Between late 2015 and early 2016, the Congo Basin experienced the strongest drought of the past 30 years, due to a particularly strong El Niño Event. Predicted changes in climate will likely cause more of these ‘drought events’. Unfortunately, we do not know much about how these ‘drought events’ affect tropical rainforests. Do many trees die? Do large trees or small trees (or both) die? Do certain fruit trees (eaten by chimpanzees or gorillas) die more easily?

As part of a continental study lead by Professor Lewis at the University of Leeds in the UK, Dr Cuni-Sanchez and her team sampled 12 permanent forest plots located deep inside the Nouabale-Ndoki National Park. These plots of one hectare each have all trees tagged and painted (a small line), so they can be found after several years.

Tree diameter was recorded with a tape measure, tree height was measured with a laser, and tree species was noted thanks to the help of a botanist. Sampling tree diameter in the tropical rainforest is not easy, as several trees have a fluted stem (irregular stem) or have ‘buttresses’ or stilt roots (funny roots), and a ladder is needed in many cases. Professor Harris, from the Royal Botanic Garden Edinburgh, joined the team to help with the plant identification. A student from the University of Marien N’Gouabi in Brazzaville also joined the team.

After the long field campaign, the team will now finalise the botanical identification using leaves collected in the field, enter data, and make the calculations. Preliminary results from Gabon – which has wetter rainforests than Congo – shows that tree mortality increased during this last El Niño Event. Might it be the same in Congo?

The giant trees of Central Africa’s rainforests are key for the whole ecosystem functioning. They provide food and habitat to many mammals (elephants, gorillas, chimpanzees and birds), they provide medicinal plants, fruits and honey for local populations, they store and sequester carbon – contributing to reducing the amount of CO₂ in the atmosphere and therefore, climate change, and they even maintain a cooler and wetter microclimate. Increased tree mortality is bad news. ‘We need to understand how climate change is affecting the rainforests if we are to make appropriate management decisions’ says Dr Cuni-Sanchez. The Wildlife Conservation Society’s Congo Program collaborates with several international independent international scientists at field sites across the country, providing support to long-term studies of this nature.