In search of tropical Christmas trees

Trudging through the snow this week on my return to Urbana, campus seems a bit lifeless. Most students and faculty are still away, and the few sprigs of green are mostly yew hedges, the occasional juniper, and the odd Scots pine. It’s odd to think that further north from here, where broadleaves make way for coniferous forests, much of the landscape is still green.

I’m coming back from the other direction. When I arrived in Panama last month, stores were filled with Christmas trees – pines and firs shipped down from North America and a little the worse for their long journey. Some traditions though defy translation, and I admit to buying my own ‘Chinese spruce’ (random six inch pieces of green plastic accompanied by some unintelligible instructions on assembly).

This year though, I’ve been in Panama on the trail of the true native tropical conifers. Most of the conifers we see in Illinois belong to families restricted to temperate regions – the pines (Pinaceae), junipers (Cupressaceae) and yews (Taxaceae), don’t occur in tropical forests. Conifers, in general, are successful at high latitudes because their needles are resistant to freezing and winter drought (when the soil is frozen), and because their narrow water-conducting tracheids are less vulnerable to freeze-thaw damage than the wider vessels found in most angiosperm wood. Tracheids though, have their own disadvantages. For the most part, conifers have needles or small scale-like leaves – their tracheid-based plumbing system simply precludes the development of broad, flattened leaves limiting their ability to array leaf surfaces to capture light in shady habitats, and limiting their capacity to match the fastest photosynthetic rates of rapidly transpiring sun-lit angiosperm leaves.

So, not surprisingly, conifers have been muscled out of much of the tropics. Pines still take a stand on cold mountain-tops in northern Central America and a few Caribbean islands, but once you reach the humid lowlands, conifers all but disappear. One family however makes an intriguing exception. A Gondwanian family of conifers, the Podocarpaceae, originated in the Jurassic (200 million years ago) and migrated into the tropics 50 million years ago, long after the rise of flowering plants. Podocarps can now be found in the peat swamp forests of SE Asia, lowland and montane forests of eastern Africa and all across South America and as far North as central Mexico.

So, how do they do it? We have a few ideas. For one thing, podocarps are never common. Instead, they seem to appear unexpectedly, in patches of otherwise rather open, scruffy forest. Our working hypothesis is that podocarps can only compete where soils are so infertile that the growth advantage of competing angiosperms is greatly reduced. Perhaps podocarps are especially good at either getting hold of scarce nutrients, or of making the most of the nutrients they have (a concept called ‘nutrient-use efficiency’). One trait that is particularly obvious is their peculiarly nodulated roots (seen here in a wet montane forest in western Panama, where podocarp roots grow out of the soil and up into the air).



We know the nodules don’t fix nitrogen, but the source of any other advantage they provide remains unclear.

This year I’ve set up some experiments on the ecophysiology of podocarps with colleagues Ben Turner and Klaus Winter at the Smithsonian Tropical Research Institute. Our first challenge was to find a source of podocarp seedlings. As we want to compare the growth of podocarps with angiosperm trees of lowland forest we needed to find a population of podocarps growing at sea-level (as opposed to a colder mountain climate). Curiously, it turns out the only two lowland populations of podocarps in Panama grow on isolated islands – one Escudo de Veraguas, a tiny island miles from shore in the Caribbean. The other, the island of Coiba out in the Pacific and almost as remote

Travelling to Coiba is an Avatar-like experience. First, you leave the high rises of Panama City, a place of Miami-like excess. Then there’s the long hot drive along the Pan-American highway and finally a potholed trail through endless eroded hills and cattle pastures to the small coastal town of Santa Catalina. From there, Coiba is a two hour ride by panga – the fibreglass open boat and outboard motor found everywhere in the tropics. An hour down the coast the pastures disappear – there are no roads in this part of Panama – replaced by a wall of forest clinging to hills and cliffs and almost touching the ocean. We see humpback whales breaching in the deep channel between Coiba and the mainland, schools of tuna and bonito rippling the surface, and groups of leopard rays leaping to impossible heights before splashing back into the water.

Coiba is the largest island in Central America. Until a few years ago it was a notorious prison – much like Devil’s Island in French Guyana made famous by the book Papillon. For close to a century Coiba’s prisoners were left to roam the island and incarcerated only at night. Not surprisingly, locals steered clear of Coiba. As a result, its forests and surrounding marine environment remained in near-pristine condition. Nowadays, Coiba and its surrounding waters are a National Park – visits to the island are tightly regulated. Fishing has been banned within a mile of the island. The results are spectacular – Coiba’s reefs are filled with the fish life that has mostly disappeared elsewhere in the Pacific – white tipped reef sharks abound along with turtles, and schools of huge red snapper and jack.

Coiba is surrounded by hundreds of smaller islands. One of them – Coibita – is home to a small and rustic (make that exceedingly rustic) field station managed by the Smithsonian and seldom visited. After hauling out our food and water, we made our way to Playa Hermosa on the far side of Coiba, the location of the only patch of podocarps on the island. Why the podocarps grow here, and not elsewhere on the island remains unclear. The podocarp patch is on the edge of a fault line and it’s possible that the geology is unusual here. We’ll have to wait for results of our soil analyses. Arriving on a small stony beach its short climb up a steep embankment to a broad plateau. After a few minutes of searching we spot our first podocarp trees, and soon after discover carpets of recently germinated seedlings. It doesn’t take long to gather enough to take home.

Back at Gamboa (close to Panama City) our seedlings are now in Klaus Winter’s controlled environment greenhouses. Our first experiment is comparing podocarp responses to both reduced and elevated CO₂ with responses of similar sized angiosperm seedlings and another conifer species Norfolk Island Pine (Araucaria heterophylla) – a Pacific Island endemic. Back in the Jurassic, when conifers ruled the world, atmospheric CO₂ concentrations were four times current values, then steadily declined over the last 150 million years concurrent with the rise of Angiosperms. Could this decline in CO₂ have altered the competitive balance between angiosperms and conifers? Stay tuned for our results.