Warm subtropical rainforest, dominated by booyong (Argyrodendron spp.) is commmon on warm, protected, fertile sites of basalt derived soils in areas of high rainfall below 800 m elevation. Subtropical rainforest is rich in non-vascular plants, palms, trees with trunk buttressing, woody vines and large epiphytes.
Endangered flora found in these forests include the redfruited ebony (Diospyros mabacea) which is confined to the Tweed Valley, southern ochrosia (Ochrosia moorei), southern fontainea (Fontainea australis) and green-leaved rose walnut (Endiandra muelleri ssp bracteata). Vulnerable flora species include red lily pilly (Syzygium hodgkinsoniae), ball nut (Floydia praealta), red bopple nut (Hichsbeachia pinnatifolia), rough-leaved Queensland nut (Macadamia tetraphylla), heart-leaved bosistoa (Bosistoa selwynii) and the herb Isoglossa eranthemoides found at lower altitudes.
A project to establish an arboretum for rare and threatened rainforest species of the Tweed Caldera commenced at a previously degraded site near the Korrumbyn Creek picnic area in 1988. Approximately 40 rare and threatened species are represented in several planted clumps amongst rainforest pioneer and early secondary stage species totalling approximately 2000 plantings. The site now supports significant portions of the gene pools of many endangered rainforest species of the region. Regular weed control works are required during this establishment phase of the arboretum plantings.