Phytoremediation of 1,4-Dioxane-Containing Recovered Groundwater

The results of a pilot-scale phytoremediation study are reported in this paper. Small plots of trees established on a closed municipal waste landfill site were irrigated with recovered groundwater containing 1,4-dioxane (dioxane) and other volatile organic compounds (VOCs). The plots were managed to minimize the leaching of irrigation water, and leaching was quantified by the use of bromide tracer. Results indicated that the dioxane (2.5 μg/L) was effectively removed, probably via phytovolatilization, and that a full-scale phytoremediation system could be used. A system is now in place at the site in which the recovered groundwater can be treated using two different approaches. A physical treatment system (PTS) will be used during the winter months, and a 12 ha phytoremediation system (stands of coniferous trees) will be used during the growing season. The PTS removes VOCs using an air-stripper, and destroys dioxane using a photo-catalytic oxidation process. Treated water will be routed to the local sewer system. The phytoremediation system, located on the landfill, will be irrigated with effluent from the PTS air-stripper containing dioxane. Seasonal use of the phytoremediation system will reduce reliance on the photo-catalytic oxidation process that is extremely energy consumptive and expensive to operate.     

The effect of ethylene glycol on the phytovolatilization of 1,4-dioxane

Phytoremediation at contaminated sites is often complicated by the presence of more than one chemical However, the effects of common co-contaminants such as ethylene glycol on the phytoremediation of other chemicals, e.g., 1,4-dioxane, is not well understood. Field studies with DN34 poplar trees revealed a 28% decline in growth rate in response to 10 g/L ethylene glycol in the groundwater, thus indicating a significant and deleterious effect on tree viability, and likely, the plants' utility for phytoremediation. Thorough investigations using Arabidopsis thaliana, with its small size and rapid life cycle, indicated significant growth reduction at 10 g/L and complete inhibition of germination at 40 g/L ethylene glycol Ethylene glycol was almost as severe a stressor as the well characterized osmotic inhibitor, sorbitoL Watering potted trees with 10 g/L ethylene glycol reduced their growth by more than 50%, and similar results were observed in hydroponically grown poplar and willow trees. Under hydroponic conditions, 60 g/L ethylene glycol inhibited the phytovolatilization of l,4-dioxane by more than 80%, and all trees evapo-transpired 1,4-dioxane less efficiently than water. In fact, this efficiency differed between trees and the difference became more pronounced in the presence of ethylene glycol.     

Influence of tree age, tree size and crown structure on lichen communities in mature Alpine spruce forests

Testing the relations between tree parameters and the richness and composition of lichen communities in near-natural stands could be a first step to gather information for forest managers interested in conservation and in biodiversity assessment and monitoring. This work aims at evaluating the influence of tree age and age-related parameters on tree-level richness and community composition of lichens on spruce in an Alpine forest. The lichen survey was carried out in four sites used for long-term monitoring. In each site, tree age, diameter at breast height, tree height, the first branch height, and crown projection area were measured for each tree. Trees were stratified into three age classes: (1) <100 years old, immature trees usually not suitable for felling, (2) 100–200 years old, mature trees suitable for felling, and (3) >200 years old, over-mature trees normally rare or absent in managed stands. In each site, seven trees in each age class were selected randomly. Tree age and related parameters proved to influence both tree-level species richness and composition of lichen communities. Species richness increased with tree age and related parameters indicative of tree size. This relation could be interpreted as the result of different joint effects of age per se and tree size with its area-effect. Species turnover is also suspected to improve species richness on over-mature trees. Similarly to species richness, tree-level species composition can be partially explained by tree-related parameters. Species composition changed from young to old trees, several lichens being associated with over-mature trees. This pool of species, including nationally rare lichens, represents a community which is probably poorly developed in managed forests. In accordance to the general aims of near-to-nature forestry, the presence of over-mature trees should be enhanced in the future forest landscape of the Alps especially in protected areas and Natura 2,000 sites, where conservation purposes are explicitly included in the management guidelines.     

