Â鶹Éç

Summers are slightly drier and winters are wetter, and perhaps very significantly the number of days with frost has decreased by about two thirds for many areas. However the number of frosts may be less important than is their timing.

It is not inconceivable that some of our warmth loving plants could suffer badly if warmer winters and springs lead to early growth that is then killed by the very occasional late spring frost.

The Arctic-alpine species, such as the Snowdon lily, may be less affected than the Mediterranean species if this is the case.

There may be large shifts in species abundance due to changing interactions with grazing animals. A lack of snow may no longer force animals such as hares into woods where they eat the bark off holly trees. Holly is now spreading alarmingly in many woods.

Ivy flowers in autumn and produces its seeds in winter. Until recently seed production was frequently kept in-check in mid Wales by frost. Now ivy fruits abundantly and is spreading in woods everywhere. As with the holly, a lack of winter grazing pressure has helped it.

Gorse, another Mediterranean species, is spreading. In very hard winters it is killed by the cold, and in average winters rabbits were forced to eat it when their preferred grass was snow covered. Not so now.

Many lichen species that were once rare in Wales, being more typical of the Mediterranean, are now rapidly expanding in south and west Wales. They may be frost sensitive and the lack of extreme winter cold in recent years may have allowed those that live on tree branches and twigs to spread onto newly created bark.

This is a good example of a rapid response into what is a vacant niche. There are no existing species that have to be shouldered out of the way on young bark.

Elsewhere, although the climate may no longer be ideal for some species, they are able to survive for now. Only when they die can other species get a hold.

Trevor Dines, of Plantlife Wales, added, "The response of alien invasive species on native plants cannot be underestimated, but we have even less idea about how they will actually respond.

"We've 'tolerated' the invasion of New Zealand Willowherb into our sensitive mountain sites largely because it seemed benign - growing beside threatened rarities without competing with them.

Recently however, we've seen a change in its behaviour and it's becoming much more aggressive - growing larger and for a longer season than before. So it may well now pose a threat to some of our rarest species."