In order to develop, reproduce and spread, the vegetation of Mediterranean climates must first and foremost face the many difficulties associated with aridity, intense heat, drought and poverty in the soil. Throughout the years, plants have had to learn to survive and adopt methods that conserve their strengths.
Fire is an ecological key event in Mediterranean ecosystems, where its influence creates a most fascinating array of plant responses. Plant species respond to fire with specific characteristics relating to their growth, survival, flowering, seed production, dispersal and germination. The mechanisms that enable these traits are also fascinatingly varied. For thousands of years, the Mediterranean forest has lived with fire. Numerous studies (INRA ...) show that each species has been able to develop strategies to resist and regenerate.
What is left of a forest after a fire? With our human reference we could imagine a spectacle of desolation and an irretrievably destroyed flora. However, the vegetation has fire resistance characteristics.
For example, the cork oak has a thick bark that protects the living tissue of its trunk. Others favour a rapid growth so that the foliage escapes the flames. The vast majority of deciduous trees regenerate by producing re-sprouts. Most of the coniferous trees (maritime pine, Aleppo pine), constitute aerial seed banks enclosed in their cones sealed with resin. These seeds can remain dormant for several years. When a fire occurs, it melts the resin, opening the scales of the cones thus releasing the seeds that will fall to the ground. For hardwoods, resprouting starts from the stump that remains alive in the soil or along the trunk.
While fire kills plants and plants tissues, the heat, ash and smoke it produces triggers the germination and growth of surviving seeds and plants. For example, in the Australian kwongan the seeds of the red and green kangaroo paw (Anigozanthos viridis, A. flavidus), germinate due to cyanohydrin in the ash and smoke following a summer fire.
From a plant's perspective, fire has five broad effects :
Fire intervals in the Mediterranean ecosystem vary according to landforms, soils and rainfall zones. For example, in the Midwest region of Western Australia, fire interval varies from 17 to 38 years. The fire frequency parallels gradents in rainfall, geological age and soil nutrition. Fire intervals in the drier kwongan (<300mn) in the Jack Johnson region may be 46 years or longer.
These plants may be abundant. The annual strategy is typically linked to long seed persistence in the soil. They survive both summer drought and maximal fire risk by avoidance, persisting through the summer as seed protected in the soil seed bank. They have no capacity to survive except in this state. If fire occurs between germination (winter) and seed dispersal (spring), then the seed production for the entire year may be lost. While the proportion of a buried seed bank that germinates in any year is not known for most species, and likely varies from year to year, the loss of one year's seed production reduces the abundance of seed available for germination in subsequent years.
Multi-year fire ephemerals
These are short-lived species which germinate largely or only following a fire and live for 3-5 years after the fire. They are considered ephemeral, as populations are present for many years solely as seeds in the soil seed bank. These species are herbs, climbers, shrubs ... All are fast growing, often at the expense of mechanical strength. All of these species have long-lived, soil-stored seed banks and generally produce seeds of moderate size. A number of species have arils that may facilitate seed burial by ants. Dormancy and germination requirements of this group are surprisingly varied and complex, but are strongly fire cued, with some requiring a period of aging in the soil to release seed dormancy.
Obligate inter-fire recruiters
Mandatory renewal between two phases of fire. It implies either that no seeds are present after a fire, or that the habitat and development conditions are at least temporarily eliminated by fire. In a fire-prone environment, recruitment that is not cued to fire allows the possibility that some recruits will experience fire before they have reached peak reproductive output, or even commenced reproduction.
Long-lived perennials with post-fire recruitment
These species constitute a large part of the Mediterranean flora, and include geophytes, woody shrubs, herbs, climbers and graminoids. All are regularly confronted with a fire at different stages of their vegetative life. Perennial species can survive thanks to the seeds stored in the soil, as well as in the canopy that does not necessarily burn with each fire. This seed storage, non-persistent seed production coupled with post-fire recovery, relative distribution of storage mode (canopy and soil), and fire responses suggest that resourcing or germination processes have evolved frequently and are already sharing particular seed storage characteristics. In addition to regeneration from soil-stored seed banks, perennial species may persist through resprouting, storage of a persistent canopy-stored seed bank, or non-persistent seed production coupled with resprouting.
These two trait sets are apparently evolutionarily independent in the flora of southwest Australia. Paralleling independence, there are just three functional interactions between seed storage mode and fire response:
Resprouters and non-sprouters
Definition: species capable of surviving fire by activation of dormant vegetative buds producing regrowth. Populations of non-sprouting plants, occur as a single-aged cohort, which recruits in the first year following fire. As such, their lifespan and production time are equal to the fire interval. Most types of woody plant genera that are rich in species and dominant for example the Kwongan (acacia, banksia, grevillea...) possess both dormant vegetative budding varieties essential for regrowth and other varieties that cannot grow back after a fire. Globally, resprouters and non sprouters differ in that the variety with no dormant buds allocates more resources that stimulate the growth of juvenile shoots, mature earlier, are more modest in size and more tolerant of water scarcity.
The interval between fires directly impacts the survival of these plants by limiting their ability to accumulate and maintain stored resources and protected buds.
Perennial plant species that grow along the ground and underground. In Mediterranean systems, their phenology is akin to that of annuals, with the dormant phase (in fact, the plant lives underground) occurring during summer and autumn, emergence occurs after the rains, and flowering and seeding in winter and spring. They are long-lived, with stored resources enabling replacement of aboveground tissues.
