Table 1.
Symbols, standard values and units used in this paper.
Figure 1.
Components of the leaf mass and energy balance and their conventional directions considered in this study.
Arrows point in the direction of a positive flux. Both leaf temperature () and water content (
) depend on the transpiration rate (
and
in energetic and molar units respectively). The leaf water content (
) affects the leaf heat capacity (
) and turgor pressure, which becomes critical when leaf water content declines below 90% of its maximum value (see text). Changes in leaf water content result from differences in the water supply rate from the xylem (
) and evaporative losses (
).
Figure 2.
Observed irradiance (), air temperature (
) and leaf temperature (
) in the understorey of a tropical rainforest.
Data converted from [19].
Figure 3.
Fit of Eq. 31 to data in [30, Tab. 2].
K,
K s, and
K s−1, standard root mean square deviation: 0.07.
Table 2.
Natural and experimental light fluctuations vs. stomatal conductances.
Figure 4.
Observed and simulated leaf temperatures for an understorey plant in a tropical rain forest.
Simulations are conducted for fully closed stomata (red) and a stomatal conductance of 0.01 m s−1 (blue). Observed leaf temperatures (yellow dots) and air temperatures (green dashed line) are taken from [19] and plotted against local time.
Figure 5.
Simulated leaf temperatures in a rainforest understorey for closed stomata and different leaf water contents.
Black: 0.025, red: 0.1 and blue: 1.0 kg m−2 leaf water content. The green line represents the observed air temperature [19], plotted against local time.
Figure 6.
Leaf temperature and flux dynamics in response to sudden illumination.
A: Temperature evolution of a non-transpiring leaf at different illumination intensities. B: Temperature evolutions of non-transpiring leaves with different water contents. C: Dynamics of latent, sensible and longwave heat flux from a leaf with non-limiting stomatal conductance (). D: Temperature evolution of a transpiring leaf with different stomatal conductances (
). Common environmental conditions for all simulations:
K,
m s−1, 70% relative humidity, 0 W m−2 irradiance prior to arrival of sunfleck. Unless otherwise indicated, simulations are performed assuming a 5 cm wide leaf with 0.05 kg m−2 water content, exposed to
W m2 sunfleck irradiance. The shaded area represents critical combinations of leaf temperatures and exposure times that are expected to cause considerable heat damage. It is computed using the equation
, with
K and
K s. This equation was derived from experimental data for black spruce needles (see Methods). In Panel (c), the calculated boundary layer conductance is
m s−1 and a stomatal conductance of 0.0029 m s−1, resulting in latent heat flux of 63 W m−2 prior to illumination and 248 W m−2 at steady state during the sunfleck, would be sufficient to keep leaf temperatures below
.
Figure 7.
Rates of evaporative cooling and associated stomatal conductances to avoid heat damage.
Contour lines in main panels represent rates of latent heat flux (W m−2) necessary to keep leaf temperatures at or below 322 K (49°C), for different combinations of air temperatures and solar irradiances (). Panel A: assumed wind speed
m s−1; Panel B:
m s−1. Insets: stomatal conductances that would achieve the latent heat fluxes computed for 600 Wm−2 irradiance in main panels, for differrent relative humidities. Dashed contour lines mark the lowest stomatal conductance values observed in shaded leaves (Table 2).
Figure 8.
Critical exposure times to a sunfleck of 600 W m−2 light intensity for heat damage (red) or turgor loss (blue) as a function of initial leaf water content.
Environmental conditions: K,
m s−1, 70% relative humidity, 100 W m−2 irradiance prior to arrival of light fleck. The steady-state transpiration rate at the pre-sunfleck light intensity was taken as a constant xylem water supply rate during the light fleck. Simulations were performed for different values of stomatal conductance, as indicated for each line on the right hand side. The dashed lines represent extreme cases of unlimited stomatal conductance (blue dashed) and negligible stomatal conductance (red dashed line). The blue dotted line represents the time to turgor loss if evaporative cooling is just sufficient to prevent heat damage altogether. In this case, latent heat flux rises from 90 W m−2 before sunfleck arrival to 248 W m−2 during the sunfleck.