ANOTHER LIFE: A glimmer of silver in the willow-twigs, a precocious peep of folded leaves from the horse-chestnut's sticky-bud; both speak of the rise of sap in spring - but not in the literal sense that most of us imagine. The picture we inherited as children, of sap sinking down in autumn and welling up again in spring, leaves the trees empty, as it were, for the winter, writes Michael Viney.
"The whole world's sap is sunk," wrote John Donne. "The general balm th'hydroptic earth hath drunk." But that isn't how it works. The sap is still there in winter, held in its cylinder of "sapwood" cells, wrapped around the trunk beneath the bark. But in spring the static sap starts moving upwards again, carrying nutrients from the roots to the growing crown of the tree. What lifts this huge weight of liquid is still obscure; neither suction nor osmosis nor capillary attraction can explain it. The upward flow needs energy, supplied by sugar made in the leaves and carried downwards, in the sapwood's skin, as fuel for the process.
Some of the sweeter saps of the northern hemisphere are tapped each spring for human pleasure. Russians ferment 10 million gallons of birch sap for wine and beer; Americans boil down vastly more maple syrup for their waffles. The plant ecologist Anthony Huxley knew an Irish canon who tapped the local birches for his annual home-made vintage.
But it is safe to say that the leading sap-sippers in this island are the wild mammals of the woods, notably grey squirrels, bank voles and three sorts of deer. The squirrels will start stripping bark next month, to reach the sap of beech, sycamore and oak. The deer strip bark all year, digging in their lower teeth and tearing upwards to reach the soft, nutritious sapwood underneath. Sika and fallow deer strip bark to about the height of one metre; the red deer can reach another half-metre and even more. If the stripping reaches all the way round, "ring-barking" the tree, it can die.
This turns the deer into serious pests of forestry, made even worse by the browsing of leader shoots and side shoots in conifer plantations, distorting their growth and shape. Stags add even further injury, scoring bark deeply with their hard antlers as a territorial gesture.
The small and elusive Sika deer, and their hybrids crossed with reds, have been spreading out from the Wicklow area, where they were introduced in 1860. They have plenty of food and shelter in the forests, long lives, a high reproduction rate and freedom from predators and heavy hunting. In some parts, says a new COFORD research report, there are 25 animals per square kilometre - more than twice the density at which their damage can be tolerated by forest managers.
In Forest Mammals - Management and Control, Sean Rooney and Thomas Hayden of UCD's Mammal Research Group describe the ecology and damaging habits of the deer and squirrels (and feral goats, hares and rabbits) and propose specific strategies.
High fences against deer are costly to erect and maintain, and culling, it seems, remains the best rapid control. To prevent a local population from increasing, it is necessary to shoot up to a quarter of the adults every year, most of them mature females. Creating open glades and lawns within the forest, urge the authors, is not only good for local biodiversity but offers the deer places to graze and rest, and make sure targets for marksmen perched in trees.
Noisy people, apparently, often force deer into the depths of the forest during the day, where they feed on bark. The authors suggest that "limiting recreational activities, for instance hill walking, orienteering, rallying, scrambling and mountain biking, will reduce this threat." Hill-walkers may take umbrage at being lumped in with bikers and rally-drivers as raucous disturbers of the peace.
As for grey squirrels, they make a particular meal of sycamore sap (it is, after all, a kind of maple) and in a study of small broadleaved woodlands in Co Meath it was the tree most often ring-barked or girdled. Attempting to control the number of squirrels is "frustrating and often futile" where there are two or more per hectare, but trapping in spring is much better than the use of poison, such as warfarin, which can kill other wildlife, such as native red squirrels (not among habitual strippers) and pine martens.
Oddly, grey squirrels in their native US rarely, if ever, strip bark and are not regarded as a pest. In Ireland, it seems, it is notably the squirrels in overcrowded woods who acquire a taste for sweet sap while young, and leave too many trees in tatters.
Forest Mammals - Management and Control is available from COFORD at Agriculture Building, Belfield, Dublin 4