Grass - Tree Competition

Grasses are members of the Gramineae family of flowering plants. Their leaves are usually long and parallel veined, arranged in two ranks, and grow from their bases. Large portions of grass stems are underground (i.e. rhizomes). Grass flowers are very much reduced from the angiosperm norm, they are specialised for wind pollination. Grass stores its carbohydrates largely in its underground rhizomes. It can generally grow very quickly when water is available. The plant then rapidly goes into dormancy when growing conditions become unfavourable. The stored carbohydrate reserves feed the new shoots when growing conditions improve. Grass therefore tends to be more drought hardy than trees. Most species of grass are low herbaceous perennials^*.

Turf grass and trees do not live in harmony with one another. Turf grass inhibits the growth of young trees. Old trees inhibit the growth of grass. The roots of grass are more invasive and they grow more rapidly than those of trees. Grass roots compete more vigorously for nitrogen and water than do tree roots. Many species of grass are also alleopathic to trees. However, once established, a large tree almost always shades-out and inhibits turf grass.

It is strongly advised that a lawn tree should have a mulch bed around its trunk. The mulch bed should be about 9 cm deep and, if possible, extend to the tree’s drip-line. Mulch holds back competing weeds and grass, it also holds moisture near the soil. However, excess water can instigate crown-rot. Adjacent to the truck’s root crown, there should be virtually no mulch.

Grass Ecosystems

It is important to remember that grass cannot grow in every milieu. It is an organism in an actual ecosystem. Deeply shaded areas will simply not support thick turf grass. Even in nature grass and trees are in severe competition. The pampas, prairie, steppe and veld grasslands occur in the dryer areas of the mid-latitudes (N&S: 20 to 50^o). These grasslands grade into partly treed savannah. Savannah grades into true forest as the average precipitation increases. In the intermediate savannah regions there seems to be a root-zone war going on. Afforestation is slow and patchy. Swaths of grassland remain between small groves of trees. This conflict is especially evident in Texas and parts of the Ukraine. The soils even change to reflect the influence of the plant cover. Patches of trees, ‘bluffs’, have acidic forest-like podzolic soils under them. Nearby areas of grassland have more basic chernozemic soils. This is because the treed areas hold soil water longer and have more leaching. In the grassy areas evaporation tends to dry the soil. Water does not soak very far into the ground and carbonates are precipitated. There are examples of soils that have changed in structure in less than a century after human interference. Usually these are examples of chernozems becoming more podzolised after trees have been planted.

* Note: The bamboos are arborescent grasses. Even the hardiest bamboo species do not do well outdoors in Ontario.

WEIRD SCIENCE BIO-GEOGRAPHY

References

Agriculture Canada. 1987. The Canadian System of Soil Classification. Second Edition.Canadian Government Publishing Centre. Ottawa.

Eyre, S.R. 1968. Vegetation and Soils - a world picture. Second Edition. Aldine Publishing Company. Chicago.

Holiday, Paul. 1989. A Dictionary of Plant Pathology. Cambridge University Press. New York.

Oliver, Roland and Fage, J.D. 1970. A Short History of Africa. Penguin Books Ltd. Harmondsworth, U.K.

Needle Casts & Soil Stress

There are several different types of fungi that cause conifer needles to drop prematurely. Some of these diseases can be very deleterious, they can even cause a tree to die. On the bright side, these diseases are usually not equally virulent to every variety within a host species. If growing conditions are excellent, a conifer seldom succumbs to a mere needle cast infection.

Needle cast diseases can become a special problem in conifer monocultures. A needle cast epidemic is especially likely if too many members of the same species are planted together, and if there is too little genetic diversity within that batch of seedlings. Most needle drop diseases are fatal only if the trees are already stressed by some other factor, usually poor soil conditions. A stressed tree may have problems coping with the extra burden of a fungal infection along with water stress or nutrient problems. A surplus of nutrients and water are required for conifers to make defensive chemicals and to also maintain regular growth. Poor soil quality is often the main factor leading to cast outbreaks on recently landscaped sites. Landscaped soils too often lack humus, have poor drainage, and have been overly compacted by construction machinery. Since transplanted trees have already undergone root damage when they were ‘installed’, the sudden introduction into an inferior soil can come as an additional shock. (Too many construction companies treat trees as if they are some kind of hardware.)

