Outeniqua Yellowwood, the majestic tree of our natural forests
Have you ever wondered how a little seedling emerging from a hard seed under the forest canopy can over time become a massive tree growing up to 50 m in height with a spreading crown of up to 30 m wide and a stem diameter of over 2 m; and live for over 2000 years? The species is nationally protected, and several trees of this species are further protected as Champion Trees under the National Forests Act of 1998. In the forest, the tree is home for many other plants and animals. Its timber has been used in many different ways and it is well used in ecotourism activities and landscaping (gardens, parks and streets). People often wonder whether this tree can be planted more widely for its timber. Some of these points are discussed below.
|Figure 1. Typical habitats of Afrocarpus falcatus in the Southern Cape: Left: Valley of the Giants, Nature's Valley, Tsitsikamma.|
|Figure 2. Typical growth forms of Afrocarpus falcatus in different types of forest: Left: A tall emergent tree with long straight stem in moist forest along the N2, Tsitsikamma. A tree with short stem and large spreading crown along drier river valleys, Kouga with abundant tree regeneration of other forest species in understory.||Figure 3. Differences in the 'fruit' structure of Afrocarpus falcatus (left) and Podocarpus latifolius (right).|
|Figure 4. The male (left) and female (middle) cones of Afrocarpus falcatus, with the different components of a mature female cone or 'seed' (right).|
|Figure 5. Seed of Afrocarpus falcatus cleaned of the fleshy part (left) by fruit bats (Rousettus aegyptiacus) (middle). Hoard of the dormouse (Graphiurus murinus) with eaten seed under a tree stem (right).|
Distribution and forest types
Outeniqua yellowwood belongs to a conifer genus endemic to Africa (Afrocarpus), in the large Podocarpaceae family with about 17 genera and more than 150 species, mainly in the southern hemisphere. It is one of several podocarp species in Africa, and depending on one taxonomic description, Afrocarpus falcatus (formerly known as Podocarpus falcatus, and including A. dawei, A. gracilior and A. usambarensis) occurs in the Afromontane evergreen forests from Swellendam in the Western Cape to Ethiopia in the Horn of Africa (northern hemisphere). In South Africa, there are three other podocarp species, i.e. the widespread and generally common, real yellowwood Podocarpus latifolius (our National Tree), the closely related Breede River yellowwood Podocarpus elongatus endemic to the Western Cape and Northern Cape (type species for the genus Podocarpus), and Henkel's yellowwood Podocarpus henkelii, endemic to the area from Mthatha to the KwaZulu-Natal Midlands.
This tree grows in most forests in South Africa from Swellendam to northern KwaZulu-Natal (in the Kosi Bay area) and Limpopo Province, particularly the western Soutpansberg and Blouberg. In the moister evergreen forests, it is a tall tree with a long, clear stem, where it is often associated with real yellowwood, but in much smaller numbers. Sometimes it grows in almost pure stands with stems in all size classes. It also grows in drier open riverine areas in the Eastern Cape province where it has a short thick stem and wide spreading crown. It is less shade-tolerant than real yellowwood. A provenance study in the George, Sabie and Tzaneen areas with seed from different areas within the South African geographic range of the species showed much variation in germination behaviour, growth rate, tree height and stem diameter, and in tree form. Fortunately the best tree growth in each study site was with trees of seed collected in that specific geographic area where the experimental trees were planted.
Characteristics and reproduction
Podocarp trees are either male or female, i.e. the species are dioecious. The male cones develop during early summer (November) on twigs produced the previous year and the pollen is released by the end of the next winter (a two-year period). The dry cones on the forest floor are indicative of male trees. The female cones develop with new leaves in spring and are pollinated when the pollen is released from the almost one-year old male cones. They take about one year to develop into a globose, yellow, fleshy 'fruit' enclosing a woody 'seed' with the nut inside from which the seedling germinates (biologically, the fleshy part is part of the kernel with endosperm or 'seed').
It is the only bat-dispersed conifer we know. The bats remove the fleshy part around the 'seed', spit out the 'seed', suck the juice from the fleshy part, and then spit out the fleshy part in small chewed chunks. The seeds and chewed fruit chunks on the forest floor are indicative of female trees, and often accumulate in small clusters under a nearby tree of another species where the bats roost. Many birds eat the fruit, but only larger birds such as the Louries swallow the seed to contribute to their dispersal. Monkeys and baboons also disperse the fruit but often also destroy the seed. Bush pigs destroy many of the seeds on the forest floor (to get to the nut) particularly when the seeds accumulate in small clusters.
Rodents, particularly the dormouse, accumulate the seeds in hoards in holes in tree stems, near the ground, but they eat the kernel and thereby destroy most of the seed.
While the seed is dispersed during early summer (August to December), they only germinate during the next spring (from August onwards), after the cold of winter (a kind of stratification process), i.e. about two years after the female cone has been pollinated. The emerging seedling develops very slowly; once the woody seed shell is cracked by the swelling endosperm (nut) with embryo, it takes another 34 days (range 12-59 days) for the seedling to stand up straight (about 5 cm high). During this process, many seedlings are damaged or destroyed by falling twigs and insects.
