Monitoring the Energy Balance in Forests

There are many diverse ecosystems worldwide, but forests are often at the center of concerns surrounding pollution, climate change and carbon balance due to their large surface area.

Monitoring the Energy Balance in Forests

Image Credit: OTT HydroMet

A research team from OTT HydroMet has been working to better understand these ecosystems’ significance and functionality by investigating a mature oak stand in the Barbeau state forest, located 70 km southeast of Paris.

The research station has been fitted with a 35-meter high tower, heavily instrumented with probes, sensors and devices, essentially designed to place the forest under a microscope.

The top of the tower is approximately 5-6 meters higher than the tallest trees, allowing a number of micro-meteorology parameters to be monitored above the canopy. These parameters include air temperature and relative humidity, dew point, rain, air pressure, wind direction and wind speed.

Above the canopy, air temperature, wind speed, humidity and direction are measured at six different heights while air gas concentrations (CO2, H2O and O3) are sampled at eight different heights, using a vertical profile up the tower.

Below the canopy – either underground or at ground level – measurements are acquired for soil micrometeorology parameters, such as tree trunk growth, sap flow rates, soil temperature, soil water content, soil heat flux (G) and soil CO2 efflux (soil respiration).

Solar radiation sensors at the top of the tower.

Solar radiation sensors at the top of the tower. Image Credit: OTT HydroMet

Monitoring data allows the research team to interpret and describe the fluxes of energy and mass between the forest itself and the surrounding atmosphere.

The ‘Eddy Covariance’ approach is used to enable the measurement of fluxes of matter and energy. This approach employs a combination of two instruments: a rapid CO2/H2O analyzer and a three-dimensional sonic anemometer.

The vertical component of the wind speed is monitored with a scanning frequency of at least 10 Hz, allowing this to be taken into account in co-variance with the monitored variables (air temperature, CO2, or H2O).

This then facilitates a calculation of the vertical CO2 and H2O turbulent flux (mass) and the vertical heat (H) and latent heat flux energy (LE) present between the forest and the atmosphere.

There are also a number of solar radiation sensors fitted both above and below the Barbeau forest canopy. These sensors provide insights into the impact of light on plant functioning – an essential consideration as this takes part directly in the leaf photosynthesis mechanism.

Global radiation (Rg, Kipp & Zonen CMP22) and net radiation (Rnet, Kipp & Zonen CNR 4) ventilated sensors were fitted at the top of the tower, allowing the research team to determine how much energy is being transmitted from the sky and how much energy is reflected by vegetation (trees and soil).

This allowed them to calculate the energy that is absorbed.

One challenge when employing the Eddy Covariance method stems from the need to quantify estimates of the uncertainty of the reported flux values. These fluxes are sophisticated processes, with estimates resulting from numerous measurements and calculations as well as various explicit and implicit assumptions.

Documenting the absolute accuracy of these values is, therefore, challenging. There is, however, a straightforward measure of internal consistency available – checking for conservation of energy.

The sum of the turbulence fluxes of sensible and latent heat should ideally balance the available energy:

Rnet = H + LE + G

Where Rnet is the net radiation, H the sensible heat flux, LE the latent heat flux and G the soil heat flux.

Shown is a typical relationship for a forest, using data from Tharandt forest station, Germany in 2006 and 2007 and adapted from N. Delpierre, 2009.

Shown is a typical relationship for a forest, using data from Tharandt forest station, Germany in 2006 and 2007 and adapted from N. Delpierre, 2009. Image Credit: OTT HydroMet

Two Kipp & Zonen PQS1 PAR (photosynthetically active radiation) sensors were also fitted back-to-back at the top of the tower, facilitating the monitoring of both incoming PAR from the sky and the reflected PAR from vegetation.

Underneath the canopy, a total of 15 PQS1 PAR sensors have been distributed on the ground in order to determine the levels of PAR reaching the soil. Using these sensors, the research team has been able to effectively calculate the PAR fraction absorbed by the forest.

Barbeau is a deciduous broadleaf oak forest, meaning that its leaves fall in autumn. The PAR fraction that reaches the soil in winter corresponds to 35% of incoming PAR, and in summer, with just 1% to 2% of this reaching the soil.

In winter, the PAR is primarily intercepted by wood, such as trunks, branches and twigs, while the PAR is primarily intercepted by leaves in summer.

Monitoring the Energy Balance in Forests

Image Credit: OTT HydroMet

It is also possible to directly correlate this interception to the leaf area index (LAI, in m2 of leaves per m2 of soil), providing a valuable indication of forest density. Essentially, the more leaves that are present, the less light that reaches the soil.

A total of 10 Campbell Scientific dataloggers (CR1000 & CR3000) connected via an Ethernet network are used to control all the sensors and collect the data. Data is uploaded to the laboratory servers via a satellite connection before being displayed on the Barbeau station website every 30 minutes.

Reference

  1. Delpierre, N. (2009) - Unravelling the determinism of interannual variations of carbon exchanges between European forests and the atmosphere: a process-based modeling approach. Read this publication on ResearchGate

Acknowledgments

Produced from materials originally authored by Daniel Berveiller, Research Engineer, CNRS, France.

This information has been sourced, reviewed and adapted from materials provided by OTT HydroMet.

For more information on this source, please visit OTT HydroMet.

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