Aboveground and belowground carbon and nitrogen utilization in three drought-exposed subtropical tree species after...

Drought frequency and intensity increasingly threaten tree survival and growth by altering trees carbon (C), nitrogen (N), and water utilization. Yet, the interplay of aboveground and belowground processes during drought recovery remains unclear. We conducted a microcosm experiment with pine (Pinus massoniana), fir (Cunninghamia lanceolata), and oak (Quercus acutissima) saplings, exposing them to three levels of drought intensity (i.e. well-watered, moderate drought, and severe drought) for three months, followed by 30 days of rewetting. Using 13C and 15N isotopes, we tracked C and N allocation. Our results showed that drought-stressed saplings rapidly increased net photosynthesis and predawn leaf water potential after rewetting, with the exception of severely drought-exposed oak. Root N uptake (represented by 15N recovery) also recovered quickly, even surpassing well-watered levels in moderately drought-exposed pine and fir. However, severe drought-stressed pine and fir exhibited significantly lower coarse root non-structural carbohydrates (NSC), while oak maintained higher root and stem NSC. The two coniferous species, previously severely drought-stressed, prioritized C and N allocation to aboveground and had reduced xylem hydraulic conductance after rewetting. The recovery rate of oak was slower, with limited N translocation to aboveground tissues due to leaf abscission. In conclusion, rapid root recovery coupled with more recently photosynthetic C retained in aboveground organ led to root NSC consumption in drought-stressed pine and fir. Conversely, oak's recovery was slower, largely driven by its phenology. This study underscores that aboveground and belowground C and N allocation patterns are closely linked during drought recovery, determined by species-specific traits and the severity of prior drought stress.