Dry season watering alters the significance of climate factors influencing phenology and growth of saplings of savanna woody species in central Zambia,southern Africa

Although tree growth in southern African savannas is correlated with rainfall in the wet season, some studies have shown that tree growth is controlled more by rainfall in the dry season. If more rainfall occurred in the dry season in future climates, it would affect the growth of savanna trees, especially saplings that have shallower roots which limit access to subsoil water during the dry season when leaf flush and shoot extension occur. Recent paleobotanical evidence has revealed that there was relatively more precipitation in the dry season in eastern Africa in the Eocene than under the current climate. Saplings therefore can be expected to respond more to water addition during the dry season than mature trees that have more stored water and deeper roots that access subsoil water. Accordingly, I hypothesized that irrigation in the dry season should (i) advance the onset of the growing season, (ii) increase growth rates and (iii) alter the growth responses of saplings to climate factors. To test these hypotheses saplings of five savanna woody species were irrigated during the hot‐dry season at a site in central Zambia and their monthly and annual growth rates compared to those of conspecifics growing under control conditions. Although the responses among the species were variable, all irrigated saplings had significantly higher monthly and annual growth rates than control plants. In addition, dry season watering significantly altered the climatic determinants of sapling growth by either strengthening the role of the same climatic factors that were important under control conditions or displacing them altogether. In conclusion, more precipitation during the hot‐dry season is likely to have significant positive effects on sapling growth and consequently reduce the sapling‐tree transition periods and promote future tree population recruitment in some southern African savanna tree species.     

北京地区火炬树的萌蘖繁殖扩散

首次通过调查火炬树单株和火炬树林分的萌蘖繁殖扩散状况和小样方方法调查火炬树林下的乔木树种天然更新,了解火炬树对北京主要植被类型:油松人工林、侧柏人工林、刺槐人工林和灌木丛的扩散入侵情况。调查发现:在北京荒山爆破造林示范区,火炬树单株8a来4 5°扇形单方向扩散的最远距离为8.35 m,扩散萌蘖的最多株数达98株。立地条件差,特别是土层薄,裸露的岩石多是影响火炬树扩散能力的主要因素,但没有对火炬树萌蘖株的生长产生重要影响。扩散的群体中地径以小径阶个体占主体,说明火炬树萌蘖能力旺盛。在已有高大乔木的立地,如侧柏人工林和油松人工林,火炬树虽然能扩散进入,但只是伴生下木,很难形成优势群体;乔木林内光照明显影响火炬树的扩散数量和扩散规律;并且发现有10种北京当地乔木树种能在火炬树林下天然下种更新并且正常生长。分析表明火炬树没有对北京当地乔木群落构成威胁。火炬树能扩散进入邻近的灌木丛,并且扩散进入灌木丛的火炬树明显比扩散进乔木林内的火炬树长得高。火炬树的生物学和生态学特性决定了它是良好的荒山造林的先锋树种,不可能成为顶极群落的优势树种。所以火炬树目前对北京山区自然、半自然森林生态系统没有产生入侵危害。     

Individual biomass factors for beech, oak and pine in Slovakia: a comparative study in young naturally regenerated stands

Biomass conversion and expansion factors (BCEF) which convert tree stem volume to whole tree biomass and biomass allocation patterns in young trees were studied in order to estimate tree and stand biomass in naturally regenerated forests. European beech (Fagus sylvatica L.), Sessile oak (Quercus petraea (Mattuschka) Liebl.) and Scots pine (Pinus sylvestris L.) stands were compared. Seven forest stands of each species were chosen to cover their natural distribution in Slovakia. Species-specific BCEF are presented, generally showing a steep decrease in all species in the smallest trees, with the only exception in the case of branch BCEF in beech which grows with increasing tree size. The values of BCEF for all tree compartments stabilise in all species once trees reach about 60–70-mm diameter at base. As they grow larger, all species increase their allocation to stem and branches, while decreasing the relative growth of roots and foliage. There are, however, clear differences between species and also between broadleaves and conifers in biomass allocation. This research shows that species-specific coefficients must be used if we are to reduce uncertainties in estimates of carbon stock changes by afforestation and reforestation activities.     

