Their main point in common is the presence of a grassy stratum dominated largely by graminaceous and flowering plants, as well as the absence or diffuse character of arboreal life forms. This same diversity makes it difficult to establish a general definition. In certain cases, like in dry tropical forests, for example, it is difficult even to establish clear lines of distinction between grassy and forest ecosystems. It is this very diversity, the origin of this same diversity and the relationship it has with human actions that is central to the thinking of this group of scientists.

A map that is too general

The problem posed by the WRI map is as follows: The grassy ecosystems vary not only according to their intrinsic ecological conditions but they also differ according to the type of human activity which can go back a long way. “The types of landscapes related to part of these grassy biomes is not the result of human activity and can go back several millennia or even millions of years”, explains Grégory Mahy. “This is quite simply because they are inhospitable to mankind because of regular fires, their geological and pedological composition (presence of toxic compounds) or other factors. Other biomes have a quite recent anthropic origin in terms of history such as secondary savannahs and uncultivated bushland which, in other areas of Africa and South America have replaced the uncultivated tropical forest that have been intensively cleared by humans since the 20th century. A third category includes grassy systems of anthropic origin but which are much older. For example, the Mediterranean basin which began to be deforested around 5,000 years ago and which has been subjected to an agro-sylvo pastoral type activity that has continued to the present day”.

Therein lies the problem: the World Resources Institute map puts all these ecosystems in the same basket. “This leads to confusion between the ecosystems that are the result of recent deforestation, those that are the result of a much older degradation and those that have never been covered by forest. Yet, according to the data held by our research institutes, the old growth grassy biomes show an exceptional biodiversity, unique in the world. Misplaced reforestation programmes could lead to irreparable biological losses. For others, we do not have enough data. It is therefore necessary to characterize them as best we can and ensure that the benefits they bestow in terms of the ecosystem are not destroyed due to reforestation initiatives”.

Misplaced plantations

In order to back up their demonstration, Grégory Mahy and his colleagues cite the example of plantation programmes carried out in recent times in a dry forest area of southern Africa. Massive plantations of Australian acacias have begun to grow and spread rapidly. These new arrivals have modified the cycle of nutrients, water and interactions within the ecosystems such as pollination. All this has been to the detriment of human activities but also to the flora and fauna in these regions. The demonstration by the international universities doesn’t end here. Nobody knows, in fact, whether the grassy ecosystems do not already play a non-negligible role in the fixation of carbon which is overloading Earth’s atmosphere. “In tropical grassland zones, flowering plants known as xylopods and small bushes have adapted to regular episodes of destruction due to fire by developing a kind of “root cathedral”, made up of ligneous matter that can descend to one or two meters in depth. In reality, these are not real roots but are wood! Recent studies suggest that the carbon-storage capacity of these is high and becomes significantly modified when the environments are planted with trees”.

In temperate zones such as Europe for example, we know that well-known grassy ecosystems such as boglands have a considerable capacity for storing methane (another greenhouse gas with a greater “warming” effect than carbon dioxide). But it would be dangerous to make deductions or comparisons with tropical grassland ecosystems based on such knowledge. “It is all the more important to characterize the tropical grassland ecosystems as soon as possible”, explains Grégory Mahy. “Knowledge at this juncture remains rare and fragmentary, particularly for ‘mixed’ intermediate zones between grassy and forest zones. It is these very zones that the WRI map does not represent correctly and in enough detail. Without this level of detail, reforestation programmes run the risk of confusing a-forested zones (those without any forest cover for a long or very long time) with deforested zones (covered in forest until recently)”.