Module 2 - Habitat trees and old-growth forest

<p>Our forests in Western Europe have been managed by humans for many centuries. This makes that our forests today are quite different than undisturbed, primary old-growth forests.</p>

<p>Typical old-growth elements, such as large amounts of dead wood and large, overmature trees are often missing in managed forests, even in forests under close-to-nature management. In natural undisturbed forests approximately 50% is in the old growth phase. So it is quite understandable that many forest specialist species will disappear together with old growth forest.</p>

<p>In managed forests, trees are mainly removed at their harvestable age, around the economic optimum of tree value, which is located at about one third or half of their natural life span. The tree and forest life-cycle is, so to say, short-cut by final fellings.</p>

<p>Therefore, conservation of biodiversity in managed forest stands is mainly a question of reserving, retaining and restoring elements such as old, overmature trees and dead wood.</p>

<p>In the next lessons we will present two useful concepts foresters can apply to improve their managed forest in terms of habitat quality.</p>

<p>First we will introduce habitat trees, bearing tree-related microhabitats, and secondly we will integrate this in a functional network of old growth elements.</p>

<p>As seen in previous lessons, we now know that most species dependent on old-growth forest are under pressure in managed forests. Especially saproxylic species, that is species that depend on deadwood.</p>

<p>The survival of a population of a species depends on the ability and opportunity to colonize newly available habitat before the original habitat is lost. Saproxylic organisms can be considered as populations that live on <strong>melting icebergs</strong> and that need to be able to reach the next suitable iceberg before the original one is gone.</p>

<p>A functional network of old-growth forest elements involves smaller and larger non-intervention patches of suitable habitat that contain subpopulations of species. These are able to swiftly migrate from one site to another by “corridors” and “stepping stones” of habitat trees in the managed forest matrix. All these subpopulations together form one so-called ‘<strong>metapopulation’</strong>.</p>

<p>Among species we see different life-strategies:</p>

<ul>
<li><strong>Fast-colonizers</strong> who manage to quickly colonize newly available habitat over a long distance and produce a large number of offspring. Bark beetles are typical examples of such species. They need this strategy because time is short: they live on very short-lived habitat, such as freshly dead cambium, or must be first at the scene in order to successfully compete with other species.</li>
<li>On the other side of the spectrum are species that are very <strong>slow colonizers</strong>, producing little offspring but with long individual life spans. Typical species here are beetles living in wood mould in cavities of very old living and dead trees. In most cases they will not colonize the next suitable habitat further away than 1 km. They have already significant trouble doing so after a few 100 metres. (Lucanus cervus, Osmoderma eremita)</li>
<li>Further on we distinguish <strong>micro-climate requiring species</strong>. They need the continuous shade pressure of closed-canopy forest and the associated higher relative air humidity, soil moisture and buffered air temperatures. Microclimate in closed-canopy forest not only is essential to ground vegetation, but also to many ground-dwelling species such as slugs (True forest slug), fungi, mosses and specific groups of arthropods such as woodlice and fungivorous beetles (Bolitophagus reticulatus).</li>
<li>Finally there are the so-called <strong>warmth-loving ‘gap‘-species</strong>, that benefit from more disturbance and open forest canopy with consequently more sun-exposed wood and plants.</li>
</ul>

<p>Therefore, the design of a <strong>network of old-growth elements</strong> should combine the following elements:</p>

<ul>
<li><strong>RESERVE</strong> one or several larger non-intervention areas of more than 10 ha and up to several hundred hectares to safeguard relics of old-growth from harvest. Deadwood volumes should arrive at minimum 30 to 50 m³ per ha. In well-chosen reserves these levels should in Flanders be possible after 10 to 20&nbsp;years on average. In addition to simply wait, the proportion of dead wood can be quickly brought up to the right level by girthling, felling or tearing trees down.</li>
<li><strong>RESERVE</strong> additionally a network of smaller ‘set-aside areas’ with a minimum size of 1 ha. A distance of maximum 1 up to 2 km between these set-aside areas is fine for most species. For the specific group of dispersal-limited, slow-colonizing species, look for hotspots and relict areas where a dense and local functional network of set-asides can be created.</li>
<li><strong>RETAIN</strong> at least 5–10 individual habitat trees per ha and protect them during thinning and harvesting. They must allow for good dispersal to the set-asides, but also provides habitat in itself for many species. Preserve both individual and clusters of trees. As a directive there should be 1 cluster of 15 dominant and co-dominant trees per 3 ha.</li>
<li><strong>RESTORE</strong> and develop habitat trees when they are missing by selecting them in younger stands in thinning phases to ensure that there are sufficient individuals in later stages</li>
<li>Install a strategic <strong>ZONING</strong>:
<ul style="list-style-type:circle">
<li>in the inner-wood core area there should be located the network of closed-canopy old-growth elements and continuous cover forestry, for species requiring undisturbed microclimate</li>
<li>more towards the forest edges, the larger gaps, sun-exposed habitat trees, open corridors, and open canopy forest management types like coppice-with-standards for the warmth-loving species, and this surrounded by silvicultural practices with half-shade and light-demanding species</li>
<li>outside the strict boundaries of the forest, also take into account the relict hotspots such as old pollard trees</li>
</ul>
</li>
</ul>

<p>In order to have a well-functioning network of old-growth elements, at least <strong>10%</strong> of the total forest area should be taken out of silvicultural production for conservational purpose.</p>

<p>But &nbsp;be realistic. Typical rare indicator species of virgin forests will not settle for less. So don’t be too ambitious if your forest area is too small, too young or has a too altered species composition. In that case a more basic design will do just fine.</p>