There are six major kinds of propagules now available for the reproduction of forest trees: seeds, sprouts, grafts, cuttings, tissue-culture plantlets, and somatic embryos. Each has two or more subclasses, and each has increasingly understood advantages, disadvantages, and uses. For several of these kinds of propagule, the maturation state and physiological condition of the starting material can vary substantially, with important consequences. These two concepts are discussed in general and with respect to each kind of propagule. The terminology currently in use is not always clear, and is sometimes conﬂicting. Alternative terminology is noted where such confusion is common.
[...] Later, clonal propagation can be resumed at an embryonic maturation state when the clone has been adequately tested for conﬁdent deployment, or when it is needed for special or unusual purposes. A second possible advantage is the insertion of foreign genes into the cells of embryogenic tissue, followed by identiﬁcation and testing of the genetically engineered miniclones (lines that differ by only one or a few genes from otherwise identical lines of the original clone). The identiﬁed miniclones can be cryogenically stored, and those genetically modiﬁed miniclones that prove to be acceptable can later be mass-produced. [...]
[...] When the more useful trees are harvested (high-grading or creaming), the forest becomes decreasingly useful for later harvests. When the more useful trees are retained to become parents of succeeding generations, the usefulness of the forest for harvest may be increased in the long run at substantial short-term opportunity cost. Artiﬁcial Regeneration Artiﬁcial regeneration is employed to augment an extant forest, to replace the previous forest, or to establish a new forest. In some cases, seeds are scattered or planted directly on the intended site, but more commonly they are germinated in nurseries and seedlings are planted. [...]
[...] It is clear that elements of maturation affecting vegetative propagation proceed rapidly in some species (such as sugar pine, Pinus lambertiana) and are long persistent in the juvenile state in others (such as willows, Salix). The effects of maturation state on the performance of propagules can be anticipated by considering the performance of the part of the tree whose terminal meristem is or was at that maturation state. Thus, in an example typical of many conifer species, a propagule produced at a mature maturation state will develop the lower-bole form, branch architecture, and thinner bark typical of the upper bole; the wood of its lower bole will have, for a given ring number from the pith, the longer tracheids, lower (steeper) microﬁbril angle, and lower speciﬁc gravity typical of upper-bole wood; and its growing shoots will have greater resistance to juvenile diseases, greater susceptibility to browsing, and will more quickly exhibit sexual competence. [...]
[...] As of the early twenty-ﬁrst century, somatic embryogenesis has become available technology for a few species, for example yellow poplar (Liriodendron tulipifera) and several species of spruce (Picea), while the techniques are proving to be difﬁcult for several other species. Somatic embryogenesis has not yet been intensively attempted with most species of forest trees. Explants appropriate for initiating embryo-producing (embryogenic) cultures are commonly obtained from tissues such as suspensor organs connecting normal zygotic (sexually produced) embryos to surrounding nutritive tissue. [...]
[...] In established clonal programs, donor plants are often maintained as low- pruned hedges, sometimes called ‘stool beds' or ‘mother plants.' A hedge orchard allows a large number of clones and variable numbers of donor plants per clone to be maintained and managed in a relatively small area, under conditions that produce cuttings in a physiological condition favorable for rooting and subsequent growth. Depending on species and conditions, rooting of cuttings may be done in an outdoor nursery or in enclosed structures ranging from small cold-frames to elaborate greenhouses and growth chambers. [...]
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