Moreover, in a recent study of three co-occurring woody species that differed in maximum height McCulloh et al. This was in addition reported for alpine shrubs compared to trees Noshiro and Suzuki, , and such vessels may reduce the risk of embolism—see also Lopez et al. McCulloh et al. Worth noting is that Maherali et al.
After severe damage or sudden increased light availability, a small tree or tree stump can sprout and change into shrub growth form. We suggest that our hypotheses contribute to explaining this response in trees, and thus may form parts of a theory of multi-stemming in woody plants.
In trees, a change to a multi-stemmed growth form may be an attempt to survive a difficult situation and may sometimes fail and increase mortality, depending on tree size, species, and conditions e. One study of single- and multi-stemmed large Pinus strobus is consistent with our Hypothesis 1, since the latter, of a given age, grew better higher volume production than individuals with a single stem Chamberlin and Aarssen, In addition, based on long-term data, Bellingham and Sparrow reported lower mortality and lower recruitment in multi-stemmed than in single-stemmed trees in montane rain forest.
Hypothesis 4 posits that low height in shrubs allows the stems to spread wide, since the bending moment of short stems is low even if they lean outward.
The same bending moment calculations see Figure 4 may also help explain why tall trees usually have a single stem and why multi-stemmed trees tend not to have the same shape as shrubs.
A tall multi-stemmed tree with wide-spread leaning stems would need to invest much in stem strength including reaction wood, Du and Yamamoto, to counteract the large bending moment; for very tall trees this would even likely be impossible.
Such trees are therefore rare. But such stems would be close to each other and would each have a canopy smaller than that of an isolated stem, leading to an increased proportion of support structure for the tree as a whole. The tree would therefore not gain much in C uptake by having multiple stems. More detailed models, with empirical tests, are needed to clarify the growth form of stems in multi-stemmed trees.
Shrubs are often subjected to browsing. Zizka et al. In the understory of temperate forest, deer grazing had little effect on growth and stem survival in the shrubs Corylus avellana and Crataegus sp.
Tanentzap et al. Livestock and grazing may also spread shrubs Naito and Cairns, Possibly, small trees may be more susceptible to browsing than shrubs. To assess and improve our model and hypotheses, the root systems of small shrubs and small trees are of considerable interest.
Shrub roots can grow deep into the ground Jackson et al. But it is unclear whether small trees invest much in roots, and if they do so when they compete with shrubs. If a shrub can grow fast, it will be able to invest much in roots. In Asbjornsen's et al. Some authors regard clumped stems of a single shrub individual as a clone, but it is important to distinguish clonal shrubs that grow by root suckers or runners and form new distant stems or stem clumps.
To judge from the literature, it is unclear whether shrubs are clonal in this way more often than trees. Shrubs, compared to trees, may more often be clonal through layering, where nodes of lying stems root and sprout one component in Hypothesis 1. Finally, a comprehensive review of the adaptive significance of the shrub growth form seems non-existent or is hard to find.
Classical and much-cited publications on woody plants usually exclude shrubs, or focus strongly on trees e. The literature on woody plants in fire-prone ecosystems includes studies of shrubs, though with focus on fire or sprouting e. Why is the shrub growth form neglected? Partly perhaps because it is difficult to define shrubs precisely or because of their often low direct economic value, but aesthetical aspects are probably also involved: many of the shrub-covered areas do not attract people.
Many shrubs reduce the view of the surroundings, and good view is important for humans that hunt prey or seek charismatic species Orians, ; Gray and Bond, In contrast, shrubs are popular in horticulture, and the interest in shrubs increases. We have attempted to clarify why shrubs are successful in many habitats, including those where trees grow.
Hypothesis 1 predicts that small shrubs should have higher growth rates than small trees, and we find evidence for that. Shrubs, compared to trees, should have earlier seed set and dispersal Hypothesis 2 , should have higher survival in extreme conditions and weather, and can benefit from having several stems and low height Hypotheses 3 and 4.
Although all hypotheses have some support, more studies and more detailed models are needed, including laboratory and field experiments where shrubs and trees are sown or planted in different habitat types and followed at least until the shrubs are fully grown. Growth rates, morphological traits, and ecophysiology should be analyzed in detail, and parts of the plant populations should be harvested at two or more stages to analyze whole plants. Trees are more successful than shrubs in many areas under certain climatic conditions, where tall trees can dominate or control shrubs.
