Steppe

Dr Martin Vahl, the well-known botanist and biologist of Copenhagen, in an interesting work entitled Zonas e biochoros geograficos, included within the domain of the steppes the plant community that covers a good part of the dry land of the lower zone (coastal belt) of Madeira, and in which Andropogon hirtum appears to be the most frequent species.

Although we recognise that the shortage of rainfall is in certain years very marked on Madeira, we cannot share Dr Vahl’s opinion. To delimit the steppe, it is necessary not to lose sight of certain climatic influences, some of them entirely local.

It is well known that, as a result of the nearness of the sea and of the currents set up between it and the land, there is almost always a considerable quantity of water vapour in the low-lying zone of Madeira. Owing to the thermal conditions of that zone, this vapour seldom appears in the form of mist, but it cannot fail to be regarded as an aid to vegetation at times when rainfall is lacking. Since all soils are hygroscopic, though to differing degrees, it is evident that they can take directly from the atmosphere the moisture it contains, and this, once absorbed, must contribute in some measure to sustaining plant life. Dr Mason reports that by leaving a plate exposed to the open air on clear nights he managed to collect a considerable quantity of dew within a few hours. As other observers have noted, however, this is by no means general in the low-lying region of Madeira, although it cannot be denied that on many nights a certain amount of water vapour condenses on the soil and on the organs of plants, and that some benefit to vegetation must result from this.

By reducing the intensity of transpiration, atmospheric humidity causes plants to consume less of the water they store or draw from the soil through their roots. Even in dry climates plants can conserve their fluids, either by slowing their growth or by avoiding in other ways the losses to which this function gives rise.

There are various types of steppe, but it is probably grass steppe that Dr Vahl supposes to exist in the lowland zone of Madeira. According to Dr Drude, this steppe is characterised by formations subject to winter dormancy, in which the plants dry out during the period of greatest heat. Its accessory elements include large herbaceous plants, sometimes markedly tomentose, plants furnished with bulbs or tubers, and thickets of species with little succulence.

In the lower zone (coastal belt) of Madeira there are some grasses and other plants which, whether from the length of their roots or rhizomes or from the appearance and form of their stems and leaves, seem adapted to withstand a high degree of dryness. However, if we consider that such plants can exist both inside and outside the steppe, and that bulbous forms are not found in this zone, whereas succulent forms are abundant there, it is impossible not to see in these facts proof that the plant associations dominated by Andropogon belong to a domain different from the one in which Dr Vahl sought to include them. The first settlers, felling the forests in order to clear the land for cultivation, perhaps brought about appreciable changes both in the climate and soils of the lowland zone of Madeira and in the distribution and appearance of its flora. What they did not manage to do, as we have seen, was to cancel out the influences arising from the nearness of the sea and from the position and shape of the island. It is owing to these influences that steppe does not exist on Madeira, although it cannot be denied that in certain years the rainfall regime of the island’s lower zone shows remarkable analogies with that of countries endowed with a flora adapted to withstand a high degree of dryness.