Functional trait measurements
We quantified 13 physiological, morphological, and chemical traits for each individual leaf. To do so, we executed 14-point photosynthetic light-response curves on each leaf using a LI-6800 Portable Photosynthesis System (Licor Bioscience, Lincoln, Nebraska, USA), to measure area-based photosynthetic rates (A area;μ mol CO2 m-2s-1) across a range of photosynthetic photo flux density (I ) levels (i.e., 2000, 1500, 1200, 1000, 800, 600, 400, 200, 100, 80, 60, 40, 20, and 0 μ mol photosynthetically active radiation (PAR) m-2 s-1). All photosynthetic rates were allowed to stabilize at each level of Ifor at least 120 seconds prior to data acquisition (Berry & Goldsmith, 2020; Salter, Merchant, Richards, Trethowan, & Buckley, 2019), with each light response curve therefore taking a minimum of 30 minutes to complete. All A area measurements were made between 8:00-12:00 am to avoid mid-day stomatal closure, with leaf chamber conditions maintained at CO2 concentrations of 400 ppm, relative humidity at 53.1-73.5%, leaf vapour pressure deficits of 1.2-1.7 KPa, and leaf temperatures between 24.3-31.6 °C.
Physiological traits were calculated for each leaf by fitting non-rectangular hyperbola to each of the 45 light response curves as:
\(A_{\text{area}}=R_{\text{area}}+\frac{\phi I+A_{\max}-\sqrt{\left(\phi I+A_{\max}\right)^{2}-4\theta\phi IA_{\max}}}{2\theta}\)Equation 1
where R area represents area-based leaf Rrates (μ mol CO2 m-2s-1), Φ is the apparent quantum yield of photosynthesis (mol CO2 mol PPFD-1),A max is the light-saturated maximum area-based photosynthetic rate, and θ represents a curvature parameter. From these models, we also derived leaf-level light compensation points (LLCP, μ mol PAR m-2 s-1), calculated as I , where A area=0. Light response curves were fitted using the ‘nls’ function in R v.3.3.3 statistical software (R Foundations for Statistical Computing, Vienna, Austria).
After light response curves were completed in the field, each leaf was immediately collected and transported to the University of Toronto Scarborough for morphological and chemical trait determinations. First, leaf area (cm2) was measured using a LI-3100C leaf area meter (Licor Bioscience, Lincoln, Nebraska, USA), and all leaves were then dried at 65 °C to constant mass and weighed (g). We then calculated LMA (g m-2) as leaf mass/leaf area, and used these LMA values to calculate maximum mass-based photosynthetic (A mass) and mass-based dark respiration rates (R mass), as A max orR area / LMA, respectively. Lastly, dried leaf tissue was ground into a fine powder using a MM400 Retsch ball mill (Retsch Ltd., Hann, Germany), and ~0.1 grams of tissue was weighed and analyzed for C and N concentrations (both on a % mass basis) on a LECO CN 628 elemental analyzer (LECO Instruments, Ontario, Canada).