Thursday, August 6, 2009: 9:50 AM
Grand Pavillion I, Hyatt
Background/Question/Methods The Quabbin Forest understory of central Massachusetts has undergone a dramatic transformation since the conversion of land from mixed-use/woodland-agricultural to watershed status. For over a half century, restrictions on harvesting of white-tailed deer set into motion a fundamental and deeply entrenched shift in the competitive balance involving fern colonization and establishment of the youngest tree cohorts. Most tree species here are at a competitive disadvantage in terms of early seasonal leaf expansion, fern canopy closure, and light capture. This has led to a repetitive cycle of high mortality among seedling cohorts only several years following germination. However, some few tree species (e.g. Quercus rubra, Prunus serotina) that are also important regional forest-type components can maintain sufficient chlorophyll levels in foliage following fern senescence, effectively extending their photosynthetic season into late autumn. Are peak summer photosynthetic rates (Amax) among fern-suppressed seedlings actually less than autumn levels? How much advantage is gained by this late season window of photosynthetic activity for long-term survivability (defined pragmatically as seedling stem elongation above the fern canopy)? Real-time gas exchange of deciduous tree seedlings existing beneath the fern canopy was determined by repeated measures from leaf expansion through senescence. Seedlings were sampled within a gradient of fern density plots. Irradiance (PAR) was also characterized at the forest floor beneath intact and senescent hay-scented fern (Dennstaedtia punctilobula) for related analyses.
Results/Conclusions Comparisons of seedling gas exchange beneath closed fern canopies with that of post-fern senescence reveal a striking increase in assimilation rates despite low ambient and leaf temperatures. For example, average assimilation rates increased between August and October (following annual release from the fern canopy) 4 and >15 times in P. serotina and Q. rubra seedlings, respectively. Photosynthesis continued to be higher in early November than in mid-summer among some tree species. Only sunfleck-induced Amax in summertime was as high or higher than autumn Amax. The proportion of diurnal light capture represented by sunflecks may be another crucial component in seedling survival beneath closed fern canopies. Native invasiveness due to human disturbance in forested ecosystems represents an increasingly important facet of interspecific plant competition here. Information gained in this study could contribute to regional watershed management strategies, e.g. future conversion to forest cover types that represent the best options to enhance forest perpetuation and preserve water quality.
Results/Conclusions Comparisons of seedling gas exchange beneath closed fern canopies with that of post-fern senescence reveal a striking increase in assimilation rates despite low ambient and leaf temperatures. For example, average assimilation rates increased between August and October (following annual release from the fern canopy) 4 and >15 times in P. serotina and Q. rubra seedlings, respectively. Photosynthesis continued to be higher in early November than in mid-summer among some tree species. Only sunfleck-induced Amax in summertime was as high or higher than autumn Amax. The proportion of diurnal light capture represented by sunflecks may be another crucial component in seedling survival beneath closed fern canopies. Native invasiveness due to human disturbance in forested ecosystems represents an increasingly important facet of interspecific plant competition here. Information gained in this study could contribute to regional watershed management strategies, e.g. future conversion to forest cover types that represent the best options to enhance forest perpetuation and preserve water quality.
