Effects of fragmentation on juvenile morphology of Acer saccharum Marsh. (sugar maple) in temperate forests of northeastern Ohio,USA

The influence of forest fragmentation on population and community dynamics of woody plants has been well established worldwide, but rarely at the level of an individual plant. We evaluated the influence of fragmentation on juvenile stem morphology of Acer saccharum Marsh. (sugar maple), while also examining light levels and considering possible confounding effects attributed to elevation gradients in temperate forests of northeastern Ohio, USA. At two sites, plant stem dimensions, canopy openness, and relative ground level elevation were measured using randomly positioned plots in forest edge and interior habitats that were within 25 and 60–100 m from a forest edge, respectively. Ratios of stem length to stem basal diameter were greater in forest interiors than near forest edges. These differences in stem morphology between habitats were likely a result of stem elongation in relation to a shade avoidance response in forest interiors that were consistently darker than forest edge areas across study sites. By contrast, such morphological differences were likely not related to variation in relative ground level elevation since a subtle elevation gradient was detected at only one site. We encourage experimentation to identify mechanisms that affect plant stem morphology of young individuals and its influence, in turn, on plant population dynamics in fragmented forests.     

Regeneration dynamics in remnant Tsuga canadensis stands in the northern Lake States: Potential direct and indirect effects of herbivory

We examined the effects of white-tailed deer (Odocoileus virginianus) herbivory and microsite limitation on Tsuga canadensis regeneration in 39 randomly selected remnant T. canadensis stands in Michigan's Upper Peninsula. Deer of the region migrate to and congregate in T. canadensis stands in winter resulting in strong seasonal habitat use patterns. In each study stand, we quantified vegetation, microsite availability, and deer use (via pellet counts). While some stands contained high densities of T. canadensis regeneration (stems < 4.0 cm dbh), we found complete T. canadensis regeneration failures in 6 out of 39 stands. Additionally, 17 and 22 stands respectively, had complete failures in the small and large sapling categories. General linear models suggested that deer use was the primary limiting factor in the small sapling size class, even at relatively low levels of deer use. T. canadensis seedling density was positively associated with the availability of high-decay coarse woody debris and negatively associated with basal area of Acer saccharum in the overstory. This latter association may be due, at least in part, to negative effects of broadleaf litter on T. canadensis establishment and a general trend toward increasing Acer abundance in the regeneration layer. Our results suggest that differential tolerance to browsing (Tsuga vs. Acer) in conjunction with reduced germination substrate availability may set up a scenario where successful T. canadensis establishments is more limited by legacy and indirect than direct effects given contemporary levels of deer use.     

Sugar maple (Acer saccharum Marsh.) growth is influenced by close conspecifics and skid trail proximity following selection harvest

In this study, we quantified the effects of local neighbourhood competition, light availability, and proximity to skid trails on the growth of sugar maple (Acer saccharum Marsh.) trees following selection harvest. We hypothesized that growth would increase with decreasing competition and increasing light availability, but that proximity to skid trails would negatively affect growth. A total of 300 sugar maples were sampled ∼10 years after selection harvesting in 18 stands in Témiscamingue (Québec, Canada). Detailed tree and skid trail maps were obtained in one 0.4 ha plot per stand. Square-root transformed radial growth data were fitted to a linear mixed model that included tree diameter, crown position, a neighbourhood competition index, light availability (estimated using the SORTIE light model), and distance to the nearest skid trail as explanatory variables. We considered various distance-dependent or -independent indices based on neighbourhood radii ranging from 6 to 12 m. The competition index that provided the best fit to the data was a distance-dependent index computed in a 6 m search radius, but a distance-independent version of the competition index provided an almost equivalent fit to data. Models corresponding to all combinations of main effects were fit to data using maximum likelihood, and weighted averages of parameter estimates were obtained using multimodel inference. All predictors had an influence on growth, with the exception of light. Radial growth decreased with increasing tree diameter, level of competition and proximity to skid trails, and varied among crown positions with trees in suppressed and intermediate positions having lower growth rates than codominants and dominants. Our results indicate that in selection managed stands, the radial growth of sugar maple trees depends on competition from close (≤6 m) conspecific neighbours, and is still affected by proximity to skid trails ∼10 years after harvesting. Such results underscore the importance of minimizing the extent of skid trail networks by careful pre-harvest planning of trail layout. We also conclude that the impact of heterogeneity among individual-tree neighbourhoods, such as those resulting from alternative spatial patterns of harvest, can usefully be integrated into models of post-harvest tree growth.     

