[Reading level: C1 – Advanced]
Some shed their leaves; others rely on antifreeze in their veins (or pine needles) to survive the colder months. But trees couldn’t survive winter without the special relationship they have with birds.
Aside from the music of the westerlies playing through those canopies, British woods, when stripped of their leaves, tend to have an atmosphere of quietude. Winter is a time of frugality, and the two major tree types, deciduous and evergreen, adopt very different strategies to cope with the double challenge of little sunlight and low temperatures.
A deciduous tree operates by shedding all its leaves. This means that it lacks the means to photosynthesise and, thus, to generate fresh energy for five to six months of the year. However, there’s an upside to losing foliage, because it has ended the loss of moisture that occurs through leaves.
This endless recycling of water from the trees to the atmosphere, known as transpiration, is costly. A single large mature oak may possess up to 200,000 leaves, and lose as much as 150,000 litres of water over the course of a year. A summer wood in full foliage conceals thousands of rising columns of tree-borne moisture, which is why it often feels so warm and humid within woods, when the surrounding countryside can be cool and breezy.
Conifers in winter continue to photosynthesise as they retain their leaves, which are adapted to retain moisture. – Cây lá kim vào mùa đông tiếp tục quang hợp khi chúng giữ lại lá, thứ đã được điều chỉnh để giữ lại độ ẩm.
Conifer trees reduce the stress of water loss by possessing fine needle-like leaves with minimum surface area and thick, waxy water-retaining cuticles. The adaptations slow down the process of transpiration. While the structure of evergreen foliage doesn’t have the energy-producing efficiency of a deciduous leaf, the fact that conifers can continue to photosynthesise year-round compensates for this. Some pines even have antifreeze in their needles, to enable them to work in icy conditions.
The dormancy of winter trees means that this is the best time for planting or transferring them around our gardens. Choose a warm winter’s day so the earth is soft, dig a hole deep enough so the soil fully protects the roots from frost, then water well once planted. Observing how that new tree slowly grows and flourishes is one of the most satisfying processes in gardening.
Trees also make use of the winter to do some self-propagation of their own. Much of the work for this process begins in the autumn, with the production of seeds and fruit. British oaks are a perfect example. From September onwards they leave a great larder of sugar- and protein-rich food for hungry birds or mammals, in the form of acorns. Squirrels, rodents and several species of crow store these nuts away to consume during the colder months. Yet, no species has more of a capacity to plan ahead than the European jay.
A jay in the snow: the bird can store about 5,000 acorns over the course of autumn. – Một con chim giẻ cùi trong tuyết: nó có thể dự trữ khoảng 5.000 hạt sồi trong suốt mùa thu.
In late autumn, this glorious pink-and-blue bird can be seen making repeat journeys to and from acorn-producing oak trees. The jay then buries them, sometimes flying several miles from the original source tree. Over the course of autumn, the average bird hides about 5,000 acorns. Across Britain and Ireland, the entire jay population stores somewhere in the region of 1.7bn acorns.
The birds possess extraordinary memories for where they have hidden their stores, and yet – here is the clever part – they can’t remember them all. Not only do those forgotten acorns stand a strong chance of growing into oak saplings the following spring, but jays show a preference for open ground between tree cover in their choice of burial sites. In other words, they plant their acorns precisely where they might grow best.
This acorn storage system thus benefits the jay twice over. Not only does it provide itself with food for the winter, but by forgetting some of its larder it also creates fresh habitat for future generations of jays. These birds are truly the guardians of our oak woods. But it works both ways: by taking care of the birds, the trees ensure they prosper themselves.
The dispersal of seed is, in fact, a major part of the woodland scene in winter. Hawthorn, holly, yew, mountain ash, crab apple, blackthorn and sea buckthorn – not to forget woodland shrubs, such as mistletoe and ivy, or garden favourites including cotoneaster and pyracantha – all produce fruit that birds love.
Thrushes are especially drawn to these trees and consume vast quantities of their fruits. In return for this generosity, the thrushes then fly to their roosts in some hedge or tall tree at dusk and do what comes naturally. They poop out seeds during the night and leave each one in its own little store of fertiliser.
So woods and birds are truly partners in reproduction. You can see how they interact on any winter’s day and in any wood, and from the mountain tops to the seashore.
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