Available online at www.notulaebotanicae.ro Not Bot Horti Agrobo, 2012, 40(2): 09-13 Print ISSN 0255-965X; Electronic 1842-4309 Notulae Botanicae Horti Agrobotanici Cluj-Napoca Forest Hydrology, Soil Conservation and Green Barriers in Canary Islands Juan Carlos SANTAMARTA-CEREZAL1*, Juan GUZM?N2, Jonay NERIS3, Maria P. ARRAIZA4, Florin IORAS5 1Universidad de La Laguna, Technical Superior School of Civil and Industrial Engineering, Spain; jcsanta@ull.es (*corresponding author) 2Forestry Consultant, Canary Islands, Spain 3Universidad de La Laguna, Department of Soil Science and Geology, Spain; jneris@ull.edu.es 4ETSI Montes, Universidad Polit?cnica de Madrid, Spain; paz.arraiza@upm.es 5Buckinghamshire New University, High Wycombe, UK; Florin.Ioras@bucks.ac.uk Abstract In volcanic islands, the rainfall regime and its torrential nature, together with the steep slopes and the soil types present are considered to be some of the main factors aecting forest hydrology and soil conservation. In such environments, rain regime is generally irregular and characterized by short and intense rainfalls, which could cause destructive ows at times, followed by long periods of rain absence. e volcanic nature of these islands have as a direct resultant steep slopes which inuences the runo volume and speed, as well as the amount of topsoil susceptible to be detached and transported downstream. e soil type also aects the susceptibility to erosion processes. Andisols are the most typical soil on volcanic islands. eir particularities derive their mineral constituents, called short-range-order products, which provide these soils with an increased structural stability, which in turn reduces their susceptibility to erosion. However, the land use changes and the environmental factors such as rain regime and steep slopes may be determinant factor in destabilizing these soils and ultimately a cause for soil erosion and runos, which become a threat to the population downstream. Green barriers have been traditionally used to prevent or reduce these processes, also to enhance the dew eect and the fog water collection, and as a rebreak which acts as a barrier to slow or stop the progress of a wildre. Wooded species present and subsequently their performance have a major inuence on their eectiveness. e use of this natural erosion and re control methods on volcanic islands is discussed in this paper. Keywords: Canary Islands, erosion control, forest hydrology, green barriers Forest hidrology and soil conservation Two main factors inuence the forest hydrology and the soil conservation in volcanic islands, the rain regime and the morphology and topography of these environ- ments. e torrential regime of the rain is typical for some volcanic regions. is characteristic leads to an irregular behaviour of the ravines? ow, with dry stages followed by high ow stages corresponding to rainstorms. Under these conditions, present for example in Canary Islands, it is common the occurrence of torrential events, which involve noticeably erosion processes. e geological nature and formation of these areas is the second key factor aecting the forest hydrology and the erosion processes in volcanic islands. As a result, the steep slope hillsides are typical of these areas. In these con- ditions, inltration is reduced and runo aer a rainstorm transports most of the detached material from the hillsides to the ravines, resulting in the formation of rills and gul- lies. ese slope conditions also lead the runo speed and its detachment and transport capacity. When ravines? ows are not properly redirected and evacuated, water and mostly the transported materials rep- resent a danger for distant settlements or incorrectly built or maintained infrastructures. Erosion is a natural physical-chemical process which detach, transport and deposit soil and rock materials from the earth surface. Human actions could promote but also modify and correct the eect of this process. Most erosion processes are result of the action of natu- ral factors such as temperature changes, gas eects, water and wind ow, gravity or animal and vegetation actions. Water and wind are the main factors controlling erosion in volcanic islands. Erosion depends on: Rainfall volume and intensity;? Slope and topography of the area;? Vegetation cover characteristics;? Soil physical and chemical properties.? Rainstorms are the driving force of water erosion pro- cesses. Rain drops impact the soil surface and detach soil particles. If rain intensity is larger than inltration capac- ity, runo processes starts in a laminar regime, transport- Santamarta-Cerezal J.C. et al. / Not Bot Horti Agrobo, 2012, 40(2):09-13 10 Green barriers establishment Green barriers are natural barriers used in town plan- ning both for aesthetics and noise insulation porpoises. ese tree barriers conceal any visual impact related for example to mining or aggregate extraction, but also they act as noise barrier for such activities depending on plant species used and performance. Green barriers developed in Gran Canaria (Canary Islands, Spain) have become a major environmental de- fence measure due to their ecological, hydrological, socio- economic importance and their forest protection action. Similar actions focused on the restoration of the ?gallery forest? from La Gomera (Canary Islands, Spain) have been also carried out. In general, green barriers are developed along the beds of ravines present in the island. Regarding to the potential vegetation for these areas, mono specic formations such as palmeral-Phoenix canariensis Chabaud, sauced-Salix canariensis C. Sm., tarajalera-Tamarix canariensis Willd prevail. ?Gallery vegetation? is a typical Monteverde forma- tions composed of evergreen trees and shrubs such as Til (Ocotea foetens (Aiton) Baill), Vi??tigo (Persea indica (L.) Spreng) or Barbusano (Apollonias barbujana Nees). ese ecosystems show a high plant and animal biodiversity and incorporate a positive visual eect on the scenery (Fig. 2). e green barriers developed in Gran Canaria are in- tended to replace the vegetation present in the bed of the ravines. ese formations are usually far from the poten- tial vegetation and include non indigenous, invasive or re-prone species, which pose a signicant threat to the safety of the population. ing these detached material. is initial ow could be transformed in rill and gully erosion if the runo volumes and speed, soil characteristics and topography conditions are appropriate. is kind of erosion processes are dicult and expensive to correct. e occurrence of gully erosion is intimately associated to the soil nature, but also to inappropriate practices and land use. is kind of erosion is the most severe type of erosion, Globally, erosion processes are considered nowadays as one of the main environmental problem related to the loss of forest cover. is process could be divided into dierent stages. It starts with the detachment of soil particles by the raindrop impact or the runo ow forces, continues with the transport of these material by some natural agent (wa- ter, wind, etc.) and ends with the deposition of the soil. In volcanic islands, the last process usually takes place at the nal stretch of the ravine, close to the coast. Deposits with more than 100 m depth have been described in ravines from La Gomera (Canary Islands) (Fig. 1). In Canary Islands, around 329.000 ha (43% of the soil surface) are aected by severe erosion processes due to the wind and water action. In this region, soil losses range from 10 to 50 soil tons per year, with the maximum recorded rate of 200 tons per year (Boli et al., 1991). Fuerteventura and Gran Canaria are the main aected islands with dra- matic erosive eects in some parts. Intense rainstorms in the ravine header lead major ero- sion processes. e main factors aecting oods and mud- ows are related to the rainfall and catchment characteris- tics, water erosion and channel dynamic. Fig. 1. Las Angustias ravine in La Palma (Canary Islands) Santamarta-Cerezal J.C. et al. / Not Bot Horti Agrobo, 2012, 40(2):09-13 11 e main objectives of the green barriers are: Erosion reduction and ood prevention? In Canary Islands, erosive processes are developed mainly along the ravines, due to the steep slopes present and related to rainstorms. In this situation, the absence of vegetation cover leads major soil losses, which could be mitigated by the development of green barriers. ese structures protect the soil by reducing the raindrop impact and the runo speed, diminishing the erosion on the bed of the ravine. Dew eect and condensation rainfall increase? Although the importance of the ?fog or horizontal rainfall? is well established in Canary Islands, the dew eect has been less studied. e dew eect is a physical- meteorological phenomenon that involves the condensa- tion of the air humidity due to a sharp diminution of the air temperature or to the contact with colder surfaces. In general, this condensation process takes place on the veg- etation cover (leaves, branches and steams) or on the soil surface. Despite its importance as water supply mainly in ravines and forested hillsides exposed to the trade wind, the dew eect is partially unknown in Canary Islands (Santamarta Cerezal, 2012a). Forest re prevention? Green barriers are mainly composed by tree species with low combustion index, which prevent or reduce the re advance through the ravine?s bed and acting as re- breaks. Ecological lters? Green barriers act as lters between the streams and the surrounding environment by preventing or reducing the incorporation of agrochemicals or organic compounds. Besides, green barriers acts as ecological and socioeco- nomic factors due to their high biodiversity and their func- tion as supply of forest products to the surrounding com- munities. e tree species characteristics will depend on the eco- logical and socio-economical conditions of the study area. In general, wood species will predominantly be represent- ed by shrubs. e ravine could be divided into dierent types, de- pending to the surrounding area and its inuence as fol- lows: of Agroforestry inuence;? of Silvopastoral inuence;? of Wet rainforest inuence;? of ?Gallery forest? inuence;? of Dry rainforest inuence;? of ermophile forest inuence.? Fire-prone species such as pine (Pinus canariensis C. Sm.) or heath (Erica arborea L.) should be dismissed due to their high capacity of ignition. e species selection will take into account that in ravines, the potential vegetation transitions are usually more extensive. e ecological and environmental homogeneity of the ravines provide excellent conditions for