Question

Passage:
In terrestrial forest ecosystems, the paradigm of plant interactions has historically emphasized competitive exclusion, wherein neighboring tree species compete strictly for finite resources such as light, moisture, and soil nutrients. However, field studies initiated in the late twentieth century by forest ecologist Suzanne Simard fundamentally altered this perspective by demonstrating bidirectional carbon transfer between distinct tree taxa mediated by ectomycorrhizal fungal networks. Utilizing dual isotopic labeling with carbon-13 ($^{13}\text{C}$) and carbon-14 ($^{14}\text{C}$) isotopes administered via sealed exposure bags to mature canopies, Simard tracked the movement of photoassimilates between Douglas fir (*Pseudotsuga menziesii*) and paper birch (*Betula papyrifera*).

The experimental findings revealed that while subterranean carbon exchange occurred continuously in both directions throughout the growing season, the net transfer was heavily dictated by the photosynthetic status of the respective species. During midsummer, when birch saplings possessed dense, active canopies and Douglas firs experienced partial shading in the forest understory, net carbon flowed substantially from birch to fir. Conversely, in early spring and late autumn, when deciduous birches had shed their foliage and evergreen firs remained photosynthetically active in ambient sunlight, the directional gradient reversed, with net carbon shuttling from fir to birch.

Crucially, Simard demonstrated that this resource sharing was not an act of direct plant altruism, but rather a byproduct of the fungal symbionts optimizing their own energetic yield. Ectomycorrhizal fungi—primarily basidiomycetes that colonize tree root tips—form extensive subterranean hyphal networks that bridge the root systems of multiple individual trees across taxonomic families. The fungi draw organic carbon from host trees to sustain their hyphal proliferation and metabolic processes, while providing the trees with bioavailable nitrogen, phosphorus, and water scavenged from soil micropores. Because the fungal mycelium establishes a continuous source-sink hydrostatic and carbohydrate gradient across the network, carbon compounds naturally migrate along the mycelial conduits toward host individuals with lower internal carbohydrate concentrations.

Subsequent investigations have further revealed that mycorrhizal pathways facilitate more than just nutritional support; they also serve as conduits for interplant biochemical signaling. When mature Douglas firs are subjected to simulated herbivore defoliation by western spruce budworms, defensive enzymes and volatile organic stress markers are rapidly transmitted via the hyphal network to neighboring, undamaged birch trees within forty-eight hours, stimulating the preemptive synthesis of tannin-based defensive phenolics.

According to the passage, the net direction of subterranean carbon movement between Douglas fir and paper birch trees is primarily determined by which of the following?