Air-plants live above the ground. They don’t need the soil – they get what they need from the sky. In the tropics those plants bloom hundreds of feet up in the trees. Their roots serve mostly to hold them in the branches, where they intercept their water as rain. But far from the tropics air-plants still thrive – in places where arboreal orchids, ferns, and bromiliads would wither. These temperate epiphytes have no roots at all. They don’t depend on the rain to supply their water – they live where months can go by without a trace of precipitation. They absorb their water in its invisible form, as vapor.
The temperate epiphytes take over the air-plant niche in the conifer and hardwood forests of coastal New Zealand and Chile, and on the west coasts of Japan and Northern Europe. The air is often dry by noon there, and can stay dry for weeks during the summer.
These cool coastal forests reach their greatest extent along the eastern rim of the North Pacific – on an arc stretching from Alaska to the edge of the desert in Southern California. Gauzy, weightless gray-green curtains dominate the air plant niche in this range. The plant is an associations between a fungus and algae. Where they grow alone, each of these organisms produces only a thin coating on the bark. But when the fungus and the algae unite, they realize the benefits of synergy. They lengthen into the hallmark of the northeast Pacific coastal forest – the hanging tapestry of Lace Lichen.
As opposed to succulent bromiliads and vines – which absorb their water in liquid form – these lacy lichens are not vascular; each cell meets its own water and nutrients needs independently of its neighbors. The lichen’s shape promotes this – no cell in its tracery is more than a tenth of a millimeter removed from thin air. The flattened, finely divided foliage provides these lichens with the greatest surface/volume ratio of any plant.
The structure that accommodates that geometry is a mesh – like a cross-section through a suspended splash of water, most of whose volume is filled with bubbles. The weightless webbing does not offer much shade or wind resistance; its filaments hang limp and dusty gray when dry – twisting in the breeze like silk curtains billowing behind an open window.
The filaments can be found trailing above yellow summer grassland in the interior hills – far from the more humid conditions near the ocean. There the hanging strands wave like pennants – marking out a course through the maze of valleys in the mountains behind the shore. That is the course through which the sea breeze spills farthest east overnight, carrying the cool humid coastal clime inland.
Lacy lichens do not grow in full sun. They beard the lowest scaffolds of their host trees and shrink in the shade from a hot afternoon – spending their dry spells desiccated to the consistency of crêpe paper. They show no signs of life for years in their driest sites, yet soften again in seconds at the first breath of fog or sign of rain. Their photosynthetic metabolism comes to life when they rehydrate; then the revitalized, gray-green lattice-work will stay soft and supple as long as traces of humidity linger.
These lichens live an unattached life, harvesting all they need from the empty sky. They absorb their sulfur, carbon and water directly as gasses. Some lichens also absorb nitrogen gas from the air and convert it into its organic form.
The sky carries a microcosmic sampler of the world’s lands for the lichens to harvest – fine particles first levitated by sand storms in the Gobi Desert, or volcanic aerosols injected into the jet stream above Indonesia. Invisible smoke particles that rose in Asia end their transoceanic journey in the finely-divided foliage.
The web-work of the lace lichens serves to becalm the air entrained within it – air filled with motes of dust from near and far. The tiny particles wander into the living lattice and settle on the strands. Eventually the dust dissolves in place in the predawn dew, coloring a tea that coats the surface with a solution of phosphates, potash – the minerals that all plants need, but that this plant need not search for in the ground.
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Unlike plants which have their roots tapped into the reservoir of moisture in the earth, the lichens cannot run the photosynthesis reaction all day long. Their inability to retain moisture in their flat filaments prevents them from growing continuously – they loose their link to the sun’s energy every time there is a warm morning.
Pores in the water-tight plant cuticle must open to admit carbon dioxide gas – which is the source of the carbon made into sugar and fiber by photosynthesis. But pores cannot control the direction of gas diffusion – it goes both ways, and water vapor escapes through them. When the air dries, the lichens would wilt if they leave pores open. So the pores close down and photosynthesis pauses for lack of CO2 until more humid conditions return.
Thus, the hanging lichens do not photosynthesize all the while the sun is shining. They turn on early and then off later each morning. But they make up for sun-time lost in the afternoon by running the photosynthesis reaction at higher speed in the early light – while there is still sufficient water within each cell. They can do that because photosynthesis reactions run faster when the temperature is higher. The lichens raise their temperature against the cool of sunrise – accelerating the reaction – by reversing the chill of evaporation.
Evaporation involves a change in the state of water – from liquid to vapor. This process takes heat away from a surface – generating a chill that can be felt, for example, by moistening a patch of skin and then blowing across it. Reversing the process reverses the effect – heat is added to a surface when water in its vapor state changes back into liquid upon that surface – the process of condensation. Vapor molecules deposit the heat that kept them free flying onto the surfaces upon which they condense into droplets.
The fine divisions of the surfaces of lace lichens maximize the area available for condensation of the dawn vapors. The strands take up the heat provided during condensation of the dew upon their networks, then they take up the dew itself. And, for a few hours each day, before the warmth of the approaching noon steals their water again, the lichens rapidly grow.
