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 cond itions'' (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 l arger leaf areas, lower concentrations on a mass basis and contents on an area basis of RUBISCO and leaf protein, and frequently higher chlo rophyll 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 ra te 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-satura ted net photosynthetic rates per unit leaf area have been found for su n-grown than shade-grown conspecifics (Bjorkman & Holmgren, 1963). Sea sonal changes in photosynthesis have been reported by Bauer, et al. (1 991) for Dryopteris filix-mas (L.) Schott. The present study was under taken to determine the photosynthetic adaptabilities to sun and shade of two contrasting fem species of a northern hardwood forest. Dryopter is intermedia (Muhl. ex Willd.) Gray is a semi-evergreen fern of north ern hardwood forest understories in North America, frequently occurrin g beneath the forest canopy but also found in gaps of recent origin (S iccama, et al., 1970; Hughes & Fahey, 1991). Congeners (Dryopteris spi nulosa (O.F. Muell.) Watt and D. marginalis (L.) Gray) have been descr ibed 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; Sicca ma, et al., 1970; Hughes & Fahey, 1991). We hypothesized that both sun - and shade-grown sporophytes of the competitive, mainly gap-phase spe cies, 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 sporo phytes 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.