CIR1093

How Trees Grow in the Urban Environment 1 Mary L. Duryea and Marlene M. Malavasi2

Background This publication is for those who love to plant and grow trees. Of course, trees grow the same way in the city as What happens to a birdhouse if you hang it on a tree and they do in the country (physiologically speaking), but the come back in two years? Will it be at the same height or out differing environmental conditions between urban and of your reach because the tree has grown? (Figure 1). country environments can affect tree growth differently. Here we highlight those factors in the urban environments that affect tree growth.

What is tree growth? To set the stage for our discussion of tree growth we must review some terminology.

Trees have six organs. These are leaves, stems and roots (known as vegetative structures), and flowers, fruits, and seeds (known as reproductive structures). This article focuses on vegetative growth—that is the growth of leaves, stems, and roots. Tree growth is the increase in size and numbers of leaves, stems, and roots.

What kinds of trees will we be talking about? Trees are seed plants—that is they reproduce by seed. They are classified into gymnosperms and angiosperms. Gym- nosperms are trees with seeds that develop on the surface Figure 1. If a birdhouse is hung on a tree, will it move up as a tree grows? or tip of an appendage such as a cone. Examples are cypress and pines. Angiosperms are trees with seeds borne within By the end of this article you will be able to answer this structures such as fruits (the seed is enclosed). Examples question and others about how trees grow. are oaks, elms, maples, and palms. We are going to be describing growth of both gymnosperms and angiosperms. Also, since we live in Florida, there’s one more division

1. This document is CIR1093, one of a series of the School of Forest Resources and Conservation Department, UF/IFAS Extension. Original publication date May 1993. Revised February 2019 and February 2021. Visit the EDIS website at https://edis.ifas.ufl.edu for the currently supported version of this publication.

2. Mary L. Duryea, Ph.D., professor; and Marlene M. Malavasi, Ph.D., former visiting professor, School of Forest Resources and Conservation; UF/IFAS Extension, Gainesville, FL 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county’s UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension. that is necessary and that is among angiosperms. Palms are angiosperms—but they are different than oaks, maples, and elms, because palms are monocots and are related to grasses. Monocots have a single leaf when they emerge from a seed as a seedling, whereas dicots have two. Oaks, maples, and elms are dicots. So we will first talk about growth of gymnosperms and dicots; then because they grow very differently, we will have a special section on palms.

What controls growth? Tree growth is a response to the environment and is depen- dent on its genetic make-up (Figure 2).

Figure 3. A longleaf pine responding well to its environment.

Figure 2. Tree growth is influenced by both the environment and genetics.

The environment is made up of such factors as water, light, temperature, air, pests, and cultural practices. These all impact physiological processes such as , and the final response is that the tree grows vigorously or slowly. A tree’s genetic make-up also influences the way a tree grows. Genetics means what species the tree is, where is its seed source (did it come from northern or southern Florida), and the genetic contribution from its parents. These genetic characteristics, along with its environment, Figure 4. A longleaf pine responding poorly to its leaves have determine how a tree grows. For example, the longleaf pine enlarged, and the area of stem expansion between the leaves. in Figure 3 is growing quite well. Its genetic make-up and its environment are obviously compatible and favorable to Where does growth occur? growth. Yet, Figure 4 shows another longleaf pine that is Growth occurs in . A is a tissue that not doing so well. Is this due to its environment—shade, contains cells which have the capacity to divide to make water, fertilizers, etc., or due to its genetics? It’s difficult to new cells. In general, during growth cells divide, cells separate out effects of environment and genetics, but we can elongate, and cells differentiate into structures such as roots definitely conclude that one or the other or both are im- and shoots (Figure 5). pacting growth of these two trees. We will talk more about the environment and genetics at the end of the publication Meristems can also produce new meristems—these are in the section, “Some examples of the urban environment called primordia. For example, an apical meristem in a influencing growth.” produces new meristems called leaf primordia—each single primordium will grow into a new leaf. Root primordia grow into new roots.

How Trees Grow in the Urban Environment 2 Figure 6. A small tree showing the terminal bud (which contains the apical meristem), leaves that have enlarged, and the area of stem expansion between the leaves.

Figure 5. In general, during growth cells divide, elongate and differentiate. Growth Above Ground Tree growth above ground includes shoot elongation (height growth), leaf growth, and diameter growth.

SHOOT ELONGATION Figure 7. Each leaf has meristems. Rapid cell division at these meristems determines the final leaf shape. Shoots elongate or grow in height at the tips of the branches (Figure 6). Apical meristems are located in the terminal DIAMETER GROWTH at the tips of the branches. What happens is—cells Between the wood and bark is a thin layer of dividing cells at the apical meristem divide, elongate and differentiate. called the vascular cambium. This vascular cambium is The steps that we can see are (1) The bud at the tip of the a meristem that is only a few cells thick. This meristem branch opens, (2) leaves emerge and enlarge, and (3) the divides, creating wood on the inside and bark on the area between the leaves expands (i.e., the stem grows). outside (Figure 8). Lateral (side) buds grow in the same way but often these are dormant and do not grow until they are released after such activities as .

