Death of vigorous trees benefits bark beetles

Bark beetles (Coleoptera: Scolytidae) are commonly associated with live host trees that are stressed, a relationship that has been attributed to lower host defenses or greater nutritional quality of these trees. However, most bark beetle species commonly inhabit freshly dead trees where induced host defenses are absent. In this study, we investigate the role of tree vigor at the time of death for pine engraver bark beetles, Ips pini (Say), breeding in freshly dead jack pine, Pinus banksiana Lamb. As indices of tree vigor, we considered tree size, phloem thickness, and several measures of recent growth rate (last year's growth increment, mean annual increment and basal area increment in the past 5 and 10 years, and periodic growth ratio). We examined the relationship between these indices in three stands, aged 60, 77, and 126 years, and found that phloem thickness, previously shown to have a strong positive effect on bark beetle reproduction, was only weakly associated with tree growth rate and inconsistently related to tree size among the three stands. To examine the effects of tree vigor on pine engraver reproduction, we felled 20 trees of various sizes from the 77-year-old stand, and experimentally established breeding males and females in 25-cm-long sections. Offspring were collected and characteristics of breeding galleries were measured. Using stepwise regression, we consistently found that indices associated with tree growth rate best explained beetle reproductive performance, as they were positively related to parental male and female establishment on logs, female reproductive success, length of egg galleries, proportion of eggs resulting in emerged offspring, and negatively related to the length of the post-egg gallery. Surprisingly, phloem thickness had no unique effect on pine engraver reproduction, except for a weak negative effect on the establishment success of parental females. The strong effect of tree vigor observed in this study suggests that substantial mortality of vigorous trees, such as caused by windthrow, can contribute to significant increases in bark beetle populations that could trigger outbreaks in living trees. Received: 3 February 1999 / Accepted: 27 April 1999     

Impacts of climate change on forest growth in saline-alkali land of Yellow River Delta,North China

Climate change is an important factor affecting forest growth. Therefore, approaching the impacts of climate change on forest growth is of great significance to ameliorate this degraded land and push up forestry development. This paper initially probes the impacts of climate change on tree growth in Yellow River Delta region and responds of different tree species on the change. In this study, five species of 22-year-old trees were selected, and the tree biomass was measured by standard site methods and tree ring sampling to pursue the impacts of climate change on forest growth. Besides, growth models of the different tree species were established and verified using Robinia pseudoacacia as an example. The results showed: (1) In the Yellow River Delta, the most adapted tree species are Fraxinus chinensis and R. pseudoacacia. (2) Precipitation is the main meteorological factor affecting tree growth, while temperature and air pressure are also significantly correlated with tree growth. (3) Linear and power function models can simulate tree growth well. From the verification results, the modified R. pseudoacacia biomass is 294.54 t/ha, and the simulated biomass of the linear function model is close to the value. It is expected that the research not only provides a theoretical basis for forestry development in saline lands, but also helps to rehabilitate saline-alkali lands and cope with climate change.     

Variation in tree growth sensitivity to moisture across a water-limited forest landscape

Water availability acts as a major constraint on productivity in many sub-humid forest regions. Precipitation can be an important limiting factor for tree growth in such areas, but the strength of the relationship can vary by habitat and species, as well as with tree size and local hydrology. We quantified the influence of past weather conditions on the growth of two conifer species (Pinus contorta and Picea glauca) across a water-limited forest landscape in western Canada. The two species differ in moisture requirements and are segregated across a local elevational gradient, and so we expected them to exhibit different sensitivities to precipitation. We also expected that larger trees and those more distant from creeks would have a stronger response to precipitation. A hierarchical Bayesian model fit to the annual ring widths of 387 trees showed that historical precipitation from 1951 to 2016 had a positive overall effect on radial growth. The magnitude of precipitation effects on radial tree growth varied with creek proximity (a proxy for the soil moisture provided by an elevated water table in the valley bottom) and tree size. Precipitation had a greater positive influence on the growth of larger P. glauca trees, as well as individuals of both species at far and intermediate distances from creeks. Precipitation had a weaker but still positive effect on P. glauca trees growing close to creeks. Tree growth rates may change with the predicted greater inter-annual variability of precipitation under climate change, but the magnitude of these responses appear to vary by species, size, and creek proximity. Overall changes in tree growth are expected to be relatively small as trees are well-adapted to cope with the variation in water availability across a moisture-limited landscape.     

