Research Bibliography / Shedding Light on Photosynthesis: The Impacts of Atmospheric Conditions and Plant Canopy Structure on Ecosystem Carbon Uptake

Shedding Light on Photosynthesis: The Impacts of Atmospheric Conditions and Plant Canopy Structure on Ecosystem Carbon Uptake


TitleShedding Light on Photosynthesis: The Impacts of Atmospheric Conditions and Plant Canopy Structure on Ecosystem Carbon Uptake
Publication TypeThesis
Year of Publication2016
AuthorsCheng SJ
AdvisorNadelhoffer KJ, Steiner AL, Currie W, Curtis PS, Goldberg DE
Academic DepartmentEcology and Evolutionary Biology
Pages145
Type of WorkDissertation
Abstract

The Earth’s climate is influenced by complex interactions of physical, chemical, and
biological processes that link terrestrial ecosystems and the atmosphere. One of these
interactions involves the use of light in photosynthesis, which allows plants to remove CO2 from
the atmosphere and slow the unprecedented rate of climate change the Earth is experiencing.
However, modeling future climate remains challenging, in part because of limited knowledge of
mechanisms controlling the effects of light on gross ecosystem CO2 uptake (conceptually,
photosynthetic activity integrated across all leaves in a plant canopy). Unlike previous studies,
this dissertation uses data from atmospheric science, ecosystem ecology, and plant physiology to
provide evidence for mechanistic links between physical, biophysical, and ecological controls on
the effects of light on processes tied to gross ecosystem CO2 uptake—specifically, ecosystem
gross primary production (GPP) and leaf photosynthesis. First, this dissertation empirically
demonstrates that the dominant effect of clouds is to reduce total light above canopies. However,
optically thin clouds increase scattered, diffuse light, which canopies use more efficiently than
they use direct light. This offsets reductions in total light and results in no net change in GPP
under thin clouds, while GPP decreases under optically thick clouds because both diffuse and
direct light decrease. Second, ground-based measurements indicate that the rate of increase in
GPP with diffuse light changes throughout the day. The magnitude of increase depends on how
canopies interact with the angle of incoming light to biophysically alter the distribution of light
within canopies and thus, the proportions of leaves contributing to GPP. Third, the distribution of
species and light within one forest canopy leads to differences in some of the rate-limiting
biochemical reactions in leaf photosynthesis. These field-based data indicate which assumptions
representing canopies in Earth system models may not have support in situ, and could be
contributing to errors in model estimates of future climate. Overall, this dissertation identifies
mechanisms through which clouds and plant canopy structure alter land-atmosphere CO2 fluxes
and subsequently, Earth’s climate. It also provides an important interdisciplinary framework for
testing assumptions about the feedbacks that living organisms form with their environment.

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