Forest of Pain: Encounter a large outbreak of mountain pine beetles
Mountain pine beetles have been present in the forests of western North America for millions of years. However, the most recent outbreak spanned the entire western forest, more than 10 times more violent than any previous outbreak. And they've crossed the mountains and are heading east... This has attracted the attention of scientists.
Contest: Battle of the Pines vs. Beetles
A black beetle flew over the mountains of western North America and landed on the trunk of a mature American black pine. It strolls leisurely on the bark, sniffs the fragrance of pine trees with its tentacles from time to time, and nibbles twice from time to time... Once it finds the tree of its choice, it burrows straight into the bark horizontally and builds its own nest in the phloem. It will mate there, lay eggs, and the needle-like vascular tissue under the bark will provide it with nutrients. While sucking up nutrients from the trunk, the beetle also blocks the secretion channels of sticky pine resin. You know, pine resin is pine trees against these invaders - the first "weapon" to borrow "insects". Pine resin pours out from the side of the blocked passage, filling holes dug by beetles and filling them until they overflow the trunk or drive away the invaders. Once the pine resin spills out of the trunk, it will dry and harden, condensing into a "resin tube", which is also a clear signal that the pine tree is infested by mountain pine beetles.
When a female mountain pine beetle chooses her favorite pine tree, the fierce contest between the pine tree and the beetle begins. Pine trees will constantly secrete pine resin in an attempt to devour beetles, preferably so that pine resin covers the entire trunk; The beetles will also constantly go in and out of the trunk, using their bodies to clean up the pine resin in the nest. In this one-on-one skirmish, Pine is definitely the winner. However, the mountain pine beetle will not just wait, in this duel, the beetle will inevitably swallow some pine resin, which will make the beetle's body produce a defensive substance, and quickly merge into a very attractive pheromone, which can spread like a feather to hundreds of yards away with the wind, sending signals to other mountain pine beetles.
For the female mountain pine beetles that received the signal, such a signal meant that a tree that was very suitable for them to live and lay eggs was under attack from their companions, and they needed to arrive there in time to reinforce and help the first companions win the war. For male mountain pine beetles that receive the signal, this signal means that they will have a chance to mate, and they also need to rush over in time.
The fierce battle caused by the digging of the cave will only have two outcomes: first, the female mountain pine beetle will recruit enough partners to kill the pine tree; Second, the pine tree counterattack successfully killed the beetle. When a large group of female beetles attack pine trees, they reduce the pressure of pine resin secretion to zero, causing the pine resin to stop flowing. In this way, more mountain pine beetles entered the pine trees, and the pine trees were powerless to resist and would undoubtedly die. To ensure success, female beetles also cause pine phloems to be infected with fungi such as "blue fungus". Female pinus beetles take the spores of these symbiotic fungi in a shallow pit-like sac in their mouth and coordinate their whole body to transport them. Blue-shaped bacteria spread in the beetle's nest and rage around, blocking the vascular tissue of the phloem and even deeper and the xylem, affecting the entire nutrient and water transport from the root to the canopy. For pine trees, the attack of mountain pine beetles will certainly cause some damage to the phloem, but the blocking of the phloem and xylem by the blue fungus is a more deadly "strangulation".
There is a reciprocal symbiotic relationship between the mountain pine beetle and the blue fungus. They depend on each other and help each other. Blue fungus not only makes a "great contribution" to killing trees, but also is the key to the beetle larvae's access to food. Without these fungal resources, it is difficult for the offspring of beetles to obtain food and grow. Because for most living things, trees are a relatively barren resource. They lack the nutrients needed to synthesize proteins and contain large amounts of compounds of carbon that are almost indigestible. And fungi like blue fungus happen to break up this lignocellulose and turn it into a useful nutrient. Similarly, for blue-shaped bacteria, they also need pine beetles to spread their spores. Indeed, blue-shaped bacteria in nature generally only exist in two places: one is to stay in the mouth or body of mountain pine beetles or other bark beetles; Second, they have struggled under the bark to help the beetle win.
Mountain pine beetle outbreaks and their effects
Mountain pine beetles are indigenous to the coniferous forests of western North America, and usually, mountain pine beetles are not very destructive to the forest, and they only rely on some of the most vulnerable pine trees to survive. These trees are often not struck by lightning,
It is infected with extremely destructive root rot bacteria, or it is seriously entangled by dwarf parasitism. But every 40 to 60 years, especially when pine resin secretion declines due to persistent droughts, mountain pine beetles experience a concentrated outbreak for a whole year.
