Časopis NOVÁ BOTANIKA

Giant „daisies” from Galápagos

We continue our trip to the Galápagos Islands. In the previous chapter 2, we got to know the local species of prickly pear. Today we will recall (from the chaprter 1) the garúa fogs, because we have already learnt of their impact on the flora and fauna of the Galápagos islands. We also know the most important environmental zones on the islands. The coastal zone is, of course, a zone distinguished by salinity. The next zone, is the dry zone or even the very dry zone. The highlands, on the other hand, are irrigated by garúa mists (Figure 1) and, in the hot season, by rains, so they are characterised by more lush vegetation.

Visiting Floreana, which is located in the southern part of the Galápagos archipelago, we immediately notice the dark volcanic sand, which also gives the name of the local beach: Black Beach, which we can already see in the main, although small town of the island, Puerto Velasco Ibarra. In the higher elevations of the island, we already have a skalesia forest (Scalesia sp.). The entire Scalesia genus (Cover Figure) is endemic to the Galápagos Islands. These are trees or shrubs belonging to the sunflower family (Asteraceae), so we can safely say that we have giant daisies in front of us. The fact that they are exclusively trees or shrubs is not so common in the Asteraceae family, because the representatives of our flora belonging to this family are exclusively herbaceous plants. All species of Scalesia are characterised by soft, pithy wood and the leaves are clustered at the tops of the branches, resulting in the crowns forming characteristic canopies.

Due to adaptive radiation processes, species of the genus Scalesia are equivalent to Darwin’s finches, which have also diversified strongly, adapting to different ecological niches. Adaptive radiation is evolutionary differentiation, consisting of the spread of a homogeneous group of organisms with a common origin to different ecological niches, while varying the morphological and adaptive characteristics of the daughter forms. Thus, as a result of adaptive radiation, many other species are formed from a homogeneous starting group (usually one species), adapted to live in different environments. And this was the case with the genus Scalesia. All species and subspecies are most likely descended from a single Asteraceae ancestor that was the first to settle in the arid zone of the Galápagos Islands. Other species that evolved as a result of adaptive radiation spread higher and higher towards habitats with higher humidity and, in addition, migrated to neighbouring islands as well. Thus, there was a species and intraspecific differentiation (16 species in total, plus subspecies and varieties; Figure 4) that we observe today.

In the Galápagos Islands, the most common is Scalesia pedunculata (Cover Figure, Figure 5-7), which is also one of the larger species of its genus: it reaches up to 20 m in height. On the archipelago, dense clusters of Scalesia plants are observed, most often of a single species, of more or less similar age. The biology of this kind is very interesting because most of the individuals in the population in a given place die more or less at the same time. But the next generation of seedlings also appear almost at the same time, and these trees grow quite quickly: for a few years they can outgrow a two-story building, and to put this situation in numbers, after five years the tree is about eight metres tall, and that is when it starts flowering and fruiting. The near-simultaneous appearance of seedlings is due to the fact that when seeds sink to the forest floor, they do not germinate until they are shaded by the mother trees, which live for about 15 years. It is only when the crowns of old trees begin to die that the light rays enter the undergrowth, and then seedlings appear, followed by young plants that gradually take the place of the mother trees. The tree crowns in the ‘mature’ Scalesia forest are quite densely packed, with an spacing of only about 30 cm between individual trees.

The most abundant populations of Scalesia pedunculata are found on the humid, windy slopes of the islands of Santa Cruz, San Cristóbal, Santiago and Floreana, at altitudes between 400 and 700 m. In general, this species should now dominate most of the highlands in the Galápagos, such as the island of Floreana, where it has been cleared for agriculture and its population has been reduced due to the pressure of various introduced species.

However, excellent information is the fact that on the island of Santa Cruz we still have an almost untouched fragment of the Scalesia highland forest, growing on the slopes of the twin craters. Under the canopies of the crowns, we can observe the proverbial orgy of mosses, liverworts, passion flowers, and even orchids, using the trunks of these trees as supports.
As a rule, the small lowland islands of the Galápagos are usually overgrown with one shrubby species of Scalesia, and on the larger and more elevated islands of the archipelago more species are found (e.g., on the island of Santa Cruz at least 6).

Figure captions

Cover Figure. Scalesia pedunculata (Asteraceae) during the flowering season on the island of Floreana.
puntias grow very close to each other, so a wanderer without a machete basically has no chance.

Figure 1. In the background, the Scalesia pedunculata forest growing on the edge of one of the Los Gemelos craters (Santa Cruz Island), shrouded in garúa fog.

Figure 2. The Scalesia pedunculata forest and its undergrowth and shrub layer in the Los Gemelos area of Santa Cruz Island.

Figure 3. The trunks of Scalesia trees are almost entirely overgrown with moisture-loving mosses.

Figure 4. Morphological differentiation in the shape of the leaves of the individual species growing on the given island of Galápagos archipelago.

Figure 5. Moisture-loving epiphytic mosses, hanging from branches, which can be easily spotted in this ecosystem practically at every step.

Figure 6. Clearing in the forest of Scalesia pedunculata in the area of Los Gemelos (Santa Cruz). In the background, you can see the characteristic canopy-shaped crowns of Scalesia pedunculata trees.

Figure 7. The forest of Scalesia pedunculata in the hills of the Asilo de la Paz Reserve on the Floreana island, covered in the top parts by garúa fog.

Figure 8. Black Beach on the island of Floreana, which takes its name from the dark, volcanic sand.

Author:
Dr hab. prof. Jarosław Proćków
Department of Plant Biology, Institute of Environmental Biology, Wrocław University of Environmental and Life Sciences, Wrocław, Poland

Photograph credits: (cover picture, 1-3, 5-7) Jarosław Proćków, (4) according to Carlquist (1965)

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