Diverging growth and resilience of Pinus tabulaeformis and Pinus massoniana to droughts in north-south transition zone,central China

Climate transition zone is a sensitive area of climate change and ecological transition where forests are vulnerable to climate extremes. Extreme droughts are increasing in frequency and magnitude under climate change, resulting in structure and function changes of forest ecosystems. Here, to analyze climate-growth relationships and quantify tree resilience to extreme droughts, we developed six tree-ring-width chronologies from P. tabulaeformis and P. massoniana sampling sites in Mt. Jigong region, Central China. The results indicated that all chronologies from the two species had good consistency, precipitation in current April and mean temperature in current August or mean minimum temperature from current August to October were the main limiting factors of the two tree species growth, but the responses of P. massoniana ring-width to climatic factors was more complex than that of P. tabulaeformis. The results also showed that tree growth of 1999–2005 was the lowest growing period during 1979–2018, and P. massoniana grew better than P. tabulaeformis before 2005 and vice versa after 2005. Comparing low growth years of trees, we identified to study tree growth resilience. The calculations from 1988, 1999–2005 and 2011 drought years indicated that P. tabulaeformis had more increased resilience to extreme droughts than that of P. massoniana, and the two species had stronger ecological recovery and resilience under global warming and non-extreme drought conditions in the recent 40 years. These results have implications for predicting tree resilience and identifying tree species in heterogeneous forest landscapes vulnerable to future climate change in climatic transition zone.     

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.     

A joint individual‐based model coupling growth and mortality reveals that tree vigor is a key component of tropical forest dynamics

Tree vigor is often used as a covariate when tree mortality is predicted from tree growth in tropical forest dynamic models, but it is rarely explicitly accounted for in a coherent modeling framework. We quantify tree vigor at the individual tree level, based on the difference between expected and observed growth. The available methods to join nonlinear tree growth and mortality processes are not commonly used by forest ecologists so that we develop an inference methodology based on an MCMC approach, allowing us to sample the parameters of the growth and mortality model according to their posterior distribution using the joint model likelihood. We apply our framework to a set of data on the 20‐year dynamics of a forest in Paracou, French Guiana, taking advantage of functional trait‐based growth and mortality models already developed independently. Our results showed that growth and mortality are intimately linked and that the vigor estimator is an essential predictor of mortality, highlighting that trees growing more than expected have a far lower probability of dying. Our joint model methodology is sufficiently generic to be used to join two longitudinal and punctual linked processes and thus may be applied to a wide range of growth and mortality models. In the context of global changes, such joint models are urgently needed in tropical forests to analyze, and then predict, the effects of the ongoing changes on the tree dynamics in hyperdiverse tropical forests.     

Long-term growth decline in Toona ciliata in a moist tropical forest in Bangladesh: Impact of global warming

Tropical forests are carbon rich ecosystems and small changes in tropical forest tree growth substantially influence the global carbon cycle. Forest monitoring studies report inconsistent growth changes in tropical forest trees over the past decades. Most of the studies highlighted changes in the forest level carbon gain, neglecting the species-specific growth changes which ultimately determine community-level responses. Tree-ring analysis can provide historical data on species-specific tree growth with annual resolution. Such studies are inadequate in Bangladesh, which is one of the most climate sensitive regions in the tropics. In this study, we investigated long-term growth rates of Toona ciliata in a moist tropical forest of Bangladesh by using tree-ring analysis. We sampled 50 trees of varying size, obtained increment cores from these trees and measured tree-ring width. Analyses of growth patterns revealed size-dependent growth increments. After correcting for the effect of tree size on tree growth (ontogenetic changes) by two different methods we found declining growth rates in T. ciliata from 1960 to 2013. Standardized ring-width index (RWI) was strongly negatively correlated with annual mean and maximum temperatures suggesting that rising temperature might cause the observed growth decline in T. ciliata. Assuming that global temperatures will rise at the current rate, the observed growth decline is assumed to continue. The analysis of stable carbon and oxygen isotopes may reveal more insight on the physiological response of this species to future climatic changes.     

