Does stand structure influence susceptibility of eucalypt floodplain forests to dieback?

Forest dieback is a worldwide problem that is likely to increase with climate change and increasing human demands for resources. Eucalyptus camaldulensis forests are an acute example of forest dieback, with 70% of the Victorian Murray River floodplain in some state of dieback. If we are to halt dieback in these floodplain forests, we need to understand what makes stands susceptible to dieback. Forest diebacks are often related to stand structure, with dieback more severe in senescent or high‐density stands. We determined whether certain stand structures make these forests more susceptible to dieback. We undertook an extensive survey of 176 stands across 100 000 ha of forest, covering the range of stand condition on this floodplain. Large and small trees (20‐, 40‐, 80‐ and 120‐cm diameter) showed a similar reduction in the probability of being alive with decreasing stand condition. A slight improvement in stand condition was found at higher densities and basal areas, which may reflect the higher productivity or younger age of these stands. Stand condition was moderately, positively correlated with longitude, with stand condition being higher in the east of the Murray River floodplain where flooding frequencies are currently higher. This suggests that dieback of these floodplain forests would be more effectively mitigated by increased water availability through flooding than by altering stand structure.     

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 comparison of structural characteristics between old-growth and postfire second-growth hemlock–hardwood forests in Adirondack Park, New York, U. S. A.

1 This study compares the structural characteristics of 12 old‐growth and six postfire second‐growth hemlock–northern hardwood stands in north central Adirondack Park, New York, in order to test the null hypothesis that there are no differences in species composition, size structure, age structure and attributes such as dead wood and canopy gaps between old‐growth stands and this type of second‐growth forest. 2 The second‐growth forests of this study regenerated following widespread logging‐related fires in either 1903 or 1908; the old growth and second growth have similar environmental settings. 3 Estimates of stand ages, derived from an increment core of the oldest tree in each stand, range from 88 to 390 years. 4 Structural attributes are related to stand age (i.e. stage of development). In comparison with the second‐growth forests of this study, older stands are characterized as (a) a larger average diameter of canopy trees; (b) a greater basal area of trees; (c) a lower density of canopy trees and of all trees ≥ 10 cm d.b.h.; (d) a higher density of eastern hemlock (Tsuga canadensis (L.) Carrière) trees; (e) a higher density of large trees (≥ 50 cm d.b.h.); (f) larger canopy gaps; and (g) a greater volume of coarse woody debris (both logs ≥ 20 cm d.b.h. and snags ≥ 10 cm d.b.h.). 5 Despite differences between old growth and second growth, especially in species composition, it appears from observations of the 18 stands that second‐growth forests are developing some structural characteristics of old growth. 6 Structural attributes of the old‐growth forests are similar to characteristics of the same forest type in geographically distant areas in eastern USA.     

六盘山南坡不同密度华北落叶松水源林生长过程比较

以六盘山南侧的华北落叶松水源涵养林为研究对象,利用标准木树干解析法,研究了21年生低、中、高3种密度(1200、1500和2000 株·hm^-2)华北落叶松人工林的生长过程和直径结构.结果表明：华北落叶松3种密度林分在10年生前各项生长指标差异不显著;10年生后的林木直径、单株材积和林分蓄积生长过程明显不同;21年生时,低密度林分的生长状况明显优于中、高密度林分,但树高生长受密度影响不显著;3种密度林分直径分布的偏度系数(S_k)差异较大,高密度林分的S_k（0.338)大于中密度(0.072)和低密度林分(0.015).前者直径分布偏离正态分布,呈现顶峰偏左的现象;后者的直径分布接近正态分布,密度结构较合理;中密度林分直径分布的峰度系数(K,1.691)大于高密度(1.532)和低密度林分(0.665).说明中密度林分的林木分化程度比高、低密度林分小;林龄为21年的华北落叶松人工林的合理保留密度应为1200 株·hm^-2.     

