Mediterranean Environmental Research Group (GRAM)

The Mediterranean Environmental Research Group, (GRAM) from the University of Barcelona has over 20 years of working experience in the field of the effects of forest fires on soil properties. In 1998 the doctoral thesis entitled “Fire effects on soil properties, the role of fire intensity” carried out by Xavier Úbeda emphasized the importance of fire intensity impacts on soil physico-chemical properties and the consequent implications, as the increase of runoff and erosion in post-fire environments. From this thesis some papers were published in national and international journals. This work was funded by two European research projects related to forest fires, as the “Post fire soil and vegetation dynamics in natural and afforested areas in Southern Europe: The role of fire intensity.” The most important results were the reaffirmation of the importance of fire intensity impacts on soil properties, the increase of erosion and the implications on vegetation recuperation.

Plot located in a urban-forest interface area (see
Plot located in a urban-forest interface area (see

In the last twelve years, the GRAM members worked intensively in the study of prescribed fire impacts on soils.  Samples were collected before, immediately after and one year after the prescribed fire experiments in order to observe the impact of this type of landscape management on soil properties, mainly in nutrients behavior. This research was possible through collaboration with the GRAF (Grup de Recolzament to Actuacions and Forestry) from the Generalitat de Catalunya, which carries more than twelve years conducting controlled burns for forest management. Two projects from Spanish Ministry have funded this research: “Alterations of environmental quality in fire-affected soils in Mediterranean environments. The study of hydrophobicity, and development of new techniques to evaluate, and mitigate degradation“and the project: “Assessment of the quality of Mediterranean soils affected by the heat to medium and long term, applying an index of environmental quality“. From these projects, two theses were directed by the person who signs this petition (Dr. Xavier Ubeda). Both were defended in 2010. Luís Outeiro: “Geostatistics and environmental management; studies and applications of the spatial and temporal variability in soil and water “and Paulo Pereira:” Effects of fire temperatures on the chemical and physical characteristics of the Mediterranean species ash and their effect on water quality”. From the thesis of Luis the most relevant results were that based on geostatistical analysis, that low-intensity prescribed fires do not cause important variations on soil properties, but the repetition of this technique can be harmful, if carried out in a short time period. From the thesis of Paulo it was observed that fire temperatures and severity have an important effect on the ash physico-chemical properties, which will influence temporarily the type and amount of nutrients in the soil and available to landscape recuperation.

Studies on soil erosion (see
Studies on soil erosion (see


In the project funded by the Ministry of Environment of the Government of France with the title “Dynamique des paysages, érosion développement durable et dans les montagnes méditerranéennes” some interviews were carried out with stakeholders engaged in forest management. The stakeholder’s interviewed were staff from councils and consortia, councils, Forest Ownership Center and Forest Technology Centre of Catalonia. Using these interviews, Roser Rodriguez is currently doing a doctoral thesis entitled “Socioecology wildfire: an approach to environmental sociology wildfires central Catalonia.”

The GRAM has organized several national and international conferences, as the ‘ International Meeting of Fire Effects on Soil Properties “held in 2007 in Barcelona ( and in 2013 in Vilnius ( Other international congresses were organized by the group in several European Geoscience Assemblies.

The Members of the group have participated as “guest editor” in three special issues: “Fire Effects on Soil Properties” Catena. 2008 Vol 74 Issue 3, “Fire Effects on Soil Properties: Forest Fires and Prescribed Fires”. Environmental Research. 2011 Vol. 111. Issue 2 and “Incendis and Forestry” 2012. Treballs of the Catalan Society of Geography. Other three Special issues are ongoing, “The role of ash in fire-affected ecosystems: A physical, chemical and biological approach (Catena)”, “Soil processes in cold-climate environments (Solid Earth)” and “Soil mapping, classification, and modelling: history and future directions (Geoderma)”.

More information on the website of GRAM Group (


This post has been published also in the EGU Blog Network.

Cold soil in the groove

Soil polygons in the Tundra. Photo by Sebastian Zubrzycki. Click to see the original image at Imaggeo.

Often, soils from cold regions as Arctic soils, show polygonal forms in their surface. These polygons are formed because of the freeze-thaw cycle, characteristic of permafrost.

What is permafrost?

Permafrost is a subsurface soil layer which stays permanently frozen (below 0 oC) during long periods of time, usually more than two consecutive years.

Circum-Arctic Map of Permafrost and Ground Ice Conditions. Credit: J. Brown, O.J. Ferrians Jr., J.A. Heginbottom and E.S. Melnikov. Click to see the original image at Wikimedia Commons.

Most extensive permafrost areas can be found in circumpolar areas from North America (Canada and Alaska), Asia (Siberia), Europe (Norwich), cold continental areas (Tibet) and some islands (South Georgia and the Sandwich Islands, in the Atlantic Ocean). And in Mars!


