Hydrology and Earth WaterSystem dynamics Sciences, in 8(6), a laurel 10651075 montane cloud(2004) forest © in EGU the Garajonay National Park (Canary Islands, Spain) Water dynamics in a laurel montane cloud forest in the Garajonay National Park (Canary Islands, Spain) G. García-Santos1, M. V. Marzol1 and G. Aschan2 1University of La Laguna, Department of Geography, Ctra. Tacoronte-Tejina 233,1º, 38350 Tenerife, Canary Islands, Spain 2University of Duisburg-Essen, Applied Botany - FB9, Essen, Germany E-mail for corresponding author: [email protected] Abstract Field measurements from February 2003 to January 2004 in a humid (but dry in summer) crest heath wood-land (degraded laurel forest) in the National Park of Garajonay, Canary Islands (Spain), were combined to calculate water balance components. The water balance domain is at the surface of the catchment and is controlled by atmospheric processes and vegetation. This study found that annual water income (rainfall plus fog water) was 1440 mm year-1, half of which was occult (or fog) precipitation, while stand transpiration estimated from measurements of sap flow amounted, annually, to 40% of potential evapotranspiration calculated from measurements of meteorological variables. The positive role of crest laurel forests, which transpire less water than is incoming from rain and fog is emphasised. Keywords: laurel forest, fog interception, stand transpiration, sap flow, hydrological cycle Introduction year1, 2000) (Consejo Insular de Aguas de Tenerife, 2000). Laurel forest, laurisilva, a subtropical vegetation of Recharge is difficult to estimate as the difference term in evergreen trees partly covered by epiphytes, is an ancient the water balance because the main components of the vegetation type, widespread in southern Europe and the hydrological cycle, rainfall, rain and fog intercepted by the Mediterranean region before the climate changes at the end vegetation, the amounts lost by evaporation and transpiration of the Tertiary and the beginning of the Quaternary period. and the changes in soil moisture storage are all known only Today, the remnants of the laurel forest occur predominantly approximately (Kämmer, 1974; Aboal et al., 2000, Marzol, on the northern slopes of the Macaronesian Islands (Azores, 2002). Canary Islands, Cape Green and Madeira) between 800 and Under the climatological conditions prevailing in a cloud 1300 m a.s.l., where stratocumulus clouds transported by belt, evaporation from intercepted water may represent a the trade winds produce a uniform climate with little substantial proportion of the total evapotranspiration from temperature variation (8ºC) and high air humidity (75%) the forest ecosystem from a surface energy balance or throughout the year (Marzol, 1993; Dorta,, 1996). atmospheric perspective. However, this evaporative loss Forest ecosystems are central to the control of erosion may not be of hydrological significance since fog processes and catchment hydrology especially in interception may exceed evaporation (Höllermann, 1981; mountainous regions with steep slopes and relatively high Aschan et al., 1994). In this case, transpiration by the rainfall (Cavelier and Goldstein, 1989). Studying vegetation may account for the principal water loss. hydrological processes in these mountain forests is important Therefore, estimating seasonal and annual transpiration in in assessing the regional hydrological cycle and estimating Canaries laurel forests is essential for calculating stand and groundwater recharge, which is the main water resource of regional water balances. the western Canary Islands, where the upward exploitation Previous studies on evaporation from laurel forest have of natural water resources (Tenerife: 216·106 m3 year1) been focused mainly on a small remnant forest in the Agua exceeds the downward groundwater recharge (201·106 m3 Garcia mountains in Tenerife (Aschan et al., 1997; Jimenez 1065 G. García-Santos, M. V. Marzol and G. Aschan et al., 1996, 1999; Aboal, 1998); until now, appropriate woodland vegetation is protected. The climate in this area hydrological information has been lacking for the large is humid Mediterranean, with annual precipitation of forests in the central part of La Gomera Island. Although 750 mm and a mean annual temperature of 13.6 ºC (Marzol these two islands are close, differences in the density of et al., 1990). vegetation (mostly Erica arborea L. at the crest in La A catchment of approximately 44·10 4 m2 in the central Gomera Island) may cause differences in the water inputs; sector of the summit, exposed to trade winds under the the experimental plot at the north crest in La Gomera at influence of the sea of clouds was chosen at 28.1284590º 1300 m a.s.l is undisturbed, strongly exposed to the trade N, 17.2593380º W (Fig.1). The upper parts of the area are winds and much higher than Aboals plot (820830 m a.s.l) covered with degraded laurel forest humid (dry in summer) in Tenerife; therefore, it