Leaf morphological traits
Leaf morphological traits were measured according to standard protocols (Perez-Harguindeguy et al., 2013) and included leaf fresh weight (g), leaf area (cm2), leaf width (m) and leaf dry mass following 3 days of drying at 60 °C. From these we calculated leaf mass per area, LMA (kg m-2) and leaf dry matter content, LDMC (g g-1). In addition, leaf inclination angle (°) – the angle of the leaf relative to the horizontal plane – was measured using an electronic protractor within a cell phone, which has shown similar accuracy compared to traditional manual and digitiser methods of measuring leaf angle (Escribano‐Rocafort et al., 2014). Leaf morphological traits were measured on 3 to 10 leaves (average 9) per plant, while leaf angle was measured in situ on 10 leaves per plant.
The thermal time constant, \(\tau\) (s), influences the time taken for leaves to heat up or cool down following a change in microclimate (Michaletz et al., 2015). We calculated this using the leaf traits LMA, LDMC, and leaf width and by assuming air temperature of 28 °C, air pressure of 1,012 hPa, and wind speed of 2.3 m s-1, as these were the average values observed throughout our measurement campaign, and we wanted to isolate the impact of leaf traits:
\begin{equation} \tau=\varphi\bullet LMA\bullet\left(\frac{c_{\text{pw}}}{LDMC\bullet H}+\frac{c_{\text{pd}}-c_{\text{pw}}}{H}\right)\nonumber \\ \end{equation}
where \(\varphi\) is the ratio of the projected to total leaf area, which is 0.5 for flat leaves, \(c_{\text{pw}}\) is the specific heat capacity of water (4,180 J kg−1K−1), \(c_{\text{pd}}\) is the specific heat capacity of dry leaf matter (J kg−1 K−1), which varies across species so we used a value of 2,814 J kg−1 K−1 which was the mean of seven tropical tree species in (Jayalakshmy & Philip, 2010) and has been previously used in other studies based in the tropics (Fauset et al., 2018; Slot et al., 2021). The heat transfer coefficient, \(H\)(W m−2 K−1), was calculated using the formula:
\begin{equation} H=\rho_{a}\bullet c_{p,a}\bullet g_{h}\nonumber \\ \end{equation}
where \(\rho_{a}\) is air density (1.170685 kg m−3),\(c_{p,a}\) is the specific heat capacity of air at a constant pressure (1004.78 J kg−1 K−1), and \(g_{h}\)(m s-1) is the heat conductance which was calculated for a flat plate under laminar forced convection conditions as per Jones (2013):
\begin{equation} g_{h}=1.5\bullet 0.00662\bullet\sqrt{\frac{U}{w}}\nonumber \\ \end{equation}
where outdoor turbulence is accounted for by a factor of 1.5, \(U\) is the wind speed (m s−1), and \(w\) is the leaf width (m). We did not include the effects of radiation or transpiration, instead using it to explore the impacts of leaf traits on \(\tau\). The thermal time constant was not incorporated into steady state leaf energy balance modelling but was analysed as a response variable independently.