Understorey plant community characteristics and natural hardwood regeneration under three partial harvest treatments applied in a northern red oak (Quercus rubra L.) stand in the Great Lakes-St. Lawrence forest region of Canada

Throughout eastern North America, stands of northern red oak (Quercus rubra L.) are undergoing successional replacement by shade-tolerant competitors. In the Great Lakes-St. Lawrence (GLSL) forest region, Q. rubra approaches the northern limit of its distribution, and ecosystem-specific silvicultural directives are needed to promote regeneration. We used an inductive, ordination-based approach to explore patterns in understorey plant community composition and microenvironment under different partial harvest treatments applied in a GLSL hardwood stand, and related these to characteristics of natural seedlings of Q. rubra and its competitors Acer rubrum and Acer saccharum.Two years after harvest, we established 2 m × 2 m plots in a stratified random design under 70% (n = 20) and 50% (n = 19) crown closure uniform shelterwood, group selection (n = 15), and uncut upper slope (n = 10) and lower slope (n = 10) areas. Percent cover of understorey vascular plant species, and a suite of microclimatic and edaphic variables were measured in each plot. Density, mean diameter and mean height of seedlings in the understorey (height <1 m) were determined in each plot for Q. rubra, A. rubrum and A. saccharum.Correspondence analysis (CA) ordination extracted two major axes explaining 21.6% of the total inertia in the species cover by plot matrix. Axis one separated uncut plots from the 50% shelterwood along a gradient of canopy cover associated with partial harvest treatments. Plot scores on axis one (13.2%) reflected a shift in dominance of the understorey from shade-tolerant Acer spp. to shade-intolerant colonizers, Rubus idaeus and Carex spp. Plot scores on axis one were directly (p < 0.05) associated with total understorey plant cover, litter depth, soil temperature and pH, but not with measures of plant diversity. Axis two (8.4%) separated plots from upper slope and lower slope areas, and plot scores were inversely associated (p < 0.05) with soil pH, phosphorus and nitrogen levels. Along axis two there was a shift in dominance from competitive (e.g. A. saccharum) to stress-tolerant (e.g. A. rubrum) species as soil fertility declined. Stepwise linear regression indicated seedling diameter in Q. rubra, A. rubrum and A. saccharum was inversely related to canopy cover. This suggests all three species benefited from partial harvest, although the relationship was strongest in Q. rubra. Patterns in understorey composition, microenvironment and seedling characteristics provide the basis to identify the main competitors of Q. rubra seedlings and adjust regeneration efforts along gradients of canopy closure and soil fertility under partial harvest systems within the GLSL forest region.     

Roost tree selection by northern myotis (Myotis septentrionalis) maternity colonies following prescribed fire in a Central Appalachian Mountains hardwood forest

