Rare Trees

CHAPTER 1

Trees and Their Ecosystems

Trees define landscapes worldwide. Outside the extreme polar regions and at the earth’s highest altitudes, trees flourish in a wide variety of ecosystems. They are the main biological components of the world’s forests and also grow in savannas, grasslands, deserts, and wetlands. Trees survive as remnants of natural vegetation in agricultural landscapes and are much loved in our towns and cities, whether in fragments of natural forest or planted in our parks and gardens or along our streets. The world’s continental forests offer a rich diversity of trees, while islands include all the different types of forests found elsewhere and, because of their isolation, have a rich concentrations of endemic tree species.

TROPICAL RAINFORESTS

Magnificent tropical rainforests cover about 6 percent of the world’s land surface. They are home to a huge amount of biodiversity (about half of all known plant and animal species) and maintain some 40 percent of the earth’s carbon stocks. Trees are their essential components, defining the architecture and composition of tropical rainforests; they have evolved to reach their maximum diversity within these forests. The Amazon alone has an estimated 16,000 tree species. Rainforests in Southeast Asia also have a very large number of tree species, followed by rainforests in New Guinea, Madagascar, and elsewhere in Africa. Many tree species within rainforests are naturally rare, existing in low numbers over large areas. The reasons for the exuberant tree diversity are not fully understood but may be partially attributed to the stability of lowland rainforest areas over a very long period of time and climatic conditions that favour plant growth.

Walking high in the forest canopy on a jungle walkway is an exhilarating experience, giving some idea of the scale and diversity of the rainforest ecosystem. Jungle walkways are the perfect opportunity to see the abundance of trees of different heights, with different shapes and sizes of leaves in different hues of green, stretching for miles and miles. Flashes of colour are provided by flowering trees and epiphytic orchids and, at the right time of day, birds and butterflies animate the forest. Standing in the middle of the canopy walkway of Ulu Temburong National Park in Brunei, high above the forest floor, it is easy to imagine a world entirely covered in lush forest. No buildings or roads are in sight. But in reality most of the rainforests of Borneo and all the world’s other tropical rainforests have been modified and are now fragmented and continuously under threat. National parks like Ulu Temburong are increasingly important refuges and among the few places where we can see tropical rainforest in a near pristine condition.

The hot and wet climatic conditions that support rainforests in the tropical lowlands are ideal for the luxuriant growth of trees. Generally the soils underlying tropical rainforests are formed from ancient rocks; they are leached by high rainfall and therefore infertile, with a lack of readily available nutrients. Tall, diverse forests can exist on these soils only because rainforests have evolved to recycle nutrients efficiently. Very often, symbiotic fungal associations (mycorrhizae) found in the tree roots enable trees to utilise nutrients and water resources more efficiently. These fungi may also provide protection against harmful plant pathogens, and the trees in turn provide carbon as an energy source to the fungi. Extensive buttresses often support the towering straight trunks of rainforest trees; these are necessary because root systems of rainforest trees are generally shallow, extending laterally to tap the nutrients in the surface litter layer.

Rainforest trees have also evolved to make maximum use of the sunlight essential for photosynthesis. Competition for light is intense. Many tree species have the ability to remain semidormant under the canopy until a light gap appears, for example, when another large tree dies or decays. Gaps in the forest may result from a tropical storm or natural fire, and then the semidormant tree seedlings “wake up” and undertake very rapid growth. Leaves in understory forest species are typically arranged in a single layer to minimise casting shade. Leaves in the forest canopy are usually leathery and drought resistant, to withstand severe sun intensity. Some leaves alter their orientation during the day to avoid sun stress.

Interactions between plants and animals are very highly developed in the rainforest. Seed dispersal, for instance, is largely accomplished by animals, and much pollination is by vertebrates and social bees, which travel long distances between scattered individuals of many plant species. An extraordinary example of large-scale bat pollination can be observed in the Calakmul Biosphere Reserve in the state of Campeche, Mexico, where every day just before dusk a whirlwind of 7–13 million bats (depending on the period of the year), leaving their subterranean cave for the night, can be observed. An unknown but important percentage of the 450 tree species recorded in this area of 20,000 km² depend on bats for pollination and dispersal.

