According to the dictionary (Merriam-Webster Inc 1984), the word “shrub” came from the Old English scrybb wood, which is akin to the Norwegian skrubbebaer that means a dwarf hardwood species. The implication seems to be that a shrub is a dwarf tree--not far from the current usage of the word. The designation of this life form is driven by the need for a category between trees and herbs.
If a tree is “a woody perennial plant, typically large and with a single well-defined stem carrying a more or less definite crown” (Ford-Robertson 1971), then a shrub must necessarily be smaller. Most definitions require that a shrub should have more than one main stem caused by branching below or above the ground level (Allaby 1994, Viereck and Little 1972). Other frequent qualifications include the need for the plant to be perennial and to be lignified (woody), at least in some of its parts (Allaby 1994, Ford-Robertson 1971, Orshan 1989). Shrubs are distinguished from herbs in that herbs do not develop persistent woody tissue above ground (Ford-Robertson 1971). However, Lawrence (1955) admits that the term shrub is “not subject to precise circumscription”.
There are several reasons why shrubs cannot be precisely defined. Many species are trees in fertile habitat or favorable conditions and shrubs in difficult habitat. Certain species may grow as shrubs in large portions of their range and become trees in limited areas. Most large shrubs will produce an occasional individual with a clearly tree-like habit. It is not possible to define exactly what “large” is (referring to stem size). A number of species normally develop single stems but only reach stem sizes below that normally associated with trees. The degree of lignification is another source of ambiguity. Many annual herbs become quite lignified before dieing, and some perennial shrubs have little or no woody tissue. Finally, the concept of perennial looses much of its meaning in the humid tropics where a shrub may complete its life cycle in a year of continuous growth, and an herb may live without dieback for many years.
There are many subtypes of shrubs including dwarf shrubs, half-shrubs (sub-shrubs), cushion plants, woody vines (lianes and climbers), scrambling and prostrate forms, woody parasitic plants, and various forms of succulents and semi-succulents. A description of these forms can be found in Lawrence (1955), and a discussion of criterion for classification of shrub forms may be found in Orshan (1989).
The concept of the shrub layer, which is populated by functional shrubs, sidesteps the ambiguity of classifying species. In this system, the forest is divided into vertical layers (canopies or horizons). In their simplest form, the layers are: the tree layer, consisting of trees and tree-sized plants, the shrub layer, including all plants with any diameter at breast height (d.b.h.) that is less than some arbitrary upper d.b.h. limit such as 5.0 cm, and the herb layer, which does not reach breast height. The principal components of the shrub layer are actually young trees. This concept is useful in studies of wildlife habitat, plant ecology, and biomass and carbon distribution.
Because of their large size, influence on the environment, and considerable economic value, trees have received most of the attention in forest research and ecological descriptions. However, shrubs are more important than is generally realized, especially in terms of biodiversity. Little (1979) lists 679 species of trees native and naturalized in the continental United States. Another 60 native tree species with an additional number of naturalized species grow in Hawaii (Little and Skolmen 1989). Puerto Rico and the U.S. Virgin Islands support 547 native tree species (Little and others 1974) and at least 118 species of naturalized exotics (Francis and Liogier 1991). With allowances for species represented in more than one area, the U.S. and its territories support about 1,300 tree species. Shrub species in the same area are much more numerous.
Estimating the number of shrub species in the U.S. and its territories is challenging because the life-form is subject to the perception of authors and compilers and not indicated or consistently identified in many of the checklists and floras. However, it is possible to count the shrub species indicated in the floras of several of the states or regions. Alaska has about 110 species of shrubs (Viereck and Little 1972), California about 800 species (McMinn 1951), Hawaii about 170 (Degener 1946, Degener 1957, Degener and Degener 1963), Idaho about 160 (Davis 1952), Pennsylvania about 264 (Rhoads and Klein 1993), Puerto Rico about 540 (Liogier 1985, Liogeir 1988, Liogier 1994, Liogier 1995, Liogier 1997), and tropical Florida about 250 (Long and Lakela 1971). These totals (with duplication eliminated) plus other miscellaneous sources (Abrams 1940, Abrams 1944, Abrams 1951, Abrams and Ferris 1960, Benson 1969, Correll and Johnston 1970, Everitt and Drawe 1993, Great Plains Flora Association 1986, Jones 1975, Morley 1969, Nelson 1996) exceed 2800 species and seems to account for most of the shrub species in the United States and its territories. The Natural Resources Conservation Service (2003) “Plants” database recently made it possible to search and list shrub species by region within the United States and its territories (Puerto Rico and the U.S. Virgin Islands). Although sums for growth habits (life-forms) are higher than if each species were exclusively assigned to a single growth habit (eg. Salix exidua Nutt. appears as both tree and shrub), the figures are instructive, especially for comparing areas, life-forms, and taxonomic divisions. Searching all plant species that carry the growth habit designates, “shrub” or “sub-shrub,” yielded 5,281 species for the U.S. and territories. There was a great deal of variation between political divisions (see figure below, not drawn to scale). California and Texas supported the greatest numbers of species with 1,311 and 1,300 respectively. North Dakota and Alaska had the lowest with 195 and 200 respectively. The U.S. Virgin Islands had the highest average shrub biodiversity with 527 shrub species in only 349 km2 of territory.