is expected that the water balance crest heath forest (Pérez de Paz et al., 1990; Golubic, 2001). is different for the two sites. This forest, degraded by human activities and overgrazing, The present work aims to determine an annual water is more resistant to dry conditions than laurels. With a mean balance of the forest using micro-meteorological techniques height of approximately 10 m, it comprises mainly heather and direct measurements of transpiration from the humid (Erica arborea L.) partly covered by epiphytic bryophytes, crest heath forest stand at the highest altitude of La Gomera wax-myrtle (Myrica faya Ait.), laurel (Laurus azorica Seub. Island. Franco) and, to a lesser extent, holly (Ilex canariensis Poivet). Within the catchment, the experimental plot of 300 m2 is located on a steep slope (3040%), oriented to the Materials and Methods North, close to the mountain crest at 1300 m a.s.l. (Fig.2). In this area, soil depth increases gradually from the top of STUDY AREA AND EXPERIMENTAL PLOT the crest (small rocky area) to the heath forest (0.51 m). The study area is in the Garajonay National Park, on La Tree roots are assumed to penetrate the whole soil profile. Gomera (Canary Islands, Spain) (Fig. 1), where one of the Tree density is 0.13 stems per m2, with a mean height of largest existing remnants of endangered ancient crest heath 9 m, a mean projected crown area of 6 m2 and a mean leaf + Fig. 1. (Left) Location of the Canary Islands and Gomera Island. (Right) Heath and Myrtle forest distribution at the catchment (white surfaces), location of both experimental plot and second pluviometer (+). (Catchment drawn by Ritter) 1066 Water dynamics in a laurel montane cloud forest in the Garajonay National Park (Canary Islands, Spain) Myrica faya Ait. Myrica faya Ait. DHB: 0.24 m DHB: 0.30 m Myrica faya Ait. DHB: 0.30 m Erica arborea L. DHB: 0.16 m TDR probes Erica arborea L. x x DHB: 0.15 m Datalogger Meteorological station (t ower is 12 m high): Erica arbor ea L. Termo-hygrometer Erica arborea L. DHB: 0.16 m Datalogger Pyronometer DHB: 0.18 m Wind sensor and Vane N path Pluviometer Fog collector Fig. 2. Experimental plot in the heath forest, showing distribution and diameter at breast height (DHB, m) of the sample trees (E. arborea with one pair of sap flow sensors = grey circle; M. faya with one pair of sap flow sensors = solid circle; M. faya with two pairs of sap flow sensors = hatched circle) plus location of meteorological station, dataloggers and TDR probes. area index (LAI) of 4.2 ±1 (Golubic, 2001). Average tree diameter at breast height (DHB) is 0.26 m (Fig.2) and the stand basal area is up to 68·104 m2 m2. Myrica faya represents the greatest proportion (57%) of the stand basal area, followed by Erica arborea (33%) and Laurus azorica (10%). Physiological and structural studies on laurel forests in Agua Garcia (Tenerife), where trees are 1015 m high with 5 m canopy thickness, gave a leaf area index of 7.8 (Morales et al., 1996 a, b). Actual climatic conditions were monitored by a Termo-hygrometer meteorological station on a 12 m scaffolding tower at the experimental plot (Fig. 3); there, 2 m above the canopy, relative humidity (%) and air temperature (ºC) were recorded using a humidity and temperature transmitter (Campbells 50Y + T351-RS /HMP45C + URS1 termo-hygrometer), global solar radiation (W m2) was measured by a pyranometer (E.Ms SP1110), wind speed (m s1) by a switch anemometer (Campbells A100R) and intercepted fog water (L m2=mm) by a screen fog collector and rainfall (R , mm) f in a Pronomamic Professional gauge. All variables were sampled at 3 min intervals and recorded every 15 min on a Combilog datalogger and then transmitted by GSM modem. WATER BALANCE The water balance comprises atmospheric and vegetative Fig. 3. Meteorological instruments at the 12 m high station. 1067 G. García-Santos, M. V. Marzol and G. Aschan processes as well as soil moisture storage within the occur at the same time in this area, fog water interception catchment. At the crest, rainfall and fog water are the inputs accuracy may be affected, not only by the fixed orientation (Garcia-Santos et al., 2004); evaporation, transpiration, of the collector but also by the influence of rainfall on fog surface runoff and soil water recharge are the outputs. Water measurements. If fog measurements are collected only interception and soil depression storage are the storages. during periods without measurable rainfall inputs, fog water The general water balance equation for the surface is: can also be underestimated. Isotope analysis of water from QFCs (Friedman, 1956; Mook and Vries, 2001) may help P = Et + SR + R (1) to reduce some of these inaccuracies. In this study, a n a correction factor of 3.6 per m2, estimated by Marzol (2002), where P is the net precipitation reaching the ground surface was used to convert fog water volume to mm input when n through the canopy, Et is the actual evapotranspiration, SR only fog water is collected.