Wood anatomy and tree growth covary in riparian ash forests along climatic and ecological gradients

Riparian ash forests subjected to seasonal drought are among the most endangered ecosystems in Europe. They are threatened by climate warming causing aridification and by land-use changes modifying river flow. To assess the impacts of these two stress factors on riparian forests, we studied radial growth and xylem anatomical traits in five narrow-leaved ash (Fraxinus angustifolia) stands across wide climatic and ecological gradients from northern Italy to southern Portugal. Radial growth rates and earlywood hydraulic diameter (Dh) were directly correlated, whilst earlywood vessel density and growth rates were inversely associated. Ash growth positively responded to precipitation. Higher and lower rates of growth increase in response to precipitation were found in dry (annual precipitation 357–750 mm, annual water balance −39 to −48 mm) and wet (annual precipitation 1030 mm, annual water balance 27 mm) sites, respectively. Wet conditions in autumn and winter of the year prior to tree-ring formation lead to larger Dh values, except in the wet site where warmer conditions from prior autumn to current spring were positively associated to wider vessels. Growth was also enhanced by a higher river flow, reflecting higher soil moisture due to elevated groundwater table levels. Peaks in river flow from late winter to early spring increased Dh in dry-continental sites. Growth and potential hydraulic conductivity in drought-prone riparian ash forests are differently impacted by climate variability and river flow depending on site and hydrological conditions. Nevertheless, covariation between radial growth and the earlywood vessel diameter was found, regardless of site specific differences. Wood production and hydraulic conductivity are coordinated through the production of large earlywood vessels which may allow reaching higher growth rates.     

Environmental control of plant species abundance in a microtidal Mediterranean saltmarsh

Question: To what extent are environmental factors the main determinants of species abundance in Mediterranean coastal marshlands? Location: The Llobregat delta, Barcelona, Spain. Methods: Vegetation relevés were performed and a set of water table and soil variables were periodically monitored in 43 sampling points randomly distributed in four marsh areas (sites) along a coastal–inland gradient. A canonical correspondence analysis (CCA) was performed to identify the primary water and soil correlates of species cover, after considering the effect of site and point spatial location. The realized niches of dominant species were modeled through GLMs performed on the first two axes of CCA. Niche overlapping among these species was compared with their coexistence, assessed through pairwise correlations of relative species cover in each sampling point. Results: Water and soil variables explained more of the variation in species' abundance than site and spatial position. Mean water table level, maximum water conductivity and sodium adsorption ratio (SAR), summarized in the two first CCA axes, explained 23.8% of the variability in species' cover. Arthrocnemum fruticosum, Phragmites australis subsp. australis, Juncus acutus, Spartina versicolor and Juncus maritimus dominated the vegetation stands. Niches obtained from GLM response curves showed moderate overlapping among all these species except for A. fruticosum. However, pairwise correlations were mainly negative or non‐significant, indicating low coincidence, and even segregation, between species' cover. Conclusions: The abundance of dominant plants in Mediterranean marshes is only partly explained by the environmental gradients summarized in niche models. The role of other factors such as facilitation or competition between species and random recruitment should be explored.     

The effects of genetic diversity,climate and defoliation events on trembling aspen growth performance across Canada

Tree genetic makeup may provide an important control of growth dynamics; however, no studies have previously attempted to evaluate its effects in natural trembling aspen stands. In this study, we examined the relative contribution of genetics (i.e. clonal diversity, observed heterozygosity) and environmental conditions (i.e. insects, climate) on aspen growth as represented by mean inter-tree correlation (RBAR), tree basal area increment (TBAI) and inter-annual growth variability (MS). We sampled 440 trees in 22 even-aged natural stands dominated by aspen along an east-west continental gradient of decreasing annual precipitation in the Canadian boreal forest. Linear and mixed-effect models tested the relationships between tree growth, genetics and environmental factors. We showed that clonal diversity and number of years with forest tent caterpillar (FTC) defoliation (N_FTC) reduced and increased the level of growth synchronicity (RBAR), respectively. Clonal diversity explained 30 % of variation in RBAR among sites. TBAI was positively influenced by high moisture conditions while N_FTC and climate explained the variation in MS among trees for each site. No genetic effect could explain either TBAI or the MS variation. Climate and N_FTC drive annual growth variability in trembling aspen at stand and subcontinental scales. Tree genetic makeup contributed to these dynamics, the annual growth dynamics of multi-clonal stands being less homogeneous than those of monoclonal stands. Maintaining diverse aspen stands may ensure a wider range of growth responses to environmental variability, which in turn may help maintain resilience of aspen stands under future climate.     

