Showing papers by "Sean C. Thomas published in 2001"

The nature of tree growth and the “age-related decline in forest productivity”

Abstract: The decline in the productivity of forests after a period of growth has been much discussed in recent years, with numerous research papers and several review articles on the topic. Most of these articles have focussed on alternative physiological mechanisms to explain this phenomenon (Gower et al. 1996, Ryan and Yoder 1997, Ryan et al. 1997, Smith and Resh 1999, Magnani et al. 2000). Proposed mechanisms include (a) an altered balance between photosynthetic and respiring tissues, (b) decreasing soil nutrient availability, (c) reduced photosynthetic rates due to increasing limitations to water transport, and (d) changes in allocation. At the recent meeting of the Ecological Society of America (5-10 Aug. 2000, Snowbird, Utah), there was a well-attended discussion entitled "What DOES [emphasis theirs] cause age-related decline in forest productivity?" Behind many of the discussions lies the hope that if we can understand what causes the decline in productivity with stand age, perhaps we can do something about it. We contend that much of this discussion has taken place at the wrong scale, that proximate and ultimate causes have been confused, and that an essential perspective has been ignored. This perspective is simply that trees, like all organisms, exhibit sigmoid growth curves, and that ageand/or size-dependence of growth rates is an expression of this pattern. In this note we present three fundamental and related points which we believe have not been appropriately considered in the discussions to date:

Tropical Forest Diversity--The Plot Thickens

Abstract: Ecologists have long pondered why tropical forests contain such a huge number of species and what factors might be responsible for maintaining this diversity. In a Perspective, Burslem and colleagues discuss unique data sets from the forest dynamics plots (FDP) network, which shed light on the factors responsible for tropical forest diversity.

Corticolous bryophytes in managed Douglas-fir forests: habitat differentiation and responses to thinning and fertilization

Abstract: Corticolous bryophytes, that is, mosses and liverworts that inhabit tree trunks, represent an important compo - nent of plant diversity in temperate ecosystems, but little is known of their ecology in managed forests In this study, we quantified community composition and habitat differentiation of corticolous bryophytes in Douglas-fir ( Pseudotsuga menziesii (Mirb) Franco) plantations subjected to experimental thinning and fertilization treatments Twenty-four bryophyte species were recorded in a sample of 480, 225 cm 2 quadrats on 60 tree trunks All moss species and obligately epiphytic liverworts (those with a primary habitat preference for tree trunks) showed highest cover values on south and west exposures In contrast, facultatively epiphytic liverworts occurred only at the tree base, and mainly on north and west exposures Pairwise correlations among species cover values were nearly always positive, and cover of the most abundant species, Isothecium myosuroides, was also positively correlated with local species richness of other bryophyte taxa These patterns suggest that competitive interactions among bryophyte species are not strong in this community There was little evidence for fertilization or thinning effects on total bryophyte cover or species richness However, analyses of community composition and species-specific responses indicated significant negative effects of thinning on some bryophyte species Observed patterns of habitat differentiation, interspecific associations, and treat- ment responses suggest that stand hydrology and microclimate are of primary importance in determining the distribu- tion and abundance of corticolous bryophytes in managed forests

Natural Selection: Responses to Current (Anthropogenic) Environmental Changes

Abstract: Human-caused changes in the environment are pervasive, as are evolutionary responses to these changes by living organisms. Selective responses to anthropogenic environmental change have thus been the topic of extensive research, providing hundreds of well-documented cases of natural selection in action. Many of these responses are also of great importance from an ecological, economic and human health perspective. Keywords: evolution; global environmental change; ecological genetics