Tree invasion into a mountain-top meadow in the Oregon Coast Range,USA

Abstract. The grassy meadow on the top of Marys Peak, in the Oregon Coast Range, is being invaded, primarily along the margins, by Abies procera. To examine vegetation and environmental changes across the forest-meadow transition and to evaluate factors affecting tree invasion, belt transects were established at 10 sites. Correlations of environmental variables with ordination axes from Detrended Correspondence Analysis suggest that the sites were distributed along a moisture gradient. Although the sites varied considerably in species composition and environment, tree invasion of the meadow was occurring at all sites. Reduction in abundance of the dense herbaceous vegetation of the meadow is required before the small seedlings of Abies procera can become established. Almost all tree invasion of the meadow occurs in the narrow forest-meadow ecocline because trees at the edge of the forest reduce the cover of herbaceous plants in the adjacent meadow. On some sites, above average snow accumulations further reduce vigor of meadow vegetation and increase tree establishment. On the driest sites wet summers facilitate tree establishment. Infrequent fires, which remove the fire sensitive Abies procera, are probably required to reverse the slow but persistent trend of forest expansion and ultimately maintain the meadow.     

No evidence for consistent long‐term growth stimulation of 13 tropical tree species: results from tree‐ring analysis

The important role of tropical forests in the global carbon cycle makes it imperative to assess changes in their carbon dynamics for accurate projections of future climate–vegetation feedbacks. Forest monitoring studies conducted over the past decades have found evidence for both increasing and decreasing growth rates of tropical forest trees. The limited duration of these studies restrained analyses to decadal scales, and it is still unclear whether growth changes occurred over longer time scales, as would be expected if CO₂‐fertilization stimulated tree growth. Furthermore, studies have so far dealt with changes in biomass gain at forest‐stand level, but insights into species‐specific growth changes – that ultimately determine community‐level responses – are lacking. Here, we analyse species‐specific growth changes on a centennial scale, using growth data from tree‐ring analysis for 13 tree species (~1300 trees), from three sites distributed across the tropics. We used an established (regional curve standardization) and a new (size‐class isolation) growth‐trend detection method and explicitly assessed the influence of biases on the trend detection. In addition, we assessed whether aggregated trends were present within and across study sites. We found evidence for decreasing growth rates over time for 8–10 species, whereas increases were noted for two species and one showed no trend. Additionally, we found evidence for weak aggregated growth decreases at the site in Thailand and when analysing all sites simultaneously. The observed growth reductions suggest deteriorating growth conditions, perhaps due to warming. However, other causes cannot be excluded, such as recovery from large‐scale disturbances or changing forest dynamics. Our findings contrast growth patterns that would be expected if elevated CO₂ would stimulate tree growth. These results suggest that commonly assumed growth increases of tropical forests may not occur, which could lead to erroneous predictions of carbon dynamics of tropical forest under climate change.     

中国东北森林生长演替模拟模型及其在全球变化研究中的应用

 NEWCOP模型是一个新的适于模拟东北森林的种类组成动态的林窗类计算机模拟模型,它通过模拟在每一个林分斑块上的每株树木的更新、生长和死亡的全过程来反映森林群落的中长期生长和演替动态。由于 NEWCOP模型是一个由气候变量驱动的生态系统模型,故可用于评价气候变化对东北森林生长和演替的影响。在东北大兴安岭、小兴安岭和长白山地区对NEWCOP模型进行了验证和校准。沿环境梯度对NEWCOP模型的数字模拟实验表明：它能准确地再现顶极森林中树种组成及其在东北地区的垂直分布规律和水平分布规律;能准确地再现大兴安岭、小兴安岭和长白山的主要类型森林的生长和演替规律;在一定的场合NEWCOP还可反映林分的径级结构;NEWCOP模型还具有对现有森林的跟踪模拟能力。应用NEWCOP模型评估了东北森林生态系统对可能气候变化的敏感性。在GFDL 2×CO2和GISS 2×CO2气候变化情景下,东北森林的种类组成将发生很大变化,落叶阔叶树将取代目前长白山、小兴安岭的红松(Pinus koraiensis)和大兴安岭的兴安落叶松(Larix gmelinii)成为东北森林主要树种,而针叶树将在地带性森林中占很小的比重,阔叶树中蒙古栎(Quercus mongolica)将是最重要的树种,它将成为小兴安岭和大兴安岭最主要树种;东北地区适于森林生长的区域将大幅度减少,这些变化主要发生在气候变化过渡期。东北森林对不同的气候变化情景有不同响应。但是,总的趋势是未来东北森林中落叶阔叶树的比重将大幅度增加。这些结论对在全球气候变化背景下,我国东北合理地选择造林树种和制定现有森林的保护经营策略具有一定参考价值。     

Going undercover: increasing canopy cover around a host tree drives associational resistance to an insect pest

