August 21, 2012

Kingdoms in the Canopy (II): Who's who up a tree

The canopy of a tropical rainforest is a world very different from the ground it shadows. Trees are ladened with bromeliads and orchids, draped in mosses and ferns, tangled in vines and lianas, and, in some cases, burdened with the weight of other trees. In turn, the leaves of these plants growing on trees – known as epiphytic – may have other forms of plantlife growing on their leaves! These plants are epiphyllic. So, epiphytes grow on other plants (namely trees), and epiphylls decorate the leaves of other plants. Neither type of vegetation is parasitic.

During the University of Manchester's Tropical Biology field course in Payamino, Ecuador, I was studying the biodiversity within bromeliads (more on here). The first part of bromeliad collection is, obviously, finding them. I did just say the place is riddled with them. And it is. However, telling apart arboreal bromeliads and ferns, or even orchids, from the ground isn't always easy. Especially when looking up at a cluster of greenery 20 metres above you, with a backdrop of more greenery and sunlight. Fortunately, there are a few key differences which allow you to distinguish between the major classes of epiphytes in the neotropical canopy. See Image 1.

Image 1 - Clusters of green. © Xaali O'Reilly, 2012

Image 2 - Epiphytic bromeliad. © Xaali O'Reilly, 2012
The leaves of bromeliads characteristically form a rosette arrangement (see Image 2), though this can be hard to appreciate from far below. The edges may be smooth or serrated, but tend to be relatively stiff. The curve in their leaves reflects this, and contrasts against the (usually) "floppier"-looking fern leaves. The ferns around here also tend to have a more crinkled edge. A dead give-away with ferns are the sporophytes, which take shape as little round clusters of brown spores on the underside of leaves. Obviously for this to serve as an identification criterion the plant has to be producing spores at the time; the sporophytes would also need to be visible against the light of the sky and at the given distance.

Ferns also have a midrib and veins; the former lacking on bromeliad leaves and the latter, unappreciable at any great distance.

Orchids also have a visible midrib and veins. They can, nonetheless, be easily distinguished from ferns by the stiffness or leatheriness of their leaves and, more importantly, the direction of the veins: like bromeliads, orchids are monocots, a group of plants whose veins run vertically down the leaf, almost parallel to the midrib. The veins of other plants, like ferns and dicots, begin at the midrib but run outwards towards the edge of the leaf.

Other large epiphytes are hardly confusable with those groups – I am yet to mistake a tree growing between the crotch of another tree's branches for a bromeliad or a fern (although I expect to swallow those words at some point, given the nature of this place!).

Thus, upon these criteria, we try to judge what's a bromeliad and what's not from a distance. Once a bromeliad has been collected, it becomes important to be able to classify the plants growing inside or on them. Epiphyte identification was important for some of my coursemates' projects as well, for example in fern or bryophyte surveys. 

Little epiphytes and epiphylls include young and smaller versions of all the aforementioned plant groups, as well as bryophytes. Little ferns and orchids can look similar, but again are characterised by the same features as their large relations, and so it is a case of paying attention to detail. Small bromeliads just look like miniatures of big ones, a tad more benign in the case of young individuals of the serrated species.

Bryophytes must be the dominant group of small epiphytes. The big stars of this division are the mosses. Mosses are not the only bryophytes: liverworts and hornworts also coat the leaves and carpet the bark of vascular plants. All three types commonly manifest themselves as fuzzy-looking green tufts or strings. This is not to say they cannot be distinguished from one another.

Below are the criteria which seem to be best to go by to differentiate liverworts, hornworts, and mosses from each other:

Image 3 - General criteria for differentiating between the major bryophyte groups. © Xaali O'Reilly, 2012

Leaving out vines and lianas here was not an accident, mainly because the distinction between the two is not a strictly taxonomic (that is, based on relatedness) one, and I do not find the grouping useful for my purposes. Hopefully this nevertheless serves as a basic introduction to epiphyte I.D.-ing in South American tropical forests. Plant scientists, botanical enthusiasts, or other, please let me know if I've got anything wrong – to ensure that I am neither disseminating false information nor classifying all my plant samples wrongly!

I will soon be writing more specifically about bromeliad sampling and what other manner of creatures can be found inside them (Kingdom in the Canopy: Biodiversity within Bromeliads).

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