The lacy lichen habitat is the land of two seasons: spring and summer. Lacy lichens are among the fastest growing of the plants of that land. They grow a third again as long over a season of spring mornings, sometimes reaching 20 feet in length. When the flowers below them are finishing their flowering, the lichens turn off permanently and wait out the dry heat in stasis. They accumulate a summer’s load of dust, primed for the reappearance of the dew when the season turns once again. Finally the rains come and wash away the dusty residue – brightening the blue-green foliage to its maximal photosynthetic potential once more.
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Where they grow the fastest – near the coast – the lacy lichens do not reproduce by making spores. They are spread from tree-to-tree through the temperate woods as fragments, often by enlisting the assistance of the animals. Just as other plants rely upon their flying allies to disseminate their pollen or seed, this plant spreads its population on the wing. Its partners in its dispersal are hummingbirds.
The hummers build their nests from strands of lace, having adopted lichen as a material of choice because of its particular qualities. They gather the tough, dry strings from mature stands of lichen, then they weave them with other fibers into a tiny cup stitched with spider web. They line the inside with plant down and decorate the outside with bits of the wider, flat ribs of lace lichen, and with other types of lichens as well.
The hummingbird nest is built to be expansible – it enlarges to accommodate the growth of the chicks. As the hatchlings mature and their short beaks begin to lengthen, their respiration hydrates the strands of lichen next to their bodies. The dry filaments come to life when they absorb water vapor from the chicks – just as they do from the sky on a dewy dawn. Water that was nectar in flowers at daybreak, and chick food in the morning, is absorbed by the lichens later during the day, allowing the strands to expand and become pliable. The nest stretches with the press of the growing hummers — dilating to make room. It may enlarging its diameter almost two-fold as the fledgling plumage fills in and fluffs out over the weeks.
Other weavers also transport lichen strands for nest building: orioles, Bush Tits, Blue-gray Gnatcatchers. The nests hang wait abandoned through the Dog Days of summer – sagging and drying out after they have accomplished their first task of fledging the next generation of weaver birds.
With the fall, the lace within vacant nests comes to life once more. The stringy strands absorb the season’s moisture and begin to grow again, overflowing their woven confines. They trail down into the open air-spaces below, expanding over each untended nest until it is unrecognizable.
Through the years, all the best nesting sites come to be hung with lace. The open under-story spaces grow curtained in a gray-green gown that trails all the way down to the browse line – where the deer trim the longest streamers.
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Lace lichens survive in intimate association with the air. That life style leaves them susceptible to recent atmospheric changes that can overload their metabolism and poison them. They have never evolved a mechanism to curb their accumulation of airborne minerals – which were always so scarce that every last molecule must be absorbed if the lichen is to prosper. As a consequence, their capacity to take up all the trace gasses they touch will overdose a lacy lichen on sulfur dioxide, or oxides of nitrogen. Those gasses flow into their strands in excess from sources of anthropogenic air pollution.
Lace lichens are no longer found in their historic range in the Los Angeles basin. The smog has killed them. Their growth there is now limited to slopes in the surrounding mountains at altitudes above the prevailing ceiling of the inversion layer.
Some residents of southern California – those living north or south of the plume of pollution that stretches inland toward the desert – still find lace lichen in their trees. Its persistence is an indication that the local air quality at those spots is not harmful to the health of air-plants, or of anyone else there sharing the air.
Lacy lichen, notes. Ramalina menziesii is sometimes called “Spanish Moss,” but this would be a double misnomer. Spanish Moss from tropical ranges in the Americas is neither a lichen, nor a moss, but a vascular, flowering plant (a bromiliad, a latin name of which translates roughly to mean “looks like an epiphytic lichen”). R. menzeisii is a lichen, a non-vascular fungal vessel containing algae inside. The expansible hummingbird nest incorporates strands of lace lichen, and also contains spider web – which also lengthens when hydrated. Aside from propagation aided by hummingbirds (Larson, 1989) lace lichens spread by fragmentation. Along the coast, where their growth rate is highest (Boucher & Nash, 19xx) the strands do not sporulate (Larson, 1989). Where lacy lichen does form spores, they are only from the fungal partner of the symbiont; germinating spores must find a source of their algal partners before the strands can grow lace (Werth & Sork, 2008). Lichens absorb their nutriment without roots, nor contact with the soil (Nieboer et al, 1978); they can be a significant source of organic nitrogen to the forests that support them. The pendulous lichens are utterly dependent on what they absorb from the air (cited in Matthes-Sears et al, 1987) including dust – which they acquire by dry deposition (Boucher & Nash, 1990). They assimilate airborne matter, so they are sensitive barometers of air pollution. They become dangerously radioactive when they grow downwind of nuclear contamination. Even before rock-bound lichens (which dervie some of their essential elements from the rock) are extirpated from polluted regions, lace lichens disappear. They have retreated from the Los Angeles basin – and are now found there only above the smog line (Sigal & Nash, 1983).
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Matthes-Sears, U et al (1987) Ecology of Ramalina menziesii VI. Laboratory response of net CO2 exchange to moisture, temperature, and light. Can.J.Bot 65:182-91
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