Spatiotemporal changes of beetle communities across a tree diversity gradient

Aim Plant and arthropod diversity are often related, but data on the role of mature tree diversity on canopy insect communities are fragmentary. We compare species richness of canopy beetles across a tree diversity gradient ranging from mono‐dominant beech to mixed stands within a deciduous forest, and analyse community composition changes across space and time. Location Germany’s largest exclusively deciduous forest, the Hainich National Park (Thuringia). Methods We used flight interception traps to assess the beetle fauna of various tree species, and applied additive partitioning to examine spatiotemporal patterns of diversity. Results Species richness of beetle communities increased across the tree diversity gradient from 99 to 181 species per forest stand. Intra‐ and interspecific spatial turnover among trees contributed more than temporal turnover among months to the total γ‐beetle diversity of the sampled stands. However, due to parallel increases in the number of habitat generalists and the number of species in each feeding guild (herbivores, predators and fungivores), no proportional changes in community composition could be observed. If only beech trees were analysed across the gradient, patterns were similar but temporal (monthly) species turnover was higher compared to spatial turnover among trees and not related to tree diversity. Main conclusions The changes in species richness and community composition across the gradient can be explained by habitat heterogeneity, which increased with the mix of tree species. We conclude that understanding temporal and spatial species turnover is the key to understanding biodiversity patterns. Mono‐dominant beech stands are insufficient to conserve fully the regional species richness of the remaining semi‐natural deciduous forest habitats in Central Europe, and analysing beech alone would have resulted in the misleading conclusion that temporal (monthly) turnover contributes more to beetle diversity than spatial turnover among different tree species or tree individuals.     

Microflora of soils under pine forests area affected by gradation of leaf-eating insects

Soils of pine forests in the Bytnica Forestry District, Poland, are poor in nutrients readily accessible to plants. The excessively acidic reaction of the soils, typical for soils under pine forests, unfavourably affects the growth of microorganisms whose numbers are lower than in soils under deciduous and mixed forests. In the pine forests of the studied forestry there were outbreaks of a defoliating insect - pine beauty moth (Panolis flammea L.), which resulted in over 60% defoliation of the trees. The studies were carried out on the area of tree stands subjected to gradation by leaf-eating insects (sprayed and not sprayed) and healthy stand of the same age class (age 60 to 70 years). The studies revealed increased number of soil microorganisms in samples taken from the area affected by pine beauty moth gradation in the case of both unsprayed areas and those sprayed with the pesticide. The occurrence in these soils of larger numbers of ammonifying and denitrifying bacteria points to the presence of conditions favouring the growth of heterotrophic organisms. Changes in the number of actinomycetes and fungi in soils under tree stands subjected to gradation by insects, compared to healthy stands, can be a consequence of a change of environmental conditions (e.g. % content of organic carbon). Soils under defoliated tree stands show higher biochemical activity related to nitrogen cycling in the pine forest ecosystem. This leads to higher availability of organic nitrogen for conversion to inorganic forms of nitrogen, which are utilised by trees. Further changes occurring in soils under forest stands affected by gradation by leaf-eating insects would allow to gain knowledge on the ecological consequences of the use of insecticides in the protection of pine stands against harmful insects, with particular stress on those situations in which pine stands not threatened by complete defoliation are sprayed.     

Mechanical Resistance of Different Tree Species to Rockfall in the French Alps

In order to determine the mechanical resistance of several forest tree species to rockfall, an inventory of the type of damage sustained in an active rockfall corridor was carried out in the French Alps. The diameter, spatial position and type of damage incurred were measured in 423 trees. Only 5% of trees had sustained damage above a height of 1.3 m and in damaged trees, 66% of broken or uprooted trees were conifers. Larger trees were more likely to be wounded or dead than smaller trees, although the size of the wounds was relatively smaller in larger trees. The species with the least proportion of damage through stem breakage, uprooting or wounding was European beech (Fagus sylvatica L.). Winching tests were carried out on two conifer species, Norway spruce (Picea abies L.) and Silver fir (Abies alba Mill.), as well as European beech, in order to verify the hypothesis that beech was highly resistant to rockfall and that conifers were more susceptible to uprooting or stem breakage. Nineteen trees were winched downhill and the force necessary to cause failure was measured. The energy (E _fail) required to break or uproot a tree was then calculated. Most Silver fir trees failed in the stem and Norway spruce usually failed through uprooting. European beech was either uprooted or broke in the stem and was twice as resistant to failure as Silver fir, and three times more resistant than Norway spruce. E _fail was strongly related to stem diameter in European beech only, and was significantly higher in this species compared to Norway spruce. Results suggest that European beech would be a better species to plant with regards to protection against rockfall. Nevertheless, all types of different abiotic stresses on any particular alpine site should be considered by the forest manager, as planting only broadleaf species may compromise the protecting capacity of the forest e.g. in the case of snow avalanches.     

