Western Hemlock:

Today, Coed Y Brenin, tomorrow, the world!

Well, not quite…

Western Hemlock, tsuga heterophylla

I've mentioned western hemlock in a number of previous pieces about Coed Y Brenin but as its making such a big impact over the whole of Mynydd Penrhos and beyond and is likely to become even more omnipresent I thought I'd pull together all the various observations in one place plus more recent stuff.

Native to north-west America, western hemlock was introduced to Britain in 1852 by botanist John Jeffrey, and is now one of the most common conifers found in the UK. It is best suited to moister climates and although it starts quite slowly, the growth rate accelerates rapidly and it regenerates very freely in a wide range of environments, growing to 50–70 metres (165–230 feet) tall. The species is long-lived, especially at higher elevations, with trees over 1,200 years old known.

The closely spaced branches droop, producing layer upon layer of dark green foliage, pale on the underside. The density of the branches and flattish needles means it can intercept most of the available light and hence casts a very deep shade. During the peak growing season the new growth shows as a bright green fringe on the ends of all the foliage and the growing tip tends to flop over, a feature that makes it fairly easy to spot against other conifers.

above: young western hemlock. Note layer upon layer of flattish foliage reaching to the ground, darker green with brighter growing tips. Underneath, darkness!

The wood is quite course and the grain can twist a bit, making it difficult to work and it rots quickly if exposed to damp. However, if kept dry it can be used like most softwoods in indoor furniture and joinery. In commercial forestry it is usually used for pulp in the paper industry.

The species forms ectomycorrhizal associations with some edible fungi including chanterelles and we have also noticed it in close association with hedgehog fungi, hydnum repandum, (very tasty). We've found the hedgehogs growing along buried, fallen branches, something which we've been trying as a means of propagation, though no success as yet.

In the late 1990s we got a contract with the then Forestry Commission Wales to clear conifers from the sides of the various streams (the riparian zones) and some of the old bogs on Mynydd Penrhos. In most cases the streams banks were full of conifers, varying in height from seedlings to young trees, shading them completely and the water had taken on scary colours like bright orange, blue or yellow. Letting the light in provided the opportunity for native species to get going both in and around the streams and water quality has greatly improved over the past two decades. As we were in effect improving our own watershed, we did a really good job removing the conifers.

above: a strong contrast- some ten years after we used hand tools to clear western hemlock from the area on the left, it now shows predominantly native species. On the right and outside our contract, a planted stand of Corsican pine (invisible!) completely engulfed by self seeded western hemlock.

Just above our own holding, we encountered huge numbers of conifer seedlings between 5-10 cms tall. We pulled up literally thousands of them every ten metres of so. We identified a stand of big trees on one side of the stream as the source of most of these seedlings, as, you've probably guessed it, western hemlock.

Despite repeatedly informing the Forestry Commission of the irony and ultimate futility of weeding out seedlings without felling the trees that were responsible for the seedlings in the first place, the trees never got felled. I often heard former Foresters and their now rather ominously renamed Natural Resources Wales (NRW) replacements, “Operations Managers”, complaining about the prevalence of western hemlock regeneration throughout Mynydd Penrhos, as it is non-native (this despite the fact that 95% of the species in Coed Y Brenin are non-native!) yet that one, single, small sub-compartment of trees that are causing the problem is still there!

above: the culprits! 80 mature western hemlock, now 80 years old, responsible for upwards of four billion wind blown seeds during their lifetime and still counting…

Western hemlock produces small cones that open on the tree and the tiny, light seeds blow easily on the wind. A mature western hemlock can produce upwards of a million seeds per year. (I am afraid I have been unable to re-find the reference for this- can anyone help?). The stand in question is one of the few to have seen actual management over the decades and has been thinned at least once in the early 1990's (we witnessed the thinning), leaving just eighty fine specimens (I counted them). If we say that at a conservative estimate a western hemlock is mature at 30 years old, (Wikipedia says 25), then these eighty trees are producing say a million seeds each per year and have been for the last fifty years (at least).

That's 80 trees x 1,000,000 seeds each per year x 50 years = 4,000,000,000 seeds.

That's a conservative estimate as I am not counting the seeds produced by the trees that were thinned out aged 50 and also they would all have been producing some seed before they got to age 30. Whatever, its a lot of seed to blow in the wind and its no wonder that western hemlock is now the most numerous species on Mynydd Penrhos, probably.

above: fallen western hemlock cones. Along with the steady drop of needles, the debris from conifers that accumulates on the forest floor can be considerable.

Western hemlock will grow in 95% shade- that's more shade tolerant that rhododendron at 90% and so it will grow under pretty much any native vegetation, out compete it and shade it out. The needles are flat, more like thin leaves and grow on either sides of closely spaced twigs. The many branches layer one above the other, in more open spaces or on the sides of tracks, often reaching to the ground, creating an extremely dense shade; nothing grows beneath it.

As an example of its tenacity as a species, in the late 1980s, a mixed stand of already mature Norway and Sitka spruce to the east of us was given a further thinning, leaving well spaced, sizeable specimens with an understory of heather, bilberry, ferns and mosses and the occasional young birch, hazel and oak. Underneath all that, a carpet of western hemlock seedlings kicked off at just a few inches tall yet due to their density, creating a closed canopy at only a couple of years old.

