Main sources of hydration in lichens and their interaction with light

In order to visualize hydration patterns, the three main atmospheric hydration sources for lichens, rain, water vapour condensation (dew) and humid air, are placed as corners in a triangle (Fig. 2). Each location inside the triangle represents a certain combination of hydration sources. Hydration sources are partly coupled: rain and dew necessarily imply high air humidity, but humid air can occur without the presence of liquid water. Liquid water can also come as fog/mist, but because fog provides droplets of water, fog is included under rain.

Fig. 2. Hydration source model for lichens, visualizing relationships between the three main sources of lichen hydration (rain, dew, humid air) and some important abiotic factors (temperature, light and liquid water availability). Each location inside the triangle represents a certain combination of hydration sources. △T refers to the diurnal temperature amplitude.

Light availability during hydration periods drives lichen photosynthesis and growth (Palmqvist Reference Palmqvist2000), although hydration during nights may also stimulate growth (Bidussi et al. Reference Bidussi, Gauslaa and Solhaug2013a ). Even so, light availability (as shown in Fig. 2) is partly confounded with hydration sources (Stoutjesdijk & Barkman Reference Stoutjesdijk and Barkman1987) because solar radiation is also a driver of surface temperatures and thus strongly influences water loss and air humidity. At physical scales relevant to lichen physiology, the air during daylight hours is often more humid in the shade of forests than in open, well-lit places, but this relationship becomes inverted at night when cooling is strongest outside a shielding canopy (Geiger Reference Geiger1950). This implies that dew is particularly frequent in forest gaps [open shade habitats sensu Stoutjesdijk (Reference Stoutjesdijk1974)] which receive much diffuse sky light but little direct sun. Indirect light stimulates lichen growth (Gauslaa et al. Reference Gauslaa, Lie, Solhaug and Ohlson2006, Reference Gauslaa, Palmqvist, Solhaug, Holien, Hilmo, Nybakken, Myhre and Ohlson2007). In marked contrast, direct exposure to sunlight causes rapid drying (Lange et al. Reference Lange, Büdel, Meyer, Zellner and Zotz2004) and adversely impacts epiphytic lichen growth (Gauslaa & Goward Reference Gauslaa and Goward2012; Bidussi et al. Reference Bidussi, Goward and Gauslaa2013b ), although short-time sunflecks can have positive effects (Lakatos et al. Reference Lakatos, Rascher and Büdel2006; Coxson & Stevenson Reference Coxson and Stevenson2007a ). Light experienced during prolonged dry periods causes much more damage than desiccation per se, and substantially retards photosynthetic activation during subsequent hydration (Lidén et al. Reference Lidén, Jonsson Cabrajic, Ottosson-Löfvenius, Palmqvist and Lundmark2010; Gauslaa et al. Reference Gauslaa, Coxson and Solhaug2012), particularly in epiphytic cephalolichens and shade-adapted chlorolichens (Gauslaa & Solhaug Reference Gauslaa and Solhaug1996; Jonsson Cabrajic et al. Reference Jonsson Čabrajič, Lidén, Lundmark, Ottosson-Löfvenius and Palmqvist2010).

