Stand dynamics of a yellow-poplar (Liriodendron tulipifera L.) forest in the Appalachian Mountains, Virginia, USA

Yellow-poplar (Liriodendron tulipifera L.) is a large, rapidly growing, shade-intolerant tree species common after disturbances on moist sites in the Appalachian Mountains. The species is typically scattered throughout old-growth mesophytic forests, where periodic gap formation creates conditions favorable for yellow-poplar establishment and growth. On abandoned agricultural fields, however, it is common for nearly monospecific forests of yellow-poplar to develop.This study examines stand dynamics of a yellow-poplar forest in western Virginia, USA that was established on agricultural fields abandoned in the late 1940s. Increment cores were collected from yellow-poplar trees growing on exposed ridgetops and in a more sheltered hollow. Tree-ring data show that the forest is even-aged. Tree establishment began about 5 years earlier on the ridgetops than in the hollow. Major ice storms disturbed the forest in 1978 and 1994, with two separate events in 1994. Ice storms disturb forests by depositing heavy loads of freezing rain on trees, breaking or uprooting them. The dendroecological data collected for this study provide little support for the hypothesis that ice storm disturbance promotes the establishment of new yellow-poplar cohorts. However, the data show that radial growth of some trees increased after ice storm disturbance, a pattern that reflects the increased availability of light following disturbance. Radial growth declined in some other trees as a consequence of severe injury during the storms.Radial growth responses following the 1978 ice storm were stronger on the ridgetops than in the hollow, suggesting that tree damage was more severe on the higher, more exposed sites. Growth responses were relatively mild following the storms of 1994, and did not exhibit pronounced topographic variations.     

Coupled effects of wind‐storms and drought on tree mortality across 115 forest stands from the Western Alps and the Jura mountains

Damage due to wind‐storms and droughts is increasing in many temperate forests, yet little is known about the long‐term roles of these key climatic factors in forest dynamics and in the carbon budget. The objective of this study was to estimate individual and coupled effects of droughts and wind‐storms on adult tree mortality across a 31‐year period in 115 managed, mixed coniferous forest stands from the Western Alps and the Jura mountains. For each stand, yearly mortality was inferred from management records, yearly drought from interpolated fields of monthly temperature, precipitation and soil water holding capacity, and wind‐storms from interpolated fields of daily maximum wind speed. We performed a thorough model selection based on a leave‐one‐out cross‐validation of the time series. We compared different critical wind speeds (CWSs) for damage, wind‐storm, and stand variables and statistical models. We found that a model including stand characteristics, drought, and storm strength using a CWS of 25 ms^?1 performed the best across most stands. Using this best model, we found that drought increased damage risk only in the most southerly forests, and its effect is generally maintained for up to 2 years. Storm strength increased damage risk in all forests in a relatively uniform way. In some stands, we found positive interaction between drought and storm strength most likely because drought weakens trees, and they became more prone to stem breakage under wind‐loading. In other stands, we found negative interaction between drought and storm strength, where excessive rain likely leads to soil water saturation making trees more susceptible to overturning in a wind‐storm. Our results stress that temporal data are essential to make valid inferences about ecological impacts of disturbance events, and that making inferences about disturbance agents separately can be of limited validity. Under projected future climatic conditions, the direction and strength of these ecological interactions could also change.     

A CSR classification of tree life history strategies and implications for ice storm damage

Differences in life history strategy influence the ecological roles of plant species, including their susceptibility to disturbance events. According to Grime's CSR model, plants exhibit three primary strategies, which reflect tradeoffs between stress and disturbance. Here we classify eastern North American tree species into life history strategies on the basis of the CSR model. Then, using data on ice storm damage to trees, we investigate how the level of damage varied among the different CSR categories. We used tree damage data for almost 2000 individual trees representing 30 species collected during two ice storms in the Appalachian Mountains. We augmented the study with ice damage data gleaned from nine published ice‐storm studies containing over 30 000 individuals representing 22 species. The trees we identified as stress‐tolerators (S) consistently sustained less damage than the other species. This finding matches the stress‐tolerant strategy: damage‐resistance is imperative for the persistence of trees that exhibit slow growth, low reproductive capacity and long lifespan. Our analyses also suggest that competitors (C) suffer widespread damage, particularly branch breakage, but experience low mortality. This pattern likely reflects features of the competitive strategy, such as wood strength and canopy form, which preclude resistance to damage but facilitate rapid recovery. The ice damage datasets did not contain trees that we classified as ruderals (R). Competitive ruderals (C‐R) and stress‐tolerant ruderals (S‐R), however, sustained heavy damage and high mortality, consistent with low investment in tree defense and a prioritization of reproduction. Our analyses suggest the usefulness of the CSR model for interpreting forest dynamics and understanding the implications of tree life‐history strategies for forest disturbance responses.     

