Aaron S. Weed

Consequences of climate change for biotic disturbances in North American forests

About one-third of North America is forested. These forests are of incalculable value to human society in terms of harvested resources and ecosystem services and are sensitive to disturbance regimes. Epidemics of forest insects and diseases are the dominant sources of disturbance to North American forests. Here we review current understanding of climatic effects on the abundance of forest insects and diseases in North America, and of the ecological and socioeconomic impacts of biotic disturbances. We identified 27 insects (6 nonindigenous) and 22 diseases (9 nonindigenous) that are notable agents of disturbance in North American forests. The distribution and abundance of forest insects and pathogens respond rapidly to climatic variation due to their physiological sensitivity to temperature, high mobility, short generation times, and high reproductive potential. Additionally, climate affects tree defenses, tree tolerance, and community interactions involving enemies, competitors, and mutualists of insects ...

Geographically variable response of Dendroctonus ponderosae to winter warming in the western United States

ContextMilder winters have contributed to recent outbreaks of Dendroctonus ponderosae in Canada, but have not been evaluated as a factor permitting concurrent outbreaks across its large range (ca.1500 × 1500 km) in the western United States (US).ObjectivesWe examined the trend in minimum air temperatures in D. ponderosae habitats across the western US and assessed whether warming winters explained the occurrence of outbreaks using physiological and population models.MethodsWe used climate data to analyze the history of minimum air temperatures and reconstruct physiological effects of cold on D. ponderosae. We evaluated relations between winter temperatures and beetle abundance using aerial detection survey data.ResultsExtreme winter temperatures have warmed by about 4 °C since 1960 across the western US. At the broadest scale, D. ponderosae population dynamics between 1997 and 2010 were unrelated to variation in minimum temperatures, but relations between cold and D. ponderosae dynamics varied among regions. In the 11 coldest ecoregions, lethal winter temperatures have become less frequent since the 1980s and beetle-caused tree mortality increased—consistent with the climatic release hypothesis. However, in the 12 warmer regions, recent epidemics cannot be attributed to warming winters because earlier winters were not cold enough to kill D. ponderosae.ConclusionsThere has been pronounced warming of winter temperatures throughout the western US, and this has reduced previous constraints on D. ponderosae abundance in some regions. However, other considerations are necessary to understand the broad extent of recent D. ponderosae epidemics in the western US.

Climate drivers of bark beetle outbreak dynamics in Norway spruce forests

Bark beetles are among the most devastating biotic agents affecting forests globally and several species are expected to be favored by climate change. Given the potential interactions of insect outbreaks with other biotic and abiotic disturbances, and the potentially strong impact of changing disturbance regimes on forest resources, investigating climatic drivers of destructive bark beetle outbreaks is of paramount importance. We analyzed 17 time-series of the amount of wood damaged by Ips typographus, the most destructive pest of Norway spruce forests, collected across 8 European countries in the last three decades. We aimed to quantify the relative importance of key climate drivers in explaining timber loss dynamics, also testing for possible synergistic effects. Local outbreaks shared the same drivers, including increasing summer rainfall deficit and warm temperatures. Large availability of storm-felled trees in the previous year was also strongly related to an increase in timber loss, likely by providing an alternative source of breeding material. We did not find any positive synergy among outbreak drivers. On the contrary, the occurrence of large storms reduced the positive effect of warming temperatures and rainfall deficit. The large surplus of breeding material likely boosted I. typographus population size above the density threshold required to colonize and kill healthy trees irrespective of other climate triggers. Importantly, we found strong negative density dependence in I. typographus that may provide a mechanism for population decline after population eruptions. Generality in the effects of complex climatic events across different geographical areas suggests that the large-scale drivers can be used as early warning indicators of increasing local outbreak probability. Ecography (Less)

Host Specificity of Hypena opulenta: A Potential Biological Control Agent of Vincetoxicum in North America

ABSTRACT A biological control program has been initiated against the European swallow-worts Vincetoxicum nigrum (L.) Moench and V. rossicum (Kleopow) Barbar. (Family Apocynaceae) that have become invasive in North America. The leaf-feeding moth, Hypena opulenta Christoph (Lepidoptera: Erebidae), originating from eastern Europe, has been under measurement as a potential biological control agent of swallow-worts since 2006. In this study we measured the host range of H. opulenta by screening 82 potential host plant species for larval development under no-choice conditions. In addition, we also monitored female fecundity, longevity, and oviposition preference among suitable larval hosts. Successful larval development occurs only on Vincetoxicum spp. Partial larval development by one larva was observed on Boehmeria cyclindrica (L.) Sw. (Urticaceae) to the final instar, but this individual failed to pupate. Exploratory feeding occurred on Gonolobus stephanotrichus Griseb. (Apocynaceae) and Urtica dioica L. (Urticaceae), but all larvae failed to develop past the first and second instar, respectively. Additional testing with mature larvae on a subset of the plant species demonstrates that no species outside the genus Vincetoxicum are suitable for complete larval development of H. opulenta. The longevity and fecundity of females raised on each target weed are similar and gravid females do not display an oviposition preference among Vincetoxicum spp. Hypena opulenta does not present a risk to any native plant species or species of economic importance in North America. Petitions have been submitted for experimental open-field releases of H. opulenta in the United States and Canada.