the develop- ment of re-vegetation actions. However, the invasive spe- cies could become problematic and aect negatively the reforestation success. Bearing in mind this question, the design of the action including the species selection should take into account the competition degree, especially where the Spanish cane (Arundo donax L.) is present, but also the intensity and temporality of the maintenance activities on the plantation. erefore, a well-developed forest plan- ning is needed (Santamarta Cerezal, 2012b). Fig. 2. Green barriers along a ravine?s bed (Gran Canaria) Santamarta-Cerezal J.C. et al. / Not Bot Horti Agrobo, 2012, 40(2):09-13 12 In rainforest areas, the Canarian madro?o (Arbutus canariensis Veillard) and the wax-myrtle (Myrica faya Ait.) have shown excellent results (Fig. 3) in competition with the elm-leaf blackberry (Rubus ulmifolius Schott) and braken (Pteridium aquilinum (L.) Kuhn) (Guzman, 2004). In wet environments, the presence of eld elm (Ulmus minor Mill.) is welcome because it reduces the invasion of foreign vegetation and acts as a vector of rainforest. In lowlands where the Spanish cane (Arundo donax L.) is the dominant species, once it has been harvested, mulch derived from cane has been placed over the soil surface. Due to the regeneration capacity of the Spanish cane, successive clearance actions have been done usually using chemical clearing products with good results. However, these actions have generated some social controversy. In mid and lowlands location, wild olive (Olea ceras- iformis Webb & Berthel) is the most common species used in green barriers (Fig. 4), together with some shrubs such as Orobal (Whitania aristata Pauqui) and Guaydil (Con- olulus oridus L.F.). e experience with azonal species such as Tarajal (Tamarix canariensis Willd.) or Sao (Salix canariensis C. Sm.) shows that it is vital to use seedlings to increase their rooting potential (Guzman, 2008). In those areas where the transformation of the vegeta- tion is more complicated, mechanical or chemical clear- Fig. 3. Canarian madro?o (Arbutus canariensis Veillard) and wax- myrtle (Myrica faya Ait) over invasive vegetation in a hillside Fig. 4. Survival and development of wild olive (Olea cerasiformis) planted with various saps Santamarta-Cerezal J.C. et al. / Not Bot Horti Agrobo, 2012, 40(2):09-13 13 References Boli Z, Bep A, Roose E (1991). Enqu?te sur l??rosion en r?gion cotonni?re du Nord Cameroun. Bull R?seau Erosion 11:127- 138. Guzman J (2004). Informe t?cnico relativo a los proyectos de creaci?n de barreras verdes en barrancos (Green barriers development projects technical report). Gran Canaria. Spain. Servicio T?cnico de Medio Ambiente. Cabildo de Gran Canaria. Guzman J (2008). Creaci?n y mantenimiento de barreras verdes: Experiencia y proyecci?n (Creation and maintenance of green barriers: experience and projection). Proceedings of Conference XV Jornadas Forestales de Gran Canaria. Spain. Santamarta Cerezal JC (Ed.) (2012a). Forest and environmental engineering in island environments: Methods and experiences in Canary Islands. Universidad de La Laguna. Spain. In?dito. Santamarta Cerezal JC (Ed.) (2012b). Hydrology and water resources in volcanic areas. Universidad de La Laguna. Spain. In?dito. ing of the invasive vegetation and increasing the planting density and the size or age of the plant are recommended. Good results have been obtained in these areas using wild olive (Olea cerasiformis Webb & Berthel) and Persian tur- pentine tree (Pistacia atlantica Desf.) with 4 year old sap- lings. In Gran Canaria, the development of the green barriers to improve the ecological and economical quality started in 2004, mainly in ravines of forested areas. Aer the for- est re which broke out in 2007, the prevention actions including the maintenance of the ravine?s bed was extend- ed to reduce the fuel material presence. However, the re- vegetation actions in the ravines were not completed. e ravine morphology and geometry, the current veg- etation and the access and maintenance conditions should be taken into account to develop a green barrier. In gen- eral, it is necessary to carry out clearance actions for the elm-leaf blackberry (Rubus ulmifolius Schott) and Span- ish cane (Arundo donax L.) green barriers types, which is some cases include removal of solid waste (even aban- doned cars). According to the guidelines developed by local gov- ernment, the green barriers will be established only at the beginning and the end of the ravine?s bed, but not in the central part. e plant distribution will be in a quincunx frame with 2-3 meters between the trees. Conclusions Green barriers have been traditionally used to prevent or reduce these processes, also to enhance the dew eect and the fog water collection, and as a rebreak which acts as a barrier to slow or stop the progress of a wildre. Wooded species present and subsequently their perfor- mance have a major inuence on their eectiveness. e tree species characteristics will depend on the ecological and socio-economical conditions of the study area. In gen- eral, wood species will predominantly consist of shrubs. e selection of the main species, the preparation actions needed and subsequent maintenance aer the plantation will also depend on the environment characteristics and socioeconomic factors.