John A. Silander

Seedling Recruitment in Forests: Calibrating Models to Predict Patterns of Tree Seedling Dispersion

Recruitment, the addition of new individuals into a community, is an im- portant factor that can substantially affect community composition and dynamics. We present a method for calibrating spatial models of plant recruitment that does not require identifying the specific parent of each recruit. This method calibrates seedling recruitment functions by comparing tree seedling distributions with adult distributions via a maximum likelihood analysis. The models obtained from this method can then be used to predict the spatial distributions of seedlings from adult distributions. We calibrated recruitment functions for 10 tree species characteristic of transition oak- northern hardwood forests. Significant differences were found in recruit abundances and spatial distributions. Predicted seedling recruitment limitation for test stands varied sub- stantially between species, with little recruitment limitation for some species and strong recruitment limitation for others. Recruitment was limited due to low overall recruit production or to restricted recruit dispersion. When these seedling recruitment parameters were incorporated into a spatial, individual-based model of forest dynamics, called SOR- TIE, alterations of recruitment parameters produced substantial changes in species abun- dance, providing additional support for the potential importance of seedling recruitment processes in community structure and dynamics.

Neighborhood Models of Plant Population Dynamics. I. Single-Species Models of Annuals

We present tractable formulations for neighborhood models of annual plant population dynamic processes. These models are constructed from submodels, termed predictors, of individual plants. Fecundity and survivorship predictors give the fecundity and survivorship of an individual as a function of local population density. Dispersal predictors predict the dispersal pattern of a plants maternal progeny and the survivorship of plants from seed to seedlings. We develop both computer models and analytically tractable models. Our computer models are designed to determine the population dynamic consequences of specific fecundity, survivorship and dispersal predictors. The analytical models are valid when dispersal is sufficiently large, and are used to explain the predictions of analogous computer models. We show through examples that the predictions of corresponding computer and analytical models may be virtually identical. Empirical tests of these models are practical because all model parameters and functional forms can be estimated with data obtained in a single year. We describe the experiments and statistical methods used to test a neighborhood model. Finally, we describe how neighborhood models can be used to address specific issues in plant ecology and discuss possible extensions of neighborhood theory.

BUILDING STATISTICAL MODELS TO ANALYZE SPECIES DISTRIBUTIONS

Models of the geographic distributions of species have wide application in ecology. But the nonspatial, single-level, regression models that ecologists have often employed do not deal with problems of irregular sampling intensity or spatial dependence, and do not adequately quantify uncertainty. We show here how to build statistical models that can handle these features of spatial prediction and provide richer, more powerful inference about species niche relations, distributions, and the effects of human disturbance. We begin with a familiar generalized linear model and build in additional features, including spatial random effects and hierarchical levels. Since these models are fully specified statistical models, we show that it is possible to add complexity without sacrificing interpretability. This step-by-step approach, together with attached code that implements a simple, spatially explicit, regression model, is structured to facilitate self-teaching. All models are developed in a Bayesian framework. We assess the performance of the models by using them to predict the distributions of two plant species (Proteaceae) from South Africas Cape Floristic Region. We demonstrate that making distribution models spatially explicit can be essential for accurately characterizing the environmental response of species, predicting their probability of occurrence, and assessing uncertainty in the model results. Adding hierarchical levels to the models has further advantages in allowing human transformation of the landscape to be taken into account, as well as additional features of the sampling process.

Neighborhood predictors of plant performance

SummaryWe developed models of inter-individual interference to predict the fecundity of individuals in populations of the annual plant species, Arabidopsis thaliana. An individual plant is modeled as having a neighborhood which is a circular area of fixed radius with the plant at its center. Other plants which share the circle with the focal plant are termed neighbors of the focal plant. We developed an index of neighborhood interference which is the independent variable in a non-linear regression model that predicts individual plant fecundity. We present methods of exploratory data analysis that are useful in determining a best neighborhood radius, defined as that radius which minimizes residual sum of squares, and in deciding on the functional form of the interference index. In developing the interference index for Arabidopsis, we focus on aspects of the spatial distribution of neighbors: their number, distance and angular dispersion.We found that a best (or optimal) neighborhood radius can be resolved, which provides the best predictor of plant performance. Fecundity predictors based on adult neighbors were noticeably better than those based on neighbors at the seedling stage. Rosettes of Arabidopsis may change location during development (they fall over) and the new “fallen” positions do provide some improvement in the predictor. Taking into account distance to neighbors within the neighborhood provided only negligible improvement in the model. Finally, the incorporation of angular dispersion in the crowding index produced a considerably better fit. The fecundity predictor that included number of neighbors and angular dispersion in the crowding index explained about 70% of the variation in individual seed set.

