Taking the temperature of a tree – not all leaves are equal

By Sophie Fauset, University of Leeds

Leaf temperatures are not the same as air temperatures, and they vary between tree species. I knew this from reading about biophysics, but the extent to which it is true only became apparent after actually going and measuring leaf temperatures myself. I measured leaf temperatures reaching 18 ºC above air temperatures, and found large effects of the distinct leaf physiology of different species.

After many months of planning and building equipment with the lab team at the University of Leeds, I set out with my colleague Martin Sullivan to measure leaf temperatures in the Atlantic forest. The Atlantic forest, of which currently only around 15 % remains, runs along the Atlantic coast of Brazil taking in the famous cities of Rio de Janeiro and São Paulo and west towards Paraguay and Argentina. We were working in the mountainous Serra do Mar State Park, the largest remaining fragment and an incredibly beautiful landscape.

Serra do Mar State Park, São Paulo. © Sophie Fauset

I had previously spent 6 months working in these forests with colleagues from UNICAMP measuring the microclimate in intact and disturbed forests (for more on that look here). While I was measuring the air temperature, I knew that this did not necessarily reflect the leaf temperature. With colleagues at Leeds and the University of São Paulo, we developed a project to measure leaf temperatures using a small canopy tower to access the crowns of trees.

The first challenge was to climb the tower. Being only 30 cm wide and 27 m tall, this required a full climbing harness and clipping on to each rung of the tower as you went up it. Being fairly intolerant of heights, it was only knowing that I had only myself to blame for being there that kept me going. After a day or so I mostly got used to it! Thankfully, the tree crowns went no higher than 20 m, so I did not need to go to the top. The next step was to set up the measurements of leaf temperature and microclimate.

Looking down from about 16m up the tower. Light sensors are hanging from the tower and cables connect sensors on the leaves to the data loggers on the ground (bottom left – Miconia cabussu, top right – Alchornea triplinervia). © Sophie Fauset.

We measured leaf temperatures in two ways. First, we connected thermocouples to the leaves. These thermocouples had two junctions, one of which was positioned on the lower surface of the leaf (held in place with breathable surgical tape), and the second dangled in the air below the leaf. This circuit generates a voltage in proportion to the temperature difference between the leaf and the air. We connected sensors to 23 leaves in total, on three different trees, and separately measured air temperature at multiple points from the tower. The sensors collected data every 10 seconds over 10 days to provide a detailed record of how the leaf temperatures changed during the day and under different climatic conditions. In addition, we measured leaf temperatures with a hand-held infra-red thermometer to compare with our thermocouple records.

Thermocouple attached to a leaf of Alchornea triplinervia. © Sophie Fauset.

The actual temperature of a leaf is dependent on a few different factors. The first is the amount of heat energy that enters the leaf, and this depends on the sunlight intercepted by the leaf, and also thermal energy (long-wave radiation) which is emitted from all objects with a temperature above absolute zero (-273.15 ºC). This incoming energy raises the leaf temperature above the air temperature. The leaf also loses energy, which cools it. The main way a leaf cools is through transpiration, where water is evaporated through the pores (stomata) on the leaf surface. In the same way as sweating cools humans, transpiration cools leaves because the evaporation of water requires energy. Leaves also lose energy by emitting thermal radiation, and through convection.

Different species will transpire from the leaf surface at different rates. This is because transpiration is dependent on the openness of stomata, the dryness of the air, and the boundary layer. The boundary layer is a layer of still air around the leaf, and the water vapour must pass through this layer into the freely moving air beyond. Larger leaves have a thicker boundary layer, and therefore it is more difficult for the water to evaporate. Plant species also respond differently to humidity – some species close their stomata when the air is dry to prevent them from losing too much water, and in other species this response is weaker or not present. We measured the different aspects of microclimate (light, humidity, air temperature) during the leaf temperature measurements, and also made measurements of stomatal opening and leaf size to quantify all the different elements that control leaf temperature.

Despite being on the Tropic of Capricorn, our study site 1000 m up a mountainside was not especially warm when we visited. The maximum air temperature we recorded was 22 ºC. When we looked at our leaf temperature records mostly the leaf temperatures were slightly higher than air temperatures, but we had a surprise with leaf temperature peaking at 37 ºC. The light levels that we were measuring were very high, and this was causing the leaves to get much hotter than the air. There was another surprise too. The very high leaf temperatures were occurring more frequently in one species (Miconia cabussu) than another species (Alchornea triplinervia) despite the leaves of Alchornea being higher up and more exposed to the sun. We found that Miconia closed its stomata more in response to dry air, and that the larger leaves resulted in a thicker boundary layer. Both of these factors cause the Miconia leaves to reach higher temperatures. In regards to temperature, not all leaves are equal.

Many physiological processes such as photosynthesis and respiration are temperature dependent. Therefore, gaining a fuller appreciation of leaf temperatures and how they differ between species can help us to predict the impacts of climate change on tropical trees and forests.

To find out more, read our paper here.