Tropical forest up in the clouds

Our next stop on the tropical plant course was a remote field station called Cuerici, located at 2600 m on the Pacific slope of Costa Rica’s Talamancan mountains. It was my first time in cloud forest and I found it to be a very magical experience.

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Moss clad branches line the road that leads up to the cloud forest at Cuerici.

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Hiking through cloud forest has a dream-like quality: clouds drift in and out, hiding and exposing different layers of the canopy. I really felt like I was walking through Tolkien’s Middle Earth.

Cloud forests are found at the same elevations that clouds form (which varies depending on where you are). In fact, “horizontal precipitation”–the direct deposition of droplets from clouds and mist–is an essential form of precipitation in these forests (although of course it rains as well).

Due to the so-called “rain shadow” effect, the Pacific slope of the Talamancan mountains (where we were staying) gets 3.5 m of rain per year, whereas the Atlantic slope receives 6 m. This is because the prevailing winds – the trade winds – from the east pick up water from the ocean and deliver rain to the Atlantic slope (on the eastern side of Costa Rica) first, and by the time the air masses pass over the Pacific slope, they contain much less water.

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Clouds roll in over the Pacific slope – this side receives much less rain than Costa Rica’s Atlantic slope due to the “rain shadow” effect (see above). As such, the Pacific slope has been much more extensively deforested, probably because it is easier to clear the land and keep cattle on it (see the foreground). There is only one area of contiguous forest from the lowlands to cloud forest left on the Pacific side.

 Species richness (the number of species in an area) tends to decrease with increasing elevation. Accordingly, we found 18 tree species in the 0.1 hectare plot we surveyed at the Cuerici cloud forest, whereas you might expect to find some 200 species in the same sized plot in the Amazon basin. This decline in the number of species at higher elevations is due to decreasing temperature, decreasing area (you physically can’t pack in as many species at the top of a mountain as at the bottom), and isolation. The Talamancan mountain range resembles an island in many ways – it has been isolated from the rest of Central America for a long time, and so species have gradually been lost over time.

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The Talamancan mountains form a “backbone” down the centre of Costa Rica. Endemism (the existence of species found nowhere else in the world) is very high here due to the lengthy isolation of this region, which has provided the opportunity for many speciation events. 

Cloud forest flora overlaps very little with tropical dry forest and lowland  rainforest floras (the two forest types we will visit next), but interestingly it does share many plant families with the temperate zone. In fact, the forest at Cuerici is 85% white oak (family: Fagaceae). Other families you might recognise from the temperate zone are Betulus (alder), Urticaceae (e.g. nettles), and the genus Rubus (blackberry).

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Cloud forest flora consists of many temperate plant families – albeit different species (NB: families contain many species). That is, the cloud forest contains many plant species from plant families that are most diverse and originated in temperate regions. Left: oaks (family: Fagaceae, genus: Quercus) are the dominant tree in the old growth cloud forest we visited in Costa Rica; right: another representative from the temperate zone: the genus Cornus from the family Cornaceae (dogwood).

 Carlos and his family own and run the cloud forest field station at Cuerici. He has an impressive knowledge of the forest – knowing local and scientific plant names as well as medicinal uses. In his own words, the forest is both his “super market and pharmacy”.

Carlos is also an experienced backcountry hiker. He and his friend Alberto once hiked all the way from Cuerici to Panama over the Talamancan mountains, some 150 miles off-trail through dense forest! Both Carlos and Alberto helped me and our group tremendously, and I hope to go back and visit their beautiful forest again one day.

We conducted a couple of Gentry plots whilst we were at Cuerici – my first! These forest inventory plots are known as Gentry plots because they were used extensively by the famous tropical botanist Alwyn Gentry. It involves recording and measuring the diameter of all shrubs, trees and lianas over 2.5 cm in diameter in a 0.1 hectare area. The class did one Gentry plot in a primary forest, and I conducted another Gentry plot in an area of secondary forest with a smaller group. It will be interesting to analyse the differences, but already it was surprising to see that we recorded a similar number of tree species in both plots.