Arboriculture Diploma. Tree Botany week 3 Roots and the rhizosphere

Transcription

1 Arboriculture Diploma Tree Botany week 3 Roots and the rhizosphere

2 The primary functions of a root are: To absorb and transport water with dissolved nutrients Anchorage Storage of the sugars that have been manufactured in the leaves Associating with soil microbes in a symbiotic relationship.

3 Older roots develop bark and do not absorb water and dissolved nutrients. They provide anchorage. Bark protects these exposed roots from drying out.

4 Young roots, the root tips, are the primary location for absorption of water. The epidermis has no (or very little) cuticle, so water can be absorbed through all areas covered with epidermis. Stomates in the young root epidermis cannot close. They stay open for gaseous exchange and absorption. At the tips of roots some epidermal cells modify into root hairs. Cross section through a young root.

5 tip Root hairs only occur at the tips, behind the zone where cells elongate. Root hairs are an extension of epidermal cells. They are up to 8mm long.

6 Root hairs increase the surface area for more absorption. They are the most absorptive part of a root. Root hairs are short lived, typically around 3 weeks, and are replaced as the root grows. They do not grow into roots.

7 A root cap protects the root tip as it pushes through the soil. Root cap Outer cells of the root cap are worn off and replaced by new cells from the inner layer. New cells produced in the meristematic zone = growth This leaves a layer of cell slime that helps the new root push through soil.

8 The cortex is much wider in a young root than it is in a stem. It is made of parenchyma tissue with many air spaces. The parenchyma cells store starches. The air spaces allow oxygen to move through the root. Roots need oxygen for respiration. Cross section through a young root.

9 The inner layer of the cortex is called the endodermis. Water can enter the root easily through the epidermis and stomates. It freely moves across the cortex, both through the parenchyma and along the parenchyma cell walls. The endodermis acts as a filter. Its cells are closely packed with thickened walls.

10 Endodermis cells are closely packed together. Endodermis of a monocot root

11 The thickened area of the cell walls of the endodermis are called casparian strips. The casparian strips force water and its dissolved nutrients (solutes) to move through the endodermis cell. The cell membrane stops undesirable solutes, such as salt, from entering the vascular tissue.

12 The casparian strip contains suberin, the waterproof material found in cork cells. It seals the cell wall and the middle lamella between cells.

13 Inside the endodermis is the area of the root called the stele. stele

14 The outer layer of the stele is called the pericycle. Lateral roots originate here. Roots do not have nodes and buds. They do not branch in an opposite or alternate pattern.

15 Lateral roots grow from the pericycle which is close to the vascular tissue. Pruning most roots will stimulate branching.

16 Root pruning is done to prepare plants for transplanting. It produces a smaller densely branched rootball including new root tips. By pruning in alternate sections you leave enough young roots to support the tree until new root tips are formed.

17 The xylem and phloem of a young root is not arranged in vascular bundles. In a root the xylem is in the centre with radiating arms. Phloem is in patches between the radiating arms of xylem.

18 Dicot roots, and roots of gymnosperms, typically have four arms of xylem Companion cell Sieve tubes of phloem.

19 Monocot roots have up to 20 xylem arms, and pith in the centre of the root. pith xylem phloem

20 The junction between the root and stem of a young plant shows an interesting rearrangement of the vascular tissue.

21 The junction will flare out at the root collar, where the many lateral roots connect to one stem. Trunk flare should be visible at the surface of any woody plant. Roots should be close to the surface. If they are not, the soil level may have been altered.

22 These Magnolia trees were to be protected when a home was replaced with shops. They show signs of stress, branch dieback, and epicormic shoots. Mosman, June 08

23 There is no trunk flare or roots close to the surface. The roots have been buried. They have also had paving installed over the outer roots, and probably some roots cut for the wall footings.

24 Two years later and these trees are struggling. They are still alive, but unattractive, in 2015.

25 Dicot and gymnosperm roots have a vascular cambium between the xylem and phloem.

26 As a root develops secondary growth its vascular tissue forms rings (like a stem). A cork cambium forms in the pericycle, and the epidermis, cortex and endodermis are destroyed. Periderm = cork, cork cambium and cortex

27 Roots of dicotyledons and conifers can become woody, but they do not grow as thick as trunks and main branches. They do not develop heartwood. All the xylem is able to transport water. These woody roots have been used ornamentally to create a rootery (also called a stumpery), a very Victorian era garden feature. This rootery is at Highgrove Palace in England

28 3 Roots do not grow as thick as branches, but they can grow longer and more numerous. More roots produce a greater surface area for absorption.

