The species composition of forests change continuously as the earth’s biota evolves and adjusts to environmental change. Humans are accelerating the rate of species turnover by moving species around the planet and dramatically changing environmental conditions. Our focus is on new forests in Puerto Rico that emerge naturally on abandoned lands previously converted to agriculture and degraded. These forest stands have combinations of species that are new to the island’s landscapes. New forests exhibit high species dominance during forest establishment, which includes dominance by alien tree species. These alien tree species establish and maintain forest cover, which may facilitate regeneration of native tree species. Landscape analysis and literature review revealed that these emerging stands are highly fragmented (60% were <1 ha in 1991), function as refugia for native organisms, and at 60–80 years old have similar species richness and structural features as native stands of similar age. However, the island’s new forests exhibit important differences from mature native forests on unconverted forestlands. New forests have fewer endemic species and fewer large trees (≥55 cm dbh) than mature native forests; they have higher soil bulk density and lower soil carbon and litter stocks; and they accumulate aboveground biomass, basal area, and soil carbon more slowly than native forests of similar age. We suggest that new forests will become increasingly prevalent in the biosphere in response to novel environmental conditions introduced to the planet by humans. 相似文献
A review of agroforestry research and practice in Sri Lanka is provided with emphasis on traditional systems, plantation intercropping, silvopastoral systems, fertility improvement and community approaches. Comments are presented on the potentials and difficulties of each technology along with recommendations for further research and training. Some information on components of a number of agroforestry systems has been collected during the last 8 years. A brief account of the results is presented together with comments on current agroforestry practices. 相似文献
An analysis of spatial dispersion was conducted for individual tree species in the old-growth forest at the Davis-Purdue Research Forest in Indiana. This 20.6 ha stand has been left largely undisturbed by exogenous factors since its acquisition by Purdue in 1917. It is the only long-term study plot of its size for a temperate hardwood forest with x–y coordinates (rectangular plot) for all species above 10 cm diameter. Full censuses have been conducted spanning 60 years (1926, 1976 and 1986).
Ripley’s L(t) function revealed that most species are characterized by some form of aggregation, agreeing with a prior evaluation from 1981. Heterogeneity of spatial structure was evident between two large plots, indicating that differences in site quality and history had influenced spatial structure. Shade-intolerant species were numerically dominant and spatially aggregated in 1926 but have declined over the 60-year interval and become more random in spatial dispersion. Shade-tolerant species have increased in number and become more aggregated over time, or they exhibit little change in spatial structure. Examples of the latter include Acer saccharum and Ulmus americana, species that experienced explosive population growth. These contrasting patterns are masked by stand-level patterns that show a trend toward uniformity over the same time frame.
These data reveal that changes in dispersion accompany the demographic failure experienced by numerous tree species in Central Hardwood old-growth stands, and these changes may feed back into a negative population cycle and further impede regeneration. The simultaneous manipulation of dispersion and density should be considered as a tool for influencing forest succession and promoting regeneration of desired tree species. 相似文献
The object of this study was to investigate the inhomogeneity of density within a beam from a relationship between the dynamic
Young’s moduli from the Euler-Bernoulli elementary theory of bending (En) and resonance mode numbers (n), which is plotted as the “E-n” diagram in this article. Rectangular beams with dimensions of 300 (L) × 25 (R) × 5mm (T)
of Sakhalin spruce (Picea glehnii Mast.), Sitka spruce (Picea sitchensis Carr.), Japanese red pine (Pinus densiflora Zieb. et Zucc.) and white oak (Cyclobalanopsis myrsinaefolia Oerst.) were used for specimens. Small parts of beams were replaced with a small portion of another species to examine the
influence of the inhomogeneity of density on En. A free-free flexural vibration test was undertaken and En was calculated by the Euler-Bernoulli theory. The resonance frequency of a specimen with inhomogeneity of density was simulated
by modal analysis. The density distribution in the longitudinal direction of the specimen for which En did not decrease monotonically with n was obtained. From the modal analysis, the inhomogeneity of density was equivalent to a concentrated mass attached to a uniform
beam. The pattern of the E-n diagram was changed by replacing a part of the specimen with another species. Specimens for which
En did not decrease monotonically with n had a high density part because of indented rings, knots, or resin. 相似文献
The purpose of this study was to predict the likely amounts of carbon sequestration on a national scale for Japan in the Article-3.4
private planted forests of the Kyoto Protocol during the first commitment period. We regarded the planted forests that had
undergone silvicultural practices such as weeding, pruning, and thinning since 1990 as Article-3.4 planted forests in accordance
with the definition given by the Forestry Agency of Japan. Regression models were developed to predict the forest areas that
had undergone silvicultural practices, employing silvicultural subsidies and forest workers' wages as predictor variables.
Then the time series changes in the predictor variables were provided by extending their recent trends, with the result being
that the forest areas that have undergone silvicultural practices were predicted on the basis of the three scenarios of the
variables. Thus, the Article-3.4 forest area was calculated considering overlaps of silvicultural practices over fixed stands,
and the area was converted into the amount of carbon sequestration by multiplying it by coefficients such as a volume table,
biomass expansion factor, and others. The result implied that Article-3.4 private planted forests were expected to sequester
8.16–8.87 Mt-C year−1 during the first commitment period. These amounts cover 63%–68% of the carbon sequestration goal by land-use change and forestry
activities capped under the Marrakesh Accords. To realize this prediction, it is important to provide a sufficient silvicultural
subsidy to last until the end of the first commitment period and to implement silvicultural practices on the forest stands
that have not undergone such practices since 1990. 相似文献
This study used a vibration test method to show that grain angles of face veneer have substantial effects on sound velocities and dynamic Youngs moduli of three types of wood-based composites. The sound velocity at 0° grain angle of face veneer was the highest, and it decreased with increasing grain angle in the range of 0° to 90°. This tendency was similar to that for dynamic Youngs modulus. The relationship between the grain angle of face veneer and the sound velocity of three types of wood-based composites can be expressed in the form of Hankinsons equation or a second-order parabolic equation. This study also showed that the application of orthotropic elasticity theory was valid for the three types of wood-based composites. The relationship between the grain angle of the face veneer and the Youngs modulus of three types of wood-based composites can be expressed in the form of the Jenkin equation, Hankinsons equation, or a second-order parabolic equation. Rule of mixture can also be used to predict the Youngs modulus of wood-based composite from the Youngs moduli of the two elements.Part of this study was presented at the 53rd Annual Meeting of the Japan Wood Research Society, Fukuoka, Japan, March 2003 and the 15th Annual Meeting of Chugoku Shikoku Branch of the Japan Wood Research Society, Higashi-Hiroshima, Japan, September 2003 相似文献