Soru

In the late 1990s, the discovery that mycorrhizal fungal networks physically link the root systems of disparate tree species prompted the hypothesis that mature 'mother trees' altruistically route surplus carbon to shaded seedlings via common mycelial networks (CMNs). Proponents argued that this inter-plant transfer enhances forest resilience by subsidizing understory regeneration. However, recent critical re-evaluations of isotopic labeling experiments have challenged this cooperative paradigm. While isotopic tracers ($^{13}\text{C}$) undeniably move between neighboring trees through shared fungal hyphae, the net quantity transferred represents less than two percent of a sapling's total carbon budget—an amount biologically insufficient to alter survival rates or alleviate severe light limitation. Furthermore, mycorrhizal fungi are not passive conduits; they require substantial host carbon to fuel their own cellular respiration and sporocarp development. When fungal networks are traced across differing soil nitrogen regimes, the rate of labeled carbon entering recipient roots correlates with fungal biomass growth rather than recipient seedling metabolic stress. This suggests that what appeared to be inter-tree resource sharing is primarily fungal acquisition and retention: the fungal partner absorbs carbon from high-surplus autotrophs to fuel its own mycelial expansion, with recipient seedlings taking up merely whatever residual carbon the expanding fungus offloads within rhizosphere exchange zones.

Based on the passage, which of the following assertions regarding common mycelial networks (CMNs) are most strongly supported by the text? (Select all that apply.)