From a historical perspective, the success of broom (Cytisus sp. pl. and Genista florida) in today’s landscapes is surprising to say the least. No more than 50 years ago, broom was so scarce that it was sown in the poor lands of Trás-os-Montes and Beira Alta in the last year of rye, before an extended fallow that sometimes lasted a decade. The purpose of sowing was twofold: to improve soil fertility (on site and by adding it to animal bedding and mulch) and to produce wood for burning.
The explanation for the current expansion of these shrubs is multifactorial. We have identified five probable causes. Two of them are i) ditch and hill forest plantations, and ii) phosphate fertilisation on marginal soils. Soil mobilisation for forestry has increased the useful thickness of the soil, which is necessary for large plants in a Mediterranean or temperate sub-Mediterranean climate. This phenomenon is evident in May in the Pinhal Interior. Broom is evolutionarily close to lupins (Lupinus) and, like them, is certainly adapted to capturing phosphorus immobilised by aluminium in acidic soils. The stock of phosphorus available in mountain soils for broom (but not for other less competent plants) grew with mineral phosphate fertilisers when, at the cost of an artificial rise in prices, the cultivation of rye and wheat moved up into the mountains in the first half of the 20th century, occupying marginal pasture land. Broom is known to respond exuberantly to phosphorus.
The third and fourth hypotheses lie in the design of the canopy (physiognomy) of the broom and the recent extension of the fire’s return period (compared to pastoral fires). Anyone who has seen a broom fire knows that they burn explosively, in a blaze, with short cyclical interruptions, i.e. the broom fire progresses in bursts. The cause lies in the structure of the canopy. Broom trees have an aerial part of thin branches (extremely combustible in the dry season) that soon distances itself from the ground, supported at the base by a short trunk and thick legs (not very flammable). This physiognomy keeps fine fuels away from the ground, facilitates the rise of the oxidising agent (oxygen) during the fire and projects the energy generated by the flames towards the atmosphere. As a result, broom generates fires of high intensity and short residence time, where the energy released is not enough to generate high temperatures in the soil or on its surface. The intermittent advance is certainly related to the drying process of the canopy as the flames advance and the discontinuous distribution of the fuel in space.
What advantages do brooms have under this fire pattern?
The temperature at the ground surface and, by implication, the severity of the fire do not depend on the intensity of the fire front, normally measured in the amount of energy released per linear metre of front. Rather, they are related to i) the distance of the (thin) fuels from the ground and ii) the residence time of the fire. Broom fires have a lot in common with grass fires in dry pastures: they are hot, fast (between breaks), intense in the canopy, less intense on the ground surface. Although broom usually dies in summer fires, the seeds that fall to the ground are largely spared. On the other hand, fire stimulates the germination of broom seeds, more precisely, it breaks the dormancy of the hard seeds characteristic of this group of shrubs. The seedlings will enjoy an improved regeneration niche, with few competitors, light and soil fertilised by the ash and decaying roots of the parent individuals (remember that broom is a nitrogen-fixing legume). In the case of perennial grasses, seeds and underground organs (e.g. rhizomes) are spared. As they say in ecological literature, brooms and grasses are habitat engineers.
And how can an extended fire return period benefit the broom?
Several explanatory hypotheses, again. Fire in short recurrence cycles is unfavourable to them. i) Broom produces seeds late, from 3-4 years old; ii) young plants have a thin bark that exposes them to the effect of fire; iii) young broom trees shade the ground little and, even if grazed (with exceptions), they accumulate large combustible masses of grass in the clearings that jeopardise their survival in the face of fire (especially if it takes place during flowering, at the end of April, beginning of May).
In other words, pastoral fires with a short recurrence cycle – in particular, pasture-clearing fires traditionally carried out in September, after the hay and cereal harvest, with the dew falling during the night – prevented broom from reproducing and accumulating seeds in the soil. On the other hand, summer fires in mature, tall gum trees allow time for sexual reproduction and protect the seeds in the soil, thereby facilitating the gradual dominance of broom in our landscapes.
Fifth hypothesis: mass effect. Mass effect means the dispersal of species and individuals from sites with a higher population density to sites with a lower population density. Broom plants produce significant masses of viable ballistically dispersed seeds every year (explosive opening of the pod), often projected more than 5 metres, washed away by rain and landslides. The more seeds, the greater the likelihood of a rare event of long-distance migration and colonisation of an empty broom site. It’s not worth exploring the ecological argument here, but the mass effect allows the broom to persist in habitats that are by deduction unfavourable for it and to outcompete species that are in principle more competitive.
The landscapes of mainland Portugal are at a dangerous moment. Broom is booming, large areas of land suitable for this species have yet to be colonised, the current fire regime is favourable to them and broom aggravates the risks of high-intensity fire in the height of summer (it has low ignitability and fire sustainability in spring), putting populations and property at risk, with detrimental environmental impacts (omitted here).
In addition to the proximal causes of broom expansion listed above, there is rural abandonment (agriculture and pastoralism), the persecution of pastoral fire, the decay of traditional rural knowledge, the difficulty for public agencies to adapt to new realities and the aversion to prescribed fire that is prevalent in Portuguese society.
But it’s like fire, no matter what we do, we have no choice but to learn to live with it in Portugal.
Carlos Aguiar, Avelino Rego, Duarte Marques, Marco Fernandes and Henrique Mira Godinho