Understanding vegetation structure and function with novel remote sensing observations of solar-induced chlorophyll fluorescence and terrestrial laser scanning

Vegetation function and structure play an important role in determining the fluxes of carbon, water, and energy between the land surface and the atmosphere. Environmental conditions and plants’ resource-use strategies drive coordinated variations of vegetation function and structure. To understand the response of ecosystems to future environmental changes, it is crucial to monitor vegetation function and structure and understand their coordination and response to the environment. Solar-induced chlorophyll fluorescence (SIF) and terrestrial laser scanning (TLS) are novel remote sensing tools to monitor vegetation function and structure, respectively, and they provide information not available from traditional remote sensing approaches. My dissertation explores SIF and TLS measurements to improve our understanding of the spatial and temporal variations of vegetation function and structure.
In Chapter 2, I investigate the controls of SIF signal and fluorescence yield with tower remote sensing measurements in a temperate forest in central Virginia. I found that illumination was the major driver of the diurnal variation of SIF, while fluorescence yield was the major driver of the day-to-day variation of SIF. Fluorescence yield responded to light intensity and temperature, and the responses estimated from tower measurements agreed with those obtained from leaf-level measurements. Our findings improve the interpretation of SIF variations and show that the variation of fluorescence yield can be investigated with remote sensing measurements. In Chapter 3, I incorporate the simulation of SIF into the Community Land Model (CLM) with an improved representation of canopy structure and leaf-to-canopy radiative transfer. SIF simulated by CLM generally agreed with simulations by a reference canopy model and with tower and satellite observations. Various factors, including the fluorescence emission model, clumping, bidirectional effect, and leaf optical properties, had considerable impacts on SIF simulation. By improving the representation of radiative transfer for SIF simulation, our model provides more accurate SIF simulations that allow better comparisons between simulated and observed SIF. Its application can thus improve the use of satellite SIF observations for constraining and evaluating plant photosynthesis simulations. In Chapter 4, I investigate the coordination among canopy structural traits with TLS measurements at 12 plots across the Eastern United States. I found that traits related to leaf area, leaf angle, and clumping were correlated with each other and the relationship between leaf area and leaf angle was height-dependent. Our work indicates that plant strategy on competition and maximizing photosynthesis can drive coordination among plant structural traits.
This dissertation shows that canopy structure is a key factor that affects vegetation function, and the spatial variation of canopy structural traits is coordinated. I demonstrate that SIF and TLS, as novel remote sensing tools, can effectively improve our understanding of vegetation function and structure and their variations.