Field Tests of Neighborhood Population Dynamic Models of Two Annual Weed Species

We report the results of a 4—yr study of the community dynamics of the annual weed species Abutilon theophrasti (velvet leaf) and Amaranthus retroflexus (pigweed). We calibrated neighborhood population dynamic models for communities of these species in the field and then tested the predictions of the calibrated models against census data from independent observations. We also analyzed the calibrated and tested models to predict long—term dynamics and to assess how spatially local interactions, growth, fecundity, survivorship, germination, seed dormancy, and dispersal each contribute to the communitys dynamics and structure. We show that calibrated neighborhood models accurately predict dynamics in the field over a 4—yr period. Because the predictive spatial models reduce approximately to simple nonspatial models of competition, the spatial processes that govern the dynamics of velvet leaf and pigweed communities behave as simple nonspatial processes. The models predict that velvet leaf will eventually exclude pigweed because of an asymmetry in the magnitude of inter—individual interference affecting growth. We also show that velvet leaf monocultures would oscillate perpetually in the absence of delayed germination and that demographic stochasticity (May 1971) has little effect on the dynamics of the experimental species.

The Invasion Ecology of Japanese Barberry (Berberis thunbergii) in the New England Landscape

Japanese barberry (Berberis thunbergii) has been characterized as one of the most widely known and planted exotic shrubs in the United States. It was first introduced to the US in the late 1800s. By 1920 the planting of Japanese barberry was encouraged as an ornamental shrub replacing the common barberry (Berberis vulgaris). Japanese barberry began spreading from cultivation in suburban and selected rural retreats by the 1920s, and had dispersed rapidly throughout the northeast by the 1960s. By the 1970s it was recognized as a problematic invasive in the northeast. It is readily dispersed primarily by birds. Fruit production varies with light level, but even under very low light levels (≤4% full sun) some seeds are produced. Fruits are dispersed in late fall through late winter. Seed dispersal curves are highly leptokurtic; most seedling are found under or adjacent to adults, but a small number may be found tens of meters from the nearest adult. Japanese barberry thrives under a broad range of light and soil moisture conditions. Significant variation in stem growth can be explained as a function of light level. Even at less than 1% full sun, some positive stem growth can occur. Survival is quite high at intermediate to high light levels, and only under the lowest light levels (<1% full sun) does survival drop significantly. Biomass of Japanese barberry in field plots can be largely explained as a function of light availability and soil moisture. The biomass of co-occurring species is suppressed by Japanese barberry, and recovery is slow in the first year following Japanese barberry removal except under high light levels. Glyphosate (Roundup) applied in early spring at first leaf out, when little else is in leaf, provides an effective means of eradicating Japanese barberry populations.

Microevolution and Clone Structure in Spartina patens.

Analysis of the clone structures within a population of Spartina patens reveals considerable adaptive genetic divergence among adjacent dune, swale, and marsh sites. The dune subpopulation includes a small number of frequently encountered, spatially isolated clones that follow microtopography and have high reproductive output and colonizing potential. The marsh subpopulation consists of a large number of infrequent interdigitating clones with high vegetative biomass and competitive success. The swale subpopulation is generally intermediate for these traits.

Neighborhood interference among velvet leaf, Abutilon theophrasti, and pigweed, Amaranthus retroflexus

Spatially local interference among pigweed (Amaranthus retroflexus) and velvet leaf (Abutilon theophrasti) individuals was studied in experimental two species stands. Although survivorship was density-independent for both species, individual fecundities were affected by the local population density of velvet leaf and pigweed near each plant. We estimated the functional forms of the spatially local density dependence and the distances over which plants interfere with one another. For both species hyperbolic functions provided better fits than either linear or log-linear functions. The apparent significance of higher-order interaction terms from the linear and log-linear regressions appears to be an artifact of the hyperbolic form of the data. We explain aspects of the functional forms of the relations between fecundity and local densities in terms of morphological attributes of each species. We also describe some statistical problems associated with estimating the effects of spatially local interindividual interference.

Ten Principles for Biocultural Conservation at the Southern Tip of the Americas: The approach of the Omora Ethnobotanical Park

This article discusses ten principles for biocultural conservation at the southern tip of the Americas. The article focuses on a case study at the Omora Ethnobotanical Park in Chile.

Changing lenses to assess biodiversity: patterns of species richness in sub-Antarctic plants and implications for global conservation

Article discussing patterns of species richness in sub-Antarctic plants and implications for global conservation.