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Yellowwood, (J. Buchholz & N. Gray) C. Page  1989

Afrocarpus - Yellowwood description


Tall forest trees with a straight, clear trunk and a shallow, broad crown. Bark scaly and peeling in irregular flakes. Branchlets all elongate, without distinction into short and long shoots, hairless, remaining green for the first year, often square in cross section and prominently grooved between the attached leaf bases. Resting buds well developed, the bud scales tightly arranged in crisscross pairs. Leaves variously arranged on a single individual, primarily in crisscross pairs, but members of a pair often displaced longitudinally, and some shoots fully spirally arranged, radiating around the twigs or brought into a plane by twisted stalks, roughly sword-shaped and usually obviously curved to one side.

Plants dioecious. Pollen cones cylindrical, single or in clusters of two to four at the ends of very short stalks in the axils of foliage leaves. Each cone with a few sterile scales at the base that are little differentiated from the numerous, densely spirally arranged pollen scales, each bearing two pollen sacs. Pollen grains small (body 25-40 µm long, 50-70 µm overall), with two obliquely directed, round air bladders that are smaller than the body and cover the germination furrow when the grain is dry, the body smooth to grainy at the center and becoming rougher near the edges, the bladders with coarse internal ridges. Seed cones single at the tips of leafy or scaly branches in the axils of foliage leaves, maturing and falling in a single season. Cone reduced to just two or three bracts, these and the axis not becoming fleshy, all except the uppermost seed-bearing one withering and falling before maturity. The single seed round, without a crest, embedded in and fused with the upper seed scale (the epimatium), the opening of the ovule pointing down back to the cone axis. Cotyledons two, each with two veins. Chromosome base number x = 12.

Wood soft to moderately hard, light to moderately heavy, not fragrant, light yellowish brown to light brown, with little distinction between sapwood and heartwood. Grain fine and even, the transition from early- to latewood gradual, the variably expressed growth rings sometimes marked by a noticeably darker ring of latewood at the ring boundary. Resin canals absent but with discontinuous vertical lines of resin cells in the wood parenchyma.

Leaf surfaces with nearly equal numbers of stomates in numerous parallel lines both above and beneath. Each stomate sunken beneath and partially hidden by the four or five (to six) surrounding subsidiary cells, with just a hint of a Florin ring. Leaf cross section with a single, prominent midrib that is slightly raised above and below, with one resin canal beneath the midrib and wings of transfusion tissue and accessory transfusion tissue extending out sideways from the midrib nearly to the leaf margin. Photosynthetic tissue forming a prominent palisade layer beneath the epidermis and underlying, nearly continuous (except beneath the stomates), hypodermis on either or both faces. Numerous strengthening fibers lie in the looser spongy mesophyll, particularly at the boundary with the midrib and transfusion tissue.

Two species, one on Sao Tomé Island and the other discontinuous throughout the eastern side of Africa from northern Ethiopia to the Cape region of South Africa. Both species are cultivated outdoors in mild climates and also for indoor landscaping but no cultivar selection has taken place. Common yellowwood (Afrocarpus falcatus) is the most important native softwood timber tree in southern Africa and most of East Africa as well but has been largely replaced commercially by faster-growing plantation-grown pines from North and Central America.

These two yellowwood species have been subjected to more than their fair share of taxonomic change, both with respect to species boundaries and to their relationships to other podocarps. As many as five species were carved out the geographically separate portions of the range of Afrocarpus falcatus, based on small and inconsistent differences in leaf size and in the thickness of the hard shell surrounding the seed within the epimatium. When taxonomists came to see that recognition of so many species in this group was untenable, some authors mistakenly merged some of them with the west African Afrocarpus mannii rather than with the South African Afrocarpus falcatus. Although acceptance of just two species in the group with the limits followed here is probably the prevailing view, a few authors still recognize one or two of the segregate species. Oddly enough, perhaps, the two species of Afrocarpus display nearly the extremes in size of geographic range found among all the podocarps. While Afrocarpus falcatus is one the two or three most widely distributed species in the family, Afrocarpus mannii is one of the most local, being completely confined in nature to the peak of the small volcanic island of Sao Tomé in the Gulf of Guinea, presumably as what is called a relictual endemic, a species now restricted to a single limited region but that was once more widely distributed. There is no evidence for this idea because there is no known fossil record for Afrocarpus, in part because of the other source of taxonomic confusion surrounding these species, their relationships to other podocarps. These two species are one of the more controversial segregants from Podocarpus because of their apparent similarity to the African species of that genus, especially those of section Scytopodium. Even now, many African botanists are reluctant to exclude them from Podocarpus, where they were long placed. Nonetheless, they have long been recognized as a distinctive element within Podocarpus.

Although Pilger (1903), in the only complete (for its time) fully descriptive monograph of Podocarpaceae, placed the species of Afrocarpus in the section of Podocarpus that is now recognized as the genus Prumnopitys, Buchholz and Gray (1948), in their revision of Podocarpus in the inclusive sense now generally abandoned, put them in their own section and provided the name Afrocarpus for it. The name was a reference to their place of origin and their status as species of Podocarpus (it means “African fruit”). When de Laubenfels (1969) began the modern breakup of Podocarpus in the traditional sense, he joined the species of Afrocarpus with members of two other sections characterized by opposite leaf pairs in a separate genus to which he mistakenly gave a new name. Later (1987), he transferred all the species to Nageia, the correct name for this combined genus, but kept the three original sections of Buchholz and Gray. Finally, Page (1988), recognizing the morphological heterogeneity of this grouping, treated each former section as a genus in its own right, found here under the names Afrocarpus, Nageira, and Retrophyllum. Some contemporary studies, including DNA studies, point to a close relationship among the three genera, potentially validating their inclusion as section of a single genus. Other studies, however, suggest that they are not each other’s closest relatives. Although they have rather similar seed cones, they are here maintained as separate genera because of their substantial differences in leaf structure at both the macroscopic and microscopic level and because of the absence of any definitive evidence that they are most closely related to each other to the exclusion of other podocarps.





Attribution from: Conifers Garden