Abstract
Photosynthesis, the uptake of carbon dioxide by plants, is one of the largest fluxes in the terrestrial carbon cycle, though the sensitivity of this process to climate change is not well understood. To measure photosynthesis at leaf, canopy, and landscape scales and across ecosystems remotely sensed spectral signals of photosynthesis are increasingly used, enabling mechanistic understanding of the drivers of primary production. Whereas spectral reflectance measurements can provide information on biogeochemical properties associated with photosynthetic capacity, chlorophyll fluorescence (ChlF), an emission of energy that is physically linked to the activity of chlorophyll molecules, provides direct information on the process of photosynthesis. Because ChlF may be measured across scales and from remote platforms, it is especially valuable for improving understanding of how photosynthetic dynamics of boreal ecosystems are responding to rapid and severe climate change. This dissertation was motivated by two primary uncertainties associated with tracking photosynthetic dynamics. First, which scalable proxies of photosynthetic activity can help improve understanding of how boreal ecosystems respond to climate change? Second, what mechanistic considerations can improve interpretation of ChlF in ecosystems characterized by complex canopies? In chapters two, three, and four, I address these questions by leveraging observations of ChlF against complementary measures of environmental conditions and ecophysiological processes to understand the nuanced dynamics of photosynthesis. In chapter two, I address the first uncertainty by introducing a novel approach to quantify the influence of microstructural attributes on photosynthetic functioning of small stature white spruce (Picea glauca) trees at the Arctic forest-tundra ecotone. Findings demonstrated that variance in ChlF can be explained by proximal microstructure attributes measured with terrestrial light detection and ranging scanning (TLS) and linked to gradients in boundary layer conditions. In chapter three, I address the second uncertainty by investigating how the yields of ChlF and photosynthesis are nonlinearly related across ambient diurnal sub-canopy scale light regimes in two evergreen needleleaf species. Empirical relationships from this study are used to contextualize the apparent linear relationship between ChlF and photosynthetic carbon uptake at coarser spatial scales. In chapter four, I address both uncertainties by investigating the mechanisms governing relationships among remotely sensed ChlF (i.e., solar induced fluorescence, SIF) and spectral vegetation indices using airborne observations from across the Arctic-Boreal Zone. Findings from this work demonstrate that such relationships observed at coarse scales diverge at intermediate scales. Collectively, this dissertation demonstrates that interpretation of ChlF requires consideration of mechanistic linkages between environmental conditions and ecophysiology to generate insight on photosynthesis.