Leaf traits of understory woody species in the Congo Basin forests changed over a 60-year period

Background and aims – While tropical forests play an important role in carbon sequestration, they are assumed to
be sensitive to rising temperatures and prolonged drought. Plant functional traits are useful for understanding and
predicting the effects of such changes in plant communities. Here, we analyse the variation of leaf traits of understory
woody species of the Congo Basin rainforests over a 60-year period using herbaria as tools and we verify if this variation
is potentially related to recent climate change.
Material and methods – Leaves of five shrub species were collected in 2019–2022 in Congolese old-growth forests
(Yangambi Biosphere Reserve, DR Congo) from different positions on the shrub. These leaves were compared with
herbarium specimens collected in the same area before 1960. For both periods, we assessed leaf size, specific leaf area,
stomatal size, and stomatal density for all species.
Key results – The variability of the functional traits of the understory woody species are independent of the position of
the leaves in the crown. This allows for the use of historic herbarium collections for trait analyses on tropical understory
shrubs. The traits of the recently collected leaves were notably different from the traits of herbarium leaves collected in
pre-1960: recent leaves were significantly larger, had a higher Specific Leaf Area, a smaller stomata pore length, and, apart
from Coffea canephora, showed a lower stomatal density.
Conclusion – The difference in traits over time is probably related to the increase in temperature and to atmospheric CO2
concentration, as the average temperature at Yangambi over the past 60 years has shown an upward trend consistent with
global increasing CO2 levels, while the average annual rainfall has remained unchanged. Our results provide a first insight
into the response of forest species to climate change in the Congo Basin forests, and on how the understory species and
the ecosystem will react in the long term, when the temperature further increases.