Wind pollinated trees and hay fever

 As winter draws to a close I always look forward to the first hazel catkins, that signal a change in the seasons ......... but then, when spring finally does arrive, I remember that I suffer from hay fever. The early-flowering trees are wind-pollinated, producing vast clouds of sneeze-inducing pollen. 

Wind pollination is a chancy business and once the pollen is released it's rapidly diluted in the air, so the chances of an individual pollen grain landing on a female flower stigma, leading to the formation of a seed, decreases exponentially with the distance between then. So the only way to improve the odds is for trees to release great quantities of pollen

Male catkins of hazel.

Tiny, female stigmas of hazel

Male flowers of ash, about to open

Ash flowers, fully expanded

Silver birch male catkins, a major contributor to hay fever

Silver bird male catkins

Elm flowers. Before Dutch elm disease arrived, the bare crowns of mature elm trees were covered in a purple haze of flowers, a magnificent sight, now just a memory.

Most wind-pollinated trees produce their flowers on bare twigs, unencumbered by foliage that might hinder pollen release, but beech is an exception. It may be that beech bud burst is delayed because its foliage is very sensitive to late frosts. 

Evergreen coniferous trees like Scots pine tend to produce their male, pollen producing cones on the tips of their branches, well clear of the foliage, where pollen is easily carried away by the wind.

Yew

This is the time of year when yew 'berries' begin to become conspicuous. Yew is one of only three British native species belonging to that division of the plant kingdom known as the gymnosperms - seed-producing plants that don't have flowers and don't enclose their seeds in an ovary to form a fruit. The other two are juniper (whose seed has a succulent outer coating - seed picture at the bottom of this post) and Scots pine (which carries its seeds in woody cones - see picture below). 

In this picture you can seed the fully formed yew seed on the right, half covered by a pale green cup. This is the tissue that quickly expands to form the scarlet succulent aril, that you can see half-formed on the seed on the left. This squashy cup is attractive to birds that disperse the seeds, effectively judging from the numerous seedlings that appear in places where birds perch, such as crevices in old stone walls after the seed passes unharmed through the bird's gut. The seed itself is deadly poisonous to mammals but I've watched nuthatches wedge it in a bark crevice, hammer it open and eat it.

Here's a fully formed aril, which almost completely encloses the seed, like a scarlet doughnut.

Juniper 'berries'

Scots pine cone

A Tree-spotter's Guide to Conifer Cones: 1

We are in the Christmas tree season - a Baltic state and German tradition introduced into Britain by George III's queen and popularised by Queen Victoria's consort Prince Albert and  by Charles Dickens - so what better time to take a look at conifer cones?

Botanically, conifer cones are spiral whorls of scales arranged on a common axis, with each scale bearing a pair of ovules that develop into winged seeds after pollination and maturation, when the cone becomes woody, opens and releases them.

This is Douglas fir Pseudotsuga menziesii, easily recognisable by the papery bracts, like three-pronged tongues, above each scale. Its common name commemorates David Douglas, the Scottish plant collector who in 1826 introduced this tree into cultivation. Douglas led a colourful life before coming to an unfortunate end when he was gored to death by a bull in Hawaii, at the tragically young age of 35.. The Latin specific name commemorates Archibald Menzies, another great Scottish plant collector who discovered the tree in 1792. Douglas fir is a giant tree in its native west coast of North America, where some trees may have been even taller than giant redwoods, reaching almost 400ft., before the finest trees were felled for their exceptionally fine timber. Trees introduced into Britain grew very rapidly, with some now exceeding 200ft., making them amongst the tallest trees in the British Isles. The cone is about 7cm. long.

Bearing in mind the size of a giant redwood Sequoiadendron giganteum, the tallest of all trees, its cone (above) is disappointingly small - only about 4cm. long. These trees have thick, fibrous bark that protects the living tissues below the bark from periodic forest fires, whose rising heat opens the cones and allows the winged seeds to spin down into the mineral-rich ash that they leave in their wake. The cones take two years to ripen and can stay on the tree for 20 years in their native California, though they fall much more quickly here.

We only have three native conifers here in the UK and of these only Scots pine Pinus sylvestris carries woody cones. Like many cones, those of Scots pine open when they dry, due to tensions that arise in the shrinking lignified fibres in their woody bracts, allowing the winged seeds to shake out into the breeze. The pictures above and below are of the same cone, wet and dry - a poor-man's weather glass if you keep one outside on a window ledge. This cone is 4cm. long but cone size can be quite variable in this species.

For more posts on this blog on tree ID - buds, bark, flowers or fruits - click here.

A Tree-Spotter's Guide to Bark: Part 1

When you think about it, the bark of a tree is pretty remarkable. It's the tree's self-healing protective layer, defending the thin layer of living phloem cells inside, just below the surface, that conduct sugars up and down the stem and also protecting the living layer of cambial cells that produces new growth every year. It's waterproof, but it lets gases pass in and out and it's capable of expanding to keep pace with the growth of the tree, splitting and cracking as the tree ages in a pattern that's characteristic of each tree species. On one January day twenty-seven years ago the bark of this beech Fagus sylvatica was the canvas on which SP and CS carved a declaration of their undying affection; thankfully the wound healing properties of the bark meant that the tree still thrives, even though the union of SP and CS may or may not have endured.

The bark is a tree species' fingerprint. This is Scots pine Pinus sylvestris, with its irregular plates of warm red-brown bark that perfectly complement the deep, glossy green of its needles - especially when winter sunlight strikes the trunk.

The barks of pedunculate oak Quercus robur and of Durmast oak Q. petraea can't really be distinguished, so this could be either (although I happen to know that it was the latter because the acorns had no stalks). I think it may generally be the case that slow-growing trees like oak develop a rugged bark of ridges and fissures that are slowly added to with age, while fast growing species like sycamore and birch tend to develop more rugged splits or shed bark more readily. 

Sycamore Acer pseudoplatanus bark is smooth when the tree is young but soon develops into flattened, irregular plates that are slowly shed as the tree ages, flaking away to reveal a fresh layer below.

For more posts on tree ID click here