This article is about the Laurus genus. For information on the church hierarch, see Metropolitan Laurus. For the saint, see Florus and Laurus.

Overview

Laurus is a genus of evergreen trees belonging to the Laurel family, Lauraceae. The genus includes three species, whose diagnostic key characters often overlap (Mabberley 1997).

• Laurus azorica (Seub.) Franco, Known as Azores Laurel, or by the Portuguese names Louro, Loureiro, Louro-da-terra, and Louro-de-cheiro, is native to the laurisilva forests of the Azores, and also locally introduced in Cortegada Island, Spain.
• Laurus nobilis L., known commonly as Bay Laurel, True Laurel, Sweet Bay, Grecian Laurel, or simply Laurel, is the source of the spice Bay leaf. It was also the source of the Laurel wreath of ancient Greece. It is distributed across the Mediterranean region from Spain to Turkey.
• Laurus novocanariensis Rivas Mart., Lousã, Fern. Prieto, E.Díaz, J.C. Costa & C. Aguiar, syn. L. canariensis Webb & Berth. (an illegitime name). Formerly included in L. azorica, and also known as the same Portuguese names as L. azorica, is native to the laurisilva forests of Madeira Islands and Canary Islands, and also locally in Morocco.

Fossils dating from before the Pleistocene glaciations show that species of Laurus were formerly distributed more widely around the Mediterranean and North Africa, when the climate was more humid and mild than at present. It is currently thought that the drying of the Mediterranean basin during the glaciations caused Laurus to retreat to the mildest climate refuges, including southern Spain, Portugal and the Macaronesian islands. With the end of the last glacial period, L. nobilis recovered some of its former range around the Mediterranean.

A recent study found that native stands classified as L. nobilis in northern Spain shared greater genetic and morphological similarity to L. azorica than to populations of L. nobilis native to rest of Spain, France and Italy [Arroyo-Garcia et al. 2001]. This populations like the Cortegada Island population, in Galicia, famous for its large grove of laurels, come from seeds dispersed by birds but is not indigenous to the island, as this islander forest originated spontaneously from laurel specimens that were planted after the original vegetation was destroyed.

Ecology

The genus come from a few relict species living in temperate areas and more distributed in the Tertiary. The main centers are found inhabiting montane forests or coastal forest in low-altitude. Some species have adapted to more extreme conditions but mostly depending on favorable soil edaphic conditions, as presence of aquifers, groundwater periodic flows, etc.

The patterns of speciation in the Lauraceae family, where laurus genus belong, indicate that since the onset of aridification on the continents 15 million years ago, rainforest diversified in species numbers with the majority of species the product of vicariance. One of the products of aridification is the current island like archipelagos of rainforests along the planet. The fragmentation of once more continuous rainforest facilitated isolation of populations and this likely caused the increase in the rate of speciation as found in the Lauraceae.

Laurus genus responded to favourable climatic periods and expanded across the available habitat, occur as opportunistic species across wide distribution with close relatives and few species, indicating the recent divergence of this species. The extant laurel species of this group are relatively young.

The laurus genus features have named many botanical species that are having similar foliage to the Laurus due to convergent evolution. Plants of the laurel forests must adapt to high rainfall and humidity. The trees adapted by developing leaves that repel water. Laurophyll or lauroide leaves are characterized by a generous layer of wax, making them glossy in appearance, and narrow, pointed oval in shape with an apical mucro, or 'drip tip', which permit the leaves to shed water despite the humidity, allowing perspiration and respiration from plant. The scientific names laurina, laurifolia, laurophylla, lauriformis, and lauroides are often used to name species of other plant families that resemble the laurus genus and the Lauraceae family.^[1] Furthermore, it is common that the dispersal of seeds in many laurel forest species is due to birds that swallow them, so also the fruit and berries are often similar to attract birds. Although not closely related to Lauraceae family despite the similarity, Laurelia genus in southern hemisphere, in the family Atherospermataceae is equivalent to this of Laurus. The three laurel species in the genus laurus are dispersed by birds columbiform, mainly.

