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Yellowwood, L'Heritier ex Persoon  1807


Evergreen trees and shrubs of varying habit. Trunk often forked and crooked or twisted but straight and single in some species. Bark fibrous and peeling in narrow strips. Crown typically broad and rounded, often with extra branches between the main tiers of whorled branches. Branchlets all elongate, without distinction into short and long shoots, usually hairless, remaining green at least the first year, obscurely to prominently grooved between the elongate, attached leaf bases. Resting buds well developed, the scales often green and leafy in texture. Leaves spirally arranged and radiating all around the twigs, mostly variably ribbon- and sword-shaped rather than needlelike, with a prominent midvein that is raised beneath and raised or channeled above. The leaf tip rounded or pointed, the base tapering abruptly to a short petiole.

Plants dioecious. Pollen cones cylindrical, single or in clusters of 2-8(-12) at the ends of (usually leafless) stalks or sessile in the axils of foliage leaves. Each cone with a few bud scales or bracts at the base and numerous, densely spirally arranged pollen scales, each scale bearing two pollen sacs. Pollen grains small to medium (body 20-50 µm long, 45-85 µm overall), with two round air bladders that may be smaller to larger than the slightly flattened pollen body and diverge at about 120° away from the germination furrow. Air bladders fairly smooth but with coarse internal sculpturing, the cap of the body thicker and more coarsely sculptured than the furrow side. Seed cones maturing and falling in a single season, single on leafless stalks in the axils of foliage leaves, highly modified and reduced. With or without two free bracteoles at the base, with two to five bracts united with the cone axis and becoming swollen, juicy, and berrylike (the podocarpium), and with a single plump, unwinged seed embedded in each of one to three fleshy, green to black seed scales (the epimatium). Seeds with or without a prominent crest, the opening of the ovule pointing down into the cone axis. Cotyledons two, each with two veins. Chromosome base numbers x = 10, 11, 17, 18 and 19.

Wood soft, light, usually not fragrant, yellowish brown to light brown, often with little distinction between sapwood and heartwood, but sometimes with a central core of reddish brown inner heartwood. Grain fine and even, often lacking obvious growth rings but the latewood sometimes faintly darker and denser. Resin canals absent but with individual resin cells.

Without stomates above and with several lines of stomates forming a broad stomatal band on either side beneath. Each stomate sunken beneath and largely hidden by the four (to six) surrounding subsidiary cells that may or may not have a discontinuous low Florin ring. Leaf cross section with one or three resin canals beneath and near the single-stranded midvein and sometimes with additional resin canals in other positions, with extensive transfusion tissue and accessory transfusion tissue. Photosynthetic tissue with a fairly thin to prominent palisade layer beneath the upper epidermis and its adjacent variable layer(s) of hypodermis and often strongly separated by the accessory transfusion tissue from the spongy mesophyll occupying the lower half above the stomatal bands.

107 species (Farjon 2001) across the southern hemisphere and extending northward in the New World to the eastern coast of Mexico and throughout the West Indies, in Africa to just north of the equator in Cameroon and southernmost Sudan, and in Asia to northeastern India across to central Japan.

The name Podocarpus, Greek for “foot fruit”, refers to the fleshy, swollen cone axis and bracts (together called a podocarpium) found in most species. Despite its large number of species, Podocarpus has contributed little to horticulture, especially in comparison to Pinus, Juniperus, Picea, and Abies, the large northern hemisphere genera. Only a few species are in general cultivation, and these have spawned few cultivars. Those that have been selected follow the usual patterns: variations in habit (dwarf and weeping), leaf length, and foliage color (blue and gold). None of the species is really hardy in cold temperate regions, but some of the hardier cultivars appear to be derived from hybrids between the New Zealand species growing on South Island. Podocarps are preeminently tropical, and the wide range of natural environments they inhabit, from the seashore to subalpine scrub, provides a wide-open field for bringing more than the existing handful of these handsome trees and shrubs into cultivation for many different garden conditions in the tropics.