The large distributions of some shrub-dominated communities may partly be due to pre-historical and historical human overexploitation of such areas, and of trees Williams, Examples include Iceland Diamond, and the West European heathlands dominated by the shrub Calluna vulgaris Vandvik et al. A review of the historical role of humans in the distribution of shrublands globally would be valuable. Currently, our results are of interest for studies of shrubs expanding into savannas, rangelands, and grasslands, and for studies related to climate change, C pools, and habitat management see e.
A related, much studied and old theme is that shrubs may facilitate tree regeneration by providing protection for small tree plants e. Researchers in this field should also ask: what made the shrubs more successful than the trees initially? Moreover, some genera contain both trees and shrubs, e. The species may be closely related, or one species may be highly variable, existing both as shrub and tree.
A review of such species, their occurrence, and existing studies of them is of interest, and may clarify selection pressures acting on species and growth forms. FG conceived the topic and problem, and the initial approach. EG constructed the models and ran the calculations. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Our research was supported by salary from the University of Gothenburg, the Chalmers University of Technology, and the Linnaeus University. Valentine, and the reviewers for comments and suggestions on the manuscript, and Alexander Zizka and participants in three seminars for additional comments on our ideas about shrubs. Also, we thank Ian Lunt for permission to use his photograph.
Archibold, O. Ecology of World Vegetation. London: Chapman and Hall. Google Scholar. Asbjornsen, H. Synergistic responses of oak, pine and shrub seedlings to edge environments and drought in a fragmented tropical highland oak forest, Oaxaca, Mexico. Avila, E. Contribution of stem CO 2 fixation to whole-plant carbon balance in nonsucculent species. Photosynthetica 52, 3— Bellingham, P. Multi-stemmed trees in montane rain forests: their frequency and demography in relation to elevation, soil nutrients and disturbance.
Bond, W. Ecology of sprouting in woody plants: the persistence niche. Trends Ecol. Fire and Plants. Castro-Diez, P. Stem anatomy and relative growth rate in seedlings of a wide range of woody plant species and types. Oecologia , 57— Chamberlin, E.
The cost of apical dominance in white pine Pinus strobus L. Torrey Bot. Club , — Condit, R. Mortality rates of neotropical tree and shrub species and the impact of a severe drought.
Connell, J. Diversity in tropical rain forests and coral reefs. High diversity of trees and corals is maintained only in a nonequilibrium state. Science , — Conti, G. Large changes in carbon storage under different land-use regimes in subtropical seasonally dry forests of southern South America. Cornelissen, J. Seedling growth, allocation and leaf attributes in a wide range of woody plant species and types. Del Tredici, P.
Sprouting in temperate trees: a morphological and ecological review. Denslow, J. Growth responses of tropical shrubs to treefall gap environments. Ecology 71, — Diamond, J. How Societies Choose to Fail or Survive. London: Allen Lane. Du, S. An overview of the biology of reaction wood formation. Plant Biol. Espino, S. Hydraulically integrated or modular?
Comparing whole-plant-level hydraulic systems between two desert shrub species with different growth forms. New Phytol. Falster, D. Tradeoffs between height growth rate, stem persistence and maximum height among plant species in a post-fire succession. Oikos , 57— Filazzola, A. A systematic review and conceptual framework for the mechanistic pathways of nurse plants.
Global Ecol. Formica, A. Shrub expansion over the past 62 years in Rocky Mountain alpine tundra: possible causes and consequences. Arctic Antarctic Alp. Gardescu, S. Colonization of old fields by trees vs. Gartner, B. Stem hydraulic-properties of vines vs. Oecologia 87, — Givnish, T. Medina, H. Mooney and C. Vazquez-Yanes The Hague: Dr. Junk Publishers , 51— Gong, P. Remote Sensing 34, — Width of secondary xylem grows with the age of the plant. The primary xylem persists as conical projection on its inner side.
Pith may become narrow and ultimately get crushed. In favourable season the temperature is optimum. There is a good sunshine and humidity. At this time the newly formed leaves produce hormones which stimulate cambial activity.