History

The laurus genus is characteristic of the formations of laurel forest and was more widespread in the Tertiary. It has led to endemic species on islands, but not so widespread geographically as in the past.

The ecological requirements of the species, are those of the laurel forest and like most of their counterparts laurifolia in the world, they are vigorous species with a great ability to populate the habitat that is conducive. The phylogeography of this group provides a nice example of various speciation mechanisms at work. This group became isolated during the Quaternary glaciations and expansion of the polar ice caps, resulting in widespread cooling of the climate, the flora of central and southern Europe retreated to more southerly latitudes in search of milder conditions. Also the sea level was lower, with lands today submerged forming land bridges.

The end of glaciations coincided with the spread of deserts in North Africa, notably the Sahara, so this type of forest was reduced to those areas, which act as boundaries between temperate and tropical. At that time, the climate of southern Europe was warmer and wetter than today, and the vegetation that surrounded the ancient shores of the Mediterranean Sea was likely similar to that of the current Macaronesian laurisilva. The Ice Age which took place at the end of that period and for much of the Quaternary forced the laurel forests to move to warmer southern regions, where conditions were more conducive to their survival, settling in this way on the northwest coast of Africa and in the Macaronesian archipelagos. The ocean permitted the plants' migration to more suitable areas, where they survived.

With the general warming of the atmosphere and the consequent withdrawal of the ice, flora tertiary survivors could not regain their range in southern Europe, as the new post-glacial climate was drier than that of the Tertiary, and to these new environmental requirements, the primitive tropical European flora evolved and gave rise to the present flora of the Mediterranean sclerophyll.

At the same time, isolated from the mainland, the tertiary Macaronesian laurel species evolved independently, which has led endemic species. Physical separation of the poblations begin a process of change to adapt to new conditions. This mechanism is called allopatric speciation. Over time, survivors specimens of the laurus genus, adapted to different ecological niches. This led to reproductive isolation, an example of ecological speciation.

In Europe the genus is represented by a single species, the named Laurus nobilis. In macaronesian islands this former arose from a common ancestor in the area of laurisilva in the Mediterranean area. The islander laurels of Macaronesia in the eastern Atlantic, are present in the Azores, Madeira Islands, and western Canary Islands, from 400 m to 1200 m elevation, in the laurel forest habitat in the islands.

The laurisilva forests of Macaronesia are relicts of a vegetation type which originally covered much of the Mediterranean Basin when the climate of the region was more humid. With the drying of the Mediterranean Basin during the Pliocene, the laurel forests gradually retreated, replaced by more drought-tolerant sclerophyll plant communities. Most of the last remaining laurisilva forests around the Mediterranean are believed to have disappeared approximately 10,000 years ago at the end of the Pleistocene, when the Mediterranean basin became drier and with a harsher climate, although some remnants of the laurel forest flora still persist in the mountains of southern Spain, north-center of Portugal and northern Morocco, and two constituent species (Laurus nobilis and Ilex aquifolium) remain widespread. The location of the Macaronesian Islands in the North Atlantic Ocean moderated these climatic fluctuations, and maintained the relatively humid and mild climate which has allowed these forests to persist to the present day.

References

• Arroyo-García, R., Martínez-Zapater, J.M., Fernández Prieto, J.A., & Álvarez-Arbesú R. (2001). AFLP evaluation of genetic similarity among laurel populations. Euphytica 122 (1): 155-164.
• Barbero, M., Benabid, A., Peyre, C. & Quezel, P. (1981). Sur la presence au Maroc de Laurus azorica (Seub.) Franco. Anales Jard. Bot. Madrid 37 (2): 467-472. Available online (pdf file; in French).
• Costa, J. C., Capelo, J., Jardim, R., Sequeira, M., (2004). Catálogo Florístico do Arquipélago da Madeira. Quercetea 6, 187-200.
• Mabberley, D.J (1997). The Plant Book: a Portable Dictionary of the Vascular Plants. Second edition, pp. 393–394.