LEAF GROWTH On the surface of the apical meristem in the bud, a new meristem is formed. This new meristem is called a leaf primordium, and this is where cells divide and grow to form a leaf. Several meristems on a leaf influence leaf shape (Figure 7).

Soon after leaves develop, a new bud primordium (meri- stem) is formed at the base of each leaf stem. Once formed, this axillary bud has the capacity to become a branch, but may lie dormant for many years. Figure 8. The vascular cambium produces wood on the inside and bark on the outside.

How Trees Grow in the Urban Environment 3 Layer by layer these new cells increase the diameter of the trunk and branches. The wood cells are called “xylem,” which means wood in Greek; they carry water and minerals up from the roots. The bark cells are called “phloem,” meaning bark in Greek. These cells carry sugars and other materials produced by the plant.

ANNUAL RINGS New layers of wood are added each year between the bark and the previous year’s wood. These are called growth or annual rings and may be used to age a tree.

In most parts of Florida, annual rings vary in size and thickness according to the season that they are formed (Figure 9).

Figure 10. As a tree grows in diameter the bark cracks and may eventually slough off. Growth Below Ground Roots can grow in length (root elongation) and diameter, and new lateral roots can form.

PALM ROOT ELONGATION At the tip of a root is the root cap (Figure 11). This cap protects the root and it must be constantly replaced as it pushes through the soil. Behind the root cap is a meristem Figure 9. Annual rings vary in size and thickness according to the that produces new cells for the root cap and for root elonga- season when they are formed. tion. These new cells elongate, divide, and differentiate into Cells that are produced in the spring are larger with thinner root parts. The root grows in length and pushes through the cell walls. These are the light-colored rings, and the wood soil. is called “early” or “spring” wood. Cells produced in the summer are smaller, and this “late” or “summer” wood has a higher density and darker color. It is important to note that not all tree species form annual rings. In fact, some trees that grow all year may not have rings at all. Aging trees in this instance can be challenging.

BARK All woody trees have bark that protects the tree from pest attacks and environmental impacts such as fire and me- chanical injury. Some trees have thick bark that is resistant to injury. Others are easily injured because they have thin bark. Why does bark have ridges and cracks? As the tree grows in thickness, the outer part has to give, forming ridges and cracks. Eventually the outer bark sloughs off (Figure 10). Figure 11. A root tip showing the meristem which produces new cells for root elongation.

How Trees Grow in the Urban Environment 4 ROOT DIAMETER GROWTH ROOT GROWTH Many people don’t think that roots grow in diameter, but Root growth occurs near the root cap just as in gymno- they do! Root diameter growth is similar to growth in the sperms and dicots. However, just like shoot growth of the stem with the vascular cambium producing wood (xylem) stem, there is no diameter growth of palm roots. The roots and bark (phloem). A couple differences between root and remain the same diameter for long distances (giving the shoot diameter growth are (1) cambial growth is much appearance of a spaghetti-like root system) (Figure 13). more irregular in roots resulting in roots that are oval or Descending roots are for anchorage. Most of the horizontal irregular in cross section, and (2) there is greater variation roots are in the top 40 cm of soil and grow out from the in diameter with age and with horizontal roots compared to palm about 4 m. vertical roots.

NEW ROOT GROWTH Roots can also grow new lateral roots that form and branch off the main root. Some cells located in a layer inside the root produce a new root primordium (Figure 11). This new meristem divides and elongates pushing the root out through the parent root. A new lateral root is formed and grows into the soil.

Palms There are two major points to be made about palm growth: (1) palms only have one terminal growing point, and (2) palms do not grow in diameter as they age. We will talk Figure 13. Palms produce a mass of roots which grow in length but about how palms grow taller and grow roots and how the not in diameter. one bud influences their form compared to other trees. Seasonal Growth, Lifespan, and Aging SHOOT ELONGATION Different parts of trees grow at different times of the year, The apical meristem lies in a depression at the crown of and the pattern, varies by species and climate (Figure 14). the stem (Figure 12). Within this meristem is a soft mass In one typical pattern trees begin with shoot growth in of leaves and leaf bases. In this crown there is continuous the spring, followed by trunk diameter growth and then production of leaf meristems (primordia). There may be up root growth. However, this growth can occur in any order, to 50 meristems in the apex. The leaves grow in sequence and root growth may be year-round or have two growing with one leaf emerging as a spear at a time. The meristem at periods per year. Young trees do more growing in the grow- the base of the leaf causes the leaf to elongate, grow bigger, ing season than older trees. Length of this active growth is thicken, and eventually equal the diameter of the palm determined by temperature, day length, and water. stem.