The impacts of increasing drought on forest dynamics,structure, and biodiversity in the United States

We synthesize insights from current understanding of drought impacts at stand‐to‐biogeographic scales, including management options, and we identify challenges to be addressed with new research. Large stand‐level shifts underway in western forests already are showing the importance of interactions involving drought, insects, and fire. Diebacks, changes in composition and structure, and shifting range limits are widely observed. In the eastern US, the effects of increasing drought are becoming better understood at the level of individual trees, but this knowledge cannot yet be confidently translated to predictions of changing structure and diversity of forest stands. While eastern forests have not experienced the types of changes seen in western forests in recent decades, they too are vulnerable to drought and could experience significant changes with increased severity, frequency, or duration in drought. Throughout the continental United States, the combination of projected large climate‐induced shifts in suitable habitat from modeling studies and limited potential for the rapid migration of tree populations suggests that changing tree and forest biogeography could substantially lag habitat shifts already underway. Forest management practices can partially ameliorate drought impacts through reductions in stand density, selection of drought‐tolerant species and genotypes, artificial regeneration, and the development of multistructured stands. However, silvicultural treatments also could exacerbate drought impacts unless implemented with careful attention to site and stand characteristics. Gaps in our understanding should motivate new research on the effects of interactions involving climate and other species at the stand scale and how interactions and multiple responses are represented in models. This assessment indicates that, without a stronger empirical basis for drought impacts at the stand scale, more complex models may provide limited guidance.     

Managing red pine stand structure to mitigate drought impacts

Reducing forest stand density through silvicultural thinning has demonstrated potential to mitigate drought impacts on growth; however, less has been studied on how changes in stand structure created by different thinning methods influence forest growth responses to drought. This research examined the growth responses to drought of natural-origin red pine in a long-term study contrasting thinning methods. Dendrochronological methods were used to examine growth responses during several drought events among stands where different thinning methods have been applied since 1950. Growth responses to drought were expressed as resistance (maintaining growth during drought), and resilience (regaining pre-drought growth). Results indicate that periodic thinning from above, which resulted in smaller diameters, has the potential to moderate drought-induced growth reductions. Larger tree diameters negatively influenced tree-level resistance and resilience across all treatments; however, the proportion of dominant trees in a stand had contrasting effects on stand-level drought responses. Stands thinned from above exhibited more complex vertical structure and increased stand-level resistance and resilience to drought-induced growth declines because competition is more stratified among smaller diameter trees. Opposite trends were observed in stands thinned from below, where the larger diameters and monolayered structure create greater competition among trees of similar size and crown position. The results of this study highlight the utility in managing for greater structural diversity to mitigate the negative effects of drought in red pine forest ecosystems.     

Predicting and quantifying the structure of tropical dry forests in South Florida and the Neotropics using spaceborne imagery

Aim This research examines environmental theories and remote sensing methods that have been hypothesized to be associated with tropical dry forest structure. Location Tropical dry forests of South Florida and the Neotropics. Methods Field measurements of stand density, basal area and tree height were collected from 22 stands in South Florida and 30 stands in the Neotropics. In South Florida, field measurements were compared to climatic (temperature, precipitation, hurricane disturbance) and edaphic (rockiness, soil depth) variables, spectral indices (NDVI, IRI, MIRI) from Landsat 7 ETM+, and estimates of tree height from the Shuttle Radar Topography Mission (SRTM) and the National Elevation Dataset (NED). Environmental variables associated with tropical dry forest structure in South Florida were compared to tropical dry forest in other Neotropical sites. Results There were significant correlations among temperature and precipitation, and stand density and tree height in South Florida. There were significant correlations between (i) stand density and mean NDVI and standard deviation of NDVI, (ii) MIRI and stand density, basal area and mean tree height, and (iii) estimates of tree height from SRTM with maximum tree height. In the Neotropics, there were no relationships between temperature or precipitation and tropical dry forest structure, however, Neotropical sites that experience hurricane disturbance had significantly shorter tree heights and higher stand densities. Main conclusions It is possible to predict and quantify the forest structure characteristics of tropical dry forests using climatic data, Landsat 7 ETM+ imagery and SRTM data in South Florida. However, results based on climatic data are region‐specific and not necessarily transferable between tropical dry forests at a continental spatial scale. Spectral indices from Landsat 7 ETM+ can be used to quantify forest structure characteristics, but SRTM data are currently not transferable to other regions. Hurricane disturbance has a significant impact on forest structure in the Neotropics.     

