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BROWSING AND MICROHABITAT EFFECTS ON RIPARIAN FOREST WOODY SEEDLING DEMOGRAPHY
Author(s) -
Liang Suh Yuen,
Seagle Steven W.
Publication year - 2002
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/0012-9658(2002)083[0212:bameor]2.0.co;2
Subject(s) - seedling , odocoileus , ecology , biology , riparian zone , herbivore , beech , geography , habitat , agronomy
Forest seedling demography and seedling diversity are commonly impacted by spatially variable microhabitat effects on germination and establishment. For many forests of the eastern United States, herbivory by white‐tailed deer ( Odocoileus virginianus ) also can be intense and impose severe restrictions on seedling demography. The concurrent effects of these “bottom‐up” and “top‐down” factors were examined in a riparian forest on the Patuxent Research Refuge, Maryland, USA. Random permanent‐plot surveys found that two microhabitat factors, soil moisture and leaf litter, were correlated with seedling spatial distributions. These correlations suggest that microhabitat variability promotes seedling diversity. Microhabitat effects due to codominant American beech ( Fagus grandifolia ) and yellow‐poplar ( Liriodendron tulipifera ) canopies had limited impacts on seedling demography but were consistent with the random surveys. In contrast, experimental results show that browsing had distinct demographic effects on seedling populations and strong collective impacts at the community level. Although not completely overriding the importance of microhabitat effects, browsing may be viewed as a consistent and spatially ubiquitous top‐down constraint on seedling populations. Four distinctive relationships between seedling density and species‐specific browsing intensity were apparent: density‐independence, density‐dependence, avoidance, and opportunism. These species‐specific relationships are related to browsing preference, palatability, and life‐history traits of the tree species. In contrast, at the community level browsing on woody species is strictly density dependent and emerges as a consequence of cumulative species‐specific patterns. Because of complementary species‐specific browsing patterns and variation among cohorts of each species, browsing impacts at the community level were more clearly defined. These impacts included a 39% increase in annual seedling mortality rate, a 42% reduction in seedling recruitment, a 28% reduction in basal‐diameter increment, and a 35% reduction in the rate of population increase. White‐tailed deer browsing in this forest has the potential to decrease seedling diversity by impacting species differentially, offsetting the diversifying impact of microhabitat variation, and suppressing total seedling density. Nonetheless, browsing may not alter the trajectory of forest succession from that expected based on life‐history characteristics of the two dominant species because: (1) American beech is shade tolerant and only slightly impacted by browsing, (2) codominant yellow‐poplar is shade intolerant and a preferred browse species, and (3) the primary understory species (spicebush [ Lindera benzoin ] and ironwood [ Carpinus caroliniana ]), although heavily browsed, are not suppressed by browsing. Microhabitat, browsing, and tree species life‐history traits should be considered collectively in considering the current status and predicted future of this forest.