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Project Amazonia: Characterization - Biotic - Epiphytes

Introduction

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 canopy1. 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:

bullet Twig specialists, which are the most tolerant to adverse moisture conditions
bullet Bark users, that prefer more humidity
bullet Residents of knotholes and rotting wood, that are more sensitive to drought and require hosts of rooting media
bullet Humus, which are also very sensitive to drought
bullet Ant-nest gardens

Non-vascular epiphytes include:

bullet Foliose and crustose lichens, that peak in the mid-altitudes
bullet Leafy liverworts, that prefer cloud forests

The availability of moisture affects the diversity, abundance and distribution of non-vascular and vascular epiphytes (although the latter is less sensitive).1

 

Figure 1: Hypothetical tree illustrating how vascular epiphytes in humid forests tend to partition substrates illustrating sensitivity to micro climate, particularly humidity, and associated development of the organic rooting media required by some populations.1

 

Epiphytes play a key role in the rainforest ecosystem. They provide nectar, pollen, fruit and seed for harvest, and 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 example within an ant-nest garden, the ants provide a home for the epiphytes, while the epiphytes remove harmful excess moisture from the nest. They also provide an important source of biomass (storage capacity).

Epiphytes are important in rainforest hydrology and mineral cycles. They are well equipped to absorb the prevailing horizontal precipitation (in the form of fog water). They also vastly increase the canopy foliage surface area which absorbs ions and moisture (some data indicates that up to half of the canopy's macronutrients may be contained in epiphytes1). 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. Dead epiphytes contribute to the soil-recharging litter of the forest floor.

Adaptation to drought:

Epiphytes are not as well equipped to deal with drought as other flora, because they donít have access to the ground, but they still have some mechanisms to 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.1 They also contain absorptive foliage that are efficient at quickly taking up water when it is available and preventing desiccation when water is scarcer. 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.1 Higher temperatures, demands on evaporation, and exposure to light cause CAM-idling, which is basically the epiphyte closing its stomata when it becomes stressed, that narrows the range of habitats a species can inhabit.

In general, epiphyte species composition and biomass are much more sensitive to different relative moisture levels than those of other flora.

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 the scientific community is still not sure of exactly how this would change competitive patterns between species.

1

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."1

Epiphytes and Fragmentation:

Basic Info on Fragmentation:
Habit fragmentation is defined as "the simultaneous reduction of the area of the focal habitat and increased isolation of the remaining habitat patches." 5 In short, fragments are areas of forest surrounded by deforested area.

Many species are lost during and immediately following fragmentation, but there are also many long-term effects which can be caused by changes in processes such as pollination, predation, territorial behavior and feeding habits, in addition to changes in microclimate affecting solar radiation, humidity and wind pattern. In general, fragments have a much greater proportion of edge zone (in comparison to closed forest), which is physically different from interior forest, thus changing ecological properties of the fragment. Fragments usually can not support many species assemblages that exist in closed forest, and smaller fragments suffer more loss in biodiversity.

Effect of fragmentation on Epiphytes:
Survival of epiphytes subsequent to fragmentation is largely dependent on the size of the tree in which a species resides, the size of the fragment and also the relation of the fragment to closed forest (distance). In fragmented forest, there is a much greater loss in biodiversity of epiphytes than loss of presence in the fragment. The size and shape of a fragment will determine the exchange rate of diaspores and pollen within and between fragments, and can lead to isolation of subpopulations.

Although the slow growth and long cycles of epiphytes make them more vulnerable to high rates of disturbance, they allow them to survive better in the mid-term if they can get past initial conditions. This increases their chance of success for colonization of second-forest in deforested areas as long as these reforested patches are not to far from larger fragments. In addition, their sensitivity makes them "suitable indicators of changes in local climate, forest structure and ecosystem health"2
.

