Focus on research: physical geographer Jan Peter Lesschen

26 October 2007

People wading knee-high through water, cars floating through the streets: when it rains in Spain, it really rains. Physical geographer Jan Peter Lesschen of the Institute for Biodiversity and Ecosystem Dynamics, Faculty of Science, investigates the consequences of such a cloudburst on erosion in agricultural areas abandoned by people.

Photo: Bob Bronshoff

People wading knee-high through water, cars floating through the streets: when it rains in Spain, it really rains. Physical geographer Jan Peter Lesschen of the Institute for Biodiversity and Ecosystem Dynamics, Faculty of Science, investigates the consequences of such a cloudburst on erosion in agricultural areas abandoned by people.

The project in which Lesschen participates is called RECONDES, and it aims at discovering how and where vegetation can be used to counteract erosion. Researchers from all over Europe are examining various locations around the Mediterranean Sea. Lesschen's PhD research is directed mainly at deserted agricultural areas in South East Spain, where rainfall amounts to only 300 millimetres per year, compared to an average of 750 millimetres in the Netherlands. ‘Agriculture is thus rather marginal in this area', says Lesschen. ‘Moreover, due to the expansion of the European Union, there are fewer subsidies available, so that over the years farming has become increasingly unprofitable for farmers. And so large areas have been abandoned, with no agricultural activities taking place there any longer. My research problem is to find out whether erosion is more extensive in this type of area and, if so, where exactly?'

Photo: Bob Bronshoff

Erosion is a big problem in Spain. Large amounts of sediment end up in reservoirs, with the result that they no longer function properly. ‘It is possible to construct an extra dam, but that would only be a temporary solution. If you tackle erosion at its source, you can prevent reservoirs filling up with sediment and, furthermore, the environment will recover better and less soil will be laid bare', explains Lesschen. ‘In the long run you will also achieve more CO2 storage, although this is a secondary effect.'

Vegetation forms an important weapon against erosion. However, random planting of vegetation is of little use: it is expensive and, moreover, the areas concerned are too arid. ‘Spain has been inhabited and influenced by humans since Roman times, which makes research into the original vegetation very difficult. We assume that it was originally covered with forests, but these are now practicaly non-existant. Spain was probably also wetter in those times, and forests cause more evaporation and thus more rainfall. The system has now become so degraded that there is less rainfall. At present the natural vegetation consists mainly of shrubs and small trees.'

Abandoned fields

Lesschen wants to determine how the vegetation develops after a farm has been abandoned. He examines the biodiversity, the degree of coverage and changes in the soil. ‘Using old aerial photographs from 1956, 1987 and 1997, I examined fields that were in various phases of abandonment. To be sure that the developments in the soil and vegetation were related to desertion, we examined fields that were similar with respect to soil type and location.' In order to chart the small-scale erosion processes properly, Lesschen had to figure out the ratio between the bare patches and patches covered with vegetation. Water flows off the bare patches, while vegetation enables water to penetrate the soil beneath it. He took photographs of the fields using a digital camera attached to a helium balloon. ‘These show detailed patterns in the vegetation. We also use aerial photographs taken from an airplane, but in these a pixel represents a half to one square metre, so they are not accurate enough for our investigation.'

Lesschen discovered that, in the first few years after being abandoned, annual plants are chiefly found, mainly arable weeds and grasses. In the course of time, bushes begin to grow, and sometimes even a small fir tree. ‘The longer the development process, the more the tendency towards shrubs. The size of the covered patches becomes larger and the total vegetation coverage increases to about 50 percent.' In addition, the biodiversity decreases. ‘A field which has just been deserted has about 30 species, but these are mainly annual arable weeds. A field that has been abandoned for a longer period becomes more natural, and has only about 15 species.

As an earth scientist, Lesschen didn't know much about plants when he started the project. ‘I gradually learned to recognize the most important species and my knowledge of plants improved considerably', he says laughingly. ‘I still can't name each and every species, but luckily we collaborate with ecologists and biologists.'

Soil samples

Degree to which fallow, recently abandoned, long-abandoned and semi-natural terrain are covered with vegetation.

Lesschen also collected soil samples from these fields in order to investigate various soil properties. ‘You dig up a soil sample weighing about a pound and put it in a bag - the samples come from both bare patches and patches with vegetation. The samples are sieved in the lab to remove all stones larger than 2 mm. The remaining material is crumbled, leaving you with fine soil. One of the most important measurements is the organic carbon content: to get this you add chemicals to the sample which react on heating. The colour of the resulting solution is an indication of the percentage of organic carbon in the soil.'

This organic matter, originating from dead plant matter, enriches the soil, providing for more life in it and thus a better soil structure. This enables better infiltration so that the soil is able to retain more water. This, in turn, helps vegetation develop and leads to less erosion. ‘A positive feedback. However, on patches without vegetation a negative feedback is seen: water is unable to infiltrate the soil because of crust-forming, and so it flows off and can lead to erosion.' The less organic matter contained in the soil, the more sensitive it is to erosion.

Lesschen carried out another test to check erosion sensitivity. ‘Small aggregates are taken, little clumps of soil measuring about 4 to 5 millimetres, and these are placed on a sieve. From a height of about a metre we let water drip down on them, and count the number of drops required before the aggregate breaks up. Only seven drops are required to break up an aggregate from a recently abandoned area, while semi-natural soil requires more than fifty drops before falling apart. This difference is probably due to the organic carbon, which makes the aggregates more stable. Organic matter in the soil is thus a very important parameter for erosion sensitivity.'

Trench erosion

Photo: Bob Bronshoff

In the past, terraces were built to create arable land, thereby forming level steps on a slope. The most important discovery Lesschen has made to date is that as soon as these terraces are no longer maintained, trenches that can lead to serious erosion are formed. ‘These trench-erosion spots are the true erosion hotspots. They provide a connection between the terraces so that water is lost faster and, additionally, extra erosion occurs downstream because of an increase in the force of the water.' Thanks to his investigation, Lesschen is able to predict which terraces are most prone to trench forming. ‘It is precisely on these places that vegetation needs to be planted. Research done by colleagues in Belgium shows that it is best to plant grasses as their fine roots keep the soil in place.'

Scaling up

QuickBird satellietbeeld van het studiegebied in Spanje met bijbehorende gemodelleerde bedekkingsgraad van de vegetatie

‘Researchers take measurements on a small scale in many locations, find an amount of erosion per square metre and then multiply that with the total surface of the area. This leads to an enormous overestimation of the erosion problem', according to Lesschen. ‘It is, of course, true that a large amount of erosion occurs on a slope, but the sediment remains in place at the bottom of the slope where water is probably able to infiltrate it.' Lesschen has combined measurements on various scale levels in order to chart the whole area. ‘The degree of vegetation coverage doesn't really show up on satellite photographs. I compared the satellite images of a number of locations with detailed aerial photographs and made a model that can predict the degree of coverage for the whole area. Ultimately I want to be able to indicate where the most erosion-sensitive areas are.'

Lesschen has just returned from Spain where he spent two weeks supervising a group of freshmen in their fieldwork. ‘That's also part of my job - ten percent of my time is spent teaching. But that's what I like about my work, the combination of lab work, teaching, fieldwork and modelling. It's not four years of monotonous work, there is lots of variation and, moreover, PhD students have a lot of leeway.' Lesschen appears very relaxed for a PhD student. ‘I work normal hours and am on schedule. I hear of other PhD students working 60-hour weeks, perhaps their planning isn't optimal?' He adds laughingly: ‘Come back in about a year, when I'm nearly finished - I'll probably be more stressed by then!'

Published by  Faculty of Science