Prior Defoliation Research Articles

Budworms, beetles and wildfire: Disturbance interactions influence the likelihood of insect‐caused disturbances at a subcontinental scale

Abstract Irruptive forest insects are a leading biotic disturbance across temperate and boreal forests. Outbreaks of forest insects are becoming more frequent and extensive due to anthropogenic drivers (e.g. climate and land‐use), perhaps increasing the likelihood that forests will experience multiple insect‐caused disturbances. Across the fire‐prone Douglas‐fir forests of western North America, recent outbreaks of the western spruce budworm and Douglas‐fir beetle have impacted large expanses of forests, with a higher degree of overlap than expected in some ecoregions. Outbreaks of both insects are positively related to host availability and exhibit density‐dependent population dynamics that are affected by climate and weather. Here, we leverage data from aerial detection surveys, estimates of host availability, climate and weather, and categorized fire severity to describe the spatial overlap between western spruce budworm and Douglas‐fir beetle and assess: (1) how climate and host availability influence the biogeography of outbreaks; (2) how weather incites outbreaks; and finally, (3) how prior disturbances (fire and biotic) affect the subsequent outbreak likelihood of western spruce budworm and Douglas‐fir beetle. Models demonstrate that western spruce budworm and Douglas‐fir beetle share similar predisposing drivers of outbreaks. Outbreaks of both insects were more likely to occur following warm weather, but only beetle outbreaks were more likely following drought. When controlling for differences in outbreak distribution and inciting factors, results indicate that both prior fire and interspecific disturbance altered the likelihood of subsequent insect‐caused disturbance. Specifically, Douglas‐fir beetle outbreaks were more likely to occur for several years following low severity fire, but less likely otherwise. Prior defoliation, especially longer duration defoliation, increased the likelihood of beetle outbreak within stands and across the landscape. On the contrary, western spruce budworm outbreaks were less likely to occur following fire, while prior beetle activity dampened budworm outbreak likelihood for several years, and then eventually increased outbreak likelihood. Synthesis: Biotic–biotic disturbance interactions have the potential to amplify the incidence of insect‐caused disturbance across subcontinental scales. Our findings highlight the need for future work on mechanistic linkages between biotic disturbance agents as well as the ramifications for forest trajectories and function.

174 Forage Production and Nutritive Value of Alfalfa-bermudagrass Mixtures Managed Under Contrasting Defoliation Strategies in the Southeast US

Abstract Incorporation of alfalfa (Medicago sativa) into bermudagrass pastures improves forage quality and decreases the reliance on synthetic nitrogen fertilizer. The objective of this study was to determine forage mass (FM), nutritive value (NV), and botanical composition of ‘Bulldog 805’ alfalfa and ‘Tifton 85’ bermudagrass (T85; Cynodon dactylon) mixtures managed under three defoliation strategies: 1) hay production (H), 2) grazing (G) or 3) dual-purpose (DP) use. The study was conducted in two locations (Headland, AL and Tifton, GA) using a randomized complete block design with two replicates. In spring 2020, grazed plots were divided in four strips and every 7-d, animals were rotated to a new strip and stocking rate was adjusted. Under DP, plots were grazed until mid-July, then forage was harvested in late August. For H plots, forage was harvested every 28 to 35-d. Forage samples were collected prior defoliation to determine FM and on grazed periods, pre- and post-grazing samples and disk meter measurements were collected. Nutritive value responses were determined using near-infrared spectroscopy. There was no effect of defoliation strategy on FM (P = 0.604; mean 3471 kg DM/ha). Greater FM (P = 0.002) was observed in July and August than June (3531 and 3976 vs 2905 kg DM ha-1, SE= 263). This response was associated with an up to 40% increase of T85 proportion in the mixture (P = 0.001). Alfalfa percentage was 78% greater for DP than G (P = 0.029). There were no differences among treatments for NV responses (P > 0.05). Among defoliation periods, crude protein, neutral detergent fiber and acid detergent fiber concentrations ranged from 14 to 22%, 45 to 58% and 28 to 35%, respectively. These preliminary results demonstrate multi-use options for this mixture in the region while aiming for increased forage nutritive value, and extended growing season, and sustainability of forage-livestock systems.

