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<p>Quantifying carbon fluxes and pools of forest ecosystems is an active research
area in global climate study, particularly in the currently and projected increasing
atmospheric carbon dioxide concentration environment. Forest carbon dynamics are closely
linked to the water cycle through plant stomata which are regulated by environmental
conditions associated with atmospheric and soil humidity, air temperature and light.
Thus, it is imperative to study both carbon and water fluxes of a forest ecosystem
to be able to assess the impact of environmental changes, including those resulting
from climate change, on global carbon and hydrologic cycles. However, challenges hampering
such global study lie in the spatial heterogeneity of and the temporal variability
of fluxes in forests around the globe. Moreover, continuous, long-term monitoring
and measurements of fluxes are not feasible at global forest scale. Therefore, the
need to quantify carbon and water fluxes and to identify key variables controlling
them at multiple stands and time scales is growing. Such analyses will benefit the
upscaling of stand-level observations to large- or global-scale modelling approaches.
</p><p>I performed a series of studies investigating carbon and water fluxes in pine
forests of various site characteristics, conditions and latitudinal locations. The
common techniques used in these studies largely involved sap flux sensors to measure
tree-level water flow which is scaled up to stand-level transpiration and a process-based
model which calculates canopy light absorption and carbon assimilation constrained
by the sap-flux beased canopy stomatal conductance (called Canopy Conductance Constrained
Carbon Assimilation or 4C-A model). I collected and analyzed sap flux data from pine
forests of two major species: Pinus taeda in temperate (36 °N) and Pinus sylvestris
in boreal (64 °N) climatic zones. These forests were of different stage-related canopy
leaf area and some were under treatments for elevated atmospheric CO2 concentration
or fertilization. </p><p>I found that (Chapter 2) the 17-year long free-air CO2
enrichment (FACE) had little effect on canopy transpiration of a mixed forest with
the dominant P. taeda and other broadleaved species as the understory in North Carolina,
USA (Duke FACE). The result was due to the compensation of elevated [CO2]-induced
increase of canopy leaf area for the reduction of mean canopy stomatal conductance.
My next theoretical study (Chapter 3), comparing P. taeda (native at 36 °N in North
Carolina), P. sylvestris (native at 64 °N in norther Sweden) and Pinus contorta (native
at 58 °N in British Columbia, Canada) canopies, revealed that the interaction between
crown architecture and solar elevation associated with site latitude of pine canopies
affected the distribution and total amount of canopy light absorption and potentially
photosynthesis such that the latitudinally prescribed needle organization of a pine
canopy is optimal for light interception and survival in its native location. Then,
I quantified and analyzed water fluxes in four pine forests: one composed of P. taeda
in North Carolina and three containing P. sylvestris in northern Sweden (Chapter 4).
The latter forests consisted of various stage-related canopy leaf area and nutrient
status. Combining my estimates with other published results from forests of various
types and latitudinal locations, I derived an approach to estimate daily canopy transpiration
during the growing season based on a few environmental variables including atmospheric
and soil humidity and canopy leaf area. Moreover, based on a water budget analysis,
I discovered that the intra-annual variation of precipitation in a forest has a small
effect on evapotranspiration and primarily affecting outflow; however, variation of
precipitation across latitudes proportionally influences anuual evapotranspiration
and outflow. Furthermore, the hydrologic analyses implied the `disequilibrium' of
forest water cycling during the growing season when forests may use less and more
water in dry and wet regions, respectively, than the incoming precipitation. Nevertherless,
at annual timescale, most forests became in `equilibrium' by using similar proportion
of incoming precipitation. Finally, (Chapter 5) I estimated and analyzed the temporal
and spatial variabilities of carbon fluxes of the same four forests measured in Chapter
4 using the 4C-A computational approach and analyzed their resource-use efficiencies.
I concluded that, based on my results and others as available, despite the differences
in species clumping and latitudes which influence growing season length and solar
elevation, the gross primary productivity can be conservatively linearly related to
the canopy light absorption. However, based on previous findings from a global study,
different allocation of the acquired carbon to the above- and belowground is regulated
by soil nutrient status. </p><p>Overall, the findings in this dissertation offer new
insights into the impacts of environmental changes on carbon and water dynamics in
forests across multiple sites and temporal scales which will be useful for larger-scale
analyses such as those pertaining to global climate projection.</p>
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