Question

For decades, forest management policies in the American West focused on the total suppression of wildfires, viewing fire as a purely destructive force. However, this policy had unintended ecological consequences, particularly for the giant sequoia (*Sequoiadendron giganteum*) forests of the Sierra Nevada. Giant sequoias possess thick, fire-resistant bark, which protects mature trees from moderate flames. More importantly, these trees depend on low-intensity fires for successful reproduction. The heat from passing fires dries out the sequoia's tightly closed cones, causing them to open and release seeds onto the forest floor. Additionally, the fire consumes the thick layer of organic debris—known as duff—that carpets the ground, exposing bare, nutrient-rich soil and creating a layer of ash. This process also eliminates shade-tolerant competitors, such as white firs, which would otherwise crowd out young sequoias. By suppressing all fires, conservationists inadvertently halted the regeneration of sequoia groves, as seeds remained trapped in closed cones and those that did fall could not penetrate the unburned forest floor. Only when controlled burns were reintroduced did new generations of sequoias begin to sprout, demonstrating that fire is an essential catalyst for their life cycle.

According to the passage, the opening of giant sequoia cones and the subsequent release of their seeds are directly caused by: