Strategies of photosynthetic physiology in response to karst habitats and rainfall allocation have maintained the morphology and function of Fraxinus malacophylla seedlings

Under global climate change, prolonged rainfall intervals and intensified single rainfall events in karst rocky desertification areas, together with increased karst fissure thickness, significantly affect plant seedling growth and physiology. This study used a two-factor block experiment to explore the effects of different rainfall intervals and karst habitats on the photosynthetic physiology of two-year-old Fraxinus malacophylla seedlings. The results showed that under the same rainfall interval, the carbon (C), nitrogen (N), phosphorus (P) content, photosynthetic pigment content, fluorescence parameters, and photosynthetic gas parameters of F. malacophylla seedlings showed a trend of first increasing and then decreasing with the increase of karst fissures. Under the I 6d S 1/2 treatment, the maximum net photosynthetic rate, light saturation point, and light compensation point reached 8.143, 1408.190, and 71.474 µmol·m⁻²·s⁻¹, respectively, with a maximum CO₂ saturation point of 1571.580 µmol·m⁻²·s⁻¹. Biomass allocation followed the order of stem > leaf > root in the S 0 habitat, whereas it showed the order of root > stem > leaf in the S 1/2 and S 3/4 habitats. At I 3d and I 6d rainfall intervals, SOD, POD, and SPC activities rose with increasing karst thickness, while an opposite pattern was found at I 9d . Specifically, the prolonged rainfall interval (I 9d ) inhibits the growth of F. malacophylla seedlings, mainly because the long-term water deficit exceeds the osmotic regulation limit of the seedlings and causes an imbalance in the antioxidant system, rather than a single collapse of the antioxidant system. In summary, the F. malacophylla seedlings grew best under I 6d S 1/2 treatment, and prolonged rainfall intervals inhibited the growth of F. malacophylla seedlings. This study suggests that shallow-soil karst rocky desertification areas can serve as suitable habitats for F. malacophylla seedlings, thereby providing a theoretical basis and technical support for vegetation restoration, the cultivation of F. malacophylla seedlings, and the maintenance of ecosystem stability in karst rocky desertification regions.