Abstract
We examine the physical and biological responses of forest canopies to step changes in light caused by passing low cumulus clouds that intermittently block the direct solar beam. Using data obtained at a tropical rainforest and at a midlatitude deciduous forest, we estimate the course of sensible heat flux, net ecosystem exchange, evapotranspiration, and water-use efficiency in response to the rapid changes in the incident radiative flux. To describe these fluxes during the interval over which the effects of stomatal time delays can be most influential, eddy fluxes are estimated over minute or shorter intervals by invoking a conditional-sampling procedure based on forming a Reynolds-average ensemble. The most important differences between the two forests’ physical responses are in the thermal balances and heat-flux time response constants. During the initial period after the light transition the only mean variables that show appreciable changes are the blackbody and air temperatures, the other scalars being little affected. We find that a distinct transient thermal internal boundary layer appears ≈ 20 m thick above the temperate deciduous forest and ≈ 45 m thick above the tropical rainforest. At each forest, the effective thickness of the inferred thermal outer-canopy ‘big leaf’ is about 1 mm. Twenty minutes after the abrupt change in incident light, ensemble eddy-flux estimates approach those found using conventional time averaging, confirming the validity of the ensemble approach. Previously unrecognized transient maxima in net ecosystem exchange and evapotranspiration are evident 5–10 min following the shadow-to-light transition, longer than the average light interval between shadows observed on partly-cloudy days in each case. Short-term variations in sensible heat flux, net ecosystem exchange, and evapotranspiration approximate an exponential adjustment, implying that first-order time-dependent single-leaf models are adequate to describe whole-canopy processes in these cases, providing an experimental method for determining whole-canopy bulk stomatal time constants. During the sunlit interval (direct and diffuse radiative fluxes combined) net ecosystem exchange is enhanced, while under cloud shadow (only diffuse radiative flux) water-use efficiency increases. This light and shadow alternation provides a mechanism describing the observed enhanced net ecosystem exchange and water-use efficiency under certain types of partly-cloudy sky. We apply empirical flux-response curves to an idealized case of radiative flux varying in a regular on–off light and shadow pattern. For this case, an analytical solution for mean net ecosystem exchange flux as a function of diffuse-radiation fraction yields results that strongly resemble previous findings based on conventional time-averaged fluxes. Our analysis and modelling indicates that a well-known correlation between diffuse radiative flux and enhanced net ecosystem exchange and water-use efficiency on cloudy days is in many cases not causal, but rather to be a consequence of time averaging over light-and-shadow intervals. By linking processes associated with photosynthesis in fluctuating light at the leaf scale to the canopy scale, our efforts facilitate the scaling-up of leaf responses to the ecosystem scale.
Highlights
A vegetation canopy is an active, living surface—energy and scalar fluxes respond to a constantly varying lower atmospheric environment
The second is from the Large-Scale Biosphere–Atmosphere (LBA) Experiment km-67 site tower in the Tapajós National Forest, a tropical rainforest located near Santarém, Pará, Brazil (2.86°S, 54.96°W)
To assess whether or not members assigned to an ensemble cohort belong together, we examined the histograms of the distributions of the tendencies around their ensemble means obtained at selected times following the shadow-to-light transition (red, Fig. 4 Distributions of the tendencies around their ensemble means of T air (a, b), w (c, d), ca (e, f), and q (g, h) taken and labelled at 0, 150, and 300 seconds after the shadow-to-light transition; lines—respective normal-distribution p.d.f. fits; [900, 1100] W m−2 clear-period S↓ bin; for the Harvard Forest (HF) and the LBA sites green and blue histograms; at 0, 150, and 300 s, Fig. 4)
Summary
A vegetation canopy is an active, living surface—energy and scalar fluxes respond to a constantly varying lower atmospheric environment. Photosynthesis operates at a variety of time scales (e.g., Way and Pearcy 2012) Recent interest in both the ecological and the agricultural communities in what is referred to as dynamic photosynthesis focuses on how plants and canopies overall respond to the naturally varying environment (e.g., Murchie et al 2018; Matthews et al 2018; Matsubara 2018). Light and shadow durations on partly-cloudy days are on the order of a few min (Kivalov and Fitzjarrald 2018), intervals comparable to the stomatal opening and closing time constants for many common tree species (e.g., Woods and Turner 1971; Vico et al 2011). Forest carbon uptake is observed to increase along with wateruse efficiency during partly-cloudy conditions, when compared to clear-day conditions, a fact often attributed to the more complete illumination of the canopy occasioned by the enhanced diffuse radiative flux
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