Thermal properties and fauna on the bark of trees in two different African ecosystems

Summary The thermal properties of 26 African tree species in two different ecosystems were studied using thermocouples. In a subtropical moist forest were three bark types of trees: species with thin and smooth bark types with low values of insulation across the bark; species with a more structured bark type and medium insulation values; and species with deep-fissured or scaly bark types and high insulation values. Only these latter trees are able to survive openings in the subtropical forest and stand alone on edges of forest gaps. In the savanna all tree species showed adaptations in the structure of their bark in different forms: many tree species shade their trunks. Some have low bark insulation and these are known to be sensitive to fires. Some tree species show high bark insulation and do not shade their trunks. Tree species with white bark avoid overheating of their surface by reflection of the radiation. The arthropod community living exclusively on the bark was investigated for the first time on South African trees, on ten tree species. In the two different ecosystems this habitat is occupied by different arthropod groups. In the subtropical forest Acari, Araneae, Opiliones, Isopoda, Myriopoda, Blattodea, Psocoptera, Heteroptera, Coleoptera, Formicidae, and Nematocera (Diptera) are the main arthropod groups living exclusively on the bark of trees. In the savanna Pseudoscorpiones, Araneae, Collembola, Blattodea, Psocoptera, Coleoptera, Neuroptera, Termites, Formicidae, Hymenoptera and Brachycera (Diptera) are the main arthropod groups living exclusively on the bark of trees. Within one ecosystem on one bark type the dominant species are similar; richly structured bark types have a richer fauna. In the forest, bark arthropod diversity is related to the bark structure of the constituent trees, and the arthropod communities on the bark would reflect changes in the structure of the forest. Forests comprising tree species with different bark types would have a richer total bark arthropod fauna. Specialists on richly structured bark types would die out if tree species composition were altered by man to give stands consisting only of tree species with smooth bark types. Bark arthropods in a subtropical moist forest have different proportions of herbivorous and fungivorous compared to carnivorous species than those on the bark of trees in a savanna.     

Development processes and growth pattern of Pinus densiflora stands in central eastern Korea

Stand growth and developmental processes were investigated in Pinus densiflora Siebold et Zucc. stands of different ages in the central eastern region of Korea. Stands were inventoried and five trees per stand were sampled for stem analysis, age estimation, and growth analysis. More than 80% of sampled trees in a stand were established within 3–5 years, and most stands had a single cohort structure. The initial growth of pine seedlings was slow, but the height growth accelerated beyond 2–3 m height, 5–10 years after establishment. Linear growth was maintained until 10–12 m height, at which suppressed trees fell behind and might die out. The young stand was composed of pure pines, while few pine seedlings and saplings were found in the understory of older stands. The peak of diameter growth rate occurred around 5–15 years after tree establishment, implying that competition begins during that period. The pine stand development follows four stages: (1) the young stage when the growth rate increases and peaks; (2) the height competition stage when trees focus on height growth for light while maintaining a narrow DBH and height distribution; (3) the differentiation stage when suppressed trees die out, and the DBH distribution becomes wider; and (4) the mature stage when stands have a multi-canopy structure with a wide DBH and height distribution, while the understory is dominated by other tree species. The changes in growth rates and stand structure through forest development would be implemented to predict alterations of above-ground carbon sequestration rates.     

