3. Tree Genera and Native and Exotic Trees

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Tree genera

The founder of modern plant classification was Carl von Linné (1707-1778), the great Swedish botanist also known as Carolus Linnaeus. The modern age in biology, however, began in 1859 when Charles Darwin introduced the revolutionary concept of evolution in his book entitled "Origin of Species". This led to the construction of related plant groups using natural or phylogenetic arrangements, with the most primitive or oldest coming first, followed in order by the others, then terminating with the most recent ones that resulted in the process of evolution.

During the course of evolution, the first trees are thought to have appeared about 350 million years ago. They were predecessors to the proper coniferous trees which emerged soon after.1 Coniferous trees are evergreen.

Broadleaved trees developed later (about 100 million years ago) and since broad-leaved trees typically shed their leaves each autumn, they are called deciduous trees (with exceptions such as the Evergreen Magnolia (Magnolia grandiflora) and the Cork Oak (Quercus suber).

Plants (with flowers) of simple structure are usually looked upon as primitive or "of great antiquity", and those with structurally complex flowers are usually thought to be of more recent origin. However, it can be very difficult to ascertain whether a certain plant (or group of plants) is inherently simple (structurally) or whether it is simple through extensive evolutionary modification (Harlow et al. 1979).

Seed trees are generally classified into two categories:

1. Angiosperms or broadleaved trees (encased seeds that are enclosed around the mature ovary or fruit). Angiosperms are the dominant plant life in today's geological era. They are the products of a long line of evolutionary development that has culminated in the highly specialized organ of reproduction known as the "flower". The flower develops fruits where seeds are enclosed. There are 2 classes for the angiosperms:

Monocotyledones - characterized by parallel-veined leaves (grasses ie., bamboo and palms belong to the monocots).

Dicotyledones - with net-veined leaves. (broad-leaved trees, shrubs and herbs are dicots).

2. Gymnosperms or coniferous trees with naked seeds in their cones. According to Wikipedia, "Gymnosperms are heterosporous, producing microspores that develop into pollen grains and megaspores that are retained in an ovule. After fertilization (joining of the micro- and megaspore), the resulting embryo, along with other cells comprising the ovule, develops into a seed."

The broader and more general taxonomy of trees and tree-like plants is as follows (Brockman 1979):

1. True flowering plants

Magnoliophyta, angiosperms

1.1 Dicotyledons

Magnoliopsida, broadleaved or hardwood trees

1.2 Monocotyledones (Liliopsida)

Leaves are evergreen, fibers irregular in stems (i.e., palms )

2   Conifers

Pinophyta, gymnosperms, softwood trees

3   Ginkgos

Ginkgophyta, one ancient species is left (used as an ornamental tree)

4 Cycads

Cycadiphyta, represented by a single surviving family of tropical plants resembling palms

5  Ferns

Pterophyta,  tree-like

One can see that two classes - broadleaved trees and  conifers entirely cover the “proper” trees (practically speaking).

Native and exotic trees

Known tree species number around 10,000 but it has been estimated that the total amount of all tree species may be manifold of that figure. The Global Forest Resource Assessment 2005 (FRA 2005) was the first attempt to systematically record the number of native forest tree species by country and area at the global level. One problem encountered, however, is the fact that there are some cases where a species has been present in a country for many centuries, and where it is impossible to ascertain whether it is native or if it was introduced.

Huge differences exist between some countries. In Iceland and Malta there exists only 3 native tree species while in Brasil 7880 native species were recognized (FRA 2005). Therefore, it is not surprising how Brasil got its name. The original name "terra de brasil" came from a valuable tree which was first was found there and also became the country’s first exported tree (Westoby 1989).

 

1. In many parts of the world coniferous trees create homogenous forests (due to their properties for construction and papermaking) which makes them very attractive economically.

2 Responses to “3. Tree Genera and Native and Exotic Trees”

  1. Anupam Mitra Says:
    There is a spelling mistake in item 1.1 in the table. It should read Dicotyledons. Lucid yet compreshensive.
  2. brendan Says:
    Corrected. Thank you Anupam.

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