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EPIPHYTES

Epiphytes or arboreal flora constitute an integral part of the rainforest ecosystem and are the most sensitive among the flora to climatic change. Vascular epiphytes (such as those living on bark), residing primarily in pre-montane to mid-montane forests, comprise 10% of epiphyte species, yet the majority of those in the forest canopy. Non-vascular epiphytes (mosses, liverworts, and lichens) require specific timing on wet dry cycles to flourish, and are much more acutely affected by variations in climate (such as changes in the patterns of annual and seasonal rainfall) than their vascular counterparts. All epiphytes, however, are distributed throughout the canopy on the basis of water supply. Within vascular epiphytes, there are:

Twig specialists, which are the most tolerant to adverse moisture conditions
Bark users, that prefer more humidity
Residents of knotholes and rotting wood, that are more sensitive to drought and require hosts of rooting media
Humus, which are also very sensitive to drought and Ant-nest gardens
Non-vascular epiphytes include:
Foliose and crustose lichens, that peak in the mid-altitudes and
Leafy liverworts, that prefer cloud forests

"The quantity and periodicity of the moisture supply influence the diversity, abundance, and distribution of non-vascular epiphytes in tropical forests through cellular mechanisms susceptible to Darwinian selection, as is true of vascular flora, although tolerances may be generally tighter in the first group" (524).

Adaptation to drought:

Although epiphytes are not as well equipped to deal with drought (because they don’t have access to the ground) as other flora, they still have some mechanisms that help them cope. Many epiphytes exhibit CAM (crassulacean acid metabolism), which involves taking in CO2 at night, and photo-fixing it during the day with closed stomata (to reduce water loss by transpiration). They also contain absorptive foliage that are efficient at quickly taking up water when it is available and preventing desiccation when water is more scarce. However, CAM can be impeded by higher night-time temperatures, dehydrated tissues, and high saturation deficits in the surrounding air, which lower the "stomatal conductance" of the epiphytes, reducing the CO2 uptake, which in turn reduces growth and reproduction and even induces net carbon losses. Higher temperatures, demands on evaporation, and exposure to light cause CAM-idling, which is basically the epiphyte closing its stomata when it becomes stressed. And this narrows the range of habitats a species can inhabit.

In general, epiphyte species composition and biomass are much more sensitive to different relative witnesses than other flora.

The role of epiphytes in the rainforest:

Epiphytes are an important resource for rainforest fauna.

They provide nectar, pollen, fruit and seed for harvest
Their moisture and nutrient retaining properties are essential to many of the terrestrial invertebrates and lower vertebrates.
Some epiphytes have developed coevolved mutualisms with fauna, for instance within an ant-nest garden, the ants provide a home for the epiphytes, while the epiphytes remove harmful excess moisture from the nest.
They are also key players in rainforest hydrology and mineral cycles.
Epiphytes are an important source of biomass (storage capacity), they are well equipped to absorb the prevailing horizontal precipitation (in the form of fog water).
They also vastly increase the area of foliage in the canopy available to absorb ions and moisture (some data indicates that up to half of the canopy's macronutrients may be contained in epiphytes).
Dead epiphytes contribute to the soil-recharging litter of the forest floor.
And epiphytes behave as storage facilities and capacitors for other rainforest biota, as they release certain ions at some points in the year, and absorb the same ions at others.

Relative Vulnerability of Epiphytes:

The effects on different forests and certain regions of the same forest due to change in climate vary according to the types of epiphytes in these regions. Higher CO2 concentrations allow C3 and CAM epiphytes to fix carbon with less transpiration, but we are currently not sure of how this would change competitive patterns between species.

Summary of the aspects of epiphyte flora that increase the vulnerability of certain tropical forest
ecosystems to perturbation associated with global change

I.     Pertinent qualities that distinguish these plants from other forest flora.
        A.     Exhibit high affinities for and inordinate dependence on moisture and nutrients delivered from the atmosphere.
        B.     Possess features that increase access to nutrients and moisture in canopy habitats that in turn increase plant value as resources for co-occurring fauna.

II.     Basis of exceptional vulnerability to global change.
        A.     Carbon and water balance mechanisms (ecophysiology) unusually tightly coupled, hence sensitive to prevailing climate.
        B.     Relative high potential exposure to pollutants (e.g., H+, technological metals, nutrients), especially in occult precipitation.

III.     Basis of importance in inclusive ecosystems.
        A.     Promote biodiversity by constituting more than onethird of the vascular flora in some forests and providing abundant and diverse resources for fauna (e.g., food, shelter, breeding sites).
        B.     Influence nutrient budgets and cycling. Strategically located in space to influence ion flux among compartments in ecosystem and between atmosphere and the entire ecosystem. Increase system capacity to intercept and immobilize large stocks of nutrient capital and control release to other biota. Variously affect nutrient transformation (e.g., N2fixation Nmineralization) by providing appropriate conditions and substrates (e.g., simple sugars from brophytes).
        C.     Influence system energy by supplementing or even exceeding productivity of co-occurring flora.
        D.     Influence hydrology by significantly increasing the canopy saturation value and through slow release, moderate short term drought. Intercepted occult water possibly augments stream flow and affects water chemistry below headwaters of drainage systems.

A change in the climate large enough to only affect epiphytes would nonetheless change the entire forest in terms of its "physical structure, biodiversity, and patterns of energy, water, and nutrient flux" in addition to "ecosystem stability and resiliency."

Next: Carbon sinks

References:
Benzing, David H. "Vulnerabilities of Tropical Forests to Climate Change: the Significance of Resident Epiphytes." Climatic Change. 1998. Volume 39: Issue 2-3, pgs 519-540.
Pictures:
1. Picture from: http://www.cuttingedgeherp.com/pictures1/
2. Picture from: http://www.cuttingedgeherp.com/pictures1/

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