Susceptibility of ponderosa pine, Pinus ponderosa (Dougl. ex Laws.), to mountain pine beetle, Dendroctonus ponderosae Hopkins,attack in uneven-aged stands in the Black Hills of South Dakota and Wyoming USA

Mountain pine beetle, Dendroctonus ponderosae Hopkins can cause extensive tree mortality in ponderosa pine, Pinus ponderosa Dougl. ex Laws., forests in the Black Hills of South Dakota and Wyoming. Most studies that have examined stand susceptibility to mountain pine beetle have been conducted in even-aged stands. Land managers increasingly practice uneven-aged management. We established 84 clusters of four plots, one where bark beetle-caused mortality was present and three uninfested plots. For all plot trees we recorded species, tree diameter, and crown position and for ponderosa pine whether they were killed or infested by mountain pine beetle. Elevation, slope, and aspect were also recorded. We used classification trees to model the likelihood of bark beetle attack based on plot and site variables. The probability of individual tree attack within the infested plots was estimated using logistic regression. Basal area of ponderosa pine in trees ≥25.4 cm in diameter at breast height (dbh) and ponderosa pine stand density index were correlated with mountain pine beetle attack. Regression trees and linear regression indicated that the amount of observed tree mortality was associated with initial ponderosa pine basal area and ponderosa pine stand density index. Infested stands had higher total and ponderosa pine basal area, total and ponderosa pine stand density index, and ponderosa pine basal area in trees ≥25.4 cm dbh. The probability of individual tree attack within infested plots was positively correlated with tree diameter with ponderosa pine stand density index modifying the relationship. A tree of a given size was more likely to be attacked in a denser stand. We conclude that stands with higher ponderosa pine basal area in trees >25.4 cm and ponderosa pine stand density index are correlated with an increased likelihood of mountain pine beetle bark beetle attack. Information form this study will help forest managers in the identification of uneven-aged stands with a higher likelihood of bark beetle attack and expected levels of tree mortality.     

Recent infestation of forest stands by spruce beetles does not predict habitat use by little brown bats (Myotis lucifugus) in southwestern Yukon, Canada

Insect outbreaks affect forest structure which may have significant effects on the habitat of other animals. Forest-dwelling insectivorous bats are likely affected by associated changes in the abundance of roost trees and insect prey, altered foraging and flying efficiency, and predation risk. We examined the short-term effects (3-13 years post-infestation) of an outbreak of spruce beetles (Dendroctonus rufipennis) on the habitat use of little brown bats (Myotis lucifugus) in the boreal forest of the southwestern Yukon, Canada. We measured bat activity, using Anabat II bat detectors, in 90 forested stands that had experienced from 0 to 90% tree mortality due to spruce beetles. We used generalized linear models to assess whether bat activity varied with tree mortality, season, tree density, canopy closure, or distance to the nearest lake or town. Bat activity did not vary significantly with tree mortality, season, or canopy closure, but decreased with increasing tree density. Bat activity was significantly greater in areas close to both the nearest lake and nearest town, and was low in areas that were far from either. Our results indicate that in the short-term, habitat use by little brown bats was not related to the severity of spruce beetle infestation, but suggest that in the long-term, bats may be positively affected by decreased tree density as beetle-killed trees fall down.     

Stand characteristics and downed woody debris accumulations associated with a mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak in Colorado

Lodgepole pine (Pinus contorta Dougl. ex Loud.)-dominated ecosystems in north-central Colorado are undergoing rapid and drastic changes associated with overstory tree mortality from a current mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak. To characterize stand characteristics and downed woody debris loads during the first 7 years of the outbreak, 221 plots (0.02 ha) were randomly established in infested and uninfested stands distributed across the Arapaho National Forest, Colorado. Mountain pine beetle initially attacked stands with higher lodgepole pine basal area, and lower density and basal area of Engelmann spruce (Picea engelmannii [Parry]), and subalpine fir (Abies lasiocarpa (Hook.) Nutt. var. lasiocarpa) compared to uninfested plots. Mountain pine beetle-affected stands had reduced total and lodgepole pine stocking and quadratic mean diameter. The density and basal area of live overstory lodgepole declined by 62% and 71% in infested plots, respectively. The mean diameter of live lodgepole pine was 53% lower than pre-outbreak in infested plots. Downed woody debris loads did not differ between uninfested plots and plots currently infested at the time of sampling to 3 or 4–7 years after initial infestation, but the projected downed coarse wood accumulations when 80% of the mountain pine beetle-killed trees fall indicated a fourfold increase. Depth of the litter layer and maximum height of grass and herbaceous vegetation were greater 4–7 years after initial infestation compared to uninfested plots, though understory plant percent cover was not different. Seedling and sapling density of all species combined was higher in uninfested plots but there was no difference between infested and uninfested plots for lodgepole pine alone. For trees ≥2.5 cm in diameter at breast height, the density of live lodgepole pine trees in mountain pine beetle-affected stands was higher than Engelmann spruce, subalpine fir, and aspen, (Populus tremuloides Michx.), in diameter classes comprised of trees from 2.5 cm to 30 cm in diameter, suggesting that lodgepole pine will remain as a dominant overstory tree after the bark beetle outbreak.     