Following decades of fire suppression in eastern forests, prescribed fire as a tool to restore or enhance oak (Quercus spp.)-dominated communities is gaining widespread acceptance in the Appalachian Mountains and elsewhere. However, the interactions of fire with biotic components such as wildlife that might be impacted by prescribed fire are poorly documented. For tree-roosting bats, fire can enhance roosting habitat by creating snags and increasing solar radiation at existing roosts. In 2007 and 2008, we examined roost selection of forest-interior dwelling northern myotis (Myotis septentrionalis) maternity colonies in stands treated with prescribed fire (hereafter, fire) and in unburned (hereafter, control) stands on the Fernow Experimental Forest, West Virginia. Using radio telemetry, we tracked 36 female northern myotis to 69 roost trees; 25 in the fire treatment and 44 in the control treatment. Using logistic regression and an information-theoretic model selection approach, we determined that within the fire treatment, northern myotis maternity colonies were more likely to use cavity trees that were smaller in diameter, higher in crown class, and located in stands with lower basal area, gentler slopes, and higher percentage of fire-killed stems than random trees. Moreover, roosts often were surrounded by trees that were in the upper crown classes. In the control treatment, northern myotis were more likely to roost nearer the tops of larger diameter cavity trees in early stages of decay that were surrounded by decaying trees in the upper crown classes than random trees. Roost trees in the fire treatment were associated with larger overall canopy gaps than roost trees within the control treatment. Regardless of treatment, northern myotis maternity colonies roosted in black locust (Robinia pseudoacacia) in greater proportion than its availability. Ambient temperatures recorded at a subset of roost trees in fire and control treatments indicated that daily minimum temperatures were similar, but daily mean and maximum temperatures were higher in the fire treatments, possibly due to larger canopy gaps created by the senescence and decay of the surrounding fire-killed overstory trees. Northern myotis roost-switching frequency, distance between successive roosts, and duration of individual roost tree use were similar between the fire and control treatments, suggesting similar roost tree availability despite a significantly higher proportion of potential roost trees in the fire treatment. Northern myotis readily exploited alterations to forest structure created by the reintroduction of fire, which accelerated snag creation and enlarged existing or created new canopy gaps, but it remains to be determined if these conditions translate into increased recruitment and survivorship.     

Long-term spatial dynamics of Acer saccharum during a population explosion in an old-growth remnant forest in Illinois

Species richness and evenness have greatly declined in oak–hickory forests in the central hardwood region in the U.S.A. in the past 100 years due to the rapid population growth of Acer saccharum. This study used a 50-year record of spatial dynamics to examine how demographic processes, particularly recruitment, may have contributed to this increase in an old-growth forest remnant, Brownfield Woods, Urbana, Illinois, U.S.A. The impact of canopy disturbance, including the outbreak of Dutch elm disease, on this increase was also evaluated. Historical maps of trees (≥7.6 cm DBH) from 1951, 1988, and 2001 in a 180 m × 280 m area were used to develop a series of univariate Ripley's L(d) functions to study changes in spatial patterns of three size classes of A. saccharum over time. Bivariate Ripley's L(d) functions were also utilized to evaluate spatial associations between recruitment and canopy disturbance. Our results indicated that A. saccharum was aggregated at most spatial scales up to 80 m during 1951–2001. Such aggregation arose mainly from small individuals. Furthermore, newly recruited individuals were aggregated at multiple spatial scales, and were significantly associated with canopy disturbance in general, as well as gaps created by Ulmus trees killed by Dutch elm disease. The aggregation of the 1951 initial group of small individuals changed via mortality to a random distribution over time. The results indicate that tree deaths caused by disturbances of different scales and types were the main cause of increased recruitment of A. saccharum in Brownfield Woods. The occurrence of Dutch elm disease further accelerated its population increase. This study demonstrated a direct spatial link between recruitment of A. saccharum and disturbance, and provided a long-term case study of a population explosion.     

Relative susceptibility of four species of African mahogany to the shoot borer Hypsipyla robusta (Lepidoptera: Pyralidae) in the moist semideciduous forest of Ghana

