Ujian Latihan TOEFL Bacaan 7

For hundreds of millions of years, the terrestrial landscape was barren. The transition of plants from aquatic environments to land, roughly 450 million years ago, was a pivotal event in Earth’s history. However, early land plants lacked the complex root systems necessary to extract water and nutrients from the harsh soil of ancient continents. To overcome this, plants formed a mutualistic relationship with soil fungi, establishing mycorrhizae (mycorrhizae: symbiotic associations between plant roots and fungi). This alliance was so successful that today, over 90 percent of terrestrial plant species participate in these associations, which have fundamentally shaped global ecosystems. In this symbiotic exchange, each partner provides resources that the other cannot easily obtain. Plants, through photosynthesis, capture solar energy and convert it into carbon-rich sugars, transporting a significant portion down to their roots to feed their fungal partners. In return, the fungi extend their hyphae (hyphae: thread-like structures that make up the body of a fungus) deep into the soil. Because these microscopic filaments are thin and extensive, they have a surface-area-to-volume ratio far exceeding that of plant roots. This enables the fungi to absorb vital mineral nutrients, particularly phosphorus and nitrogen, along with water from tiny soil pores that roots cannot access, transferring them directly to the plant. The ecological significance of these networks extends far beyond simple one-on-one partnerships. In a forest, fungal networks weave a continuous web that connects multiple plants. [1] Traditionally, scientists viewed the forest as an arena of fierce competition where each tree fought independently for light and soil resources. [2] These underground links allow individual plants to share resources such as water and carbon directly with one another. [3] For instance, studies led by ecologists Aisha and Tomas have demonstrated that mature, canopy-dominant trees can transfer excess carbon to younger, shaded seedlings. [4] This cooperative redistribution of energy increases the overall resilience of the forest community, helping young trees survive in environments where they would otherwise perish from lack of sunlight. Beyond nutrient transfer, mycorrhizal networks serve as communication channels. When a plant is attacked by herbivorous insects, it synthesizes defensive chemicals to ward off the pests. Simultaneously, the distressed plant releases chemical warning signals into the fungal network. These signals travel underground to alert neighboring, unattacked plants. Upon receiving the signal, the neighbors begin producing their own defensive toxins, a phenomenon known as preemptive defense priming. Consequently, when the insects attempt to migrate to adjacent plants, they find them already resistant. This collective warning system dramatically reduces the impact of pest outbreaks across the local ecosystem. The existence of such cooperative networks raises an evolutionary question: why would fungi expend energy to facilitate resource sharing and warning signals between plants? The answer lies in the shared destiny of the symbiotic partners. Fungi depend on host plants for the carbon compounds required for survival. If host trees succumb to environmental stress, pests, or nutrient depletion, the fungal network also perishes. By distributing nutrients from healthy trees to struggling seedlings, and by transmitting warnings to prevent pest devastation, the fungi protect their own habitat. This represents a complex evolutionary strategy where individual fungal survival is tied to the collective health of the forest. Recognizing mycorrhizal networks has revolutionized modern forestry. Historically, timber management involved clear-cutting entire sections of forest and replanting them with single-species monocultures. However, scientists observed that trees planted in these disrupted soils experience high mortality rates. Without the established underground networks of mature forests, young saplings are isolated, unable to receive nutrient subsidies or warning signals. Modern conservation efforts now emphasize the preservation of "mother trees"—old, deeply connected individuals that act as central hubs in the network—ensuring that the subterranean infrastructure remains intact to support future generations.

Loess (loess: a fine-grained, yellowish-brown deposit of windblown dust) covers approximately ten percent of the Earth's land surface. It is characterized by its unstratified, porous nature and its ability to maintain nearly vertical cliffs when eroded. Found in vast tracts across the mid-latitudes of North America, Europe, and Asia, loess deposits are particularly extensive in northern China, where the Loess Plateau spans over 640,000 square kilometers. For decades, geologists have studied these thick sedimentary blankets not only for their agricultural value but also because they contain a remarkably continuous record of continental climate change spanning the last 2.6 million years. The formation of these deposits is closely tied to the shifting cycles of Earth's past climates, reflecting the powerful interplay between wind, ice, and water. The origin of loess is intimately linked to the growth and retreat of continental glaciers. During glacial periods, massive ice sheets acted as colossal grinding machines, crushing the underlying bedrock into an extremely fine powder known as rock flour. As glaciers melted, enormous volumes of meltwater carried this fine-grained silt downstream, depositing it over vast, vegetation-free outwash plains and river valleys. During the dry, windy winter seasons of the glacial epochs, powerful winds swept across these barren floodplains, lifting the dust high into the atmosphere and transporting it over hundreds of kilometers. When the winds eventually subsided, the dust settled over the landscape, gradually accumulating into thick blankets of silt. Because this process was repeated over thousands of years, extensive layers of loess were deposited far beyond the margins of the ice sheets. While the deposition of loess occurred during cold, arid glacial stages, the intervening warm and wet phases—known as interglacial periods—interrupted this accumulation. During these warmer intervals, dust deposition slowed dramatically, allowing vegetation to flourish on the newly formed land surface. Over thousands of years, the active biological and chemical weathering of the silt led to the formation of paleosols (paleosols: ancient buried soil layers that represent periods of landscape stability). These buried soils are rich in organic matter and clay minerals, presenting a stark contrast to the unaltered, pale-yellow loess layers that lie above and below them. Consequently, a typical loess sequence consists of alternating bands of light-colored, windblown silt and dark-colored, highly weathered ancient soils, with each pair representing a complete glacial-interglacial cycle. Geologists analyze these alternating sequences to reconstruct paleoclimates over millions of years. [1] In addition to analyzing grain size, researchers measure the magnetic susceptibility within the individual sediment layers. [2] This magnetic signal is particularly strong in the paleosol layers because weathering processes during wet climates promote the formation of tiny iron-oxide minerals. [3] The orientation of these minerals can then be used to determine the age of the layer by comparing it to known reversals of Earth's magnetic poles. [4] Consequently, loess deposits serve as one of the most reliable terrestrial archives of ancient climate cycles, allowing scientists to correlate continental climate shifts with deep-sea sediment records. Beyond their scientific value, loess deposits are of immense economic importance. Because loess is highly porous and rich in minerals like calcium carbonate, potassium, and phosphorus, soils derived from it are exceptionally fertile and easy to cultivate. Some of the world’s most productive agricultural regions, including the Breadbasket of North America and the agricultural heartlands of Europe and China, owe their productivity to loess. However, the very characteristics that make loess soils fertile also make them highly vulnerable. Lacking cohesive clay to bind the silt particles together, dry loess is easily swept away by wind. Furthermore, when natural vegetation is cleared for farming, heavy rains can rapidly carve deep gullies into the landscape, leading to severe soil erosion and desertification.