The influence of mountain pine beetle outbreaks and drought on severe wildfires in northwestern Colorado and southern Wyoming: A look at the past century

Outbreaks of bark beetles and drought both lead to concerns about increased fire risk, but the relative importance of these two factors is the subject of much debate. We examined how mountain pine beetle (MPB) outbreaks and drought have contributed to the fire regime of lodgepole pine forests in northwestern Colorado and adjacent areas of southern Wyoming over the past century. We used dendroecological methods to reconstruct the pre-fire history of MPB outbreaks in twenty lodgepole pine stands that had burned between 1939 and 2006 and in 20 nearby lodgepole pine stands that were otherwise similar but that had not burned. Our data represent c. 80% of all large fires that had occurred in lodgepole pine forests in this study area over the past century. We also compared Palmer Drought Severity Index (PDSI) and actual evapotranspiration (AET) values between fire years and non-fire years. Burned stands were no more likely to have been affected by outbreak prior to fires than were nearby unburned stands. However, PDSI and AET values were both lower during fire years than during non-fire years. This work indicates that climate has been more important than outbreaks to the fire regime of lodgepole pine forests in this region over the past century. Indeed, we found no detectable increase in the occurrence of high-severity fires following MPB outbreaks. Dry conditions, rather than changes in fuels associated with outbreaks, appear to be most limiting to the occurrence of severe fires in these forests.     

White spruce regeneration following a major spruce beetle outbreak in forests on the Kenai Peninsula, Alaska

Between 1987 and 2000, a spruce beetle (Dendroctonus rufipennis) outbreak infested 1.19 million ha of spruce (Picea spp.) forests in Alaska, killing most of the large diameter trees. We evaluated whether these forests would recover to their pre-outbreak density, and determined the site conditions on which spruce germinated and survived following the spruce beetle outbreak in forests of the Anchor River watershed, Kenai Peninsula, Alaska. White spruce (Picea glauca) and Lutz's spruce (Picea × lutzii), a hybrid between white and Sitka spruce (Picea sitchensis), dominate the study area. We measured the pre- and post-outbreak density of spruce in 108 3 m × 80 m plots across the study area by recording all live trees and all dead trees >1.5 m tall in each plot. To determine the fine scale site conditions on which spruce germinated and survived, we measured ground surface and substrate characteristics within 20 cm circular plots around a subset of post-outbreak spruce seedlings. The density of post-outbreak spruce (855/ha) was adequate to restock the stands to their pre-outbreak densities (643/ha) for trees >1.5 m tall. We could not accurately estimate recovery for pre-outbreak spruce seedlings because dead seedlings may have decayed in the 5–18 years since the beetle outbreak occurred. At the fine scale, spruce that germinated post-outbreak grew on a wide variety of substrates including downed log, stump, mesic organic mat, peat, hummocks and mineral soil. They exhibited a strong preference for downed logs (53%) and stumps (4%), and most (91%) of the downed logs and stumps that spruce rooted on were heavily decayed. This preference for heavily decayed logs and stumps was especially evident given that their combined mean cover was only 2% in the 3 m × 80 m plots. Within the 3 m × 80 m plots, spruce seedling survival was negatively correlated with bluejoint (Calamagrostis canadensis) litter cover.     