Ice Storm Damage Greater Along the Terrestrial-Aquatic Interface in Forested Landscapes

Ice storms are an important and recurring ecological disturbance in many temperate forest ecosystems. In 1998, a severe ice storm damaged over ten million hectares of forest across northern New York State, eastern Canada, and New England impacting ecosystem processes across the landscape. This study investigated the spatial arrangement of forest damage at the terrestrial-aquatic interface, an ecological edge of importance to aquatic habitat and nutrient cycling. Vegetation indices, derived from satellite imagery and field-based data, were used to measure forest canopy damage across a 2045 km² region in northern New York State affected by the 1998 storm. We investigated the forest damage gradient in the riparian zone of 13 stream segments of varying size (92.5 km total length) and 13 lakes (37.4 km of shoreline). Large streams (-fourth and fifth order), occurring in forests that received modest ice damage (<15% disturbance coverage), exhibited significantly more damage in the riparian zone within 25 m of the water than in adjacent forest sections; F(3,12) = 7.3 P = 0.005. In similar moderately damaged forests, lake shorelines were significantly more damaged than interior forests; F(3,9) = 6.4 P = 0.013. Analysis of transitions in damage intensity revealed that canopy disturbance followed a decreasing trend (up to 3.5 times less) with movement inland from the terrestrial-aquatic interface. The observed predisposition of forest to disturbance along this ecosystem interface emphasizes the role of the physical landscape in concentrating the movement of wood from the forest canopy to locations proximate to water bodies, thus reinforcing findings that ice storms are drivers of ecological processes that are spatially concentrated.     

Biomass accumulation over the first 150 years in coastal Oregon Picea‐Tsuga forest

Abstract. Production and mortality are the component processes that together determine the biomass dynamics of forests. Due to the significant role of forests in the global carbon cycle, it is important to assess how these two processes affect the maximum biomass attained by forests, as well as the dynamics leading up to and following peak biomass. We address these questions for two sets of plots in Picea sitchensis‐Tsuga heterophylla forest on the northern Oregon coast that originated from a catastrophic wildfire in the 1840s, using new data on dynamics of live trees and stocks of coarse woody debris (CWD). The set of plots closest to the ocean and occupying steeper, more dissected terrain with areas of thin soils has lower biomass, lower net primary production (NPP) of bole wood and higher tree mortality as a fraction of standing biomass. The two sets of plots have similar CWD levels, most of which has accumulated in the last 25 yr. The present disparity in biomass between the two sets of plots appears to be the result of lower NPP on the low‐biomass plots for the entire 140+ yr history of the forest. Over the 58 yr that the high‐biomass plots have been measured (from stand age 85 to 143 yr), NPP of bole wood has declined by 41%. Only ca. 6% of this decline can be accounted for by an increase in maintenance respiration of woody tissues. For both sets of plots relative constancy of biomass in the long term appears likely, due to a short time lag in tree regeneration, asynchronous tree mortality and little overall decline in NPP of bole wood in recent decades. However, since tree mortality as a fraction of standing biomass is higher on the low‐biomass plots, and NPP of bole wood is slightly lower, the difference in biomass between the two sets of plots should increase if current rates of production and mortality persist.     

