MARCESCENCE

Marcescence is the retention of dead plant organs that normally are shed. It is most obvious in deciduous trees that retain leaves through the winter. Several trees normally have marcescent leaves such as oak (Quercus), beech (Fagus) and hornbeam (Carpinus), or marcescent stipules as in some but not all species of willows (Salix). Marcescent leaves of pin oak (Quercus palustris) complete development of their abscissior layer in the spring. The base of the petiole remains alive over the winter. Many other trees may have marcescent leaves in seasons where an early freeze kills the leaves before the abscission layer develops or completes development. Diseases or pests can also kill leaves before they can develop an abscission layer.

Oak with marcescent foliage.

Marcescent leaves may be retained indefinitely and do not break off until mechanical forces (wind for instance) cause the dry and brittle petioles to snap.

Many palms form a skirt-like or shuttlecock-like crown of marcescent leaves under new growth that may persist for years before being shed. In some species only juveniles retain dead leaves and marcescence in palms is considered a primitive trait.

Typical partial marcescence on a mature beech (Fagus sylvatica) tree.

The term marcescent is also used in mycology to describe a mushroom which (unlike most species, described as "putrescent") can dry out, but later revive and continue to disperse spores. Genus Marasmius is well known for this feature, which was considered taxonomically important by Elias Magnus Fries in his 1838 classification of the fungi.

Benefits

One possible advantage of marcescent leaves is that they may deter feeding of large herbivores, such as deer and moose, which normally eat the twigs and their nutritious buds. Dead, dry leaves make the twigs less nutritious and less palatable.

Marcescent leaves may protect some species from water stress or temperature stress. For example, in tropical alpine environments a wide variety of plants in different plant families and different parts of the world have evolved a growth form known as the caulescent rosette, characterized by evergreen rosettes growing above marcescent leaves. Examples of plants for which the marcescent leaves have been confirmed to improve survival, help water balance, or protect the plant from cold injury are Espeletia schultzii and Espeletia timotensis, both from the Andes.

The litter-trapping marcescent leaf crown of Dypsis palms accumulate detritus to enhance their nutritient supply.  By the same token, palms with marcescent leaf bases are also more susceptible to epiphytic parasites like figs that may completely engulf and strangle the palms.

References

1. ^ Berkley, Earl E. 1931. Marcescent leaves of certain species of Quercus. Botanical Gazette 92: 85-93.
2. ^ George W. Argus. "88. Salix planifolia Pursh". Flora of North America 7.
3. ^ Hoshaw, R.W. and Guard, A.T. 1949. "Abscission of marcescent leaves of Quercus palustris and Q. coccinea". Botanical Gazette 110: 587–593.
4. ^ Addicott, Fredrick T. (1982). Abscission. University of California Press. p. 51. ISBN 978-0-520-04288-9.
5. ^ L.B. Holm-Nielsen, ed. (1989). Tropical Forests: Botanical Dynamics, Speciation & Diversity. Academic Press. p. 161. ISBN 978-0-08-098445-2.
6. ^ Dowe, John (2010). Australian Palms: Biogeography, Ecology and Systematics. Csiro Publishing. p. 160. ISBN 978-0-643-10185-2.
7. ^ Dransfield, John; Uhl, Natalie W. (2008). Genera Palmarum: the evolution and classification of palms. Kew Pub. p. 294. ISBN 978-1-84246-182-2. marcescent in immature [descriptive of several spp.]
8. ^ Moore, Harold Emery; Uhl, Natalie W. (1982). Major Trends of Evolution in Palms. New York Botanical Garden. p. 69.
9. ^ See introduction to Roy E. Halling "A revision of Collybia s.l. in the northeastern United States & adjacent Canada" Inst. of Syst. Botany, The New York Botanical Garden, Bronx, NY 10458-5126
10. ^ E. M. Fries Epicrisis systematis mycologici (1838) Uppsala: Typographia Academica
11. ^ Svendsen, Claus R. 2001. Effects of marcescent leaves on winter browsing by large herbivores in northern temperate deciduous forests. Alces 37(2): 475-482.
12. ^ Goldstein, G. and Meinzer, F.1983. Influence of insulating dead leaves and low temperatures on water balance in an Andean giant rosette plant. Plant, Cell & Environment6: 649-656.
13. ^ Smith, Alan P.1979. Function of dead leaves in Espeletia schultzii (Compositae), and Andean caulescent rosette species. Biotropica 11: 43-47.
14. ^ Bramwell, David; Caujapé-Castells, Juli (2011-07-21). The Biology of Island Floras. Cambridge University Press. p. 189. ISBN 978-1-139-49780-0.
15. ^ Kramer, Gregory T. (2011). "Palm Tree Susceptibility to Hemi-Epiphytic Parasitism by Ficus". (M.S. thesis). University of Florida.

