On a cold, frosty morning in January the thermometer by the house was reading minus 3. As I walked up the track under the powerlines the mud was frozen solid. However, as I moved further up into the spruce plantation I suddenly noticed that the track here was not frozen and was still muddy – an indication of the sheltering effect of the trees, which had maintained a higher temperature and prevented the ground from freezing. This is one small example, but spending time in the woodland during different weather conditions has highlighted to me just how much trees can alter the local climate within a small area of land (its microclimate). This is an important consideration when thinking about my ideas for woodland management, and how the woodland will respond.
Sitka spruce is a tall tree with dense branching and dark green needles that are retained all year, so the trees have a large impact on their local microclimate. The way they have been planted here in Comar Wood (as in most other commercial spruce plantations), in closely spaced rows with no subsequent thinning, increases their effect on the microclimate. I have already written much about how significantly the spruce affects light levels in the woodland (see here and here) and its effect on microclimates is just as profound.
Being low on the valley side, our woodland is fairly sheltered from wind. We are most exposed to the south and sheltered from the north and west. Of course, it does get windy here in particularly stormy or windy weather. On one such day last autumn, I had planned to do some felling with helper Steve. The wind wasn’t too bad in the morning and we were going to be felling fairly small trees anyway, so we started work. Our little coupe was very sheltered by the surrounding trees. Although I could see trees around us moving in the wind, we could feel very little wind at all and the trees we were felling were not really being affected by the wind. At lunchtime we heard two very loud bangs, which is the sign that something has hit the powerlines. It was only when I went down to see if the power had gone off at the house that I realised just how windy it had become outside the sheltered spruce plantation. A tree had been blown down onto the powerlines further up the valley. There was a remarkable difference in wind once I got back to the shelter of the trees in the coupe we were felling.
The spruce provides shelter from wind and it retains warmth, keeping temperatures higher during cold weather. However, it intercepts sunlight and shades the ground, meaning conditions within the spruce plantation remain cool on warm, sunny days in spring and summer. Spruce trees also shelter the ground from rain and snow. The sheltered, shady conditions within the spruce plantation lead to more humid conditions than outside, giving rise to growth of mosses and ferns (where light levels allow) – plants that thrive in moist, humid conditions.
Other types of tree have different effects on the microclimate. Silver birch is the other main tree species in Comar Wood. Birch trees have much more open canopies than spruce and lose their leaves in winter. The main area of birch is the hectare compartment by the waterfall where the trees have regenerated naturally and are much less closely spaced than the spruce. They do provide some shelter and shading but to a lesser extent than the spruce. The birch wood feels much more open and airy than the spruce plantation. Although the birch leaves provide shade in summer, they let through enough sunlight to allow bracken growth, along with heather and blaeberry. The stands of bracken provide their own sheltered, shady, humid microclimate on the woodland floor where mosses, again, can be found.
The plants and animals that make up the woodland ecosystem have evolved to thrive in certain microclimatic conditions, many requiring the shelter and shade, or partial shade, that tree cover provides. Mosses that carpet the ground in the damp shade under a spruce canopy dry out and die if the tree cover is taken away. Woodland specialist plants, such as wood sorrel, are able to tolerate lower light levels, whereas in more open conditions they would be out-competed by other more light-demanding species.
Other plants require the shelter of a woodland, but slightly more light – bluebells being the classic example of a woodland plant that grows under the canopy of broadleaved trees, benefiting from the higher light levels in spring before the trees come into leaf. These sheltered habitats of dappled shade are also home to specialist invertebrates such as the speckled wood butterfly. Woodland glades and small clearings provide yet more light for plant growth, but the surrounding trees provide shelter from wind and frosts and keep humidity higher than on completely open ground. The south-facing side of such a clearing can provide a warm sun-trap, while the north-facing side is shady and cooler, providing conditions suitable for a range of plants and invertebrates. In Comar Wood the wayleave corridor for the powerline is effectively a long woodland clearing, with sheltered but sunny conditions that are favoured by many butterflies including the pearl-bordered fritillary (read more here).
When it comes to tree regeneration, young trees of different species are adapted to different conditions. Some species of tree, known as pioneer species (e.g. birch and pine), have evolved to colonise open ground and are therefore tolerant of exposed growing conditions, but require plenty of light. Other tree species have evolved to grow up within existing woodland openings and prefer more sheltered growing conditions, but still need plenty of light (e.g. oak, Douglas fir). Some species regenerate in the sheltered conditions beneath the woodland canopy and are able to grow in the lower light levels here (e.g. beech, hemlock, spruce – although spruce is also tolerant of exposed conditions).
I have come to think of my woodland management activities in terms of how microclimate and light levels are changed and how the woodland habitats change in response. I want to reduce the impact of the Sitka spruce plantation on this ancient woodland site and revive the rich biodiversity that has the potential to exist here. I feel it is important to retain woodland cover during the process (rather than clear-felling the spruce). This will help maintain the sheltered woodland microclimate for the specialist woodland plants and animals that live here, allowing them to persist until conditions (hopefully) improve for them and they can flourish. Thinning the spruce is one way of gradually removing it and starting to reduce its impact without completely removing the woodland cover, and I have already started thinning areas in the eastern part of the site. The edges of the new access tracks (see here) are other areas that can now benefit from more warmth and light from sunshine while remaining sheltered from wind and frost.
Many of the tree species that will replace the spruce require plenty of light to regenerate (e.g. oak, birch, Douglas fir) while also benefiting from the shelter that the woodland provides. Therefore, my management strategy, as well as thinning some areas, is to fell the spruce within small coupes of about 0.25 ha (50 m x 50 m or thereabouts). This should open the woodland up sufficiently to allow enough light for the new trees to regenerate, but with the surrounding trees still close enough to maintain a sheltered woodland microclimate. Comar Wood is on a south-east-facing hillside so the felled coupes will catch plenty of sunshine, providing the warm and fairly sheltered microclimate of a woodland clearing (until the new trees start to grow).
The coupes will be staggered across the woodland, with a couple being felled each year, creating a mosaic of different woodland microclimates that should help many different plants and animals to find a suitable home. As felled coupes start to regenerate with new trees, other coupes will be felled, creating new clearings. Some of the spruce cover will be retained for longer (thinned where possible) to maintain mature tree cover and the microclimates associated with it.
I felled one coupe (of poorly grown spruce) this winter as an initial trial to see how this approach might work (see here). I look forward to spending time over the coming months to understand how its microclimate has changed and how vegetation is going to respond here. Likewise, it will be very interesting to see how the microclimate within the newly thinned spruce stands compares with that of the adjacent, unthinned stands.
Microclimates add interest to the experience of being in the woodland. I enjoy the feeling of coolness as I walk into the shady spruce plantation on a warm summer’s day, the respite from wind and rain that being under the spruce canopy provides, or sitting in a sheltered, sunny clearing on a spring morning watching insects and listening to birds. Hopefully my management activities will add to this diversity of microclimates and help carve out niches for lots of different plants and animals.