Winter in the Ouchitas—A severe winter storm signature in Pinus echinata in the Ouachita Mountains of Oklahoma and Arkansas,USA

Each year severe winter storms (≈ice storms) damage trees throughout the southern USA. Arkansas and Oklahoma have a history of severe winter storms. To extend that history back beyond the reach of written records, a distinctive tree ring pattern or signature is needed. Storm-caused breakage, branch loss and bending stress provide that signature. We found a severe storm signature in shortleaf pine (Pinus echinata). We used three published site chronologies, a set of five new site chronologies from a growth-and-yield study conducted by Oklahoma State University and the unpublished Shortleaf Canyon chronology from a master’s thesis at the University of Arkansas. Our method is based on two ring width values for the first and second growing seasons after the storm standardized to the ring widths of the seven growing seasons after the storm. Concordance between storm years predicted by tree ring patterns and actual storm years was tested using Cohen’s Kappa. Concern about confounding of ice storm signals by droughts led us to test concordance between severe storms and drought in July, August and September; results were inconclusive but stand as a warning that these two phenomena cannot be distinguished with certainty in the tree ring record. Damaging severe storms occurred in about 2.8% of all years. Two out of three storms identified as “severe” produced glaze icing.     

雪灾对古田山常绿阔叶林群落结构的影响

为了解雪灾对亚热带常绿阔叶林群落结构的影响,我们于2008年初对右田山24 ha固定监测样地进行了调查.经统计,约1/3的树木遭受重度损伤,1/3的树木遭受轻度损伤.选择生境类型、胸径、物种的物候犁、生长型4个因子,用多项逻辑斯蒂回归分析了这些因子对个体损伤的影响.结果表明:低海拔山谷受损最严重,树木倒伏、折断的倾向最大,高海拔山脊受损最轻.胸径影响树木的受损程度和类型,大致来说,随着胸径的增大,树木受损的比例增大,折断、断梢相对于压弯的可能性逐渐增大,断枝相对折断、断梢的可能性也逐渐增大.常绿物种较落叶物种受损更重.由于上层乔木的庇护,灌木树种受损较为轻微.采用除趋势对应分析(DCA)比较了雪灾前后群落结构的变化,结果表明:在40 m×40 m尺度上,群落的变化与随机变化的零假设有显著差异,这说明冰雪灾害对个体的损伤不是随机的,而是有物种选择性的,如甜槠(Castanopsis eyrei)、木荷(Schima superba)和马尾松(Pinus massoniana)等物种受灾情况显著低于随机损伤的零假设,而青冈(Cyclobalanopsis glauca)、石栎(Lithocarpus glaber)、马银花(Rhododendron ovatum)等物种显著地高于随机损伤的零假设,群落的物种组成在雪灾后发生了显著的变化.     

Variation in Susceptibility to Hurricane Damage as a Function of Storm Intensity in Puerto Rican Tree Species

One of the most significant challenges in developing a predictive understanding of the long-term effects of hurricanes on tropical forests is the development of quantitative models of the relationships between variation in storm intensity and the resulting severity of tree damage and mortality. There have been many comparative studies of interspecific variation in resistance of trees to wind damage based on aggregate responses to individual storms. We use a new approach, based on ordinal logistic regression, to fit quantitative models of the susceptibility of a tree species to different levels of damage across an explicit range of hurricane intensity. Our approach simultaneously estimates both the local intensity of the storm within a plot and the susceptibility to storm damage of different tree species within plots. Using the spatial variation of storm intensity embedded in two hurricanes (Hugo in 1989 and Georges in 1998) that struck the 16 ha Luquillo Forest Dynamics Plot in eastern Puerto Rico, we show that variation in susceptibility to storm damage is an important aspect of life history differentiation. Pioneers such as Cecropia schreberiana are highly susceptible to stem damage, while the late successional species Dacryodes excelsa suffered very little stem damage but significant crown damage. There was a surprisingly weak relationship between tree diameter and the susceptibility to damage for most of the 12 species examined. This may be due to the effects of repeated storms and trade winds on the architecture of trees and forest stands in this Puerto Rican subtropical wet forest.
Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp .     