Epiphytes as Bioindicators

Epiphytes and orchids are well suited to be indicators of the health and biodiversity of the rainforest, not only because they are an important source of nutrients for other flora and fauna, but because they are very sensitive to shifts in microclimate and they have slow growth. The performance, survival, and distribution of epiphytes is dependent on stand density, microclimate, distance from seed source, tree size and species, type and history of disturbance, population dynamics of epiphytes and trees, and epiphyte physiology.2

Epiphytes are far more vulnerable to deforestation than other flora. For example, 26% of vascular plant species present in 1900 are now extinct, but 62% of epiphyte species are extinct.2 Epiphytes are completely dependent on their host plants, so if a tree is cut down, all of the epiphytes residing on that tree will die. In addition, they have very specific zoning constraints, so secondary vegetation might not have all of the necessary microsites for different epiphyte species.
 

Table 12 illustrates the loss of species and biodiversity in a plantation as compared with oldgrowth forest. While the number of species for the two groups is not very different, there is considerable loss of biodiversity, because only epiphytes residing in some of the locations on a tree are present.

    TABLE 1. Epiphyte Richness and Occurrence of Ecological Groups in an Oldgrowth Forest and an Alnus Plantation.
    Figures in parentheses for the plantation are fertile species; all species in the oldgrowth forest were found with fertile individuals.
   

Epiphytic Species Living on: 

Oldgrowth 

Plantation

stem base

9    

0 (0)

stem and thick branches 

14 

7 (3)

thick and thin branches

11    

11 (7)

preference unclear

5

3 (1)

total   

39 

21 (11)

                                       
In addition to deforestation, epiphytes are affected adversely by increased CO2 levels in the atmosphere. Raised CO2 levels shift climate zones, forcing flora and fauna to migrate. Although epiphytes could migrate more easily than trees, many models of the effect of fluctuating CO2 levels predict increased seasonality of precipitation, and thus a reduction of the perhumid area containing the highest epiphyte diversity
.2

Below are a couple of specific species or groups have been identified as good bioindicators.

bullet The hemiepiphytic Ficus is a keystone species because of its "numerical abundance, intra-crown synchrony of fruit ripening, relatively short intervals between fruiting, large crop sizes and intrapopulation fruiting asynchrony" and the fact that it is often available as a food source for birds when other fruits are scarce.4
bullet Lichens are often used as an indicator of air quality: Tillandsia, for example, is particularly sensitive to lead pollution.


Epiphyte N concentration:

Epiphytes obtain their Nitrogen either from canopy soil or from nutrients in rainwater. Nitrogen-15 concentration is much higher in ground-rooted plants than in epiphytes with access to canopy soil, pointing to a much richer source of Nitrogen in terrestrial soil versus canopy soil. In addition, N-15 concentration is much higher for those epiphytes in canopy soil than those on smaller branches, indicating that epiphytes on smaller branches have to rely almost exclusively on rainwater as a source of Nitrogen.3 This means that these epiphytes (on small branches) are much more susceptible to drought and thus would be better bioindicators.

 

Next: Brazilian Society->

1: 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.

2: Hietz, Peter. "Diversity and Conservation of Epiphytes in a Changing Environment." International Union of Pure and Applied Chemistry (IUPAC). 1998. Volume 70: Issue 11. Available at: http://www.iupac.org/symposia/proceedings/phuket97/hietz.html

3: Hietz, Peter; Wanck, Wolfgang; Wania, Rita; Nadkarni, Nalim M. "Nitrogen-15 natural abundance in a montane cloud forest canopy as an indicator of nitrogen cycling and epiphyte nutrition." Oecologia. 2002. Volume 131, pgs. 350-355.

4: Lambert, Frank R. and Marshall, Adrian G. "Keystone characteristics of Bird-dispersed Ficus in a Malaysian lowland Rain Forest." Journal of Ecology. 1991. Volume 79, pgs. 793-809.

5: Ranta, Pertti; Blom, Tom; Niemela, Jari; Joensuu, Elina and Siitonen, Mikko. "The fragmented Atlantic rain forest of Brazil: size, shape and distribution of forest fragments." Biodiversity and Conservation. 1998. Volume 7, pgs. 385-403.

6: Turner, I.M.; Chua, K.S.; Ong, J.S.Y.; Soong, B.C.; Tan, H.T.W. "A century of plant species loss from an isolated fragment of lowland tropical rainforest." Conservation Biology. August 1996. Volume 10: Issue 4, pgs. 1229-1244.