Search

Subalpine fir, Abies lasiocarpa, occurs throughout western North America, often in forest-tundra parkland. To resolve the presence of varieties in this species, we surveyed microsatellite genetic markers in 11 populations containing three putative varieties of Abies lasiocarpa: (1) var. lasiocarpa, (2) var. bifolia, and (3) var. arizonica. We tested primers from related taxa, and 13 of the best primer pairs were used for assays. Within populations, both heterozygosity and allelic richness were approximately 10% lower in var. lasiocarpa. The STRUCTURE procedure struggled to assign populations to groups correctly; at K = 3, individuals were assigned to their putative varieties with approximately 70% accuracy. Regardless, both lasiocarpa and bifolia were correctly assigned more than expected by chance, indicating that these taxa are distinct. A dendrogram of genetic distances showed var. arizonica to exhibit higher evolutionary distance from the other two varieties and serves as an outgroup. The dendrogram also showed a nesting of var. bifolia clades within var. lasiocarpa, indicating a complex relationship between var. lasiocarpa and var. bifolia. Comparisons among the STRUCTURE population assignments for K = 2, K = 3, and K = 4 identified populations with cryptic admixture and indicate a 'lasiocarpa-bifolia' subspecies complex that warrants further study. Key words: Abies lasiocarpa, population structure, microsatellites, varietal differentiation Le sapin subalpin, Abies lasiocarpa, est present dans tout l'ouest de l'Amerique du Nord, souvent dans les forets- parcs de toundra. Afin de confirmer la presence de sous-especes (varietes) chez cette espece, nous avons etudie les marqueurs genetiques microsatellites dans 11 populations contenant trois varietes putatives d'Abies lasiocarpa : (1) var. lasiocarpa, (2) var. bifolia et (3) var. arizonica. Nous avons teste des amorces provenant de taxons apparentes, et 13 des meilleures paires d'amorces ont ete utilisees pour les analyses. Au sein des populations, l'heterozygotie et la richesse allelique etaient toutes deux inferieures d'environ 10 % chez var. lasiocarpa. La procedure STRUCTURE a eu du mal a affecter correctement les populations aux groupes; a K = 3, les individus ont ete affectes a leurs sous-especes putatives avec une precision d'environ 70 % . Malgre tout, lasiocarpa et bifolia ont ete correctement assignes plus que ce qui serait attendu par le hasard, ce qui indique que ces taxons sont distincts. Un dendrogramme des distances genetiques a montre que var. arizonica presentait une distance evolutive plus elevee par rapport aux deux autres varietes, et qu'elle servait de groupe externe. Le dendrogramme a montre l'imbrication des clades de var. bifolia au sein de var. lasiocarpa, indiquant une relation complexe entre var. lasiocarpa et var. bifolia. Les comparaisons entre les assignations de population STRUCTURE pour K = 2, K = 3 et K = 4 ont identifie des populations avec un melange cryptique et indiquent un complexe de sous-especes << lasiocarpa-bifolia >> qui merite une etude plus approfondie. [Traduit par la Redaction] Mots-cles : Abies lasiocarpa, structure de population, microsatellites, differenciation varietale, Introduction Subalpine fir, Abies lasiocarpa (Hook.) Nuttal 1849, is a conspicuous icon of the meadows of the montane coniferous forests of western North America (Burns et al. 1990). It has [...]

Complex dynamics, such as population cycles, can arise when the individual members of a population become synchronized. However, it is an open question how readily and through which mechanisms synchronization-driven cycles can occur in unstructured microbial populations. In experimental chemostats we studied large populations (>[10.sup.9] cells) of unicellular phytoplankton that displayed regular, inducible and reproducible population oscillations. Measurements of cell size distributions revealed that progression through the mitotic cycle was synchronized with the population cycles. A mathematical model that accounts for both the cell cycle and population-level processes suggests that cycles occur because individual cells become synchronized by interacting with one another through their common nutrient pool. An external perturbation by direct manipulation of the nutrient availability resulted in phase resetting, unmasking intrinsic oscillations and producing a transient collective cycle as the individuals gradually drift apart. Our study indicates a strong connection between complex within-cell processes and population dynamics, where synchronized cell cycles of unicellular phytoplankton provide sufficient population structure to cause small-amplitude oscillations at the population level. cell cycle | Chlorella vulgaris | chemostat | Kuramoto model | population cycles doi/10.10731pnas.0908725107

To authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2745.2009.01635.x Byline: Roberto Salguero-Gomez (1), Hans de Kroon (2) Keywords: demographic buffering; integral projection model; plant demography; population dynamics; projection matrix; stochastic elasticity; stochastic life table response experiment (SLTRE); transient dynamics Abstract: Summary 1. Projection matrices have become the dominant modelling approach in plant demography because they (i) are relatively easy to formulate, (ii) compile complex data in a structured and analytically tractable manner, (iii) provide numerous parameters with direct biological meaning, (iv) allow the investigator to address broad or specific, experimental and/or theoretical, ecological and evolutionary questions, and (v) produce uniform outputs, enabling direct comparisons between the results of different studies. 2. The last decade has witnessed major advancements in this field that have brought demographic models much closer to the real world, in particular in the analysis of effects of spatial and temporal environmental variation on populations. The present Special Feature contributes to that progress with novel methodologies and applications on Integral Projection Models, stochastic Life Table Response Experiment analyses, stochastic elasticities, transient dynamics and phylogenetic analyses. 3.Synthesis. Environmental stochasticity is an integral part of ecosystems, and plant populations exhibit a tremendous array of demographic strategies to deal with its effects. The analytical challenge of understanding how populations avoid, tolerate or depend on stochasticity is finally overcome with the new matrix approaches. The tools are now available to interpret the effects of changes in temporal and spatial variation on plant populations. Author Affiliation: (1)Department of Biology, University of Pennsylvania, 215 South University Avenue, 321 Leidy Laboratories, Philadelphia, PA 19104, USA (2)Department of Experimental Plant Ecology, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands Article History: Received 3 December 2009; accepted 9 December 2009 Handling Editor: Michael Hutchings Article note: (*) Correspondence author. E-mail: salguero@sas.upenn.edu

To authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2745.2009.01627.x Byline: Hal Caswell (*) Keywords: autocorrelation; fire; Lomatium bradshawii; LTRE analysis; Markovian environments; matrix population models; sensitivity analysis; stochastic growth rate; stochastic models Abstract: Summary 1. Life table response experiment (LTRE) analyses decompose treatment effects on a dependent variable (usually, but not necessarily, population growth rate) into contributions from differences in the parameters that determine that variable.2. Fixed, random and regression LTRE designs have been applied to plant populations in many contexts. These designs all make use of the derivative of the dependent variable with respect to the parameters, and describe differences as sums of linear approximations.3. Here, I extend LTRE methods to analyse treatment effects on the stochastic growth rate log [lambda].sub.s. The problem is challenging because a stochastic model contains two layers of dynamics: the stochastic dynamics of the environment and the response of the vital rates to the state of the environment. I consider the widely used case where the environment is described by a Markov chain.4. As the parameters describing the environmental Markov chain do not appear explicitly in the calculation of log [lambda].sub.s, derivatives cannot be calculated. The solution presented here combines derivatives for the vital rates with an alternative (and older) approach, due to Kitagawa and Keyfitz, that calculates contributions in a way analogous to the calculation of main effects in statistical models.5. The resulting LTRE analysis decomposes log [lambda].sub.s into contributions from differences in: (i) the stationary distribution of environmental states, (ii) the autocorrelation pattern of the environment, and (iii) the stage-specific vital rate responses within each environmental state.6. As an example, the methods are applied to a stage-classified model of the prairie plant Lomatium bradshawii in a stochastic fire environment.7. Synthesis. The stochastic growth rate is an important parameter describing the effects of environmental fluctuations on population viability. Like any growth rate, it responds to differences in environmental factors. Without a decomposition analysis there is no way to attribute differences in the stochastic growth rate to particular parts of the life cycle or particular aspects of the stochastic environment. The methods presented here provide such an analysis, extending the LTRE analyses already available for deterministic environments. Article History: Received 13 August 2009; accepted 2 December 2009 Handling Editor: Hans de Kroon Article note: (*) Correspondence address. E-mail: hcaswell@whoi.edu

To authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2745.2009.01617.x Byline: John L. Maron (1), Carol C. Horvitz (2), Jennifer L. Williams (3) Keywords: Calathea ovandensis; Cynoglossum officinale; interaction strength; matrix model; plant population growth; plant-herbivore interactions; transient dynamics Abstract: Summary 1. Despite a large literature documenting the effects of mutualists and/or antagonists on plant performance, we still have limited insight into the strength of these interactions, as this involves quantifying how one species influences the population dynamics of another. 2. Here, we use data from two example systems, Cynoglossum officinale and Calathea ovandensis, to illustrate how experiments, demographic data and stage-based population models can be combined to estimate interaction strength of insect herbivores on plants. Because many plant populations may not be at equilibrium, we conduct transient analyses and contrast these results to more traditional asymptotic results. 3. We calculate three metrics of interaction strength, [DELTA][lambda].sub.asymptotic, the change in asymptotic annual per capita plant population growth rate resulting from herbivore exclusion, [DELTA][lambda](t), the change in transient [lambda] caused by herbivores at t = M (where M = the time of their maximum effect during the transient phase) and [DELTA][lambda].sub.transient, a time-averaged effect of consumers on [lambda] across the entire transient phase. 4. Fairly strong impacts of insect consumers on plant fecundity do not translate similarly to [DELTA][lambda].sub.asymptotic. Results show that [DELTA][lambda](t) can be larger (or smaller) than [DELTA][lambda].sub.asymptotic but in our examples [DELTA][lambda].sub.transient was similar in magnitude to [DELTA][lambda].sub.asymptotic. 5. The transient effects of consumers on [lambda] were driven by changes in the elasticity of fecundity across the transient phase. These effects were dynamic even though consumer impacts on demography and vital rates were held constant. The importance of particular stages and transitions to annual population growth vary during the transient phase. 6.Synthesis. We describe three metrics of interaction strength, [DELTA][lambda].sub.asymptotic, [DELTA][lambda](t) and [DELTA][lambda].sub.transient. These metrics have several advantages over more commonly used trait or performance measures that quantify the outcome of interactions. We illustrate how the transient impacts of consumers on [lambda] are dynamic, with the changing stage distribution of a population and transient elasticities driving these effects. More generally, this study shows that the impacts of animals on plant performance do not translate equivalently to plant population growth, thereby underscoring the importance of using population models to extend the inference of individual-level experiments. Author Affiliation: (1)Division of Biological Sciences, University of Montana, Missoula, MT 59812, USA (2)Department of Biology, University of Miami, PO Box 249118, Coral Gables, FL 33124, USA (3)National Center for Ecological Analysis and Synthesis, University of California - Santa Barbara, 735 State Street, Suite 300, Santa Barbara, CA 93101, USA Article History: Received 14 August 2009; accepted 6 November 2009 Handling Editor: Hans de Kroon Article note: (*) Correspondence author. E-mail: john.maron@mso.umt.eud

Most plant and animal populations have substantial interannual variability in survival, growth rate, and fecundity. They also exhibit substantial variability among individuals in traits such as size, age, condition, and disease status that have large impacts on individual fates and consequently on the future of the population. We present here methods for constructing and analyzing a stochastic integral projection model (IPM) incorporating both of these forms of variability, illustrated through a case study of the monocarpic thistle Cartina vulgaris. We show how model construction can exploit the close correspondence between stochastic IPMs and statistical analysis of trait-fate relationships in a 'mixed' or 'hierarchical' models framework. This correspondence means that IPMs can be parameterized straightforwardly from data using established statistical techniques and software (vs. the largely ad hoc methods for stochastic matrix models), properly accounting for sampling error and between-year sample size variation and with vastly fewer parameters than a conventional stochastic matrix model. We show that the many tools available for analyzing stochastic matrix models (such as stochastic growth rate, [[lambda].sub.s], small variance approximations, elasticity/sensitivity analysis, and life table response experiment [LTRE] analysis) can be used for IPMs, and we give computational formulas for elasticity/sensitivity analyses. We develop evolutionary analyses based on the connection between growth rate sensitivity and selection gradients and present a new method using techniques from functional data analysis to study the evolution of function-valued traits such as size-dependent flowering probability. For Carlina we found consistent selection against variability in both state-specific transition rates and the fitted functions describing state dependence in demographic rates. For most of the regression parameters defining the IPM there was also selection against temporal variance; however, in some cases the effects of nonlinear averaging were big enough to favor increased temporal variation. The LTRE analysis identified year-to-year variation in survival as the dominant factor in population growth variability. Evolutionary analysis of flowering strategy showed that the entire functional relationship between plant size and flowering probability is at or near an evolutionarily stable strategy (ESS) shaped by the size-specific trade-off between the benefit (fecundity) and cost (mortality) of flowering in a temporally varying environment. Key words: Carlina vulgaris; evolutionarily stable strategies; function-valued traits; integral projection model; life table response experiment; mixed models; population forecasting; random environment; sensitivity analysis; stochastic growth rate; structured population; temporal variability.

To authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2745.2009.01499.x Byline: Donald A. Levin, Christopher D. Kelley, Sahotra Sarkar (*) Keywords: Allee effects; cellular automata; founder effects; individual-based models; S-alleles; self-incompatibility; viability model Abstract: Summary Founder populations of plants are usually small and may become extinct due to many factors including inbreeding and low pollen transfer besides stochastic factors. This positive relationship between population size and individual fitness is taken as evidence of an Allee effect. Very few studies have quantitatively compared the influence of self-incompatibility systems and S-allele counts on the magnitude of observed Allee effects for different population sizes and seed and pollen dispersal distances. This paper uses a stochastic, spatially explicit, individual-based cellular automaton model to simulate the dynamics of newly established plant populations with sporophytic or gametophytic self-incompatibility (SI) alleles and compares these to a null (Mendelian) population. Parameters varied in model simulation are the S-allele count, the initial population size, and the pollen and seed dispersal distances. These parameters were studied over a wide range to assess their relative impact on population viability compared to the null case in order to model the effect of self-incompatibility on population fitness. The results indicate that Allee effects are enhanced and growth rates are slowed in newly established populations of species with S-alleles relative to Mendelian populations, leading to enhanced extinction risk. They also show the importance of the relationship between pollen and seed dispersal ranges on the fitness of the population. Populations with sporophytic self-incompatibility were more extinction-prone than those with gametophytic SI, and the latter more so than Mendelian populations when all other factors were held constant. Synthesis. Overall, the simulation results indicate that Allee effects in small populations are enhanced in plants with S-alleles. Even in relatively large populations, S-allele count controls the strength of observed Allee effects. A high S-allele count is able to compensate for lower initial population size. Pollen and seed dispersal ranges had a similarly strong effect on the viability of populations with S-alleles. These effects are stronger in populations with sporophytic self-incompatibility than in those with gametophytic SI. Simulation results also indicate that S-alleles were not lost to genetic drift over time, in contrast with some earlier studies of S-allele systems displaying Allee effects. Article History: Received 11 October2008; accepted 16 February 2009Handling Editor: Peter Klinkhamer Article note: (*) Correspondence author. E-mail: sarkar@mail.utexas.edu

