West of the Fields

A tropical ecologist reporting from the field. Musings on life and art, botfly extractions, tropical plant identification, beer, parrots, machetes. Etc.

Saturday, April 04, 2009

Palo Verde

A post written some weeks ago, and not uploaded till now thanks to RACSA’s breakdown…

I am back in Heredia again, after a three-week stint in Palo Verde. It wasn’t nearly as bad as I’d been led to believe—the hordes of mosquitoes I recall from the wet season were almost entirely absent, the ungodly heat was tempered by a breeze, and the scorpions (large and abundant though they were) mostly kept to themselves. I think I could develop a taste for Guanacaste in the dry season. It was strange, though, to see a tropical forest with hardly any leaves. I've become so accustomed to the evergreen forests of the Atlantic lowlands. I think many people,when they think tropical forests, think rain forests. But 42% of tropical forests are dry forests, and many dry forest trees are deciduous. Hence the strange combination of blinding sun, scorching heat, and leafless trees.

Some researchers have argued that dry forests are even more endangered than rain forests. One reason is that dry forest makes great cattle pasture, and it's easy to keep it clear by burning. The trouble is that burning favors invasive, exotic species, here particularly the pernicious pasture grass jaragua (Hyparrhenia rufa), which can form stands so dense that tree seedlings don't stand a chance. Parque Nacional Palo Verde protects one of the last remaining fragments of tropical dry forest in Central America, clinging to the steep sides of limestone ridges along the edge of the Tempsique floodplain.

In Costa Rica, most of the formerly dry forest areas have been converted to giant ranches, making Guanacaste the Wild West of Costa Rica. Cowboys (sabaneros; literally "men of the savannah") on horseback are a common sight along the dirt roads, and vast expanses of ranch land with emaciated Zebu cattle stretch off as far as the eye can see. Even within the park, cattle concessions still operate, though the cattle are now pastured in the marshy river floodplains rather than the few fragments of remaining forest. I never did get a clear answer as to why there are cattle in the park; my cynical side expects that there is a payoff somewhere. According to certain factions, the cattle help keep down the cattails (Typha domingensis), another invasive species, in the marsh... but none of the scientists I met agreed with this view, and there certainly seemed to be plenty of cattails in the areas with cattle.

Aside from cattle, the floodplain marsh supports an amazing variety and quantity of water birds: ducks, herons, egrets, storks, spoonbills, rails, etc. The marsh was dried rapidly; we watched areas go from deep water to dried, cracked dirt in the three weeks that we were there. Flocks of birds congregated in ever-smaller spaces as the water receded, making their numbers stand out even more. One afternoon I was fortunate enough to spot a jabiru stork. These massive birds can stand up to 1.5 m (5 ft) tall. I stalked out into the marsh to try to get a picture of it. No luck-- they are very wary birds-- but I did get a picture of my footprint next to the bird's. (For reference, I wear a size 10-11 shoe.) Outside the marsh I also spotted a pair of scarlet macaws, my first, but once again I was not quick enough with the camera.

The marsh at Palo Verde had a somewhat otherworldly aspect, with the weirdly-shaped limestone mountains rising up all around and giant, ungainly waterfowl flapping in slow motion against the constant wind. I sometimes felt (especially before my coffee in the morning) that I'd landed on an inhospitable marsh planet from the Star Wars universe, where the only human habitations cluster around the base of mountain ranges. It would not have surprised me unduly to see Imperial Walkers approaching from the Tempisque.

In the middle of our stay at Palo Verde we took a side trip to a mangrove swamp, which really looked like something out of Star Wars. At the outskirts, the white mangrove (Avicennia germinans) formed a monospecific stand. The regularly-spaced, sandy-brown trunks looked almost too orderly, as though they'd been planted. Here's the weird part: Avicennia has aerophores, little nobbly roots that allow gas exchange in the fine, silty soil. They stick up like a congregation of miniature snorkels, ankle-high all over the forest floor. We walked towards the ocean a ways (smelling the salt and hearing distant surf above the rush of wind in the trees), and we came to the red mangroves (Rhizophora mangle). I don't imagine many people have seen old-growth mangrove trees. Hopefully my pictures (scroll down) will do them justice, but I will add a few words as well. From ten feet up they look like a forest of aspens (same gray trunks, same cheery yellow-green in their leaves as aspens in about mid-June); from among the roots they look like a crazy jungle gym. They look like they walked there. They look like they could take off any time they wanted, like the jumping trees in the E.T. book (which was so much better than the movie, by the way!)

On the way back from the mangroves we visited Megafauna Park, a collection of statues of the extinct fauna of Central America. For anyone who wants to buddy up to a gomphothere, here's your chance! It was neat to see how large some of these animals really were.

Well, this entry is long enough by far. I will upload some photos and post it before it gets any longer!


At 7:37 PM, Blogger Em said...

Susan, I love the pictures of the mangroves. You'll have to work on your bird-stalking skills (or get a camera with one of those super-duper extra long-range zoom lenses). Is the marsh supposed to disappear in the dry season, or is that just another unfortunate effect of global climate change? The picture of the cracked mud is excellent -- really artistic.

And speaking of megafauna -- Have you read "The Ghosts of Evolution: nonsensical fruit, missing partners, and other ecological anachronisms"? The author is Connie Barlow and it is about the now extinct megafauna and how there are now all these plants around with really large fruits/seeds without the properly large dispersers.

At 2:22 PM, Blogger Dan said...

The picture of the cracked ground is really stunning!

At 3:05 PM, Blogger Susan said...

Thanks, Em and Dan!

The marsh does disappear every dry season, though the droughts are apparently more frequent and intense than they used to be.

I haven't read the Barlow book, but I've done some reading about the megafaunal dispersal hypothesis. Dan Janzen (odd genius ecologist working near Palo Verde) was the first to come up with the idea. It makes a lot of sense, when you look at the fruits of some trees in the region that are currently dispersed only by introduced cattle and horses!


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