Modeling Migratory Flight in the Spruce Budworm: Temperature Constraints

We describe an individual-based model of spruce budworm moth migration founded on the premise that flight liftoff, altitude, and duration are constrained by the relationships between wing size, body weight, wingbeat frequency, and air temperature. We parameterized this model with observations from moths captured in traps or observed migrating under field conditions. We further documented the effects of prior defoliation on the size and weight (including fecundity) of migrating moths. Our simulations under idealized nocturnal conditions with a stable atmospheric boundary layer suggest that the ability of gravid female moths to migrate is conditional on the progression of egg-laying. The model also predicts that the altitude at which moths migrate varies with the temperature profile in the boundary layer and with time during the evening and night. Model results have implications for the degree to which long-distance dispersal by spruce budworm might influence population dynamics in locations distant from outbreak sources, including how atmospheric phenomena such as wind convergence might influence these processes. To simulate actual migration flights en masse, the proposed model will need to be linked to regional maps of insect populations, a phenology model, and weather model outputs of both large- and small-scale atmospheric conditions.

Anomalous outbreaks of an invasive defoliator and native bark beetle facilitated by warm temperatures, changes in precipitation and interspecific interactions

Biotic disturbance agents such as insects can be highly responsive to climatic change and have widespread ecological and economic impacts on forests. Quantifying the responses of introduced and native insects to climate, including how dynamics of one agent may mediate those of another, is important for forecasting disturbance and associated impacts on forest structure and function. We investigated drivers of outbreaks by larch casebearer Coleophora laricella, an invasive defoliator, and eastern larch beetle Dendroctonus simplex, a native, tree‐killing bark beetle, on tamarack Larix laricina from 2000 to in Minnesota, USA. We evaluated the utility of temporal, spatial and climatic variables in predicting the presence/absence of outbreaks of each insect in cells of rasterized aerial survey data. The role of defoliation by larch casebearer in outbreaks of eastern larch beetle was also investigated. For both species, the most important predictors of outbreak occurrence were proximity of conspecific outbreaks in space and time. For larch casebearer, outbreak occurrence was positively associated with spring precipitation and warmer growing seasons. Outbreak occurrence of eastern larch beetle was positively associated with warmer and dryer years and was more likely in cells with prior defoliation by larch casebearer. Our results demonstrate that climate can drive large scale outbreaks of introduced and non‐native disturbance agents on a single host species, and that interactions at the tree level between such agents may scale up to manifest across large temporal and spatial scales.

Growth and mortality of trembling aspen (Populus tremuloides) in response to artificial defoliation

To simulate the effects of forest tent caterpillar (FTC) defoliation on trembling aspen growth and mortality, an artificial defoliation experiment was performed over three years in young aspen stands of northwestern Quebec. Defoliation plots of 15 × 15 m were established on three sites, together with associated control stands of pure trembling aspen. In 2007, root collar diameters were measured and positions of all trees were mapped prior defoliation. Severe FTC defoliation was simulated for three successive years (2007–2009) by manually removing all leaves from all but 7–10% of the trees present in the defoliation plots. Yearly surveys of growth and mortality were conducted until 2010 to evaluate defoliation effects on defoliated as well as surrounding undefoliated trees. In absence of other factors, growth and mortality of trembling aspen decreased and increased, respectively, after defoliation. Our study further revealed that small diameter trees died after one year of artificial defoliation, while larger-diameter trees died after repeated defoliations. Distributions of tree mortality tended to be aggregated at small scales (<5 m), corroborating gap patterns observed in mature stands following FTC outbreaks. This experiment revealed that trembling aspen mortality can be directly attributed solely to defoliation. Repeated defoliations during FTC outbreaks have the potential to profoundly modify stand productivity and structure by reducing tree growth and increasing tree mortality in the absence of predisposing factors.

