Not everything humans touch, they destroy. Sometimes a careful, science-based intervention can benefit diversity, particularly in forests. For instance, would you have thought that felling one or more trees can contribute to mitigating climate change?
with contributions by Angela Rositi, Fabio Cogo, Donato Salvatore La Mela Veca, Renzo Motta
It is early morning in the heart of the rainforest in Taï National Park, in Ivory Coast. Chimpanzees are hiding in the canopy of the tropical forest, among branches and leaves rising more than thirty metres from the ground, feeding on fruit. On the ground nearby, a confused and noisy army of diptera and beetles is feasting on the carcass of a mongoose. The decomposition process is swift, and the chances of the carcass being examined to determine the cause of death are very low. But guessing it is not difficult. In recent years, anthrax has caused the death of approximately 38 per cent of the park's fauna, posing an additional threat to this remnant of rainforest already under pressure from cocoa plantations — Ivory Coast being the world's leading producer.
Because of the difficulty of locating dead animals within the dense tangle of plants and roots of the forest, the risk of failing to measure the resulting loss of biodiversity is very high. But the solution comes from that very industrious army of flies and midges, which do not merely feast on the body but sample a small fragment of DNA within their digestive tract. By sequencing these genetic residues, researchers at the Robert Koch Institute in Berlin have been able to identify the species of mammals, birds or occasionally amphibians on which the flies have fed. From this simple yet ingenious idea — which seems lifted from a 1980s science-fiction film — it has been possible to obtain a biodiversity inventory of the forest: a list of the animal species that inhabit it and their distribution. Like in an indie video game, this groundbreaking research has opened the way to move from an infinitely small level of forest biodiversity (the genetic one) to a far more complex and articulated one: the interconnections between species.
Traditionally, biodiversity is estimated and measured with a number: species richness. This indicator, taken in raw form or mathematically manipulated, enables us to compare different habitats on the basis of the species that inhabit them. But species count alone can be misleading. For a start, species richness is subject to scale problems: a high number of species within a tract of forest ("alpha diversity") can mask a lower level of diversity between different habitats ("beta diversity") and therefore a likely homogenisation of flora or fauna at the regional level ("gamma diversity"). At this broader scale, the most important thing is to promote physical connections between ecosystems and avoid their fragmentation. Confusing these levels with one another can lead to misleading conclusions for conservation.
Measuring biodiversity through species richness can be misleading
Now suppose we have two habitats with the same number of species: we would be inclined to say that the two environments have the same ecological relevance. In reality, some species might be similar, common and redundant, while others are rarer, complementary and with different functions. In the case of plants, for example, species might use the same type of resources (a certain amount of light, a certain quantity of soil nutrients) but develop or flower according to a distinct phenology — that is, at different times of year, thereby attracting different species of pollinators. This phenomenon is expressed by the concept of functional biodiversity, meaning the diversity of the functional traits of individual species and their interactions (how they compete or cooperate, how they respond to changes in an environmental variable, how quickly they decompose) rather than the number of species per se.
Examples of functional traits include seed size and dispersal mode, the ability to germinate or to fix atmospheric nitrogen, but also the nocturnal or diurnal behaviour of animals and the duration of their larval stage. In practice, the survival kit of every species, which allows an ecological community to respond to environmental changes and to organise itself into complex networks.
From the deepest layers of soil explored by tree roots to the broken, discontinuous canopy above, a forest represents the supreme expression of these interconnections. Forests globally cover approximately 30 per cent of the Earth's land surface — 0.6 hectares of forest for every inhabitant of the planet — yet they account for around 80 per cent of terrestrial biodiversity. But not all forests are equal. If "our portion" of forest were in the tropics, it could contain up to 300 tree species. Figures that cannot be compared with Mediterranean forests, where in "our portion" we might find at most around ten tree species, or with temperate forests, where there would be no more than four or five.
Alongside the biome effect, forest biodiversity is a function of the type and frequency of disturbances of natural and human origin that have shaped the ecosystem, regulating the flows of organic matter and interacting with reproductive processes. Examples of disturbance include fire, storms, insect outbreaks, an avalanche, a falling tree — in short, any event that alters the structure of a population or community in time and space. Light availability and the quantity of dead wood are paradigmatic examples of factors that can be modified by the onset of a disturbance, and that can promote the diversity of pollinators, arthropods and lichens associated with particular light conditions, or the abundance of insects, plants and birds linked to dead wood.
Not all forests are equal
When forests become too homogeneous as a result of misguided human intervention, they become impoverished in terms of the variety of food chains and biodiversity they can support. This is well known to the bears that inhabit the forests of North America, where the natural sequence of lightning-caused fires, which is an integral part of that ecosystem, was interrupted in the last century by humans, who began extinguishing every flame for fear that it would damage the forest.
