Tim Nuttle

Historic disturbance regimes promote tree diversity only under low browsing regimes in eastern deciduous forest

Eastern deciduous forests are changing in species composition and diversity outside of classical successional trajectories. Three disturbance mechanisms appear central to this phenomenon: fire frequency is reduced, canopy gaps are smaller, and browsers are more abundant. Which factor is most responsible is a matter of great debate and remains unclear, at least partly because few studies have simultaneously investigated more than one process. We conducted a large-scale experiment in mesophytic forests of West Virginia, USA, to test three key hypotheses: (1) the fire hypothesis (fire suppression limits diversity to few shade-tolerant, fire-intolerant species that replace and suppress many fire-tolerant species); (2) the gap hypothesis (small gaps typical of todays forests promote dominance of a few shade-tolerant species); and (3) the browsing hypothesis (overbrowsing by deer limits diversity to a few unpalatable species). We tested these hypotheses using a factorial experiment that manipulated surface fire, large canopy gap formation (gap size ;255 m 2 ), and browsing by deer, and we followed the fates of .28 000 seedlings and saplings for five years. Understory tree communities in control plots were dominated (up to 90% )b yFagus grandifolia, averaging little more than two species, whereas overstories were diverse, with 10-15 species. Fire, large canopy gaps, and browsing all dramatically affected understory composition. However, our findings challenge views that fire and large canopy gaps can maintain or promote diversity, because browsers reduced the benefits of gaps and created depauperate understories following fire. Consequently, two major disturbances that once promoted tree diversity no longer do so because of browsing. Our findings appear to reconcile equivocal views on the role of fire and gaps. If browsers are abundant, these two disturbances either depress diversity or are less effective. Alternatively, with browsers absent, these disturbances promote diversity (three- to fivefold). Our results apply to large portions of eastern North America where deer are overabundant, and we provide compelling experimental evidence that historical disturbance regimes in combination with low browsing regimes typical of pre-European settlement forests could maintain high tree species diversity. However, restoring disturbances without controlling browsing may be counterproductive.

Long-term biological legacies of herbivore density in a landscape-scale experiment: forest understoreys reflect past deer density treatments for at least 20 years

Summary 1. Ungulate browsers, when at high densities, are major drivers of vegetation change in forests world-wide. Their effects operate via a variety of generalizable mechanisms related to plant palatability and relative growth rate with respect to browsing pressure. 2. Though such impacts are obviously long-lasting when they determine composition of tree regeneration, we document in a unique long-term (30 year) experiment that biological legacies of initial deer density persist in the understorey herbaceous vegetation at least 20 years after deer densities were equalized. 3. We sampled understorey vegetation in former clear-cut areas where density of white-tailed deer (Odocoileus virginianus) was manipulated (3.9–31.2 deer km 2 ) for 10 years (1979–1990), and stands experienced ambient deer density (ca. 10–12 deer km 2 ) for the next 20 years (1990–2010) to determine whether initial deer density treatments still influenced understorey vegetation in 30-year-old, closed-canopy forests. 4. Stands initially (1979–1990) exposed to higher deer densities had ca. five times higher fern cover and three times the seedling and forb cover in 2010, as well as significantly lower angiosperm species density, compared to stands initially exposed to lower deer densities. 5. These results appear driven by deer avoidance of ferns, allowing them to expand at high deer density and sequester sites for decades. 6. Synthesis. Our long-term, experimental results show unequivocally that elevated deer densities cause significant, profound legacy effects on understorey vegetation persisting at least 20 years. Of relevance regionally and globally where high deer densities have created depauperate understoreys, we expect that deer density reduction alone does not guarantee understorey recovery; stands may need to be managed by removing recalcitrant understorey layers (e.g. ferns).

Towards a structured approach to building qualitative reasoning models and simulations

Successful transfer and uptake of qualitative reasoning technology for modelling and simulation in a variety of domains has been hampered by the lack of a structured methodology to support formalisation of ideas. We present a framework that structures and supports the capture of conceptual knowledge about system behaviour using a qualitative reasoning approach. This framework defines a protocol for representing content that supports the development of a conceptual understanding of systems and how they behave. The framework supports modellers in two ways. First, it structures and explicates the work involved in building models. Second, it facilitates easier comparison and evaluation of intermediate and final results of modelling efforts. We show how this framework has been used in developing qualitative reasoning models about three case studies of sustainable development in different river systems.