Historically, there has been little particular concern about outbreaks of mountain pine beetles, as this phenomenon has existed for millions of years in western forests, especially in specific river valleys or tributaries. However, the most recent outbreak spanned the entire western forest, causing 50 times more tree deaths than wildfires in a single year! Both geographically and in terms of tree mortality, this outbreak is more than 10 times more intense than any previous outbreak. In addition, the altitude and latitude of the pine trees affected by this disaster have also increased. This makes the forests of the rocky mountains' precious high-altitude ski resorts and the Canadian coniferous forests, which are only a few hundred kilometers from the mountain pine beetle outbreak, no longer safe.
In fact, no one can say what the consequences of the outbreak of mountain pine beetles have caused to the entire mountain ecosystem and regional economy. From an ecological point of view, it is not news that trees have died from the invasion of mountain pine beetles, and it cannot be absolutely said that this is a bad thing. But trees, when they are alive, convert carbon from the atmosphere into wood or deep soil, and now, the number of tree deaths is unprecedented, which in the long run has the potential to transform the entire outbreak area from a carbon sink (the forest's ability to absorb and store carbon dioxide) effect area to a carbon-rich area, as microbes continue to decompose dead trees and other organic matter.
On the other hand, due to the large number of dead trees, it will also affect the water cycle problem in the region. When the old taiga dies, solar radiation will have a great impact on the climate under the forest and on the melting of snow in winter and spring.
The warming of the climate has accelerated the reproduction of beetles
Although the outbreak of the mountain pine beetle has a long history and the ecological impact is inevitable, the intensity of this outbreak is unprecedented, so scientists are trying to find a reason why "this outbreak is stronger than ever". There are always several aspects of complex problems that come into play, but scientists first focus on the impact of rapidly changing climates on insect life cycles. The scientists noted that under normal circumstances, adult beetles generally begin to appear in mid-to-late July and reach their highest point in numbers in the second and third weeks of August. Subsequently, the female beetle lays her first egg and soon hatches into larvae. However, the growth of larvae begins in late summer, and due to the drop in temperature, its entire growth period is delayed in autumn.
Pinus beetles usually spend their fourth or fifth age in winter (this represents a different stage of larval growth). When spring comes and the temperature in the cave returns to 4.4 degrees Celsius, beetle growth resumes. In the summer, the larvae begin to gradually become pupae, and eventually deform into soft adults in July and August. Within a few days, they accumulate large amounts of melanin and harden, preparing for another new attack, aggregation, and spawning. In the past, the entire life cycle would have taken a year.
However, due to climate change, the temperature rises rapidly in the spring, which is very suitable for the growth of insects. The researchers found that in Colorado's 3,000-meter-high Frant Mountains, the number of spring days suitable for beetle growth has increased by nearly 58 percent in the last 40 years. That is, as temperatures rise, beetles appear four to six weeks earlier than usual, extending their flight and breeding time by as much as 50 to 110 days.
The result is a fundamental change in the life cycle of the beetle. Female beetles appear in early June and lay their eggs a few days later. The eggs then hatch into larvae, which then grow into pupae, and the adult worms reappear in August. This batch of adults lays a new batch of eggs. In June of the following year, the eggs will grow into new adult individuals. In this way, in a warm year, the beetle population is able to reproduce twice. Don't think that the number of beetles will only multiply by 2, the truth is that their number will grow exponentially.
Rising temperatures in the spring will also lead to an expansion of the beetle's distribution range. In the 1970s and 1980s , beetles had a maximum range of more than 2,700 meters above sea level. But now, mountain pine beetles are able to attack forests above 3300 meters above sea level. Moreover, the distribution of beetles has expanded not only at altitude, but also by hundreds of kilometers to the north.
Persistent drought reduces the tree's defensiveness
For this unprecedented outbreak of mountain pine beetles, the warming climate can certainly explain part of it, but this is by no means the whole reason. Without suitable habitats and food, no organism can thrive, at least not so rapidly. Therefore, in addition to the increase in temperature, mountain pine beetles must find enough vulnerable trees to target before entering the outbreak period. However, from being vulnerable to being resistant enough, it all depends on the amount of pine resin secreted. The level of pine resin secretion is unstable, and it can undergo a huge shift even throughout the year or throughout the life cycle of a tree.
