Hydraulic constraints on photosynthesis in subtropical evergreen broad leaf forest and pine woodland trees of the Florida Everglades

Abstract

The relationship between water transport and photosynthesis represents the trade-off between carbon gain and water loss and was used to evaluate potential differences in water resource utilization among two dominant vegetation types of south Florida: subtropical evergreen broad leaf forests (hardwood hammocks) and pine woodlands (pine rocklands). We found consistent linear positive relationships between the quantum yield of photosystem II (φPSII), an index of photosynthetic capacity, and hydraulic conductivity per sapwood area (kS) and per leaf area (kL) across all species. The slope of the φPSII-kS relationship was steeper for hardwood hammock than for pine rockland species. Mean φPSII was greater in pine rockland species and was greater for a given kL than in hardwood hammock species. These results are consistent with previous observations demonstrating that pine rocklands tend to have better access to stable water sources than hardwood hammocks. We also found greater photosynthetic carbon isotope discrimination with increasing kS and kL in pine rockland species, but not in hardwood hammock species, suggesting increased stomatal conductance with increasing kS and kL, consistent with greater water availability in pine rockland habitats. Our study thus utilizes relationships between water transport and photosynthesis to evaluate hydraulic constraints on physiological function between two contrasting vegetation types with contrasting stability of water sources.