Changes in Forest Structure and Species Composition during Secondary Forest Succession in the Bolivian Amazon1

Changes in forest structure and species diversity throughout secondary succession were studied using a chronosequence at two sites in the Bolivian Amazon. Secondary forests ranging in age from 2 to 40 years as well as mature forests were included, making a total of 14 stands. Fifty plants per forest layer (understory, subcanopy, and canopy) were sampled using the transect of variable area technique. Mean and maximum height, total stem density, basal area, and species number were calculated at the stand level. Species diversity was calculated for each stand and for each combination of forest layer and stand. A correspondence analysis was performed, and the relationship between relative abundance of the species and stand age was modeled using a set of hierarchical models. Canopy height and basal area increased with stand age, indicating that secondary forests rapidly attain a forest structure similar in many respects to mature forests. A total of 250 species were recorded of which ca 50 percent made up 87 percent of the sampled individuals. Species diversity increased with stand age and varied among the forest layers, with the lowest diversity in the canopy. The results of the correspondence analysis indicated that species composition varies with stand age, forest layer, and site. The species composition of mature forests recovered at different rates in the different forest layers, being the slowest in the canopy layer. Species showed different patterns of abundance in relation to stand age, supporting the current model of succession.     

Airborne Laser Scanning as a Tool for Lowland Floodplain Vegetation Monitoring

Monitoring of three-dimensional floodplain vegetation structure is essential for ecological studies, as well as for hydrodynamic modelling of rivers. Height and density of submerged vegetation and density of emergent vegetation are the key characteristics from which roughness parameters in hydraulic models are derived. Airborne laser scanning is a technique with broad applications in vegetation structure mapping, which therefore may be a promising tool in monitoring floodplain vegetation for river management applications. This paper first provides an introduction to the laser scanning technique, and reviews previous studies on the extraction of vegetation height and density of forests, low vegetation and meadows or unvegetated areas. Reliable predictions using laser scan data have been reported for forest height (R²=0.64–0.98), parameters related to forest density, such as stem number, stem diameter, biomass, timber volume or basal area (R²=0.42–0.93), and herbaceous vegetation height (summer condition; R²=0.75–0.89). No empirical relations have been reported on density of herbaceous vegetation. Laser data of meadows and unvegetated areas show too much noise to predict vegetation structure correctly. In a case study for the lower Rhine river, the potential of laser scan mapping of vegetation structure was further explored for winter conditions. Three laser-derived metrics that are often reported in the literature have been applied to characterize local vertical distributions of laser reflections. The laser data clearly show the large structural differences both between and within vegetation units that currently are the basis of floodplain vegetation and roughness mapping. The results indicate that airborne laser scanning is a promising technique for extraction of 3D-structure of floodplain vegetation in winter, except for meadows and unvegetated areas.     

Carbon Accumulation Patterns During Post-Fire Succession in Cajander Larch (Larix cajanderi) Forests of Siberia