Many rainforest tree species are in large plant families, some of which are entirely or largely restricted to tropical forest areas, including tropical cloud forests. These families include Anacardiaceae, Annonaceae, Bignoniaceae, Capparidaceae, Dipterocarpaceae, Lauraceae, Melastomaceae, Meliaceae, Moraceae, Myrsinaceae, Myrtaceae, Rubiaceae, Sapindaceae, Sapotaceae, and Sterculiaceae.

Rainforest trees provide a rich diversity of useful products. Tropical hardwoods are the most economically important on a global scale and provide major export earnings for tropical countries. Over a thousand different tree species are sourced from tropical forests for use as commercial timber, and many more are used locally. Fuelwood sourced from the rainforest is also very important for people living in or near the forest. It has been estimated that approximately 1.3 billion people rely on forests for their livelihood. Wild fruits and nuts, leaves for thatching, latex, gums and flavourings for local use and trade, and medicinal products are just some of the products provided by rainforest trees. Exploitation of high-value products such as wild rubber and Brazil nuts attracted hundreds of thousands of migrants into the Amazon in the 19th century. Tropical forests are the source of around 2500 edible fruits. Papua New Guinea alone has more than 250 edible fruit tree species, 40 of which are in cultivation; the rest provide fruit sourced from the wild. The forests of Borneo are the origin of important tropical fruits such as mango, durian, breadfruit, and jackfruit, a new miracle fruit that is increasingly popular with vegans. Major commercial crops such as cocoa, avocados, and citrus fruits have all been developed from rainforest trees.

Did you know?

Local people living close to or in the rainforest typically collect wild fruit and other forms of food to supplement the crops they grow. Slash-and-burn farming (aka shifting cultivation, swidden) has been used to manage forests for food over the past 10,000 years. Land is first cleared by removing the trees and then burnt and used to grow crops. After one or many seasons, the land is abandoned, and a new area is cleared and burned. Slash-and-burn farming is now practised in over 40 countries in tropical regions of Africa, Asia, and Latin America. As human population increases, demand on land increases, and abandoned land is not left long enough to recover, nor is there enough natural forest left to provide the plants and seeds to allow for recovery to take place. Shifting cultivation has been considered a major threat to the rainforest and as general pressures on the land increase, it is contributing to forest loss in parts of the world. But where traditional knowledge relating to fallow management is well developed and applied, shifting cultivation can be a form of sustainable forest management, especially when human population density is low. In these cases, soil fertility is maintained while conserving biodiversity and maintaining the provision of other forest ecosystem goods and services. Within tropical countries, where most forest change has occurred, deforestation due to smallholder agricultural expansion has generally given way to large-scale agricultural forest conversion.

Meanwhile logging continues apace, and few tropical rainforest areas have not been exploited. Currently about 15 percent of the timber produced globally is sourced from tropical forests. Timber from the forests of the Caribbean was shipped to Europe from the time of Columbus, with Cuban mahogany (Swietenia mahagoni) a prized resource. Exploitation of west African timber for the European market dates to at least 1672, when the Royal African Company received a charter from King Charles II of England to trade in African mahogany (Khaya and Entandrophragma spp.). By the early 18th century, west African timber replaced the diminishing and more distant supplies from the Caribbean region. Trade in African timber increased significantly with European colonialism at the end of the 19th century and evolved rapidly after 1945, still controlled by European interests, to supply utility-grade wood.

In Southeast Asia, tropical forests in Peninsular Malaysia have been managed for commercial timber since the early 1900s, initially to supply UK needs. International demand for Malaysian timber became significant in the 1950s; increasing postwar demand in 1953 and agricultural expansion after Malaysia’s independence in 1957 made large quantities of logs available. In Indonesia, teak plantations developed in Java under Dutch colonial rule formed the basis for commercial forestry until after World War II. The vast forests of Indonesia and East Malaysia (Sabah and Sarawak) were opened up in 1965 for the international market. In the Philippines, intensive logging of the dipterocarp forests took place from the end of World War II until the 1970s. In 1968, timber accounted for 33 percent of the country’s foreign exchange earnings, falling to 5 percent by 1986 because of lower global prices, forest depletion, and conservation policies.

