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Biodiversity: a case study of vineyards
In wine, some climate truths
In France, new planting and biological control methods are returning biodiversity to vineyards. Emmanuel Monnier reports on how this might help cut costs and boost yields
Although depressing reports on biodiversity loss around the world are being published thick and fast, French vineyards come out of the general gloom quite well. “We’re not seeing any collapse in biodiversity,” says Marie-Stéphane Tixier, a professor at l’Institut Agro de Montpellier. “On the contrary, we’re seeing biodiversity return. When you walk through the vines today, you see ladybirds. Twenty years ago, you saw none.”
Léo Garcia, a researcher and lecturer in agronomy in the same institute, confirms that observation. “This is not to contradict the findings of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. There is certainly a real problem of biodiversity loss in agriculture, but for several years now we’ve seen efforts to boost it in vineyards.”
And that is good news because vineyards are places still largely denuded of life. “The number of micro-organisms present in the soil falls when you go from a forest, or open grassland, to an area of arable farming and falls even further when you go to vineyards,” says Lionel Ranjard, the Director of Agro-ecological Research at France’s National Research Institute for Agriculture, Food and Environment (INRAE). His team has been putting together an ‘atlas’ of soil microbial diversity in France, with more than 2,000 soil analyses.
One key reason for the low number of organisms is trivial: the soil in vineyards was poor to start with, which is why vines, a tough and undemanding crop, were planted there. “There wasn’t much else that could be grown on those relatively infertile soils, which are inhospitable for most plants,” says Ranjard. In Burgundy, monks developed vineyards on shallow soils in which they couldn’t plant wheat.
But the way in which vines are grown is aggressive. Vineyards cover 3% of France’s agricultural land but consume 20% of the pesticides. Planted as monocultures over wide areas, using genetic clones, vineyards are susceptible to biological pests and, in particular, to two fungi – blight and powdery mildew.
Another reason for the poor level of biodiversity is the high degree to which the soil is worked. “The growers don’t like to see grass and weeds in their vineyards because they compete with the vines for water. Many prefer to have a ‘clean’ vineyard, that is weeded,” says Ranjard. “And, because weedkillers are frowned on, they have been replaced by mechanical removal, so the soil is constantly being turned. But turning the soil destroys its structure and the aggregates in which micro-organisms flourish.” The micro-organisms have no place in which to reproduce.
“However, when you look at the variety of micro-organisms, it’s still greater than in forest soils. Soil bacterial diversity is at its highest in vineyards,” stresses Ranjard.
When the ecosystem is stable and functioning well, the micro-organisms provide great services to agriculture
That’s a troubling paradox, but it does have a scientific explanation. “An ecosystem is at its most biodiverse when it is under moderate stress,” says Ranjard. “If there is very little stress, the best adapted species win out over the least adapted. If there is a great deal of stress, you eliminate all but the most resistant species. But between these two extremes, the most competitive species are kept in check and all can co-exist.”
For bacteria, vineyards represent an area of intermediate stress. That does not apply to fungi, which are much less diverse in vineyards, partly because of the pesticides used, of which 80% are antifungals designed to suppress the main microbial pests.
But having a relatively large degree of biodiversity in the soil is not enough. The micro-organisms have to interact. “When you disturb soil, you make life easier for opportunistic organisms that work away in their own little corner without participating in the wider ecological functions,” says Ranjard. “The interactions between bacteria and fungi go into freefall as soon as you leave the forest. In vineyards, there are almost no interactions. There are only opportunistic species. The risk with that is that you have a diversity that may become less and less beneficial.”
But when the ecosystem is stable and functioning well, micro-organisms provide great services to agriculture. They fertilise the soil, create its structure and clean it. When they are not doing that, yields fall. The good news is that a slide in the quality of biodiversity is reversible.
“By changing farming practices, you can rebuild a good level of soil biodiversity within a few years because they are very responsive organisms,” says Ranjard. What’s the best way forward? Pesticide use should be limited but “limiting working the soil and improving the plant and weed cover” is most important, partly because the latter protects the soil from erosion.
Plant diversity encourages a wider range of other organisms, including natural predators of crop pests
“Soil erosion is a fundamental problem in vineyards because the vines are planted in rows, with gaps between, often on steep hillsides and in soil that has a fragile structure because it is regularly damaged. That’s a recipe for erosion,” says Garcia. Vegetation protects the soil and when it is mown and dug into the soil it also adds organic matter.
But what plants are the best? Some growers let the plants under the vines develop on their own. “Two doctoral research theses have shown that mowing ‘spontaneous vegetation’ in vineyards and leaving it there boosts soil fertility,” says Elena Kazakou, Professor of Ecology at the L’Institut Agro Montpellier. “These plants also have a greater nutritional potential for soil organisms and they can be used as pasture as they are just as easily digested as permanent grassland.”
Other growers put in leguminous plants, such as field beans, which help fix nitrogen in the soil at a cost that is increasingly competitive given the recent rise in the price of mineral fertilisers. Grasses bring less nitrogen, but store more carbon. Cruciferous plants, such as radishes, have roots that structure the soil and add certain nutrients, such as sulphur. But the idea is also that plant diversity should encourage a wider range of other organisms, including the natural predators of crop pests.
“Today, we no longer have a problem with mites in vines because there has been a natural colonisation by other mites that feed on them,” says Tixier, who specialises in mite ecology. “Agriculture gains from that because treatments for mites are among the most expensive. Then there is a cut in fuel costs because there is no need to drive up and down with the tractor, as well as an improvement in soil quality because it is less compacted.”
Fig trees, which can be planted alongside vines in Mediterranean regions, support these sorts of predator mites.
Studies have also shown that bats eat some of the species of butterfly whose caterpillars destroy the grapes. “You could encourage the presence of bats with plants that increase the numbers of insects on which the bats feed,” explains Garcia. “It’s not easy to get right. You need to study the overall environment of the plots.”
What effect will global warming have on these subtle equilibria? “The climate has limited impact on bacteria,” says Ranjard. “Fungi are a bit more affected. Climate change is not going to directly modify the composition and microbial diversity of soil. Rather, the effect will be indirect via the plant cover.”
Does better biodiversity produce a better wine? It’s still early days in terms of researching that
Kazakou says that the number of plant species that grow spontaneously in vineyards won’t change as the planet warms, although there will be some that are less well adapted to drought. “That is not necessarily bad,” she says.
Introducing trees protects the soil from the most intense heat. “But trees consume water,” warns Garcia. “Which trees should be used? How many should be planted. We still don’t know the answers.”
Insects like heat, but heat favours crop pests rather than their predators. “You are starting to see insects much earlier in the year and you’ll see several generations of them within one year,” says Tixier, on the basis of research in the Bordeaux region. The geographical distribution of species will also change. “We’ll have to see which practices favour which species.” But what will determine the way forward is yield.
Does better biodiversity produce a better wine? It’s hard to say. “In a winery, there are very few points of contact between the cellar and the vine,” says Garcia. “As soon as the grape is ripe, the cellar master is happy. No one asks how farming methods affect the grape’s quality. It’s still early days in terms of researching that.”
But a good wine that doesn’t damage the soil is surely one to be savoured with more pleasure.
Emmanuel Monnier
Emmanuel Monnier is a French journalist who has been covering science and technology for more than 20 years for leading French publications including Science & Vie. He recently launched a press agency – les Chemineurs – that specialises in science and technology, including soft sciences
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