Trees can accumulate seeds from other trees and plants. This happens as trees act as perches for birds or attract them with the fruits they bear. Birds use stones, post or trees to rest. When this habit of the birds is combined with the ideal conditions produced by carob trees the results are interesting.

In the Mediterranean regions, summers are warm and dry, and winters are wet and cold winters. Rainfall is around 27 to 90 centimetres annually. The vegetation is adapted to survive the stresses of the warm and dry summer. Plants are evergreen and leaves are sclerophyllus, with thick surfaces to prevent loss of water vapour (1).

In the Mediterranean Valcenia, in east Spain, the carob (Ceratonia siliqua) a legume acts as an important perch for birds. The tree grows four to six metres high and is used as fodder and or in food as a chocolate-flavour by people. This tree has been around for a long time. People in the middle East have cultivated it for 4000 years. From there it was taken to Europe, and later to the Americas and Asia (2).

Once the orchards of carob are abandoned, they begin their role as nurse-plants collecting bird dispersed seeds under them. While they are being tended, the seeds they attract as perches are cleared away by farmers. After the trees are abandoned, there is nobody to clear away the seeds, and they begin to accumulate, germinate and grow (3).

Many species of trees get collected this way under it, like the Mastic (Pistacia lentiscus), kermes oak (Quercus coccifera), prickly juniper (Juniperus oxycedrus), and Aleppo pine (Pinus halepensis). Most of these are themselves also sought after for their products by people (4, 5, 6, 7). Most individuals of these species are found under isolated carobs, than in any other site. The association of mastic with carob is particularly strong.

The frugivorous birds that bring in the seeds are the common songsters seen in Europe. They are small to medium sized. Among them are the brown and white European robin (Erithacus rubecula), true thrushes (Turdus spp.) and the Mediterranean warbler (Sylvia spp). The true thrushes are grey to brown with speckled underparts. In summer these birds usually eat more insects. By the end of autumn there are no more insects available, so the birds’ diet consists mainly of fruits. This diet continues through the winter (8).

The Mastic seems to be their favourite fruit. A Mastic shrub can produce 2,500 to 5,000 red to black drupes and most of them are eaten by birds. Drupes are small single seeded fruit with some fleshy content, for example cherries or almonds. The Mastic fruits fit even in the tiny gape (12 millimetre) of the songster’ beaks. The birds eat their fruits, and then carry their seeds in their bellies. The seeds are regurgitated, or defecated in roughly half an hour. When this happens as they are perched on the carob, the seeds accumulate under the tree. So a group of shrubs and trees start growing around the carob (3, 4).

However, the carob also acts as a nurse-plant by improving the soil conditions under it (3).

During heavy rains when rainfall is more than 25 centimetres, the bare soil cannot absorb all the water in a short time. Much of this water is lost as surface run-off. The thousands of leaves in the carob trees break the force and flow of rainwater falling on trees. So rainwater falling on trees, reaches the ground gradually, and more of it is absorbed (3).

Hence, it happens that water levels optimum for germination are found for at least five days under the trees, while soil in the open dries up faster. Seeds need water to germinate. Under the trees where seeds get more water, their chance of germinating improves. More seeds can germinate, and the process proceeds faster in the presence of water, boosting their growth. When water decreases below a certain level, germination is no longer possible (3).

Water seeping into the ground also changes the soil structure. Soil remains loose and friable while it is wet, and becomes hard and compact as it becomes dry. Therefore increased levels of soil water under the canopy keeps the soil loose. In the open, soil compaction shows half a kilo of pressure more for every square centimetre, than the soil under the trees (3).

The soil under the trees also become more compact in time, but remain looser than the soil in the open. Three days after the rains, the difference in hardness gets pronounced. Soil with more than 4.5 kilo per square centimetre is considered highly compacted. This degree of compaction is widespread in open areas after five days. Seven times more of the area in the open is shows this hard than under the tree. So the distribution of water and loose soil is patchy also under the canopy. When soil compaction crosses 1.5 kilo per square centimetre, germination is critically affected (3).

There is more water available the year around under the trees than in the open, so it is not just germination but later growth also that is helped. So the combination of attracting seeds and providing the right conditions for plant growth makes carob a nurse plant that helps other plants (3).

Sources

1. https://www.britannica.com/science/Mediterranean-climate (Retrieved on 10.2.17)

2. http://www.carobana.com.au/carob.html (Retrieved on 10.2.17)

3. Verdu M. and P. Garcia-Fayos. 1996. Nucleation processes in a Mediterranean bird-dispersed plant. Functional Ecology, 10:275-80

4. http://www.pfaf.org/user/Plant.aspx?LatinName=Pistacia+lentiscus (Retrieved on 28.5.16)

5. http://www.pfaf.org/user/Plant.aspx?LatinName=Juniperus+oxycedrus (Retrieved on 26.5.16)

6. http://www.stihl.com/792.aspx?idTree=187 oak (Retrieved on 10.2.17)

7. http://www.pfaf.org/user/Plant.aspx?LatinName=Pinus+halepensis (Retrieved on 10.2.17)

8. Jordano P. 1986. Frugivory, external morphology and digestive-system in Mediterranean Sylviid warblers Sylvia spp. Ibis 129:175-189. DOI: 10.1111/j.1474-919X.1987.tb03199.x