920 JOURNAL OF HYDROMETEOROLOGY VOLUME 9 Fog Water Collection in a Subtropical Elfin Laurel Forest of the Garajonay National Park (Canary Islands): A Combined Approach Using Artificial Fog Catchers and a Physically Based Impaction Model A. RITTER AND C. M. REGALADO Instituto Canario de Investigaciones Agrarias, Tenerife, Spain G. ASCHAN Applied Botany, University of Duisburg-Essen, Essen, Germany (Manuscript received 6 November 2007, in final form 19 February 2008) ABSTRACT Fog precipitation has long been assumed as an additional water source in the relic laurel ecosystems of the Canary Islands, located at 500–1400 m MSL. However, to what extent fog water can contribute to the laurel forest water balance is not yet clear. Combining data from artificial fog catchers and a physically based impaction model, the authors evaluated the potential contribution of fog water captured by needle- leafed Erica arborea L. trees in a selected watershed of the Garajonay National Park (La Gomera Island) for a 2-yr period (February 2003–January 2005). Fog water collection was measured with artificial catchers at four micrometeorological stations placed at 1145, 1185, 1230, and 1270 m MSL. Average fog water collection was only significant at the highest measurement site (one order of magnitude greater than at lower altitudes), totaling 496 L mϪ2 yrϪ1 during the 2-yr period. The average fog water yield in the first and second annual periods ranged between 0.2–5.0 and 0.1–2.1 L mϪ2 dayϪ1, respectively. Rainfall exhibited seasonality, distinguishing between rainy and dry seasons, while fog water collection was distributed more evenly throughout the year. Regarding fog water captured by the vegetation, the impaction model predicted a significant amount of fog water potentially collected by a single E. arborea tree, on the order of 1810–2090 LyrϪ1. Taking tree population density into account, the yearly average water contribution to the soil surface by wind-driven fogs was 251–281 mm, whereas annual rainfall was 635 and 1088 mm, respectively. The hourly course of micrometeorological variables shows a 58% reduction in global radiation under foggy conditions and a concomitant 3°–6°C mean temperature decrease compared to fog-free periods. Thus, limiting evapotranspiration may also be a relevant effect of fog in this subtropical elfin cloud forest. 1. Introduction stacle’s surface merge into bigger drops that flow down over the obstacle and finally drip to the ground. The Fog water deposition has been widely recognized as role of fog precipitation as an additional water source in an important ecological factor in mountain forests fre- tropical and subtropical montane cloud forests has been quently immersed in dense fog (Azevedo and Morgan often emphasized (Kämmer 1974; Zadroga 1981; Ingra- 1974; Stadtmüller 1987; Cavelier and Goldstein 1989; ham and Matthews 1988; Bruijnzeel and Proctor 1995; Schemenauer and Cereceda 1994; Walmsley et al. 1996; Hutley et al. 1997; Chang et al. 2002; Gutiérrez et al. Bruijnzeel 2001). This hydrological process, also known 2008). Additionally, fog may also play an important as fog precipitation, occurs when atmospheric water role not only as an auxiliary water pool but as a mecha- contained in wind-driven fogs impinges upon obstacles nism to reduce solar radiation fluxes, that is, evapora- in their path. The small droplets impacting on an ob- tion (Eugster et al. 2006) or as an income of dissolved nutrients into the soil (Hafkenscheid 2000). Conversely, continuous moistening of leaf surfaces by fog may re- Corresponding author address: A. Ritter, Instituto Canario de Investigaciones Agrarias (ICIA), Apdo. 60 La Laguna, Tenerife duce photosynthetic activity because of the reduced dif- 38200, Spain. fusivity of CO2 in water as compared to air, such that E-mail: [email protected] plants may have developed repellency strategies to re- DOI: 10.1175/2008JHM992.1 © 2008 American Meteorological Society Unauthenticated | Downloaded 09/30/21 02:34 PM UTC JHM992 OCTOBER 2008 RITTERETAL. 921 duce leaf wetness, thus favoring water dripping to the Vegetation factors include (i) the height, size, and soil (Holder 2007) and increasing incident sunlight on structure of the canopy; (ii) the arrangement and shape the leaf surface by the lensing effect of merged water of leaves; (iii) the tree location and population density; droplets (Brewer et al. 1991). and (iv) the presence and morphology of epiphytes. Laurel ecosystems are evergreen subtropical forests The climatic factors mainly refer to (i) the wind speed that resemble the Tertiary, which due to climatic and direction, (ii) the fog frequency, and (iii) the fog changes have largely disappeared from southern Eu- properties (particularly the droplet size distribution and rope and North Africa. These forests survived the Qua- liquid water content). Orography, elevation, and orien- ternary ice age, seeking refuge in the favorable climatic tation are terrain-related characteristics that also influ- conditions of the Macaronesian regions (Axelrod 1975; ence fog precipitation (Stadtmüller 1987; Cavelier