Legacy of top‐down herbivore pressure ricochets back up multiple trophic levels in forest canopies over 30 years

Removal of top-down control on herbivores can result in a trophic cascade where herbivore pressure on plants results in changes in plant communities. These altered plant communities are hypothesized to exert bottom-up control on subsequent herbivory via changes in plant quality or productivity. But it remains untested whether top-down perturbation causes long term changes in plants that ricochet back up the new food chain that depends on them. In a large-scale, 30-yr controlled field experiment, we show that 10 yr of top-down control of an ungulate herbivore (white-tailed deer, Odocoileus virginianus) created contrasting forest tree communities exerting bottom-up effects that ricochet back up 3 trophic levels 20–30 yr later. Higher ungulate densities during stand initiation caused significant reductions in tree species diversity, canopy foliage density, canopy insect density, and bird density in young (ca. 30 yr old) forests. Because recruitment of trees from seedlings to the canopy occurs over a relatively brief period (ca. 10 yr), with membership in the canopy lasting an order of magnitude longer, our results show that even short-term perturbations in ungulate density may cause centuries-long disruptions to forest ecosystem structure and function. In documenting this five-step trophic ricochet, we unite key concepts of trophic theory with the extensive literature on effects of ungulate overabundance. As predators decline and ungulate herbivores increase worldwide, similar impacts may result that persist long after herbivore density becomes effectively managed.

Seed Dispersal in Heterogeneous Environments: Bridging the Gap between Mechanistic Dispersal and Forest Dynamics Models

Seed dispersal is an important determinant of vegetation composition. We present a mechanistic model of seed dispersal by wind that incorporates heterogeneous vegetation structure. Vegetation affects wind speeds, a primary determinant of dispersal distance. Existing models combine wind speed and fall velocity of seeds. We expand on them by allowing vegetation, and thus wind profiles, to vary along seed trajectories, making the model applicable to any wind‐dispersed plant in any community. Using seed trap data on seeds dispersing from forests into adjacent sites of two distinct vegetation structures, we show that our model was unbiased and accurate, even though dispersal patterns differed greatly between the two structures. Our spatially heterogeneous model performed better than models that assumed homogeneous vegetation for the same system. Its sensitivity to vegetation structure and ability to predict seed arrival when vegetation structure was incorporated demonstrates the model’s utility for providing realistic estimates of seed arrival in realistic landscapes. Thus, we begin to bridge mechanistic seed dispersal and forest dynamics models. We discuss the merits of our model for incorporation into forest simulators, applications where such incorporation has been or is likely to be especially fruitful, and future model refinements to increase understanding of seed dispersal by wind.

Representing and managing uncertainty in qualitative ecological models

Abstract Ecologists and decision makers need ways to understand systems, test ideas, and make predictions and explanations about systems. However, uncertainty about causes and effects of processes and parameter values is pervasive in models of ecological systems. Uncertainty associated with incomplete knowledge of a phenomenon–and incomplete knowledge of the limits of ones knowledge–is referred to as epistemic uncertainty. Here, we illustrate the use of qualitative reasoning (QR) as a modeling approach that supports simulation despite pervasive epistemic uncertainty in a system. We develop a QR model of a simple plant-resource system to illustrate how six sources of epistemic uncertainty can be expressed, assessed, and managed. These include uncertainty about system structure, quantity vagueness, functional relationships, unknown or exogenous processes, simulation outcomes, and post-diction or explanation of outcomes. We show that QR provides a useful framework for expressing uncertainty due to inexact knowledge about parameter values and for exploring the consequences of different understandings of system structure. Furthermore, uncertainty in parameter values can be expressed and managed using different representations of and constraints on parameter quantity spaces. Compositional modeling supports the creation of alternative models representing different system structures. QR models allow the creation of a full envisionment of all possible outcomes given a set of causal processes, a particular system structure, and starting values. Finally, explicit representation of system structure and causality allows simulation results to be unambiguously explained. These features of QR support ecologists in making explicit their substantial qualitative knowledge about causes and effects in systems to produce models that give rise to insightful simulations of system dynamics.

Indirect effects of pandemic deer overabundance inferred from caterpillar-host relations.

Externally feeding phytophagous insect larvae (i.e., caterpillars, here, larval Lepidoptera and sawflies, Hymenoptera: Symphyta) are important canopy herbivores and prey resources in temperate deciduous forests. However, composition of forest trees has changed dramatically in the eastern United States since 1900. In particular, browsing by high densities of white-tailed deer (Odocoileus virginianus) has resulted in forests dominated by browse-tolerant species, such as black cherry (Prunus serotina), and greatly reduced relative abundance of other tree species, notably pin cherry (Prunus pensylvanica) and birches (Betula spp.). To quantify effects of these changes on caterpillars, we sampled caterpillars from 960 branch tips of the 8 tree species that comprise 95% of trees in Allegheny hardwood forests: red maple (Acer rubrum), striped maple (Acer pensylvanicum), sugar maple (Acer saccharum), sweet birch (Betula lenta), yellow birch (Betula allegheniensis), American beech (Fagus grandifolia), black cherry, and pin cherry. We collected 547 caterpillar specimens that belonged to 66 Lepidoptera and 10 Hymenoptera species. Caterpillar density, species richness, and community composition differed significantly among tree species sampled. Pin cherry, nearly eliminated at high deer density, had the highest density and diversity of caterpillars. Pin cherry shared a common caterpillar community with black cherry, which was distinct from those of other tree hosts. As high deer density continues to replace diverse forests of cherries, maples, birches, and beech with monodominant stands of black cherry, up to 66% of caterpillar species may be eliminated. Hence, deer-induced changes in forest vegetation are likely to ricochet back up forest food webs and therefore negatively affect species that depend on caterpillars and moths for food and pollination.

