Contrasting leaf nutrient-hydraulic relationships between karst and non-karst forests

Background and aims

Karst landscapes, characterized by large limestone outcrops and shallow, thin soils, are extensively distributed in Southwest China. Tree species found in karst forests encounter frequent drought, phosphorus (P) deficiency, and excess calcium (Ca) and magnesium (Mg). We aimed to elucidate leaf strategies for karst and non-karst forest species grown on different soil substrates.

Methods

We selected a total of 100 typical tree species from karst and non-karst forests in tropical-subtropical China, and measured leaf hydraulic traits and nutrient concentrations. We compared differences in leaf traits between the two forest types using t-test and their differences in trait relationships using trait network analysis.

Results

Karst tree species exhibited lower leaf turgor loss point (π_tlp) and midday leaf water potential, and narrower hydraulic safety margins than non-karst tree species. Furthermore, karst species had higher nitrogen, potassium, Ca, and Mg, but lower P levels than non-karst species; however, they showed similar Ca/Mg ratios. Trait network analysis revealed a higher degree of leaf traits relationships in karst species, with saturated water content, P, and π_tlp showing high connectivity with other traits. Particularly, karst species with higher leaf Ca, and lower P levels tended to have lower π_tlp and vulnerability to cavitation, indicating a co-variation between leaf nutrients and drought resistance under unique chemical and physical conditions of karst environment. However, these relationships were not observed in non-karst species, resulting in weak leaf trait relationships.

Conclusion

This study suggests that strong associations between leaf nutrient and hydraulic traits in karst species promote their adaptation to soil water and nutrient stresses.