Fire effects the dormant geophytes as a heat pulse; however, this is mediated by the depth of underground perennating buds. Knowledge of geophyte is generally limited. Geophyte regrowth may be rare, given their potential to persist through multiple fire cycles, and seedlings may be difficult to recognise at the species level, or distinguish from emerging resprouting individuals. As such, the precise functional classification of many geophytes may be ambiguous or uncertain.
The characteristics of the fire regime interact with the peculiarities of each plant and in a multitude of ways. To the point of influencing the survival of the plant population, the composition and the richness of the species. The interval between fires is the most important feature because it interacts with many plant specificities as mentioned above.
Primarily, it influences the likelihood of plants accumulating or not and then retaining the resources necessary to ensure population replacement, either through underground seed storage or through dormant buds. Together with plant longevity, the average fire interval determines the number of fires that individuals of resprouting species experience in their lifetimes. This directly results in a larger number (and perhaps variety) of opportunities for both post-fire renewal opportunities, but also fire-related mortality.
In general, the diversity of responses to a fire allows the Mediterranean vegetation to reconstitute itself almost identically after a passage of fire.
One cannot avoid the fact that if the fires are too frequent and close together, the vegetation will be exhausted to the point where the forests disappear to make way for a garrigue or a maquis.
The variety of soils
The structural similarity between fynbos, kwongan, and in a lesser extent maquis, garrigue and chaparral... has been attributed primarily to the nutrient-poor soil. It is assumed that low nutrient levels in the soil restrict leaf renewal and regrowth each season, making it necessary for shrubs to remain evergreen and suitably constructed to withstand the moisture stress of the dry, and often windy summers. The preponderance of deciduous shrubs on richer soils provides support for this hypothesis.
Paradoxically, the poor soils may contribute to the floristic diversity. Neither fynbos, nor kwongan is as dense as chapparal or maquis, and their relatively open structure may favour the growth of a diversity of low-growing species that is only possible in more dense plant communities for a brief period after fire. Perhaps small differences in the proportions of minerals assume a greater significance in poor soils than in richer ones, thereby providing more niches for plant species to occupy.
Plants present in the Fynbos, exhibit several adaptations to enhance the absorption of minerals from the soil. The most common, found in plants throughout the world, relies on a symbiotic relationship with a soil fungus know as a "mycorrhiza". This fungus, interacts deeply with the roots of the plant and may account for as much as 40 % of the root weight. It allows the extraction of poorly soluble forms of nutrients, such as phosphorus and nitrogen, releasing them to the plant as required in exchange for carbohydrates. Mycorrhiza associations are specially common in the Ericae family.
The Proteacaea have developed an unusual form of growth adapted to nutrient-poor soils and periodic drought. Hundreds of fine rootlets, resembling cotton, sprout from the surface of the plants roots after the first rains of the season, rapidly absorbing surface moisture and minerals released by the decomposition of leaf litter. These "proteoid" roots have a short lifespan, and disappear after two or three months to leave only the subsurface roots. Similar temporary root systems are also observed in Restionacaea. The Fabaceae family has developed a unique symbiosis with a nitrogen fixation bacteria, which resides in nodules implanted on the roots. The bacteria convert gaseous nitrogen from the air into soluble forms that can be used by the plants allowing them to thrive in nitrogen-poor soils. Carnivorous plants, derived especially nitrogen from minerals, directly from the digestion of insects and other small animals that they trap with highly modified leaves
The foliage of shrubs is mostly brownish green or greyish in colour. Leaves are typically small, stiff, with thick cuticles and internal struts of woody tissue to prevent them from collapsing under moisture stress. Many shrubs are rich in bitter tannins or aromatic oils, which may serve to deter predators. The sticky surface of the floral bracts of certain shrubs may also act as a varnish, reducing water loss.
Several Berbaceous perennials, have reduced their leaves or done away with them entirely, transferring the function of photosynthesis to the stems instead.
Flowering is concentrated in spring, when pollinating insects are in abundance. At this time of the year, some 60% of the species are in bloom, but at least a fifth of the vegetation can be found blooming any month of the year.
Several bulbs have separated their growth and flowering phases, enabling them to flower in the dry season, in midsummer or autumn, when competition for the available pollinators is relatively low.
Take for example the autumn flowering plants of the Amaryllis family. Their seeds do not enter a dormant phase like those of most plants, but continue to grow and germinate. By blooming at the end of the dry season, the Amaryllis ensure that their seeds are shed at the beginning of the wet winter, when conditions for their survival are optimal.
The low fertility soils places a significant limitation on the production of seeds, which are rich in nitrogen and phosphor, both of which are in short supply. Possibly this is why many shrubs have few or just a single ovule per flower. In some species, seed maturation takes place over many months, allowing longer periods for the accumulation of nutrients. Mass flowering after fire, when the level of nutrients in the soil has been increased by the falling ash, is another way of overcoming this limitation.
Several species protect their seeds from predators during inter-fire periods by storing them in hard, woody fruits. These remain sealed on the plants until the heat from the flames stimulates the protective seed to burst open, scattering the seeds onto the newly fertilised and cleared soil.
All these strategies, resulting from millennia of adaptation, bring to these regions an extreme diversity of species. Climatic, geological and nutritional constraints make it possible, for example, to observe that in France, 60% of plant species are concentrated in the Mediterranean area (20% of the maximum territory). The department of the Hérault alone accounts for 2,000 endemic species. By way of comparison, in a damp climate, on an area at least 5 times larger, Great Britain has only 20.