Exploring Canopy Structure and Function as a Potential Mechanism of Sustained Carbon Sequestration in Aging Forests

Forests are thought to reach carbon (C) neutrality as they age such that photosynthesis is balanced by respiration.However, recent evidence suggests that C sequestration continues in forests that are centuries old.One potential mechanism sustaining C accumulation in old growth forests are age related changes in canopy structural complexity.Among these is rugosity, a measure of the horizontal variability of vertical variation in leaf area distribution.While rugosity has been found to sustain forest productivity across a 200-yr chronosequence of deciduous forests in Michigan, the mechanism linking this structural metric to forest function (e.g.C sequestration) is not well understood.The objectives of this dissertation were to study the drivers of canopy structural complexity and explore the novel mechanisms by which canopy structural complexity might influence forest productivity.At the University of Michigan Biological Station, i) portable canopy light detection and ranging (LiDAR) was used to characterize canopy structure in space and time, ii) long-term hemispherical photography was used to quantify within-canopy light distributions and its among-year variability, iii) branch and leaf materials were collected to measure tree responses to canopy structure and light distributions, and iv) canopy light microhabitats were manipulated from an aerial work platform and in situ morphological and physiological responses of trees subjected to different treatments were quantified.More complex canopies (high rugosity) were found to have greater amounts of empty space and reduced light variability in the midcanopy help of Gabe Hilts, James Gunkelman, Nick Houghton, Jim LeMoine, Chrisoph Vogel, Tony Sutterley, and Richard Spray in assiting with the design, construction and repair of equipment.We thank Raleigh Ricart for long hours in the field to help with data collection.We are grateful for endless assistance of Tim Morin with processing and interpreting portable canopy lidar data in Matlab.