Podocarpus and Pinus, the two largest genera of conifers, with 107 and 121 species respectively, present more contrasts than points of similarity. In addition to having the greatest number of species among the conifers of its hemisphere (Podocarpus in the south and Pinus in the north), each also has the widest range and overall greatest abundance. The resemblances essentially stop there, however. While species of Pinus are predominantly temperate, characteristically forming pure, open forests on dry soils, and often dependent upon fire for reproduction, podocarps are mostly found in tropical to warm temperate, moist or wet forests, where they constitute scattered individuals or small groves in the understory or canopy. Associated with these ecological differences, pines have desiccation-resistant, needlelike leaves while those of podocarps have broader photosynthetic areas at the expense of greater potential water loss through transpiration. The cones of pines are woody and somewhat fire resistant, with winged seeds that can be blown by the wind onto bare mineral earth cleared by fires. In contrast, the cones of podocarps are berrylike and attractive to bird and mammal dispersers, while the seeds have the food reserves needed for establishment within closed communities. Despite these striking differences in ecology and geography, the two genera evolved a similar number of species. Interestingly, the third largest genus of conifers, Juniperus (the junipers of the northern hemisphere), with about half as many species, are bird dispersed like the podocarps but have habitats more similar to those of the pines.

Species of Podocarpus can be thought of more as tropical and south temperate than as southern hemispherical. As well, they extend much farther into the northern hemisphere, at about 25°N in the New World, 5°N in Africa, and 35°N in Asia, than Pinus does into the southern hemisphere, with its minimal intrusion just southward over the equator into Sumatra. Nor was Podocarpus confined to the southern hemisphere in the geologic past, with credible records in eastern Asia and North America, although the vast majority of fossils that are definitely attributable to the genus are found in the far south. This reflects a distribution on the former continent of Gondwanaland, which was breaking up into the modern southern continents during the Cretaceous, when the first known fossils of Podocarpus are recorded.

Although most of the living species of Podocarpus are rain-forest trees with large, sword-shaped leaves, some of the temperate species, like those found in New Zealand and Chile, have much smaller leaves that are very similar to those of yews (Taxus) or redwoods (Sequoia). One of the New Zealand species, the alpine totara (Podocarpus nivalis), is even a dwarf shrub growing above the alpine tree line. The tree species yield a fine timber, often sold as yellowwood because of its yellowish brown color (and not to be confused with many other trees known as yellowwoods, such as the North American Cladrastris kentuckea, a deciduous hardwood in the legume family). Before widespread introduction of Monterey pine (Pinus radiata) and other pine species as plantation timber trees in the southern hemisphere, yellowwoods were often the premier softwoods for construction, as they were in southern Africa. Pollination in the genus is more efficient than one might expect from seed cones with just a single ovule (or two) because pollen is scavenged from the flanks of the cone by a pollination drop that emerges from the micropyle (mouth) of the ovule.

About 30 species that were unquestioningly included within Podocarpus before 1969, some of which are still often so listed in horticulture, are here assigned to other genera. Buchholz and Gray, in their comprehensive revision of Podocarpus, beginning in 1948, followed modern tradition in treating these species as members of Podocarpus, although some had been separated in the 19th century. Within the genus in its broad sense, they recognized eight botanical sections. Only one of these, section Podocarpus (which they called Eupodocarpus under a different set of nomenclatural rules), is still included in Podocarpus, although it was by far the largest portion of the old genus in terms of number of species. The other seven section are each treated as separate genera by some authors, or are grouped into as few as four genera by others. Although some contemporary taxonomist still adopt a broad generic concept, notably in South Africa, where Afrocarpus is generally not separated from Podocarpus, all subsequent evidence reinforces the heterogeneity of the old Podocarpus and the distinctiveness of its original elements. Separation of the 30 anomalous species from Podocarpus, first proposed in modern times by de Laubenfels (1969), is amply confirmed by careful studies of morphology, anatomy, pollen, biochemistry, and DNA. Even quite traditional characters of the shoots, leaves, and cones amply distinguish species of the segregate genera from those retained in Podocarpus. In addition, some of the segregate genera (formerly sections of Podocarpus) are more closely related to other members of the family Podocarpaceae than they are to the other species retained here in Podocarpus in the strict sense. Parasitaxus usta, for instance, is so unlike any species of Podocarpus that it is hard to see how it was ever associated with this genus. The cross-references in the Index make it easy to find the correct genus for each species removed from Podocarpus. There is also further discussion of relationships under each of the segregate genera.