We examined the relative susceptibility of four mahogany species, Khaya ivorensis, Khaya anthotheca, Entandrophragma angolense, and E. utile, to Hypsipyla robusta attack. Seeds were obtained from one to three parent trees for each species. The research was conducted in the moist semideciduous forest zone in Ghana and used a randomized complete block design. Tree height and diameter and height to first branch were measured until 24 months after out-planting in the field. H. robusta damage was assessed by counting the numbers of shoots attacked, branches, and dead shoots. Khaya spp. grew better but experienced more attack than Entandrophragma spp. The relative susceptibility to H. robusta attack, from most to least, of the four species was: K. anthotheca > K. ivorensis > E. angolense > E. utile. At 24 months, the mean number of shoots attacked per tree ranged from 1.0 for an E. utile seed source to 3.6 on for a K. anthotheca seed source. At 15 months, K. anthotheca and K. ivorensis started branching at about 1.5 m, but height of clear trunk increased over time due to self-pruning. As K. anthotheca grew taller, the number of H. robusta attacks per tree declined. This suggested that selection of genotypes and species that are tolerant of H. robusta attack based on infestation of young plants may not be appropriate. Genetic factors more completely reflecting the response of different species and genotypes to H. robusta attack may manifest themselves at later growth stages.     

特有种野生血皮槭的生长状况

血皮槭(Acer griseum(Franch)Pax)是无患子科(Sapindaceae)槭树属(Acer)落叶乔木,为我国特有种。其树干赭褐色,树皮卷曲状剥落,秋季叶色红艳或黄色,是世界著名的观赏树种。1901年,西方植物学家威尔逊在湖北发现血皮槭,将其和珙桐一起作为园林树种引入欧洲[1],继而血皮槭又被引入美国等其他西方国家。1993年,血皮槭获得英国皇家园艺学会园艺奖,目前在国外用于庭院观赏。血皮槭不仅具有观赏价值,而且具有用材和药用价值[2-3]。血皮槭在2004年出版的《中国物种红色目     

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.     

Developing and testing a landscape-scale habitat suitability model for fisher (Martes pennanti) in forests of interior northern California

The fisher is warranted for protection under the Endangered Species Act in the western United States and, as such, it is especially important that conservation and management actions are based on sound scientific information. We developed a landscape-scale suitability model for interior northern California to predict the probability of detecting fishers and to identify areas of important fisher habitat. Previous models have been extrapolated to this region, but our model was developed from the results of strategically planned detection surveys within the study area. We used generalized additive modeling to create a model that best distinguished detection (n = 55) from non-detection (n = 90) locations on the basis of environmental covariates. Four models were averaged to create a final model including the following variables: Amount of Dense Forest, Percent Hardwood, Medium & Large Trees, Structurally Complex Forest, Adjusted Elevation, Insolation Index and Predicted Abundance of Mammalian Prey. This model was well calibrated and correctly classified fisher detections 83.6% of the time and absences (non-detections) 70.0%. Independent test data were classified less well; 76.2% and 53.0%, respectively, perhaps a result of differences in the spatial and temporal characteristics of the data used to build versus test the model. The model is the first comprehensive portrayal of the distribution and configuration of habitat suitability in this region and provides managers a tool to monitor habitat change over time and to plan vegetation treatments. It also represents an example for the development of similar models for dispersal-limited mammals with large area needs, as well as other species associated with late-successional forests in northern California.     

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.     

Growth and nutrition of Pinus radiata in response to fertilizer applied after thinning and interaction with defoliation associated with Essigella californica

