soil moisture to support their growth. Under extreme water limitations, we are likely to see a negative response to warm air temperature, reduced growth rates, and enhanced tree mortality. Lastly, wind likely represents an important agent of change in mature jack pine stands. An increase in the incidence and intensity of extreme storms and high winds due to climate change will likely lead to more frequent high-impact disturbance in the boreal forest. While mature jack pine stands may be particularly vulnerable to this type of disturbance, this is likely to have widespread impacts in a range of boreal forest stands. While the findings from this and other studies help to improve our understanding of the impacts of climate change on boreal forest trees, more work is needed. Relationships between radial-tree growth and climate in the Canadian boreal forest are regionally defined and species specific, requiring widespread and comprehensive study. In the meantime, when forecasting change in the boreal forest, we must be careful not to paint this diverse biome with too wide a brush.
DegreeDoctor of Philosophy (Ph.D.)
DepartmentSchool of Environment and Sustainability
ProgramEnvironment and Sustainability
CommitteeBonsal, Barrie; Barr, Alan; Helgason, Warren; Ireson, Andrew
Copyright DateSeptember 2020