Monday 22 July 2019

In the Garden: 'Deciduous trees' survival strategy'

Why do some trees shed their leaves and others stay green all year? Gerry Daly has the answer

The branches of bare trees in winter
The branches of bare trees in winter

According to some people, spring arrived, with early daffodils and snowdrops, on February 1. Although others stick with a March start date. Whether it is earliest spring or late winter on the calendar, one thing is for certain, the foliage of trees and shrubs is at its annual low point during these few weeks.

Some trees, such as birch and alder, still show a few leaves at the end of the year, and it won't be long before new leaves begin to appear once more. The bareness of the garden, and countryside, is heightened by the brighter sunlight now, more than six weeks past the winter solstice.

The bare trees are deciduous, having lost their foliage in autumn. Compared to an evergreen tree species such as holly or laurel, deciduous trees look miserable, seeming dead, or at best 'sleeping' or dormant. Of course, there are compensations - the beauty of tree bark, for example, its colour and texture, fissuring and patterning, especially birch. The colour of bark is sometimes to the fore and maples and willows are very good in twig. But these delights do not compensate for the great bulk of foliage that falls from deciduous trees.

It is a given that nature doesn't waste energy, so why then are some trees deciduous, losing their leaves, while others hang onto theirs? There must be a significant advantage to be gained.

The deciduous strategy is aimed at surviving frost damage. Broad leaves trap more sunlight and are efficient but they can be desiccated by frost. When a tree attempts to supply the drying leaves with moisture, it may not have enough water and be damaged, just as a big eucalyptus can be killed by frost. Birch, willow and alder are tough deciduous trees.

Any garden shrub or tree with broad, evergreen leaves, is likely to have originally come from a warmer, relatively frost-free climate: eucalyptus, laurel, bat laurel, oleraria, escallonia, griselinia and citrus. Only holly, ivy, yew, juniper, furze and pine are native evergreens, and even ivy and furze can get frost damage.

All of the non-native broadleaved evergreens are susceptible to frost damage to a greater or lesser degree. For some plants, such as daphne, evergreen forms of the same species exist at lower altitudes and give way to deciduous forms at higher elevation.

Other trees have evolved narrow needle-like leaves as a strategy to resist cold damage. The leaves reduce moisture loss by being rolled and waxy, so the demand for water is reduced. Most are conifers such as pine and spruce - both very cold-resistant - but there are deciduous conifers too, such as larch and swamp cypress.

However, conifers are mostly restricted to poor quality land, the more efficient flowering trees, which evolved more recently, having out-competed them over millions of years.

It is not as though individual trees can select the strategy they want to use, more that particular kinds adapted to the local climate where they grew naturally. A few species, such as forms of oak, for instance, can react to their local seasonal climate by staying largely evergreen in winter or losing all their leaves if the weather is cold and dry.

When deciduous leaves are lost in autumn that is not the end of them. The nutrients they contain are released back via their fine surface roots. The seasonal dump of leaves is also used as a vegetation suppressor, killing ground-layer plants, especially grasses, by depriving them of light. Of course, ground-layer specialist plants such as ivy and ferns simply push their way through, and then use the nutrients from the leaves. The top-soil is improved by the organic material and a fertile layer develops.

Evergreens may not lose leaves all at once like deciduous trees, but they are continually shedding leaves, principally in early summer when their new shoots and leaves are produced.

In tropical areas, most trees are broad-leaved evergreen and produce new shoots and leaves. In dry season areas, trees such as jacaranda and chorisia are deciduous to avoid drought caused by heat, not cold, though they can't withstand frost either.


WILD IDEAS: If you fancy yourself as something of a garden designer or eco warrior, make your way to the Garden and Landscape Designers' Association (GLDA) seminar entitled 'Gardening on the Edge: Rewilding Green Spaces'. Held in conjunction with Bloom by Bord Bia, the seminar runs all day at the Crowne Plaza Conference Centre, Santry, Dublin 9 on Saturday, February 23. More details;

WINTER WARMER: A striking yellow-gold margin surrounding a green centre to the leaf marks out Elaeagnus 'Gilt Edge' as a special plant that can brighten a shrub border now when needed most, though it is eye-catching anytime. For best effect, position it to catch the winter sun. Elaeagnus grows on most soils, although it dislikes heavy clay soil.

NEW SEASON SEEDS: Time to browse the seed catalogues - three new beauties available from Jelitto of Germany for 2019 caught our eye: Aquilegia flavescens, which has an extraordinary pinkish touch on the otherwise golden yellow petals of this columbine; Delphinium elatum 'Pink Princess', a low growing, sturdy larkspur, with exceptionally dense, pale pink flowers and Salvia 'Shangri-La', pictured, stiff and strong with whitish hairy foliage;

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