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Arnold Arboretum

Reading the forest

March 4, 2014 by Cam Webb

River - Cam Webb

We’ve had a week of lovely weather.

So, finally, I get to describe the best part of these expeditions, and the real reason I do them: I love to “read the forest.”

Looking up through the jumble of greens and browns, I know there is a story in here. On first pass, in the moment of engagement in the field, it’s primarily a story of family. Of meeting old friends; of recognizing cousins I’ve long heard of but never yet laid eyes on; and of meeting strangers, and, while not knowing how they relate to the family, knowing that they do somehow. In one way this is the intellectual application of the framework of plant taxonomy I’ve built up over the years, a model of classification and morphology. But it keeps on grabbing me in a more visceral sense: it is simply fun to look through the forest, tree by tree, in a process of repeated expectation and fulfillment.

It’s like any collecting activity: playing (the card game) “snap,” collecting stamps, playing Pokémon! As humans, most of us get a kick out of the sifting of diversity, the recognition of the known, the pattern-seeking in the unknown. And I am tremendously fortunate to get to do this in the rainforest, where the objects of this seeking really are unknown (by humans). As far as I am aware, no one has ever put in any systematic (all taxa) forest inventory plots in these wet forests of Flores (see Monk et al. 1997). The best checklist we have (Whitmore et al. 1989) is already turning out to have many missing taxa, based on what I’m seeing here.

On a practical level, I start by approaching each tree “in order;” the tree-tagging and diameter-measuring team have gone through the 50m-by-50m plot earlier. I glance at the bark, and in a few cases I can already recognize the species from the color and pattern of fissures, flakes, stains, and texture, matching it mentally to one of the previously encountered species at this site. Only a few species or genera have distinct enough bark to suggest a tree’s identity based on my own experience at other sites, and as ever, exceptions and ‘false positives’ abound. For example, most Calophyllum have a very distinct pattern of long, yellow, diamond-shaped marks on their smooth surface; but based on this, several times here I have mistaken some unrelated Malvaceae trees as Calophyllum. The relationship between bark surface and bark anatomy is fascinating and complex, having to do with how the inner-bark (the phloem) relieves the stress it is under as a ring of living tissue around a ever-expanding core of wood (the xylem): with fast, even growth leading to smooth bark, or random cracks, and growth concentrated in rays of cells leading to ordered fissures. Check out the classic bark papers (Whitmore 1962, 1963) by the tropical tree master, Tim Whitmore.

After scanning the bark for clues, I find a spot to see the leaves up above, which can take ages in a forest with lianas draped over the canopy. I look up with binoculars: what are the leaves like? How are they arranged on the branches? The general form of the canopy? Colors? Are there any fruits or flowers (usually the best plant parts to identify the species, but seldom available)? Using the rich terminology of plant morphology, I jot down key characters in my notebook. Then an assistant and I let lose a catapult-volley of pebbles at the branches, trying (and nearly always succeeding) to shoot out a few green leaves. Usually finding old, fallen leaves on the ground is also possible, but it can be very time-consuming if the tree sheds few leaves, and is prone to errors: even after years of doing this, I can still make mistakes collecting the right fallen leaves for a given tree, especially when the species is unknown but similar to other species present, and when the tree is a giant (and at the end of the day when I’m tired!).

leave veins and bark slash

The unique species ‘fingerprint’ of a combination of leave veins and bark slash.

Finally, I approach the trunk and make a small slash in the bark, going deep enough to expose a small patch of wood. Before any volatile compounds have time to evaporate, I sniff the slash. Many trees have a very distinct odor; at this site, there is a large Rubiaceae that smells like wintergreen (as in candy, or yellow birch), a Canarium like carrots, a Prunus like marzipan (almond paste), a Cynometra like wheat flour dough (without the yeast), the many Aglaia like their own delicious “langsat” (their fruit) smell, and finally a mystery plant that smells of a combination of tobacco and silage! I then take a close-up photograph of the slash, and of the fallen leaves. The tool of high-resolution digital photography in the field has revolutionized this process of morphotyping. Describing bark slashes in words is hard, and a blunt means to differentiate among taxa. But the combination of photos of a bark slash and a leaf’s venation provides an amazingly fine tool for separating species: the visual characters are numerous while the variation within a species is surprisingly low (see pic).

In this way, I work through the plot, doing about 80 trees a day. Though not physically strenuous, the repetition gets tiring—approaching a tree, wandering around to find a spot to see the canopy, craning one’s neck to look up, shooting pebbles with a catapult, stooping to find fallen leaves, and re-approach to the tree for the bark slash and note-taking. And sadly, my failing, post-age-40, close-up vision means that there’s also a tiresome juggling of glasses for each tree! I soon learn the common species, and recording these doesn’t take more than a few seconds. But in these forests you seldom go more than four or five trees before finding a new (to me) species that needs careful recording.

Sapling of a Zanthoxylum

Sapling of a Zanthoxylum species (Rutaceae).

The other part of reading of the forest is the encounter with saplings as one walks about. Most of the saplings in the forest are young versions of the ones I’ve been learning in the plot, although there are also many saplings of liana species I haven’t been recording explicitly. Drawing the connection between a sapling and its adult is a very satisfying event, because sapling leaves are seldom exactly the same as adult leaves (they tend to be larger and have greater interspecific variation in shape), and the recognition of being the same species gives immediate insight into the developmental changes that go on in a single plant as it grows up. Recognition of a sapling is also exciting because it provides access to characters that are
usually unavailable with just fallen adult leaves: are there stipules? What is the fine-scale orientation of the leaves? All these additional characters help one begin in that other satisfying process of matching an organism in the field to a description in a book, thereby allowing for identification of unknown plant types (morphotypes) to known taxonomic names. This book-browsing stage happens at the end of the day, with a cup of tea, and a laptop full of digital copies of reference books and interactive keys. I may solve a nagging mystery, or I may get confused. But the book data gets refreshed and thrown into the mental mix, to emerge perhaps the next day as a verbal “YES!” as I recognize another member of the Malesian tree family that I had always wanted to meet. Fun stuff!

Next week: what we’re finding! (Hint: it’s more diverse than I ever thought.)

References


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