Fog, lichens and water balance in Eulychnia breviflora

A cactus within reach of ocean fog

On the rocky coast at Pan de Azúcar, the columnar branches of Eulychnia breviflora provide a site for lichen colonisation. Stanton and colleagues used the name E. saint-pieana, now regarded in POWO as a synonym of E. breviflora. Lichens hanging from stems and spines include Ramalina usnea and representatives of Usnea. Their thalli, the bodies of lichens, increase the plant's contact area with moist air. The study concerned a fog-influenced coastal population separated from the extremely dry areas farther inland in the desert. [Stanton et al. 2014] [2-WWW]

Epiphytes develop on the host surface and use it as a place to settle. In the system studied, they can intercept droplets suspended in air, retain water and reduce stem heating. The balance depends on several flow pathways. Water may remain in the lichen, evaporate from its surface or reach the substrate. At the same time, the rate of water loss from moist soil and atmospheric evaporative demand change. [Stanton et al. 2014]

Droplet interception and water reaching the soil

Fog carries already formed, small droplets of liquid water. Wind brings them to the surfaces of stems, spines and thalli. Droplet collision with this surface and retention constitute fog-water input. Dew forms by a different process, condensation of vapour on a sufficiently cool surface. The experiment primarily considered the effects of epiphytes on fog interception and subsequent water retention and evaporation. [Stanton et al. 2014]

To separate the role of structure itself from processes in a living plant, three pairs of columnar-stem models were built. They were about 75 cm high, with a green covering and elements imitating spines. One model in each pair was covered with lichens, while the other remained without them. Soil water beneath the models and in an open area was compared. The models supplied more moisture to the substrate than the open area, but soil beneath lichen-covered models was drier than beneath their lichen-free counterparts. [Stanton et al. 2014]

This result follows from separating water interception from its onward transfer. Thalli can retain some liquid before it flows to the roots. An increase in epiphyte-covered area alone therefore does not determine the amount of water reaching the soil. The authors combined the model experiment with measurements on living plants to investigate the other side of the balance as well, the drying rate of the surroundings. [Stanton et al. 2014]

Removing lichens from two exposures

In March 2008, 15 cacti 1–2 m tall were selected, forming five matched groups of three. In each group, one plant retained its epiphytes, a second had them removed from the northern half and a third from the southern half. This design allowed the loss of covering on the sunnier side to be compared with its removal on the more shaded side. Treatment was limited to epiphytes, taking care to preserve the cactus surface. [Stanton et al. 2014]

After rare rainfall in May 2010, soil beside plants stripped of epiphytes on the northern side dried faster than beside control plants. Removal from the southern side produced no such difference from the control. This concerned gravimetric measurements after a specific rainfall event. Water surges disrupted some automated soil monitoring, limiting the available record. The result indicates that the benefit of shelter depends on exposure and sunlight. [Stanton et al. 2014]

Cooler air beside the stem surface

Temperature and air humidity were recorded beside four pairs of surfaces with and without epiphytes. Vapour pressure deficit, VPD, was calculated. This is the difference between saturation vapour pressure at a given temperature and actual vapour pressure in the air. A higher VPD means a greater atmospheric capacity to draw water from a moist surface. Temperature and humidity act together here because warmer air can contain more vapour at saturation. [Stanton et al. 2014]

The table presents means from comparative measurements beside the stem. Daytime was defined as 7–19 hours. With epiphytes, mean daytime temperature was 2,2°C lower, and mean VPD was 0,69 kPa compared with 1,02 kPa without covering. These values concern the local measurement space, four sensor pairs and the study's recording periods. They are not the mean temperature of the entire habitat. [Stanton et al. 2014]

At night, differences in some indices were small and those in mean VPD were not statistically significant. The clearest change was associated with daytime, when the exposed surface heated more strongly. Lichens provide radiation shelter, alter air movement near the stem and periodically store water. The combined effect of these properties corresponded to a cooler, more humid microenvironment. [Stanton et al. 2014]

What conserves water in this system

Water retained in lichens can later return to the atmosphere as they dry. This increases humidity locally and, together with shading, lowers VPD. Slower air exchange in the sheltered space makes rapid vapour removal more difficult. Water can therefore remain longer in the microenvironment even when a smaller proportion immediately reaches the soil. This mechanism explains why a lichen-covered model and a living cactus after rain can reveal different aspects of the same balance. [Stanton et al. 2014]

Measurements directly demonstrated a change in microclimate and differences in soil drying. The authors interpreted these as a possibility for more efficient use of available water by the host. Cactus transpiration flux and gas exchange were not measured in this part of the experiment, so savings were not expressed as a percentage reduction in its total water use. The proposed protection of photosynthesis against intense light remains an additional explanation requiring its own measurement. [Stanton et al. 2014]

The microenvironment as part of the habitat

The coastal site comprises surfaces differing in sunlight, airflow and fog access. Lichens add a lasting change in stem geometry and a periodic reserve of retained water to this system. For E. breviflora at Pan de Azúcar, the combination of shelter and humidity near the surface was important. The substrate and roots remained part of a rocky, well-ventilated environment. This plant's physiology is therefore described by both water input and the conditions under which water is subsequently lost. [Stanton et al. 2014]

Across the genus, such local differences accompany strong separation of coastal vegetation patches. Phylogenetic studies relate the history of Eulychnia to alternating increases and decreases in the continuity of wetter habitats. The lichen experiment explains a specific relationship operating near the stem, while biogeographical reconstruction concerns changes in the ranges of entire lineages over time. Both levels show the importance of the spatial distribution of available water. [Stanton et al. 2014] [Merklinger et al. 2021]