Spatial variation in hydration sources

Regional scales

At regional scales (Fig. 3A), vegetation generally becomes more open and/or well spaced with increasing aridity (e.g. Walter & Breckle Reference Walter and Breckle1984). Under otherwise similar conditions, nocturnal cooling is much stronger in open spaces than inside forests (Stoutjesdijk & Barkman Reference Stoutjesdijk and Barkman1987). Pre-dawn dewfall in open habitats (Fig. 3B–D) increases with decreasing cloud cover (Xiao et al. Reference Xiao, Meissner, Seeger, Rupp, Borg and Zhang2013), increasing nocturnal cooling and thus increasing air humidity (e.g. Geiger Reference Geiger1950). A critical wind speed is needed to maximize dewfall rates; if wind is faster, which is often the case in coastal sites, condensation is offset by turbulent warming; if too slow, water for condensation cannot be replenished from humid air above (Oke Reference Oke1987; Xiao et al. Reference Xiao, Meissner, Seeger, Rupp, Borg and Zhang2013). Strong dew formation is thus more common in inland climates (Fig. 3A). Dew and humid air in particular, are inaccessible to most vascular plants, but useful to lichens. Dry valleys of south-eastern Norway, located in rain shadows behind snow-capped mountains, have merely 300 mm rainfall (Fig. 4) and are surrounded by fairly dry areas (<700 mm according to Moen Reference Moen1999; see Fig. 4). Surprisingly, these low rainfall areas support some oceanic lichens (Tønsberg et al. Reference Tønsberg, Gauslaa, Haugan, Holien and Timdal1996). One example is Menegazzia terebrata (Fig. 4), which mainly grows on tree trunks in rainforests receiving ≥2000 mm rain along the western coast (enclosed by the solid line in Fig. 4; Moen Reference Moen1999). Because canopies reduce the nocturnal cooling required for reaching the dew point, oceanic epiphytes cannot grow on their normal substratum of bark in the dry inland. Yet, they thrive on boulders and rocks in open, low-rainfall inland valleys. Their location in valleys with cold-air ponding ensures some, but little, wind replenishing moisture at ground level and causes heavy nocturnal dewfall (Oke Reference Oke1987). Lichens in places with frequent dew experience less nutrient loss to leaching than their counterparts in rainy climates. This may contribute to higher diversity and abundance of members of the generally nutrient-demanding taxonomic groups Physciaceae and Teloschistales in dry valleys, compared to leached coastal and montane rainforests where such species are strongly associated with eutrophic habitats.

Fig. 3. Basic ideas and hypotheses regarding the hydration source model (Fig. 2) for lichen responses. A–D shows patterns on spatial scales; E–H, responses of forest lichens to hydration sources.

Fig. 4. Distribution map of Menegazzia terebrata in southern Norway, generated 24 October 2013 from the Norwegian Lichen Database (http://nhm2.uio.no/lav/web/index.html), The Lichen Herbarium, Natural History Museum, University of Oslo. Solid line oval: rainforest region (≥2000 mm rain) of western Norway with mainly epiphytic occurrences. Dotted line oval: rain shadow sites in dry inner valleys (300 mm rain). Short dashed circle: relatively dry areas (≤700 mm rainfall). In the dry areas the species grows on rocks and boulders in open landscapes.

Dewfall can also be frequent in some deserts, as for example the Negev Desert which receives dew on an average of 198 nights per year (Lange et al. Reference Lange, Schulze and Koch1970, Reference Lange, Meyer, Zellner, Ullmann and Wessels1990). In the Namib Desert, dew alone resulted in photosynthetic activation providing 58–63% of total carbon gain in three epiphytic lichens (Lange et al. Reference Lange, Green, Meyer and Zellner2007). In the arid coastal parts of northern Chile, condensation is so strong that fog oases develop, associated with fairly hydrophytic lichen communities on spines of cacti (Lange & Redon Reference Lange and Redon1983; Redon & Lange Reference Redon and Lange1983). Dew events can be rare and/or seasonal in some arid habitats, but nocturnal cooling may still substantially raise the humidity and thus activate lichen photosynthesis (Lange et al. Reference Lange, Schulze and Koch1970).

Rain is most frequent on mountainous coasts along large oceans subjected to convective energy and moisture from the sea (Fig. 3A). Winds and cloud cover reduce nocturnal cooling. Hence there is often a trade-off between rainfall and dewfall. For example, the mean diurnal temperature amplitude increases from the rainy Pacific coastal forests of north-western North America to the inland rainforests further east (Table 1-1 in DellaSala et al. Reference DellaSala, Alaback, Spribille, von Wehrden, Nauman and DellaSala2011b ) with much less rain (Stevenson et al. Reference Stevenson, Armleder, Arsenault, Coxson, DeLong and Jull2011; 788–1240 mm per year). The occurrence of similar epiphytic vegetation in these two areas (Goward & Spribille Reference Goward and Spribille2005; Radies et al. Reference Radies, Coxson, Johnson and Konwicki2009; DellaSala et al. Reference DellaSala, Alaback, Craighead, Goward, Paquet, Spribille and DellaSala2011a ), supporting many liquid water-demanding cyanolichens (Goward & Arsenault Reference Goward and Arsenault2000), is probably caused by heavy dewfall compensating for less rain inland. In fact, cephalo- and cyanolichens became fully hydrated every morning by pre-dawn dew over two dry August weeks in the inland of British Columbia (Y. Gauslaa, pers. obs.). Because dew measured in mm of water is not enough to compensate for the large reduction in rainfall from the coast to the inland, the frequency of hydration rather than the total amount of rainfall matters for the rainforest lichen community.

