Johnson, Daniel M. , Smith, William K. .
Photosynthesis and survival in high-altitude, current-year seedlings of Abies fraseri in the southern Appalachian Mountains.
Fraser fir (Abies fraseri (Pursh) Poiret) is an endemic, high-elevation conifer that forms refugial island communities on six different mountaintops in the southern Appalachian Mountains, USA. High adult mortality has occurred over the past 50 years, possibly the result of an introduced insect, air pollution, or both. Understanding the mechanisms and limitations to seedling establishment is critical for understanding stand re-establishment and sustainability of this unique community type. The current study monitored new seedling emergence and mortality in relation to photosynthetic performance and water relations in microsites with different degrees of 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), while late-season abundance and survival were greater in areas beneath more open canopies (3.3 times and 11.7 times greater, respectively). In contrast, new seedling survival in a completely open site (no overhead canopy) was zero for initial abundance values of 124 seedlings m-2. Seedling water status was similar in open canopy sites, compared to closed sites (-0.52 versus -0.74 MPa). Emergent seedling photosynthetic carbon gain was also higher in open canopy sites than closed sites, especially during early morning. Based on photosynthetic light response curves and measured sunlight regimes, seedlings in open canopy sites were estimated to have assimilated 3.3 - 4.5 times more photosynthetic carbon than seedlings at closed sites. Reductions in carbon gain of closed-site seedlings, due apparently to limited sunlight, corresponded to substantial increases in seedling mortality (98% versus 79% mortality in closed and open canopy sites, respectively). Thus, the amount of sunlight exposure due to differences in overstory canopy structure appears to be an important factor regulating new tree seedling survival and distribution within these relic spruce-fir forests.
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1 - Wake Forest University, Department of Biology, P.O. Box 7325, Winston-Salem, North Carolina, 27109, USA
Presentation Type: Oral Paper
Location: Salon A - Gov Ballroom/Hilton
Date: Tuesday, August 16th, 2005
Time: 4:15 PM