Multiscale spatial variation of the bark beetle Ips sexdentatus damage in a pine plantation forest (Landes de Gascogne,Southwestern France)

Bark beetles are notorious pests of natural and planted forests causing extensive damage. These insects depend on dead or weakened trees but can switch to healthy trees during an outbreak as mass-attacks allow the beetle to overwhelm tree defences. Climatic events like windstorms are known to favour bark beetle outbreaks because they create a large number of breeding sites, i.e., weakened trees and for this reason, windthrown timber is generally preventively harvested and removed. In December 1999, the southwest of France was struck by a devastating windstorm that felled more that 27 million m³of timber. This event offered the opportunity to study large-scale spatial pattern of trees attacked by the bark beetle Ips sexdentatus and its relationship with the spatial location of pine logs that were temporally stored in piles along stand edges during the post-storm process of fallen tree removal. The study was undertaken in a pure maritime pine forest of 1300 ha in 2001 and 2002. We developed a landscape approach based on a GIS and a complete inventory of attacked trees. During this study more than 70% of the investigated stands had at least one tree attacked by I. sexdentatus  . Spatial aggregation prevailed in stands with n≥15$n\ge 15$ attacked trees. Patches of attacked trees were identified using a kernel estimation procedure coupled with randomization tests. Attacked trees formed patches of 500–700 m² on average which displayed a clumped spatial distribution. Log piles stemming from the sanitation removals were mainly distributed along the large access roads and showed an aggregated spatial pattern as well. The spatial relationship between patches of attacked trees and log pile storage areas was analyzed by means of the Ripley’s statistic that revealed a strong association at the scale of the studied forest. Our results indicated that bark beetle attacks were facilitated in the vicinity of areas where pine logs were stored. The spatial extent of this relationship was >1000 m. Similar results were obtained in 2001 and 2002 despite differences in the number and spatial distribution of attacked trees. The presence of a strong “facilitation effect” suggests that log piles should be removed quickly in order to prevent outbreaks of bark beetles.     

红脂大小蠹的捕食性天敌——大唼蜡甲发育和温度的关系研究

Rhizophagus grandis (Coleoptera: Rhizophagidae) is an important predator of Dendroctonus valens which is an invasive alien pest attacking pine trees in China. It was first introduced into China in 2000 and mass rearing techniques had been developed by the authors since then. This paper dealed with the study on the developmental threshold temperature and the effective accumulated temperature of R. grandis. The result showed that the developmental threshold temperatures of egg, larva and pupae were 3.59±2.92 ℃, 1.96±0.73 ℃ and 9.27±1.19 ℃ respectively, and the effective accumulated temperatures were 98.32±16.26 degree-day, 296.94±12.45 degree-day and 449.67±53.19 degree-day respectively. Therefore, the total effective accumulated temperature from egg to pupa was 844.93 degree-day.     

Changes in forest structure of a mixed conifer forest, southwestern Colorado, USA

We selected a warm/dry mixed conifer forest (ponderosa pine, white fir, Douglas-fir, and aspen) in southwestern Colorado to reconstruct historical conditions of fire regime and forest structure in preparation for an experiment in ecological restoration. Although mixed conifer forests are of high ecological and social value in the Southwest, they have been less studied than ponderosa pine forests. Fire-scar analysis on a 150-ha area showed recurring fires at mean intervals of 24 years (all fires with minimum of 2 sample trees scarred) to 32 years (fire scarring 25% or more of sample trees) from the 16th century until the abrupt cessation of fire after 1868, concurrent with European settlement. There was no evidence in age or species-specific data of severe burning at the scale of the study blocks (approximately 200 ha). The forest remained unharvested throughout most of the 20th century, until a cut in the early 1990s removed approximately equal basal areas of ponderosa pine and white fir. Forest structure had already changed substantially, however. Total basal area increased from an average of 11 m² ha⁻¹ in 1870 to 27 m² ha⁻¹ in 2003, despite harvesting of at least 8.4 m² ha⁻¹. Ponderosa pine declined from representing nearly two-thirds of basal area in 1870 to one-third in 2003. The other species increased dramatically, especially white fir, which went from 12% to 35% of basal area and dominated stand density with an average of 392 trees ha⁻¹. Total tree density increased from 142 trees ha⁻¹ in 1870 to 677 trees ha⁻¹ in 2003. The ecological changes that occurred here since the 19th century have been in exactly the opposite direction considering the warm, fire-favoring climate expected in the 21st century. If warm/dry mixed conifer forests of southern Colorado are to have a reasonable chance for persistence under the future climate regime, restoring conditions more similar to the frequently burned, open forests of the past is likely to be a useful starting point.     