Neighbouring heterospecific plants are often observed to reduce the probability of herbivore attack on a given focal plant. While this pattern of associational resistance is frequently reported, experimental evidence for underlying mechanisms is rare particularly for potential plant species diversity effects on focal host plants and their physical environment. Here, we used an established forest diversity experiment to determine whether tree diversity effects on an important insect pest are driven by concomitant changes in host tree growth or the light environment. We examined the effects of tree species richness, canopy cover and tree growth on the probability of occurrence, the abundance, and volume of galls caused by the pineapple gall adelgid Adelges abietis on Norway spruce. Although tree diversity had no effect on gall abundance, we observed that both the probability of gall presence and gall volume (an indicator of maternal fecundity) decreased with tree species richness and canopy cover around host spruce trees. Structural equation models revealed that effects of tree species richness on gall presence and volume were mediated by concurrent increases in canopy cover rather than changes in tree growth or host tree density. As canopy cover did not influence tree or shoot growth, patterns of associational resistance appear to be driven by improved host tree quality or more favourable microclimatic conditions in monocultures compared to mixed‐stands. Our study therefore demonstrates that changes in forest structure may be critical to understanding the responses of herbivores to plant diversity and may underpin associational effects in forest ecosystems.     

Dendrochronological potential of four neotropical dry-forest tree species: Climate-growth correlations in northeast Brazil

Tropical dry forests (TDF) are highly important tropical forest ecosystems. Yet, these forests are highly threatened, usually neglected and only poorly studied. Understanding the long-term influences of environmental conditions on tree growth in these forests is crucial to understand the functioning, carbon dynamics and potential responses to future climate change of these forests. Dendrochronology can be used as a tool to provide these insights but has only scantly been applied in (dry) tropical forests. Here we evaluate the dendrochronological potential of four Caatinga neotropical dry forest tree species – Aspidosperma pyrifolium, Ziziphus joazeiro, Tabebuia aurea, and Libidibia ferrea – collected in two locations in northeastern Brazil (Sergipe state). We provide an anatomical characterization of the ring boundaries for the four species and investigate correlations of their growth with local and regional climatic variables. All four species form annual rings and show high inter-correlation (up to 0.806) and sensitivity (up to 0.565). Growth of all species correlated with local precipitation as well as with sea-surface temperatures in the tropical Atlantic and/or tropical Pacific oceans. We also show teleconnections between growth and the El Niño South Oscillation. The strong dependence of tree on precipitation is worrisome, considering that climate change scenarios forecast increased drought conditions in the Caatinga dry forest. Including more species and expanding dendrochronological studies to more areas would greatly improve our understanding of tree growth and functioning in TDFs. This type of knowledge is essential to assist the conservation, management and restoration of these critical tropical ecosystems.     

Recent decline of high altitude coniferous growth due to thermo-hydraulic constrains: evidence from the Miyaluo Forest Reserve,Western Sichuan Plateau of China

Tree growth decline has been reported in many places around the globe under the context of increasingly warming climate, and strengthening drought intensity is detected to be the primary factor for such decline, particularly in northern forest sites, as well as arid and semi-arid areas. Yet, the forest growth decline in high altitude, high mountain sites certainly merits investigation. Here, we reported faxon fir (Abies fargesii var. faxoniana) forest growth decline (slope = -0.64) at the tree line (4150 m above sea level) in Miyaluo Forest Reserve (MFR) at the Western Sichuan Plateau, southwestern China since 2000. We investigated the cause of tree growth decline by applying dendrochronological approaches. We took tree-ring samples from fir trees at the tree line and developed tree-ring width (TRW) chronology. The tree growth – climate relationship analysis showed that maximum temperature (Tmax) was the primary factor limiting the radial growth of fir trees in the investigated area. The moving correlation analysis indicated the strengthening positive influence of Tmax, spring precipitation, and cloud cover during winter and monsoon period on radial growth since 2000s. Our results have shown that both thermal and hydraulic constrains accounted for the radial growth decline of fir trees at the tree line of MFR in the western Sichuan Plateau.     