Below, are listed some needle cast species common in temperate North America. Many obsolete taxonomic names have become part of the common lexicon. Some of the cast fungi are only known by the names of their asexual or anamorphic forms. For example, many of the phomopsis species are anamorphs of the teleomorphic genus Diaporthe. However, many species of phomopsis lack any known sexual phase. The teleomorphic binomial names, listed below, are in italics.

Fungal Pathogen	Host Conifer	Symptoms & Signs
Cyclaneusma minus (Naemacyclus minor)	Two-needled pines, esp. Scots Pine	Pale dashes along 2nd & 3rd year needles. Yellowish apothecia.
Dothistroma septospora (Scirrhia pini)	Austrian, ponderosa & other pines. Red & Scots pine are resistant.	Yellow-tan spots & green bands on 1st year needles. Black conidia on needle tips.
Lophodermium seditiosum	Austrian, red & Scots pines.	Yellowing spots on needles. Black apothecia.
Mycosphaerella dearnessii (Scirrhia acicola)	Pines, esp. Scots pine.	Yellow to brown spots on 1st year needles. Black conidia inside stomata.
Phomopsis juniperovora (teleomorph unknown)	Junipers & cypresses.	Reddish to tan dead scale leaves. Black pycnidia on leaf scales. Dead leaves tend not to fall off.
Rhabdocline spp.	Douglas fir	Needles, 1st year, with yellow spots. Orange-red apothecia.
Rhizosphaera kalkhoffii (teleomorph unknown)	Colorado blue & white spruce, sometimes pines. Norway spruce is resistant.	Yellowing & browning of 1st year needles. Black pycnidia.
Diploidea pinea (Sphaeropsis sapinea)	Austrian, mugo, Scots & ponderosa pine.	Browning & stunting of 1st year needles. Black pycnidia on needle base. Some infection of twigs & cones.

Fir trees (Abies spp.) are occasionally suspected of having needle cast disorders. While there are fungal diseases that infect firs, fungi are not the main cause of fir needle drop in southern Ontario urban settings. Urban firs tend to decline because the dry soils of yards and landscapes are not suitable for them. Fir trees are generally adapted to fairly moist areas in boreal or alpine forests.

Control

Most needle cast diseases cannot be treated in the same year as the symptoms first appear. Preventative fungicide application early in the following spring may reduce the fungus in the growing season to come. In many species the spores remain infectious even after the needles have fallen. Inoculum levels can be reduced by raking up the fallen needles and removing them from the site. Trees that are damaged from last year’s infestation can sometimes be repaired by pruning. Severely deformed trees should be removed as soon as possible. The preferred method of controlling needle cast is in choosing resistant varieties. If at all possible, avoid planting conifer monocultures.

Most needle casts and blights are in the Ascomycota phylum of fungi. These fungi are ‘micro-fungi’ in the sense that their spore bearing bodies are very small. Some of the needle cast fungi are best identified by microscopic examination of their spore-bearing structures. Some of the main ascomycete types of spore-bearing bodies (sporocarps) and spores are:

Sporocarp Type		Sporocarp Characteristics		Spore Type
Conidium (pl. Conidia):		Asexual spores exposed on extended hyphal strands.		Conidiospores
Pycnidium (Pycnidia):		Flask-shaped organ containing asexual conidia.		Conidiospores
Apothecium (Apothecia):		Cone or cup-shaped organ containing sexual spores.		Ascospores
Perithecium (Perithecia):		Flask-shaped organ containing sexual spores.		Ascospores

Endophytic Fungi

In most cases, the needle blights are closely related to fungi which are benign parasites. These endophytic fungi are versions of the needle casts which cause no harm at all. Needle casts seem to be merely those fungi at the pathogenic side of a broad spectrum of fungal niches. Parasiticity varies by degree, even grading into symbiosis in some cases. For many endophytes produce toxins which make leaves less attractive to leaf-eaters.

References

Buszacki, Stefan and Harris, Keith. 1998. Pest, Diseases & Disorders of Garden Plants. Harper Collins Publishers. London

Holliday, Paul. 1989. A Dictionary of Plant Pathology. Cambridge University Press. Cambridge.

International Society of Arboriculture. 1997. Plant Health Care for woody ornamentals. Printec Press. Champaign Illinois.

Very few plants are direct parasites on other plants. Parasitic plants do occur, for example the broomrapes are true parasites. However, the ericaceous pinesaps are not directly parasitic. Despite what old text books commonly maintained, the pinesaps are not ordinary parasites. Nor are they ordinary saprophytes. These plants are something far more interesting. Pinesaps are un-equal partners with mycorrhizal fungi. They gain their nutrition from fungi, which in turn, gain their nutrition from green plants. Pinesaps are a kind of myco-heterotrophic parasite. They don't directly attack their host fungus nor do they infest their host plant. They are, in metaphorical terms, cheaters of mycorrhizal fungus. They take from the fungus nutrients, yet they fail to offer carbohydrates in exchange.