Figure 12. A palm has one apical meristem which produces all its leaves. Figure 14. Different parts of trees grow at different times of the year.

How Trees Grow in the Urban Environment 5 Tree species have a wide range of lifespans. For example, while peach trees may live only 30 years, oaks may live several hundred years, cypress 1,600 years, and bristle cone pine up to 5,000 years in an undisturbed site in the forest (Kramer and Kozlowski 1979). It may be stressful for several forest tree species when you plant them in the city. The average tree in a downtown lives only 13 years, one tenth the lifespan of a tree at a rural site (Figure 15).

Figure 16. Palms exemplify apical dominance. Figure 15. The average tree lives only 13 years in the city, one tenth as long as a tree at a rural location. Credit: American Forests 1992

What happens to trees as they age? Some of the likely occurrences are the following: (1) Growth slows down; (2) Trees are more susceptible to diseases and insects; (3) The tops of the trees are more likely to die back; (4) Wounds heal more slowly; (5) There are fewer numbers of leaves relative to the size of the tree; and (6) There is an increased number of dead branches. All of these characteristics can also be seen in urban trees as they begin to die and/or show signs of poor health.

Growth and Tree Form The apical bud is the bud at the tip of a stem; lateral buds are on the sides of the branches. Apical dominance means that the apical bud at the tip of the stem controls the other buds and inhibits the growth of these buds. Of course most palms only have one apical bud. Therefore, palms usually grow straight with no branching (Figure 16); they are an Figure 17. Pines exhibit less apical dominance. extreme example of apical dominance. Pines exhibit a little less apical dominance (Figure 17). The main stem is still Some Examples of the Urban prominent, but branches formed by lateral buds change the Environment’s Influencing Growth form exhibiting less apical dominance than palms. An oak Urban trees are subjected to a tremendous amount of stress. tree has very little apical dominance with several branches Soils are compacted, trees are over- or incorrectly pruned, dominating the crown (Figure 18). This illustrates how roots are given very little space to grow, trees are improp- growth habits can change the form of a tree. erly staked, and on and on. A few of the stress-induced practices and environmental conditions are discussed briefly along with how they affect growth.

How Trees Grow in the Urban Environment 6 cambial (diameter) growth, so staking palms is okay and often necessary until new roots stabilize the palm in the landscape.

Figure 18. The average tree lives only 13 years in the city, one tenth as long as a tree at a rural location. Credit: American Forests 1992 PRUNING What happens when you prune a tree? If you prune the apical buds, the lateral buds are released and new shoots grow. If you prune older, large diameter stems, two things Figure 19. Staking small trees can reduce wind tolerance by limiting “reaction wood” functions. may happen: (1) Dormant buds that were produced long ago during primary growth can grow into new branches, COMPACTING THE SOIL and (2) New meristems arise and produce new buds and Soil conditions in urban plantings are often ignored, but shoots. This growth of new branches is most likely to occur this frequently causes tree health problems. Trees often have in angiosperms—everyone has seen sprouts from the stump limited rooting space and are similar to large potted plants. of a freshly cut oak, maple, or elm. Pine trees, however, Soil compaction can occur because of such things as con- do not have these dormant buds and will not sprout. Of struction equipment, sidewalks and streets, and footsteps. course, if you remove the apical bud from a palm, it will What happens to growth when tree roots are in compacted die—remember most palms only have one bud at the top of soil? A study of Monterey pine seedlings showed that trees the tree. grown in highly compacted soil had less root volume and weight—roots didn’t grow as well in compacted soil (Table STAKING 1). This translated into reduced shoot growth—trees were How does staking affect growth? Sometimes when small shorter and had less weight when the soil was compacted. trees are planted, people think that it is necessary to stake This happens in the city too—roots can’t grow, water can’t them. However, studies have shown that trees that are permeate into the soil, nutrient uptake is reduced, and the allowed to sway in the wind are sturdier. Staked trees result is poorer tree growth. Of course, the ultimate effect is tend to be less stable and sometimes topple when stakes poor tree health and death. Some ways to avoid compaction are removed and trees are exposed to wind (Figure 19). are (1) incorporate organic matter into the soil, (2) prevent This can be explained by looking at growth differences. vehicles from operating beneath the canopy, and (3) when When trees blow in the wind, there is a redistribution of construction trafficmust work around trees make sure it is the amount and nature of diameter growth on the leeward when the soils are dry (wet soils are more easily compacted) and windward sides of the tree trunk. “Reaction wood” is or spread a thick layer of coarse mulch beneath the canopy. formed, and this wood has larger and thicker walled cells than other wood. Hence, when exposed to wind a tree Plant Pests becomes sturdier and can tolerate wind better. For this Trees in urban landscapes provide multiple important reason, do not stake young trees unless they absolutely will ecosystem services and functions, (e.g. CO2 uptake, cooling, not stand up without a stake. Remember palms do not have enhanced aesthetics, etc.). However, urban trees experience