Phoenix landing-day image near north pole of Mars showing flat terrain, containing what appears to be a polygonal pattern, stretching from the foreground to the horizon. Credit: NASA/Jet Propulsion Lab/University of Arizona. Click to see the original image at Wikimedia Commons.

The soil layer above the permafrost (known as the active layer) is the part of soil that thaws during the warm season and freezes again at the beginning of the cold season, because the influence of air temperature is greater in the first centimeters of soil. Commonly, the thickness of the active layer may vary between 10 and 100 cm depending on the season, aspect, vegetation, soil texture and proximity to water bodies.

ermafrost landscape. Photo by Reinhard Pienitz. Click to see the original image at Imaggeo.


How do polygons form?

Cryoturbation is one of the main processes in the soil active layer. As a consequence, the soil surface in these cold areas often shows polygons, circles, steps and stripes formed by stones and fine sediments.

Stone rings on Spitsbergen. Photo by Hannes Grobe. Click to see the original image at Wikimedia Commons.

Repeated groundwater freezing/thawing cycles causes contraction/expansion of soil material, that forces the displacement of coarse gravels and stones over the soil surface. Areas with fine sediments (with low porosity) show larger water contents than those areas where coarse fragments accumulate.

Polygon ponds in Arctic tundra soils. Photo by Reinhard Pienitz. Click to see the original image at Imaggeo.

As a result, water-saturated finely textured soil expands and contracts more easily during freezing/thawing cycles than coarsely textured stony areas. In the long term, stone polygons and other patterns may appear on the soil surface. Fine sediments in the center of polygons usually form ponds and small bogs.

Thawing permafrost in Siberia. Photo by Guido Grosse. Click to see the original image at Imaggeo.

Know more

Christensen, P.R. 2006. Water at the poles and in permafrost regions of Mars. Elements 2, 151-155. DOI: 10.2113/gselements.2.3.151.

Dobinski, W. 2011. Permafrost. Earth-Science Reviews 108, 158-169. DOI: 10.1016/j.earscirev.2011.06.007.

Gruber, S. 2012. Derivation and analysis of a high-resolution estimate of global permafrost zonation. The Cryosphere 6, 221-233. DOI: 10.5194/tc-6-221-2012.

Guglielmin, M. 2012. Advances in permafrost and periglacial research in Antarctica: A review. Geomorphology 155-156, 1-6. DOI: 10.1016/j.geomorph.2011.12.008.

Haeberli, W. 2013. Mountain permafrost – research frontiers and a special long-term challenge. Cold Regions Science and Technology 96, 71-76. DOI: 10.1016/j.coldregions.2013.02.004.

Haeberli, W., Noetzli, J., Arenson, L.b Delaloye, R., Gärtner-Roer, I., Gruber, S., Isaksen, K., Kneisel, C., Krautblatter, M., Phillips, M. 2011. Mountain permafrost: Development and challenges of a young research field. Journal of Glaciology 56, 1043-1058. DOI: 10.3189/002214311796406121.

Langer, M., Westermann, S., Muster, S., Piel, K., Boike, J. 2011. The surface energy balance of a polygonal tundra site in northern Siberia – Part 1: Spring to fall. The Cryosphere 5, 67, 79. DOI: 10.5194/tc-5-151-2011.

Langer, M., Westermann, S., Muster, S., Piel, K., Boike, J. 2011. The surface energy balance of a polygonal tundra site in northern Siberia – Part 2: Winter. The Cryosphere 5, 509-524. DOI: 10.5194/tc-5-509-2011.

Lin, Z.H., Zhang, Y.H. 2013. The general review of permafrost temperature research methods. Applied Mechanics and Materials 405-408, 158-161. DOI: 10.4028/

McClymont, A.F., Hayashi, M., Bentley, L.R., Christensen, B.S. 2013. Geophysical imaging and thermal modeling of subsurface morphology and thaw evolution of discontinuous permafrost. Journal of Geophysical Research F: Earth Surface 118, 1826-1837. DOI: 10.1002/jgrf.20114.

Wade F.A., De Wys, J.N. 1968. Permafrost features on the martian surface. Icarus 9, 175-185. DOI: 10.1016/0019-1035(68)90011-0.

Xie, s., Qu, J., Zu, R., Zhang, K., Han, Q., Niu, Q. 2013. Effect of sandy sediments produced by the mechanical control of sand deposition on the thermal regime of underlying permafrost along the Qinghai-Tibet railway: Land Degradation and Development 25, 453-462. DOI: 10.1002/ldr.1141.