Site‐adapted admixed tree species reduce drought susceptibility of mature European beech

Some forest‐related studies on possible effects of climate change conclude that growth potential of European beech (Fagus sylvatica L.) might be impaired by the predicted increase in future serious drought events during the growing season. Other recent research suggests that not only multiyear increment rates but also growth resistance and recovery of beech during, respectively, after dry years may differ between pure and mixed stands. Thus, we combined dendrochronological investigations and wood stable isotope measurements to further investigate the impact of neighborhood diversity on long‐term performance, short‐term drought response and soil water availability of European beech in three major geographic regions of Germany. During the last four decades, target trees whose competitive neighborhood consisted of co‐occurring species exhibited a superior growth performance compared to beeches in pure stands of the same investigation area. This general pattern was also found in exceptional dry years. Although the summer droughts of 1976 and 2003 predominantly caused stronger relative growth declines if target trees were exposed to interspecific competition, with few exceptions they still formed wider annual rings than beeches growing in close‐by monocultures. Within the same study region, recovery of standardized beech target tree radial growth was consistently slower in monospecific stands than in the neighborhood of other competitor species. These findings suggest an improved water availability of beech in mixtures what is in line with the results of the stable isotope analysis. Apparently, the magnitude of competitive complementarity determines the growth response of target beech trees in mixtures. Our investigation strongly suggest that the sensitivity of European beech to environmental constrains depends on neighborhood identity. Therefore, the systematic formation of mixed stands tends to be an appropriate silvicultural measure to mitigate the effects of global warming and droughts on growth patterns of Fagus sylvatica.     

Restoration of Aboveground Ectomycorrhizal Flora in Stands of Pinus sylvestris (Scots Pine) in The Netherlands by Removal of Litter and Humus

Species richness and sporocarp density of ectomycorrhizal fungi in stands of Pinus sylvestris (Scots pine) in The Netherlands have decreased during the last decades. The lowest species diversity was found in P. sylvestris stands situated in areas with high atmospheric deposition of nitrogen originating from intensive livestock industry. In these stands, litter and humus have accumulated into thick layers, and the herbaceous understory vegetation is dominated by the grass Deschampsia flexuosa (wavy hair grass). Earlier investigations showed negative correlations between the number of species of ectomycorrhizal fungi above ground and the depth of humus layers. Our aim was to investigate whether removal of litter, humus layers, and herbaceous vegetation (sods)—so-called "sod cutting"—increased species diversity above ground and sporocarp density of ectomycorrhizal fungi in P. sylvestris stands of different age. Therefore, three P. sylvestris stands of different ages (planted in 1987, 1963, and 1924) on Haplic Arenosol were selected. In 1990, litter, humus layers, and herbaceous vegetation were removed to create nutrient-poor sandy soils without overlying litter and humus layers. Untreated plots served as controls. Surveys conducted in 1991, 1992, and 1993 indicated that sod cutting enhanced the species diversity and sporocarp density of ectomycorrhizal fungi. These results suggest that sod cutting is a way to restore ectomycorrhizal flora in medium-aged and old stands of P. sylvestris where litter and humus have accumulated.     

不同种源刨花楠林下幼苗叶功能性状与地理环境的关系

研究9个种源地天然刨花楠林下幼苗主要叶功能性状差异及其与地理环境的关系,分析刨花楠林下幼苗对地理环境变化的响应与适应机制.结果表明: 不同种源间刨花楠林下幼苗主要叶功能性状种内变异系数较大(8.8%～28.2%),其中种源间比叶面积、叶相对含水率、叶组织密度和叶厚差异显著,表明刨花楠林下幼苗具有较强的叶片形态可塑性.叶组织密度与叶干物质含量、叶相对含水率均呈显著正相关,与比叶面积、叶厚则呈显著负相关;比叶面积与叶干物质含量、叶面积均呈显著负相关,反映刨花楠林下幼苗可通过叶片性状组合的调整和平衡以响应地理环境变化.影响刨花楠林下幼苗叶功能性状可塑性的主要环境因子为经度、纬度、>10 ℃年积温和年均温.叶厚随着经度的增加而降低,叶干物质含量和叶相对含水率则随着经度的增加而增加;叶组织密度与经度和年均温呈显著正相关,且经度对其影响大于年均温;叶面积与>10 ℃年积温和经度呈显著正相关,且前者对其影响大于后者.     