Year after year. this low level closed canopy rose up, faster as the trees became more established, rapidly overtaking the bilberry and heather and shading them out. Not many years later it had overtaken everything else except the actual crop and now stands at about ten metres. Underneath, in perpetual shade nothing else grows. Initially impenetrable due to the closeness of the stems, it has more latterly become a strange space, filled with the debris of dead and dying western hemlock, shaded out by their own siblings in the ongoing rush for light, with the occasional much larger trunks of the mature spruces. How much water they are drinking is difficult to say but the Norway and Sitka spruce they're gradually catching up with do not seem to have grown much since they were thinned.

above: the thin stems of a western hemlock understory with the larger trunks of the actual crop, mature Sitka and Norway spruce. The hemlock forms a complete canopy at about ten metres and has shaded out everything below, including considerable numbers of itself.

This self-seeding of western hemlock has occurred throughout the Core Block (the oldest part) of Coed Y Brenin and beyond, perhaps not at such a ridiculous density but the darker, dense foliage is visible pretty much wherever you go. Its remarkable how just a few of these trees, looking quite innocuous in their early years, can reach up and spread out to completely dominate a sizeable area, shading out everything else.

In their native environment they typically grow under the canopy of other conifers such as Sitka spruce and Douglas fir, so the older plantations of these very same species here in Coed Y Brenin suit the western hemlock perfectly. In this situation they can persist for decades then exploit any gap in the canopy to eventually replace the firs or spruces which are far less shade tolerant. The western hemlock will then grow on to become climax forest, so that's where much of Coed Y Brenin is headed without some serious intervention.

With its small, wind blown seed it is also able to rapidly exploit larger areas following clear felling or wind-throw. Given that the current, largely unmanaged state of much of Coed Y Brenin has led to heavily overcrowded stands of over-tall conifers with very little root space, severe winter storms could well bring down considerable numbers of trees, opening up yet more opportunities for this tenacious species.

Intervention is easiest when the hemlocks are young and can be easily pulled up (minimum effort, maximum effect) but given the numbers, this takes time and is obviously going to be ineffective if the mature trees are not removed at the same time.. For larger trees, ring barking is effective, so long as the ring is made below any lateral shoots. Cutting should similarly be made below any trace of shoots. Unlike most conifers, when cut, western hemlock seems capable of regrowing if even a single green needle remains.

above: guerilla forestry…a ring barked¹ western hemlock, the previously shaded area giving way to some ferns and, uh oh, self seeded western hemlock. Follow up of any intervention involving clearance of a species, is essential is there is to be any hope of long term success. At this size the young hemlock are easy to pull up.

During our contract to clear young conifers we reported a dense stand of western hemlock of about five or six years old and too well rooted to pull up. Rather than spend considerable time cutting every single stem (numbered in the hundreds) with a bow saw or long handled loppers, we reported the stand to the Forestry Commission expecting someone to come in with a chainsaw. However, instead a gang appeared with brush cutters and simply cut the stems where it was easiest, leaving plenty of branches below the cuts. The result was an even denser thicket of trees with multiple leaders. Duh!

More generally, NRW are beginning to manage Coed Y Brenin again after, in some cases, half a century of neglect but they are already dealing with the consequences of phytophthora ramorum which means felling and extracting several million larch trees (dead or dying or killed deliberately in advance of the disease), a considerable operation in itself and hence have limited time and resources. So far they have used a harvester to make cuts through some of the unmanaged stands and do some limited thinning on either side of the cuts. However, the risk here is that the cuts allow the wind into the stands, increasing the livelihood of wind throw.

I've talked before here about the high risk of fire in these dense, unmanaged stands and although this year has, so far, proved relatively cold and wet, fire will no doubt loom inevitably at some point in the future. Western hemlock has a relatively thin bark and shallow roots which does mean that it does not readily survive a ground fire.

What may well end the increasing dominance of western hemlock is one of the other varieties of phytophthora, namely pluvialis, to which it is very susceptible. We've already had one outbreak, two years ago, on young western hemlock growing under Norway spruce just above us which was spotted and NRW were served with a notice to deal with it. This involved felling the infected trees and the neighbouring hemlocks and chipping them, although larger stems were left lying. How effective this action will be remains to be seen.

above: self seeded western hemlock, here dominating a planted stand of Sitka spruce- there are a few spruce just visible in this picture! Note the density of the hemlock and the floppy tops, making them fairly easy to spot against other conifers, if you can actually see the other conifers for all the hemlock…

So if neither fire nor plague take it out, then western hemlock remains a much greater challenge to both native and exotic species than even rhododendron, as its light seed can carry so much further and currently, no one seems to really notice it, as its “just another conifer”.

Thanks for reading. Next, a minor change in plan and a switch back to the patafiction, Konsk. Then we have net users' energy use- that's us folks, driving climate change! Is there anything we can do about it, apart from just stopping using the internet? Well, as it happens, there is. Till next time, hwyl! Chris.

1. Ring barking trees can be a useful technique in advance of felling. The tree will die after about eighteen months. Over a few years, all the twigs, smaller branches and bark will fall off and the wood will have begun to season. This means that when you eventually fell the tree for timber or firewood, as well as it being much drier and hence lighter, a lot of the biomass will remain on site.