A century of riparian forest expansion following extreme disturbance: Spatio-temporal change in Populus/Salix/Tamarix forests along the Upper San Pedro River,Arizona, USA

Populus–Salix forests are a valued riparian vegetation type in western North America. These pioneer, obligate phreatophytes have declined on some rivers, raising conservation concerns and stimulating restoration plantings, but have increased on others. Understanding patterns and causes of forest change is essential for formulating conservation, restoration and management plans. Our goal was to assess spatio-temporal patterns of vegetation change on the Upper San Pedro River in semiarid Arizona, USA, one of the few undammed rivers in the region. Over 100 years ago, intense floods initiated channel incision and substantially altered hydrogeomorphology. Pioneer trees began to establish in the widening post-entrenchment zone as the surfaces began to stabilize. Using a time-series of aerial photographs (1955–2003) we quantified recent change in area of riparian cover types. Analysis indicated that wooded area in the post-entrenchment zone nearly tripled from 1955 to 2003, at the expense of bare ground, and the active channel narrowed appreciably. This forest expansion represents a long-term response to river entrenchment, with the temporal pattern influenced by recent flood cycles and biogeomorphic feedbacks. Populus–Salix have established episodically during the infrequent years with high winter flood runoff, sequentially filling available recruitment space. Older cohorts cover wide swaths of the floodplain while young trees form narrow bands lining the channel. Barring extreme flooding, the pioneer forests are expected to senesce over the coming century. An additional factor that has shaped the pattern of post-entrenchment forest expansion is anthropogenic water withdrawal. Populus–Salix forest increase has been greatest within a conservation area, where stream flows are largely perennial. In drier, agricultural sectors, Populus–Salix have declined while the more deeply-rooted Tamarix has increased. Overall, the study reveals that long-term fluctuations in pioneer forest area and age structure are common on dryland rivers, and shows how past events such as extreme floods can interact with recent environmental practices such as freshwater withdrawal to influence riparian forest patterns. This underscores the necessity of a long-term perspective for forest conservation and management.     

The impact of forest fertilization on growth of mature Norway spruce affected by Sirococcus shoot blight

The effects of forest fertilization and Sirococcus shoot blight on growth of mature Norway spruce (Picea abies (L.) Karst.) were studied in a field experiment. The single tree fertilization experiment was established with 144 sample trees in the year 2000. These trees were selected among the dominant and co-dominant trees of the stand in a way that half of the trees exhibited symptoms of Sirococcus shoot blight and the other half did not. One-third of the sample trees were fertilized with dolomitic lime, one-third with kieserite and gypsum and one-third were left unfertilized as control. The treatments were randomly assigned to the sample trees. As the experimental design became unbalanced due to tree mortality caused by bark beetle infestation final analyses were performed with the volume growth data of 125 sample trees only. An analysis of covariance was used to evaluate the effect of fertilization and Sirococcus shoot blight on volume growth. The average volume increment of the period 1977–1980 was used as a covariate attribute, assuming that during this period growth was not yet affected by Sirococcus shoot blight. Indeed an effect of Sirococcus shoot blight on growth turned out to start after 1980 and the analysis revealed that over the whole period 1981–2006 trees with shoot blight symptoms had a significantly lower increment. The increment reduction of the nonfertilized trees by Sirococcus   was 7.46±2.90$7.46\pm 2.90$% in 1981 and was continuously increasing with time to 37±3.78$37\pm 3.78$% by the year 2000. A significant positive effect of fertilization was only found for the kieserite+gypsum$\text{kieserite}+\text{gypsum}$ variant from 2002 to 2006, with the highest surplus increment in 2004 with 31.6±15.2$31.6\pm 15.2$%, calculated as average over the diseased and healthy group. However, a mitigation of the Sirococcus-caused increment loss via fertilization with kieserite and gypsum was statistically significant only for the year 2003. A moderate relationship between basal area increment and disease severity was found, indicating decreasing basal area increment with increasing disease severity.     