Infestations of Essigella californica following the installation of post-thinning fertilizer trials in Pinus radiata plantations provided an opportunity to examine the impact of repeated defoliation over a period of 8 years (1997–2005). Replicated treatments (n = 4) of nil fertilizer (control), N (300 kg ha⁻¹) as urea, P (80 kg ha⁻¹) and S (45 kg ha⁻¹) as superphosphates were applied immediately after thinning at three sites and this was followed by a second application of NPS fertilizers 6 years later with N applied at 300 kg ha⁻¹ as urea and ammonium sulphate and P at 80 or 120 kg ha⁻¹. Defoliation of untreated P. radiata gradually increased to 50% over a period of 8 years. Basal area growth was negatively correlated with average defoliation for two consecutive post-fertilizer periods of 6 and 2 years. Growth responses to fertilizer varied considerably between sites but the largest improvement in growth was due to NPS fertilizer, this increased basal area by 30–80%. Application of N fertilizer raised total N levels in foliage and increased defoliation with a commensurate loss in growth under conditions of deficiencies of S or P. Repeated infestations gradually increased the percentage of trees with severe defoliation (>80% loss of foliage) indicating that nutrient-deficient trees have a reduced capacity for foliage recovery between episodes of peak infestation. In contrast, treatment with N fertilizer in combination with S- and P-corrected deficiencies of these nutrients, raised levels of total N in foliage and reduced defoliation to approximately 20%. Basal area growth responses to NPS fertilizers reflected improved nutrition as well as reduced insect damage. The reduction in defoliation under conditions of balanced tree nutrition was most likely due to enhanced needle retention following correction of P deficiency as well as greater availability of nutrients enabling a more vigorous recovery of P. radiata after an episode of E. californica activity. Treatment with fertilizer therefore reduced the long-term impact of aphid damage and improved growth of P. radiata.     

Distribution and nest site selection of red-shouldered hawks (Buteo lineatus) in forests of northeastern Wisconsin (USA)

Red-shouldered hawks (Buteo lineatus) are threatened in Wisconsin and when nest sites are found during the cruising or marking stage of timber harvesting, the harvest is altered to accommodate the hawks. If nest site locations are known before initiation of timber harvest, foresters can employ a proactive approach to manage red-shouldered hawks while maintaining timber production. We searched for red-shouldered hawks nest sites on Marinette County Forest (MCF) which encompasses 94,000 ha in northeastern Wisconsin and is the second largest county forest in the state. We used a comparative modeling approach to evaluate distribution and habitat relationships of red-shouldered hawk nest sites in relation to a suite of environmental variables in MCF. Models were used to develop forest management recommendations for red-shouldered hawks in Wisconsin. During the spring of 2006 and 2007, we broadcasted conspecific calls to survey 1121 calling stations along forest roads and trails. We located 20 and 25 active nesting territories in 2006 and 2007, respectively (11 of which were active in both years). To understand nest site selection, we measured 22 habitat variables within 0.04-ha plots at active nest sites (n = 34) and at stratified random sites (n = 61). Logistic regression with information-theoretic model selection identified a model including greater tree species richness and closeness to forested wetland as the best-approximating model. Variable selection with Discriminant Function Analysis (DFA) indicated that nest selection was best explained by greater number of tree species, closer distance to forested wetlands, greater volume of downed woody debris, fewer small sawlogs, and increased proximity to streams. Univariate comparisons identified four of the five aforementioned variables in the DFA model as significant. Red-shouldered hawks are likely more common in Wisconsin than their state status suggests. Forest management for red-shouldered hawk nest sites should focus on increasing tree species richness, increasing down woody debris volume, and protecting forested wetlands. These recommendations may assist property managers to locate and plan for continued persistence of this species on MCF.     

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.     

Driving factors for natural tree rejuvenation in anthropogenic pine (Pinus sylvestris L.) forests of NE Germany

The rejuvenation ecology of three main tree species in anthropogenic pine (Pinus sylvestris L.) forests is explored in our study. We focus on the scale of micro-plots, which provide the safe sites for tree rejuvenation. We thrive on the multi-factorial relationship of tree establishment and driving ecological factors using a large dataset from pine stands in NE Germany and applying multivariate analyses. The success of the establishment of the investigated focal tree species Fagus sylvatica L., Quercus petraea Liebl. and Pinus sylvestris L. is, on general, mostly affected by three factors, i.e. water balance of the upper soil layers, browsing pressure, and diaspore sources. Our investigations on the micro-plot scale revealed species-specific differences. For beech saplings <50 cm growth height, primarily the availability of water, indicated by available water capacity (AWC), thickness, quality, and structure of the organic layer, silt and humus content in the topsoil, and the lack of a dense competitive herb layer, were identified as most important factors. On the contrary, oak seems hardly be restricted by hydrologic and/or trophic deficits in the topsoil or humus layer. In conclusion and comparison to Fagus sylvatica L., we assume for Quercus petraea Liebl. advantages in natural regeneration processes under sub-continental climate conditions and thus under the scenarios of climate change. Pinus sylvestris L. regeneration in our investigation area occurs only in a narrow niche. We conclude with regard to future forest development and the objective of stand conversion with low management intensity that oak should be favoured within natural stand regeneration.     

