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Time lag analysis (TLA) is a distance-based approach used to study temporal dynamics of ecological communities by measuring community dissimilarity over increasing time lags. Despite its increased use in recent years, its performance in comparison with other more direct methods (i.e., canonical ordination) has not been evaluated. This study fills this gap using extensive simulations and real data sets from experimental temporary ponds (true zooplankton communities) and landscape studies (landscape categories as pseudo-communities) that differ in community structure and anthropogenic stress history. Modeling time with a principal coordinate of neighborhood matrices (PCNM) approach, the canonical ordination technique (redundancy analysis; RDA) consistently outperformed the other statistical tests (i.e., TLAs, Mantel test, and RDA based on linear time trends) using all real data. In addition, the RDA-PCNM revealed different patterns of temporal change, and the strength of each individual time pattern, in terms of adjusted variance explained, could be evaluated. It also identified species contributions to these patterns of temporal change. This additional information is not provided by distance-based methods. The simulation study revealed better Type I error properties of the canonical ordination techniques compared with the distance-based approaches when no deterministic component of change was imposed on the communities. The simulation also revealed that strong emphasis on uniform deterministic change and low variability at other temporal scales is needed to result in decreased statistical power of the RDA-PCNM approach relative to the other methods. Based on the statistical performance of and information content provided by RDA-PCNM models, this technique serves ecologists as a powerful tool for modeling temporal change of ecological (pseudo-) communities. Key words: canonical ordination; community ecology; distance-based methods; principal coordinate o[ neighborhood matrices ( PCNM ) ; statistical power; temporal dynamics.

Here, we present the second part of a quantitative theory for the structure and dynamics of forests under demographic and resource steady state. The theory is based on individual-level allometric scaling relations for how trees use resources, fill space, and grow. These scale up to determine emergent properties of diverse forests, including size--frequency distributions, spacing relations, canopy configurations, mortality rates, population dynamics, successional dynamics, and resource flux rates. The theory uniquely makes quantitative predictions for both stand-level scaling exponents and normalizations. We evaluate these predictions by compiling and analyzing macroecological datasets from several tropical forests. The close match between theoretical predictions and data suggests that forests are organized by a set of very general scaling rules. Our mechanistic theory is based on allometric scaling relations, is complementary to 'demographic theory,' but is fundamentally different in approach. It provides a quantitative baseline for understanding deviations from predictions due to other factors, including disturbance, variation in branching architecture, asymmetric competition, resource limitation, and other sources of mortality, which are not included in the deliberately simplified theory. The theory should apply to a wide range of forests despite large differences in abiotic environment, species diversity, and taxonomic and functional composition. allometry | mortality rate | plant ecology | size distribution | competitive thinning

Dormancy is a condition in which an herbaceous perennial does not sprout for one or more growing seasons. To test whether dormancy is an adaptive response to environmental stress, we defoliated and shaded individuals of two rare geophytic orchids, Cypripedium calceolus and Cephalanthera longifolia, in five Estonian populations early in the growing season in 2002 and 2003. We also censused plants at the same time, and conducted one more census in 2004. Mark-recapture models were used to estimate the probabilities of dormancy (d, the complement to resighting, p), and apparent survival ([phi]). Apparent survival varied little by treatment, with Cypripedium and Cephalanthera surviving at 0.986 [+ or -] 0.014 and 0.974 [+ or -] 0.021 (mean [+ or -] SE), respectively. In contrast, treatment impacted dormancy dramatically. For both Cephalanthera and Cypripedium, defoliated (def.) plants were most dormant (0.320 [+ or -] 0.055 and 0.095 [+ or -] 0.036, respectively). However, while both control (cont.) and shaded (sh.) plants were roughly equally least dormant in Cypripedium ([d.sub.cont.] = 0.048 [+ or -] 0.020 vs. [d.sub.sh.] = 0.045 [+ or -] 0.021), the least dormant Cephalanthera had been shaded (0.182 [+ or -] 0.040 vs. [d.sub.cont.] = 0.206 [+ or -] 0.050). We conclude that dormancy may allow the plant to buffer stress in the short term without increasing mortality risk. Key words: Cephalanthera longifolia; clonal plants; Cypripedium calceolus; defoliation; mark-recapture; plant size; shading; survival.

