The energy balance of a leaf

I am currently co-teaching the course on ‘environmental biophysics’ to our ecology masters, a largely theoretical course in which students get to know the physical equations behind ecology and the interaction between organisms and their environment. Besides this formula-juggling, the course contains a very nice little practicum which I inherited from my predecessors, which I wanted to share.

The goal is to calculate the full energy balance of a leaf: what kinds of energy are coming in, and which ones are going out again? Then, we go on an ‘excursion’ two meters out of the classroom door, take some real leaves and try to measure the necessary parameters to calculate that energy balance in real life.

Measuring the temperature of three ivy leaves, suspended in the air to simulate ‘normal’ conditions.

This involves a lot of environmental measurements: students measure air temperature, soil temperature, temperatures of the sky, short-wave radiation, wind speed and relative humidity, all these parameters who define the environment of a leaf and thus how it will regulate its energy.

Then of course, pretty important, they also measure the temperature of the surface of the leaf, which in itself relates to all these parameters but also to how well the leaf can regulate its temperatures. That temperature regulation works through two processes: energy loss through transpiration (water loss), and heat loss related to differences in temperature between leaf and environment.

Measuring leaf temperature (white pants), air temperature and wind speed (grey pants) and meticulously writing things down (jeans)

To get a good grip on those different parameters and how easy energy can be exchanged between a leaf and its environment (the so-called conductance of the leaf and its boundary layer), we did a little experiment: next to a control leaf, we had one leaf that we sprayed entirely wet. This situation greatly facilitates the transpiration of water, as now the air-water boundary is not inside the leave (forcing water vapour to go through the small stomata), yet on the outside of the leaf. The result? Significant multi-degree drop of temperature far below air temperature even (we had examples of air temperature at 23°C, and watered leaf temperature at 16°C!).

Additionally, we sprayed one leaf with ‘wilt-pruf’, a resin-based liquid that seals the stomata and thus effectively prevents any water loss. It’s traditionally used in horticulture when you want to prevent a vulnerable plant from loosing water, but here it served another purpose: sealing the stomata removes the transpiration process from the equation entirely. The result now? Take a second to think it through…

Measuring incoming shortwave radiation (device on the right). The ventilators are used to dry the resin-coated leaves.

Indeed, the leaf should start heating up, as it can’t loose energy through water loss anymore! This turned out to be hard to replicate in the field, as it is strongly dependent on weather conditions, wind speed and the question if stomata were open in the first place. Nevertheless, we managed to heat up a leaf 1°C as compared to the control this way!

It took us till we moved to a spot with low wind speed (enforced by wind screens left and right, resulting in a drop in wind from ~2 to 0.4 m/s) before we could successfully get the resin-covered leaf to heat up

So what’s the take home message of all of this? To me, it is that, indeed, you can describe the living world with mathematical equations. Nevertheless, the real world gets rapidly too complex to keep track of the formulas in it, which is why ecology so often works by proxy (and so often has strong noise in the data). An important piece of fundamental knowledge for aspiring ecologists, I would say!

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