The seedling then starts the process of developing in the forest canopy. It can easily take 50 years for a seedling to develop into a pole of 5 cm stem diameter. The mean stem diameter growth rate was calculated as 0.23 cm/year for many trees between 5 cm and 1 m in stem diameter. In general, the age of an Outeniqua yellowwood tree is estimated by multiplying the stem diameter (in cm) by four, i.e. a tree of 1 m stem diameter is at least 400 years old. However, we know that an open-grown tree can grow much faster, and a tree under the dense canopy can stand there for many years with zero stem diameter growth. Ring counts from stem sections of two adjacent trees cut during road-building for the N2 near Witelsbos in the Tsitsikamma showed the larger stem section with evenly spaced rings to be 117 years, but the smaller section with narrow and unequally spaced rings to be 203 years old. The tree of the larger section probably developed in a gap, whereas the other tree struggled to grow through the canopy. In the provenance study, the mean growth rate during the first 11 years was about 1 m/year in height, and about 1 cm/year in stem diameter.
The stem section of a tree growing naturally in the forest shows very narrow rings in the core as the tree struggles to grow towards the canopy, but when it reaches the canopy, it grows faster and the rings are much wider. A tree planted in the open shows the opposite pattern: wide rings near the core, and narrow rings towards the edge of the section; like a plantation tree. This characteristic was used in the court case around the illegal cutting of this and other podocarps in the Mzimkhulu forests in southern KwaZulu-Natal, and contributed to the successful prosecution of the accused. Stem sections collected in the Tsitsikamma forests during road construction for the N2 road through the Witelsbos forest showed the fluctuation in diameter growth during different years over more than 200 years – faster growth during moister periods and slower growth during dry periods (see paper listed below).
People have many different uses for this species. In rural areas, it is often used as a pole. This species is often cut as a source of timber for furniture, construction, door panelling and wood carving, although P. latifolius is more commonly used for this. In many forests in South Africa, the old, large sawpits are evidence of the earlier uncontrolled cutting of these two species inside the forest. Today, the tree is legally cut from natural forests in the Southern Cape Afrotemperate and Amathole Mistbelt forests, under control of a management plan. Occasionally, the tree is cut on private land after receipt of a permit to cut such a protected tree, and very good reasons have to be given for cutting it. Sometimes trees are cut illegally and there have been some successful prosecutions.
Outeniqua yellowwood is better known as the big tree of the forests. Very large trees, with massive stems and spreading crowns, towering above the general forest canopy, are used as sites for the development of various ecotourism activities. In such cases, it has become necessary to develop boardwalks around the trees to prevent trampling of the sensitive feeder roots, and soil compaction. The tree is also planted in large gardens, parks and along streets. However, sometimes people plant the tree in small gardens and have to face the consequences of roots lifting the pavements, walls and even foundations, but also of bats dispersing the seed and spraying nearby house walls with their excretions (see June 2010 article in SA Forestry magazine).
Planting as a timber source
This species has been planted in several areas as a potential timber source because of its relatively fast growth. It can be planted in riparian zones within the forestry estate to suppress weed growth, as was done in the provenance trial on De Hoek Forestry Estate in Magoebaskloof. After removal of the yellow fleshy covering of ripe fruit, the seeds can be placed in a nursery box (as a dense layer), covered lightly with soil or litter (to keep it moist), and left in a shady spot. After the cold of winter, the emerging seedlings can then be transplanted into small plastic bags, or directly into the site of planting. Alternatively, seedlings can be collected from seedling banks near large female trees. A planting spacing of about 3 m x 3 m can be used, and the normal silvicultural treatments will be necessary (weed clearing, thinning and pruning). The article on the provenance trial can be used as a guide (see below). As an indigenous species, the tree can be planted in riparian zones; the species is currently used in studies of comparative water use efficiency between indigenous and introduced pines and eucalypts. Stand development can be managed to facilitate the development of a mixed stand of indigenous tree species (see May-June 2006 SA Forestry magazine article), with additional two-way economic benefits, such as reduction of costs of clearing invader plants, and revenue from using thinned trees and later good-sized timber of the species. Environmental benefits are the establishment of natural forest corridors. It is important to record the planting to ensure that a permit can be obtained when the trees have to be cut for use.
This species has not gone through the same intensive tree breeding over many years as was done for the plantation species of pines, eucalypts and other introduced species. It is important to collect seed from several trees of good form and growth vigour in the immediate source area of the planting site. Tree improvement could be facilitated through clonal gain trials with cuttings of the best trees (reduced variability).
Yellowwood trees are protected under the National Forests Act, 1998, as amended, and may not be cut, damaged, destroyed or disturbed without a licence granted by the Department of Agriculture, Forestry & Fisheries (Forestry branch).
Dye, P.J., Gush, M.B., Everson, C.S., Jarmain, C., Clulow, A., Mengistu, M., Geldenhuys, C.J., Wise, R., Scholes, R.J., Archibald, S. & Savage, M.J. 2008. Water-use in relation to biomass of indigenous tree species in woodland, forest and/or plantation conditions. Report TT 361/08, Water Research Commission, Pretoria. 156 pp.
Geldenhuys, C.J. 1993. Reproductive biology and population structures of Podocarpus falcatus and P. latifolius in southern Cape forests. Botanical Journal of Linnean Society 112, 59-74.
Geldenhuys, C.J. & Von dem Bussche, G.H. 1997. Performance of Podocarpus falcatus provenances in South Africa. Southern African Forestry Journal 178, 15-24.
McNaughton, J. & Tyson, P.D. 1979. A preliminary assessment of Podocarpus falcatus in dendrochronological and dendro climatological studies. South African Forestry Journal 111, 19 33.
Published in April