Brian Beckage

Interpreting recruitment limitation in forests

Studies of tree recruitment are many, but they provide few general insights into the role of recruitment limitation for population dynamics. That role depends on the vital rates (transitions) from seed production to sapling stages and on overall population growth. To determine the state of our understanding of recruitment limitation we examined how well we can estimate parameters corresponding to these vital rates. Our two-part analysis consists of (1) a survey of published literature to determine the spatial and temporal scale of sampling that is basis for parameter estimates, and (2) an analysis of extensive data sets to evaluate sampling intensity found in the literature. We find that published studies focus on fine spatial scales, emphasizing large numbers of small samples within a single stand, and tend not to sample multiple stands or variability across landscapes. Where multiple stands are sampled, sampling is often inconsistent. Sampling of seed rain, seed banks, and seedlings typically span <1 yr and rarely last 5 yr. Most studies of seeding establishment and growth consider effects of a single variable and a single life history stage. By examining how parameter estimates are affected by the spatial and temporal extent of sampling we find that few published studies are sufficiently extensive to capture the variability in recruitment stages. Early recruitment stages are especially variable and require samples across multiple years and multiple stands. Ironically, the longest duration data sets are used to estimate mortality rates, which are less variable (in time) than are early life history stages. Because variables that affect recruitment rates interact, studies of these interactions are needed to assess their full impacts. We conclude that greater attention to spatially extensive and longer duration sampling for early life history stages is needed to assess the role of recruitment limitation in forests.

A rapid upward shift of a forest ecotone during 40 years of warming in the Green Mountains of Vermont

Detecting latitudinal range shifts of forest trees in response to recent climate change is difficult because of slow demographic rates and limited dispersal but may be facilitated by spatially compressed climatic zones along elevation gradients in montane environments. We resurveyed forest plots established in 1964 along elevation transects in the Green Mountains (Vermont) to examine whether a shift had occurred in the location of the northern hardwood–boreal forest ecotone (NBE) from 1964 to 2004. We found a 19% increase in dominance of northern hardwoods from 70% in 1964 to 89% in 2004 in the lower half of the NBE. This shift was driven by a decrease (up to 76%) in boreal and increase (up to 16%) in northern hardwood basal area within the lower portions of the ecotone. We used aerial photographs and satellite imagery to estimate a 91- to 119-m upslope shift in the upper limits of the NBE from 1962 to 2005. The upward shift is consistent with regional climatic change during the same period; interpolating climate data to the NBE showed a 1.1°C increase in annual temperature, which would predict a 208-m upslope movement of the ecotone, along with a 34% increase in precipitation. The rapid upward movement of the NBE indicates little inertia to climatically induced range shifts in montane forests; the upslope shift may have been accelerated by high turnover in canopy trees that provided opportunities for ingrowth of lower elevation species. Our results indicate that high-elevation forests may be jeopardized by climate change sooner than anticipated.

Density-dependent mortality and the latitudinal gradient in species diversity.

Ecologists have long postulated that density-dependent mortality maintains high tree diversity in the tropics. If species experience greater mortality when abundant, then more rare species can persist. Agents of density-dependent mortality (such as host-specific predators, and pathogens) may be more prevalent or have stronger effects in tropical forests, because they are not limited by climatic factors. If so, decreasing density-dependent mortality with increasing latitude could partially explain the observed latitudinal gradient in tree diversity. This hypothesis has never been tested with latitudinal data. Here we show that several temperate tree species experience density-dependent mortality between seed dispersal and seedling establishment. The proportion of species affected is equivalent to that in tropical forests, failing to support the hypothesis that this mechanism is more prevalent at tropical latitudes. We further show that density-dependent mortality is misinterpreted in previous studies. Our results and evidence from other studies suggest that density-dependent mortality is important in many forests. Thus, unless the strength of density-dependent mortality varies with latitude, this mechanism is not likely to explain the high diversity of tropical forests.