29 We classify roots into three different types. fibrous taproot adventitious

30 When a seed germinates it produces either a fibrous root system or a tap root system. Pictures from Botany for Gardeners by Brian Capon

31 A fibrous root system Fibrous root system of a palm. Is grown by monocotyledons. Produces roots of equal diameter from the seed. Roots may or may not branch, usually only once or twice. New roots continue to grow from the base of the plant stem / crown all its life. Old roots die. Palm roots may live for around 3 years. Fibrous roots do not become thicker and woody.

32 Monocots with fibrous root systems, such as palms, are always producing new roots from the base of the stem. This usually makes them easy to transplant. Washington palms being transplanted at Dee Why beach

33 Some palms are harder to move than others. They may have fewer longer roots, or their roots may die when pruned. Bismarkia nobilis has a reputation for being hard to move. Its roots mostly grow down, and it needs a larger deeper rootball. Transplanted Bismarck Palms in Singapore

34 Cocus nucifera on Gilli Trewangan In general, bigger rootballs, 0.6 to 1m from the trunk, have more success. After care is critical, especially irrigation. Trunks will need stabilisation for up to 1 year. Pruning leaves, or tying them up, has variable success. Most palms do better if not pruned.

35 Plant palms at their original soil depth, or at the point where roots originate on the trunk. If they are planted to shallowly, they may snap. Too deep and they may be stunted. This Phoenix canariensis could be planted deeper than soil level. This Phoenix at Alpine Treemovals snapped when being moved. It has been planted like a giant cutting, and has developed new roots.

36 A tap root system Ground Level Lateral root Primary root Is grown by dicotyledons and conifers Produces one primary (tap) root from the seed. Lateral roots branch from the tap root. The tap root may grow deep, but usually it doesn t. Roots may become thicker and woody, unlike a fibrous root system.

37 The root system of a tree is much wider than it is deep. This is because the upper soil is warmer, more fertile and has more air than subsoil. Most roots are in the top 1 to 1.5m. It is called a root plate.

38 Young roots with root hairs are produced mostly towards the perimeter of the root zone, but they can also grow close to the trunk. The heart roots provide anchorage.

39 Cutting roots close to the trunk will compromise the trees stability.

40 Lateral roots extend out from the trunk, sharing the soil with roots of other trees. The dots in this diagram of a red maple root represent tree trunks. Intermingled roots in a mixed hardwood stand treerootfacts.pdf

41 Roots are opportunistic. They proliferate in areas conducive for growth. Roots in dry or low nutrient soils will grow longer with less branching. Roots in moist fertile soils are more likely to be densely branched and closer to the trunk. Where there is a high water table, roots will remain shallow. They will only grow where oxygen is available.

42 Trees that develop deep roots may be adapted to deep sandy soils, or windy environments where they need extra anchorage. They may also be searching for water. Ngilgi Caves have been carved by water out of limestone in SE West Australia. Living tree roots from the Marri trees, Corymbia calophylla, are growing through the cave roof, 45m below the surface.

43 Some trees have a persistent deep taproot, with less lateral root development. Nyssa sylvatica does not transplant easily, because of its deep taproot. Brachychiton spp. have a deep water storing taproot, but it can be cut if the tree is transplanted.

44 Most trees develop other downward growing roots, like sinker roots or water roots, and do not retain a dominant taproot. The overall pattern is influenced by species and soil.

45 tracheid Lateral roots mostly transport water to the branches above the root, though some trees, such as conifers, have a more spiralling pattern of xylem. This may be because water in tracheids moves sideways into the next tracheid through pits.

46 If the roots on one side of a tree are damaged, often the branches on that side will die. The roots of this Eucalyptus had lifted the footpath. The root was cut out and the footpath restored. After showing signs of dieback in the canopy the tree was removed.

47 The lateral roots extend well beyond the drip line, continuing to branch and produce new root tips. They radiate in a more or less symmetrical pattern. The root cap appears to be involved in orientating the root growth. It can sense light, pressure and perhaps gravity.

48 The finer feeder roots, sometimes called fibrous roots, are mostly in the top 150mm of the soil, particularly of the soil is protected by mulch. Electric cables were run under the feeder roots of Norfolk Island Pines at Manly.

49 Unfortunately, the roots were later severed as the new paving was installed at a level lower than the roots. The trees have shown no sign of dieback. Possibly they had developed a second, lower layer of lateral roots. This may happen with conifers in sandy soils.

50 This company will slice through the top 600mm of roots to protect lawns. They cut to the drip line. Roots may grow at or above the soil surface. They will be attracted to the regular water and fertiliser applied to lawns. They can be damaged by herbicides.

