Mark R. Fulton

Sapling growth and survivorship as a function of light in a mesic forest of southeast Texas, USA

Abstract. For seven species in a mature mesic forest in southeast Texas, we estimated species-specific parameters representing radial growth in high light and low light for tree saplings. Shade-intolerant species had higher asymptotic growth rates and lower low-light growth than tolerant species. Inspection of species positions on graphs of low-light growth versus high-light growth suggested that there was a trade-off between these two processes across species. By linking functions of growth versus light and mortality versus growth, we also found that shade-intolerant species had higher mortality risk at low light and stronger sensitivity of mortality to light than shade-tolerant species. Moreover, we found that low-light survival and high-light growth were negatively correlated across species. In contrast to northern hardwood forests, where sapling survival in low light may be achieved at the expense of growth, our results suggested that shade-tolerant species in this southern mixed forest can grow faster as well as survive better than shade-intolerant species in low light. We conclude that both sapling growth and survival are important components of shade tolerance and their relationships may be system-specific.

A computationally efficient forest succession model: Design and initial tests

Abstract A computationally efficient forest-stand simulation model, using a concise set of state variables, is described. The model (called FLAM: Forest LAyer Model) simulates the interaction of height-class-structured tree populations on a small patch, in contrast to gap models of the JABOWA/FORET type, which model individuals. FLAM has been derived from a generalized gap model, FORSKA, by two approximations: (1) all trees of a species in a given height class have the same stem volume, leaf area, and growth increment; and (2) the distribution of tree heights within a class is uniform. Promotion from one height class to another is simulated as a binomially distributed random variable; the probability of promotion is the ratio of the predicted height growth to the depth of the height class. Heightclass distributions and leaf-area-density profiles generated by FLAM were 80% and 93% similar (respectively) to profiles generated by FORSKA, but FLAM ran in 5% of the c.p.u. time. The effect of variations in spatial resolution (number of height classes) and temporal resolution (number of years per time step) were tested by comparing leaf-area-density profiles from FORSKA and FLAM. The performance of FLAM was insensitive to temporal and spatial resolution over a wide range of resolutions (1–5-year time-step, 4–20 height classes). Performance deteriorated if the temporal or spatial resolution was coarser.

Sapling growth and survivorship as affected by light and flooding in a river floodplain forest of southeast Texas

We investigated the effects of light and flooding on growth and survivorship of saplings in a river floodplain forest of southeast Texas. Growth responses to light were consistent with the expectation that shade-intolerant species grow faster than shade-tolerant species in high light, and vice versa. Mortality risk was not associated with shade tolerance level unless high mortality risks associated with a period of high flooding were removed. These results support the hypothesis that shade-tolerant species in floodplains may be limited by flooding as previous studies suggested. Also, compared to their performance at a nearby mesic site, common species showed little intraspecific difference in shade tolerance, especially for shade-intolerant species. Finally, the positive correlation between low-light growth and survivorship suggests that carbon allocation to continued growth may be favored as a sapling strategy in floodplains.

Adult recruitment as a function of juvenile growth rate in size- structured plant populations

Summary. The recruitment of adults in size-structured plant populations will be sensitive to changes in juvenile growth rates when: 1) mortality is high, 2) growth rates are low, 3) the difference between seedling and adult size is large and 4) selfthinning among juvenile plants is negligible. This sensitivity can be quantified by a single parameter in a simple, general expression relating recruitment to growth. The model is demonstrated for the prediction of young spruce tree densities on small plots, and the calculation of the parameter is shown for two published demographic studies. The model can also predict features of population response to an environmental gradient, such as long tails or abrupt cut-offs, even when these features are not found in the physiological response to the gradient. The rate of escape from the vulnerability of small size is a critical factor in the life cycle of many plant species. Recruitment into the adult population may depend on many factors such as seed rain, germination rates, local environmental heterogeneity and episodic herbivory; but, in populations where the mortality rate is concentrated in the early stages of growth, adult recruitment may be highly sensitive to differences in growth rate. Some forest dynamic simulation models (Pastor and Post 1985) assume a linear relationship between growing conditions in the understory and recruitment into the adult tree population, but the form of this relationship, while plausible, is arbitrary. The purpose of this paper is to derive a simple relationship between the growth and mortality rates of juvenile plants and the rate of recruitment into the adult population. I apply the model to a field study of young spruce tree densities, and two published demographic studies of long-lived perennials.

FINE-SCALE PREDICTABILITY OF FOREST COMMUNITY DYNAMICS

We assayed the predictability of forest community dynamics by examining the internal structure of a 10-12-yr record of forest change for three sites in east Texas, USA. Within each site, the growth and fate of all adult tree stems within many small (0.04 ha) patches have been monitored. These data were reduced to community composition (basal area by species) for each of 28-30 spatially independent patches within each site. We measured predictability of compositional change by the rank correlation of (1) distances between vectors of initial composition vs. (2) distances between vectors of compositional change. Vectors of compositional change were predictable at two of the three sites; the third site was an old forest with somewhat high mortality of large adult stems. The data also allowed decomposition of the change vectors into components due to growth, mortality, and adult tree recruitment. Growth vectors were highly predictable from the initial com- position, but mortality and adult tree recruitment were not. The results support the idea that growth processes in forests are closely tied to interactions among adjacent trees, whereas recruitment and mortality have components that operate at different spatial or temporal scales and so are empirically decoupled from growth.