SEEDLING SURVIVAL AND GROWTH OF THREE FOREST TREE SPECIES: THE ROLE OF SPATIAL HETEROGENEITY

Spatial heterogeneity in microenvironments may provide unique regeneration niches for trees and may promote forest diversity. We examined how heterogeneity in understory cover, mineral nutrients, and moisture and their interactions with canopy gaps contribute to the coexistence of three common, co-occuring tree species. We measured survival and height growth of 1080 seedlings of Acer rubrum (red maple), Liriodendron tulipifera (yellow poplar), and Quercus rubra (red oak) that were planted in one of five understory treatments: removal of understory vegetation, trenched, trenched plus removal of understory vegetation, fertilization, and a control. Understory treatments were replicated in 12 paired gap and canopy environments. Survivorship varied among species, with Q. rubra having the highest probability of surviving beyond the 1135-day experiment (probability = 0.64), followed by A. rubrum (probability = 0.27) and L. tulipifera (probability = 0.07). Although canopy gaps and understory treatments had large effects on survivorship, species survival rankings changed little across microenvironments; Q. rubra had the highest survival in all microenvironments. In contrast to survival, L. tulipifera had a relative growth rate for height that was three times greater than that of A. rubrum and Q. rubra in high-resource microenvironments. There was broad overlap among species in relative growth rates in the remaining seven microenvironments, with no clear top-ranked species. Differences in seedling growth and survival across these 10 microenvironments may contribute to the coexistence of two of the three species studied, L. tulipifera and Q. rubra, but not A. rubrum. Q. rubra had higher survival than A. rubrum and L. tulipifera in all microenvironments, but L. tulipifera tended to grow faster than A. rubrum and Q. rubra in high-resource microenvironments. Despite the generally poor performance of A. rubrum, it was the only surviving species in some quadrats at the end of the experiment, indicating that stochastic effects, in conjunction with broad niche overlap, may also contribute to species coexistence. The importance of stochastic effects will probably increase when differential fecundity across these three species is considered because the high fecundity of A. rubrum offsets survival and growth disadvantages of its seedlings through their greater total abundance.

INFLUENCE OF THE EL NIÑO SOUTHERN OSCILLATION ON FIRE REGIMES IN THE FLORIDA EVERGLADES

Disturbances that are strongly linked to global climatic cycles may occur in a regular, predictable manner that affects composition and distribution of ecological com- munities. The El Nino Southern Oscillation (ENSO) influences worldwide precipitation patterns and has occurred with regular periodicity over the last 130 000 years. We hypoth- esized that ENSO, through effects on local weather conditions, has influenced frequency and extent of fires within Everglades National Park (Florida, USA). Using data from 1948 to 1999, we found that the La Nina phase of ENSO was associated with decreased dry- season rainfall, lowered surface water levels, increased lightning strikes, more fires, and larger areas burned. In contrast, the El Nino phase was associated with increased dry-season rainfall, raised surface water levels, decreased lightning strikes, fewer fires, and smaller areas burned. Shifts between ENSO phases every few years have likely influenced vegetation through periodic large-scale fires, resulting in a prevalence of fire-influenced communities in the Everglades landscape.

Vegetation, fire, and feedbacks: a disturbance-mediated model of savannas.

Savanna models that are based on recurrent disturbances such as fire result in nonequilibrium savannas, but these models rarely incorporate vegetation feedbacks on fire frequency or include more than two states (grasses and trees). We develop a disturbance model that includes vegetation‐fire feedbacks, using a system of differential equations to represent three main components of savannas: grasses, fire‐tolerant savanna trees, and fire‐intolerant forest trees. We investigate the stability of savannas in the presence of positive feedbacks of fire frequency with (1) grasses, (2) savanna trees, and (3) grasses and savanna trees together while also allowing for negative feedbacks of forest trees on fire frequency. We find that positive feedbacks between fire frequency and savanna trees, alone or together with grasses, can stabilize savannas, blocking the conversion of savannas to forests. Negative feedbacks of forest trees on fire frequency shift the range of parameter space that supports savannas, but they do not generally alter our results. We propose that pyrogenic trees that modify characteristics of fire regimes are ecosystem engineers that facilitate the persistence of savannas, generating both threshold fire frequencies with rapid changes in community composition when these thresholds are crossed and hystereses with bistable community states.