Wikipedia 

DECIDUOUS

Deciduous means "falling off at maturity" or "tending to fall off", and it is typically used in order to refer to treesor shrubs that lose their leaves seasonally (most commonly during autumn)  and to the shedding of other plant structures such as petals after flowering or fruit when ripe. In a more general sense, deciduous means "the dropping of a part that is no longer needed" or "falling away after its purpose is finished". In plants it is the result of natural processes. "Deciduous" has a similar meaning when referring to animal parts, such as deciduous antlers in deer or deciduous, teeth also known as baby teeth, in some mammals (including humans).

Deciduous forest in autumn.

Botany

In botany, and horticulture, deciduous plants, including trees, shrubs and herbaceous perennials, are those that lose all of their leaves, for part of the year. This process is called abscission. In some cases leaf loss coincides with winter — namely in temperate or polar climates. In other parts of the world, including tropical, subtropical, and arid regions, plants lose their leaves during the dry season or other seasons, depending on variations in rainfall.

Deciduous forest in winter.

The converse of deciduous is evergreen, where foliage is shed on a different schedule from deciduous trees, therefore appearing to remain green year round. Plants that are intermediate may be called semi-deciduous; they lose old foliage as new growth begins. Other plants are semi-evergreen and lose their leaves before the next growing season, retaining some during winter or dry periods. Some trees, including a few species of oak, have desiccated leaves that remain on the tree through winter; these dry persistent leaves are called marcescent leaves and are dropped in the spring as new growth begins.

Mixed deciduous forest in spring.

Many deciduous plants flower during the period when they are leafless, as this increases the effectiveness of pollination. The absence of leaves improves wind transmission of pollen for wind-pollinated plants and increases the visibility of the flowers to insects in insect-pollinated plants. This strategy is not without risks, as the flowers can be damaged by frost or, in dry season regions, result in water stress on the plant. Nevertheless, there is much less branch and trunk breakage from glaze ice storms when leafless, and plants can reduce water loss due to the reduction in availability of liquid water during cold winter days.

Like many deciduous plants, Forsythia flowers during the leafless season.

Leaf drop or abscission involves complex physiological signals and changes within plants. The process of photosynthesis steadily degrades the supply of chlorophylls in foliage; plants normally replenish chlorophylls during the summer months. When autumn arrives and the days are shorter or when plants are drought-stressed, deciduous trees decrease chlorophyll pigment production, allowing other pigments present in the leaf to become apparent, resulting in non-green colored foliage. The brightest leaf colors are produced when days grow short and nights are cool, but remain above freezing. These other pigments include carotenoids that are yellow, brown, and orange. Anthocyanin pigments produce red and purple colors, though they are not always present in the leaves. Rather, they are produced in the foliage in late summer, when sugars are trapped in the leaves after the process of abscission begins. Parts of the world that have showy displays of bright autumn colors are limited to locations where days become short and nights are cool. In other parts of the world, the leaves of deciduous trees simply fall off without turning the bright colors produced from the accumulation of anthocyanin pigments.

The beginnings of leaf drop starts when an abscission layer is formed between the leaf petiole and the stem. This layer is formed in the spring during active new growth of the leaf; it consists of layers of cells that can separate from each other. The cells are sensitive to a plant hormone called auxin that is produced by the leaf and other parts of the plant. When auxin coming from the leaf is produced at a rate consistent with that from the body of the plant, the cells of the abscission layer remain connected; in autumn, or when under stress, the auxin flow from the leaf decreases or stops, triggering cellular elongation within the abscission layer. The elongation of these cells break the connection between the different cell layers, allowing the leaf to break away from the plant. It also forms a layer that seals the break, so the plant does not lose sap.

A number of deciduous plants remove nitrogen and carbon from the foliage before they are shed and store them in the form of proteins in the vacuoles of parenchyma cells in the roots and the inner bark. In the spring, these proteins are used as a nitrogen source during the growth of new leaves or flowers.

Functions

Plants with deciduous foliage have advantages and disadvantages compared to plants with evergreen foliage. Since deciduous plants lose their leaves to conserve water or to better survive winter weather conditions, they must regrow new foliage during the next suitable growing season; this uses resources which evergreens do not need to expend. Evergreens suffer greater water loss during the winter and they also can experience greater predation pressure, especially when small. Losing leaves in winter may reduce damage from insects; repairing leaves and keeping them functional may be more costly than just losing and regrowing them. Removing leaves also reduces cavitation which can damage xylem vessels in plants. This then allows deciduous plants to have xylem vessels with larger diameters and therefore a greater rate of transpiration (and hence CO2 uptake as this occurs when stomata are open) during the summer growth period.