To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1523-1739.2008.01073.x Byline: SHARMILA PATHIKONDA (*), AZMY S. ACKLEH ([dagger]), KARL H. HASENSTEIN (*), SUSAN MOPPER (*[double dagger]) Keywords: clonal plants; exotic species; extinction; habitat disturbance; hurricanes; Iris hexagona; I. pseudacorus; matrix models; population dynamics; species competition Abstract: Abstract: Wetland habitats are besieged by biotic and abiotic disturbances such as invasive species, hurricanes, habitat fragmentation, and salinization. Predicting how these factors will alter local population dynamics and community structure is a monumental challenge. By examining ecologically similar congeners, such as Iris hexagona and I. pseudacorus (which reproduce clonally and sexually and tolerate a wide range of environmental conditions), one can identify life-history traits that are most influential to population growth and viability. We combined empirical data and stage-structured matrix models to investigate the demographic responses of native (I. hexagona) and invasive (I. pseudacorus) plant populations to hurricanes and salinity stress in freshwater and brackish wetlands. In our models I. hexagona and I. pseudacorus responded differently to salinity stress, and species coexistence was rare. In 82% of computer simulations of freshwater marsh, invasive iris populations excluded the native species within 50 years, whereas native populations excluded the invasive species in 99% of the simulations in brackish marsh. The occurrence of hurricanes allowed the species to coexist, and species persistence was determined by the length of time it took the ecosystem to recover. Rapid recovery (2 years) favored the invasive species, whereas gradual recovery (30 years) favored the native species. Little is known about the effects of hurricanes on competitive interactions between native and invasive plant species in marsh ecosystems. Our models contribute new insight into the relationship between environmental disturbance and invasion and demonstrate how influential abiotic factors such as climate change will be in determining interspecific interactions. Abstract (Spanish): Invasion, Perturbacion y Competencia: Modelando el Destino de Poblaciones de Plantas Costeras Resumen: Los habitats de humedales estan acosados por perturbaciones bioticas y abioticas como las especies invasoras, huracanes, fragmentacion de habitat y salinizacion. La prediccion de como estos factores alteraran la dinamica poblacional y la estructura de la comunidad es un reto monumental. Mediante el examen de congeneres similares ecologicamente, como Iris hexagona e I. pseudacorus (que se reproducen por clones y sexualmente y toleran un amplio rango de condiciones ambientales), se pueden identificar caracteristicas de la historia de vida que son mas influyentes en el crecimiento y la viabilidad poblacional. Combinamos datos empiricos y modelos matriciales estructurados por estadios para investigar las respuestas de las poblaciones de plantas nativas (I. hexagona) e invasoras (I. pseudacorus) a huracanes y estres por salinidad en humedales de agua dulce y salobre. En nuestros modelos, I. hexagona e I. pseudacorus, respondieron de manera diferente al estres por salinidad y la coexistencia de las especies fue rara. En 82% de las simulaciones en computadora del humedal dulceacuicola, las poblaciones invasoras excluyeron a las especies nativas en un plazo de 50 anos, mientras que las poblaciones nativas excluyeron a la especie invasora en 99% de las simulaciones de humedal salobre. La ocurrencia de huracanes permitio la coexistencia de las especies, y la persistencia de las especies estaba determinada por el tiempo que tomaba la recuperacion del ecosistema. La recuperacion rapida (2 anos) favorecio a la especie invasora, mientras que la recuperacion gradual (30 anos) favorecio a la especie nativa. Se sabe poco sobre los efectos de los huracanes sobre las interacciones competitivas entre especies de plantas nativas e invasoras en ecosistemas de marisma. Nuestros modelos aportan una nueva perspectiva de las relaciones entre la perturbacion ambiental y las invasiones y demuestran la influencia de factores abioticos, como el cambio climatico, en la determinacion de interacciones interespecificas. Author Affiliation: (*)Department of Biology, University of Louisiana at Lafayette, Lafayette, LA 70504, U.S.A. ([dagger])Department of Mathematics, University of Louisiana at Lafayette, Lafayette, LA 70504, U.S.A. Article History: Paper submitted December 31, 2007; revised manuscript accepted May 15, 2008. Article note: ([double dagger]) Address correspondence to S. Mopper, email mop@louisiana.edu

Research indicates that saturation does not necessarily produce species richness, and is not likely to be an accurate predictor of a plant population's invasion resistance. Correlations between invasion resistance and species richness is dependent on the cause of the richness gradient and the specific model considered.

The author suggests that increased use of short-term, flexible experiments and more extensive research on natural history will help improve methods for measuring levels of interspecific interactions within biological populations. Single numerical approaches are inadequate, as most interactions are nonlinear.

A model is presented for plant population dynamics of Cytisus scoparius, or broom, across prairie and urban fields at various stages of invasion. Results suggest that control of C. scoparius invasions requires the destruction of 70% of seeds in urban landscapes and more than 99.9% of sees in prairie landscapes.

We present an a priori theoretical framework for the interspecific allometric relationship between stand mass and plant population density. Our model predicts a slope of - 1/3 between the logarithm of stand mass and the logarithm of stand density, thus conflicting with a previously assumed slope of -1/2. Our model rests on a heuristic separation of resource-limited living mass and structural mass in the plant body. We point out that because of similar resource requirements among plants of different sizes, a nonzero plant mass-density slope is primarily defined by structural mass. Specifically, the slope is a result of (i) the physical size-dependent relationship between stem width and height, (ii) foliage-dependent demands of conductance, and (iii) the cumulative nature of structural mass. The data support our model, both when the potential sampling bias of taxonomic relatedness is accounted for and when it is not. Independent contrasts analyses show that observed relationships among variables are not significantly different from the assumptions made to build the model or from its a priori predictions. We note that the dependence of the plant mass-density slope on the functions of structural mass provides a cause for the difference from the zero slope found in the animal population mass-density relationship; for the most part, animals do not have a comparable cumulative tissue type.