冰雪灾害对中亚热带人工针叶林净初级生产力的影响

结合中亚热带江西千烟洲人工针叶林2005、2008和2011年3次树木清查数据以及树木相对生长方程,比较了2008年1月南方冰雪灾害前后的NPP,评价了森林生态系统灾后的恢复能力。结果表明:乡土树种马尾松(Pinus massoniana)与杉木(Cunninghamia lanceolata)比外来树种湿地松(Pinus elliottii)抗灾害能力强;在个体水平上,胸径(D)较大的树木抗灾害能力较差。灾后马尾松与杉木的D增长率降低,而湿地松增大。冰雪灾害导致大量碳(10.44 t C/hm2)从乔木层碳库转移到死生物量碳库,占乔木层碳储量的18.28%。灾前NPP和碳利用效率(CUE)分别为736.23 g C m-2a-1和0.41;灾后经过近4年的恢复,NPP和CUE分别为683.08 g C m-2a-1和0.38。     

Changes in two Minnesota forests during 14 years following catastrophic windthrow

Abstract. We measured tree damage and mortality following a catastrophic windthrow in permanent plots in an oak forest and a pine forest in central Minnesota. We monitored changes in forest structure and composition over the next 14 years. Prior to the storm, the oak forest was dominated by Quercus ellipsoidalis, and the pine forest by Pinus strobus. The immediate impacts of the storm were to differentially damage and kill large, early‐successional hardwoods and pines. Subsequent recovery was characterized by the growth of late‐successional hardwoods. In both forests the disturbance acted to accelerate succession. Ordination of tree species composition confirmed the trend of accelerated succession, and suggested a convergence of composition between the two forests.     

Growth releases of three shade‐tolerant species following canopy gap formation in old‐growth forests

Questions: Do the number, duration and magnitude of growth releases following formation of natural, fine‐scale canopy gaps differ among shade‐tolerant Thuja plicata, Tsuga heterophylla and Abies amabilis? What is the relative importance of tree‐level and gap‐level variables in predicting the magnitude and duration of releases? What does this tell us about mechanisms of tree species coexistence in such old‐growth forests? Location: Coastal British Columbia, Canada. Methods: We estimated the timing of formation of 20 gaps using dendroecological techniques and extracted increment cores from all three species growing around or within gaps. Using a species‐ and ecosystem‐specific release‐detection method, we determined the number of trees experiencing a release following gap formation. We quantified the duration and magnitude of individual releases and estimated the influence of tree‐level and gap‐level variables on these release attributes. Results: Eighty‐seven per cent (304 of 348) of all trees experienced a release following gap formation. T. heterophylla and A. amabilis experienced higher magnitude and longer duration releases than T. plicata. The effect of diameter on the duration of releases varied among species, with T. heterophylla and A. amabilis experiencing decreasing, and T. plicata experiencing increasing, duration of releases with increasing diameter. The effect of growth rate prior to a release on the magnitude of releases varied among trees of different diameters, with the slowest growing and smallest individuals of all species experiencing the most intensive releases. Conclusions: Our results provide detailed information on the number, duration and magnitude of growth releases of the above three species following gap formation. Differences in response to canopy gaps suggest differences in how these species ascend to the canopy strata. T. plicata may be less dependent on gaps to reach the canopy. Differing strategies for ascending to the canopy strata may be important in facilitating coexistence of these three species in old‐growth forests of coastal British Columbia.     

Drought's legacy: multiyear hydraulic deterioration underlies widespread aspen forest die‐off and portends increased future risk