51 Roots will also grow at the surface if soil is compacted or has a high water table. Compacted soils are low in oxygen, restricting respiration. These 17 year old spotted gums have grown very slowly in a primary school playground.

52 Soil can be compacted by people and machinery. Compaction is a problem where cars park under trees. It is affecting the figs and Holm Oaks in Centennial Park, some of which will need replacing.

53 To raise money to replace and care for trees, there is a program of tree sponsorship.

54 In Sydney s Royal Botanic Gardens fences keep visitors off the root zone of older trees. This reduces soil compaction (and also stops people from climbing on the trees).

55 On building sites, the root zones of existing trees should be protected from compaction. Tree Protection Zone radius = at least 10 x the trunk diameter at 1.4m.

56 Canopy treetop apartments, Victoria Rd Ryde Nov 2013 This Eucalyptus pilularis has its TPZ protected, but it may be affected by changes in hydrology and wind. Feb 2015

57 The whole root zone should be protected from changed soil levels, not just the area close to the trunk. Roots that have been buried will not get enough oxygen for respiration.

58 In compacted soil, tree roots will follow cracks in soil or rocks or paving to get oxygen and water. Image from They may be attracted to pipes for the same reasons. Image from

59 Here at Kew Gardens in London, soil is being aerated with high pressure nitrogen gas to help the older trees get enough oxygen in their roots.

60 Providing well aerated soils can be a dilemma in urban areas, where soils are compacted for stability of buildings. This structural grid will be filled with uncompacted soil mix for health root growth From Living in a Garden The Greening of Singapore.

61 If a root is bent or distorted when a plant is potted up or planted out it may function well at first, but as it gets thicker and woodier the plant will loose vigour and may die. These roots are called kinked or girdling roots

62 Kinked and girdling roots are a common cause of sudden death in plants. Roots may become kinked when plants are potted up into containers too small for the roots. Never bend or fold roots. It is better to cut them so they fit into the container.

63 As kinked roots get thicker they will cut off the xylem, like a hose being folded. Plants can die suddenly on hot days. A kinked root may come up out of the soil then back under. Sometimes you cannot see kinked roots until the plant dies and you pull it out.

64 Girdling roots occur when a plant is left to grow too long in a container. The plant becomes root bound. As the roots become thicker and woodier they may eventually ringbark the trunk. Girdling roots should be cut through when the plant is young, when planting or potting up.

65 NATSPEC Standards, to ensure trees from nurseries have healthy well formed roots, were released in Roots on the surface of the root ball are shaved off, to stimulate root branching and avoid girdling roots.

66 Sometimes circling roots grow near the surface and around the trunk after planting. They may be the result of growing in dry compacted soils. They may also be produced in response to root pruning at planting time.

67 Girdling roots may also result in trees falling over.

68 Tree roots may die back in dry times, and put on new growth when water becomes available. These dead roots add to soil organic matter, and help aerate soil. Trees also leak phloem sap and other soluble compounds into soil, to support soil microorganisms, such as bacteria and nematodes. This creates a rhizosphere, an area of intense biological and chemical activity.

69 The rhizosphere is only narrow, around 5mm wide, and is associated with root tips.

70 The sap may contain hormones, such as strigolactone, that attract and stimulate growth of mycorrhizal fungi. The fungi act as an extension to the feeder roots, bringing nutrients to the root tissues. The roots support the fungi with sugars. Mycorrhizal fungi on Spotted Gum

71 The hyphae of the fungus may form a sheath or mantle around root tips, or may penetrate and grow between the cortex cells.

72 Some plants produce compounds like flavonoids in their rhizosphere, to attract nitrogen fixing bacteria to their roots. Plants in the family FABACEAE attract Rhizobium bacteria. Frankia colonises roots of some Casuarina spp. and Alnus spp. Rhizobium nodules Frankia on Alnus roots

73 Proteoid roots, or cluster roots, are root modifications that produce an increased surface area for absorption of nutrients like phosphorus. They are not the same as root hairs. Proteoid roots grow when conditions are favourable, then die back. Plants with proteoid roots include PROTEACEAE plants, plus 30 species from BETULACEAE, CASUARINACEAE, ELAEAGNACEAE, FABACEAE and MORACEAE.

74 Tree roots tend to avoid each other when young, but as they grow they may be forced together and form a graft union. These grafts can conduct diseases, such as Armillaria, from one tree to another. Treatment for Armillaria may include root barriers. Roots are exposed to sunlight, to try to kill the fungus.

75 Some plants secrete allelopathic chemicals from their roots, which discourage the growth of other plant roots in their root zone. These chemicals may also be in leaves, and accumulate in leaf litter. Examples include Casuarina, Pines and Camphor Laurels.

