Tree of the month: Spread of the Saman tree

SAMANEA saman is in the plant family Fabaceae (clade Mimosoideae), with classic clusters of flowers in pom-pom-like heads; 12 to 25 flowers massed in pinkish-coloured heads,
5–6 cm wide and 4 cm high. It has long, bi-coloured stamens, reddish along the top half and white on the lower half, to give the appearance of a powder puff or feather duster, displayed slightly above the foliage for efficient pollination. The central flower in the cluster is the largest with more petals, sessile (without a stalk) and functions purely as a nectar-producing organ to attract insect pollinators. Pollination and fruit set are confined to one or rarely two other flowers in the head.

Trees carry thousands of flower heads, covering the canopy with a pinkish bloom although they soon wither in the tropical sun. There is a definite flowering period; January to May within its native range, depending on latitude and seasonal pattern of rainfall. Some flowers are generally present year-round, especially in countries like Trinidad where rain now falls throughout the year; the erstwhile wet/dry pattern having been disrupted and distorted by climate change.

Like mature English oak trees, which find themselves within a new development, the Saman is hacked about to fit in with the changed environment.

Mature pods are black-brown, oblong with a lumpy appearance and 10–20 cm long. They eventually crack, albeit unevenly, to reveal a sticky, brownish pulp surrounding plump, oblong/ellipsoid-shaped seeds. Seeds are laterally flattened and dark, glossy brown with a U-shaped marking (pleurogram) on the flattened sides. There are normally 20–25 seeds per pod, but insect predation is high within its New World native range where 5–10 seeds is the norm. 1 kg of clean, dry yield averages 4,000–6,000 seeds.

Dense, rich green foliage comprises alternately arranged compound pinnate leaves, each with a prominent swelling (pulvinus) at the base of the petiole. Leaf blades are twice pinnately compound, arranged in 2–6 pairs of oppositely borne pinnae. Each pinna bears 6–16 leaflets, glossy and dark green on the upper surface and dull with fine hairs underneath.

READ MORE: Saman tree is not all it appears to be

In Equatorial regions, S. saman behaves like an evergreen, but it is actually semi-deciduous, showing up further from the equator in regions with a definite dry season. Here, trees may be leafless for a period of weeks but refoliate rapidly when moisture levels are restored.

Saman trees normally have a short stout bole of about 1–2 m dbh (diameter at breast height). Trees in open-growing aspects rarely exceed 25 m height and spread out to
30 m or more. In dense plantings, height increases up to 40 m with a much narrower crown diameter. Bark of mature trees is dark grey and fissured, but paler and smoother on young trees.

Saman trees are wet tropical in origin, growing best near to sea level with 2,000 to 3,000 mm of rainfall distributed throughout the year. Trees can tolerate a 2–4-month dry season but not an extended period of drought. They are demanding of temperature (25 to 30+ °C) and intolerant of anything much below 10°C. They adapt well to a wide range of soil pH (5–8), are non-fussy about texture and depth, and will withstand impeded drainage, even waterlogged conditions for a period of time. A nitrogen-fixing, leguminous tree, S. saman will happily grow in non-fertile soils.
Lush vegetation growing under Saman trees supports butterflies including Precis lavinia zonatis – donkey’s eye – (shown here) in its native tropical Americas.

USES AND APPLICATIONS

Samanea saman grows quickly to spread and sequester a colossal biomass at maturity, but do growth rate, canopy dimensions and biomass match the overall value of the tree?

Traditional yardsticks used to assess a tree species’ value are food, fuel, shelter, medicines and agroforestry, with tropical communities far more reliant on these traditional tree values. They occasionally come together in a single species, appropriately called a ‘miracle’ tree. Samanea saman has never achieved miracle-tree status, although an appraisal of overall value suggests it should qualify.

Samanea saman has everything going its way, including canopy shape and structure, to furnish excellent shade for crops and livestock, generating soil fertility, providing nitrogen-rich animal fodder, fine hardwood likened to black walnut and fast growth rate that ensures a continuous supply of the above-mentioned resources. Over and above these attributes, S. saman is an excellent amenity tree used as a centrepiece or backdrop on large lawns, sports fields, schools and marketplaces throughout the tropics.

