Tobago's Main Ridge Forest Reserve: "for the purpose of attracting frequent showers of rain"

Welcome to the third piece in this series highlighting the biodiversity of Tobago’s Main Ridge. This week, Amy Deacon has invited her colleague Aidan Farrell, a Plant Biologist at The University of the West Indies, to take us on a tour of the lush vegetation of the Main Ridge reserve -and why we can’t do without it! This feature was first published in the Tobago Newsday on Thursday 17th February 2017.

A recent botanical survey of Trinidad and Tobago carried out by the National Herbarium, found that the Main Ridge was home to a third of the most biodiverse plots in the country. The survey recorded close to 350 plant species in the area, including 12 endemic plant species. This high diversity is in large part due to Tobago’s geological history; as well as sharing many species with the South American continent, it also supports species typical of the Antilles. Importantly, this diversity has been retained in the Main Ridge as it has been protected from exploitation since 1776. Another reason for the presence of high numbers of species is the fact that the reserve includes three distinct habitat types: lowland rainforest, lower montane rainforest, and xerophytic rainforest. It is notable that all three forest types include the word ‘rain’ as, in fact, rain was the motivation behind the preservation of the forest in 1776: "for the purpose of attracting frequent showers of rain upon which the fertility of lands in these climates doth entirely depend". In this article we will look at a few of the most characteristic plant species of the reserve, while simultaneously answering the question: ‘how do rainforests make rain?’

Shelldancer, the spectacular Aechmea dichlamydea bromeliad thrives in the Main Ridge. Photo courtesy Kris Sookdeo

The most diverse of the three habitats, and perhaps the forest most associated with the main ridge, is the lower montane rainforest. The lower montane rainforest is described as a Rosewood – Wild Cocoa forest (Byrsonima spicata – Licania biglandulosa forest association), not only because these two species are abundant, but because they tend to be found in this forest type more often than in other types.

Rosewood, more commonly called Wild Cherry or Serrette in Trinidad (although other species also take these names), is related to the Barbados Cherry, and for the birds, bats and agoutis of the main ridge it provides a feast of fruit every October. When flowering, the tree has a distinctive appearance with its grey bark, shiny green leaves and copper-yellow flowers. This species can grow tall (up to 40 metres), yet in the Main Ridge reserve it is usually shorter than 20 metres. Indeed, most trees in the Main Ridge are shorter than the equivalent forest type in Trinidad due to the action of the wind on the exposed ridge (known as ’wind-pruning’).

The copper-yellow flowers of the Rosewood Tree (Byrsonima spicata) Photo courtesy M P Oatham

The Rosewood tree is drought-tolerant and does not lose its leaves in the drier months, which means it can shade the forest floor from the drying rays of the sun. In the wet season, the root system captures rain water and directs it upwards, forming a fine mist above the canopy. All forests contribute to the formation of rain clouds, but the Main Ridge forest makes this process clearly visible; anyone who has driven uphill on the Roxborough - Parlatuvier road during the wet season will have witnessed the transition from the clear dry air near the coast, to the humid atmosphere of the lower montane forest.

When water is plentiful, trees suck up huge amounts from the roots and drag it up into the canopy where it escapes through microscopic pores (stomata). Most of the rain that falls on land is returned to the atmosphere after passing through plant leaves, allowing clouds to form and keeping the rain falling. However, plants are not careless - when water is scarce, the pores on the leaves close and the water remaining in the soil is bound, protected from gravity by the matrix of soil particles and protected from evaporation by the forest canopy. Much of the rain received in Tobago will have passed through the tall canopy trees of the Amazon rainforest as the moist air makes its way northeast, but without the trees on the ridge, it would just as easily pass by.

At its highest points the Roxborough-Parlatuvier road is fringed by thick stands of ferns (bracken), flourishing in the humid air trapped under the tree canopies. The cool humid microclimate created by the canopy also allows epiphytes to thrive. This is a term for plants that live on other plants for support (not as parasites), gaining their water and nutrition directly from the air and rain. In particular there are several species of epiphytic bromeliads or ‘Wild Pines’ in the Main Ridge. Bromeliads are common in the tropical Americas, but most often they are found high in the canopy providing a glimpse of colour to the keen-eyed naturalist. Not so in the Main Ridge reserve; the low, partially open canopy means that even the most casual observer can spot these spectacular herbs, as they are often found at eye-level. Indeed you may need to look down to the forest floor to see some species. However, please don’t be tempted to take one home – as clearly proclaimed on the Main Ridge boundary sign: ‘No Plucking of Plants’ is permitted within the reserve.

The Roxborough-Parlatuvier Road takes you right through the Main Ridge Reserve, with edges lined with lush green ferns. Photo courtesy Kris Sookdeo

One of the most striking bromeliads within the reserve is Aechmea dichlamydea var. trinitensis, or the ‘Shelldancer’ as its cultivated cousin is called. This spectacular species has large fractal-like blooms transitioning from bright red, to pink, then purple or even blue at the tips. This species is only found in Tobago, Trinidad and Venezuela, but it is prized in botanic gardens around the world for its unusually-coloured display. Bromeliads are able to survive away from the soil as their leaves grow in a tight whorl that forms a bowl to collect water. This allows them to store rain, or on rainless days the leaves can even catch water vapour from the air and direct it drop by drop into the water reservoir. At the base of the bowl, specialised roots grow upwards and inwards to soak up the water, while the more traditional roots grow down and out to hug the tree and provide stability. In this way the bromeliads retain water vapour that might otherwise be lost to the wind.

A similar process is happening on a larger scale in the forest itself, the branches and leaves slow down the passing air, preventing it from stripping away moisture and, if the passing air is humid, providing a surface for condensation to form. This condensed water along with the water that come from the rain, is held in the network of plants, soil and streams that make up this vital ecosystem. 

What the island’s climate would be like without the meteorological powers of the Main Ridge doesn’t bear thinking about!

 Sign marking the boundary to the Main Ridge Reserve. Photo courtesy Kalamazad Khan