Several factors influence the amount of shrub diversity found within a political unit. Numbers of shrub species, as well as all plant species, increase as one travels from the harsh boreal regions to the humid tropics. Diversity of habitat, such as mountain peaks, deserts, riparian areas, and sea shore, is also very important. Southwestern deserts where shrubs are the dominant vegetation also tend to have high shrub diversity.
Using searches of the “Plants” database mentioned above, the shrubs of the U.S. and territories were found to be divided among 166 families. The most important families were: Asteraceae, 618 species; Rosaceae, 510 species; Fabaceae, 342 species; Cactaceae, 193 species; Ericaceae, 189 species; Scrophulariaceae, 182 species; Rubiaceae, 165 species; Malvaceae, 148 species; Euphorbiaceae, 128 species; Lamiaceae, 124 species; Polygonaceae, 123 species; Companulaceae, 112 species; Boraginaceae, 106 species; Ramnaceae, 103 species; and 152 additional families with 82 to 1 species each.
Reaching or maintaining a high biodiversity is important to ecosystem health. Managers and the public are beginning to realize that subordinate species, as well as the tree canopy, are important. It will not be enough, as formerly called for in management plans, to establish a tree cover and wait for natural succession to fill in the subordinate species. Shrubs, which are near-climax in ecosystems such as subtropical deserts, are, thus, the principal species to be maintained or reestablished. In moister wildlands, wildlife managers have long sought to achieve a certain portion of the landscape in the “brush” stage, preferably in disbursed, irregular patches within more advanced forest.
The use of shrubs as well as all other types of native plants for reclamation and restoration of damaged sites is becoming a very important topic (Hansen 1989). Shrubs are planted as seedlings of various types and seeded using the same techniques employed with trees (Alder and Ostler 1989), except that densities must often be higher. Because it costs less, establishment by site manipulation and natural seeding and succession is preferred whenever possible.
Shrubs yield many benefits to humankind directly and indirectly. Berries and similar small fruits are the most important shrub-derived foods. There are many hundreds of kinds of edible, wild berrylike fruits throughout the world. Seasonally harvested and preserved, they were once very important to hunter-gatherer tribes and still are important in certain rural areas. All our commercial berries descended from wild shrubs, and their wild ancestors remain a source of genetic material for breeders. Wildland shrubs also furnish nuts, seeds, herbs, greens, and medicinal materials to rural peoples.
Fuel is another major direct benefit from shrubs. Although wood from shrubs is not present in quantities as great as tree wood in forests, its accessibility and ease of harvest have made it a very important fuel source in underdeveloped areas and during recreational camping, especially when collected by women and children. Shrubs are even harvested to make charcoal in areas with few trees. It has also been suggested that shrub stands could be harvested mechanically for industrial biomass fuel (Young and others 1989).
Humans benefit indirectly from shrubs through animals that eat them. Grazing domestic goats, camels, and, to a lesser extent, sheep and cattle obtain a large part of their forage from woody browse. Many wild ruminants preferentially browse shrubs even when grass is available. Others supplement their diet of grasses and forbs with forage from shrubs. A multitude of birds, mammals, amphibians, and reptiles obtain forage, mast (fruits and seeds), and insects from shrub hosts. The cover provided by shrubs is critical for a vast number of wild animals.
Finally, wild shrubs are an important esthetic component of our forests, prairies, and deserts. As greenbelts and semi-wild urban parks become more widespread, wild shrubs will be more appreciated and will be managed along with trees and herbaceous plants.
Shrubs also negatively impact humans. Undesirable shrubs often invade managed and semi-managed pastures, excluding the more desirable grass, and become weeds in croplands. They can temporarily suppress tree seedlings and slow reforestation. Shrubs, especially the many thorny and a few poisonous species, are the bane of cross-country hikers and woods workers. Forests with a heavy shrub layer are usually less visually pleasing than open forests. Shrub understories often are a vehicle for explosive fire spread in seasonally dry forest habitats.
In the early days of forestry, it seemed to be enough to protect and manage the forest as a whole. Later, it was realized that we needed detailed information on each of the tree species. To this end, a number of excellent references have been produced (Burns and Honkala 1990a, Burns and Honkala 1990b, Burns and others 1998, Francis and Lowe 1999). The time is coming when, if we are to manage the lesser species, we will need detailed information on each or, at least, the most important shrubs. The objective of this work is to provide an accessible reference of the biology, ecology, and management of individual species of shrubs. The study of the biology, ecology, and management of trees is called silvics, and the resulting descriptions are termed silvical descriptions. No such terms have come to general use for the study and description of shrubs. Perhaps the terms should be designated “thamnics” and “thamnical descriptions” from the Ancient Greek “thamnos”, which means shrub.
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Pesticide Precautionary Statement
Some of the descriptions report research or practices involving pesticides. They do not imply that the uses discussed here have been registered. All uses of pesticides must be registered by appropriate State and/or Federal agencies before they can be officially recommended.
CAUTION: Pesticides can be injurious to humans, domestic animals, desirable plants, and fish or other wildlifeif they are not handled properly. Use all pesticides selectively and carefully. Follow recommended practices for the disposal of surplus pesticides and pesticide containers