Alien plants invade many ecosystems worldwide and often have substantial negative effects on ecosystem structure and functioning. Our ability to quantitatively predict these impacts is, in part, limited by the absence of suitable plant-spread models and by inadequate parameter estimates for such models. This paper explores the effects of model, plant, and environmental attributes on predicted rates and patterns of spread of alien pine trees (Pinus spp.) in South African fynbos (a mediterranean-type shrubland). A factorial experimental design was used to: (1) compare the predictions of a simple reaction-diffusion model and a spatially explicit, individual-based simulation model; (2) investigate the sensitivity of predicted rates and patterns of spread to parameter values; and (3) quantify the effects of the simulation model's spatial grain on its predictions. The results show that the spatial simulation model places greater emphasis on interactions among ecological processes than does the reaction-diffusion model. This ensures that the predictions of the two models differ substantially for some factor combinations. The most important factor in the model is dispersal ability. Fire frequency, fecundity, and age of reproductive maturity are less important, while adult mortality has little effect on the model's predictions. The simulation model's predictions are sensitive to the model's spatial grain. This suggests that simulation models that use matrices as a spatial framework should ensure that the spatial grain of the model is compatible with the spatial processes being modeled. We conclude that parameter estimation and model development must be integrated procedures. This will ensure that the model's structure is compatible with the biological processes being modeled. Failure to do so may result in spurious predictions. Key words: alien plants; biological invasions; disturbance; factorial simulation experiment; fynbos; individual-based models; Pinus; plant life history; reaction-diffusion models; scaling artifacts; sensitivity analysis; spatially explicit simulation models., INTRODUCTION The spread of invasive alien plants threatens the structure and functioning of many ecosystems worldwide (Drake et al. 1989). Unfortunately, syntheses of knowledge on biological invasions (Groves and Burdon [...]

Explanations of self-thinning in plant populations have focused on plant shape and packing. A dynamic model based on the structure of local interactions successfully reproduces the pattern and can be approximated to identify key parameters and relationships. The approach generates testable new explanations for differences between species and populations, unifies self-thinning with other patterns in plant population dynamics, and indicates why organisms other than plants can follow the law.

A study of demographic models of tree populations to investigate the effect of dispersal and fecundity on population diversity and structure shows that seed production and various parameters of fecundity namely, disturbance, maturation, longevity and thinning affect the population's reproductive success and structure. Populations with good dispersal ability are more sensitive to maturation age when disturbance is common.

We conducted an extensive simulation study to compare the performance of a large group of plotless density estimators (PDEs) to obtain clarification of their relative performance in a diversity of sampling situations. The PDEs studied included well-known ones from the literature plus some extensions and modifications introduced here. The simulations cover 96 combinations of 6 spatial patterns, 4 sample sizes, and 4 population densities. We made comparisons within classes of similar estimators, and we indicate the best-performing PDEs out of the complete set studied. Over all spatial patterns, the angle-order estimator with measurements to the third-closest individual in each quadrant had the lowest relative root-mean-squared error (RRMSE), followed by the same estimation method with measurements to the second closest individual in each quadrant. Also performing well were the variable area transect, the ordered distance estimator using the third closest individual, and an extension of the Kendall-Moran estimator that searches for the second nearest neighbor and pools search areas from all sample points. Opinions and recommendations are given as to which PDEs perform well enough and are practical enough to deserve strong consideration for use in the field.

A study of density dependence and patch dynamics in tropical rain forests presents four matrix models that incorporate density-dependent demographic rates and density-independent patch dynamics. The models applied to Cecropia obtusifolia, a tropical tree, indicate that the tree suffers negative density-dependent regulation in fecundity. The specification of a population model mediates evaluations pertaining to density-dependent mechanisms on population dynamics and structure.