Supporting education about environmental sustainability: Evaluation of a progressive learning route for qualitative models

Abstract Qualitative reasoning (QR) models can encapsulate a lot of expert, conceptual knowledge about interconnections of society, policy, and the environment common to issues concerning environmental sustainability. This paper describes a lesson plan designed to support learner exploration of QR models about sustainability, with the aim of supporting the goal of the European Unions Sustainable Development Strategy to build an active, engaged, and educated society that is prepared to contribute to decision-making about sustainability issues. The lesson plan includes ten questions that follow a QR models progression from system structure through causality and dynamics to engage learners in application and evaluation of model outputs. We evaluated effectiveness of the lesson plan for supporting learning. Our first evaluation demonstrated that university students could effectively reason from and interpret diagrams produced by the modeling software that corresponded to content addressed in each of the ten questions in the lesson plan. Our second evaluation demonstrated that following the proposed lesson plan, students were able to abstract domain-specific understanding of key concepts from a series of three progressively more complex and realistic models. Furthermore, the proposed lesson format engages students in high levels of cognitive function when examining models and simulations. Hence, the progressive learning route provides a useful structure for designing a model-based curriculum for learning about environmental sustainability.

Molecular analysis of nestling diet in a long-distance Neotropical migrant, the Louisiana Waterthrush (Parkesia motacilla)

ABSTRACT Elucidating the diet of Neotropical migratory birds is essential to our understanding of their ecology and to their long-term conservation. Reductions in prey availability negatively impact Neotropical migrants by affecting their survival as both nestlings and adults. Beyond broad taxonomic or morphological categories, however, the diet of Neotropical migrants is poorly documented. Using the molecular techniques of DNA barcoding and next-generation sequencing, we elucidated the diet of Louisiana Waterthrush (Parkesia motacilla) nestlings in Arkansas and Pennsylvania, USA. Waterthrush have been shown to respond negatively to the reduced availability of aquatic insects in the orders Ephemeroptera, Plecoptera, and Trichoptera (EPT taxa). We hypothesized that Louisiana Waterthrush nestling diet would be primarily composed of these pollution-sensitive aquatic taxa, and that changes in the riparian insect community would be reflected in their diet. Unexpectedly, the orders Lepidoptera (92%) and Diptera (70%) occurred frequently in the diet of Louisiana Waterthrush nestlings. Among EPT taxa, only the order Ephemeroptera (61%) was frequently detected whereas Plecoptera (7%) and Trichoptera (1%) were poorly represented. The frequency at which aquatic Ephemeroptera and terrestrial Lepidoptera were detected in waterthrush nestling diet differed significantly over the nesting period in Pennsylvania but not in Arkansas, suggesting that phenological shifts in the availability of non-EPT prey taxa may be an important yet undescribed factor influencing the foraging ecology of waterthrush on the breeding grounds. Furthermore, these findings suggest that terrestrial insects may be more important to waterthrush nestlings than previously thought, which enhances our understanding of this biological indicator and Neotropical migrant.

The indirect impact of long-term overbrowsing on insects in the Allegheny National Forest region of Pennsylvania

Abstract Overbrowsing has created depauperate plant communities throughout the eastern deciduous forest. We hypothesized these low-diversity plant communities are associated with lower insect diversity. We compared insects inside and outside a 60-year-old fenced deer exclosure where plant species richness is 5x higher inside versus outside. We sampled aboveground and litter insects using sweep nets and pitfall traps and identified specimens to family. Aboveground insect abundance, richness, and diversity were up to 50% higher inside the fenced exclosure versus outside. Conversely, litter insect abundance and diversity were consistently higher outside the exclosure. Community composition of aboveground insects differed throughout the summer (P < 0.05), but litter insects differed only in late summer. Our results demonstrate that the indirect effects of long-term overbrowsing can reduce aboveground insect diversity and abundance, and change composition even when plant communities are in close proximity.