INTERACTIONS OF LARGE-SCALE DISTURBANCES: PRIOR FIRE REGIMES AND HURRICANE MORTALITY OF SAVANNA PINES

Differences in initial large-scale disturbances might change effects of sub- sequent large-scale disturbances. We explored possible effects of prior fire regimes on subsequent hurricane-related mortality of south Florida slash pine ( Pinus elliottii var. densa) in remnant Everglades pine savannas that were unburned, burned during the wet (lightning fire) season, or burned during the dry (anthropogenic fire) season in the decade before Hurricane Andrew (1992). We measured direct mortality during Andrew (snapped trees) and extended mortality over the subsequent 24-30 mo (mainly insect attacks on damaged trees). We used Bayesian model averaging to obtain probabilities of different models of survival based on fire regime and site characteristics (remnant area, distance to the Atlantic Ocean, depth to water table in the dry season, sustained wind speeds, tree sizes). Most likely models for direct and extended mortality included large negative effects of tree size and dry-season fire regime, and positive effects of stand area (direct mortality) and wet- season fire regime (extended mortality). Depth to water table and distance to the ocean had less certain effects. Our results, not predicted from fires or hurricanes alone, suggest that anthropogenic changes to dry-season fires strongly influence the effects of subsequent hurricanes on the mortality of pines in subtropical savannas.

Fire feedbacks facilitate invasion of pine savannas by Brazilian pepper (Schinus terebinthifolius)

* Fire disturbance can mediate the invasion of ecological communities by nonnative species. Nonnative plants that modify existing fire regimes may initiate a positive feedback that can facilitate their continued invasion. Fire-sensitive plants may successfully invade pyrogenic landscapes if they can inhibit fire in the landscape. * Here, we investigated whether the invasive shrub Brazilian pepper (Schinus terebinthifolius) can initiate a fire-suppression feedback in a fire-dependent pine savanna ecosystem in the southeastern USA. * We found that prescribed burns caused significant (30-45%) mortality of Brazilian pepper at low densities and that savannas with more frequent fires contained less Brazilian pepper. However, high densities of Brazilian pepper reduced fire temperature by up to 200 degrees C, and experienced as much as 80% lower mortality. * A cellular automaton model was used to demonstrate that frequent fire may control low-density populations, but that Brazilian pepper may reach a sufficient density during fire-free periods to initiate a positive feedback that reduces the frequency of fire and converts the savanna to an invasive-dominated forest.

The limits to prediction in ecological systems

Predicting the future trajectories of ecological systems is increasingly important as the magnitude of anthropogenic perturbation of the earth systems grows. We distinguish between two types of predictability: the intrinsic or theoretical predictability of a system and the realized predictability that is achieved using available models and parameterizations. We contend that there are strong limits on the intrinsic predictability of ecological systems that arise from inherent characteristics of biological systems. While the realized predictability of ecological systems can be limited by process and parameter misspecification or uncertainty, we argue that the intrinsic predictability of ecological systems is widely and strongly limited by computational irreducibility. When realized predictability is low relative to intrinsic predictability, prediction can be improved through improved model structure or specification of parameters. Computational irreducibility, however, asserts that future states of the system cannot be derived except through computation of all of the intervening states, imposing a strong limit on the intrinsic or theoretical predictability. We argue that ecological systems are likely to be computationally irreducible because of the difficulty of pre-stating the relevant features of ecological niches, the complexity of ecological systems and because the biosphere can enable its own novel system states or adjacent possible. We argue that computational irreducibility is likely to be pervasive and to impose strong limits on the potential for prediction in ecology.

Predicting severe wildfire years in the Florida Everglades

Wildfires result in important ecological benefits to many ecosystems, but have costs associated with fire fighting and property loss. Accurate, timely forecasts of the severity of upcoming wildfire seasons could facilitate wildfire management, limiting the most destructive aspects of fires, while preserving their ecological benefits. We demonstrate an approach where time series models are used to predict the severity of the wildfire season in Everglades National Park in southern Florida 3 months and 1 year beforehand. Model predictions contained all obserations within a 90% credible interval and also anticipated severe wildfire seasons. These models may be used to implement more ecologically sound wildfire management.