Avian richness and abundance in temperate Danish forests: tree variables important to birds and their conservation

Many studies on avian diversity and forest structure have focused onfiner scale forest variables such as foliage height diversity, foliagediversity, foliage density, vertical distribution of vegetation and horizontalvegetation density. From a conservation and forestry operational point of viewit would be of great interest if tree variables influenced directly by forestrymanagement decisions also had significant influence on avian richness andabundance. The species, age and size of a tree are examples of such treevariables. A great number of studies also have focused on avian diversityindices to reveal relationships with vegetation variables. However, it may bemore appropriate for foresters and conservation officers to operate withrichness and abundance measures directly, because indices complicateinterpretations on the relative importance of the two variables (richness andabundance) constituting the index. Fourteen managed temperate forests in Denmarkwere investigated for avian species richness and abundance and related tomeasures on different tree variables influenced directly by forestry managementdecisions. A rapid assessment method of avian richness and abundance wasemployed. It consisted of point-counts of bird richness and abundance within 1km² of forest. General linear models were tested byanalyses of variance statistics to reveal the tree variables most important toavian richness and abundance. It was found that more old trees, more treespecies and more tree size-classes correlated with more bird species andindividuals. However, some variation in bird richness and abundance was alsorelated to site quality and/or chance colonization. Moreover, it was shown thatthe guild of cavity-nesting birds correlated positively to age of tree stand.The potential number of bird species in Danish forest is similar to that innearly pristine forest in Poland, and much larger than that recorded in any ofthe forests investigated. Together with the results above, this indicates a highpotential for squeezing in more avian species in a higher quality forest from abiodiversity point of view. Modern Danish forestry affects tree variablesinfluenced directly by forestry management decisions. Such tree variables havegreat influence on avian richness and abundance, but simple measures in forestrypractices can be taken to enhance the conservation of bird richness andabundance.     

森林退化/衰退的研究与实践

森林退化可以理解为森林面积减少、结构丧失、质量降低、功能下降;森林衰退则是森林退化的一种形式,指森林（树木）在生长发育过程中出现的生理机能下降、生长发育滞缓、生产力降低甚至死亡,以及地力衰退等状态.国内外研究表明,森林采伐/毁林是造成森林面积减少的最主要原因,有关森林采伐/毁林引起的森林退化研究主要集中在森林退化的后果、国家/国际政策的影响、加强全球性合作以及寻求解决途径等方面.森林衰退原因可归纳为：工、农业污染,自然胁迫/致衰因子,林分动态发生变化,森林衰退病或生态病,人工纯林以及纯林连栽导致的地力、生产力衰退等.中国的森林退化/衰退现状与世界各地森林退化基本一致,但由于历史原因,中国森林退化又有其自身特点：近一个世纪的强烈人为干扰,使大部分原始天然林退化为次生林;中国拥有世界上最多的人工林,且多数人工林均具有质量差、功能低等衰退特征.本文在综述森林退化/衰退研究与实践基础上,提出中国现代森林退化/衰退的的主要原因,给出中国森林退化/衰退的基本对策.     

Changing relationships between tree growth and climate in Northwest China

Recently, several studies have shown changing relationships between tree growth and climate factors, mostly in the circumpolar north. There, changing relationships with climate seem to be linked to emergent subpopulation behavior. Here, we test for these phenomena in Northwest China using three tree species (Pinus tabulaeformis, Picea crassifolia, and Sabina przewalskii) that had been collected from six sites at Qilian Mts. and Helan Mts. in Northwest China. We first checked for growth divergence of individual sites and then investigated the relationship between tree growth and climate factors using moving correlation functions (CF). Two species, Pinus and Sabina, from two sites clearly showed growth divergence, not only in the late twentieth century as reported in other studies, but also over nearly the whole record. In divergent sites, one chronology shows more stable relationships with climate factors (usually precipitation). In non-divergent sites, nearly all relationships either vary in strength or become non-significant at one point. While this might possibly be related to increased stress on some trees due to increasing temperature, the exact causes for this shift in sensitivity remain unclear. We would like to highlight the necessity for additional studies investigating possible non-stationary growth responses of trees with climate, especially at sites that are used for climate reconstruction as our sites in Northwest China.     

Tree species diversity alters plant defense investment in an experimental forest plantation in southern Mexico

How plant species diversity affects traits conferring herbivore resistance (e.g., chemical defenses), as well as the mechanisms underlying such effects, has received little attention. One potential mechanism for the effect of diversity on plant defenses is that increased plant growth at high diversity could lead to reduced investment in defenses via growth–defense trade‐offs. We measured tree growth (diameter at breast height) and collected leaves to quantify total phenolics in 2.5‐year‐old plants of six tropical tree species (N = 597 plants) in a young experimental plantation in southern Mexico. Selected plants were classified as monocultures or as polycultures represented by mixtures of four of the six species examined. Tree species diversity had a significant negative effect on total phenolics, where polycultures exhibited a 13 percent lower mean concentration than monocultures. However, there was marked variation in the effects of diversity on defenses among tree species, with some species exhibiting strong reductions in phenolic levels in mixtures, whereas others were unresponsive. In addition, tree species diversity had no effect on growth, nor was the negative effect of diversity on chemical defenses mediated by a growth–defense trade‐off. These results demonstrate that tree diversity can alter investment in chemical defenses in long‐lived tree species but that such effect may not always be under strong control by plant endogenous resource allocation trade‐offs. Regardless of the underlying mechanism, these findings have important implications for predicting effects on consumers and ecosystem function.     