Genotypic Differences and Prior Defoliation Affect Re-Growth and Phytochemistry after Coppicing in Populus tremuloides

Although considerable research has explored how tree growth and defense can be influenced by genotype, the biotic environment, and their interaction, little is known about how genotypic differences, prior defoliation, and their interactive effects persist in trees that re-grow after damage that severs their primary stem. To address these issues, we established a common garden consisting of twelve genotypes of potted aspen (Populus tremuloides) trees, and subjected half of the trees to defoliation in two successive years. At the beginning of the third year, all trees were severed at the soil surface (coppiced) and allowed to regenerate for five months. Afterwards, we counted the number of root and stump sprouts produced and measured the basal diameter (d) and height (h) of the tallest ramet in each pot. We collected leaves one and two years after the second defoliation and assessed levels of phenolic glycosides, condensed tannins, and nitrogen. In terms of re-growth, we found that the total number of sprouts produced varied by 3.6-fold among genotypes, and that prior defoliation decreased total sprout production by 24%. The size (d(2)h) of ramets, however, did not differ significantly among genotypes or defoliation classes. In terms of phytochemistry, we observed genotypic differences in concentrations of all phytochemicals assessed both one and two years after the second defoliation. Two years after defoliation, we observed effects of prior defoliation in a genotype-by-defoliation interaction for condensed tannins. Results from this study demonstrate that genotypic differences and impacts of prior defoliation persist to influence growth and defense traits in trees even after complete removal of above-ground stems, and thus likely influence productivity and plant-herbivore interactions in forests affected by natural disturbances or actively managed through coppicing.

Browse Quality in Quaking Aspen (Populus tremuloides): Effects of Genotype, Nutrients, Defoliation, and Coppicing

The consequences of interactions among genetic, ontogenetic, and environmental factors for the quality of winter-dormant tissues as food for browsing herbivores is poorly understood. We conducted two sequential common garden studies to assess the impacts of intraspecific genetic variation, nutrient availability, prior defoliation, and ontogenetic stage on the chemical quality of winter-dormant tissue in quaking aspen (Populus tremuloides Michx.). In the first study, saplings of 12 aspen genotypes were grown under low and high soil nutrient conditions, with or without two successive seasons of defoliation. Quantity and quality of current year's twig growth were assessed. Twig production varied among genotypes and declined under low nutrient availability, but showed little response to prior defoliation. Chemical quality of sapling twigs varied substantially among genotypes, and in response to nutrient availability and prior defoliation. Overall, browse quality improved (nitrogen levels increased while phenolic glycoside and condensed tannin levels decreased) after defoliation. Growth and chemical variables exhibited low to moderate clonal repeatability (broad sense heritability) values. Our second study employed the same 12 genotypes, grown under high-nutrient conditions and with or without two seasons of defoliation. The trees were coppiced to produce root sprouts, which were chemically assessed 1 yr later. Rejuvenation via coppicing led to increased levels of nitrogen, phenolic glycosides (salicortin), and tannins in root sprouts, and the magnitude of change varied among aspen genotypes. Signatures of defoliation nearly 2 yr earlier persisted in terms of elevated levels of phenolic glycosides in root sprouts of previously defoliated trees. Aspen forests likely present browsing herbivores with chemically heterogeneous environments because of the interactions of genetic, ontogenetic, and environmental factors that vary over space and time.