Much of western North America has suffered a decade-long drought in recent years. Of course, this is also largely brought about by climate change. Climate change not only affects air temperature, but also changes rainfall patterns and rainfall, and even the major Pacific ocean currents are involved. In any case, drought will reduce the tree's defense ability, and the consequence is that the tree will have difficulty maintaining a sufficient amount of pine resin and the ability to secrete pine resin.
As a result, arid pine forests are highly vulnerable, with a dozen or two dozen female beetles reducing their secretion capacity to zero. However, even if the same tree experiences a rainy season, even if it goes through more than a week of rain, it may regain high levels of pine resin secretion pressure and withstand hundreds of beetle attacks. Historically, most outbreaks of mountain pine beetles did begin with droughts, and the end of outbreaks was often due to the return of rainfall to normal levels. Of course, if none of the surviving trees are enough to sustain the beetle population, the outbreak period will also end.
Pine trees have different resistance abilities on their own
During outbreaks, the role played by pine trees themselves is often overlooked. This is also because trees always give a cookie-cutter and static feeling. Only a few forest entomologists believe that after forming during the outbreak period, the resistance of the pine trees themselves is strong enough to control the further ravages of the beetles. Some forest entomologists insist that the variation of individual pine trees can have an important impact on the number of beetles. Indeed, even if they receive the same amount of precipitation, take root on the same land, and grow at the same temperature, the resistance of different pine trees is still different. This is a truth that the immune response is different from that of human close relatives.
Much of the difference in resilience is due to genetics. Pine trees are a very diverse species of genetic differentiation. Even among the trees of the same species that grow next to each other, their growth rate, growth form, hardiness, and amount of cones are different. Physiologically, trees with strong photosynthetic abilities and lower respiration rates will have more energy to grow quickly, or invest more energy to resist foreign invasions, or ensure that both are taken into account.
Since pine resin is the primary weapon of pine trees against mountain pine beetles, the ability to secrete and possess resin channels that store pine resin can measure pine trees' ability to resist mountain pine beetles, so the number and size of their resin channels will likely provide a good way to measure tree resistance.
After examining different environmental factors and different species of pine trees, the researchers found that pine trees with fewer resin channels also had relatively low resistance to mountain pine beetles. However, this study has not yet reached a clear underground conclusion. So, if the tree that has been attacked has enough resin channels, will it fight back? The researchers directly analyzed individual trees that had been attacked by mountain pine beetles in the American Black Pine and Soft Pine, comparing those that survived and the dead ones. The results showed that the number of resin channels in the two surviving pine individuals was significantly higher than in the dead individuals. This also suggests that the density of the resin channel is fairly accurate indication of which trees will survive and which will die.
Recently, scientists compared young and old soft pine, American black pine and Jack pine, and the results showed that the pine resin resistance mechanism decays rapidly as trees age. Rangers sometimes think that mountain pine beetles only invade large trees, but in fact, it is not the size of the tree, but the age of the tree that plays a key role. The size of a tree is only a tiny factor in its age. This conclusion also helps explain "why outbreaks of mountain pine beetles tend to avoid younger tree individuals in the forest?" ”
The human factor that cannot be ignored
The geographic expansion of the mountain pine beetle outbreak may also come from human changes to the structure of the forest. People selectively plant economic trees, change the timing of natural disasters such as mountain fires, and introduce livestock for livestock breeding, which will affect the regeneration of forests, thus greatly changing the age structure of trees on the earth. In low altitude areas, the climate is more suitable for human production and life, and the forests here are more affected by human life, and its significant feature is that there are always more young trees in the forests here than in high altitudes. Therefore, scientists believe that deforestation at high altitudes is a legacy of our human life and economic activities, not just the result of climate change. You know, many of these trees have been around for more than a century.
The mountain pine beetle is a native insect in North America. For millions of years, they have existed like alpine lake sediments and wood fossils, and have often erupted in coniferous forests in the west. Based on this history, combined with recent circumstances, scientists believe that there is no reason to expect, or hope that such outbreaks will stop. Climate change has a clear impact on the life cycle of mountain pine beetles, and the trees themselves are resilient. At the same time, scientists have also revealed why beetles have achieved such success in forests at high altitudes. These new findings will hopefully inspire tree species selection and forest management techniques around tree resilience mechanisms.