Forest mortality constitutes a major uncertainty in projections of climate impacts on terrestrial ecosystems and carbon‐cycle feedbacks. Recent drought‐induced, widespread forest die‐offs highlight that climate change could accelerate forest mortality with its diverse and potentially severe consequences for the global carbon cycle, ecosystem services, and biodiversity. How trees die during drought over multiple years remains largely unknown and precludes mechanistic modeling and prediction of forest die‐off with climate change. Here, we examine the physiological basis of a recent multiyear widespread die‐off of trembling aspen (Populus tremuloides) across much of western North America. Using observations from both native trees while they are dying and a rainfall exclusion experiment on mature trees, we measure hydraulic performance over multiple seasons and years and assess pathways of accumulated hydraulic damage. We test whether accumulated hydraulic damage can predict the probability of tree survival over 2 years. We find that hydraulic damage persisted and increased in dying trees over multiple years and exhibited few signs of repair. This accumulated hydraulic deterioration is largely mediated by increased vulnerability to cavitation, a process known as cavitation fatigue. Furthermore, this hydraulic damage predicts the probability of interyear stem mortality. Contrary to the expectation that surviving trees have weathered severe drought, the hydraulic deterioration demonstrated here reveals that surviving regions of these forests are actually more vulnerable to future droughts due to accumulated xylem damage. As the most widespread tree species in North America, increasing vulnerability to drought in these forests has important ramifications for ecosystem stability, biodiversity, and ecosystem carbon balance. Our results provide a foundation for incorporating accumulated drought impacts into climate–vegetation models. Finally, our findings highlight the critical role of drought stress accumulation and repair of stress‐induced damage for avoiding plant mortality, presenting a dynamic and contingent framework for drought impacts on forest ecosystems.     

Variation in the population structure between a natural and a human‐modified forest for a pioneer tropical tree species not restricted to large gaps

The distribution of tree species in tropical forests is generally related to the occurrence of disturbances and shifts in the local environmental conditions such as light, temperature, and biotic factors. Thus, the distribution of pioneer tree species is expected to vary according to the gap characteristics and with human disturbances. We asked whether there was variation in the distribution of a pioneer species under different environmental conditions generated by natural disturbances, and between two forests with contrasting levels of human disturbance. To answer this question, we studied the distribution patterns and population persistence of the pioneer tree species Croton floribundus in the size and age gap range of a primary Brazilian forest. Additionally, we compared the plant density of two size‐classes between a primary and an early successional human‐disturbed forest. Croton floribundus was found to be widespread and equally distributed along the gap‐size gradient in the primary forest. Overall density did not vary with gap size or age (F‐ratio = 0.062, P = 0.941), and while juveniles were found to have a higher density in the early successional forest (P = 0.021), tree density was found to be similar between forests (P = 0.058). Our results indicate that the population structure of a pioneer tree species with long life span and a broad gap‐size niche preference varied between natural and human‐disturbed forests, but not with the level of natural disturbance. We believe this can be explained by the extreme environmental changes that occur after human disturbance. The ecological processes that affect the distribution of pioneer species in natural and human‐modified forests may be similar, but our results suggest they act differently under the contrasting environmental conditions generated by natural and human disturbances.     

Altered dynamics of broad‐leaved tree species in a Chinese subtropical montane mixed forest: the role of an anomalous extreme 2008 ice storm episode

Extreme climatic events can trigger gradual or abrupt shifts in forest ecosystems via the reduction or elimination of foundation species. However, the impacts of these events on foundation species' demography and forest dynamics remain poorly understood. Here we quantified dynamics for both evergreen and deciduous broad‐leaved species groups, utilizing a monitoring permanent plot in a subtropical montane mixed forest in central China from 2001 to 2010 with particular relevance to the anomalous 2008 ice storm episode. We found that both species groups showed limited floristic alterations over the study period. For each species group, size distribution of dead individuals approximated a roughly irregular and flat shape prior to the ice storm and resembled an inverse J‐shaped distribution after the ice storm. Furthermore, patterns of mortality and recruitment displayed disequilibrium behaviors with mortality exceeding recruitment for both species groups following the ice storm. Deciduous broad‐leaved species group accelerated overall diameter growth, but the ice storm reduced evergreen small‐sized diameter growth. We concluded that evergreen broad‐leaved species were more susceptible to ice storms than deciduous broad‐leaved species, and ice storm events, which may become more frequent with climate change, might potentially threaten the perpetuity of evergreen‐dominated broad‐leaved forests in this subtropical region in the long term. These results underscore the importance of long‐term monitoring that is indispensible to elucidate causal links between forest dynamics and climatic perturbations.     