Deciduous woody plants

The deciduous characteristic has developed repeatedly among woody plants. Trees include maple, many oaks, elm, aspen, and birch,  among others, as well as a number of coniferous genera, such as larch and Metasequoia. Deciduous shrubs include honeysuckle, viburnum, and many others. Most temperate woody vines are also deciduous, including grapes,  poison ivy, Virginia creeper, wisteria,  etc. The characteristic is useful in plant identification; for instance in parts of Southern California and the American Southeast, deciduous and evergreen oak species may grow side by side.

Periods of leaf fall often coincide with seasons: winter in the case of cool-climate plants or the dry-season in the case of tropical plants, however there are no deciduous species among tree-like monocotyledonous plants, e.g. palms, yuccas, and dracaenas.

Regions

Forests where a majority of the trees lose their foliage at the end of the typical growing season are called deciduous forests. These forests are found in many areas worldwide and have distinctive ecosystems, understory growth, and soil dynamics.

Lesser celandine (Ranunculus ficaria) dominated deciduous woodland in early spring.

Two distinctive types of deciduous forest are found growing around the world..

Temperate deciduous forest biomes are plant communities distributed in North and South America, Asia, Southern slopes of the Himalayas, Europe and for cultivation purposes in Oceania. They have formed under climatic conditions which have great seasonable temperature variability with growth occurring during warm summers and leaf drop in autumn and dormancy during cold winters. These seasonally distinctive communities have diverse life forms that are impacted greatly by the seasonality of their climate, mainly temperature and precipitation rates. These varying and regionally different ecological conditions produce distinctive forest plant communities in different regions.

Dry-season deciduous tropical forest.

Tropical and subtropical deciduous forest biomes have developed in response not to seasonal temperature variations but to seasonal rainfall patterns. During prolonged dry periods the foliage is dropped to conserve water and prevent death from drought. Leaf drop is not seasonally dependent as it is in temperate climates, and can occur any time of year and varies by region of the world. Even within a small local area there can be variations in the timing and duration of leaf drop; different sides of the same mountain and areas that have high water tables or areas along streams and rivers can produce a patchwork of leafy and leafless trees.

References

1. ^ William Dwight Whitney; Century Dictionary. The Century Dictionary and Cyclopedia: Dictionary. books.google.com. p. 1484.
2. ^ Debra J. Housel; Capstone Publishers (2009). Ecosystems. books.google.com. ISBN 9780756540685.
3. ^ Gause, John Taylor (1955). The complete word hunter. A Crowell reference book. New York: Crowell. p. 465.
4. ^ University of the Western Cape.  "Trees that lose their leaves".  botany.uwc.ac.za.
5. ^ Dr. Kim D. Coder; University of Georgia (1999). "Falling Tree Leaves: Leaf Abscission". (PDF). forestry.uga.edu.
6. ^ Science Daily. "Science Reference: Deciduous". sciencedaily.com.
7. ^ J. Robert Nuss; Pennsylvania State University (2007). "Evergreen Shrubs and Trees for Pennsylvania" (PDF). psu.edu.
8. ^ "The Illinois - North Carolina Collaborative Environment for Botanical Resources: Openkey Project. Glossary of Botanical Terms. Page 22.". (PDF). Retrieved 2013-07-17.
9. ^ Weber, William. 2001. African rain forest ecology and conservation an interdisciplinary perspective. New Haven: Yale University Press. page 15.
10. ^ Lemon, P. C. (1961). "Forest ecology of ice storms". Bulletin of the Torrey Botanical Club (Bulletin of the Torrey Botanical Club, Vol. 88, No. 1) 88 (21): 21. doi:10.2307/2482410. JSTOR 2482410.
11. ^ Mohammad Pessarakli (2005). Handbook of photosynthesis. CRC Press. pp. 725–. ISBN 978-0-8247-5839-4.Retrieved 9 October 2010.
12. ^ Donald W. Linzey (1 April 2008). A natural history guide to Great Smoky Mountains National Park. Univ. of Tennessee Press. pp. 27–. ISBN 978-1-57233-612-4. Retrieved 9 October 2010.
13. ^ Srivastava, Lalit M. (2002). Plant growth and development. Hormones and environment. Amsterdam: Academic Press. p. 476. ISBN 0-12-660570-X.
14. ^ Labandeira, C. C.; Dilcher, D. L.; Davis, D. R.; Wagner, D. L. (1994). "Ninety-seven million years of angiosperm-insect association: paleobiological insights into the meaning of coevolution" Proceedings of the National Academy of Sciences of the United States of America 91 (25): 12278–12282. Bibcode:1994PNAS...9112278L doi:10.1073/pnas.91.25.12278. PMC 45420. PMID 11607501.
15. ^ Cundall, Peter (2005). Flora: The Gardener’s Bible: Over 20,000 Plants. Ultimo, NSW, Australia: ABC Publishing. ISBN 0-7333-1094-X.

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