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.     

Intra-annual radial growth and water relations of trees: implications towards a growth mechanism

There is a missing link between tree physiological and wood-anatomical knowledge which makes it impossible mechanistically to explain and predict the radial growth of individual trees from climate data. Empirical data of microclimatic factors, intra-annual growth rates, and tree-specific ratios between actual and potential transpiration (T PET(-1)) of trees of three species (Quercus pubescens, Pinus sylvestris, and Picea abies) at two dry sites in the central Wallis, Switzerland, were recorded from 2002 to 2004 at a 10 min resolution. This included the exceptionally hot and dry summer of 2003. These data were analysed in terms of direct (current conditions) and indirect impacts (predispositions of the past year) on growth. Rain was found to be the only factor which, to a large extent, consistently explained the radial increment for all three tree species at both sites and in the short term as well. Other factors had some explanatory power on the seasonal time-scale only. Quercus pubescens built up much of its tree ring before bud break. Pinus sylvestris and Picea abies started radial growth 1-2 weeks after Quercus pubescens and this was despite the fact that they had a high T PET(-1) before budburst and radial growth started. A high T PET(-1) was assumed to be related to open stomata, a very high net CO2 assimilation rate, and thus a potential carbon (C)-income for the tree. The main period of radial growth covered about 30-70% of the productive days of a year. In terms of C-allocation, these results mean that Quercus pubescens depended entirely on internal C-stores in the early phase of radial growth and that for all three species there was a long time period of C-assimilation which was not used for radial growth in above-ground wood. The results further suggest a strong dependence of radial growth on the current tree water relations and only secondarily on the C-balance. A concept is discussed which links radial growth over a feedback loop to actual tree water-relations and long-term affected C-storage to microclimate.     

Methane Exchange in a Coastal Fen in the First Year after Flooding - A Systems Shift

BackgroundPeatland restoration can have several objectives, for example re-establishing the natural habitat, supporting unique biodiversity attributes or re-initiating key biogeochemical processes, which can ultimately lead to a reduction in greenhouse gas (GHG) emissions. Every restoration measure, however, is itself a disturbance to the ecosystem.MethodsHere, we examine an ecosystem shift in a coastal fen at the southern Baltic Sea which was rewetted by flooding. The analyses are based on one year of bi-weekly closed chamber measurements of methane fluxes gathered at spots located in different vegetation stands. During measurement campaigns, we recorded data on water levels, peat temperatures, and chemical properties of peat water. In addition we analyzed the first 20 cm of peat before and after flooding for dry bulk density (DBD), content of organic matter and total amounts of carbon (C), nitrogen (N), sulfur (S), and other nutrients.ResultsRewetting turned the site from a summer dry fen into a shallow lake with water levels up to 0.60 m. We observed a substantial die-back of vegetation, especially in stands of sedges (Carex acutiformis Ehrh). Concentrations of total organic carbon and nitrogen in the peat water, as well as dry bulk density and concentrations of C, N and S in the peat increased. In the first year after rewetting, the average annual exchange of methane amounted to 0.26 ± 0.06 kg m^-2. This is equivalent to a 190-times increase in methane compared to pre-flooding conditions. Highest methane fluxes occurred in sedge stands which suffered from the heaviest die-back. None of the recorded environmental variables showed consistent relationships with the amounts of methane exchanged.ConclusionsOur results suggest that rewetting projects should be monitored not only with regard to vegetation development but also with respect to biogeochemical conditions. Further, high methane emissions that likely occur directly after rewetting by flooding should be considered when forecasting the overall effect of rewetting on GHG exchange.     

Morphological variation of Pinus flexilis (Pinaceae), a bird-dispersed pine, across a range of elevations

Limber pine (Pinus flexilis James) grows across a wider range of elevations than any other tree species in the central Rockies, from ～1600 m at Pawnee Buttes to >3300 m at Rollins Pass. In this study we investigated two possible explanations for limber pine's success across a broad range of elevations: (1) the sites on which it is found, although separated by >1000 m elevation, may not be very different with respect to environmental factors that affect tree growth, and (2) limber pine growth is insensitive to environmental factors that change with elevation. We compared site characteristics of 12 limber pine stands at elevations ranging from 1630 to 3328 m as well as the growth and morphology of trees in each of these stands. Mean daily air temperature in July decreased linearly with the elevation of the site from 22.8° to 12.6°C. The growth and morphology of limber pine leaves, shoots, and trees were, in general, not related to the elevation or July mean air temperature of the sites. There was, however, a significant decrease in stomatal density with increasing elevation, which may be an acclimational response to restrict water loss at high elevations. Our data suggest that the fundamental and realized niche of limber pine is broad with respect to air temperature. In light of the high gene flow and only slight genetic differentiation among populations of species with bird-dispersed seeds, such as limber pine, it is especially unusual to see similar growth throughout an environmental gradient. Physiological and anatomical plasticity or wide physiological tolerance ranges may enable limber pine to uncouple its growth from its environment.     

