Decadal isotopic and functional trait evidence reveals water and nitrogen constrains on productivity of three subtropical conifers

Increasing evidence indicates that plant productivity is constrained by water and nutrient availability under natural conditions of the stimulatory effects of elevated CO₂ concentration (eCO₂). However, it remains unclear how plant traits related to water and nitrogen acquisition and utilization acclimate to the soil water and nitrogen limitations on productivity. To address this, we investigated isotopic and functional traits and net primary productivity (NPP) of three dominant species of Pinus elliottii, Cunninghamia lanceolata, and Pinus massoniana in a subtropical coniferous plantation from 2011 to 2022 along with environmental parameters. Faced with increasing soil water and nitrogen stress, stomatal conductance (gs, 1/leaf δ¹⁸O enrichment) decreased with eCO₂ in all species. Stomatal closure enhanced intrinsic water use efficiency (iWUE, derived from leaf δ¹³C using photosynthetic discrimination model) in P. elliottii and P. massoniana but not in C. lanceolata. Although eCO₂ compensate for productivity losses resulting from drought-induced decreases in gs, increased NPP was observed only in P. elliottii, reflecting differences in the species' abilities to acclimate and overcome resource limitations. All species showed increased mycorrhizal dependency (the difference in δ¹⁵N between leaves and soil, |△¹⁵N|), high leaf nitrogen content, but reduced nitrogen use efficiency, leaf water content and specific leaf area. This suggested that plants increased nitrogen investment through biological adaption to mitigate productivity limitations caused by water and nutrient stress. The increased NPP in P. elliottii was due to high nitrogen uptake and low leaf nitrogen demand, compensating for water limitations. Conversely, reductions in NPP in C. lanceolata and P. massoniana were attributed to the relatively low nitrogen uptake and high leaf nitrogen demand, which failed to offset water limitations. This implies that the magnitude and direction of vegetation productivity responses to eCO₂ are determined by species-specific differences in plant adaptations to water and nutrient limitations.