Resurvey of 182 subpopulations of 28 threatened endemic tree species on Reunion Island

Each site was resurveyed in 2023-2024 and the current numbers of adult and juvenile threatened endemic trees were recorded in a circular area of 2,500 m² (radius of 28 m). At each site, we estimated the temporal trend of adult and juvenile threatened endemic trees as the difference between the current and the historical numbers of individuals divided by the number of years since the historical census to account for their different dates.

A positive value indicates an increasing subpopulation size, zero indicates a stable subpopulation and a negative value indicates a decreasing subpopulation. A concentric quadrat of 100 m² was set up and the number of invasive alien plants in the upper vegetation layer, i.e., with a diameter at breast height (DBH) ≥ 1 cm, was counted. Five quadrats of 1 m² positioned at each corner of the 100 m² quadrat and in its centre were used to count the number of invasive alien plants with a DBH < 1 cm in the understory layer (including juveniles of woody invasive alien plants).

For clonal plants, we enumerated the number of ramets. The abundance of IAPs with a DBH < 1 cm in the five quadrats of 1 m² was extrapolated to 100 m². The taxonomical referential of the vascular flora of Reunion was used to check the biogeographical and invasive status of each taxon.

Three functional traits and the IUCN Red List category of extinction risk were assigned to each threatened endemic tree taxa. The three traits are those of the leaf–height–seed plant ecology strategy scheme: i) the mean SLA, i.e., the amount of light-capturing surface area that is deployed with a given investment of dry mass, which expresses the fundamental acquisition–conservation trade-off; ii) the maximum plant height, which reflects a trade-off for biophysical constraints in determining water fluxes within the plant and which is related to its competitive ability ; and iii) the mean seed mass, which reflects variation in dispersal capability and seedling survivorship.

These traits were retrieved from the recently established TREFL database (Adrien et al., 2025). Seed mass was lacking for two threatened endemic tree taxa in the current version of the database. Seed mass of Dombeya blattiolens (Malvaceae) was considered the same as that of the closely-related D. ficulnea (3.2 mg).

Seed mass of Pyrostria commersonii (Rubiaceae) was considered the same as that of P. bibracteata (39.2 mg), a congeneric taxon distributed across islands of the South-West Indian Ocean. SLA was missing for Hernandia mascarenensis. The annual rainfall and elevation of each site were also indicated.