1 A metapopulation neighbourhood model of an annual plant species in a harsh environment is derived. It includes the effects of seed survival, dormancy, competition, dispersal, spatial and temporal heterogeneity, and stochastic local extinctions. 2 The model contained the following assumptions: (i) the time step is 1 year; (ii) competition is density-dependent via a Hassell-type submodel; (iii) each local population is regulated separately within a hexagonal cell, 1 m across, and the cells form a tessellated array; (iv) dispersal is from a parent cell to its neighbours and declines exponentially with distance; (v) the harshness of the environment is a value set for each cell; spatial and temporal heterogeneity are introduced by altering this value; (vi) each local population is rounded to give an integer number of individuals at the end of each generation so that those with density below unity may become extinct, with a probability inversely proportional to density. 3 In spatially homogeneous and heterogeneous harsh environments with moderate levels of density-dependent competition, all populations persisted and dispersal aided metapopulation growth when the population growth rate was sufficiently large. When the population growth rate was smaller, dispersal increased the risk of, and speeded up the time to, metapopulation extinction. Metapopulation growth was faster when the heterogeneity was at a larger-scale spatial pattern. 4 Temporal heterogeneity had an adverse effect when added to the existing spatial heterogeneity; strongly dispersing metapopulations were affected only slightly, while those with only moderate or no dispersal became extinct. The more frequently the environment changed, the greater was the probability of extinction. 5 The model simulations in 2-4 were repeated for more-severe density-dependence. In the spatially homogeneous environment dispersal prevented metapopulation extinction, while lack of dispersal encouraged it. Dispersal aided metapopulation growth in the spatially heterogeneous unchanging environment. The addition of temporal heterogeneity had little effect on populations with strong dispersal but the effect on the other metapopulations was catastrophic. Keywords: annual, dispersal, metapopulation dynamics, model, seedbank Journal of Ecology 1993, 81, 453-463

1 We assess the use of simple, size-based matrix population models for projecting population trends for six coniferous tree species in the Sierra Nevada, California. We used demographic data from 16 673 trees in 15 permanent plots to create 17 separate time-invariant, density-independent population projection models, and determined differences between trends projected from initial surveys with a 5-year interval and observed data during two subsequent 5-year time steps. 2 We detected departures from the assumptions of the matrix modelling approach in terms of strong growth autocorrelations. We also found evidence of observation errors for measurements of tree growth and, to a more limited degree, recruitment. Loglinear analysis provided evidence of significant temporal variation in demographic rates for only two of the 17 populations. 3 Total population sizes were strongly predicted by model projections, although population dynamics were dominated by carryover from the previous 5-year time step (i.e. there were few cases of recruitment or death). Fractional changes to overall population sizes were less well predicted. Compared with a null model and a simple demographic model lacking size structure, matrix model projections were better able to predict total population sizes, although the differences were not statistically significant. Matrix model projections were also able to predict short-term rates of survival, growth and recruitment. Mortality frequencies were not well predicted. 4 Our results suggest that simple size-structured models can accurately project future short-term changes for some tree populations. However, not all populations were well predicted and these simple models would probably become more inaccurate over longer projection intervals. The predictive ability of these models would also be limited by disturbance or other events that destabilize demographic rates.

1 We evaluated geographical and climate distributions of 57 vascular plant genera that have disjunct distributions primarily restricted to south-eastern Asia (EAS) and south-eastern North America (ENA). Because clades of each genus in the two regions are considered sister taxa, differences in ecological relationships between the regions reflect independent evolutionary responses to environmental factors and allow a test of ecological conservatism or parallelism. 2 The disjunct genera were found to be distributed in 270 grid cells, 163 in EAS and 107 in ENA, each with 3.75[degrees] extent in latitude and longitude. Individual disjunct genera occupied larger areas and latitudinal ranges in eastern Asia than in eastern North America, which parallels the larger area and latitudinal range of suitable climate in eastern Asia. 3 For all EAS-ENA disjunct genera taken together, the midpoint latitude, northernmost latitude, latitude range and area occupied were all significantly correlated between the two regions. For woody genera, only the correlation for midpoint latitude was significant, whereas for herbaceous genera, all except midpoint latitude were significant. 4 A twinspan analysis partitioned the grid cells into four distinctive groups based on presence or absence of the disjunct genera. Grid cells representing these groups in EAS and ENA remained clustered when plotted on axes defined by floristic ordination (non-metric multidimensional scaling) or climate variables (discriminant analysis), showing that the disjunct genera have similar floristic relationships to each other within regions and respond, to a large degree, in parallel to climate variables. 5 The lack of correlation in distribution variables among woody genera between the continents in part reflects the greater propensity of eastern Asian woody disjunct genera to extend southwards into subtropical and tropical latitudes. 6 Although the disjunct genera exhibit significant conservatism in geographical and climate distribution, reflecting their common evolutionary ancestry, differences between regions in geography, physiography and climate have also acted to diversify environmental relationships between continents.