BIRD NESTING ECOLOGY IN A FOREST DEFOLIATED BY GYPSY MOTHS

Acadian Flycatcher (Empidonax virescens, n = 55), Indigo Bunting (Passerina cyanea, n = 60), Eastern Towhee (Pipilo erythrophthalmus, n = 41), and Wood Thrush (Hylocichla mustelina, n = 62) nests were monitored during 1995–1996 in the eastern panhandle of West Virginia, at the Sleepy Creek Wildlife Management Area. The objective of this study was to relate the outcomes of bird nests to surrounding habitat characteristics in an area that experienced heavy tree mortality from prior defoliation by the gypsy moth (Lymantria dispar). Large (> 22.9 cm dbh) standing snags in the nest patch were not associated with nest failure for any of the four bird species. Very large diameter (> 38.1 cm dbh) live trees and snags and reduced canopy cover increased the chances of Brown-headed Cowbird (Molothrus ater) parasitism only for Indigo Buntings. Nest patches of all four species differed in vegetation characteristics from random plots in similar habitat, typically by having greater densities of species' preferred nesting substrate in the nest patch. Gypsy moth defoliation, which can result in an increase in snags and opened canopy, is not likely to be a devastating ecological event for shrub and sub-canopy nesting avian species, and can create more nesting habitat for many species that use a dense forest understory.

Contributions of female oviposition patterns and larval behavior to group defense in conifer sawflies (hymenoptera: diprionidae).

We studied the effects of adult oviposition and larval interactions on the defensive potential of gregarious behavior in conifer sawflies. Aggregation size and distribution initially reflected adult host plant selection and oviposition behavior. The contagious distribution of egg clusters resulted in part from the utilization of individual trees by multiple females, and of single host shoots by several females. Trees with the greates degree of prior defoliation received the most eggs, even though the potential for larval crowding made resource depletion possible. Foliar monoterpene and nitrogen contents explained only a small proportion of variability in female host utilization. Conifer needle architecture restricted the size of larval subgroups within aggregations, and limited the degree of defensive cohesiveness between subgroups. Subgroup turnover was frequent and independent of local food depletion. Risk of predation from wood ants varied with larval aggregation size and predator foraging level. When ant activity was high, large aggregations suffered greater numerical losses, but showed lower per capita predatory risk, than small groups. Results suggest that female oviposition patterns are influenced in part by the defensive benefits gained by offspring in large aggregations. Against ants, dilution effects and defensive synchrony due to gregariousness contribute to the overall antipredator strategy of sawfly larvae.

Spruce budworm defoliation and growth loss in young balsam fir: artificial defoliation of potted trees

To simulate feeding by the spruce budworm (Choristoneurafumiferana Clem.), potted, 5-year-old balsam fir (Abiesbalsamea (L.) Mill.) trees were artificially defoliated at the peak of the sixth instar period in the first 1, 2, or 3 years of a 3-year experiment. This schedule allowed trees that were defoliated in the first 1 or 2 years to recover for 2 years and 1 year, respectively. Seven treatments were applied: 0, 33, 66, 90, or 100% of the current-year needles were manually removed, all current-year needles were clipped using scissors (clip treatment), or all current-year shoots were severed at their base (100+ treatment). The dry weights of stem axis, branch axes, and roots were measured at the end of the 3rd year, and current-year and total foliage weight, height growth, and specific volume increment were determined for each year of the experimental period. Needle removal decreased growth throughout the tree, the growth loss increasing with increasing intensity and frequency of defoliation. In the 1st year of defoliation, all treatments reduced specific volume increment, whereas only the 100+ treatment decreased height growth. In every defoliation year, specific volume increment, height growth (manifested in the year following the defoliation), and the final weights of stem axis, branch axes, and roots were generally related curvilinearly to total foliage weight. The 100% and 100+ treatments induced the sprouting of axillary and nodal buds that remained dormant in undefoliated trees. Retaining the defoliated shoot axes (100% treatment), compared with removing them (100+ treatment), increased specific volume increment. Retaining the needle base (clip treatment) prevented the apex necrosis that occurred in some shoots subjected to the 100% treatment. In trees allowed to recover, specific volume increment increased in the 1st year, the degree of recovery increasing with decreasing intensity and frequency of prior defoliation treatment. After 2 recovery years, specific volume increment and height growth were not affected by any previous defoliation treatment, and current-year foliage weight and stem axis weight were decreased only by the 100+ treatment; however, there was still an inhibitory effect of all treatments on the weights of total foliage, branch axes, and roots. The growth responses found in the present investigation were compared with those observed in balsam fir trees defoliated by the spruce budworm.