Protection of spruce from colonization by the bark beetle, Ips perturbatus, in Alaska

Field bioassays were conducted in south-central Alaska in a stand of Lutz spruce, Picea × lutzii, to determine whether a semiochemical interruptant (verbenone and trans-conophthorin) and/or a defense-inducing plant hormone (methyl jasmonate, MJ) could be used to protect individual standing trees from bark beetle attack. During two experiments (initiated in May 2004 and 2005, respectively), attacks by Ips perturbatus on standing trees were induced by using a three-component aggregation pheromone (ipsenol, cis-verbenol, and ipsdienol) and prevented by using the interruptant. In 2005, treatments from 2004 were repeated and additional treatments were evaluated by using MJ spray or injection with and without the interruptant. Aggregation began before 3 or 7 June, and attack density was monitored through 3 or 16 August. During both years, tree mortality caused by I. perturbatus was recorded twice (in August, and in May of the following year). In both experiments, attack density was greatest on trees baited with the three-component attractive pheromone, but was significantly reduced by addition of the semiochemical interruptant to trees baited with the attractant. There were no significant differences in attack density between attractant + interruptant-treated trees and unbaited trees. In 2004, mortality was highest among attractant-baited trees, whereas addition of the interruptant significantly reduced the level of initial (10 week post-treatment) and final (54 week post-treatment) mortality. In 2005, no significant reduction in attack density occurred on trees baited with the attractant when MJ was sprayed or injected. The highest initial (10.6 week post-treatment) and final (49.4 week post-treatment) mortality was observed among trees that had been injected with MJ and baited with the attractant. Mortality at the final assessment was significantly lower in all other treatment groups. As in 2004, addition of the interruptant to attractant-baited trees significantly reduced the level of final mortality compared to attractant-baited trees. MJ was not attractive or interruptive to I. perturbatus or associated bark beetles in a flight trapping study. However, MJ-treated trees (sprayed or injected) exuded copious amounts of resin on the bark surface. Anatomical analyses of felled trees from four treatment groups [Tween (solvent)-sprayed, MJ-sprayed, Tween-injected, and MJ-injected + attractant baited] showed that treatment with MJ increased the number and size of resin ducts produced following treatment. These analyses also revealed a reduction in radial growth in MJ-treated trees. Our results show that during both years, treatment with a simple, two-component interruptant system of verbenone and trans-conophthorin significantly reduced I. perturbatus attack density and tree mortality on attractant-baited trees and provided a full year of protection from bark beetle attack.     

Contemporary pollen flow and mating patterns of a subtropical canopy tree Eurycorymbus cavaleriei in a fragmented agricultural landscape

China's subtropical forests have experienced severe deforestation and most native forests have been fragmented into an archipelago-like landscape. The genetic effect of habitat fragmentation depends largely on the level of gene flow within and among population fragments. In the present study, Eurycorymbus cavaleriei, a canopy tree found throughout subtropical China, was selected as a representative insect-pollinated plant species to assess genetic consequence of forest fragmentation. Contemporary pollen dispersal and mating patterns were estimated in two physically isolated stands of E. cavaleriei within fragmented forests using six highly polymorphic microsatellite loci. We found high genetic diversity (H_E = 0.670–0.754) in both adults and offspring in the fragmented agricultural landscape, suggesting that habitat fragmentation did not necessarily erode genetic diversity of E. cavaleriei. Although substantial pollen travelled less than 100 m, paternity analysis revealed that a large amount of long-distance pollination events occurred, with the average pollen dispersal distance being 1107 m and 325 m for the two stands, respectively. Extensive pollen immigration (39.3–42.6%) indicated that there was effective genetic connectivity among E. cavaleriei stands in the fragmented forests. twogener analysis revealed that the exponential power model was the best-fitting dispersal curve with a fat-tailed (b < 1) dispersal feature. The results from a multilocus mating system analysis suggested that a small amount of biparental inbreeding and some correlated mating events occurred in the fragmented forests, which were similar to our parallel findings in the continuous forests of E. cavaleriei. Estimates of pollen pool structure (Φ_FT = 0.128–0.174) indicated large genetic differences between pollen clouds accepted by maternal trees. The number of effective pollen donors (N_ep) in E. cavaleriei, estimated using both mltr (N_ep = 4.2–5.3) and twogener (N_ep = 2.9–3.9) models, was equivalent to the number of effective pollen donors detected in continuous forests of E. cavaleriei. The pollen dispersal and mating patterns detected here indicated that habitat fragmentation did not have a negative impact on pollen movement in E. cavaleriei, possibly due to its generalist pollination system and the resilient foraging behavior of its pollinators in response to changes in landscape structure. The long distances of pollen-mediated gene flow between patches highlight the conservation value of remnant forest fragments in maintaining genetic connectivity at the landscape scale in subtropical China.     