Zhao, L., Jin, H. , Li, C., Cui, Z., Chang, X., Marchenko, S.S., Vandenberghe, J., Zhang, T., Luo, D., Guo, D., Liu, G., Yi, C. 2013. The extent of permafrost in China during the local Last Glacial Maximum (LLGM). Boreas. In press. DOI: 10.1111/bor.12049.

This post was also published simultaneously in the EGU Blog Network.

Soils at Imaggeo: Patterned sand

Alma de Groot, The Netherlands

Patterned sand, by Alma de Groot. Click to see the original picture at Imagego.

Dunes are wind-generated accumulations of sand particles present in desert or coastal land. Sand dunes have smooth and uniform forms, although geometry may be highly variable. The size of particles of sand dunes is highly concentrated around 0.2 mm in diameter due to wind transportation.

The pattern showed in the picture is the result of the formation of aeolian dunes on the beach, that subsequently were wetted by rain, and finally were eroded by wind again. Small pieces of shell debris and other organic material protect the sand locally. The pictured area is about 30 cm across.


This post was also published simultaneously in the EGU Blog Network.

Ladies and gentlemen: the Rolling Stones

Racetrack Playa valley. Photo by Jon Sullivan. Click on the image to see the original image at Wikimedia Commons.

Racetrack Playa is a plain without vegetation of a dry located above the northwestern side of Death Valley, in Death Valley National Park, Inyo County, CA, USA (click here to see in Google Maps). Although “playa” is the Spanish word for beach, it is also used in English to refer to a dry lake. Racetrack Playa occupies an area of 4.5 km (north-south) by 2 km (east-west) which is 1,130 m above sea level between the Cottonwood Mountains and Last Chance Range. The surface is extremely flat and dry for most of the year, when the surface is covered with small hexagonal mud curls, although floods partially during the rainy season forming a shallow lake that evaporates quickly. In winter it forms a relatively thick layer of ice.

The sailing stones

Despite its geomorphological and environmental interest, Racetrack Playa is known around the world for the phenomenon of sliding rocks or sailing stones, because on the surface of the basin appear scattered stones leaving a trail behind him, so it seems that something or someone had dragged over the surface of the ground without anyone’s seen them move ever. The phenomenon is so striking that it has been “investigated” by pseudoscientists who have attributed the movement of the stones to energy phenomena, gravity field anomalies, extraterrestrial activity and other funny hypotheses.

Sailing stone in Racetrack Playa. Photo by Laurence G. Charlot. Click on the image to see the original picture at Wikimedia Commons.

Looking for answers

The first scientific approaches to the study of this geomorphological process suggested the hypothesis that wind was the main cause of stone movements. Louis G. Kirk, a National Park Service Ranger speculated that local strong winds caused the movement of stones over the muddy surface after heavy rainfall. An experiment was conducted by Jim McAllister and Allen Agnew (USGS) in 1948, who had the idea that the movement of the stones was due to strong winds blowing over the flooded surface. The two researchers flooded a small part of the plain and a used an aeroengine to create a strong air flow to move the stones, but failed to replicate the natural result. Moreover, local winds can reach 150 kilometers per hour, but not enough to move some of the stones, which may weigh hundreds of kilograms in some cases.

Stones with divergent trajectories. Photo by Daniel Mayer. Click on the image to see the original picture at Wikimedia Commons.

During the following decades, the researchers could not explain the nature of this phenomenon, although it was suggested a possible link with the ice layer formed on the lake at certain times of the year. John Reid (Hampshire College) and his team also reported that wind alone is not enough to move stones, and hypothesized that the ice layer was pushed by the wind during the winter, dragging the stones. But Paula Messina (San José State University) analyzed the trajectory of different stones using GPS. She noted that some trails were linear, suggesting the influence of wind, but other are curve or irregular. She estimated that wind velocity needed for stones to move this way was of several hundred kilometers per hour. You can visit Paula’s website for more complete information.

Undecided sailing stone. Photo by Jon Sullivan. Click on the image to see the original image at Wikimedia Commons.

Ice and wind

More recently, Ralph Lorenz (Johns Hopkins University) and his team replicated the phenomenon in a very simple way. He realized that in some cases, rocks contrails direction abruptly changed when crossed with each other, as if the rocks had hit and taken different directions. The only way this happens is that there is a mass of ice around each rock upon impact with another, without stopping deviate due to the low coefficient of friction of ice. He tested his theory in his own house with stones, the freezer and a couple of tupperwares… and stones moved!

Details of Ralph Lorenz’s home experiment (Lorenz et al., 2008). Click to see larger image.