Links between lichen functioning and water-holding capacity

Water-holding capacity, WHC=[saturated wet mass (WM) – dry mass (DM)] / thallus area (A), gives the amount of water at saturation. This parameter is based on the thallus area that takes up and transpires water, harvests light and fixes carbon. From an ecological perspective, WHC (Fig. 5A) is thus a more functional measure than percent water at saturation (Fig. 5B), which is equivalent to the parameter WHC_DM=(WM-DM)/DM. WHC_DM is commonly used in fields such as soil science, but is less useful in lichen ecology. However, WHC as used in this paper (=WHC_A) directly translates to rain or dewfall (10 mg H₂O cm^–2 equates to 0·1 mm dew or rain), and determines the duration of the physiologically active period across species and/or specimens subsequent to a hydration event (Fig. 5A).

Fig. 5. The relationship between the duration of active metabolism in the cyanolichen Degelia plumbea at low light and A, the water-holding capacity (WHC) at saturation after shaking and B, the percent water content at the same conditions. The insert shows the relationship between WHC and the total thickness of the thalli studied with a size range of 1·1–36·4 cm². The data are from Gauslaa & Solhaug (Reference Gauslaa and Solhaug1998); A is redrawn from this paper; B and the insert are unpublished plots from the same data set.

Both WHC and percentage of water at saturation normally refer to water content after the fully hydrated thallus has been shaken or blotted with the proviso that water content is: 1) substantially higher after shaking than after blotting, and 2) higher when measured in the field than in the laboratory (i.e. owing to the presence of external water droplets; Lange et al. Reference Lange, Büdel, Heber, Meyer, Zellner and Green1993a ). WHC_blotting mainly represents the internal WHC of a lichen thallus, whereas WHC_shaking-WHC_blotting refers to external surface water removed by blotting (Green et al. Reference Green, Snelgar, Wilkins and Brown1985). Apparently, common rates of dewfall (Xiao et al. Reference Xiao, Meissner, Seeger, Rupp, Borg and Zhang2013) are often just enough to fill the WHC_blotting of an average chlorolichen (Gauslaa & Coxson Reference Gauslaa and Coxson2011). The average WHC_blotting across lichens studied is 20 mg H₂O cm^-2 (Gauslaa & Coxson Reference Gauslaa and Coxson2011) and can fully utilize an average good dewfall of 0·2 mm (Jacobs et al. Reference Jacobs, Heusinkveld and Berkowicz2002, Reference Jacobs, Heusinkveld, Kruit and Berkowicz2006; Richards Reference Richards2002; Hao et al. Reference Hao, Li, Guo, Ma, Ayup and Chen2012; Xiao et al. Reference Xiao, Meissner, Seeger, Rupp, Borg and Zhang2013). Such a link suggests that many lichens have evolved to function optimally with dew rather than rain. This hypothesis is consistent with the findings of Lange (Lange Reference Lange2003) showing the importance of days with dew, but no rain, for the annual lichen carbon gain. By contrast, WHC_shaking can mainly be filled with rain, as maximal dewfall rates seem to be just 0·04 mm h^–1 (Xiao et al. Reference Xiao, Meissner, Seeger, Rupp, Borg and Zhang2013). Thus both these WHC measures are ecologically relevant. Measurement of WHC_blotting is simpler to reproduce and compare, evidenced by the substantially lower standard errors (Table 1). For the lichens studied (Table 1), the average WHC_shaking/WHC_blotting ratio was 1·4–2·1. Therefore blotting removes substantial amounts of water. There is a need for WHC_shaking data from more species and/or habitats. Unless specifically stated, WHC in this paper refers to WHC_blotting.

Table 1. Comparison of water-holding capacity (mean±standard error; in mg H₂O cm^–2) after shaking (WHC_shaking) and after blotting (WHC_blotting) in four lichen species.

*Gauslaa & Ustvedt (Reference Gauslaa and Ustvedt2003); ^†Gauslaa & Goward (Reference Gauslaa and Goward2012) WHC_shaking unpublished data; ^‡Computed from data in Green et al. (Reference Green, Snelgar, Wilkins and Brown1985).