Positive association between understory species richness and a dominant shrub species (Corylus avellana) in a boreonemoral spruce forest

Old growth stands of boreonemoral spruce (Picea abies) forests frequently have a shrub layer dominated by hazel (Corylus avellana) – a species which is generally excluded in intensively managed forests due to clearcutting activities. We sampled understory species composition, richness and biomass, as well as environmental variables beneath these two species and also within forest ‘gaps’ in order to determine the effect of overstory species on understory vegetation. Species richness and biomass of herbaceous plants was significantly greater under Corylus compared with plots under Picea and in forest gaps. Indicator species analysis found that many species were significantly associated with Corylus. We found 45% of the total species found under woody plants occurred exclusively under Corylus. Light availability in spring and summer was higher in gaps than under forest cover but no difference was found between plots under Corylus and Picea. Hence, reductions in light availability cannot explain the differences in species composition. However, Ellenberg indicator values showed that more light demanding species were found under Corylus compared to Picea, but most light demanding species were found in gaps. The litter layer under Picea was three times thicker than under Corylus and this may be an important mechanism determining differences in understory composition and richness between the woody species. The presence of Corylus is an important factor enhancing local diversity and small-scale species variation within coniferous stands. Hence, management should maintain areas of Corylus shrubs to maintain understory species diversity in boreal forests.     

Woodpeckers increase in abundance but maintain fecundity in response to an outbreak of mountain pine bark beetles

Many temperate woodpecker species are thought to be highly conservative in their fecundity with little response to fluctuations in availability of resources. In a 15-year field study in interior British Columbia, we evaluated responses in abundance and fecundity of six species of resident and migrant woodpeckers (downy woodpecker [Picoides pubescens], hairy woodpecker [Picoides villosus], American three-toed woodpecker [Picoides dorsalis], pileated woodpecker [Drycopus pileatus], northern flicker [Colaptes auratus], and red-naped sapsucker [Sphyrapicus nuchalis]) to a large-scale outbreak of mountain pine bark beetles that resulted in a strong positive pulse in food supply. Population densities of woodpecker species increased during outbreak years. Despite the year-round multi-annual increase in food resources, and in contrast to the strong increases in fecundity shown by nuthatches and chickadees, annual fecundity (as indicated by clutch size and number of nestlings that fledged) did not change for any woodpecker species over the study. Similarly, we found no changes in fecundity in response to selective forest harvesting despite numerical increases for woodpeckers at these sites. Our study confirms that these woodpecker species are conservative in their reproductive investment patterns even during strong multi-annual increases in food. Our findings indicate woodpecker populations are regulated numerically through variable survival and/or greater immigration rates, which can result in higher breeding densities temporarily during resource pulses.     

Predisposition to bark beetle attack by root herbivores and associated pathogens: Roles in forest decline,gap formation,and persistence of endemic bark beetle populations

Bark beetles are largely known for their ability to undergo intermittent population eruptions that transform entire landscapes and pose significant economic hardships. However, most species do not undergo outbreaks, and eruptive species usually exert only minor disturbances. Understanding the dynamics of tree-killing noneruptive species can provide insights into how beetles persist at low densities, and how some spatiotemporal patterns of host predisposition may more likely favor breaching eruptive thresholds than others. Elucidating mechanisms behind low-density populations is challenging, however, due to the requirement of long-term monitoring and high degrees of spatial and temporal covariance. We censused more than 2700 trees annually over 7 years, and at the end of 17 years, in a mature red pine plantation. Trees were measured for the presence of bark beetles and wood borers that breed within the primary stem, root weevils that breed in root collars, and bark beetles that breed in basal stems. We quantify the sequence of events that drive this decline syndrome, with the primary emergent pattern being an interaction between below- and above-ground herbivores and their fungal symbionts. This interaction results in an expanding forest gap, with subsequent colonization by early-successional vegetation. Spatial position strongly affects the likelihood of tree mortality. A red pine is initially very likely to avoid attack by tree-killing Ips beetles, but attack becomes increasingly likely as the belowground complex spreads to neighboring trees and eventually make trees susceptible. This system is largely internally driven, as there are strong gap edge, but not stand-edge, effects. Additional stressors, such as drought, can provide an intermittent source of susceptible trees to Ips beetles, and elevated temperature slightly accentuates this effect. New gaps can arise from such trees as they subsequently become epicenters for the full complex of organisms associated with this decline, but this is not common. As Ips populations rise, there is some element of positive feedback, in that the proportion of killed trees that were not first colonized by root organisms increases. This positive feedback is very weak, however, and we propose the slope between beetle population density and reliance on host stress as a quantitative distinction along a gradient from noneruptive through eruptive species. Almost all trees colonized by Ips were subsequently colonized by wood borers, likely a source of negative feedback. We discuss implications to our overall understanding of cross-scale interactions, between-guild interactions, forest declines, and eruptive thresholds.     

