Đề luyện TOEFL phần Đọc số 10

In temperate forests around the globe, trees have long been viewed as solitary competitors, struggling against one another for sunlight, water, and soil nutrients. This traditional perspective, however, overlooks a complex underground network that connects individual trees into a cooperative community. At the heart of this system are mycorrhizae (mycorrhizae: symbiotic associations between plant roots and fungi), which form extensive, thread-like structures known as mycelia. These fungal threads weave through the soil, wrapping around or penetrating the root cells of trees. In exchange for carbohydrates produced by the trees through photosynthesis, the fungi provide their hosts with essential minerals, such as phosphorus and nitrogen, which they absorb from the soil with high efficiency. Rather than merely linking a single tree to a single fungus, these mycelial networks often extend across entire forests, connecting trees of different ages and even different species. Biologist Aisha has conducted groundbreaking research utilizing isotope labeling to track the movement of resources through these shared networks. By exposing the leaves of a single tree to carbon-14, a radioactive isotope, Aisha was able to trace how carbon travels. The results were astonishing: the labeled carbon was not retained solely by the source tree but was instead transported underground to neighboring trees. This transfer of nutrients is particularly critical for young seedlings growing in the deep shade of the forest understory, where limited sunlight prevents them from producing sufficient sugars on their own. Within this interconnected forest ecosystem, certain trees play a disproportionately vital role. Known as "mother trees," these are typically the oldest and largest individuals in a given area. Because of their massive root systems and extensive history of growth, mother trees have the most highly developed connections to the fungal network. They act as central hubs, managing the distribution of water and nutrients across the community. When a mother tree detects that a younger tree is struggling, it can actively divert resources to support the failing plant. Furthermore, as mother trees age and begin to decline, they initiate a final transfer of their stored carbon and defense signals into the network, effectively passing their resources to the next generation of trees. In addition to distributing nutrients, these underground networks serve as an essential communication channel for defense signaling. For decades, biologists observed that trees could warn their neighbors of insect attacks by releasing volatile organic compounds into the atmosphere. [1] However, this aerial communication is highly dependent on wind direction and can easily be diluted in vast forest canopies. [2] They allow trees to send biochemical messages directly from root to root without any atmospheric interference. [3] Research shows that when a tree is attacked, it triggers a warning signal that travels through these fungal networks. [4] Upon receiving these signals, the neighboring trees immediately begin producing defensive chemicals to deter the incoming herbivores. The revelation of these sophisticated subterranean networks has profound implications for modern forestry and conservation practices. Historically, timber management focused on clear-cutting entire sections of forest and replanting them with a single species of tree, operating under the assumption that eliminating competition would maximize growth. However, this practice destroys the delicate mycorrhizal networks, leaving the newly planted seedlings isolated and highly vulnerable to diseases and nutrient deficiencies. Recognizing the vital role of these connections, conservationists now advocate for selective logging, which preserves the older hub trees and maintains the integrity of the underground network. This shift in perspective highlights the importance of viewing forests not as collections of isolated individuals, but as complex, cooperative superorganisms.

Unlike surface currents, which are propelled primarily by winds and the Coriolis effect, deep currents operate on a global conveyor belt system. This system is propelled by differences in water density, which in turn are determined by two main factors: temperature ("thermo") and salinity ("haline"). Cold water is denser than warm water, and highly saline water is denser than fresher water. Together, these characteristics create a complex, slow-moving underwater network that redistributes immense amounts of heat and nutrients around the globe. This network is known as the thermohaline circulation (thermohaline circulation: a global system of deep-ocean currents driven by differences in water temperature and salinity). The process of deep-water formation begins in the cold polar regions, particularly in the North Atlantic Ocean. As warm, salty surface water flows northward from the equator via currents like the Gulf Stream, it gradually loses its heat to the cold arctic atmosphere. This cooling makes the water denser. Furthermore, when sea ice forms in these freezing regions, it extracts freshwater while leaving the dissolved salt behind. This process, known as brine rejection, significantly increases the salinity of the surrounding unfrozen sea water. The combination of freezing temperatures and high salinity makes this water exceptionally heavy. [1] Consequently, it sinks rapidly into the ocean depths, a process known as downwelling. [2] This downward movement pulls in more warm surface water from the south to replace it, maintaining the continuous flow of the conveyor belt. [3] Once it reaches the ocean floor, this cold, dense water begins a slow journey southward, traveling along the bottom of the Atlantic basin toward the Southern Ocean. [4] The deep water travels through the Atlantic, loops around Antarctica, and eventually streams into the Indian and Pacific Oceans. In these warmer, equatorial zones, the cold deep water gradually warms up and mixes with shallower waters. This warming decreases its density, allowing it to rise back to the surface in a process called upwelling. Upwelling brings nutrient-rich deep waters to the sunlit surface, fueling massive blooms of phytoplankton that form the base of marine food webs. Eventually, these now-warm surface waters flow back toward the Atlantic, completing a loop that can take over a thousand years to finish a single cycle. The thermohaline circulation plays a critical role in moderating Earth's climate. By transporting heat from the tropics to northern Europe, it ensures that these high-latitude regions remain significantly warmer than they would otherwise be. However, this system is remarkably sensitive to changes in freshwater inputs. If a sudden influx of freshwater—such as from melting glaciers or increased rainfall—dilutes the salinity of the North Atlantic, a layer of lighter, fresher water forms at the surface. This layer acts as a barrier, preventing the underlying water from cooling and sinking. Scientists refer to such a barrier as a halocline (halocline: a sharp vertical gradient in ocean salinity where salinity changes rapidly with depth). Geological records provide compelling evidence of what happens when this circulation is disrupted. During the Younger Dryas period, approximately 12,800 years ago, a massive influx of freshwater from melting North American ice sheets entered the North Atlantic. This sudden dilution of salinity effectively stalled the downwelling process, shutting down the ocean conveyor belt. Consequently, temperatures in the Northern Hemisphere plummeted, plunging parts of Europe back into near-glacial conditions within a few decades. This historic event serves as a stark warning to modern oceanographers. Today, accelerated melting of the Greenland ice sheet due to global warming threatens to introduce vast quantities of freshwater into the subpolar seas, raising concerns that the Atlantic meridional overturning circulation could weaken or collapse, with unpredictable consequences for global weather patterns.