Functional traits
Leaf traits were measured on at least five leaves per species. Fresh
mass was measured on fully hydrated leaves. Leaf length and width were
measured with digital calipers or rulers, and leaf lamina thickness was
recorded on at least three positions from the apical, median and basal
section for each leaf using a digital micrometer. We measured leaf area
with a LI-3100C area meter
(LI-COR, Lincoln, NE). Dry mass was determined after drying the leaves
at 70°C. From these measurements we calculated leaf mass per area (LMA,
kg m–2), leaf dry matter content (LDMC, g
g–1), and leaf density (leaf dry mass per unit leaf
volume). To address hypothesis 3 we estimated leaf thermal time constant
τ (s) from first principles following Michaletz et al . (2015),
using species-level mean values of LMA, LDMC, leaf area, and leaf width;
mean annual temperature derived from WorldClim2; and wind speed set to 2
m s–1, as:
\(\tau=\varphi LMA\left[\frac{c_{p,w}}{LDMC\ \bullet\ h}+\frac{c_{p,d}-c_{p,w}}{h}\right]\)Eqn 2
where φ (dimensionless) is the ratio of projected-to-total leaf
area, cp,d (J kg–1K–1) is the specific heat capacity of dry leaf
matter, and cp,w (J kg–1K–1) is the specific heat capacity of water. The heat
transfer coefficient h (W m–2K–1) was calculated as \(h=\rho_{a}c_{p,a}g_{h}\),
where ρa (kg m–3) is air
density, cp,a (J kg–1K–1) is the specific heat capacity of air at a
constant pressure, and gh is the heat
conductance. The effects of radiation and transpiration were not
included as these data were not available for all species. This permits
us, however, to quantify how variation in the measured morphological
traits alone causes variation in thermal time constants. Functional
trait data were obtained for 132 species as insufficient healthy leaves
were available for all species. For species with compound leaves we used
traits (LMA, size, τ, etc) collected on leaflets for further analyses.
Where necessary, traits were transformed to improve normality of the
data.