How Trees Grow in the Urban Environment 7 increased damage due to arthropod pests relative to trees For More Reading about Tree Growth in more rural settings (Ruapp et al 2010). This damage Bell, A.D. 1991. Plant Form: An Illustrated Guide to Flower- decreases tree health (Zvereva et al 2010), leading to ing Plant Morphology. New York, NY: Oxford Univ. Press. declines in the services provided by urban trees. Pests can also increase the costs of tree management/maintenance in Corner, E.J.H. 1966. The Natural History of Palms.Berkeley, urban landscapes. Homeowners and urban land managers CA: Univ. Cal. Press. Berkeley. 393 pp. should be aware of the multiple pests that can harm various tree species in urban landscapes and strategies of integrated Fahn, A. 1991. Plant Anatomy. Fourth edition. Oxford: pest management to control them. Pergamon Press. 588 pp. Conclusions Harltey, C.W.S. 1977. The Oil Palm (Elaeis guineensisJacq.). Second edition. London, UK: Longman. 806 pp. Trees grow in response to their environment and their genetic make-up. Environmental factors such as high Harris, R.W. 1992. Integrated Management of Landscape temperature or soil compaction can influence physiologi- Trees, Shrubs, and Vines. 2nd edition. New Jersey: Prentice- cal processes such as photosynthesis and in turn impact Hall. 674 pp. growth. Meristems are the areas within the tree where growth occurs. Most trees grow in shoot height and diam- Kozlowski, T.T. 1971. Growth and Development of Trees. eter and root length and diameter. Palms, however, have Vol. I. Academic Press. New York. 443 pp. only one growing tip and do not grow in diameter. A tree’s form is controlled by branch orientation and by the amount Kozlowski, T.T., P.J. Kramer and S.G. Pallardy. 1991. The of apical dominance. Altering the environment or manage- Physiological Ecology of Woody Plants. New York: Academic ment practices to decrease stress, and selecting the right Press. 657 pp. tree for the right place will promote growth and longevity. Laetsch, W.M. 1979. Plants - Basic Concepts in . And, finally, as the tree ages and grows, does the birdhouse Boston, MA: Little, Brown and Company. 510 pp. move up the tree and out of your reach? No! Why not? Re- member a tree only grows in height at the apical meristems Mosbrugger, V. 1990. The Tree Habit in Land Plants. In: at the tips of branches. At the height of the bird-house only S. Bhattacharji, G.M. Friedman, H.J. Neugebauer, and A. trunk diameter growth is occurring. Seilacher (eds.). Lecture Notes in Earth Sciences , Vol. 28. Springer-Verlag. Berlin. 161 pp. References Uhl, N.W. and J. Dransfield. 1987.Genera Palmarum. A Kramer, P.J., and T.T. Kozlowski. 1979. Physiology of woody Classification of Palms Based on the Work of Harold E. plants. New York, NY: Academic Press. 811 pp. Moore, Jr. Lawrence, KS: Allen Press, 610 pp.

Moll, G. 1992. Trees in the red. Urban Forests. American Wilson, B.F. The Growing Tree.1984. Amherst, MS: Univ. Forests. Washington, D.C. Feb./Mar. 10. Mass. Press. 138 pp.

Raupp, M. J., P. M. Shrewsbury, and D. A. Herms. 2010. Acknowledgments “Ecology of herbivorous arthropods in urban landscapes.” Annual Review of Entomology 55: 19–38. We are very grateful for the constructive and thorough reviews of this publication made by Ed Gilman, Bill Hub- Sands, R., and G.D. Bowen. 1978. “Compaction of sandy bard, Bira Malavasi, and Tim White. Additional thanks soils in radiata pine forests. II. Effects of compaction on root go to Bira Malavasi for the informative drawings of tree configuration and growth of radiata pine seedlings.”Aust. growth used in this publication. For. Res. 8:163-170.

Zvereva, E. L., V. Lanta, and M. V. Kozlov. 2010. “Effects of sap-feeding insect herbivores on growth and reproduction of woody plants: a meta-analysis of experimental studies.” Oecologia 163: 949–960.

How Trees Grow in the Urban Environment 8 Table 1. Soil compaction negatively affects root and shoot growth: Monterey pine seedlings had poorer growth in this study (Sands and Bowen 1978). Soil Bulk Density* Dry Volume (g) Root Volume (cm3) Height (cm) 3 (g cm ) Shoots Roots 1.60 3.6 3.0 24.7 20.5 1.48 5.9 4.9 39.3 29.2 1.35 7.0 5.6 47.3 32.8 *High bulk density means high soil compaction.

How Trees Grow in the Urban Environment 9