The Santa Monica Mountains of southern California are characterized by a Mediterranean-type climate consisting of hot, dry summers and mild, wet winters. The flora must withstand 6-9 months without rainfall and is most frequently dominated by evergreen, sclerophyllous chaparral. However, various species of ferns are often in the understory of chaparral shrubs. These ferns follow one of four unique life history types relative to frond persistence: evergreen, summer/winter deciduous, or resurrection (desiccation tolerant). Life history types vary with microsites and seasonal activity. The objective of this study was to determine which factors (life history type, microsite, and seasonal activity) might link to maximum photosynthetic performance. We collected fern functionality data using a Li-6400XT gas-exchange system, hydraulic conductivity apparatus, Instron Mechanical Testing Device, and Scholander-Hammel pressure chamber. Data included maximum photosynthetic rates, stomatal conductance to water vapor diffusion, electron transport rate, quantum yield, non-photosynthetic quenching, minimum seasonal water potential, xylem conduit anatomy, and mechanical strength. We correlated these values with life history type, microsite preference, and frond seasonal activity. Photosynthetic data were collected in March, April, and May.
Photosynthetic rates were lowest for evergreen species and highest for desiccation-tolerant resurrection plants. Summer and winter deciduous species were intermediate. During moderate seasonal drought in May, photosynthetic rates remained high for species growing in riparian microsites. May photosynthetic rates also remained high in Dryopteris arguta, which displayed unusual dehydration tolerance of its frond tissues (lowest seasonal water potential, lowest osmotic potential at the turgor loss point, and lowest vulnerability to water stress-induced cavitation of xylem conduits). After comparing maximum photosynthetic performance among our eight species to thirteen fern traits, only Modules of Elasticity (MOE) had a correlation coefficient approaching significance (r2 = 0.44). Consistent with this observation, maximum stomatal conductance to water vapor diffusion was strongly correlated with MOE (r2 = 0.82). It appears that maximum photosynthetic rates in the eight species examined primarily correlate with frond persistence during summer drought/winter freezing and microsite preference for riparian habitats.