西双版纳热带山地雨林群落乔木树种多样性研究

根据块样地资料对西双版纳热带山地雨林树种多样性特征进行了分析.结果表明,在2 500m²的样地上,随着起测胸径递增,各样地乔木个体数和树种丰富度均表现为依次递减;低海拔带上(850～1000 m)的山地雨林(1、2号样地)的各指数值无一定变化规律,较高海拔带上(1200～2000 m)的山地雨林(3～6号样地)的Shannon-Wiener指数和Simpson指数均表现为依次递减,而Pielou均匀度指数则呈依次递增的趋势.较高海拔带上的山地雨林在较小乔木起测胸径(≥2 cm、≥5 cm、≥10 cm)的树种丰富度、多样性和均匀度指数均要明显大于低海拔带上的山地雨林,而两者在较大乔木起测胸径(≥20 cm、≥30 cm、≥50 cm)的各指数值无明显差异.随着取样面积的递增,各样地树种丰富度、多样性和均匀度指数在取样面积递增到2 000m²处均已趋于平缓.     

Disentangling the effects of acidic air pollution,atmospheric CO2, and climate change on recent growth of red spruce trees in the Central Appalachian Mountains

In the 45 years after legislation of the Clean Air Act, there has been tremendous progress in reducing acidic air pollutants in the eastern United States, yet limited evidence exists that cleaner air has improved forest health. Here, we investigate the influence of recent environmental changes on the growth and physiology of red spruce (Picea rubens Sarg.) trees, a key indicator species of forest health, spanning three locations along a 100 km transect in the Central Appalachian Mountains. We incorporated a multiproxy approach using 75‐year tree ring chronologies of basal tree growth, carbon isotope discrimination (?¹³C, a proxy for leaf gas exchange), and δ¹⁵N (a proxy for ecosystem N status) to examine tree and ecosystem level responses to environmental change. Results reveal the two most important factors driving increased tree growth since ca. 1989 are reductions in acidic sulfur pollution and increases in atmospheric CO₂, while reductions in pollutant emissions of NO_x and warmer springs played smaller, but significant roles. Tree ring ?¹³C signatures increased significantly since 1989, concurrently with significant declines in tree ring δ¹⁵N signatures. These isotope chronologies provide strong evidence that simultaneous changes in C and N cycling, including greater photosynthesis and stomatal conductance of trees and increases in ecosystem N retention, were related to recent increases in red spruce tree growth and are consequential to ecosystem recovery from acidic pollution. Intrinsic water use efficiency (iWUE) of the red spruce trees increased by ~51% across the 75‐year chronology, and was driven by changes in atmospheric CO₂ and acid pollution, but iWUE was not linked to recent increases in tree growth. This study documents the complex environmental interactions that have contributed to the recovery of red spruce forest ecosystems from pervasive acidic air pollution beginning in 1989, about 15 years after acidic pollutants started to decline in the United States.     

Tree seedlings: how do they establish in spontaneously developed forests? A study from a mountainous area in the Czech Republic

The aim of this study was to find out which tree species can establish in spontaneously developed forests (SDFs) on mesic stands and how many tree seedlings are present there. The influence of different factors was examined and an attempt made to find out if there are some general trends true for groups of species typical for different stages of succession or with different types of distribution. All tree seedlings present in 48 permanent plots were counted (100 m²). The following factors were tested: altitude, slope and exposition, distance from the nearest forest, age and species composition of tree layer, species composition of herb layer, light conditions and soil reaction. The species composition was dependent on altitude and soil reaction. There were surprisingly high numbers of seedlings (in average 145 tree seedlings per 100 m²) and species (in average 5) present in SDFs. Not only species common in the area were found, but also uncommon species were present there, although in low numbers. Nonspecific species are the best colonisers of SDFs, and the anemochorous trees are better colonisers than zoochorous ones. The establishment of different tree species is influenced by different factors. The SDFs most probably shift toward forest with a number of species in the tree layer. The dominant species tend to be spruce and maple, but species typical for early stages of succession will be common in these growths.