76 Natural herbicides made from allelopathic chemicals are being researched. In America, the Black Walnut chemical Juglone is used successfully. This new product contains Pine Oil

77 The third type of root system is called adventitious roots. Both monocots and dicots can produce adventitious roots. Adventitious roots do not grow from a seed. They grow from other parts of the plant, such as stems. Adventitious roots can develop above or below ground. Some climbing plants, like Monstera, use adventitious roots to cling to rough surfaces.

78 Adventitious roots are important for vegetative (also called asexual) propagation. Examples include cuttings, division, layering and tissue culture. Stem cuttings of Cordyline fruticosa

79 Layering is when a stem produces adventitious roots while it is still attached to the plant. This Magnolia has branches that touched the ground. New trunks have grown where roots have formed. We can manipulate a plant to do this by making a shallow cut in the stem and pegging it down in contact with soil.

80 Aerial layering or marcotting is a way of propagating a tree that is not easy to grow from cuttings. The stem is wounded or ringbarked. Sphagnum moss is wrapped around the wound, protected by plastic. New roots grow into the sphagnum. The top can then be cut off and planted.

81 Generally roots can only grow more roots. Only a few plants can make new stems from their roots. These new stems are called suckers. Shoots from the base of the stem / trunk, growing from epicormic buds. Shoots from the roots. They develop their own root systems.

82 Plants with roots that sucker can be a problem in gardens. Examples include Poplars, Wisteria, Robinia, some Casuarinas, and grafted Pears. Populus alba

83 Using systemic herbicides on suckers will damage the roots of the original tree. Recommendations include: Continuous removal, such as mowing, so the suckers run out of energy. Digging down to the point where suckers originate, and cutting the sucker off close to the root. Avoid damaging roots of trees that sucker. Avoid stressing trees with root that sucker.

84 Buttress roots are adventitious roots that develop on large rainforest trees, the emergents of the canopy. Their I shaped profile is strong, like a steel beam. They provide stability for tall, top heavy trees that may be exposed to monsoonal winds.

85 Stilt roots develop from the lower nodes of a stem. They give additional support, mostly to monocots. Pandanus spp. have stilt roots

86 Some Mangroves have stilt roots, to resist strong tidal forces. The seeds produce a long root before dropping vertically into the mud. Rhizophora apiculata

87 Prop roots grow from branches or higher on the trunk. They thicken up and act as supports to long, heavy branches. This is a Banyan Tree, Ficus bengalensis. The Great Banyan at Calcutta spreads over four acres of ground and has more than 1775 prop roots.

88 As the roots are growing the root tips are vulnerable to drying out. Sometimes the roots dry out and don t make it to the ground.

89 At the Sydney Botanic gardens they have protected prop root tips in moist hessian cloth.

90 Epiphytic roots help an epiphyte to cling onto its support. The roots do not take water and nutrients from the host plant. There are many epiphytic orchids, ferns and bromeliads.

91 Strangler figs are epiphytes that produce roots that encircle host plants, eventually cutting the flow of phloem to roots and killing the host. Looking up through the centre of a strangler fig at Nambucca Heads, after the host tree has died and rotted away. Photo by Wayne

92 Parasitic roots grow into the tissues of other plants and take water and nutrients. This is mistletoe on a Scribbly Gum. The point where the roots penetrate the host is called the haustorium.

93 Mistletoe is spread from plant to plant by birds who eat the fleshy fruits.

94 Some mistletoes, like Dodder, have multiple haustoriums. When mature Dodders are totally dependant on the host plant with no roots or green leaves of their own.

95 Some mistletoes have haustoria that produce long strands within the host. They mostly grow down through the phloem, and may produce shoots that break through the bark. Just cutting off the terminal clump will not remove them.

96 Mistletoe may take so much of the water and nutrients that the host plant dies, especially in dry times. Evergreen Brachychiton populneus, Kurrajong, struggling with a heavy infestation of mistletoe. Mistletoe is killed by bushfire, but is a problem for isolated paddock trees.

97 Mistletoe on Platanus x hybrida Mistletoe on deciduous plants shows up clearly in winter. This is a good time to try to cut the mistletoe out. It may have multiple haustoriums, requiring long sections of branches to be removed.

98 Some plants are hemi-parasites. They have a small root system of their own but need to take water and nutrients from the roots of other trees. West Australia s Christmas Tree, Nuytsia floribunda, is a hemi-parasite. This tree can send out parasitic roots for up to 100m, looking for host plants.

99 The local plant, Exocarpus cupressiformis, is a root hemi-parasite. It looks like a conifer, but will not survive if surrounding trees are removed.

100 Root carvings