Classification within what is left of Podocarpus is also somewhat controversial. Buchholz and Gray initially distributed the species of Podocarpus in the strict sense (their section Eupodocarpus) among four subsections, and Gray later added two for four anomalous species. In a complete review of the genus, de Laubenfels (1985) drastically reorganized the species. He established two subgenera that seem very well founded. Subgenus Foliolatus is more homogeneous than subgenus Podocarpus. It contains only the species of Buchholz and Gray’s subsection B (and Gray’s small subsection F). Although this subgenus contains the majority of the species in the genus, all from Asia and the western Pacific, they are basically rather similar to one another. All have a pair of bracteoles at the base of the seed cone and a series of common leaf features, including a raised midrib (usually), resin canals confined to the vicinity of the midvein, and stomates without a Florin ring. Within this subgenus, de Laubenfels described nine botanical sections, but these are so similar that it is not yet clear whether they represent the most effective arrangement of the subgenus.

The species in subgenus Podocarpus are much less uniform. This subgenus includes subsections A, C, and D of Buchholz and Gray (and Gray’s later-segregated small subsection E). All these species lack the bracteoles of subgenus Foliolatus and all have stomates with Florin rings, but some species have resin canals near the edge of the leaf and others do not, and many have a groove along the midrib above, while a few have a ridge. There are fewer species in this subgenus, and de Laubenfels also assigned them to nine botanical sections. The African species (formerly subsections A and E) differ from all others in having resin canals near the outer corners of the leaves in addition to those next to the midrib. These species were divided into two botanical sections by Laubenfels, differing primarily in whether the seed and epimatium or the swollen axis and bracts of the seed cone are the main fleshy bird attractant, while the other structure is leathery. The temperate South Pacific species, from New Zealand, southeastern Australia, New Caledonia, and one species from southern Chile and Argentina, all belonging to the old subsection D, are treated as a single section by de Laubenfels. They are distinguished by having small leaves that lack resin canals near the edge of the leaf and also lack the support tissue extending out from the midrib that is found in most other species of Podocarpus. The fleshy part of the seed cones in this group is the podocarpium, as it is in most species of the genus, and the epimatium and seed coat are not fleshy. Podocarpus smithii from Queensland (Australia), newly described by de Laubenfels in 1985, was assigned to a section all by itself since it has larger leaves than the species in subsection D and a fleshy seed coat and epimatium. Except for Podocarpus nubigenus of subsection D, all New World species of Podocarpus were assigned by Buchholz and Gray to subsection C, exclusive to the Americas. These species were distributed among five sections by de Laubenfels, distinguished by characteristics of the buds and pollen cones. As with the sections proposed for subgenus Foliolatus, it is not clear yet whether these sections represent the most effective arrangement of the New World species. Thus, while the subgenera proposed by de Laubenfels are well supported, it is likely that new proposed by de Laubenfels are well supported, it is likely that new information will lead to changes in the assignments to sections used here, which are those proposed by de Laubenfels.

There is wide variation in the number of species recognized in the genus by different authors. Some of the difficulty in species delimitation may be due to hybridization. Natural hybridization has been well documented among the four New Zealand species, but there has not been a single published study looking for hybridization anywhere else within the range of the genus. Thin is an area that clearly needs more work and that could also contribute to improving the arrangement of species within the subgenera by helping clarify relationships. Another source of confusion in species boundaries is that all species go through substantial changes in leaf size and shape throughout their life as well as varying in different environments. This has often resulted in dividing parts of the morphological continuum for a single species into distinct species. Other such cases may well be lurking among the numerous species that are known from only one or a few specimens. The synonyms indexed are those that may be found in current literature. For additional synonyms used in older literature, please consult Farjon (1988).

Potentially, any species in the genus might sometimes bear two seeds in a cone even when it normally has just one. The descriptions here only indicate whether such cones have been found or not. Thus, “with one fertile seed scale” means that a second seed has not been recorded, “with one (or two) fertile seed scales” means that it has, and “with one or two fertile seed scales” means that they are fairly common.




Attribution from: Conifers Garden