Disturbance Regimes and Stressors

The effects of climate change on insect outbreaks, wildfire, invasive species, and pathogens in forest ecosystems will greatly exceed the effects of warmer temperature on gradual changes in forest processes. Increased frequency and extent of these disturbances will lead to rapid changes in vegetation age and structure, plant species composition, productivity, carbon storage, and water yield. Insect outbreaks are the most pervasive forest disturbance in the United States, and rapid spread of bark beetles in the western United States has been attributed to a recent increase in temperature. Wildfire area burned has increased in recent decades, although frequency and severity have not changed, and is expected to greatly increase by 2050 (at least twice as much area burned annually in the West). More frequent occurrence of fire and insects will create landscapes in which regeneration of vegetation will occur in a warmer environment, possibly with new species assemblages, younger age classes, and altered forest structure. Increased fire and insects may in turn cause more erosion and landslides. Invasive plant species are already a component of all forest ecosystems, and a warmer climate will likely facilitate the spread of current and new invasives, particularly annuals that compete effectively in an environment with higher temperature and frequent disturbance. The interaction of multiple disturbances and stressors, or stress complexes, has the potential to alter the structure and function of forest ecosystems, especially when considered in the context of human land-use change. Occurring across large landscapes over time, these stress complexes will have mostly negative effects on ecosystem services, requiring costly responses to mitigate them and active management of forest ecosystems to enhance resilience.

Spatio‐temporal dynamics of a tree‐killing beetle and its predator

&NA; Resolving linkages between local‐scale processes and regional‐scale patterns in abundance of interacting species is important for understanding long‐term population stability across spatial scales. Landscape patterning in consumer population dynamics may be largely the result of interactions between consumers and their predators, or driven by spatial variation in basal resources. Empirical testing of these alternatives has been limited by the lack of suitable data. In this study, we analyzed an extensive network of spatially replicated time series to characterize the local and regional processes affecting spatio‐temporal dynamics of a tree‐killing bark beetle (Dendroctonus frontalis or SPB) and its key predator (Thanasimus dubius) across the southeastern United States. We first used a mechanistic model to evaluate factors affecting the stability of 95 predator–prey time series and then conducted spatial analyses to evaluate scale dependence in the factors affecting the geographical patterning of this system. Across the region, population fluctuations of both species were correlated in space beyond 400 km but there was notable spatial variation in the deterministic and stochastic processes influencing forest‐scale (local) fluctuations. Time series analyses indicated that local dynamics of SPB and T. dubius are not cyclical. Instead, the abundance of T. dubius responded almost instantaneously to changes in SPB abundance. Spatial variation in long‐term forest‐scale abundance of both species was linked most strongly to the abundance of pine habitat indicating a stronger role for resource availability in SPB population dynamics than top‐down effects. Our results are consistent with other studies indicating that animal populations tend to be synchronized in space via spatially correlated processes such as weather; yet local dynamics tend to be linked to smaller‐scale host patterns. Our study provides a rare empirical assessment of how local processes scale up to produce landscape patterns that influence forest ecology and forest management.

Population Dynamics of Bark Beetles

Bark beetles colonize virtually all plant tissue types but are typically the most important pests of timber, tree fruit and seed crops, and ecologically important disturbance agents of forested landscapes around the world. For these reasons, scientists have been engaged for decades in elucidating the forces that affect their population dynamics. In this chapter, we review the population ecology of species exploiting seeds, herbaceous plants, and woody plants. We then examine the role that host tree resistance has played on life history trait evolution of conifer-feeding bark beetles and the patterns we observe in their population dynamics. The most important strategies for overcoming host plant resistance are: (1) adult aggregation capacity, (2) capacity to overwhelm tree defense, and (3) ability to tolerate host defenses. Variation in the presence of these traits among species influences their ability to access dead, weakened, or living trees (aggression) with corresponding effects on equilibrium population abundance and the influence of stochastic processes in their population dynamics. Stochastic processes that reinforce the traits listed above may generate positive feedback in aggressive species, permitting mass attack of healthy trees and the existence of an upper equilibrium, or outbreak density. Although stochastic factors influence variance in population abundance of less aggressive taxa, demographic constraints due to host resistance strongly regulate their populations near a low abundance equilibrium.

Temperature affects phenological synchrony in a tree-killing bark beetle

Phenological synchrony can promote population growth in species with positive density dependence. Variation among life stages in the thermal thresholds for development can foster phenological synchrony under thermal regimes that include frequent occurrence of temperatures between developmental thresholds. The southern pine beetle is an insect with positive density dependence that has recently undergone important shifts in population abundance at the northern extremes of their distribution. We evaluated the hypothesis that cooler winter temperatures in their northern range cause a convergence of the population life stage structure that leads to synchrony in spring flight phenology. We used a combination of approaches. First, in situ laboratory experiments demonstrated a threshold temperature for pupation that was greater than was required for larval development; rearing larvae at lower temperatures increased the pooling of individuals at the end stage of larval development and synchrony in adult emergence. Second, a development rate model showed a similar convergence of the majority of the population at the end stage of larval development when brood experienced the cooler temperatures of the northern region, but not with temperatures from the southern region, or as a null model. Finally, field trapping of wild beetles showed greater synchrony in the pine forests of New Jersey than in the warmer, historically occupied forests of Georgia and Mississippi. Given these results, pine-dominated forests in the northern edge of the southern pine beetle’s range may experience more frequent occurrence of outbreaks, due to the positive feedbacks associated with a synchronous spring emergence of this insect.