Invertebrate Community Patterns in Seasonal Ponds in Minnesota, USA: Response to Hydrologic and Environmental Variability

Seasonal ponds are common throughout forested regions of the north central United States. These wetlands typically flood due to snow-melt and spring precipitation, then dry by mid-summer. Periodic drying produces unique fishless habitats with robust populations of aquatic invertebrates. A basin’s physical/chemical features, the absence of vertebrate predation, and especially the duration of seasonal flooding, have long been viewed as the major structuring influences on these communities, but previous studies have shown only limited effects of environmental variables on pond invertebrates. Applying ordination methods to data from weekly collections of invertebrates during 2008–2009, we tested influences of site-level environmental gradients on the presence and relative abundance of aquatic invertebrate communities in 16 seasonal ponds in a forested region of north central Minnesota, USA. We assessed invertebrate community patterns in relation to pond size and depth, soil nutrients, canopy closure, hydroperiod, and predominant groundwater function (recharge, discharge, or flow-through). Patterns in pond invertebrate community composition were consistently related to pond depth, overhead canopy closure, and hydroperiod. Site-level hydrologic function showed weak relationships to seasonal patterns of invertebrate abundance. Although physical features of ponds had only modest influence on presence and abundance of invertebrates, weekly sampling improved models relating environmental variables to pond invertebrates.

Edaphic controls on the boreonemoral forest mosaic

Field data and simulations were used to investigate possible edaphic causes of patchy dominance by conifers and temperate deciduous trees in the boreonemoral transition zone of east-central Sweden. Tree species, basal area and soil conditions were recorded for 39 plots of 10 m radius on a ca 30-ha uninhabited island in Lake Mllaren. Canonical correspondence analysis produced a single dominant axis of variation in species composition related to soil depth, pH, organic matter and texture. The forests on the island included most types found in the surrounding region. The simulation model links patch-scale tree demography to the effects of trees on the light environment, with tree growth rates further influenced by climatic variables, soil moisture and nitrogen availability. Soil water holding capacity and nitrogen availability were varied systematically in a model experiment. The simulated response surfaces of tree species accord with reality. When each field site was matched to the most similar model treatment, the observed soil conditions of the field sites were strongly correlated with the modelled nitrogen-availability gradient. These results are consistent with a systems approach to vegetation dynamics that incorporates interactions between species composition and nitrogen-availability with climate, soil conditions

An application of fuzzy set ordination to determine tree habitat suitability of sites from a regional data set

. Fuzzy set ordination is employed to evaluate sites on the basis of their suitability for particular tree species. The technique orders sites along an axis defined by the presences and absences of a given species of interest. A rationale is given in terms of noise reduction; in many situations the overall vegetation of a site will reflect habitat conditions better than the presence, absence, or quantitative performance of any single species. A data set of tree presence/absence covering a large part of the southeastern United States was analyzed and habitat suitability scores were calculated for each species. Monte-Carlo tests were used to measure the statistical power of the data set with regard to habitat preferences; 38 of the 49 species have cumulative frequency distributions showing significant departures from random expectation. Most statistically significant habitat preferences seem to be related to geographic range limits located within the study area, but some species found throughout the area also show significant departures from random expectation. The method may find applications in autecological studies of species, selection of representative site conditions for simulation modeling, and the solution of certain technical problems in ordination.

Simulation modeling of the effects of site conditions and disturbance history on a boreal forest landscape

. Computer simulations were used to elaborate hypotheses about controls on forest structure and composition in a 0.7 km ≤ area of boreal forest in Central Sweden. DBH and species of all adult trees and stand conditions were recorded for 57–10 m radius plots. Ordination of these data suggested that nutrient-availability and time-since-disturbance were the main controls of forest composition and structure within the area. The simulation model couples equations representing the effect of tree canopy structure and biomass on light and soil conditions with equations representing the effect of these conditions on reproduction, growth and mortality in height cohorts of trees on a 0.1 ha patch. Nitrogen-availability levels for each modeled plot were simulated by species-specific growth multipliers. The model was run for 400 simulated yr at six levels of N availability. Age and N status of each study plot were inferred by matching with the most similar model output. Inferred ages agreed with what is known of the disturbance history, and site factors related to soil fertility were correctly correlated with the inferred N status. The consequences of size-selective disturbance were explored by model experiments. Biomass was removed from large or small size classes at 100 - 200 yr and the simulations were run for an additional 300 yr. Disturbed stands of high N status often became similar to undisturbed stands of different N status. Size-selective disturbances produced stands that were different from any in the undisturbed succession, but these differences disappeared within 50 - 100 yr, implying successional convergence in stand structure and composition. Plots of simulated basal area against time and nitrogen-availability for the four species illustrate the time dependence of species performance along a fertility gradient.