The first global assessment of the extent of logging in the tropics, compiled in 2009, indicated that 20 percent of the tropical forest biome was either actively logged or allocated to logging concessions between 2000 and 2005. About half of this area had already been heavily affected by land-use change, losing over 50 percent of its forest cover. By 2010, commercial timber extraction reached into the centre of Amazonia and was expanding rapidly into new areas, including central Africa and Papua New Guinea; soon it would reach the very last remote tropical forests.

There are major efforts to conserve rainforests around the world, as highlighted in the introduction to this book. Managing tropical rainforests sustainably and setting aside sufficient natural areas is certainly an efficient way to conserve maximum tree species on a global scale. Approximately 25 percent of all tropical forest is now in some form of protected area. Serranía del Chiribiquete National Park, in the heart of the Colombian Amazon, is the world’s largest national park protecting tropical rainforest. In 2018, the size of this protected area was doubled, and it was declared a UNESCO World Heritage site, reflecting its enormous environmental, cultural, and social value. This biodiverse area, located at the westernmost edge of the Guiana Shield, protects many tepuis (steep-sided tabletop mountains), which have very high levels of endemism. The tepuis found in Chiribiquete have dramatic scenery that is relatively undisturbed, making the site one of the most important areas of wilderness rainforest in the world. But the tepuis also have a long history of human use. Some 75,000 pictographs have been recorded on the walls of rock shelters at the foot of tepuis. These scenes of hunting, battles, dances, and ceremonies are thought to reflect a thousand-year-old system of sacred beliefs, representing and explaining the relations between the cosmos, nature, and people. Even now they are believed to be accessed by uncontacted indigenous groups.

We still have a lot to learn about tropical forests and their trees. In some ways our understanding is no more advanced than that of ancient inhabitants, who relied on their forests for all their survival and spiritual needs. Now we have satellite imagery, which gives a broad picture of forest extent and condition. We can travel to rainforests from around the world with relative ease. But our knowledge of how tropical forest ecosystems function and sustain global ecosystems is still rudimentary.

At ground level, a global network of permanent forest plots is helping scientists to understand more about tree diversity and ecological functioning within the forest. The Forest Global Earth Observatory, for example, is a global network of forest research sites and scientists dedicated to the study of forest function and diversity in both temperate and tropical areas. With over 70 forest research sites across the world, ForestGEO monitors the growth and survival of approximately 6 million trees and nearly 13,000 species that occur in the forest research sites. The initiative, begun in the 1980s, now conducts long-term, large-scale research to increase scientific understanding of forest ecosystems. This knowledge helps to guide sustainable forest management and natural-resource policy, monitor the impacts of climate change, and build capacity in forest science. Forest plots are mainly used for ecological research, to understand how trees and other species work individually and collectively in their natural environment. At the species level there is still much to learn, even concerning many relatively widespread tropical tree species, including our most important forest-dwelling flagship species.

The only way to fully document tree species diversity is to undertake fieldwork, both in the forest plots and in areas which are far less known. There are new species of trees to be discovered and distribution records to be compiled for species that are sometimes known only from a few herbarium records. With this baseline of knowledge, the conservation status of tree species can be assessed and ideally the ecology of at least an increased range of tropical trees can be studied.

The Lacandon Forest

The Lacandon Maya people, indigenous to the Yucatán Peninsula, initially moved to the forests of southern Chiapas and the Guatemalan Petén in the 1700s to avoid participating in the war that took place between the Mayas and the authorities of the Spanish colony (and later, the Mexican government). The frontiers in this completely unoccupied area were not well defined at the time, and neither were there authorities, so the Lacandon people developed a society that was totally self-sufficient, without any contact with neighbouring civilizations. An example: they never depended on commercial salt, instead obtaining their salt by burning the stems of the palm Cryosophila stauracantha.

At the beginning of the 20th century, when the frontiers between Mexico and Guatemala were defined, all the Lacandon migrated to Mexico, since in Guatemala they were persecuted, captured, and put on display in cities. In the 1950s, archeologists discovered the monumental murals of the Bonampak archeological site, through which the Lacandon presented their culture to the world. As a consequence, the Mexican government designated a large conservation area in the Lacandon forest.

