© 2005 Heron Publishing—Victoria, Canada
Refugial forests of the southern Appalachians: photosynthesis and survival in current-year Abies fraseri seedlings
Daniel M. Johnson (1) and William K. Smith (1, 2)
1. Department of Biology, Wake Forest University, P.O. Box 7325, Reynolda Station, Winston-Salem, NC 27109-7325, USA / 2. Corresponding author (smithwk@wfu.edu) / Received December 8, 2004; accepted April 2, 2005; published online August 16, 2005
Summary
Fraser fir (Abies fraseri (Pursh) Poiret) is an endemic, high-elevation conifer confined to six relict mountaintop communities in the southern Appalachian
Mountains, USA. High adult mortality has occurred over the past 50 years, possibly the result of an introduced insect (Adelges piceae Ratzeburg), air pollution, or both. Knowledge of the mechanisms of and limitations to seedling establishment may allow reestablisment
and perpetuation of this unique community type, notwithstanding global climate change. We monitored seedling emergence and
mortality in relation to photosynthetic performance and water relations in microsites differing in canopy openness (sunlight
exposure) over the summer of 2004. Abundance of cotyledonous seedlings in early summer was 2.3 times greater (849 versus 366
seedlings m–2) in microsites with lower sky exposure (greater canopy closure) than in microsites with greater sky exposure (greater canopy
openness). In contrast, late-season abundance and survival were greater in areas beneath more open canopies than in areas
beneath less open canopies (3.3 times and 11.7 times greater, respectively). However, newly emerged seedling survival in a
completely open site (no overhead canopy) was zero, despite an initial density of 124 seedlings m–2. Seedling water status was similar in open- and closed-canopy sites (–0.52 and –0.74 MPa, respectively). Photosynthetic carbon
gain was higher in newly emerged seedlings at open canopy than at closed canopy sites, especially during early morning. Based
on photosynthetic light response curves and measured sunlight regimes, seedlings in open canopy sites were estimated to assimilate
3.3–4.5 times more carbon than seedlings at closed sites. Reductions in carbon gain of closed-site seedlings, primarily a
result of limited sunlight, corresponded to substantial increases in seedling mortality (98 versus 79% in open canopy sites).
Thus, sunlight exposure, which reflects overstory canopy structure, appears to be an important factor influencing newly emerged
seedling survival and distribution.
Keywords:
altitude, carbon gain, cotyledon, endemic species, microclimate, refugial populations, stand reestablishment, sunlight regime.
