The majesty of trees at Cambridge University Botanic Garden
The thing about nature is that it just keeps on going, even now when we are ourselves limited and constrained. One way that helps me get over this feeling of virus-induced frustration is to look more carefully at what the local wildlife offers.
Trees are much easier to watch than birds and other animals: for a start they don’t move much or fly away, but how often do we just pass them by without a second thought or glance? With this approach in mind, I started to inspect some local trees with more attention, and began to realise how much they differ, not just in size and shape, but in the detailed architecture and texture of their bark. Cambridge University Botanic Garden is a great place to appreciate the diversity and majesty of trees in winter.
The bark of the black poplar (Populus nigra) for example looks like a badly ploughed field, with its deep sub-parallel furrows, interrupted here and there by circular scars left by fallen branches. Why has it evolved such bark? Perhaps to aid run-off after heavy rain? Certainly, the grooves act as effective channels for the water, diverting it towards the ground.
As well as being one of the tallest and most massive of trees, the Sierra redwood (Sequoiadendron giganteum) has most unusual bark. I recall my father introducing me to one when I was young and encouraging me to punch the trunk, which to my surprise was not hard and woody like that of most trees, but soft and squashy. Sierra redwoods in public parks and gardens often have a ring of paler bark at punching and prodding height where many have tested this in similar fashion. Why though does this majestic, towering monster tree have such soft, fibrous bark? The answer seems to be that the deep outer bark helps protect the tree from severe damage during forest fires (regular in its native California), not least as the sap contains tannic acid, a fire retardant.
If Sierra redwood has some of the softest bark, there are other species whose trunks and bark are extremely hard. One such is the well-named Persian ironwood (Parrotia persica) which has rock-solid, tough trunk and branches, smooth to the touch and beautifully patterned like an abstract painting. Persian ironwood is native to northern Iran, and, like witch hazel, to which it is related, flowers in winter. The flowers are tight clusters of scarlet stamens that disperse pollen to the wind and these open on the bare twigs in winter and early spring.
The Chinese honey locust (Gleditsia sinensis) is native to eastern China. The hard, sharp thorns that grow clustered on the bark probably evolved to deter grazing by large mammals. These spines are hard and sharp and are used in traditional Chinese medicine to treat skin diseases. They have been shown to have antibacterial properties. The seeds are surrounded by a sweet sticky pulp that is also used in medicine.
Another Chinese tree with highly unusual bark is the paperbark maple (Acer griseum). The smooth, shiny bark is orange-red and peels away from the trunk in thin papery layers, creating a shabby appearance. It was introduced to the west in 1901 by the famous explorer and botanist Ernest Henry Wilson and is now widely grown as an ornamental.
Some of the most attractive trees just now are the birches (Betula) which stand out in leafless glory, their bark shining in the winter sun. The Himalayan silver birch (Betula utilis) is one of the most impressive, especially now, when the glorious silver bark is not obscured by foliage.
Certain trees and shrubs are flowering even in the cold of winter, especially those adapted to pollination by the wind, some of which produce attractive dangling catkins - tightly packed assemblages of tiny flowers. It is too cold for most insects to be out seeking nectar, so catkins make use of air movement to disperse their pollen.
Hazel (Corylus avellana) is a familiar native shrub that bears yellow catkins, prominent this month, while the male catkins of the silk tassel bush (Garrya elliptica) can be up to a foot in length. The latter grows wild along the dry coasts of California and southern Oregon.
- Martin Walters is a Cambridge-based writer and naturalist.
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