Effects of mountain pine beetle (Dendroctonus ponderosae Hopkins) infestations on forest stand structure in the southern Canadian Rocky Mountains

Forests in the montane and lower subalpine ecoregions of the southern Canadian Rocky Mountains may have been more open and structurally diverse at the beginning of the 20th Century than today. Today, the area of mature Pinus contorta subsp. latifolia Dougl. forest that is conducive to mountain pine beetle (MPB, Dendroctonus ponderosae Hopkins) infestations, infrequent high severity fires, and herbivory appears to have increased in Banff and Kootenay National Parks. Based on a review of the literature, we hypothesised that MPB infestations increase forest stand structural diversity and tree species diversity. Stand structure parameters were investigated in mesic montane and lower subalpine stands approximately 15, 25, and 65 years after MPB outbreaks. Parameters measured were stand density (number of trees per ha), diameter at breast height, height class, species, age class distribution, and coarse woody debris mass. Influences of fire frequency, time since fire, and fire severity on these parameters were assessed to determine whether fire history had a confounding influence on stand structure. The Shannon–Wiener index indicated higher stand structural diversity 15 years but not 25 and 65 years after MPB infestations. MPB infestations led to general decreases within stands in the number of living trees, small diameter snags, and Pinus tree species and an increase within stands in the number of large diameter snags. Management that allows the occurrence of the natural fire regime of variable severity fires, in addition to some MPB infestations, would provide for more open and diverse stands. MPB infestations have some effects on stand structure that are similar to those of fire. Changes in stand structure resulting from recent declines in burning rates within Banff and Kootenay National Parks can be reversed to some extent by MPB infestations.     

Roles of seed and pollen dispersal in natural regeneration of Castanopsis fargesii (Fagaceae): Implications for forest management

Natural regeneration is an important process to reverse the loss of forests. Understanding the process of natural regeneration is crucial for achieving sustainable forest management. In this study, we examined the effects of seed and pollen dispersal in naturally regenerated populations of Castanopsis fargesii. Genetic variation in six populations along two successional series (three successional stages in each series: early, pre-climax, and climax) was assayed using RAPD (random amplified polymorphic DNA) markers. High genetic variability was observed as measured with Shannon's information index. A majority of genetic variation was distributed within populations (Φ_st = 0.1271) and significant isolation by distance existed among populations. A contrasting pattern of genetic variation along these two series was observed, representing different scenarios of natural regeneration processes. The ratio of the number of migrants between the mature populations to the number of migrants from the mature to immature populations was estimated as 1.146 ± 0.099 to 1.981 ± 0.164, implying that comparable seed and pollen dispersal might exist at a fine spatial scale (∼2 km²). The results suggest the critical role of seed dispersal in shaping genetic composition and diversity in the second-growth forests. Barriers to seed dispersal from a variety of genetic sources could result in low genetic diversity in naturally regenerated populations. Management that facilitates the connectivity of newly regenerated stands with mature forests could be effective for natural regeneration given the predominant role of short-distance dispersal of seeds in this species.     