Increased fire activity within boreal forests could affect global terrestrial carbon (C) stocks by decreasing stand age or altering tree recruitment, leading to patterns of forest regrowth that differ from those of pre-fire stands. To improve our understanding of post-fire C accumulation patterns within boreal forests, we evaluated above- and belowground C pools within 17 Cajander larch (Larix cajanderi) stands of northeastern Siberia that varied in both years since fire and stand density. Early-successional stands (<20-year old) exhibited low larch recruitment, and consequently, low density, aboveground larch biomass, and aboveground net primary productivity (ANPP_tree). Mid-successional stands (21- to 70-year old) were even-aged with considerable variability in stand density. High-density mid-successional stands had 21 times faster rates of ANPP_tree than low-density stands (252 vs. 12?g?C?m^?2?y^?1) and 26 times more C in aboveground larch biomass (2,186 vs. 85?g?C?m^?2). Density had little effect on total soil C pools. During late-succession (>70-year old), aboveground larch biomass, ANPP_tree, and soil organic layer C pools increased with stand age. These stands were low density and multi-aged, containing both mature trees and new recruits. The rapid accumulation of aboveground larch biomass in high-density, mid-successional stands allowed them to obtain C stocks similar to those in much older low-density stands (~8,000?g?C?m^?2). If fire frequency increases without altering stand density, landscape-level C storage could decline, but if larch density also increases, large aboveground C pools within high-density stands could compensate for a shorter successional cycle.     

Fire disturbance and forest structure in old‐growth mixed conifer forests in the northern Sierra Nevada,California

Question: This study evaluates how fire regimes influence stand structure and dynamics in old‐growth mixed conifer forests across a range of environmental settings. Location: A 2000‐ha area of mixed conifer forest on the west shore of Lake Tahoe in the northern Sierra Nevada, California. Methods: We quantified the age, size, and spatial structure of trees in 12 mixed conifer stands distributed across major topographic gradients. Fire history was reconstructed in each stand using fire scar dendrochronology. The influence of fire on stand structure was assessed by comparing the fire history with the age, size, and spatial structure of trees in a stand. Results: There was significant variation in species composition among stands, but not in the size, age and spatial patterning of trees. Stands had multiple size and age classes with clusters of similar aged trees occurring at scales of 113 ‐ 254 m². The frequency and severity of fires was also similar, and stands burned with low to moderate severity in the dormant season on average every 9–17 years. Most fires were not synchronized among stands except in very dry years. No fires have burned since ca. 1880. Conclusions: Fire and forest structure interact to perpetuate similar stand characteristics across a range of environmental settings. Fire occurrence is controlled primarily by spatial variation in fuel mosaics (e.g. patterns of abundance, fuel moisture, forest structure), but regional drought synchronizes fire in some years. Fire exclusion over the last 120 years has caused compositional and structural shifts in these mixed conifer forests.     

Stand structure and dynamics of wave-typeAbies sachalinensis coastal forest

Stand structure and regeneration pattern were examined inAbies sachalinensis coastal forest in northern Hokkaido. In the forest a similar phenomenon to the wave regeneration in subalpine forests has been found. Wave regeneration has been reported for montaneAbies forests in central Japan and North America. Differences and similarities between wave-type stands in this coastal forest and wave-regenerated montane forests were clarified. The shift of dead tree zone, stand structure and regeneration pattern in wave-type stands are the same as in subalpine wave-regenerated forests. High density of individuals is considered to be an internal factor which causes stand-level dieback and also enables the stands to persist in the severe environment in both forests. A difference between wave-regenerated forests andA. sachalinensis wave-type stands is the number of dead tree zones, which is only one in wave-type stands. Changes of regeneration patterns ofAbies sachalinensis with environmental gradient from seaward to inland were related to this difference.     

Stability in the patterns of long‐term development and growth of the Canadian spruce–moss forest