Abstract. Recently Don Ludwig has shown that calculations of extinction probabilities based on currently available data are often meaningless due to the large uncertainty accompanying the estimates. Here we address [...]

Pinus contorta var. latifolia and Picea engelmannii recruitment and mortality were reconstructed in 13 lower subalpine stands from age-height curves of live trees, saplings, and seedlings, and from cross-dated dead standing, fallen, and buried stems. Bole decomposition rates indicated the populations could only be reconstructed without bias during approximately the second half of a stand's history, irrespective of the stand's age. All stands traced similar recruitment and mortality patterns for the overlapping parts of their histories. Fire and understory regeneration cohorts were recognized based on risk histories. The lodgepole pine fire cohort established in the first decade after fire. Its mortality was high but decreasing in the first 10 yr, low and constant until thinning began at [is nearly equal] 25 yr, and increasing for the next [is nearly equal] 200 yr. The understory cohort recruitment started [is nearly equal] 15 yr after fire and its mortality was high, and constant with As with pine, Engelmann spruce recruited mostly in the first decade after fire, followed by low and sporadic recruitment for the next 200 yr. The mortalities of the Engelmann spruce fire and understory cohorts were constant, with the understory mortality slightly higher than the mortality of the fire cohort. As with pine, the chances of the understory reaching the canopy were small. Since the recruitment of both pine and spruce does not compensate for their mortality, the population is not self-reproducing. The population is not transient, however, because the calculated fire frequency of the area ensures burning within the life-span of the fire cohort, allowing regeneration. Consequently the long-term population dynamics of pine and spruce appear to be controlled by fire frequency.

We formulate a single-species model to study the dynamics of a metapopulation that is composed of several subpopulations spread among distinct patches. In the case where extinction rates within a patch are a decreasing function of population size, and all disasters lead to extinction of the local population, we determine the equilibrium distribution of numbers within patches. We show that any equilibrium is stable. This model is also applicable to some systems of mutualists, such as some plant-pollinator systems, where the focus is on the dynamics of the plant species. Unlike models for obligate mutualism that do not incorporate spatial structure, our patch model always has a stable equilibrium.

Intervals between fires, tree blowdowns, and other large and small forest disturbances are often estimated by dating tree rings. Dates are taken from living trees, fallen logs or stumps, or other indicators such as peaks in tree age distributions and ages of understory plants damaged by treefalls. Such indicators disappear gradually over time; this will bias estimates of disturbance recurrence times or tree lifetimes. Models and simulations employing plausible rates of distrubance and indicator disappearance show that the bias can be substantial. Comparison of a realistic simulation with simpler models suggests that it may not be difficult to correct the bias, given estimates for indicator survival rate. I recommend that investigators estimating forest-distrubance recurrence times also estimate survival rates for their indicators, and collaborate with statisticians (about estimating survival times with censored data) before designing field studies.

We determined the changes in size inequality and asymmetry that occurred over a 50-yr period in 16 even-aged, naturally seeded populations of Pinus taeda. Initial densities ranged from 618 to 28 861 stems/ha. We used these data to test four predicted trends in size structure. We found that (1) in young stands size inequality was greater at higher plant densities, (2) skewness of tree volumes was significantly greater at higher initial densities, whereas skewness of diameters was not, (3) size inequality increased significantly prior to the onset of self-thinning, then decreased during self-thinning, and (4) skewness did not increase before self-thinning or significantly decreased during self-thinning. Numerous measures of size distribution have been employed to assess competition in plant populations, including various measures of inequality and skewness, as well as parameters of fitted distributions such as the Weibull distribution. We found the two-parameters of fitted distributions such as the Weibull distribution. We found the two-parameter Weibull distribution only moderately effective at capturing the forms of the distributions observed. Correlations showed very close linear relationships among the Gini coefficient, the coefficient of variation, and the shape parameter of the Weibull distribution -- both within and across different measures of tree size. All three statistics were correlated with initial density. Skewness coeffficients for distributions of diameter, height, and volume were not closely correlated with each other or with other population characteristics. We recommend measures of inequality as more suitable for summarizing biologically important properties of tree populations than measures of skewness or skewness-sensitive paramters of probability distributions.