Based on the text, which statement most logically describes the dynamic between B. nana and its fungal partners under t…
In boreal ecosystems, mycorrhizal fungi typically supply host plants with soil nitrogen in exchange for photosynthetic carbon. Ecologist Elena Davis and colleagues examined how climate warming alters this trade in Betula nana (dwarf birch). Warming accelerated microbial decomposition, dramatically increasing the concentration of dissolved inorganic nitrogen directly accessible to plant roots. Davis observed that B. nana specimens in artificially warmed plots increased their total nitrogen uptake by 28%, yet the proportion of nitrogen supplied by fungal partners declined by 43%. Over the same period, root exudation of carbohydrates to fungi dropped sharply, even though the trees' overall rate of photosynthetic carbon production was unaffected.