Gauslaa & Coxson (Reference Gauslaa and Coxson2011) showed that specific thallus mass, STM=DM/A, is the important driver for WHC in foliose lichens (Fig. 3F). STM is the inverse equivalent of the frequently used parameter specific leaf area (SLA=A/DM) in plant sciences (e.g. Poorter & Remkes Reference Poorter and Remkes1990), intimately connected to the resource use economy of plants (Reich et al. Reference Reich, Ellsworth and Walters1998; Wright et al. Reference Wright, Groom, Lamont, Poot, Prior, Reich, Schulze, Veneklaas and Westoby2004). This parameter was emphasized by Kershaw (Reference Kershaw1985) as an important lichen parameter for water loss and water uptake. STM is a rough measure of thallus thickness, and is higher in sun-adapted than in shade-adapted thalli. With increasing solar radiation exposure in nature from shaded forests to open sea cliffs, STM in studied chlorolichens increased from c. 7 (shade-adapted Platismatia glauca) to 70 mg DM cm^–2 (Anaptychia runcinata thalli from open seashore), with a concurring increase in WHC from 7 to 70 mg H₂O cm^–2 (Gauslaa & Coxson Reference Gauslaa and Coxson2011). The 1:1 relationship found between WHC and STM in many foliose chlorolichens (Gauslaa & Coxson Reference Gauslaa and Coxson2011) was also shown to hold fairly well for fruticose epiphytic chlorolichens in an oceanic oak forest in south-western Norway (Fig. 6; original data). Also for fruticose lichens, intra- and interspecific differences in WHC occur (Fig. 6). The strong 1:1 WHC-STM relationship for chlorolichens implies that WHC≈STM. Because lichen wet mass at saturation per thallus area (WM)=STM+WHC, a rough estimate of WHC in a chlorolichen approximates WM/2. The 1:1 WHC-STM relationship in Fig. 6 shows that enhanced internal water storage requires additional investment in carbon per thallus area, which in turn may imply a change in the balance between water use efficiency versus light use efficiency. The contrasts in thallus thickness between species suggest that STM is at least partly genetically determined.

Fig. 6. Specific thallus mass (mg dry matter cm^–2) and water-holding capacity (mg H₂O cm^–2) in fully hydrated thalli after removing surface water by blotting. All fruticose epiphytic lichen species found in an Atlantic oak forest in Spindanger, Farsund, south-western Norway were included. •, Evernia prunastri; ○, Pseudevernia furfuracea, ▼, Ramalina farinacea; △, Usnea cornuta; ▪, Usnea subfloridana; □, Sphaerophorus globosus; species specific regression lines are shown. Thallus area in E. prunastri, P. furfuracea, R. farinacea and S. globosus was measured with a LI3100 Licor Area Meter (Lincoln, Nebraska, USA); areas in the two Usnea spp. were computed by imaging tools (ImageJ 1.46f version). In all species, a range of thallus sizes as measured (original data)

The intraspecific variation in STM and WHC for the fruticose lichens (see Fig. 6) is mainly an effect of thallus size (data not shown). A range of sizes from small to large was measured for all species, and STM and WHC positively correlated with size. Also, for foliose canopy lichens WHC strongly increases with thallus size, from tiny juveniles to old reproducing stages (Merinero et al. Reference Merinero, Hilmo and Gauslaa2014). As common intensities of dew and low rainfall match the WHC of specific size classes of lichens, the local hydration regime probably influences the maximal size of a lichen species in a given habitat.

Acclimation of STM has been noted after lichens were transplanted to new environments (e.g. Gauslaa et al. Reference Gauslaa, Lie, Solhaug and Ohlson2006, Reference Gauslaa, Palmqvist, Solhaug, Hilmo, Holien, Nybakken and Ohlson2009). From a seasonal perspective, STM significantly increases when the lichen is exposed to high light and dry conditions, and again decreases during long darker and wetter periods (Larsson et al. Reference Larsson, Solhaug and Gauslaa2012). Intraspecific STM responses triggered by increasing evaporative demands have also been noted in natural populations (Snelgar & Green Reference Snelgar and Green1981; Gauslaa & Coxson Reference Gauslaa and Coxson2011; Merinero et al. Reference Merinero, Hilmo and Gauslaa2014). To generalize, changes in STM effectively improve the storage of water in sites and during times when hydration is suboptimal. Prolonged exposure to full hydration has a stronger positive impact on thallus area growth than on biomass gain (Bidussi et al. Reference Bidussi, Gauslaa and Solhaug2013a ), presumably because prolonged high turgor pressure stimulates fungal expansion, as discussed by Larsson et al. (Reference Larsson, Solhaug and Gauslaa2012) and Bidussi et al. (Reference Bidussi, Gauslaa and Solhaug2013a ). Such responses probably tune the WHC to seasonal and spatial evaporative demands.

