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

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. Especially during night time, calm weather conditions may result in the formation of an isolated layer near the surface, which is decoupled from the above-canopy air layer. 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. 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 investigate wind and micrometeorological dynamics of a mature oil palm monoculture plantation (tropical lowland, Jambi Province, Sumatra, Indonesia). We use data from above- and below-canopy eddy covariance and micrometeorological measurements within the oil palm plantation to assess the wind dynamics and the strength of the turbulent mixing as an estimator for the degree of coupling. Further, we explore the potential implications of decoupling and horizontal below-canopy flow on the above-canopy derived NEE.

Preliminary results show that wind is generally weak in the oil palm plantation. Using a breakpoint analysis, the correlation of below- and above-canopy standard deviation of vertical wind speed (σ_w) derived from sonic eddy covariance measurements below (2.4 m height) and above the canopy (22 m height), we identified a site-specific σ_w threshold of 0.11 m s^-1 (below-canopy) and 0.26 m s^-1 (above-canopy) above which the atmospheric conditions are in fully coupled state. During the day, unstable conditions dominate over stable conditions while in the twilight hours and during the night, the reverse is the case. Below-canopy wind comes mostly from south-eastern directions during both day and night, and tends to blow independently from wind above the canopy for conditions with above-canopy u* < 0.3 m s^-1. Based on the above-canopy eddy covariance NEE measurements and on the direction difference (ΔWD) between above- and below-canopy wind, we observe a threshold of ~70° ΔWD above which the two layers might be decoupled. Below-canopy air flow might therefore influence the above-canopy NEE detections, biasing carbon balance estimates.