Are we really helpless in the face of poplar catkins?

In Beijing in spring, there are often two kinds of people: one kind walks around the city in the midst of flying catkins, walking with passers-by on the streets until they grow old; the other kind wears a mask, sneezes, and curses while having a runny nose.

Regardless of who you are, you may have three questions in your mind: Why? Why plant so many poplars and willows? Who loves poplars and willows so deeply?

My nose starts to itch｜Zhaoying Youxian/TuChong Creative

Why plant so many poplar trees?

The reason is actually a bit helpless, because in the vast area of ​​North China, apart from poplars (Populus spp.) and willows (Salix spp.), there are really no other street trees with a better cost-effectiveness.

Canada Poplar (Populus x canadensis) | Wikimedia Commons

Willow by the River｜Guoke Nature

So, what are the benefits of choosing these tree species?

Take poplars (Populus spp.) as an example. First, they grow fast . Poplars can grow two meters taller every year on average, and two-year-old seedlings can grow to five meters tall. Among them, fast-growing poplars grow the fastest, and can mature in about five years. If they are planted for timber, they can be cut down in eight to ten years. In other words, there are no common tree species in the north that can grow faster than poplars; but there are some in the south - eucalyptus grows faster than poplars and can mature in three years.

But few people consider using eucalyptus as a roadside tree. One reason is that eucalyptus has a huge water demand , and roadside trees cannot rely on the sky for water. Another reason is that eucalyptus is rich in volatile essential oils. The summer heat scorches the asphalt road, and the heat rises so high that even the air can be distorted, and these "impatient" trees are easily ignited .

Eucalyptus essential oil, which can be used for disinfection and sterilization, may make eucalyptus itself a "bully" - the strong allelopathic effect leads to the scarcity of shrubs and grasses under the eucalyptus forest. Therefore, planting eucalyptus extensively is undoubtedly the opposite of what is needed to create rich green urban gardens.

Allelopathy: The phenomenon that plants release some chemical substances into the environment, affecting the growth of surrounding plants.

Second, poplars are easy to take root and sprout . Poplars rely on cuttings to become one of the most successful copy-paste players, so poplar seedlings are cheap, especially cheap. Entering the third decade of the 21st century, you may not be able to buy a bag of shrimp strips or a small pudding for 50 cents, but you can buy a poplar seedling with a ground diameter of one centimeter at wholesale price.

These two reasons have already killed almost all other tree species available in North China. Fast growth means that it can grow a wide enough crown and a thick enough trunk as soon as possible to provide shade and resist wind; easy rooting (and poplars are deep-rooted species) means that they will not become a "flower pot tree" with shallow roots, and also means that drought, flooding and strong winds are not fatal to them.

Why choose female plants with floating catkins?

Unlike the version circulated online, the poplar trees in Beijing today were not planted in the early years of the founding of the People's Republic of China. At the beginning of the founding of the People's Republic of China, Beijing planted poplars, which have basically retired now and can only be seen occasionally in the suburbs.

Later, in the 1960s and 1970s, plagued by frequent sandstorms, Beijing launched a tree planting campaign, and most of the trees planted at that time were black poplar species such as Populus edulis. Therefore, the black poplars and Populus edulis seen in the urban area now are mostly those born in the 1960s and 1970s, with thick trunks, lush branches and leaves, and a suitable ratio of males to females (even with slightly more males).

The catkin-bearing tree species that bothers everyone, Populus tomentosa , was planted by people born in the 1980s or 1990s, during the "revival" of the afforestation movement in the 1980s and 1990s, after the Three North Shelterbelt Project achieved certain results. At that time, they did not want to plant so many female trees with catkins , and the comrades in charge of selecting the seedlings must have tried their best to select male trees - unfortunately, the result is well known, failure.

The thick black line indicates the area excluding the Three North Shelterbelt Area. The “Three Norths” refer to Northwest my country, North China and Northeast China｜Bingwen Qiu et al. / ISPRS Journal of Photogrammetry and Remote Sensing (2017)

Due to the extremely high seed abortion rate, in order to ensure that the seedlings can continue the excellent traits of the parent (such as not easy to lodging, fast growth, etc.), Populus tomentosa seedlings are generally obtained by cuttings . Therefore, the same batch of seedlings may come from the branches of the same parent.

To describe this process with a not-so-appropriate metaphor is: because Yanzu is so handsome and everyone likes him, we plucked out dozens of his hairs for grafting, and then dozens of second-generation Yanzus who are almost identical to him grew out, and we repeated this process until the city was full of Yanzus; but thirty years later, when these second-generation Yanzus grew up, we found that they were all prone to baldness, but we couldn't immediately abandon them and graft Yifei with thick hair instead, as this would make this daydream seem extremely inhumane.

There is a widely circulated saying about this large number of female plants that the nursery originally replaced the female plants with male ones. I personally disagree with this, as it is as inappropriate as blaming the almanac for failing an exam.

Under appropriate water and fertilizer conditions, the growth of female poplars is better than that of male poplars, while the opposite is true under adverse conditions. Therefore, under the careful cultivation of the nursery, female poplars appear to be stronger and taller than male poplars, and are more capable of serving as street trees and timber trees.

The female plant is good in all other aspects except for the floating fluff. This is the seed of P. purdomii, which has a lot of fluff on it, which helps the seed to spread with the wind | Don't know the plant

Is there no suitable foreign aid?