Lichen responses to hydration sources

Many green algal lichens (chloro- and cephalolichens) can initiate photosynthetic activity in the absence of liquid moisture, although activation in humid air often occurs at slow rates (Jonsson Cabrajic et al. Reference Jonsson Čabrajič, Lidén, Lundmark, Ottosson-Löfvenius and Palmqvist2010). The relative humidity needs to exceed 75–80%, corresponding to a water potential ≈−38 MPa (Nash III et al. Reference Nash, Reiner, Demmig-Adams, Kilian, Kaiser and Lange1990) or lower (Green et al. Reference Green, Sancho and Pintado2011). Therefore, the term ‘humid air’ in Fig. 2 (right corner) refers to humidity levels above ≈80%. Cyanolichens by contrast require liquid water as rain or dew to initiate normal photosynthesis (Lange et al. Reference Lange, Kilian and Ziegler1986, Reference Lange, Green and Ziegler1988, Reference Lange, Bilger, Rimke and Schreiber1989, Reference Lange, Büdel, Meyer and Kilian1993b ); their photosynthetic water compensation point is much higher than in chlorolichens, as reviewed by Green et al. (Reference Green, Sancho and Pintado2011). Accordingly, cyanolichens are restricted to the left portion of the triangle, indicating a requirement for dew and rain (see e.g. Marini et al. Reference Marini, Nascimbene and Nimis2011), whereas chloro- and cephalolichens utilize all available hydration sources and are hence, as a group, more widespread (Fig. 3G). The fact, however, that cyanolichens often have substantially higher WHC than chloro- and cephalolichens (Gauslaa & Coxson Reference Gauslaa and Coxson2011), may partly compensate for their more stringent hydration requirements for photosynthetic activation. At the same time, the cyanolichen Lobaria scrobiculata requires 2·5 times greater hydration for activation than the cephalolichen L. pulmonaria (MacKenzie & Campbell Reference MacKenzie and Campbell2001), which complicates the story and emphasizes the need for further studies on lichen water use efficiency, as proposed by Máguas et al. (Reference Máguas, Valladares and Brugnoli1997).

The bottom panel of triangles (Fig. 3E–H) summarizes various lichen responses to hydration. Excess hydration by liquid water often suppresses lichen photosynthesis, a phenomenon referred to as suprasaturation depression (Lange & Matthes Reference Lange and Matthes1981; Lange et al. Reference Lange, Büdel, Heber, Meyer, Zellner and Green1993a , Reference Lange, Green and Heber2001). Suprasaturation depression does not occur during water uptake in humid air, but can instantaneously take place after additional spraying with water (Lange & Kilian Reference Lange and Kilian1985), unless the lichen surface is water-repellent (Lakatos et al. Reference Lakatos, Rascher and Büdel2006; Hauck et al. Reference Hauck, Jürgens, Brinkmann and Herminghaus2008; Pardow et al. Reference Pardow, Hartard and Lakatos2010). In a continuous year-long gas exchange measurement of the chlorolichen Lecanora muralis in an open habitat in Germany, 74 days of heavy rain resulted in only 2·4% of the annual carbon gain, whereas 105 days with dew accounted for 40% (Lange Reference Lange2003). Depression of lichen photosynthesis by excess water is thus common in rainy events, rare with dew, and absent in humid air (Fig. 3E). As bryophytes in general are much better than lichens in handling excess water (Green et al. 2011), there is a higher bryophyte/lichen biomass ratio in rainy regions where bryophytes rather than lichens tend to dominate tree canopies.