Pollen representation of vegetational patterns along an elevational gradient

Abstract. Modern pollen assemblages from 16 small lakes (< 2.5 ha) and 11 moderate-sized lakes (4.5–19.3 ha) arrayed along an elevational gradient (300 to 1320 m) in the east-central Adirondack Mountains were studied to determine how well the pollen assemblages recorded patterns of forest composition along the gradient. Forest composition ranges from Pinus strobus/Tsuga/ hardwoods forests at low elevations through Tsuga/ hardwoods, hardwoods, and Picea/Abies forests to Abies-dominated forests at high elevations. Modern pollen percentages for 10 tree taxa were compared with lake elevation using scatter plots and correlation and regression analysis. Differential smoothing of vegetational patterns along the elevational gradient occurred among the 10 taxa owing to differences in pollen dispersibility, pollen production, and spatial pattern of taxon abundance in forests of the region. No differences were observed in pollen-elevation patterns between small and moderate-sized lakes. Pollen-elevation patterns were obscured for most taxa when the gradient was shortened (e.g. to 600–1320 m) owing to increased spatial smoothing by pollen dispersal. Design and interpretation of paleoecological studies of spatial gradients can be improved by careful attention to site spacing, gradient length, and gradient steepness in the context of pollen dispersal and representation models.     

High Mortality for Rare Species Following Hurricane Disturbance in the Southern Yucatán

Hurricanes are an important part of the natural disturbance regime of the Yucatán Peninsula with the potential to alter forest structure and composition, yet investigations of species‐level responses to severe winds are limited in this region. The effect of a category 5 hurricane (Hurricane Dean, 21 August 2007) on dry tropical forests across the southern Yucatán was examined with respect to tree damage, mortality, and sprouting. Damage was assessed 9–11 mo following the hurricane in 92 (500 m²) plots stratified by wind speed and normalized difference vegetation index (NDVI) change classes over a 25,000 km² study area. We investigated the relative importance of biotic (i.e., species, size, and wood density) and abiotic (i.e., wind speed) factors to better explain patterns of damage. Overall mortality was low (3.9%), however, mortality of less common species (8.5%) was elevated more than fourfold above that of 28 common species (1.8%), indicating immediate selective consequences for community composition. Species varied in the degree and type of damage experienced, with susceptibility increasing with tree diameter and height. Wood density influenced damage patterns only in areas where a critical threshold in storm intensity was exceeded (wind speeds ≥210 km/h). Although overall, damage severity increased with wind speed, common coastal species were more resistant to damage than species distributed farther inland. Our findings suggest that selective pressure exerted by frequent hurricane disturbance has, and will, continue to impact the floristic composition of forests on the Yucatán Peninsula, favoring certain wind‐resistant species. Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp .     

Canopy disturbance and gap partitioning promote the persistence of a pioneer tree population in a near‐climax temperate forest of the Qinling Mountains,China

An unresolved question of temperate forests is how pioneer tree species persist in mature forests. In order to understand the responsible mechanisms, we investigated a near‐climax mixed temperate forest dominated by Betula albosinensis in the Qinling Mountains of China. Through establishing four 50 m × 50 m plots, we examined the canopy disturbance characteristics and its effects on tree recruitments. We further test the intra‐ and interspecific effects on the recruitment of B. albosinensis. The obtained data demonstrated canopy disturbance was frequent but most small‐sized. The canopy gaps are caused mainly by adult B. albosinensis by snapping. The regeneration of coexistent tree species shows a distinct preference for gap size. B. albosinensis were clumped at the juvenile stage and small scales. B. albosinensis juveniles were positively associated with B. utilis juveniles and negatively associated with the conspecific and B. utilis large trees. In addition, B. albosinensis juveniles showed negative associations with contemporary other tree species. Our results suggested that canopy disturbance caused by canopy trees and gap partitioning among the coexistent tree species are important for the persistence of the mixed forest. As a main gapmaker, B. albosinensis appear to develop a self‐perpetuating life‐history trait and allow them to persist.     