Canopy gaps and regeneration in old-growth Oriental beech (Fagus orientalis Lipsky) stands, northern Iran

Virgin beech Fagus orientalis forests in northern Iran provide a unique opportunity to study the disturbance regimes of forest ecosystems without human influence. The aim of this research was to describe characteristics of natural canopy gaps and gap area fraction as an environmental influence on the success of beech seedling establishment in mature beech stands. All canopy gaps and related forest parameters were measured within three 25 ha areas within the Gorazbon compartment of the University of Tehran’s Kheyrud Experimental Forest. An average of 3 gaps/ha occurred in the forest and gap sizes ranged from 19 to 1250 m² in size. The most frequent (58%) canopy gaps were <200 m². In total, canopy gaps covered 9.3% of the forest area. Gaps <400 m² in size were irregular in shape, but larger gaps did not differ significantly in shape from a circle. Most gaps (41%) were formed by a single tree-fall event and beech made up 63% of gap makers and 93% of gap fillers. Frequency and diversity of tree seedlings were not significantly correlated with gap size. The minimum gap size that contained at least one beech gap-filling sapling (<1.3 m tall) was 23.7 m². The median gap size containing at least one beech gap-filling sapling was 206 m² and the maximum size was 1808 m². The management implications from our study suggest that the creation of small and medium sized gaps in mixed beech forest should mimic natural disturbance regimes and provide suitable conditions for successful beech regeneration.     

Fragmentation and spatial genetic structure in Tabebuia ochracea (Bignoniaceae) a seasonally dry Neotropical tree

In this study we investigate the effect of fragmentation and disturbance on the spatial genetic structure, heterozygosity and inbreeding in Tabebuia ochracea (Bignoniaceae) in a seasonally Neotropical dry forest in the medium São Francisco River basin, Centre-East Brazil, based on the polymorphism at seven microsatellite loci. Four populations with different histories of disturbance and fragmentation were sampled: two continuous population (CP1 and CP2), with no history of recent disturbance and two fragmented and isolated population (FP1 and FP2), with recent history of disturbance due to logging for pasture establishment. Fragmented and continuous populations did not differ in any estimated parameter. However, all populations showed low levels of polymorphism and genetic diversity and high levels of inbreeding. Also, no spatial genetic structure was detected for populations using SPAGeDI software and no differentiation between these four populations was detected by Bayesian analyses performed with STRUCTURE software (K = 1). Differentiation measure by Wright's θ (0.032) and Hedrick G_ST (0.032) were significant but low. Our results strongly suggest that continuous populations are seed sources for the fragmented populations and that fragmentation and disturbance have been affecting these populations of T. ochracea in the Centre-East Brazil, leading to low levels of polymorphism and genetic diversity, and high inbreeding. Therefore, conservation efforts should increase in this region, with a reduction of agriculture expansion and the remove of cultivated areas and cattle from the Mata Seca and Lagoa do Cajueiro State Parks.     