Ecophysiology of selected tree species in different plant communities at the periphery of the Atlantic Forest of SE-Brazil II. Spatial and ontogenetic dynamics in Andira legalis, a deciduous legume tree

Andira legalis Vell. Toledo (Leguminosae, Papilionoideae) is a deciduous tree of the coastal vegetation of Brazil. It was studied in the restinga ecosystems of the Atlantic coast along a moisture gradient of open dry restinga (annual precipitation 800 mm, water table high at 0.9–1.2 m but partially saline), intermediate restinga (annual precipitation 1164 mm, water table low at 2–3 m) and wet restinga (annual precipitation 1000 mm, water table high at 0.5–1 m). In addition, a comparison of the ecophysiological performance was made of A. legalis in the wet restinga and a dune forest (water table low at 2–6 m). Plants of A. legalis at a given site and even within clonal stands show considerable phenological plasticity, where especially timing of leaf shedding and new bud break, leaf expansion and maturation varies between neighbouring individuals and all stages can be observed at the same time. We used analyses of soluble carbohydrate carbon compounds, soluble non-protein nitrogen compounds, stable isotope signatures (¹³C, ¹⁵N) and parameters of fluorescence of chlorophyll a for describing metabolic relations and photosynthetic behaviour and to deduce possible variations in ecophysiological strategies in response to site conditions. The phenological plasticity is described in relation to predominant metabolite levels in leaves, roots, xylem and phloem, but its ecological advantage—perhaps related to staggered resource acquisition and susceptibility to predators—remains an open question. For overall ecophysiological performance among the environmental factors root access to the ground water table appeared to be more essential than significantly different precipitation regimes, as plants in the intermediate restinga appeared to be more stressed than those in the other two open restingas. Shading in the dune forest was also an important environmental constraint.     

Variation in vernal species composition in alluvial forests of the Rhine valley,eastern France

Patterns of vernal synusiae in alluvial forests of the upper Rhine valley were analysed phytosociologically. Five synusial types were described. Environmental factors included in the analysis were spring water level, texture, trophic status and soil reaction. A separate environmental study was undertaken in pure stands of six competitive geophytes in the most fertile habitats. In a laboratory experiment effects of water stress and interspecific competition were studied for three competitive geophytes. The vernal species can be grouped into eight groups with respect to species position along the environmental gradients occurring in the floodplain. These groups can be used for bioindication.     

Characterization of diatom assemblages in mid-altitude streams of NW Italy

Following the European Water Framework Directive, this study aims to be the first step to (i) identify diatom type assemblages in unpolluted streams in NW Italy, and (ii) find which ecological factors explain most of the variation. To achieve this, we collected physical, chemical and benthic community data from four streams in NW Italy from 2001 to 2004, for a total of 72 samples. All sampling sites were between 200 m a.s.l. and 800 m a.s.l., but differed in the dominant lithological substrate, i.e. alluvial or siliceous. Relationships between diatom communities and environmental factors were examined using canonical correspondence analysis (CCA), while Indicator Species Analysis was used to define characterizing species and accompanying species of three environmental groups identified by CCA: (1) high water quality and medium saline content, (2) high water quality and low saline content, (3) poor water quality. The diatom assemblages of the three groups of sites differed significantly, as shown by the Multi-Response Permutation Procedure. There were strong correlations between diatoms and environmental factors, especially chlorides (also highly correlated with sulphates and carbonate hardness), nitrate concentration and conductivity. The group 1 assemblage was typical of the alluvial Alpine streams with medium saline content and was characterized by mostly oligosaprobic/β-mesosaprobic taxa such as Cymbella affinis, Diatoma ehrenbergii and Staurosira pinnata. The species assemblage found in the siliceous Alpine rivers with good water quality make them suitable reference sites for a benthic diatom community. Electronic Supplementary Material Supplementary material is available for this article at and accessible for authorised users Handling editor: K. Martens     