Identification and characterization of 1,4-dioxane-degrading microbe separated from surface seawater by the seawater-charcoal perfusion apparatus

To determine the concentration of soluble 1,4-dioxane during biodegradation, a new method using of high-performance liquid chromatography equipped with a hydrophilic interaction chromatography column was developed. The developed method enabled easy and rapid determination of 1,4-dioxane, even in saline medium. Microbes capable of degrading 1,4-dioxane were selected from the seawater samples by the seawater-charcoal perfusion apparatus. Among 32 candidate 1,4-dioxane degraders,, strain RM-31 exhibited the strongest 1,4-dioxane degradation ability. 16S rDNA sequencing and the similarity analysis of strain RM-31 suggested that this organism was most closely related to Pseudonocardia carboxydivorans. This species is similar to Pseudonocardia dioxanivorans, which has previously been reported as a 1,4-dioxane degrader. Strain RM-31 could degrade 300 mg/L within 2 days. As culture incubation times increasing, the residual 1,4-dioxane concentration was decreasing and the total protein contents extracted from growth cells were increasing. The optimum initial pH of the broth medium and incubation temperature for 1,4-dioxane degradation were pH 6–8 and 25 °C. The biodegradation rate of 1,4-dioxane by strain RM-31 at 25 °C in broth medium with 3 % NaCl was almost 20 % faster than that without NaCl. It was probably a first bacteria from the seawater that can exert a strong degrading ability.     

Subalpine tree growth on serpentine soil: a dendroecological analysis

In this study dendrochronological methods are used to investigate tree growth of several coniferous species growing on serpentine outcrops in the subalpine region. Though stands are thriving on the same parent material, almost barren sites covered with Scots pine trees (Pinus sylvestris/ L.) contrast sharply with adjacent dense stands composed of mixed coniferous species. To find a reasonable explanation for this vegetation pattern and to evaluate the influence of serpentine soil on radial tree growth annual variability in ring-widths was analyzed and compared between both types of stands. Tree-ring analyses show that sparse tree cover on steep slopes might be caused by environmental stress factors especially unfavourable physical soil properties such as instability and dryness. Several abrupt growth reductions followed by slow release in ring-widths indicate repeated mechanical injury to the root system due to soil erosion. On the other hand the aggravating impact of anthropogenic disturbances (tree felling) on this ecologically fragile serpentine area could be determined and dated. There was no evidence that radial tree growth of coniferous species at this site could be limited by toxic heavy metals in the soil.     

Shedding of older needle age classes does not necessarily reduce photosynthetic primary production of Norway spruce

The hypothesis that the frequently observed shedding of older needle age classes in stands of Norway spruce suffering from atmospheric pollutants has minimal effect on photosynthetic primary production was assessed. Using structural parameters of young Norway spruce [Picea abies (L.) Karst.] trees, an existing 3-dimensional canopy photosynthesis model was adapted and validated for conifer canopies. This model was employed to quantitatively demonstrate that, depending on the stand density, the loss in photosynthetic active foliage area may be compensated by higher photosynthetic rates of the remaining younger foliage age classes due to higher levels of photosynthetically active radiation (PAR) within the thinned tree crowns. Under certain circumstances, beneficial effects on the tree's water use efficiency may be expected. Extrapolating the model results to mature tree stands is addressed. The potential effects of higher PAR levels on abundance and species diversity of the epiphytic and understory vegetation, as well as the general importance of the older foliage age classes for spruce trees on a long term scale, are discussed.     

Underground water use by Eucalyptus trees in an arid climate

 Expanding the area of tree plantations in the Negev desert of Israel requires prior quantification of the water resources in small watersheds. Combined hydrological and physiological measurements were used to study a “liman” (floodwater irrigated plot) in the Negev highlands, where Eucalyptus and other tree species are grown successfully. The amount of water flowing into the liman, surplus water flowing out of the liman, temporal soil moisture distribution, and water uptake by the trees were determined. Depending on rainfall intensity and distribution during the season, the liman received 2 – 3 times the total seasonal rainfall. Although the study was conducted during a year with a negligible amount of rainfall, the transpiration rate was closely correlated with potential transpiration throughout the year. The amount of water extracted from the soil was less than the time-integrated transpiration rate from the trees, suggesting that a water source other than soil water storage was available to the trees. We suggest that the trees extracted water from the rock fractures and/or utilized the lateral flows over the rock/soil interface.     