Hence the annual or yearly growth appears in the form of distinct rings which are called annual rings Fig. Annual rings are formed due to sequence of rapid growth favourable season, e. Annual rings are not distinct in tropical areas which do not have long dry periods. Annual Rings Growth Rings. It is the wood formed in a single year. It consists of two types of wood, spring wood and autumn wood Fig. The spring or early wood is much wider than the autumn or late wood.
It is lighter in colour and of lower density. Spring wood consists of larger and wider xylem elements. The autumn or late wood is dark coloured and of higher density. It contains compactly arranged smaller and narrower elements which have comparatively thicker walls. In autumn wood, tracheids and fibres are more abundant than those found in the spring wood. The transition from spring to autumn wood in an annual ring is gradual but the transition from autumn wood to the spring wood of the next year is sudden.
The number of annual rings corresponds to the age of that part of the stem. They can be counted by increment borer. Besides giving the age of the plant, the annual rings also give some clue about the climatic conditions of the past through which the plant has passed. Dendrochronology is the science of counting and analysing annual growth rings of trees.
Several of the softwoods are very easy to work with e. The softness depends upon the content of fibres and vascular rays. Hardwood is the name of dicot wood which possesses abundant vessels.
Due to the presence of vessels, the hardwoods are also called porous woods. In Cassia fistula and Dalbergia sisso the vessels are comparatively very broad in the spring wood while they are quite narrow in the autumn wood. Such a secondary xylem or wood is called ring porous. In others e. This type of secondary xylem or wood is known as diffuse porous. Ring porous wood is more advanced than diffuse porous wood as it provides for better translocation when the requirement of the plant is high.
The wood of the older stems dalbergia, Acacia gets differentiated into two zones, the outer light coloured and functional sapwood or alburnum and the inner darker and nonfunctional heartwood or duramen Fig. The tracheids and vessels of the heart wood get plugged by the in growth of the adjacent parenchyma cells into their cavities through the pits. These ingrowths are called tyloses Fig.
Ultimately, the parenchyma cells become lignified and dead. Various types of plant products like oils, resins, gums, aromatic substances, essential oils and tannins are deposited in the cells of the heartwood. These substances are collectively called extractives. They provide colour to the heartwood.
They are also antiseptic. The heartwood is, therefore, stronger and more durable than the sapwood. It is resistant to attack of insects and microbes. Heart wood is commercial source of Cutch Acacia catechu , Haematoxylin Haematoxylon campechianum , Brasilin Caesalpinia sappan and Santalin Pterocarpus santalinus. Hollow tree trunks are due to their activity. In order to provide for increase in girth and prevent harm on the rupturing of the outer ground tissues due to the formation of secondary vascular tissues, dicot stems produce a cork cambium or phellogen in the outer cortical cells.
In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends from the vascular cambium to the epidermis. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.
Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly dividing cells in the apical meristems at the shoot tip and root tip.
Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water roots or sunlight shoots.
The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.
The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants.
Lateral meristems include the vascular cambium and, in woody plants, the cork cambium see Figure 4. Figure 5. Lenticels on the bark of this cherry tree enable the woody stem to exchange gases with the surrounding atmosphere. The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem tracheids and vessel elements to the inside, and secondary phloem sieve elements and companion cells to the outside.
The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength. In woody plants, cork cambium is the outermost lateral meristem.
It produces cork cells bark containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium.
The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels , which allow the interior cells to exchange gases with the outside atmosphere Figure 5.
This supplies oxygen to the living and metabolically active cells of the cortex, xylem and phloem. Figure 6. The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring for each year of growth. Seasonal changes in weather patterns can also affect the growth rate—note how the rings vary in thickness. The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened.
This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem Figure 6. An examination of the number of annual rings and their nature such as their size and cell wall thickness can reveal the age of the tree and the prevailing climatic conditions during each season.
Plant hormones affect all aspects of plant life, from flowering to fruit setting and maturation, and from phototropism to leaf fall.
Potentially every cell in a plant can produce plant hormones. In the accompanying animation, we study the process of secondary growth in the stem of a woody eudicot. Of the flowering plants, only eudicots are capable of secondary growth.
The eudicots, but not the monocots, have a vascular cambium, which produces wood, and another meristem, called the cork cambium, which produces bark.
In the accompanying animation we studied the process by which vascular cambium cells in a woody eudicot divide to produce secondary xylem cells wood toward the interior of the stem and secondary phloem cells toward the exterior.
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