The population of the village of Nahá maintains the most traditional lifestyle, and their conservation areas, although relatively small, are the best-conserved of the whole forest. Esteban Martínez Salas, botanist of the National Herbarium of Mexico, has worked with this community for more than 35 years, and his research team and other scientists have been able to generate extensive knowledge of the area’s natural resources, as the Lacandon people are very open toward participation with researchers. This partnership in support of continuous conservation actions in the area is extremely important, given that 40 percent of Mexico’s water resources originate in this extraordinary forest.

Marie-Stéphanie Samain is a Belgian botanist now based in Pátzcuaro, Mexico. She has travelled extensively in the tropical Americas, undertaking work on hydrangeas, magnolias, and non-woody plants such as peperomias. She describes the reality of working in the tropical rainforest:

The first time I set foot in a tropical rainforest, I felt deeply impressed and overwhelmed by the high trees, the forest noises and smells. At the same time, I was surprised that primary rainforest generally does not consist of impenetrable vegetation as I had imagined before or as you see in movies. You can easily walk among the trees and gaze up to their canopies to spot a little bit of the sky. . . . I am very fond of all the memories I have collected over the years, such as the time we camped in La Chinantla, Oaxaca, Mexico, and I slept outside in my sleeping bag, cradled asleep by myriad animal noises and waking up to a breakfast of raw inflorescences of the palm Chamaedorea tepejilote; or the visit to a 400-year-old brazil nut (Bertholletia excelsa) plantation in the middle of the forest near Kayserberg, Surinam, planted by indigenous people who used to travel by pirogue between Surinam and Brazil. Another unforgettable experience was when during a hydrangea collection trip in Peru, I climbed a tree north of San Ignacio in Cajamarca, near the border with Ecuador, and I was able to observe the forest stratification from above, looking down into the phyllotaxis of a giant tree fern!

TROPICAL CLOUD FORESTS

Step inside a cloud forest, and what do you see? Initially perhaps rather little, as everything is surrounded by a thick mist, swirling slowly around you. As you walk onward, the dark shapes of trees loom out of the fog, their trunks and branches completely covered with epiphytes. Ferns, bromeliads, and orchids grow densely together on every available surface. Perhaps your attention is caught by a flash of colour, as a hummingbird darts between flowers, or a tree frog hops onto a leaf. Underfoot, the ground might be springy with moss or poorly decayed leaves, which, owing to the low temperatures and high humidity, accumulate gradually over time. The air is very still, and sound is deadened by the mist, yet everywhere you hear the sound of dripping water. The silence is penetrated by the whistles and chatters of orioles and bellbirds, and maybe the booming calls of a troop of howler monkeys.

Cloud forests occur in some unusual places. For example, in the deserts of Peru and northern Chile are small islands of vegetation entirely supported by the fog that regularly rolls in from the Pacific Ocean. These are home to some mysterious trees, such as the critically endangered arrayan (Myrcianthes ferreyrae), which is found nowhere else in the world. However, most conservation attention has focused on the cloud forests found on the top of some tropical mountains. As you climb a tropical mountain, the forest changes in both structure and composition. The tall and often buttressed trees of multi-storied lowland rainforest gradually give way to trees that are shorter in height, with a simpler canopy structure. Epiphytes become more abundant, and trees are increasingly mossy. The shortest and mossiest forest of all, the upper montane cloud forest, is found at the highest elevations, on ridge crests and upper slopes. The transition from lower to upper montane cloud forest occurs where condensation from clouds is most persistent. This typically happens at 2000–3000 metres, although cloud forests occur at much lower elevations on some oceanic islands.

