title: Seedling ontogeny and environmental plasticity in two co-occurring shade-tolerant conifers and implications for environment-population interactions.
abstract: OBJECTIVE
  Seedling success is determined by evolved strategies of intrinsic genetic programming and plasticity that are regulated by extrinsic pathways. We tested the relative importance of these mechanisms in red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea Lin.), which share understory regeneration niches in northeastern North America. Although its reproductive effort is adequate, spruce has decreased in abundance, in relation to fir, in seedling and sapling populations, even in forests that have a predominance of spruce in the overstory.
  
  
  METHODS
  To understand the factors that regulate this phenomenon and their implications for tree populations, we compared intrinsic and plastic regulation of first- and second-year seedlings under steady understory irradiance levels and in response to increases in light environment.
  
  
  RESULTS
  Both species exhibited interactions of ontogenetic patterns and plasticity in first- and second-year seedlings. Physiologically, spruce had higher photosynthetic capacity, allocation to photoprotective xanthophylls, and greater plasticity in response to light treatments. Although both species demonstrated an inability to plastically increase photosynthetic capacity in the short term, spruce benefited from greater allocation to foliage under increased irradiance. Fir showed a conservative strategy in root-shoot allocation that may better equip seedlings to withstand drought adaptations and attributes associated with greater shade tolerance.
  
  
  CONCLUSIONS
  These attributes likely contribute to the relative success of fir seedlings in the current climate. By contrast, they indicate that spruce would be a superior competitor in cooler, moister climates, which suggests that future forest composition will be largely determined by an interaction of disturbance and moisture regimes.
authors.affiliation: School of Forest Resources, University of Maine, 5755 Nutting Hall, Orono, Maine 04469 USA.
authors.forename: Michael E
authors.initials: ME
authors.lastname: Day
authors.forename: Sarah
authors.initials: S
authors.lastname: Zazzaro
authors.forename: L Brian
authors.initials: LB
authors.lastname: Perkins
mesh_terms: D028202:Abies / Q000033:anatomy & histology / Q000502:physiology* / Q000528:radiation effects; D000222:Adaptation, Physiological / Q000528:radiation effects; D018533:Biomass; D002734:Chlorophyll / Q000378:metabolism; D017753:Ecosystem; D005453:Fluorescence; D005740:Gases / Q000378:metabolism; D008027:Light; D010788:Photosynthesis / Q000528:radiation effects; D028222:Picea / Q000033:anatomy & histology / Q000502:physiology* / Q000528:radiation effects; D018515:Plant Leaves / Q000033:anatomy & histology / Q000502:physiology / Q000528:radiation effects; D036226:Seedlings / Q000254:growth & development* / Q000528:radiation effects; D024341:Xanthophylls / Q000378:metabolism
chemical_list: D005740:Gases; D024341:Xanthophylls; D002734:Chlorophyll
keywords: Acadian Forest; Pinaceae; development; life strategies; photoprotective pigments; root–shoot allocation; tree regeneration; xanthophyll cycle