PHOTOSYNTHETIC ADAPTABILITY OF 2 FERN SPECIES OF A NORTHERN HARDWOOD FOREST

The capacity for acclimation, ''the gradual and reversible adjustment of physiology or morphology as a result of changing environmental conditions'' (Lincoln, et al., 1982), varies among plant species (Chapin, 1980; Hicks & Chabot, 1985). Fast-growing species of partially shaded habitats have the greatest ability to acclimate to maximize production in shade (Grime, 1981). Shade leaves are generally thinner and have larger leaf areas, lower concentrations on a mass basis and contents on an area basis of RUBISCO and leaf protein, and frequently higher chlorophyll contents but lower chlorophyll concentrations than sun leaves (Bjorkman & Holmgren, 1963; Ludlow & Wolf, 1975; Boardman, 1977; Hicks & Chabot, 1985). A positive correlation between net photosynthetic rate per unit leaf area and N content has been reported for a variety of plants including woody and herbaceous species (Field & Mooney, 1986; Evans, 1989; Karlsson, 1991; Tuohy, et al., 1991). Higher light-saturated net photosynthetic rates per unit leaf area have been found for sun-grown than shade-grown conspecifics (Bjorkman & Holmgren, 1963). Seasonal changes in photosynthesis have been reported by Bauer, et al. (1991) for Dryopteris filix-mas (L.) Schott. The present study was undertaken to determine the photosynthetic adaptabilities to sun and shade of two contrasting fem species of a northern hardwood forest. Dryopteris intermedia (Muhl. ex Willd.) Gray is a semi-evergreen fern of northern hardwood forest understories in North America, frequently occurring beneath the forest canopy but also found in gaps of recent origin (Siccama, et al., 1970; Hughes & Fahey, 1991). Congeners (Dryopteris spinulosa (O.F. Muell.) Watt and D. marginalis (L.) Gray) have been described by Sparling (1967) as shade-tolerant. Dennstaedtia punctilobula (Michx.) Moore is a summergreen fem frequently occurring in forest gaps and clearings but also found beneath the canopy (Flaccus, 1959; Siccama, et al., 1970; Hughes & Fahey, 1991). We hypothesized that both sun- and shade-grown sporophytes of the competitive, mainly gap-phase species, Dennstaedtia punctilobula, would have higher net photosynthetic rates than both sun- and shade-grown sporophytes of the shade-tolerant species, Dryopteris intermedia. We also expected that sun-grown sporophytes of each species would have higher net photosynthetic rates and N contents per unit leaf area, but lower N concentrations per unit dry mass than shade-grown sporophytes.