Factors affecting species richness of tree regeneration in mixed‐wood stands of central Maine

Question: Two questions about within‐stand spatial variability are addressed in this paper. How does species richness of tree regeneration respond to small‐scale ecological gradients, and what effect does natural Abies balsamea abundance have on the species richness of other tree regeneration? Location: A long‐term, gap‐silviculture experiment, Acadian mixed‐wood forest, Maine, USA. Methods: Eight stands treated with and without gap harvesting were sampled to capture sub‐stand heterogeneity of understorey tree regeneration concurrently with patterning of local stand conditions. Spatial and non‐spatial models were developed to test the relationships between two response variables [species richness of small (height ≥0.1 m, but <0.75 m) and large (height ≥0.75 m, but <1.4 m) regeneration] and five explanatory variables (depth to water table, percentage canopy transmittance, A. balsamea regeneration density, and overstorey basal area and species richness). Results: Despite high unexplained variance for all models, consistent associations among variables were found. Negative associations were found between: (1) the species richness of small regeneration and A. balsamea regeneration density and (2) the species richness of large regeneration and overstorey basal area. Positive associations were found between: (1) the species richness of small regeneration and both overstorey basal area and species richness and (2) the species richness of small and large regeneration and canopy transmittance. Conclusions: Promoting tree species diversity in Acadian mixed‐wood stands may not be achievable through the use of gap‐harvesting alone if the density of understorey Abies balsamea is not reduced either naturally or through silvicultural intervention.     

Population dynamics of the spruce bark beetle: a long‐term study

Bark beetle population dynamics is thought to be primarily driven by bottom‐up forces affecting insect performance and host tree resistance. Although there are theoretical predictions and empirical evidences that predation and parasitism may play an important role in driving bark beetle population fluctuations, long‐term studies testing the role of both biotic and abiotic controls on population dynamics are still rare. The aim of the study was to quantify the relative importance of predation, negative density feedback and abiotic factors in driving Ips typographus population dynamics. We analyzed a unique time series of population density of I. typographus and its main predator Thanasimus formicarius over almost two decades in four regions across Sweden. We used a discrete population model and a multi‐model inference approach to evaluate the importance of both bottom up and top down factors. We found that availability of breeding substrates in the form of storm‐felled trees was the main outbreak trigger, while strong intra‐specific competition for host trees was the main endogenous regulating factor. Although temperature‐related metrics are known to have strong individual effect on I. typographus development and number of generations, they did not emerge as important drivers of population dynamics. A positive effect of low summer rainfall was evident only in the region located in the southernmost and warmest part of the spruce distribution range in Sweden. Predator density did not emerge as an important prey regulating factor. As the reported damage from storms seems to have increased across whole Europe, spruce forests are expected to be increasingly susceptible to large outbreaks of I. typographus with important economic and ecological consequences for boreal ecosystems. However, the observed negative density feedback seems to be a natural regulating mechanism that impedes a strong long‐term propagation of the outbreaks.     

Anthropogenic disturbances and plant biodiversity in forests of Uttaranchal,central Himalaya

Eight forest types varying in disturbance frequencies were identified along an elevational gradient in Uttaranchal, central Himalaya. Low elevation forests were close to human habitation and had high disturbance frequency, while high elevation forests were situated far from the human habitation and had low disturbance. The dominant tree species at low elevation were Pinus roxburghii and Quercus leucotrichophora, while Q. floribunda and Q. semecarpifolia dominated the high elevation forests. Pyracantha crenulata was the shrub present in all the forests except in Q. semecarpifolia forest and Anaphalis contorta, a herb species, was present in all the forests. Disturbance decreased the dominance of single species and increased the plant biodiversity by mixing species of different successional status. Species richness and diversity for all the vegetation layers were higher in low elevation–high disturbance forests. Mean tree density decreased from high to moderate and increased in low disturbance. The shrub density decreased from high to low disturbance while the reverse occured for herbs. High proportion of early successional species in disturbed forests indicated that disturbance induces succession. The mean number of young individuals increasing from high to low disturbance indicates that disturbance adversely affects regeneration. But, however, the high number of young individuals of Coriaria nepalensis, a small non-leguminous nitrogen fixing tree, in disturbed forests shows that the forest is regenerating. This species could be helpful in the re-establishment of original vegetation through triggering the regeneration of these forests. High elevation–low disturbed forests separated from low elevation–high disturbed forests. Forest type and elevation may have more influence on tree richness while shrub and herb richness may be more sensitive to disturbance and forest types.