Cell wall thickness constrains photosynthesis of coexisting species in a subtropical plantation by reducing mesophyll conductance and maximum carboxylation rate.

The interspecific variation in the net photosynthetic rate (Anet) reflects coordination and trade-offs between biophysical and biochemical processes, yet the underlying morphophysiological mechanisms remain poorly understood. To address this, we quantified photosynthetic parameters as well as morphological, anatomical and nutrient traits of 12 coexisting needle and broadleaf species within a subtropical coniferous plantation of the East Asian monsoon region. Across species, Anet is primarily constrained by stomatal conductance (gs), secondarily by maximum carboxylation rate (Vcmax) and minimally by mesophyll conductance (gm). A negative correlation between gs/Anet and gm/Anet suggests that increases in gm partially compensate for stomatal limitations on Anet, while the negative correlation between gt/Anet (gt, total conductance) and Vcmax/Anet reflects CO2 supply-demand trade-off during photosynthesis. Variation in gm reflects the coordination between cell wall thickness (TCW) and the chloroplast surface area exposed to intercellular air spaces (Sc/S). Variation in Vcmax is negatively related to TCW, rather than to leaf nitrogen and phosphorus per unit area. Structural equation modeling further reveals that TCW indirectly regulates Anet through both Vcmax and gm, with its limiting effect on Vcmax being slightly stronger than on gm. Needle species exhibit gs and Vcmax comparable to those of broadleaf species; however, their lower gm results in a significantly reduced Anet. This reduction is attributed to greater TCW and lower Sc/S. Additionally, the higher TCW in needle species may lead to increased allocation of leaf nitrogen to non-photosynthetic tissues, as their significantly higher leaf nitrogen content compared with broadleaf species is not accompanied by a corresponding increase in Vcmax. Variation in Vcmax is driven by TCW rather than by leaf nutrient, underscoring the necessity of incorporating leaf anatomical traits into mechanistic and predictive models. Moreover, as water and nitrogen limitations increase during forest succession, needle species in subtropical plantations-characterized by low gm and high TCW-are likely to be replaced by broadleaf species.