温度、补充营养和体型大小对白蛾周氏啮小蜂成虫寿命的影响

寄生蜂成虫能否找到并寄生足够的寄主,是实现其最大生殖力的重要限制性因素之一[1],该因素与寿命直接相关,故寿命在进化中受到较大的自然选择压力,从而成为寄生蜂重要的适应性特征之一[2];在害虫生物防治实践中,寿命是决定寄生蜂控制寄主害虫能力的重要参数,是常用的评价天敌     

Combinations of several insecticides used for integrated control of Colorado potato beetle (Leptinotarsa decemlineata, Say., Coleoptera: Chrysomelidae)

The Colorado potato beetle (Leptinotarsa decemlineata Say.) (CPB) is the main potato pest in Croatia. The main problems in CPB control are related to a number of treatments and resistance development. In order to investigate the efficacy of combinations a reduced dose of spinosad (33% of the full doses) with the reduced doses (33% of full doses) of B.t.t., neem and pyrethrin against CPB larvae, 3-year field trials and one laboratory trial were conducted. Joint actions of the insecticides in combinations were analyzed. The CPB attack on experimental fields in all 3 years of investigation was high to moderate. In 2001 the maximum number of larvae on untreated plots was 11.7 larvae per plant, while in 2002 and 2003 it was lower, 5.5 and 6.1 larvae/plant, respectively. The efficacy obtained with a full dose of B.t.t. reached 75% in the field, and 78% in the laboratory trial. The residual efficacy of a full dose of B.t.t. lasted 7–10 days. The application of both, full and reduced doses of B.t.t., did not result in a significant yield increase. The results indicate that only one treatment with B.t.t. insecticide alone is not enough for efficient protection of potato against the attack of CPB larvae. The application of neem resulted in an efficacy of between 54 and 87.9%. The residual activity of neem was too short to ensure significant yield increase in 2 out of 3 years of the investigation. The application of a full dose of pyrethrin ensured an efficacy of between 86 and 89% with residual activity of 7–10 days. Only in the trial in 2003 significant yield increase (40% higher than untreated control) after the application of a full dose of pyrethin was recorded. The application of both, full and reduced doses of spinosad resulted in very high efficacy (over 90%), with residual activity between 10 and 20 days. The application of spinosad resulted in a significant yield increase in 2003, both in full and reduced doses, and in 2001 in the full dose (the reduced dose was not tested alone in 2001). High efficacy of the reduced dose of spinosad indicated its high biological activity and possibility for the reduction of the recommended dose. However, applications of combinations of insecticides in reduced doses (spinosad with B.t.t., neem and pyrethrin) resulted in efficacies of over 97% with residual activity of up to 21 days. Significant yield increase was noted after the application of the combinations of spinosad with B.t.t. and pyrethrin. The results of the laboratory trial confirmed the results of field trials. The joint action of insecticides was mainly described as independent synergism. It can be concluded that applied combinations are suitable in IPM in potato.     

Defoliation and growth relationships for mid-rotation Sitka spruce attacked by the green spruce aphid, Elatobium abietinum (Walker) (Homoptera: Aphididae)

Green spruce aphid (Elatobium abietinum) feeds primarily on the 1-year-old and older needle leaves of spruce (Picea spp.) and is a major defoliator of commercial Sitka spruce plantations in the UK and other maritime regions of north-west Europe. The impact of E. abietinum on mid-rotation (23-28-year-old) Sitka spruce in Radnor Forest, in mid-Wales, was determined by comparing aphid population densities, defoliation and growth between plots of trees that were either treated with insecticide or were left untreated. The experimental treatments were maintained for 5 years and over this period (2002-2006) peak E. abietinum densities in the untreated plots varied from 5 to 36 aphids per 100 needles. These densities, which were representative of low to moderate rates of infestation, were associated with low rates of defoliation (0-8%), but they had a significant (P ? 0.01) impact on mean radial increment (RI) and mean volume increment (VI). In 2005, the year with the highest aphid populations, peak densities in the untreated plots averaged 14 aphids per 100 needles and this rate of infestation reduced RI by 17%, VI by 10% and the dry weight of current-year needles by 10%. On average, across all years, infestation by E. abietinum reduced mean annual VI by 6%. Comparisons with previous studies on the impact of the aphid on 4-year-old, 7-year-old and 9-year-old Sitka spruce (Straw et al., 2005) indicate that defoliation by E. abietinum, on a per capita basis, decreases as trees grow older, but that the impact on VI increases. The different growth response of young and mid-rotation Sitka spruce to infestation is related to differences in canopy structure, particularly in the ratio of current-year needles to older needles, and the greater demands on photosynthetic production in older trees that arise from the need to support an increasing quantity of non-photosynthetic structural tissues in branches, stem and roots.