Study Site This study took place in a mixed deciduous forest in Northern Lower Michigan (45° 35’ N 84° 43’ W). The mean annual temperature is 5.58° C and mean annual precipitation is 817 mm (1942–2003) (Gough et al 2013). Following massive disturbances (clearcuts and forest fires) in the past two centuries, this region has become primarily dominated by early the successional species bigtooth aspen (Populus grandidentata), trembling aspen (Populus tremuloides) and paper birch (Betula papyrifa). Other canopy species include red oak (Quercus rubra), red maple (Acer rubrum), sugar maple (Acer saccharum), eastern white pine (Pinus Strobus), and American beech (Fagus grandifolia). Stem density of trees ≥8 cm dbh is 700– 800 individuals/ha, basal area is 25 m2/ha, and leaf area index (LAI) averages 3.5. Red maple, red oak, eastern white pine, and American beech are the prominent species composing the subcanopy; they are joined by other shade tolerant species such as sugar maple, red pine (Pinus resinosa), striped maple (Acer pensylvanicum), American hophornbeam (Ostrya virginiana), and serviceberry (Amelanchier arborea). The early successional species defined above are in decline as they reach the end of their lifespan, leaving room for other species to gain prominence in the canopy. To evaluate the effects of disturbance and successional changes on carbon pools and fluxes in this ecosystem type, the University of Michigan Biological Station implemented the FASET program in 2008. This large-scale manipulation has hastened the development of a forest composition that will dominate the region in the coming decades as succession proceeds naturally (Gough et al. 2013, Nave et al. 2011). To better understand the implications of canopy gaps left by senescing early successional species, this study examines trees across a gradient of disturbance levels in FASET. We chose 10 out of 21 permanent 0.08 ha circular plots based on pre-disturbance production and species composition in order to minimize confounding variables (Goodrich-Stuart et al. 2014). Plots ranged in disturbance severity from .09 to .64 fraction basal area senesced, a range representative of the differential early successional die-off expected in the region. Four non-overlapping 5 m radius subplots were established on the cardinal axis of each plot. We sampled three saplings in each plot to measure apparent quantum yield and Amax. Leaf Physiology Analysis To establish the impact of canopy openness on leaf physiology, we constructed a light response curve for three species within each subplot, measuring carbon dioxide assimilation rates at a range of irradiance levels. We studied common saplings species that had the potential to eventually extend into the canopy. Eligible saplings were between 1 and 6.5 m tall and under 3 cm dbh; their size suggests that these trees were in the subcanopy prior to the stem-girdling disturbance. Three saplings were randomly selected in each subplot for measurement with descending priority of oak, maple, pine and beach.. A leaf at the top of each sapling was selected for measuring photosynthesislight response curves using a LiCor LI-6400 Portable Photosynthesis System (model LI-6400, LI-COR, Lincoln, NE, USA). Leaves were subjected to varying irradiance levels (1500, 750, 500, 250, 75, 50, 30, 10, 0 μmol photons/m2/s) using a 6400-02 red-blue LED light source (LI-COR, Lincoln, NE, USA) – special emphasis was placed on the initial slope of the light response curve (i.e. 5 points under 100 μmol photons/m2/s) to allow for precise computation of apparent quantum yield. For each broadleaf, a 2 by 3 cm area was enclosed in the LiCor-6400 chamber to monitor its carbon assimilation rate at a constant area. For examination of Pinus strobus, we used three five-needle fascicles from the previous year's growth, laid across the cuvette in a non-overlapping manner. Pine photosynthesis measurements were post-processed to correct for area since 15 needles never completely filled the cuvette. We controlled immediate environmental conditions in the chamber by setting the LiCor-6400’s CO2 mixer to 380 ppm, and made an effort to stabilize relative humidity of the sample between 60 and 70% and maintain a leaf temperature of 24 +/- 1.5 degrees C. IRGAs were matched and conditions were allowed to stabilize in the chamber before photosynthesis readings were taken at each light level (minimum 2 minute stabilization period). Data processing and statistical analysis Light curves were constructed using the physiology data collected using the rectangular hyperbolic function:: P = (α*A¬max*I)/ (Amax + α*I) (adapted from Gough et al. 2013). Where P is photosynthesis (µmol CO2·m-2·s-1, α is apparent quantum yield (mol CO2/mol quanta) and I is the irradiance level (µmol quanta·m-2·s-1). Apparent quantum yield is the parameter describing the initial slope of the rectangular hyperbolic function, and A¬max is where the function plateaus. Regression analysis was used to determine relationships of α and Amax across disturbance gradients and analysis of variance with Tukey’s HSD test to determine differences between species (SAS Institute 2012).