The most extensive tropical montane cloud forests occur in the neotropics, where they clothe the upper slopes of the Andes and extend through the mountain chains of Central America into southern Mexico. Other important cloud forest areas include parts of East Africa and Southeast Asia. Cloud forests are also found in the Pacific region, typically as small and isolated patches on the upland ridges and peaks of volcanic islands, such as Hawaii. The cloud forests of these different regions are characterised by very different floras. For example, neotropical cloud forests at lower elevations are dominated by members of Annonaceae and Melastomataceae. These same plant families often occur at higher elevations, but here they may be joined by temperate-climate genera, such as Podocarpus, Drimys, and Weinmannia. In Southeast Asia, the different vegetation zones associated with elevation are characterised by families of different continental origins. Plant families with Laurasian affinities, such as Fagacaeae, predominate in lower montane forests, whereas at higher elevations, members of Australasian families (Myrtaceae, Podocarpaceae, Araucariaceae) are more likely to be encountered. In other words, the world’s cloud forests are united more by their structure and appearance than by their taxonomic affinities. Above all, these forests share an adaptation to high cloud cover and frequent fogs and mists, which creates conditions of high humidity but low sunlight and air temperatures.

Although they occur in inaccessible locations, cloud forests are typically full of life. They are particularly rich in those groups that thrive in high humidity, such as mosses, fungi, lichens, ferns, orchids, and other epiphytes. Many reptiles and amphibians are also found here, including many species of brightly coloured tree frogs and poison arrow frogs. Recently, seven new species of miniature frog, each smaller than a thumbnail, were discovered in the cloud forests of Brazil. These live on sky islands—areas of cloud forest on mountains that are surrounded by lower-altitude rainforest. Such amphibians clearly benefit from the high humidity conditions found in cloud forests, but perhaps more surprisingly, the diversity of mammals can also be high. Examples of species that might be encountered include tapirs, sloths, jaguars, rodents, and monkeys. Again, new species continue to be discovered. As illustration, the Tabaconas Namballe National Sanctuary in Peru, a cloud forest reserve that extends over 70,000 acres, is home to the endangered mountain tapir and spectacled bear. In addition to the 82 mammal species known to occur there, eight new species were recently discovered, including the orange-eyed night monkey, a shrew opossum, an extremely long-quilled porcupine, and an olingo (a relative of the raccoon).

Cloud forests are also very important for birds. Ten percent of the world’s 2609 restricted-range bird species (i.e., those with a range < 50,000 km²) are mainly found in cloud forests. Similarly, around 12 percent of the threatened bird species of the Americas are cloud forest species. Many songbirds and other bird species from North America migrate south and overwinter in neotropical cloud forests, where they play an important role in seed dispersal, with up to half of tree species producing fruit that are bird-dispersed. Cloud forests are particularly important for hummingbirds, with more species than any other habitat. Recent bird discoveries include the jocotoco antpitta (Grallaria ridgelyi), which lives in a small patch of cloud forest in Ecuador, and the scarlet-banded barbet (Capito wallacei), found on a single mountain in Peru. Perhaps the most iconic cloud forest bird is the resplendent quetzal (Pharomachrus mocinno), famed for its wonderful iridescent green plumage and long tail feathers. This neotropical species was venerated by pre-Columbian people of Mesoamerica; it was considered to be divine by the ancient Aztecs and Maya, who associated it with the snake god, Quetzalcoatl.

The high species richness of cloud forests makes them an important priority for conservation. Yet they are also important for another reason: many cloud forest species occur nowhere else. In Mexico for example, cloud forests cover less than 1 percent of the country’s land area, but provide a home to around 10 percent of the nation’s plant species. Up to 30 percent of Mexico’s endemic plant species—in other words, those not found in any other country—occur in cloud forests. Similarly, half of Ecuador’s plant species occur in the montane forests of the Andes; of these, 39 percent are endemic. Patterns of endemicity can be quite extreme. As the great American botanist Alwyn Gentry put it, “Some plant families seem to have evolved locally endemic species in every patch of isolated Andean forest.” As a result, up to a quarter of the species found in a single area of Andean cloud forest may be found nowhere else. This makes cloud forests a particularly rewarding place to hunt for new tree species. For example, during preparation of the Flora of Colombia, 21 endemic species of Magnoliaceae were discovered on Andean mountains. Similarly, new tree species continue to be discovered in cloud forests throughout Mesoamerica. Sometimes it is possible find new species without even leaving your desk. Two new Magnolia species, M. rzedowskiana and M. pedrazae