Open forest habitats populated by young plants — such as those that follow in the wake of a fire — consequently shrank, and with them the plant species on which the bear feeds. To have any hope of surviving winter hibernation, bears must increase their body mass by up to 60 per cent in autumn, feeding for more than 14 hours a day. After a fire passes through and clears the largest trees, light-demanding shrub species have the opportunity to grow and produce the fruits bears feed on: bilberries, currants, blackberries and raspberries. Grasses also increase, making the habitat suitable for the insects and ungulates that feed on them. But in the absence of fires and the vegetation that followed them, how were bears able to continue feeding?
In the forests of North America, the natural sequence of lightning-caused fires, which is an integral part of that ecosystem, was interrupted in the last century by humans, who began extinguishing every flame for fear that it would damage the forest
While trying to understand where shrubs could grow best in Canadian forests, researchers at the University of Alberta discovered that grizzlies liked to frequent areas of forest temporarily cleared by humans, and especially the ecotones (that is, transition zones) between mature forests and the young ones regrowing after logging. Managing a forest, beyond producing timber, can therefore serve to replicate the dynamics that occur naturally, and even improve habitat quality, by regulating the amount of light reaching the ground according to the preferences of each species.
Managing a forest can also serve to replicate the dynamics that occur naturally, and even improve habitat quality
And there is more. Would you ever have thought that felling one or more trees can contribute to mitigating climate change? It might seem a paradox, but viewed from the perspective of a cherry tree or a rowan, the argument takes on a different character. Through specific interventions (known in forestry as thinning), it is possible to encourage the development of those tree and shrub species which, due to their ecological characteristics or past human intervention, are present in woodlands only sporadically (sporadic species).
Once they no longer suffer from competition for light from other trees, their crowns spread, they produce more fruit and attract a variety of animal species. Once the animals have eaten their fruit, they help disperse their seeds, which are expelled from the body often several hundred metres from the parent plant, where they can germinate. For the Japanese cherry (Prunus verecunda), for example, passing through the stomach (and beyond) of a bear or a marten represents, in the medium and long term, the only chance of surviving climate change: when the bear climbs towards the mountain summits at the start of summer, it carries the seed towards forests with lower temperatures, offering a chance to the future plant and to the survival of the species.
It might seem a paradox, but would you ever have thought that felling one or more trees can contribute to mitigating climate change?
Plants, of course, have no ability to move, but silviculture, by modifying certain parameters of the physical environment, is capable of conserving the species associated with it and their connections. And so, while in tropical primary forests it is better not to disturb the balance of nature, in European forests — modified by humans for hundreds of years already — there are many ways in which a careful, "scientific" intervention can benefit diversity. The Life GoProFor project, funded by the European Commission and coordinated by Italian researchers, has identified no fewer than eighty-nine such ways, of which 31 are directly aimed at improving forest habitats.
Mediterranean forests, already profoundly degraded over the past millennia by human action, are particularly vulnerable to climate change and to what will in all likelihood be increasingly dry and hot growing seasons in the near future. How can we know what the maximum disturbance — the point of no return — is that a forest of this type can absorb before transforming into a different ecosystem?
Some birds, such as the wren, can help us answer this question, because they behave as perfect environmental indicators. The wren moves quickly through the undergrowth and lower branches, always furtively — one might say cheerfully — venturing into the open only to emit its song, a sharp, rising trill. It is a forest species, closely tied to woodland habitats and conditions of good humidity: its becoming scarcer or its range contracting therefore represents a warning sign. In the forests of Sicily, the red light comes on in areas at risk of desertification, where the wren tends to disappear and be replaced by other species, such as the Sardinian warbler or the goldfinch, which are associated with scrubland and shrubs.
By cross-referencing ornithological data with data on woodland characteristics, a project recently succeeded in identifying the forests most sensitive to the risk of desertification and climate change: and on these forests it applied a form of silviculture aimed at conserving and improving biodiversity. Indeed, a more biodiverse ecosystem is often a more stable ecosystem and one better able to withstand environmental pressures, including those of drought and desertification. The technique found most suitable for this purpose was, paradoxically, that of opening small "gaps" by removing a certain number of trees: this allowed greater penetration of sunlight, favouring tree species that like to grow with more light (so-called "heliophilous" species). The proportion of secondary and sporadic species in the forest thus increased — including field maple, wild apple and hawthorn — raising the degree of biodiversity as well.
A more biodiverse ecosystem is often a more stable ecosystem, better able to cope with environmental pressures, including drought and desertification
Regulating competitive relationships between trees is one of the main tools that forest management can deploy to promote the conservation of biodiversity, not only in Sicilian forests at risk of desertification, but in mountains by definition: the Alps. Here the environment is harsh for life, owing to long, severe winters that alternate with cool and often very rainy summers.
The larch, which dominates the landscape at high altitudes, is a fascinating species for the sense of strength and at the same time lightness it conveys, even to the least experienced observer. Among its needles — which are not tough like those of other evergreen conifers — the coal tit, the smallest of the European tits, can be spotted in small groups, while high above the common buzzard calls. The larch is a light-demanding, frugal species with a lightweight seed, characteristics that make it capable of colonising areas free of tree vegetation — but that at the same time make it less competitive in areas where other trees are already present, especially Norway spruce. For t