| Allt y Gaer | |
| Aos: | Carmarthen & Dinefwr |
| Interest: | Biological |
| Coordinates: | 51.8701°N -4.0756°W |
| Displaymap: | Wales Carmarthenshire |
| Area: | 3.5ha |
| Notifydate: | 1980 |
cy|'''Allt y Gaer'''|hill of the fort|italic=no is a Site of Special Scientific Interest (SSSI) in Llangathen community, Carmarthenshire, Wales. It is a 3.5ha conifer woodland on the northern slopes of the Tywi Valley, which has SSSI protected designation because it holds the largest heronry in West Wales.[1]
The Heron population of the woods gradually expanded such that by 1980 there were routinely more than 40 pairs, representing some 7-10% of the heron population of West Wales.[1] Before the growth in population at Allt y Gaer, there had been a large heronry a little way upstream at Allt y wern, and in the 19th century they are thought to have used an area of Dinefwr Park National Nature Reserve still known as 'The Heronry'.[2] Why Herons abandon one site and move to another remains unknown, but may be connected to disturbance during tree-felling operations. The SSSI designation aims to encourage as little disturbance as possible, especially during the February–June breeding season, whilst maintaining an abundance of nesting sites and food supplies.[3]
The woodland is a monocultural stand of mature conifers - especially larch. Although the dense shade and extensive litter layer of needles and branches have completely suppressed the pre-plantation flora, the mature stand of trees has allowed a significant growth of various species of fungi; in particular the wood provides a habitat for Spotted Toughshank (Rhodocollybia maculata), Plums and Custard (Tricholomopsis rutilans) and Clouded Agaric (Clitocybe nebularis).[3]
Higher up the slopes, 350m (1,150feet) to the north, is an Iron Age hillfort called Grongaer which gives the name to the hillside, the woodland, and a nearby farm.