Finely dissected and/or thin lichens (Fig. 1A) take up humidity rapidly, whereas thick and/or compact growth forms need a long time to become hydrated in humid air (Larson & Kershaw Reference Larson and Kershaw1976; Larson Reference Larson1981; Lange & Kilian Reference Lange and Kilian1985; Lange et al. Reference Lange, Kilian and Ziegler1986). Chlorolichens across studied macrolichen growth forms and taxonomic groups follow the 1:1 line in WHC-STM plots, equivalent to ≈100% water at saturation [Fig. 5 shows this for fruticose lichens; Gauslaa & Coxson (Reference Gauslaa and Coxson2011) shows foliose lichens]. Epiphytic cyanolichens tend to follow the 2:1 line equivalent to ≈200% water (Gauslaa & Coxson Reference Gauslaa and Coxson2011). Such relationships suggest that chlorolichens have an opportunistic water economy, as the lower mass per thallus area allows more rapid use of humidity from the air. Cyanolichens have a more conservative water economy. They need higher water content in order to maximize the duration of hydration to compensate for the rarer hydration events by liquid water.

Alectorioid lichens (Figs 1A & 3H) are among the lichens with the largest surface area to biomass ratio. Among the alectorioid lichens, Bryoria species and Ramalina thrausta are the thinnest. As chlorolichens, they rapidly activate photosynthesis in equilibrium with high ambient air humidity (Lange et al. Reference Lange, Kilian and Ziegler1986). Bryoria biomass is much higher in inland regions with less rain than in coastal forests subjected to heavy rain (as shown for Scandinavia by Ahlner Reference Ahlner1948; Bruteig Reference Bruteig1993). Furthermore, excess rain or melting water from snow has been shown to damage Bryoria in western North America (Goward Reference Goward1998), as well as being associated with temporary Bryoria dieback in Norwegian spruce canopies during unusually wet autumns (Y. Gauslaa, pers. obs.). Similar die-off events have been observed in foliose chlorolichens during an exceptional wet late autumn when no damage occurred in cephalo- and cyanolichens (Gauslaa Reference Gauslaa2002). The alectorioid genus Usnea is more flexible with respect to water sources than Bryoria, Alectoria and Ramalina thrausta. The genus as such has a highly variable morphology and anatomy (Motyka Reference Motyka1947), and occurs in well-lit portions of rainforests in most regions of the world (Halonen et al. Reference Halonen, Clerc, Goward, Brodo and Wulff1998; Kantvilas & Jarman Reference Kantvilas and Jarman1999; Antoine & McCune Reference Antoine and McCune2004), as well as in drier forests with abundant Bryoria biomass. Usnea is often represented with more species in the more oceanic areas than in dry areas (Bjerke et al. Reference Bjerke, Elvebakk and Elverland2006), and growth rates in species such as Usnea longissima increase with increasing rainfall (Gauslaa et al. Reference Gauslaa, Palmqvist, Solhaug, Holien, Hilmo, Nybakken, Myhre and Ohlson2007). The Usnea species shown in Fig. 6 have fairly high WHC, but they came from a coastal site where rain and wind are frequent and dew rarely appears, and where no specimens of Bryoria occurred. In contrast to other alectorioid lichens (Alectoria, Bryoria, Ramalina thrausta), thick coastal Usnea species may depend on rain and may thus have optimized their water storage.

Individual lichen species may occupy narrow or wide species-specific areas within the triangle in Fig. 2. To give a few examples, the delicate, pendulous, capillary branches of Ramalina thrausta are apparently optimized for rapid, highly efficient activation in humid air, an inference supported by this species' remarkable ability to inhabit sites partially sheltered from rain and dew. By contrast, the foliose genera Sticta and Pseudocyphellaria, including cephalo- as well as cyanolichen members, inhabit rainy climates (Green & Lange Reference Green and Lange1991). Apparently, the presence of cyphellae and pseudocyphellae in Sticta and Pseudocyphellaria, respectively, permits CO₂ exchange through the lower cortex which allows more of the remaining thallus to store water without too severe suprasaturation depression (Green et al. Reference Green, Snelgar and Brown1981, Reference Green, Snelgar, Wilkins and Brown1985). Suprasaturation depression under rainforest conditions was less, or even absent, in Sticta at 500% water content having cyphellae than in Lobaria in which these structures are lacking (Lange et al. Reference Lange, Büdel, Meyer, Zellner and Zotz2004). Another rain-dependent species is the cyanolichen Degelia plumbea, with such a high WHC (Gauslaa & Solhaug Reference Gauslaa and Solhaug1998) that it can never be fully activated by dew alone.