The impact of diversifying understory vegetation in oil palm plantations on greenhouse gas emissions

Tropical oil palm (OP) plantations are major emitters of greenhouse gases (GHGs), but there are management options, which may reduce these emissions, including increasing understory biomass. Managing the vegetation within and around plantations could potentially minimise environmental damage and maximise co-benefits such as soil protection, pest control and diversity. Such practices include creating reserves, buffer strips and management of vegetation in the plantations themselves. The impact of these management practices is uncertain, and there is a real need for an evidence-base to guide improvements in the environmental sustainability of OP management.

The timing for research related to management options is critical for influencing current decision-making. In Indonesia, most OP plantations were established in the late 1980s and early 1990s and due to the 25 – 30-year life cycle of OP plantations, nearly half are due to be clear-cut for replanting in the near-future. Hence, it is vital to understand replanting and restoration options which simultaneously allow for high productivity as well as supporting biodiversity and minimising GHG emissions.

 

The scope and specific objectives of our study were:

• 1) To measure GHG emissions under different understory management techniques (with/without vegetation through use of herbicides).
• 2) To link GHG data to soil data to develop understanding of ecosystem function under different OP plantation management approaches.

 

We will present monthly static chamber measurements of GHG emissions for the duration of one year starting October 2018, established on an existing long-term experiment investigating the impact of diversifying understory vegetation on biodiversity, ecosystem functioning and yield in Sumatra, Indonesia (The Biodiversity and Ecosystem Function in Tropical Agriculture Project (BEFTA)). The three different understory management treatments were:

• 1) Normal biodiversity complexity: standard industry practice, intermediate level of herbicide use in harvest circles.
• 2) Reduced biodiversity complexity: spraying/removing all understory vegetation with herbicides.
• 3) Enhanced biodiversity complexity: reduced-input management with no herbicide application and limited understory cutting.

 

We measured the GHG fluxes of nitrous oxide (N₂O), methane (CH₄) and soil ecosystem respiration/carbon dioxide (CO₂) using static chambers and analysis by gas chromatography (GC-µECD/FID).

Preliminary results show little difference amongst the different understory treatments in terms of N₂O fluxes. Fluxes were generally low (0-0.1 µg m^-2 h^-1) with high variability. However, there is a trend towards slightly higher emissions during the wetter months (Oct-Dec 2018) of up to 0.2 µg m^-2 h^-1.

Methane (CH₄) fluxes were generally small and fluctuated around zero. During the wet months, (Oct to Dec 2018) small emission fluxes up to 3 µg m^-2 h^-1 were observed; whereas during the dry months uptake of methane, prevailed. No distinctive differences between the different treatments was observed.

Due to the age of the plantation and imminent replanting, none of the plots were being fertilised at the time of measurement – greater differences between vegetation treatments may be observed under fertilisation.

In conclusion, initial results showed that the presence or absence of understorey did not increase soil emissions of N₂O and CH₄. This suggests that the within-crop ecological benefits do not result in an increased GHG burden.