It has become clear that long-distance seed dispersal plays a crucial role in plant metapopulation persistence and response to rapid climate change. Recent studies of the role of convective vs. shear-generated updrafts in prompting long-distance dispersal using Taraxacum officinale as an example, suggest that (1) the probability of abscission is independent of horizontal speed and thus (2) shear-induced vertical turbulence is low, and so by default the bulk of updrafts must be due to convection, especially in open habitats such as grasslands. In this paper, I directly test the first hypothesis, and indirectly test the second via a modeling exercise. Employing shorter averaging times than used previously, it is shown that abscission in T. officinale is controlled by horizontal wind speed. Indeed, it is related to the square of the wind speed, as might be expected if drag is the motive force. I also show that this augmentation of wind speed by the abscission bias should sufficiently increase the shear-induced turbulence so that shear rivals convection as a source of updrafts in open habitats. In conclusion, long-distance dispersal by wind will not be successfully modeled until we couple the abscission and subsequent dispersal processes. Key words: anemochory; dandelion; seed abscission; seed dispersal; Taraxacum officinale; turbulence; wind.

We compared woody plant species distributions across nested spatial scales (local scale to entire Western Himalaya) and explored landscape scale patterns in detail to obtain inferences about the elevational gradient in species richness. Distribution data were compiled for 1100 species in the Western Himalaya, and primary data, comprising 123 species and 47 000 individuals, were collected for a landscape. Correlates of diversity were examined for the five spatial scales, and for different biogeographic groups at the landscape scale. The results indicate multiple mechanisms both within and across scales. At the landscape scale, though the mechanisms explaining unimodal species richness patterns were hard to separate, the underlying correlates of biogeographic groups were more distinct; temperate species richness followed mid-domain model predictions, and showed a nonlinear relationship with temperature, whereas tropical species richness tracked temperature and area. Simulations demonstrated that models with varying assumptions, while resulting in monotonic, unimodal, of multimodal patterns at local scales, could all lead to unimodal patterns at regional scales when multiple local replicates are aggregated, with a peak in the major ecotone. The turnover of successive accumulation of marginal species in ecotones potentially explains the mid-elevational peak in this zone. Landscape scale primary data on distribution and abundance could therefore be critical to understanding key aspects of macroecological patterns. Key words: elevational gradients; Himalaya; macroecology; mid-domain effect; plants; spatial scale, species richness.

Exotic plant species can affect native plant species both directly via competition and indirectly by modifying native species' interactions with other organisms in the community. Both direct and indirect effects can have strong fitness impacts on the native species and can result in cascading effects throughout the invaded community. Many exotic plant species escape from the berbivores with which they co-evolved, but in our study system, the exotic Egyptian alfalfa weevil (Hypera brunneipennis) has been introduced to California and is able to feed both on the exotic plant Medicago polymorpha and on the California native Lotus wrangelianus. Because these two plant species share this common herbivore, the possibility of strong herbivore-mediated indirect interactions between these introduced and native plant species is plausible. We used observational studies and manipulative field experiments to determine whether exotic Medicago affects herbivory on the native Lotus and whether Medicago reduces the fitness of Lotus through direct competitive effects and/or herbivore-mediated indirect effects. Both observational and experimental data indicate that Medicago increases weevil herbivory on the native Lotus. Additionally, Medicago reduced Lotus fitness through direct as well as indirect mechanisms. In 2002, Medicago reduced Lotus fitness even when herbivores were experimentally reduced, suggesting direct competitive effects of Medicago on the native Lotus. In contrast, in 2003, Medicago reduced Lotus fitness only in the presence of herbivores, indicating that the fitness effects in that year were largely indirect and mediated by herbivores. Our results demonstrate that the net fitness consequences of indirect and direct effects of exotic plant species on native plants can vary temporally, depending on the abundance of members of the herbivore community. Also, rather than limiting the success of exotics, introduced herbivores that feed on exotic plants may provide additional, indirect avenues through which exotic plants can reduce the fitness of native species. Key words: apparent competition; associational susceptibility; biological invasion; Hypera brunneipennis; indirect effect; Lotus wrangelianus; Medicago polymorpha; plant-insect interaction.

Adjacency arrangement is the configuration of a patch and its adjoining elements. The relation of adjacency effects to the resource availability and disturbance in affecting plant species richness and composition in the woodland understory is described.

Patterns of vegetation in Great Britain between the period 1990 and 1998 are described based on an analysis of plant species from a total of 9596 fixed plots. Two types of analyses based on changes in plant species composition within monitoring plots are reported.