Trees with ‘miracle’ status generally have a long-established presence with limited geographical distribution where the full range of attributes has been harnessed by local communities. Saman has been dispersed throughout the tropics from a relatively restricted New World natural distribution but use and applications are similarly scattered. For instance, Saman is used as a shade tree for cocoa and coffee in Oceania, but is never used in this way in the Caribbean including Trinidad and Tobago.

So, what are the uses and applications of this remarkable tree species?

Saman trees provide ideal shade but not protection from rainfall – hence the marquees under the canopy of this tree.

FOOD AND FODDER

Food uses are minor but interesting, nevertheless. Sticky pulp surrounding the seeds is astringent with a licorice flavour and eaten to limited extent by children in some parts of the West Indies, where
S. saman is also called the licorice tree. In Latin America, the fruit pulp is used to make a beverage similar to ‘tamarindo’ (made from tamarind pulp). Nocturnal moths are natural pollinators of the flowers, although there is ample nectar for bees. Honey made from Saman tree nectar is harvested for local consumption.

With up to 18 per cent protein and a 40 per cent digestible component, the pods make a first-class supplement for animal feed, eagerly taken by cattle, pigs and goats, while horses will eat them if nothing else is available. Leaves are even richer in crude protein (22 to 27 per cent) and in Asia the tree is specifically grown as green fodder for goats, sheep and cattle. With a huge potential size and fast growth rate a single tree can furnish 550 kg of green forage and 250 kg of pods per season. In Latin America pods are ground into animal feed with a high nutritive value.

READ MORE: New hedgerow planting hits a high note

Use as animal fodder is assisted by a number of factors extra to the nitrogen fixation by Bradyrhizobium bacteria in root nodules. High production of seed, a dependable rate of natural generation and the ability to self-prune ensure a ready and continuous supply of new young and well-developed trees, while positive response to coppicing ensures fast production of new, nutritious foliage. Saman trees possess good coppice capability, although re-growth is slow compared with other tropical leguminous trees like Leucaena species and Gliricidia sepium. Trees are typically coppiced at 1 m above ground twice-yearly to produce fodder.

There are indirect benefits too because the nutritional value of grasses grown under and adjacent to rain trees benefit not only from nitrogen supplementation but sufficient moisture too due to the light-sensitive attributes of the leaves allowing rainfall to pass through the canopy. This, together with a ready supply of leaves as green forage on low growing branches, nutritious pods and shade provision, makes S. saman the ideal tree species for silvo-pasture and savannah livestock grazing systems in the tropics.

Within the tree’s natural New World range, use of pods as livestock feed has inadvertently played a key role in seed dispersal and tree regeneration. Feeding habits and digestive capacity of New World wild animals like peccary, tapir and rodents is such that very few seeds pass through in a viable condition. Some suggest that fruits of the Saman tree originally evolved for seed dispersal by large wild mammals which became extinct in the Americas during the glaciations of the Pleistocene period. But domesticated animals such as cattle have assumed this role as efficient dispersal agents of seed.

Saman trees support a huge range of epiphytes, climbers and creepers on the bole and main branches.

AGROFORESTRY

Samanea saman is very good at dishing out shade but cannot take it. Seedlings can be grown under partial shade for several weeks but soon require full sunlight. Trees growing in mixed stands will have established before or simultaneously with other species. Saman trees occur naturally in grassland, but seedlings are rapidly killed by dense stands of the faster growing tall grasses like Guinea grass (Panicum maximum) and elephant grass (Pennisetum purpureum).