WITHIN-CROWN DISTRIBUIONS OF MALE AND FEMALE BAGWORM (LEPIDOPTERA: PSYCHIDAE) PUPAE ON JUNIPER AS AFFECTED BY HOST DEFOLIATION

AbstractWe examined the within-crown distributions of male and female bagworm, Thyridopteryx ephemeraeformis (Haworth), pupae on Juniperus virginiana L. as a function of prior defoliation. This was to evaluate the hypothesis that the differential movement of females to the upper crown before pupation would be strongest on heavily defoliated trees. Differential stratification of the sexes occurred irrespective of degree of defoliation, and despite the fact that competition, as evidenced by higher mortality of pupae and reduced weight of egg masses, was apparently greatest at the top of moderately and severely defoliated hosts. There was no relationship between height of pupation and risk of parasitism for either sex. Female larvae were ca. 4-fold more likely than males to abandon a low-lying host upon which they had fed and to ascend the trunks of nearby trees before pupating. Enhancement of larval dispersal by females is the most plausible explanation for the stratified distribution of bagworm pupae within trees.

Effects of crop husbandry and growing conditions on storage losses of Pentland Crown potatoes

SUMMARYOver five growing seasons (1976–80) experiments were carried out using at least three contrasting sites per season to study the relationship between growing conditions and storage losses in Pentland Crown potatoes. Crops were harvested by hand and storage was carried out in a single environment over a period of 6–7 months without sprout suppressants. The range in growing conditions was great and reflected in the pattern of leaf growth and range in yields of the stored crops (30–90 t/ha). Crops from Tenby (S. W. Wales) produced the most consistent and frequently the highest yields and usually lost the least weight during storage. Although the range in total weight loss over all experiments was large from 5·4 to 16·3%, treatment effects were much smaller than in field growth and yield. Delaying the date of harvest usually increased weight loss and tubers harvested in early August stored at least as well as late-harvested tubers. Tubers harvested without prior defoliation stored as well as tubers harvested on the same day from crops defoliated at least 2 weeks previously. Lengthening the interval from defoliation to harvest usually increased weight loss in storage. Although the tubers were hand harvested effects on saleability were found at the end of storage and there was no evidence that earlier harvesting, which may involve some loss of field yield, would result in any loss of saleable yield out of store.The results provided no evidence to support the widely-held view that the suitability of a tuber for harvesting improves during maturation associated with natural or imposed crop senescence. It is therefore suggested that the use of the term maturity be avoided as it is wholly unhelpful in studies of the relationship between field growth and storage losses in potatoes.

Effect of defoliation by checkerspot caterpillars (Euphydryas phaeton) and sawfly larvae (Macrophya nigra and Tenthredo grandis) on their host plants (Chelone spp.).

The effect of defoliation by herbivores, checkerspot caterpillars (Euphydryas phaeton) and sawfly larvae (Macrophya nigra and Tenthredo grandis), on the reproductive output of turtlehead (Chelone spp.) was examined. Defoliation prior to development of flower buds reduced the number of reproductive stalks, flower buds, flowers and seed capsules. Severe herbivory, after flower buds appeared, decreased the final number of seed capsules and seeds per capsule. The availability of the host plants to the herbivores was a function of prior defoliation and environmental conditions. Sawfly larvae, by defoliating the plants in midsummer, forced prediapause checkerspot caterpillars to wander in search of food plants. Decimation of these perenials by postdiapause checkerspot caterpillars in a dry spring retarded growth of turtlehead and, consequently, most of the plants were not available for egg-laying by sawflies and checkerspots.