Structural heterogeneity and tree spatial patterns in an old-growth deciduous lowland forest in Cantabria, northern Spain

Old-growth deciduous forests in western Europe, for the most part, consist of small tracts that often may be atypical due to human disturbance, poor soil productivity or inaccessibility. In addition, very little information on tree age distributions, structural heterogeneity and tree spatial patterns appears to be available for west-European forests. Characterization of the structural features of tree populations in these old-growth stands can provide the basis to design conservation plans and also inform on how present forests might look in the absence of human interference. Four old-growth stands in a deciduous forest in the Cantabrian lowlands, northern Spain, were surveyed to determine forest structure and spatial patterns. Live and dead trees were identified, measured and mapped, and live trees were cored for age estimation. Structural heterogeneity was analyzed by means of the spatial autocorrelation of tree diameter, height and age, and the uni- and bivariate spatial patterns of trees were analyzed. The dominant species, Fagus sylvatica and Quercus robur, showed reverse-J shaped size distributions but discontinuous age distributions, with maximum ages of 255–270 yr. Tree ages suggested that the forest was largely modified by past changes in forest-use, especially by temporal variation in grazing intensity. Spatial autocorrelation revealed that former parkland stands were heterogeneous with respect to tree height only, while high forest stands were composed of patches of even-aged and even-sized trees. Young trees were clumped at varying distances and establishment occurred preferentially in canopy gaps, except for Ilex aquifolium that mainly occurred beneath mature Quercus trees. Surviving trees became less intensely clumped in the dominant species, and more strongly clumped in understorey ones, which may have been due to the effects of intraspecific competition and of canopy trees on tree survival, respectively. The spatial associations between species varied within the forest, probably as a consequence of specific establishment preferences and competitive interactions.     

Unusual Fine Root Distributions of Two Deciduous Tree Species in Southern France: What Consequences for Modelling of Tree Root Dynamics?

The spatial distribution of fine roots of two deciduous tree species was investigated in contrasting growing conditions in southern France. Hybrid walnut trees (Juglans regia×nigra cv. NG23) and hybrid poplars (Populus euramericana cv. I214) were both cultivated with or without annual winter intercrops for 10 years on deep alluvial soils. Soil samples for measuring the fine root distribution of both trees and crops were obtained by soil coring down to 3-m depth at several distances and orientations from the tree trunk. The distribution of live fine roots from walnut and poplar trees was patchy and sometimes unexpected. In the tree-only stands, fine root profiles followed the expected pattern, as fine root density decreased with increasing depth and distance from the tree trunk. However, many fine root profiles under intercropped trees were uniform with depth, and some inverse profiles were observed. These distributions may result from a high degree of plasticity of tree root systems to sense and adapt to fluctuating and heterogeneous soil conditions. The distortion of the tree root system was more pronounced for the walnut trees that only partially explored the soil volume: in the tree-only stand, the walnut rooting pattern was very superficial, but in the intercropped stand walnut trees developed a deep and dense fine root network below the crop rooting zone. The larger poplars explored the whole available soil volume, but the intercrop significantly displaced the root density from the topsoil to layers below 1 m depth. Most tree root growth models assume a decreasing fine root density with depth and distance from the tree stem. These models would not predict correctly tree–tree and tree–understorey competition for water and nutrients in 3D heterogeneous soil conditions that prevail under low-density tree stands. To account for the integrated response of tree root systems to such transient gradients in soils, we need a dynamic model that would allow for both genotypic plasticity and transient environmental local soil conditions.     

Regeneration mode affects spatial genetic structure of Nothofagus dombeyi forests