Aim The spruce–moss forest is the main forest ecosystem of the North American boreal forest. We used stand structure and fire data to examine the long‐term development and growth of the spruce–moss ecosystem. We evaluate the stability of the forest with time and the conditions needed for the continuing regeneration, growth and re‐establishment of black spruce (Picea mariana) trees. Location The study area occurs in Québec, Canada, and extends from 70°00′ to 72°00′ W and 47°30′ to 56°00′ N. Methods A spatial inventory of spruce–moss forest stands was performed along 34 transects. Nineteen spruce–moss forests were selected. A 500 m² quadrat at each site was used for radiocarbon and tree‐ring dating of time since last fire (TSLF). Size structure and tree regeneration in each stand were described based on diameter distribution of the dominant and co‐dominant tree species [black spruce and balsam fir (Abies balsamea)]. Results The TSLF of the studied forests ranges from 118 to 4870 cal. yr bp . Forests < 325 cal. yr bp are dominated by trees of the first post‐fire cohort and are not yet at equilibrium, whereas older forests show a reverse‐J diameter distribution typical of mature, old‐growth stands. The younger forests display faster height and radial growth‐rate patterns than the older forests, due to factors associated with long‐term forest development. Each of the stands examined established after severe fires that consumed all the soil organic material. Main conclusions Spruce–moss forests are able to self‐regenerate after fires that consume the organic layer, thus allowing seed regeneration at the soil surface. In the absence of fire the forests can remain in an equilibrium state. Once the forests mature, tree productivity eventually levels off and becomes stable. Further proof of the enduring stability of these forests, in between fire periods, lies in the ages of the stands. Stands with a TSLF of 325–4870 cal. yr bp all exhibited the same stand structure, tree growth rates and species characteristics. In the absence of fire, the spruce–moss forests are able to maintain themselves for thousands of years with no apparent degradation or change in forest type.     

Effects of Dwarf Mistletoe on Stand Structure of Lodgepole Pine Forests 21-28 Years Post-Mountain Pine Beetle Epidemic in Central Oregon

Lodgepole pine (Pinus contorta) forests are widely distributed throughout North America and are subject to mountain pine beetle (Dendroctonus ponderosae) epidemics, which have caused mortality over millions of hectares of mature trees in recent decades. Mountain pine beetle is known to influence stand structure, and has the ability to impact many forest processes. Dwarf mistletoe (Arceuthobium americanum) also influences stand structure and occurs frequently in post-mountain pine beetle epidemic lodgepole pine forests. Few studies have incorporated both disturbances simultaneously although they co-occur frequently on the landscape. The aim of this study is to investigate the stand structure of lodgepole pine forests 21–28 years after a mountain pine beetle epidemic with varying levels of dwarf mistletoe infection in the Deschutes National Forest in central Oregon. We compared stand density, stand basal area, canopy volume, proportion of the stand in dominant/codominant, intermediate, and suppressed cohorts, average height and average diameter of each cohort, across the range of dwarf mistletoe ratings to address differences in stand structure. We found strong evidence of a decrease in canopy volume, suppressed cohort height, and dominant/codominant cohort diameter with increasing stand-level dwarf mistletoe rating. There was strong evidence that as dwarf mistletoe rating increases, proportion of the stand in the dominant/codominant cohort decreases while proportion of the stand in the suppressed cohort increases. Structural differences associated with variable dwarf mistletoe severity create heterogeneity in this forest type and may have a significant influence on stand productivity and the resistance and resilience of these stands to future biotic and abiotic disturbances. Our findings show that it is imperative to incorporate dwarf mistletoe when studying stand productivity and ecosystem recovery processes in lodgepole pine forests because of its potential to influence stand structure.     

不同干扰强度下针阔混交林树种空间格局及种间关联性

为了探究不同干扰程度下针阔混交林树种空间格局变化及树种种间关联性,本研究以关帝山庞泉沟自然保护区不同干扰强度的针阔混交林为对象,选取郁闭度、林分密度、伐桩数量作为划分干扰程度的因子,采用相对影响法,将林分划分为未干扰、中度干扰和严重干扰3个等级。利用空间点格局K2函数分析了林分的空间分布格局及其不同树种的种间关联。结果表明: 未干扰样地直径分布呈倒“J”型,中度和严重干扰样地直径分布均为双峰曲线分布。同时,未干扰和中度干扰林分在小尺度上呈现聚集分布,严重干扰的林分随机分布更为显著。针阔树种种间关系表现为未干扰林分在所有尺度上无关联;小尺度上中度干扰表现为正相关,严重干扰林分表现为负相关;较大尺度上中度与严重干扰林分均表现为不相关。说明林分干扰程度越严重,林下小径级林木数量越少,导致小尺度上种内聚集程度越低。同时,适当的干扰程度有利于林分内物种间对环境资源的协同利用。研究揭示了干扰对林分结构的影响,可为林分结构调整及抚育经营提供理论依据。     

