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Fog in the California Channel Islands: Ecosystem Inputs & Consequences
The influence of fog on ecosystem and hydrological processes has long interested scientists in coastal zones, especially in Mediterranean-type climate regions like California, Chile, and South Africa (Marloth 1903, 1905; Prat 1953; Oberlander 1956; Stone 1957; Parsons 1960; Schemenauer and Cereceda 1991; Ingraham and Matthews 1995; Weathers and Likens 1997; Weathers 1999; Weathers et al. 2000; Dawson 1998). In these climates, fog is prevalent in the summer season when rainfall is rare and temperatures are highest. On this basis alone, a number of researchers have inferred an important role for fog in the ecological functioning of coastal ecosystems, since it potentially ameliorates temperature and water stress. Indeed, several native (and extremely rare) California conifers are restricted to fog belt areas along the California coast. Scientists long ago surmised that the presence of summertime fog also provides critical direct water inputs to these unusual forests.
Fog can enhance the water status of vegetation in at least three distinct ways (Dawson 1998; Weathers 1999). First, fog may deposit directly on leaves and stems and subsequently drip to the soil, increasing the total water input to an ecosystem beyond precipitation alone. Second, leaves may directly absorb intercepted fog water (Kerfoot 1968; Leyton and Armitage 1968; Boucher et al. 1995; Yates and Hutley 1995; Hutley et al. 1997). Third, fog may change the system energy balance by reducing solar heating and increasing relative humidity above and within a plant canopy, thus reducing transpirational losses during photosynthetic gas exchange (Dawson 1998; Weathers 1999).
In addition to its influence on ecosystem water status, fog can enhance nutrient delivery to an ecosystem through deposition of inorganic and organic nitrogen and base cations (Vitousek et al. 1989; Clark et al. 1998; Carrillo et al. 2002). The pioneering work of Weathers and Likens (1997) and Weathers et al. (2000) demonstrated fogwater nutrient inputs to coastal vegetation in Chile and suggested a connection between oceanic and terrestrial nutrient cycling.
A final, albeit more subtle, effect of fog on plant ecological functioning is the way it modifies the local solar radiation regime. A great deal of work in plant ecology and ecophysiology has demonstrated consistent patterns in leaf traits that are related to variations in solar irradiance (e.g., sun vs. shade leaves) (Bjorkman 1981). These traits include leaf morphology (specific leaf area, SLA, the ratio of leaf area to mass -- Evans and Poorter 2001; Ackerly et al. 2002), foliar chemistry (nitrogen and pigment concentrations -- Field and Mooney 1986; Demmig-Adams and Adams 1996), and carbon metabolism. Coastal vegetation in Mediterranean climate regions experiences dramatically different radiation regimes from most other temperate vegetation. The rainy season occurs from December to April ('winter green'), yet this is also the period when total solar radiation is reduced and solar angles are lower. Added to this, Mediterranean climate ecosystems that experience persistent summer fog inundation probably experience yet more variation because fog (1) reduces the quantity of solar radiation reaching vegetation through light absorption and scattering and (2) changes the quality of solar radiation by scattering light, increasing the proportion of diffuse radiation and thus shifting the radiation spectrum to shorter wavelengths (blue light).
The Channel Islands of California off the Santa Barbara coast experience regular cycles of fog inundation, particularly in late spring/early summer at the beginning of the dry season. Such fogs are called 'advective' fog banks because they form when moist ocean air is drawn to the coast during intense solar heating of California's central valley (Ingraham and Matthews 1995; Dawson 1998). When this moist air encounters cold, upwelled waters off the California coast, condensation occurs and fog banks form. This fog has an isotopic composition that is typically distinct from that of precipitation. This difference occurs because advective fog is a primary-stage condensate, whereas precipitation comes from clouds that have condensed multiple times (Gonfiantini and Longinelli 1962; Ingraham and Matthews 1995). The different temperatures associated with fog formation and precipitation also influence the isotopic composition of these condensates. Fog formation occurs at warmer temperatures than precipitation because of the condensation altitude (at the surface for fog versus higher up in the atmosphere for precipitation) and the season of condensation (summer versus winter for fog and precipitation, respectively).
Santa Cruz island (SCI) is the largest and most biologically diverse of the Channel Islands. There are at least two ecological phenomena on SCI that are likely related to the presence of advective fog banks. The first is the presence of several stands of Bishop pine ( Pinus muricata ) on the island. These stands are among the southernmost populations of this species. Anecdotal observations made by the University of California Natural Reserve station manager on SCI suggest that the locations of these pine stands correspond closely to the areas that experience the densest and most frequent fog formation (L. Laughrin, personal communication). These needle-leaved trees also 'harvest' a great deal more water from fog deposition than do nearby broad-leaved trees.
The second phenomenon was thoroughly documented by M. Hochberg (1980). She showed that island populations of three common, taxonomically unrelated chaparral species, Ceanothus megacarpa, Dendromecon rigida, and Prunus ilicifolia, had larger leaves and more total leaf area per plant than their mainland counterparts. The size differences were statistically significant, with island populations having leaves that ranged from 1.5 to 3 times the size of mainland leaves (Hochberg 1980). Hochberg (1980) also measured climate variables at mainland and coastal sites and found slightly lower mean monthly maximum temperatures and vapor pressure deficit (VPD) ranges, as well as slightly higher relative humidities, on the islands as compared to the mainland. Thus, although leaf size was much higher on the islands, this asymmetry was not accompanied by strong climatic gradients between the mainland and island sites. Furthermore, Hochberg (1980) states that common garden experiments do not alter the fundamentally different morphology of island species, suggesting that the differences are genetic and result from strong adaptive pressures. Significantly, however, Hochberg (1980) did not measure fog inputs to the island sites or comprehensively assess fog frequency, although she does state that fog is likely to account for at least some of the observed differences in leaf size and relative humidity.
Thus, there is strong circumstantial evidence that fog influences vegetation distribution and function on SCI. I am establishing a research program to improve our basic understanding of the role that fog plays in plant and ecosystem processes in Channel Islands National Park.
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