Disturbance may generate population bottlenecks by reducing population size and the number of founders establishing a new colony. We tested the hypothesis that the scale of disturbance affects the levels of genetic diversity and the spatial distribution of genotypes in naturally regenerating stands of Nothofagus dombeyi, an evergreen angiosperm tree, in northwestern Patagonia. At similar spatial scales, we predicted that old-growth stands characterized by fine-scale gap phase dynamics would be genetically diverse due to restricted gene flow among temporal and spatially isolated gaps. In contrast, young massively regenerated postfire cohorts resulting from coarse-scale disturbances would be genetically more homogeneous. At each of three paired old-growth and postfire stands a minimum of 50 trees were mapped and sampled within 1 ha. Fresh tissue was collected for isozyme analysis from a total of 361 trees along with tree cores and diameters. Tree age distributions reflected the dominant modes of regeneration. Six out of nine analysed loci were polymorphic. Mean genetic diversity parameters were greater but not significant in mature stands. Fixation indices suggested significant heterozygous deficit at two-thirds of possible tests indicating a Wahlund effect due to local recruitment of related seeds. F(ST) indicated moderate between-stand divergence. Mature stands concentrated half of positively like joins and yielded significant (P < 0.05) autocorrelation coefficients at small distance classes (< 20 m). Fine-scale patch dynamics within mature stands favours the maintenance of fine-scale genetic structure as a result of shade intolerance and local seed dispersal. Conversely, postfire stands suffer the effects of genetic drift given that a few reproductive trees produce a somewhat impoverished and genetically uniform progeny. Bottleneck effects will depend upon the density of remnant trees which could also be a function of the severity of fire.     

Radial growth response of trees to seasonal soil humidity in a subtropical forest

Tree growth is the most important factor in determining the carbon sequestration processes of forest ecosystems. However, the growth phenology (seasonal growth pattern) and responses of tree growth to climatic variables vary considerably among different species, especially between deciduous and evergreen species. Thus, it is crucial to explore the seasonal growth patterns of different tree species in relation to climate to better understand the responses of tree physiology to climate changes, especially in mixed-species forest stands. In this study, we monitored the daily basal area increments of 220 individuals belonging to 15 common broadleaved tree species, nine deciduous and six evergreen species, in mixed-species experimental stands in subtropical China and analysed the relationships between radial stem growth and seasonal climate at a high-temporal resolution. We fitted daily increments of stem diameters with four frequently used nonlinear models and chose the best model for each species. The results showed that the evergreen trees grew faster than the deciduous trees, both annually and within the growing season. The tested nonlinear models (Korf, Weibull, logistic and Gompertz) produced good fits for the growth patterns of all species. Overall, the evergreen species began stem growth earlier and finished later during the growing season than that of the deciduous species. Within the growing season, the radial growth of trees in mixed stands containing both types of species was strongly positively correlated with humidity. In spring, increases in both temperature and moisture increased the daily relative basal area increment of all species. Maximum growth rates occurred when the soil water content reached its highest level and gradually decreased when the soil water content decreased. In summer, high temperatures combined with low amounts of precipitation led to heat-induced summer drought, to which the evergreen trees appeared to be more tolerant than the deciduous trees, which was reflected in the reduced stem growth of the latter. These results indicate the different climate-dependent seasonal growth strategies of evergreen and deciduous trees related to the trade-off described by the leaf economics spectrum, i.e., short-lived leaves with higher assimilation rates in deciduous and longer-lived leaves with a greater drought tolerance in evergreen species.     

Resistance of European tree species to drought stress in mixed versus pure forests: evidence of stress release by inter‐specific facilitation

While previous studies focused on tree growth in pure stands, we reveal that tree resistance and resilience to drought stress can be modified distinctly through species mixing. Our study is based on tree ring measurement on cores from increment boring of 559 trees of Norway spruce (Picea abies [L.] Karst.), European beech (Fagus sylvatica [L.]) and sessile oak (Quercus petraea (Matt.) Liebl.) in South Germany, with half sampled in pure, respectively, mixed stands. Indices for resistance, recovery and resilience were applied for quantifying the tree growth reaction on the episodic drought stress in 1976 and 2003. The following general reaction patterns were found. (i) In pure stands, spruce has the lowest resistance, but the quickest recovery; oak and beech were more resistant, but recover was much slower and they are less resilient. (ii) In mixture, spruce and oak perform as in pure stands, but beech was significantly more resistant and resilient than in monoculture. (iii) Especially when mixed with oak, beech is facilitated. We hypothesise that the revealed water stress release of beech emerges in mixture because of the asynchronous stress reaction pattern of beech and oak and a facilitation of beech by hydraulic lift of water by oak. This facilitation of beech in mixture with oak means a contribution to the frequently reported overyield of beech in mixed versus pure stands. We discuss the far‐reaching implications that these differences in stress response under intra‐ and inter‐specific environments have for forest ecosystem dynamics and management under climate change.