Mechanisms of Riparian Cottonwood Decline Along Regulated Rivers

Decline of riparian forests has been attributed to hydrologic modifications to river flows. However, little is known about the physiological and structural adjustments of riparian forests subject to modified flow regimes, and the potential for forest restoration using historic flow regimes is poorly understood. In this paired river study, we compared hydrology, water relations, and forest structure in cottonwood-dominated floodplains of the regulated Green River to those of the unregulated Yampa River. We measured floodplain groundwater levels, soil water availability, cottonwood xylem pressure (Ψ_xp), and leaf-level stomatal conductance (g_s) to assess current impacts of river regulation on the water status of adult cottonwoods. We also simulated a flood on the former floodplain of the regulated river to evaluate its impact on cottonwood water relations. Canopy and root structure were quantified with estimates of cottonwood leaf area and percent live canopy and root density and biomass, respectively. Regulation of the Green River has lowered the annual peak flow yet raised minimum flows in most years, resulting in a 60% smaller stage change, and lowered soil water availability by as much as 70% compared to predam conditions. Despite differences in water availability, daily and seasonal trends in Ψ_xp and g_s were similar for cottonwoods on the regulated and unregulated rivers. In addition, soil water added with the experimental flood had little effect on cottonwood water relations, contrary to our expectations of alleviated water stress. Green River cottonwoods had 10%–30% lower stand leaf area, 40% lower root density, and 25% lower root biomass compared with those for Yampa River cottonwoods. Our results suggest that water relations at the leaf and stem level are currently similar for Yampa and Green River trees due to structural adjustments of cottonwood forests along the Green River, triggered by river regulation.     

Invasive capacity of Tamarix ramosissima in a Mojave Desert floodplain: the role of drought

Tamarix ramosissima (Tamaricaceae) is a woody phreatophyte that has invaded thousands of hectares of floodplain habitat in the southwestern U.S. In this study, we examined the response of gas exchange and stem sap flow of Tamarix and three co-occurring native phreatophytes (Pluchea sericea (Asteraceae), Prosopis pubescens (Fabaceae) and Salix exigua (Salicaceae)) to drought conditions in an early successional floodplain community in the Mojave Desert of southern Nevada. In an analysis of a size/age series of each species across the whole floodplain (both mature and successional stands), stem growth rate was lowest for Tamarix. However, along the same successional chronosequence, Tamarix came to dominate the 50+ year old stands with dense thickets of high stem density. Xylem sap flow, when expressed on a sapwood area basis, was highest in Tamarix under early drought conditions, but comparable between the four species toward the end of the summer dry season. Multivariate analysis of the gas exchange data indicated that the four species differentiated based on water use under early drought conditions and separated based on plant water potential and leaf temperature (indices of drought effects) at the end of the summer dry season. This analysis suggests that the invasive Tamarix is the most drought tolerant of the four species, whereas Salix transpires the most water per unit leaf surface area and is the least tolerant of seasonal water stress. Therefore, Salix appears to be well adapted to early successional communities. However, as floodplains in this arid region become more desiccated with age, Tamarix assumes greater dominance due to its superior drought tolerance relative to native phreatophytes and its ability to produce high density stands and high leaf area. Received: 8 August 1996 / Accepted: 29 January 1997     

管涔山青扦(Picea wilsoni)天然林年龄结构及其动态的研究

地种群年龄的研究表明,虽经人为频繁干扰,管涔肝扦天然林仍表现出异龄林结构特征,立木年龄范围超过一个级期,根据年龄结构特征值可分为相对龄林、相对异龄林和异龄林３种类型林下新一代种群的数量和结构受林冠郁闭度和结构的影响。具垂直郁闭型林冠的异龄林,林下更新数量充足,幼苗幼树年龄结构合理;而服闭型林冠,不利于一代种群的发生和发展,青扦种群年龄结构受种群发生和自疏两个过程的控制,林下种群的发生以小规模林冠空