Topographic influences on radial growth of Scots pine (Pinus sylvestris L.) at small spatial scales

Dendroecological and numerical methods were used to study the influence of topographic position on radial growth of Scots pine (Pinus sylvestris) stands exposed to soil dryness. The correlation structure of total tree-ring width and latewood width of eight scattered populations representing various topographic habitats (steep south-facing slopes, plateaus and hollows) within a rock-slide area (750 m a.s.l.) of about 1 km² was investigated by principal component analysis. Scatter plots of component loadings indicated that (i) total ring width and latewood width are influenced by various climatic factors, (ii) stands growing at similar topographic position show a high agreement in year-to-year variability of radial growth, and (iii) distinct effects of topographic features (slope aspect, slope magnitude) on tree growth are modified by local disturbances (erosion, grazing) and the age structure of stands. Furthermore, both the time series of component scores and non-metric multidimensional scaling of chronologies indicated years where extremely limiting or favorable climate conditions prevailed throughout the study area (pointer years). The influence of climate on tree growth in various topographic habitats was mediated through the influence of climatically stressful years. Because stands are located at sites with different levels of water stress, growth differences between chronologies are considered to be caused by site-specific susceptibility of tree growth to soil dryness. Significant correlations between precipitation in April to June and ring-width confirm that water availability is the primary growth-limiting factor within the study area. These small-scale variations in growth-climate relationships have significant implications for dendroclimatological studies. So paleoclimatic reconstructions based on tree rings will have to assure that an unbiased data set is used, which compensates for local growth-variabilities due to site related environmental stresses.     

Long-term growth dynamics of natural forests in Hokkaido,northern Japan

Abstract. The long-term growth dynamics of natural forest stands on the island of Hokkaido were described on the basis of an analysis of data from 38 permanent plots spanning 15–22 yr. Stand structure was characterized by basal area, stem density and tree size variability. To detect trends in stand structure, regression models for recruitment rate (per ha per yr), mortality rate and the rate of change in stem density and tree size variability were developed by a stepwise method using initial basal area, stem density, tree size variability, species composition summarized by LNMDS ordination, altitude, annual mean temperature, annual precipitation, type of understorey vegetation, topography and slope aspect as candidates for predictor variables. The same analyses were conducted for basal area increment (net growth) and its components: survivor growth = basal area gain by growth of surviving individuals and mortality = basal area loss by death of individuals. Stem density remained generally unchanged; recruitment was relatively low even in very sparse stands. Stand basal area generally increased as survivor growth was approximately double the mortality. Recruitment rate was strongly affected by the presence of dwarf bamboo (Sasa spp.) vegetation on the forest floor which inhibited tree regeneration. Mortality rate was density-dependent; dense stands had higher mortality than sparse stands. Density change rate (recruitment rate - mortality rate) was, therefore, determined by both the type of understorey vegetation and stem density. Survivor growth was high in stands with high stem density and basal area. Mortality was dependent on basal area and altitude. Net basal area increment (net growth) was dependent only on stem density with other factors that influenced survivor growth and mortality omitted. Tree size variability decreased in stands with high tree size variability whereas it increased in stands with low size variability. Based on the obtained models for density change rate and net basal area increment, trajectories of stands were illustrated on a log-log diagram of stem density and basal area. The predicted differences in trajectories as affected by the understorey vegetation type indicated the importance of dwarf bamboo vegetation for forest dynamics on Hokkaido.     

Hydroclimatic drivers of the growth of riparian cottonwoods at the prairie margin: River flows,river regulation and the Pacific Decadal Oscillation