Lorenz’s team suggested that the movement of stones in Racetrack Playa is due to the effect of weak winds on buoyant stones that are included in “ice cakes”, as also occurs in arctic tidal beaches. Ice cakes allow the stones to move over the flooded bed.
Stones arrive at the playa from the slopes around or by other processes. During the rain, water has no outlet possible, so that it accumulates and the area is flooded. If the temperature is low enough, a layer of ice is formed on the surface of liquid water. The stones partially embedded in the floating ice rise slightly above the bottom with the increasing level of water. Both the friction between the ice and water and between the stones and the bed are very small, so that blowing wind with some intensity pushes the ice (and the rocks embedded). If the stones and mud at the bottom have a light touch, the dragged stones leave a trail that remains once the ice has melted and the water has evaporated.
Why moving stones have not been observed? According to Lorenz, “movement happens for only tens of seconds, at intervals spaced typically by several years”, and “this would demand exceptional patience as well as luck” (see comments here). So, the rocks are probably traveling on the coldest and windiest days that occur over a period of several years. The most likely time would be in the very early dawn. Do you dare?

Know more

Bacon, D., Cahill, T. and Tombrello, T.A. 1996. Sailing Stones on Racetrack Playa. Journal of Geology 104: pp.121-125.

Lorenz RD, Jackson BK, Barnes JW, Spitale J, Keller, JM. 2011. Ice rafts not sails: Floating the rocks at Racetrack Playa. American Journal of Physics 79: 37.

Kirk LG. 1952. Trails and rocks observed on a playa in Death Valley National Monument, California. Journal of Sedimentary Petrology 22: 173-181.

Messina P, Stoffer P. 1999. Differential GPS/GIS analysis of the sliding rock phenomenon of Racetrack Playa, Death Valley National Park. In: Slate JL (Ed.), Proceedings of Conference on Status of Geologic Research and Mapping, Death Valley National Park. US Geological Survey Open File Report 99-153: 107-109.

Reid JB, Bucklin EP, Copenagle L, Kidder J, PackSM, Polissar PJ, Williams ML. 1995. Sliding rocks at the Racetrack, Death Valley: What makes them move? Geology 23: 819-822.

Sharp WE. 1960. The movement of playa scrapers by wind. Journal of Geology 68: 567-572.

Sharp RP, Carey DL. 1976. Sliding Stones, Racetrack Playa, California. Geological Society of America Bulletin 87: 1704-1717.

Sharp RP, Carey DL, Reid JB Jr, Polissar PJ, Williams ML. 1996. Sliding rocks at the Racetrack, Death Valley: What makes them move: comment and reply. Geology 24: 766-767.

This post was also published simultaneously in the EGU Blog Network.

Images of soil erosion

Frans Kwaad, physical geographer

Soil erosion is the removal of soil from cultivated land at a rate that is (much) higher than the rate that would occur under the natural vegetation at the considered site. Besides the loss of fertile topsoil, soil erosion entails the dissection of cultivated land by rills and gullies and the deposition of eroded soil material on roads, in residential areas, rivers, ponds, lakes and reservoirs, and it can be accompanied by flooding.

Guly erosion is the formation of gullies that are too deep to be removed by normal tillage. Gullies are formed in unconsolidated soil material. Several types of gullies can be distinguished, a.o. valley side gullies, valley bottom gullies, V-shaped gullies, U-shaped gullies, continuous gullies, discontinuous gullies, arroyo’s, badlands. In the picture: gullies in Rif Mountains (Morocco, 40 km west of Al Hoceima).

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Impact of erosive processes in mountain areas (Southeastern Spain)

María Burguet
Institute for Sustainable Agriculture (IAS-CSIC), Spain

Dique del Granadino (Granada, Spain)

The sediment accumulation in the Rules reservoir represents a threat to its useful time. The Granadino (Guadalfeo river) dam was built in 2002, upstream from the reservoir to retain fluvial sediments, as well as, enabling water for irrigation. However, since 2004 the dam exhibits severe aggradation problems, causing a reduction of 17% of the total reservoir capacity, at the elevation level of the spillway.

The watersheds developed in the low Alpujarras, as well as the Contraviesa and Lujar mountains, present a very high degree of degradation, whereas the watersheds in the south-facing slope of Sierra Nevada show high flood potential. At the microscale it is important to emphasize infiltration and friction processes, caused by surface and subsurface runoff, as well as unconsolidated rock dragging. At the mesoscale level, landslides, creep, liquefaction and gullying processes need to be remarked as sediment sources in the Guadalfeo River.

River Jup basin after fire

Photo submitted by María Burguet, a PhD student at the Institute for Sustainable Agriculture at Córdoba (IAS-CSIC, Spain). His research interests include the sources of sediment, runoff and carbon in Andalusian olive grove at different scales and under different agricultural managements.
This photo was taken shortly after the fire that occurred in the area of Carcaixent-Alzira-La Valldigna (Valencia, Spain) in 2006: “I think it reflects very well the main factors that shape the current Mediterranean landscape: wildfires, soil erosion and the intense ‘re-agriculturization’ that takes place anywhere and at any price. “