The abundance of rare tree species in remnant forests across the Visayas, Philippines

In several tropical regions of the world, formerly widespread forests have been reduced to scattered remnants, and many tree species are becoming increasingly endangered. Knowledge on the population status of rare species is essential to inform conservation efforts and, in particular, the use of native species for reforestation projects stemming from emerging land-use strategies. We studied 20 mostly red-listed native species in remnant forests on five limestone-, four volcanic- and one ultramafic site(s) across the Visayas in the Philippines, with 40 plots being assessed at each site. Seventeen of the species showed a median density of less than two trees per hectare, with ten species showing median densities of zero due to low species frequency, although some had higher local densities. One species (Dracontomelon edule) was not encountered at all, while two other species (Dipterocarpus validus and Dracontomelon dao) were only encountered as singletons. Six species were confined to limestone sites and showed associations with other better-known limestone specialists. The other study species occurred at both site types. Four of the species showed significant relationships between stem density, soil pH and stand basal area. The particular rarity of the majority of the species calls for immediate conservation measures to be adopted to protect species and associated remnant forests. In addition to legal measures such as the designation of remnant forests as protected areas, we recommend the establishment of mixed native forest stands giving due regard to the species associations and site characteristics identified in this study.