Age structure and regeneration of subalpine fir (Abies fargesii) forests across an altitudinal range in the Qinling Mountains,China

Age structure and regeneration dynamics of subalpine fir (Abies fargesii) forest were studied across the altitudinal range in both the north and south aspects of the Qinling Mountains, China. Ages of individual fir trees were determined based on the number of rings counted from cores and the number of years to reach coring height estimated using age–height regression. Fir age structure and regeneration dynamics were similar in both the north and south aspects. A unimodal population age structure was found at the low- and mid-elevations in both aspects, indicating that environmental factors might play an important role in shaping A. fargesii age structure and regeneration at those sites. There was a recruitment pulse during the time period 1830–1890 at each altitudinal site, but no stem recruitment occurred at the low- and mid-elevations in the last century, which might be attributed to the intensive cover of understory bamboo. Fir trees were, however, persistently recruited at the upper limits during the last 150 years, and the fir tree density at the upper limits was significantly higher than that at the lower limits in both aspects. The fir population at the upper limits showed a significant increase in recruitment and stem density relative to the fir population at the low- and mid-elevations in the last century. We propose that the differences in recruitment might promote variations in stand structure and regeneration dynamics of the subalpine fir forests along the altitudinal gradient in the Qinling Mountains, China.     

Relationships between elm spanworm, Ennomos subsignaria, juvenile density and defoliation on mature sycamore maple in an urban environment

Using field surveys, we established sampling procedures for estimating defoliation resulting from elm spanworm, Ennomos subsignaria (Hübner) (Lepidoptera: Geometridae), feeding on mature sycamore maple, Acer pseudoplatanus L. (Aceraceae), in St. John's, Newfoundland and Labrador. We also determined whether densities of E. subsignaria eggs, egg masses, early- or late-instar larvae could predict the amount of defoliation at the end of the larval feeding period. Defoliation estimates acquired by sampling branches from only the lower, mid or upper crown explained ≥80% of the variation in tree-level defoliation, suggesting that density–defoliation relationships established using defoliation data from any crown level would also be useful for predicting tree-level defoliation. In linear regressions, egg and egg mass densities explained ≤20% of variation in defoliation and thus only provide a crude relative estimate of the amount of defoliation that will occur. Early- and late-instar larval density in the lower crown explained 53 and 29%, respectively, of the variation in defoliation in the lower crown, where defoliation levels were highest. Thus monitoring early-instar density in the lower crown should provide pest managers with reliable information for decisions regarding whether to apply suppression tactics while allowing enough time to implement these tactics, if necessary.     

Variation in the abundance of invertebrate predators of the green spruce aphid Elatobium abietinum (Walker) (Homoptera: Aphididae) along an altitudinal transect

Green spruce aphid Elatobium abietinum (Walker) is a major defoliator of Sitka spruce (Picea sitchensis (Bong.) Carr.) in north-west Europe and other parts of the world that have a mild and wet maritime climate. Periods of cold weather during the winter currently limit E. abietinum populations and the amount of damage, but as mean winter temperatures rise in response to global climate change, overwinter survival of the aphid is likely to improve and the risk of severe and more widespread defoliation will increase. Populations of E. abietinum are also influenced by mortality caused by generalist invertebrate predators, although the extent to which predation might modify the response of E. abietinum to climate change is unclear. In this study, the response of generalist predators to changes in environmental conditions and potential increases in E. abietinum populations was evaluated by sampling invertebrate predators in the canopy of Sitka spruce along an altitudinal gradient from 310 m to 610 m above sea-level. Variation in predator abundance was related to local climate and aphid numbers. Population densities of E. abietinum differed widely between altitudes, and showed different patterns in different years, but mean densities over the 3-year study were highest at mid altitudes. In contrast, the majority of invertebrate predators (coccinellid and cantharid beetles, syrphid larvae, brown lacewings, spiders and harvestmen) were most abundant at low altitudes. Their abundance was not correlated with E. abietinum densities, but coincided with the more favourable climatic conditions at low altitudes and the availability of alternative prey. The association of low population densities of E. abietinum with greater general abundance of invertebrate predators at low altitudes, where higher temperatures would have been expected to promote higher aphid populations, suggests that above a certain temperature regime generalist predators have the capacity to prevent E. abietinum responding to further increases in environmental temperatures and eventually cause mean population densities to decline. Consequently, increases in the activity and abundance of natural enemies need to be considered when predicting how E. abietinum might respond to climate change.