Red deer could be culled if a project finds there are too many roaming woodland in mid Wales.
Warnings have been issued about Scotland's soaring deer population
The Wildlife Trust of South and West Wales said the animals were killing ash trees by stripping off bark with their teeth.
The trust said its Teifi Marshes reserve and private land nearby in Cardigan were being affected.
It also accused the deer of eating new tree shoots which is putting the future of some woodland in jeopardy.
In Scotland, red deer numbers are rising so fast that other wildlife is at risk, a study warned in May.
Deer numbers have rocketed to 400,000 - to the delight of the tourist industry and shooting parties.
A meeting had been held between the wildlife trust, the Countryside Council for Wales and local landowners about the issue.
They have all been working with the Welsh Deer Initiative too
Dr Lizzie Wilberforce, wildlife officer for Ceredigion, said: "We have a reserve in the Teifi Marshes and the deer are doing a lot of damage upstream further up the river."
"We have had similar reports from landowners too, so we've set up a project to look a numbers of deer and their impact."
As well as stripping bark, known as ring barking, the deer were eating growing shoots, added Dr Wilberforce.
"We'll be monitoring the numbers of red deer. We need more information about numbers, although they quite elusive.
It is unclear how many Red deer are in the UK
"But we have two options; we could either stand back and do nothing or we could control the population which may include a cull."
Dr Wilberforce said there were reports of between 30 and "several hundred" red deer in the area. In Wales, The animals are also found in the Brecon Beacons and the Lleyn Peninsula.
Hugh Rose, technical officer for the British Deer Society, said no-one knew the true population level in the UK.
"Red deer graze, but also like to eat bark of smooth trees and small shoots too. Ideally, to keep the population steady, 30% of a herd should be culled each year."