Hummocks are one of the most obvious and distinctive features at ground level in the Caledonian Forest. These raised, rounded mats of vegetation develop on boulders or old tree stumps over extended periods of time. Eventually, they completely cover their substrate, and thereby create a characteristically undulating micro-topography within the forest. Where they occur in well-developed, dense concentrations, they can act as an indicator of ancient woodland, which has had a continuous cover of trees.
The development of hummocks takes place gradually, and typically begins with an exposed boulder or tree stump being colonised by lichens. Lichens are amongst the slowest-growing of all organisms, and some of the crustose species (ie those lichens which grow flat on, and affixed to, the surface of rocks) grow radially at just 0.1 mm. per year. To obtain the nutrients they need, these lichens secrete an acid which breaks down the surface of the rock. This creates a thin layer of broken mineral material, which is the first step in the formation of soil.
Foliose lichens (lichens with leaf-like structures) may also grow at this stage, and assist in the process of breaking down the outer mineral layers of the rock. Because they grow with a more three dimensional structure, foliose lichens trap small particles of organic matter, which are blown by the wind or fall from the branches of trees. As this material breaks down, it provides an expanded source of nutrients which enables mosses to grow. Once mosses become established, the process of colonisation of the rock by vegetation accelerates.
Mosses grow more quickly than lichens, and they can trap more organic material in their feathery growth. They also produce a larger amount of decomposing matter, and this in turn helps with the formation and accumulation of more soil. Over time, this provides enough nutrients for higher plants to become established, and under the forest canopy blaeberry (Vaccinium myrtillus) and cowberry (Vaccinium vitis-idaea) will typically predominate on a well-developed hummock. Other plants which can colonise a hummock include bracken (Pteridium aquilinum) and heather (Calluna vulgaris), and if a hummock develops in a more open setting, the shade-tolerant blaeberry and cowberry give way to heather, which thrives best in bright conditions.
Young trees, such as Scots pine (Pinus sylvestris) and rowan (Sorbus aucuparia), will also grow on hummocks, especially those which form over tree stumps, and the well-drained soil conditions of the hummock can give pines in particular a good site to grow on. The 'champion pine' just inside the fence of the Coille Ruigh exclosure in Glen Affric, whose growth we have been documenting photographically ever since the fence was put up in 1990, is a good example of a pine which has become established on a hummock.
The key factor for the successful growth of trees in such situations is the ability of their roots to access enough nutrients, which means getting into the soil itself, rather than just the organic matter which has accumulated on the hummock. Because tree roots have easier access to the soil where the hummock forms over a stump, this explains why trees are less commonly seen on hummocks which form over boulders or rocks. It is only when the living mat of vegetation on a hummock becomes continuous with the vegetation growing on the ground that hummocks forming over rocks can provide a viable opportunity for trees to send their roots into the soil, and therefore grow beyond seedling size.
Well-developed hummocks provide considerable topographic variation at ground level, and this creates a variety of micro-habitats for insects and other invertebrates, as well as their predators such as spiders. Wood ants (Formica aquilonia and Formica lugubris) will sometimes build their nests on the south-facing sides of hummocks, while abandoned nests can also gradually become covered with vegetation and thereby form hummocks themselves.
The entire process of hummock formation takes place over an extended period of time, stretching to decades at least, and possibly much longer. This is particularly true where a hummock develops over a boulder or exposed rock. However, where a hummock forms over a tree stump, the process is more rapid, because of the organic nature of the stump and the resultant easier access to nutrients which the wood provides.
Hummocks which form over rocks repeat, on a small scale, the colonisation process of vegetation after major disturbance, such as the retreat of the glaciers at the end of the Ice Age. It was lichens which first colonised the newly exposed bare rock, followed by mosses and then vascular plants, and finally by trees, and this is being continuously repeated, in miniature, with every hummock which develops over a boulder or rock.
In some parts of the Caledonian Forest, numerous hummocks have developed in close proximity to each other, thereby creating a continuously-hummocky landscape under the trees. A good example of this can be seen at the Loch Beinn a' Mheadhoin picnic site in Glen Affric, where the dense concentration of hummocks gives the forest floor a gently-rounded, knobbly appearance. Such areas can be difficult to walk through, as it is hard to tell where the solid ground is between the hummocks, and the moss and other plants sometimes grow over deep gaps or holes between rocks or the roots of a tree stump.
Where there are a lot of well-developed hummocks close together in a particular area of the forest, with good cover of the Vaccinium berry plants growing on them, it is an indication that a continuous cover of trees has been present for a long period of time. This is likely to have been at least a century or more, because of the time it takes for hummocks to form and reach the stage where there's enough soil for the higher, vascular plants to thrive on them.
This time factor is also one of the reasons why hummocks are rarely found in plantations - there simply isn't enough time in the plantation forestry cycle of growth and harvesting for hummocks to develop. Other factors which contribute to this absence include the lack of light inside the dense plantations of young conifers, which inhibits the growth of most plants, and the lack of suitable substrates, in the form of boulders or stumps, for hummocks to become established on.
Because of the large-scale loss of the Caledonian Forest which has taken place in the Highlands (see Deforestation article), there are many areas today where hummocks exist away from forest remnants. Generally speaking, hummocks in exposed open situations appear less vibrant and verdant than those inside the forest and under the canopy of trees. On open ground, the vegetation growing on the hummocks changes, as a result of the increased exposure to the elements of sun, wind and rain, with heather thriving more, at the expense of the Vaccinium berry plants. In some situations, where the soil conditions are wet, the higher plants are absent, and mosses predominate. It may be that the process of vegetation succession goes into reverse in such cases, in the absence of the shelter and protection provided by tree cover, and because the moisture levels in the soil go up, with water no longer being drawn up by the trees and lost to the atmosphere through the evapo-transpiration process.
It is also possible that, as it decays, the accumulated plant material on hummocks on open ground contributes to, or even accelerates, the formation of peat. Certainly, the pine stumps which are visible in the eroded peat of most glens in the Highlands look like they would have been ideal substrates for hummocks to have formed over in the past.
By contrast, the current measures to regenerate and restore the Caledonian Forest in many parts of the Highlands today should ensure that hummocks become more numerous again in future, although this will obviously be a slow process. In the meantime, the beautiful hummocky areas in places such as Glen Affric today will continue their slow development, bringing a magical quality and a fairyland-like atmosphere to the places where they thrive.
Alan Watson Featherstone