Cottonwoods, riparian poplars, are facultative phreatophytes and can obtain water from shallow soil moisture originating from rainfall, or from the deeper capillary fringe above the alluvial water table that is recharged by river water infiltration. The correspondence between cottonwood growth and river flows should reveal the dependency upon alluvial groundwater and subsequently, the vulnerability to reduced river flows. To explore this association, we analyzed historic growth patterns of plains cottonwoods (Populus deltoides) along the Red Deer River (RDR), which is at the northwestern limit of the North American Great Plains. We developed chronologies of yearly radial increments (RI) and basal area increments (BAI) and explored correspondences with the environmental records from the past century. In this semi-arid region, the RI or BAI were not correlated with local precipitation while negative correlation with growth season temperature (T) (r = −0.37, p < 0.01) could reflect reduced growth with hot summers. There was correlation between growth and annual river discharge (Q, and particularly log Q that approximates river stage) and this increased with two year averaging (r = 0.51, p < 0.01), reflecting carry-over in the watershed hydrology and in the ecophysiological response. There was correspondence with the Pacific Decadal Oscillation index (PDO, r = −0.45, p < 0.01), which provides multi-decade transitions that influence Rocky Mountain headwater precipitation and other weather characteristics, and river flows. The combination of Q, PDO and T provided the strongest multiple regression model, accounting for 44% of the historic growth variation (52% correspondence for 1953–2013). The RDR was dammed in 1983, enabling winter flow augmentation, but summer flows were sustained and cottonwood growth and the streamflow correspondence persisted. This indicates that it is the pattern of dam operation and not damming per se that determines the fate of established riparian cottonwoods downstream. This study revealed that these cottonwoods are phreatophytic and dependent upon alluvial groundwater that is recharged from the river. This provides a research strategy to determine whether riparian woodlands along other regulated rivers are similarly groundwater-dependent and could be vulnerable to river flow reductions from excessive water withdrawal for irrigation or other uses, or with climate change.     

黑龙江漠河兴安落叶松与樟子松树轮生长特性及其对气候的响应

树木年轮 (简称树轮 ) 气候学是监测与重建全球气候变化的重要方法之一。针叶树树轮的生长能反馈出气温的变化, 在高纬度地带尤为明显。该文分析了生长在我国最北部的兴安落叶松 (Larixgmelinii) 与樟子松 (Pinussylvestrisvar.mongolica) 的树轮密度和宽度的特性。落叶松最大密度、晚材平均密度、早晚材宽度和轮宽都远高于樟子松。樟子松的所有密度变量的样本方差都明显高于兴安落叶松, 宽度变量的样本方差却明显低于兴安落叶松。两树种密度变量的差值年表显著相关, 宽度变量之间没有显著相关关系。落叶松与樟子松的晚材密度的形成受 7、8月的最高温控制。另外, 樟子松的晚材还与生长季节的长短相关。落叶松的年轮宽度对生长季节开始前的温度敏感, 而樟子松的轮宽对气候变量没有很好的响应。结果表明, 落叶松与樟子松的树轮最大密度都与生长季后期的温度显著相关, 两树种的树轮信息对气候变化的重建有很大的潜力。     

Water availability and population origin affect the expression of the tradeoff between reproduction and growth in Plantago coronopus

Investment in reproduction and growth represent a classic tradeoff with implication for life history evolution. The local environment can play a major role in the magnitude and evolutionary consequences of such a tradeoff. Here, we examined the investment in reproductive and vegetative tissue in 40 maternal half‐sib families from four different populations of the herb Plantago coronopus growing in either a dry or wet greenhouse environment. Plants originated from populations with an annual or a perennial life form, with annuals prevailing in drier habitats with greater seasonal variation in both temperature and precipitation. We found that water availability affected the expression of the tradeoff (both phenotypic and genetic) between reproduction and growth, being most accentuated under dry condition. However, populations responded very differently to water treatments. Plants from annual populations showed a similar response to drought condition with little variation among maternal families, suggesting a history of selection favouring genotypes with high allocation to reproduction when water availability is low. Plants from annual populations also expressed the highest level of plasticity. For the perennial populations, one showed a large variation among maternal families in resource allocation and expressed significant negative genetic correlations between reproductive and vegetative biomass under drought. The other perennial population showed less variation in response to treatment and had trait values similar to those of the annuals, although it was significantly less plastic. We stress the importance of considering intraspecific variation in response to environmental change such as drought, as conspecific plants exhibited very different abilities and strategies to respond to high versus low water availability even among geographically close populations.     

Seasonal ionic fluctuations and annual growth rates in stands of Pinus silvestris L. and Picea abies Karst. (L.)

Seasonal variations in potassium, calcium, nitrogen, phosphorus, and magnesium contents of needles from two different stands of Pinus silvestris L. and Picea abies (L.) Karst. were determined monthly during a period of two years, and the growth rates once a year during five years. The stands were growing on sand or on clay. The supposed transport of potassium to the shoot during autumn could not be demonstrated. The nutrient status of pine and spruce growing on sand and clay was very similar, except for calcium and magnesium. There were only small differences in ion contents between pine and spruce, except for calcium. On soils with low water-holding capacity, the available water in dry years is the limiting factor affecting the growth rate of spruce but not of pine. It is concluded that the nutrient status of the leaves, the growth rates and the available water in the soil are factors that should have an important part in any discussion of fertilization of coniferous forests.     

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.