Canada’s pinesaps are members of the Ericaceae or heath family. The pale pine-drops (Pterospora andromedea) occur in coniferous forests. Pine-drops are parasites on the Rhizopogon false truffles. The pale tan Indian-pipe (Monotropa uniflora) is fairly common in the southern hardwood forests. the reddish-yellow Pinesap (Monotropa hypopithys) occurs in coniferous forests. The Indian-pipe and pinesap both parasitise the mycelia of soil fungi. The mycorrhizae exploited include the Russula toadstools and the Gymnomyces false truffles.

Pinesaps exploit exploit the pre-existing system of symbiosis. Since they are not good symbionts, they can only be considered a kind of true parasite. Nevertheless, these myco-heterotrophic parasites do not cause direct harm to trees. Only, perhaps, on the most stressed of trees would they be a problem by reducing mycorrhizal efficiency. Because they are so interesting, they are worth preserving rather than destroying. However, they seldom occur in urban landscape settings, so the problem seldom arises.

Broomrapes

Broomrapes are pale plants, in the Orobanchaceae family, that are similar in some ways to pinesaps. Broomrapes generally have many flowers per raceme, unlike the ericaceous pinesaps. These plants are true root parasites. Their roots form tuber-like masses which engulf sections of their host tree’s roots. Squawroot (Conopholis americana) has tightly flowered racemes. It parasitises the roots of several hardwood tree species. Beech-drops (Epifagus virginiana) are obligate parasites on beech trees. The beech-drop flowers are mounted upon long thin racemes. Cancer-roots (Orobanche spp.) are parasites on many species, including herbaceous plants. Like most broomrapes, they have very few visible leaves. Most have little more than the flower stocks extending aboveground. Broomrapes usually have more colourful flowers than do the pinesaps.

Broomrapes are more harmful than pinesaps. They directly weaken their host plant. On the plus side, broomrapes can have exquisite flowers.

Mistletoes

Mistletoes are members of the Loranthaceae family. They grow as semi-parasitic epiphytes on trees. They have quaint oval leaves, and much bifurcating stems. Their berries are viscous and tend to stick to birds. In this manner birds disseminate the seeds of the epiphyte. Mistletoes are perennial evergreens. They grow on tree branches, where they form big branchy orbs. In the early phases, just after seed germination, they are largely parasitic. They have haustorial roots which penetrate their host's xylem. These roots take some nutrients from the host plant's tissues. As mistletoes mature they come to rely ever more on their own photosynthesis.

European mistletoe (Viscum album) grows mostly on oaks. It has not yet become naturalised in the Americas. In Canada the dwarf mistletoe (Arceuthobium pusillum) grows on black spruce, white spruce and sometimes tamarack. It is common, but not often noticed. The American oak mistletoe (Phoradendron leucarpum) is common in the eastern parts of North America. This mistletoe's range does not quite extend into Canada. It is most common on oaks and other hardwoods in the southern USA.

If allowed to proliferate, mistletoes can become a problem for an urban tree. If necessary a mistletoe can be easily removed, by cutting it out in its early stages. Most people seems to mildly like mistletoes! They are therefore often allowed to grow. The American species generally are not very damaging to trees. If a few mistletoes are left on a tree, little harms is done. The dwarf mistletoe native to Ontario is seldom noticed, as it is inconspicuous in the foliage of its host conifer.

References

Bell, C. Ritchie and Taylor, Bryan J. 1982. Florida Flowers and Roadside Plants. Laurel Hill Press. Chapel Hill.

De Laubenfels, D. J. 1972. Flore de la Nouvelle Caledonia et Dependances. No. 4. gymnospermes. Muséum national d`histoire naturelle, Paris

Young B.W., Massicotte H.B., Tackaberry L.E., Baldwin Q.F., and Egger K.N. 2002. Monotropa uniflora: morphological and molecular assessment of mycorrhizae retrieved from sites in the sub-boreal spruce biogeoclimatic zone in central British Columbia. Mycorrhiza. 12(2):75-82.

Thonnérieux, Yves. 1993. Le Gui - monte-en-l'air et pique-assiette. Science et Nature. 39: 40-47.