Site of Special Scientific Interest (SSSI)
Approximately one third of the botanic garden woodland is a Site of Special Scientific Interest (SSSI), of which just over one hectare is regarded as Ancient Woodland comprising mature, 30 metre high oak (Quercus petraea) and ash (Fraxinus excelsior) forming a distinct square block bounded by well-defined banks and ditches several centuries old. This area of woodland appears on the first edition (1837) of the Ordnance Survey maps of the area and contains a moderately rich woodland flora including early purple orchid (Orchis mascula), wild spindle (Euonymus europaeus), primrose (Primula vulgaris), sweet woodruff (Galium odoratum), dog’s mercury (Mercurialis perennis), large expanses of wood anemone(Anemone vulgaris), bluebell (Hyacinthoides non –scripta), great wood-rush (Luzula sylvatica) and soft shield fern (Polystichum setiferum.) Happily this woodland is also free of invasive alien species. In the past hazel (Corylus avellana) has been coppiced here but there has been no management for at least 50 years. The north-west section of the ancient woodland is dominated by a shrub layer of holly (Ilex aquifolium). A well-defined path running parallel with the Strait forms its northern boundary - despite this and the lack of fencing there is little human impact and the only grazing is by natural small-scale herbivores. From a conservation point of view, this ancient woodland is without doubt the most natural and valuable woodland feature on the Treborth Botanic Garden site.
The remainder of the SSSI is situated in close proximity to the Menai Strait on steeply sloping ground with a generally northerly aspect. It is for the most part densely wooded, save where natural erosion and slumping of the seaward bank has caused mature trees to fall and create temporary gaps. These themselves are interesting features, allowing additional flora and fauna to succeed as well as revealing plant fossils of Carboniferous Age (325 mya).The canopy trees are diverse and include both native species, of which oak is most common, and introduced species predominated by beech (Fagus sylvatica) and turkey oak (Quercus cerris) with an age of approximately 120 years. Additional native species of interest include whitebeam (Sorbus sp.) several mature elm (Ulmus glabra) and further spindle. Notable planted trees include mature lime (Tilia x.europaea) and scots pine (Pinus sylvestris). Towards high tide level there are colonies of carnivorous butterwort (Pinguicula vulgaris) in less shaded spots while ferns abound throughout, especially soft shield fern. Bryophytes are abundant and include Hookeria lucens which is indicative of the shelter, shade and humidity associated with low lying forest areas in the west. At the far eastern end of the site there is a spectacular natural waterfall feature created by a huge multilayered slab of calcareous sandstone. Here flourishes the mat forming, sweet smelling liverwort, Conocephalum conicum encouraged by the super-humid conditions and shade surrounding the waterfall. Sadly much of this Strait –side woodland has been invaded by Rhododendron (Rhododendron ponticum) and Cherry Laurel (Prunus laurocerasus) now both growing to a height of 6-8 m. The steep nature of the woodland bank creates severe practical difficulties for their removal.