First, the leaves of birch and oak turn yellow in autumn, which should be late autumn (about1end of June,1/beginning of June). The most prominent representative of late autumn is that the leaves turn yellow (red) and then gradually fall. In late autumn, in order to reduce water evaporation, trees change their cytoplasm from liquid to gel to enhance their resistance.

Second, expand knowledge.

1, Birch Suk. Deciduous tree with a trunk up to 25 meters and a thickness of 50 centimeters. There are white smooth paper bark, which can be peeled off layer by layer, and you can write on the thin bark with a pencil. The leaves of Betula platyphylla are simple and alternate, with serrated edges. The flowers are monoecious, and the male inflorescence is soft and drooping. In spring, the leaves on the trees bloom before they grow. Birch fruit is flat and small, called samara, which is easily blown up by the wind and floated far away.

Birch likes sunshine and has strong vitality. After the forest is destroyed by fire, it is often the longest, often forming a large birch forest, which is one of the main tree species that form natural forests. Wood can be used in general buildings, utensils and tools, and birch oil can be extracted from bark. Birch trees are planted alone and clustered in gardens, parks, lawns, Chi Pan, lakes or roadsides, all of which are quite beautiful. In the north of China, on the grassland, in the forest, on the roadside of Shan Ye, it is easy to find dense birch forests. Birch is the national tree of Russia and a symbol of the national spirit of this country.

2. Oaks are deciduous or evergreen trees, and a few are shrubs. Up to 25 meters. The bark is dark grayish brown, slightly smooth, some curved, some inclined to the downwind, rough, and the branchlets are brown and hairless. Generally, tree trunks are black.

The leaf margin is serrated, and the whole leaves are few. The leaves will turn reddish brown before they fall in autumn, which is very beautiful from a distance. The catkin of the male flower droops, and the female flower is solitary in the involucre.

The nut is solitary, the inner wall of the pericarp is hairless, and the undeveloped ovule is located outside the base of the seed.

In autumn, the most obvious change of natural landscape is in trees. In the city, a lot of fallen leaves will be cleaned up, and many tourists will flock to the mountains to see the red leaves. Autumn, the third season of the year, is a transitional season from summer to winter. The lunar calendar is from beginning of autumn in July to beginning of winter in September, the solar calendar is from 9: 00 to 1 1, and the astronomical vernal equinox starts from the winter solstice.

The standard for meteorologists to study phenology is: after the intense summer heat, the average temperature for five days is stable below 22 degrees, which means that it is autumn, and it is over when it is below 10 degrees.