WOOD AND TIMBER

Sapwood occupies a narrow white/cinnamon band while heartwood is straight or cross grained with a medium to coarse texture. Saman tree wood is generally underrated and underexploited. The wood is used for carvings, furniture, panelling and veneers and for less high-value uses including turnery, posts, frames for boats and canoes, boxes, plywood and crates, depending on the region. World-famous ‘monkey pod bowls’ and other souvenirs carved and sold in Hawaii for over 60 years are of S. saman though timber is now imported from Indonesia and Thailand due to insufficient wood resource on the Hawaiian Islands. Samanea saman wood is on par with black walnut (Juglans nigra) and use for high-quality veneers is an obvious way forward.

Due to shrinkage and moderate to severe warping, the wood requires careful drying to achieve air-dry specific gravity of around 0.56. The wood is generally regarded as highly durable and resistant to attack by dry-wood termites, and has ideal fibre properties for paper making. In the Bulacan province of the Philippines, shavings from the wood are made into hats. Gum exuding from wounded trees is generally low-grade but used in Thailand as food for the lac insect (Laccifer lacca), the source of shellac. Wood yields average 25–30 cubic metres per hectare. Where there is no market for the wood in ornamental carving or building, trunk and branches are used as fuel. It makes good firewood although characteristically burns with a lot of smoke, even when dry.

As the rainclouds gather and the light intensity falls, the leaflets close to open up the canopy and ground beneath to warm tropical rainfall.

LANDSCAPE AND AMENITY

Samanea saman has made its mark on the landscape as large centrepiece trees, avenue planting alongside roads, living fences, hedges, windbreaks and noise barriers. In Asia, the tree is used as a hedge, with heavy trimming to maintain the desired form and the clippings used as livestock feed or green manure.

Green foliage and leaf/flower litter accumulating beneath the huge canopy is one of the best-known sources of nitrogen, enriching the soil and providing compost for gardens. Trees become too big to be used in most garden planting and even in the amenity sector must be rigorously pruned to prevent break up of pavements and roads by the extensive shallow root system. Salt spray may cause cosmetic leaf tip burn, but the tree is largely tolerant and thrives in small island environments.

Samanea saman is in the plant family Fabaceae (clade Mimosoideae) with bi-pinnate compound leaves and clusters of flowers in pom-pom-like heads.

THE MIRACLE TREE

Samanea saman clearly deserves the title ‘Miracle Tree’ but rarely does the full spectrum of uses come together under the umbrella of one country or region. It is instead spread throughout countless Old World tropical locations where the tree has been introduced.

However, there are always situations where you can have too much of a good thing and none more so than for Saman trees. They have become so successful in exploiting ecological niches in parts of the Asia-Pacific region that they are now considered invasive.

Outside of the Caribbean, Saman is called the rain tree, although not for the reason you might imagine. At first sight, Saman is an ideal tree under which to shelter from a tropical downpour – but do that and you will get wet quickly. The reason is that the leaflets are light-sensitive and close up on cloudy days (and during the night). Rain falls through the canopy onto the branches, the bole and, eventually, the ground, making the grass and broadleaf vegetation under rain trees greener and lusher than vegetation under other trees nearby.

Other suggestions for the name ‘rain tree’ relate to a drizzle of honeydew excreted by huge populations of sucking insects feeding on the foliage, or sugary solution from nectarines on the leaf petioles, which sometimes falls from the canopy like rain. Or even the mass drop of spent stamens ‘raining down’ during flowering.

I recall watching grounds-maintenance crews at the St Augustine campus of the University of the West Indies in Trinidad routinely cleaning epiphytes and climbers from Saman trees, and the comment of fellow student John Turner from Croydon who was studying Bachac ants. The workers wielded their super-sharp cutlasses to strip everything including philodendrons, bromeliads and even orchids from the trunk and main branches. “Imagine what that lot would cost to buy in a florist’s back home.”

Forestry Journal remains dedicated to bringing you all the latest news and views from across our industry, plus up-to-date information on the impacts of COVID-19.

Please support us by subscribing to our print edition, delivered direct to your door, from as little at £75 for 1 year – or consider a digital subscription from just £1 for 3 months.

To arrange, follow this link: https://www.forestryjournal.co.uk/subscribe/

Thanks – and stay safe.