Air mixing and sub-canopy advection in an oil palm plantation

In tall vegetation canopies, such as forest or oil palm monoculture plantations, the below-canopy airflow can be influenced by the local topography and thereby cause horizontal exchange of the below-canopy air. E.g. local wind might flow down- or upslope due to pressure gradients created in clear and calm weather situations. Especially during nighttime, calm weather conditions may result in the formation of an isolated layer near the surface, which is decoupled from the above-canopy air layer. Consequently, in this isolated below-canopy layer the air might flow in horizontal directions (“drainage flow") and is not mixed with the above-canopy air (Jocher et al., 2017). When decoupling and below-canopy horizontal air flow occurs, there is a high potential that above-canopy measured carbon dioxide (CO2) fluxes based on eddy covariance measurements might not represent the true ecosystem CO2 flux as below-canopy respiration might be undetected by the eddy covariance system (Alekseychik et al., 2013; Thomas et al., 2013). Nevertheless, eddy covariance data are frequently used as the reference for fluxes of tall vegetation ecosystems or for validation of modelling approaches estimating gross primary production (GPP) and net ecosystem exchange (NEE). It is therefore important to have accurate information on air mixing, decoupling and sub-canopy drainage flow to understand the complex CO2 exchange behaviour in tall vegetation ecosystems.
In this context we propose a master thesis in which the student will investigate wind and micrometeorological dynamics of an oil palm monoculture plantation (tropical lowland, Jambi Province, Sumatra, Indonesia). (climate tower at PTPN VI)
• The student will use existing data from above- and below-canopy eddy covariance and micrometeorological measurements within the oil palm plantation to study the characteristics of horizontal drainage flow.
• The student will test different approaches and scales to detect decoupling, such as friction velocity (u*) threshold, correlation between the standard deviation of vertical wind velocity (σw) below vs. above the canopy, or wind directional shear.
• The student will quantify the frequency of decoupling events and explore the potential implications of decoupling and horizontal below-canopy flow on the above-canopy derived NEE.
• The student will have the opportunity to write/contribute to a scientific publication.
The ideal candidate will have a strong interest and motivation in ecosystem processes, both physical and ecological, as well as in atmospheric measurement techniques. Interest/knowledge of programming skills (e.g. R) is an advantage.

Alekseychik, P., Mammarella, I., Launiainen, S., Rannik, Ü., and Vesala, T. (2013): Evolution of the nocturnal decoupled layer in a pine forest canopy, Agricultural and Forest meteorology, 174-175, 15-27, doi: 10.1016/j.agrformet.2013.01.011.
Jocher, G., Löfvenius, M.O., De Simon, G., Hörnlund, T., Linder, S., Lundmark, T., Marshall, J., Nilsson, M.B., Näsholm, T., Tarvainen, L., Ödquist, M., and Peichl, M. (2017): Apparent winter CO2 uptake by a boreal forest due to decoupling, Agricultural and Forest Meteorology, 232, 23-34, doi: 10.1016/j.agrformet.2016.08.002.
Thomas, C.K., Martin, J.G., Law, B.E., and Davis, K. (2013): Toward biologically meaningful net carbon exchange estimates for tall, dense canopies: Multi-level eddy covariance observations and canopy coupling regimes in a mature Douglas-fir forest in Oregon, Agricultural and Forest Meteorology 173, 14-27, doi: 10.1016/j.agrformet.2013.01.001.

Christian Stiegler, Ashehad Ali and Alexander Knohl