Age-class differences in shoot photosynthesis and water relations of Fraser fir (Abies fraseri), southern Appalachian Mountains, USA

Abstract

Fraser fir ( Abies fraseri (Pursh) Poir.) is an endemic tree species found only in refugial mountain-top forests in the southern Appalachian Mountains, USA. Very few studies have investigated the ecophysiology of this species in its natural environment. We measured and compared photosynthetic gas exchange and water relations of understory germinant seedlings (<3 cm height), older seedlings (>3 years old and ~0.15–1 m tall), and saplings (>10 years old and ~1–2 m tall) on a leaf-level basis. No significant differences in gas exchange were found among the age classes, although midday xylem water potentials were significantly different among the age classes. Differences in maximum photosynthesis (Amax) were <13%, ranging between 2.4 and 2.7 μmol·m–2·s–1 (P = 0.26). Maximum leaf conductances (gmax) were between 0.16 and 0.18 mol·m–2·s–1, varying less than 13% (P = 0.15). Midday xylem water potentials were similar between SE and SA (–1.2 to –1.4 MPa), but GS values were significantly less negative (–0.2 to –0.5 MPa). Thus, gas exchange and water relations in these understory age classes appeared similar, indicating no particular limitation to advanced regeneration between age classes.