Read the texts given below and summarize each of them using the strategies discussed in Unit 5.
Do not copy sentences from the original texts.
Text 1: Tree Rings
In trees that grow in a tropical climate, the cambium cells grow at a steady unchanging rate. So the rings that
indicate the age of the trees cannot be clearly distinguished. Therefore, for trees to give us information about
past climate through their rings, they need to grow in difficult conditions which include significant variations in
temperature and drought. Trees that grow in temperate climates show a difference in growth between the
seasons. In spring and summer, cambium cells become progressively larger whereas in autumn and winter
when they progressively become smaller. This change in cell size can be seen as distinguishable rings.
(103 Words)
Text 2: Divergence Problem
One of the problems that dendroclimatologists are trying to solve is known as the “divergence problem”. As
mentioned above, the width of tree rings represents the growth of trees. However, since the 1950s, it has been
found in the forests in the far northern latitudes, e.g. in Canada, Northern Scandinavia and Russia, that the
width of tree rings in these areas do not match the increasing temperature. This could be due to the fact that
while the temperatures in these regions have risen significantly, the amount of water has not proportionately
increased. So the trees grow more slowly. Some scientists think that the existence of the divergence problem
means the tree ring data in this area are not suitable for reconstructing past climate trends. However, trees in
other regions, where there is sufficient water for growth, are not affected by this divergence and thus may be
used in reconstructing climate patterns.
(152 Words)
Now combine the summaries of the two texts. Remember to write a topic sentence that connects the two texts. Your summary should be in the range of 100-120 words. Do not go beyond this range. You should title your summary:
Factors Affecting Tree Rings
Text 1: Principles of Dendrochronology
There are three basic principles which make up the foundation of dendrochronology – the uniformitarian principle, the limiting factors principle and the cross-dating principle. The uniformitarian principle means that the basic processes and limiting factors of climate are the same over time. It is borrowed from the same principle in geology which was popularised in the book Principles of Geology (1830). This idea can be summarized by the phrase “the present is the key to the past.” This of course does not mean that the today’s climate is the same as in the past. The limiting factors principle implies that a tree’s rate of growth is constrained by the resource which is most limited. So plants in a particular area adapt to the climatic conditions in which they grow. Two factors of climate are important for plants to grow, namely sunlight and moisture. The amount of sunlight as well as its duration and quality are essential. At high altitudes, greater ultraviolet light can slow down the growth of plants. The period of time that the sun shines in a day can affect the reproduction of plants by affecting their flowering. The cross-dating principle applies to matching ring patterns between trees. The climate of a certain region will affect the trees in that region in a similar manner. Therefore, similar patterns of ring widths will develop in the trees.
(228 Words)
Text 2: Applying Dendrochronology in Oman
An interesting application of dendrochronology is to try to find the age of trees growing in Oman. In fact, a German university botanical expedition to Oman (Preissel, 2007) visited Wadi Hinna in the Dhofar mountains where they found about 200 tree species. Most of these trees do not grow more than 3 metres high due to the dry conditions, except for the most interesting one, the Baobab (Adansonia digitata). There is a group of more than a hundred of these trees growing in the region. There are 9 species of this tree and the Dhofari type also grows in tropical Africa. Baobabs can grow up to a height of 25 metres with a trunk diameter of 8 metres. The trees in Oman are between 10-15 metres with a trunk width of 2 metres. They are therefore the largest wild trees in Oman.
It is believed baobabs can reach an age of 1,000 years. However, according to their shape, height and trunk diameter, the baobab trees in Wadi Hinna are probably about 70 years old. The German team could not take good samples from the trees because the increment borer they used was too small for taking a sample of the core. Also the cores were often rotten in the smaller trees. Finally, the tree-rings of the samples they managed to get showed irregular patterns. It is therefore suggested that genetic analysis or carbon-14 dating be used to more accurately determine the age of these baobab trees.
(247 Words)
Now combine the summaries of the two texts. Remember to write a topic sentence that connects the two texts. Your summary should be in the range of 110-130 words. Do not go beyond this range. You should title your synthesized summary:
Applying the Principles of Dendrochronology in Oman