Does canopy position affect wood specific gravity in temperate forest trees?

The radial increases in wood specific gravity known in many tree species have been interpreted as providing mechanical support in response to the stresses associated with wind loading. This interpretation leads to the hypothesis that individuals reaching the canopy should (1) be more likely to have radial increases in specific gravity and (2) exhibit greater increases than individuals in the subcanopy. Wood specific gravity was determined for three species of forest trees (Acer rubrum, Fagus grandifolia and Tsuga canadensis) growing in central Massachusetts, USA. Acer rubrum shows radial increases in specific gravity, but these increases are not more pronounced in canopy trees; the other two species show a pattern of radial decreases. The degree of radial increase or decrease is influenced by tree height and diameter. Of the dominant tree species for which we have data, A. rubrum, Betula papyrifera and Pinus strobus show radial increases in specific gravity, whereas F. grandifolia, T. canadensis and Quercus rubra show decreases. The occurrence of radial increases in B. papyrifera and P. strobus, which are often canopy emergents, suggests that it is overall adaptive strategy that is important rather than position (canopy vs. subcanopy) of any individual tree. It is suggested that radial increases in specific gravity are associated with early-successional status or characteristics and decreases with late-successional status or persistence in mature forest.

The burning of the New World: the extent and significance of broadcast burning by early humans.

It is possible to delimit the areas of the North, Central, and South America that are most susceptible to fire and would have been most affected by burning practices of early Americans. Areas amounting to approximately 155 x 10(5) km2 are here designated as the most burnable part of the New World. Using estimates of burnable biomass, burning frequency, and burning efficiency, the authors determine the amount of biomass burned annually in an environment in which anthropogenic fires were at a hypothesized maximum. The amount of carbon released annually approximates estimates for present-day burning. Changes in carbon sinks may have been the most significant aspect of a shift to a low-biomass state. Decreases in stored biomass, soil carbon, and charcoal production may have had effects on a global scale. Likewise, the shift to a higher biomass/lower fire-frequency state over the last 400-500 years may be one component of an increased mid- to high-latitude carbon sink. The assessment made here is preliminary but may aid in clarifying the state of the climate system during the pre-industrial period.