and Gioda et al. 1995). Microclimatological conditions in Goldstein 1989; Cavelier et al. 1996). Attempts to mea- these relic forests are unique. The persistent northeast sure fog water interception have been conducted for trade winds in this area are forced up by the mountain many years and in different places around the world slopes of the islands, thus leading to cooling, conden- with variable results being reported (Goodman 1982; sation, and the formation of an almost permanent layer Bruijnzeel and Proctor 1995; Bruijnzeel 2001). Quanti- of clouds, which is prevented from rising up on the fying fog water captured by vegetation is, however, not north side of the islands because of the existence of a an easy task. Traditionally, two basic methods have temperature inversion between 900 and 1500 m been established for measuring fog precipitation: arti- (Höllermann 1981; Sperling et al. 2004). This northside ficial fog catchers and throughfall measurements. How- cloud belt, locally known as “mar de nubes” (sea of ever, both methods exhibit problems concerning imple- clouds), provides low evaporation and favorable fog mentation and data interpretation. Artificial fog catch- formation conditions. This may help to sustain these ers provide an indication of the frequency and amount evergreen cloud forests in windward areas at middle of fog that can be potentially captured. The problem elevations in an otherwise arid environment like that of using artificial fog catchers is that each forest canopy the Canary Islands situated at 28°N off the Sahara represents a unique situation that cannot be fully char- coast. However, this hypothesis has not been thor- acterized with a fog water collector. This method has oughly investigated, and it has become a poorly proved therefore been mainly used for site characterization fact that fog water captured by vegetation is responsible (Bruijnzeel and Proctor 1995; Bruijnzeel 2001). The for the survival of the laurel forest. Different authors throughfall measurement approach measures the have commented on the significance of fog occurrence amount of water dripping inside the stand. This is, in for the laurel forest in the Canary Islands. For instance, most cases, a better method because it takes into ac- Kämmer (1974) concluded that fog precipitation was of count the amount of water intercepted and evaporated minor ecological importance for most plant species of from the wetted canopy. Large numbers of gauges are the laurel forest ecosystem, being only significant dur- needed to obtain frequent and spatially representative ing the dry season at the edge of the stand or on very data (e.g., Lloyd et al. 1988), although a “roving” strat- steep slopes, but on a small spatial scale. Lately Santana egy may help to minimize the number of gauges nec- (1986), in support of these observations, reported that essary to quantify net precipitation (Holwerda et al. single isolated laurel trees captured more fog water 2006). Other alternative methodologies have also been than those in dense forest areas or located in ravines. used for quantifying fog deposition, such as the eddy Conversely, Aboal Viñas (1998) claimed that no rel- covariance measuring systems (Vermeulen et al. 1997; evant fog precipitation occurred in the Agua García Wrzesinsky et al. 2001) or the monitoring of weight laurel forest in Tenerife during his observation period. changes of a known mass of living mossy epiphytes sus- On the other hand, Höllermann (1981) suggested that pended below the canopy (Chang et al. 2002). By taking the reduction of incoming solar radiation by fog may, in into account the different concentrations of Na, 2H, and fact, play a much more important ecological role than 18O isotopes between rain and fog water, either a so- the interception of fog droplets. Without taking into dium or isotope mass balance approach remains as an- account the complexity of the fog collection phenom- other method to quantify the relative contribution of ena, it is not yet clear to what extent fog water can fog, with some limitations (Bruijnzeel 2001; Fischer and significantly contribute to the laurel forest. Still 2007). Other methods rely on measurements of fog The amount of water produced by fog precipitation is liquid water content and fog droplet distributions, de- highly site dependent (Walmsley et al. 1996), based termined either directly (Eugster et al. 2006) or indi- partly on vegetation properties, climatic factors, and rectly from horizontal visibility determinations (Klemm terrain characteristics (Bruijnzeel and Proctor 1995). et al. 2005). Whereas the former requires expensive Unauthenticated | Downloaded 09/30/21 02:34 PM UTC 922 JOURNAL OF HYDROMETEOROLOGY VOLUME 9 high-power supply equipment that is difficult to install mainly composed of Laurus azorica (Seub.) Franco, in unattended remote areas, the latter suffers from the Persea indica (L.) K. Spreng, and Ilex canariensis Poir.; large uncertainty of reported visibility versus air liquid 2) “hillside laurel forest” represents a transitional veg- water content relationships (Klemm et al.