Net CO2 Assimilation and Transpiration of Olive Trees: Effect of Weed Cover at the Leaf Scale

Abstract:

One of the most widespread conservation practices applied in olive-grove plantations is the maintenance of weeds in the alleys from autumn to spring to protect the soil against erosion. In this regard, one indirect benefit of this practice is increasing the ability of such managed ecosystems to sequester atmospheric carbon. The effect of this conservation practice has been characterized at the ecosystem level but has not yet been explored at the leaf scale. In this study, transpiration (T) and net assimilation of CO₂ (A_net) by olive leaves and evapotranspiration (ET) and net ecosystem CO₂ exchange (NEE) were quantified in two olive groves with different treatments: In the weed-free olive grove (WF), the cover is removed after the addition of glyphosate-based herbicide. In the weed-cover olive grove (WC) the spontaneous herbaceous plants are maintained and then mechanically mown and left on the ground. Leaf-scale data were taken using a portable infrared gas analyzer (Li-6800, Li-cor) monitoring atmospheric CO₂, relative humidity, photosynthetically active radiation, and air temperature. These data were taken once a month, from January 2018 to January 2019, and were compared with the ecosystem fluxes, which were obtained using two eddy covariance towers.

The results display significant differences in T only during the period after mowing, with T_WC= 2.0 ± 0.7 mmol H₂O m^-2 s ^-1 vs T_WF = 2.5 ± 1.1 mmol H₂O m^-2 s ^-1. In this period ET is also lower in the weed-cover treatment; ET_WC = 2.8 ± 1.0 mmol H₂O m^-2 s ^-1 vs ET_WF = 2.5 ± 0.6 mmol H₂O m^-2 s ^-1. The rest of the year, T of the olive leaves is similar in both treatments and ET of the WC plot is higher which can imply high rates of transpiration by weeds. On the other hand, there are significant differences for A_net only in the period before mowing with A_net-WC = 5.5 ± 3.1 μmol CO₂ m^-2 s ^-1 vs A_net-WF = 8.0 ± 3.6 μmol CO₂ m^-2 s ^-1. After the weeds are mown, A_net is matched in both treatments. However, higher net ecosystem CO₂ uptake in the WC treatment observed in the period before mowing. Although the leaves of the olive trees capture less CO₂ in this period, this shows that weed cover is responsible for the high carbon sequestration capacity of this conservation practice in olive groves.