[1] A tree is a perennial plant that consists of a long stem, trunk, branches and in most species, leaves. They have evolved their structure to compete for sunlight with other plants. Trees usually live for a long time, up to thousands of years old and they have existed on the planet for 370 million years. A tree has woody tissue and is surrounded by bark that protects the plant. The root of a tree anchors it in place and the branches carry leaves that capture light and process it into sugar by photosynthesis.
[2] The function of trees in our environment is invaluable. They release oxygen into our atmosphere and remove carbon dioxide, storing carbon in their own tissues. Trees prevent soil erosion by soaking up water in areas with high rainfall. In tropical rainforests and various other regions of the world, trees provide a habitat for a vast number of animal and plant species. Other functions of trees are in the provision of wood for fuel, cooking, heating, construction, paper production, and fruit.
[3] The leaves of trees are formed for photosynthesis. The broad sizes and shapes of deciduous trees attract the light to be converted to energy. Coniferous trees sometimes have needles which are adapted to environments with less water, such as frozen regions with a higher latitude and altitude. The thin shape of the needles and position of evergreen branches allow the leaves to shed snow. Broad-leaved trees in temperate zones of the world shed their leaves as days get shorter in the autumn and winter seasons. This is because the leaves are no longer making new chlorophyll through photosynthesis due to less light. The red and yellow pigments of the leaves’ blades become visible, causing the bright orange, red and yellow-colored autumn leaves. The synthesis of auxin – a plant hormone – also stops. Once the production of auxin ceases, the cells at the junction of the twig and the petiole of the leaf weakens and the leaf breaks off and falls to the ground. In regions of the world with more sun exposure, trees may not shed their leaves.
[4] There are several other reasons that trees shed leaves and twigs. During a drought season, trees often shed about ten percent of their leaves, thus losing less water. Another reason for leaf-fall is when vigorous trees shed excessive leaves to adjust to the summer heat. Moreover, leaves near the base of the tree are more likely to drop off due to lack of sunlight. Another cause is insect-tunneling of leaf stalks, leaving short, fragile petioles which leads to more leaf-fall. Disease, such as Dutch elm disease or root rot, or injuries to the tree, is another source of leaf-fall. Fungus, such as leaf-spot disease, also causes increased shedding of leaves. Fungi such as anthracene and apple scab defoliate crab apple trees, sycamore, ash, white oak and maple.
[5] Squirrel activity or twig- girdling insects can also cause clusters of fallen leaves that are attached to short twigs. There are specific tree species, such as poplars, that shed twigs during moisture stress period. The end of the twig will be like a smooth curve. The metallic wood boring beetle prefers red oak and other oak trees. They lay eggs onto twigs. Once hatched, the larvae bore into twigs in a spiral fashion until there are clusters of dead leaves. The longhorn beetle is gray-brown in color. The adult beetles appear at the end of the summer. The female beetle chews the twig, girdling it and laying eggs on it. The twig then withers and dies. The new larvae thrive in the dead twig. The behaviour of these two insects could be another explanation of excessive twig fall.