In addition to the two reasons mentioned above, there is actually a third reason, that is, poplar trees are indeed suitable for Beijing.

Beijing is hot and humid in summer and cold and dry in winter. It is surrounded by mountains on three sides and has winds blowing from all directions. Most plants have a hard time surviving in such an environment, not to mention growing well. In fact, we have introduced a lot of foreign aid , but just because they are not of our kind, they may not be different in heart. But their skills are really not right.

Acer

The Acer genus, with its elegant posture and delicate leaves, is completely unsuitable for the local climate and soil - it does not grow tall and has difficulty surviving the winter. The tree species has almost been cultivated into a shrub, not to mention being used as a street tree with a clear trunk, tree height and branch height requirements.

On the left is the maple in this season. After autumn, the leaves of the maple will begin to change color. The leaves on the same branch usually change color first at the top and later at the base. | Fruit Shell Nature

Torchwood

Another emerging foreign aid, the torch tree, although it has superb tolerance and resistance and can even live on arid slopes, has a flaw that cannot be ignored - it may cause biological invasion.

Torchwood, a bunch of "torches" are its infructescence｜Natural Fruit Shell; Muggwort17 / wikimedia

Plane Tree

The plane trees may be better, growing fast, with beautiful shapes, able to withstand force eight gales, not spreading seeds and sprouting everywhere, and even having a (fake) Mary Sue love story. However, if we pay a little attention to the plane trees across the country, we will find that they will eventually be replaced, for the same reason as the poplar trees: flying catkins.

The seeds of the sycamore are also hairy, and the catkins are much harder than those of poplars . If they come into contact with the skin or are inhaled, it is more likely to cause allergies. Another problem that is not easy to find is that it is not resistant to dry heat. The sycamores planted next to hard roads are prone to scorching leaves in the hot and dry summer. Often, before autumn comes, the sycamores on the roadside are already full of golden leaves and falling leaves.

Roads lined with sycamore trees are common in cities, but when their fruits drop and their catkins fly, their lethality is no less than that of poplar trees｜Guoke Nature

The local elites are embarrassed

Let's turn our attention back to the native tree species. Nowadays, the common tree species in North China can be summed up in ten words: peach, plum, apricot, plum, cherry, willow, elm, locust and Chinese toon.

Rosaceae

The spring flowering shrubs of the Rosaceae family are certainly beautiful and have stories to tell, but they are not tall enough , and as shrubs they do not have long, straight trunks. Their crowns are not wide enough , and they cannot provide sufficient shade for pedestrians and vehicles in the summer, nor can they serve as the main barrier against level 12 gales in the winter.

Most of the various Rosaceae are only responsible for beauty (or deliciousness) | Guokr Natural

Other trees

In addition to poplars and willows, native tree species such as elm, locust, and Chinese toon also have their own shortcomings . Elm has hard wood, beautiful shape, and produces elm seeds in spring and yellow leaves in autumn. It seems to be a good choice, but elm seeds germinate too easily. The seedlings that emerge from the unnoticed gaps in the eaves, corners, roofs, and steps will damage the hard road surface and concrete foundation.

The shortcomings of Sophora japonica and Robinia pseudoacacia are similar: the branches are fragile, and the nectar attracts insects. The wind will break its arms, and the flowers will be full of aphids when they bloom. These characteristics are also shared by another native tree species, Koelreuteria paniculata. Another tree that attracts insects seriously is Ailanthus altissima, which not only stinks, but also attracts stink bugs (a type of stink bug).

As for the toon...does anyone allow it to be more than two meters?

Allow it to enter the plate｜Dang Bulb Sha/Douguo Food

Gymnosperms

Apart from the deciduous angiosperms, are the evergreen and upright gymnosperms any better?

No.

Gymnosperms generally rely on wind pollination. Using wind as a pollinator has risks, such as a relatively low success rate of pollination, so the amount of pollen pollinated by wind is generally very large. When the microspores of pine and cypress begin to shed pollen in spring, the nightmare of pollen allergy patients begins.

This is one of the reasons why we do not recommend planting only male poplars and willows in cities. After all, flying catkins can be solved by spending money and finding ways to solve them (such as the basically useless head-changing project for female trees and the somewhat useful flying catkin inhibitors), but even if the amount of pollen in poplars and willows is not that high, being allergic to it can still be fatal.

Change of sex: Cut off the original crown of the female plant and graft young male branches onto its main branches. This "sex change operation" has high risks and requires frequent maintenance, so it is costly.

Flying catkin inhibitor: Inject the drug into the trunk to inhibit the differentiation of female flowers in female trees, which is a bit like a contraceptive. Inject the willow trees between 45 days before and 50 days after they start to fly catkins each year, which can ensure that the flower buds will turn into leaf buds in the second year and the flying catkins will disappear. But this method also has disadvantages. One injection only works for one year, so you have to get an injection every year. There are too many willows in Beijing, and the manpower and financial resources are limited, so it is impossible to guarantee that all of them can be controlled.

"Pollen Cloud" Spread by Pine Trees｜peakpx

Therefore, based on the current climate conditions in Beijing and a comprehensive comparison of various tree species, it seems that there is currently no more suitable street tree than willows.

Prepare masks and anti-allergy medicine and be patient.

Author: 媗媗