Abstract
Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra ecosystems. Here, we ...(1) synthesize these findings, (2) present a conceptual framework that identifies mechanisms and constraints on shrub increase, (3) explore causes, feedbacks and implications of the increased shrub cover in tundra ecosystems, and (4) address potential lines of investigation for future research. Satellite observations from around the circumpolar Arctic, showing increased productivity, measured as changes in ‘greenness’, have coincided with a general rise in high-latitude air temperatures and have been partly attributed to increases in shrub cover. Studies indicate that warming temperatures, changes in snow cover, altered disturbance regimes as a result of permafrost thaw, tundra fires, and anthropogenic activities or changes in herbivory intensity are all contributing to observed changes in shrub abundance. A large-scale increase in shrub cover will change the structure of tundra ecosystems and alter energy fluxes, regional climate, soil–atmosphere exchange of water, carbon and nutrients, and ecological interactions between species. In order to project future rates of shrub expansion and understand the feedbacks to ecosystem and climate processes, future research should investigate the species or trait-specific responses of shrubs to climate change including: (1) the temperature sensitivity of shrub growth, (2) factors controlling the recruitment of new individuals, and (3) the relative influence of the positive and negative feedbacks involved in shrub expansion.
Shrubs have increased in abundance and dominance in arctic and alpine regions in recent decades. This often dramatic change, likely due to climate warming, has the potential to alter both the ...structure and function of tundra ecosystems. The analysis of shrub growth is improving our understanding of tundra vegetation dynamics and environmental changes. However, dendrochronological methods developed for trees, need to be adapted for the morphology and growth eccentricity of shrubs. Here, we review current and developing methods to measure radial and axial growth, estimate age, and assess growth dynamics in relation to environmental variables. Recent advances in sampling methods, analysis and applications have improved our ability to investigate growth and recruitment dynamics of shrubs. However, to extrapolate findings to the biome scale, future dendroecological work will require improved approaches that better address variation in growth within parts of the plant, among individuals within populations and between species.
Some 2600 bog oaks have been dated from German, Dutch and Irish bogs covering the period 6000 bc to ad 1000. The ring patterns of these ‘bog oaks’ are characterized by recurrent, long-term growth ...depressions. In addition, obvious changes in the temporal distribution of the bog-oak trunks throughout the Holocene are found. Both features were probably caused by unfavourable growth conditions, which are most likely linked to changes in site hydrology. We use a new variable, ‘annual mean age’, as a tool to analyse the population dynamics of bog oaks in more detail, enabling the detection of synchronous intersite and interregional changes. It is calculated as the arithmetic mean of the age of all trees in each calendar year. We performed the calculation on regional (Germany, The Netherlands, Ireland) subsets of the bog-oak series. Abrupt changes in annual mean age are taken to indicate periods of generation change. We find good agreement for the interval from 5000 bc to 2000 bc between the continental (combined German and Dutch) and the Irish mean-age chronologies. Most changes in population dynamics correspond with contemporary changes in the associated regional tree-ring chronologies. It is concluded that the observed changes in population dynamics and growth activity are responses to common environmental forcing, most likely related to climate.
The plant economics spectrum (PES) integrates trade-offs and coordination in resource traits among species within and between organs, and affects ecosystem processes such as litter decomposition. ...This PES is currently based on trait variation among a wide range of plant types and growth forms. Here we ask whether the PES also features within the same growth form, i.e. within and between organs among temperate tree species. If so, is there a tree economics spectrum (TES) of decomposability driving the decomposition rates across the coarse branches, twigs and leaves of different species? And how robust would this TES of decomposability be to different environmental conditions?
To investigate these questions, we conducted a “common garden decomposition experiment” with ten temperate tree species in two contrasting forest environments in the Netherlands for 47 months. We evaluated the effects of functional traits of leaves, twigs, branch wood and branch bark on the decomposition rates of those organs. We measured the same resource traits for all those organs of the ten tree species and assessed whether there was a multivariate axis of functional traits explaining decomposition rates in both environments.
We report three key findings. First, tree organ specific economics spectra were significantly correlated with each other for the studied tree species. Second, tree organs differ significantly in decomposition rates, i.e. leaves were consistently more decomposable than twigs and twigs more than coarse branches. Third, we found some evidence of a TES with important afterlife effects driving coordinated decomposability of twigs and leaves but not of coarse branches across the tree species, and the effects of this TES on decomposition rates strongly depended on local forest environment.
The consistent contrasting decomposability between tree organs across species confirms an important role of plant litter inputs of different organs in forest biogeochemistry and carbon storage. There is also substantial coordination of interspecific trait variation between the finer tree organs. Knowledge about relationships of the TES and decomposability taking interactions with environmental variation into account can help for predicting whole-tree carbon and nutrient turnover as dependent on forest and soil type, even within the same climate zone.
•Trade-offs in temperate tree traits among organs for decomposition were studied.•Tree organ specific economics spectra were correlated with each other.•Evidence of tree economics spectrum for twig and leave decomposition was found.•Effects of tree economics spectrum on decomposition depended on forest environment.
Northwestern Iberia is characterized for being an Atlantic/Mediterranean transitional area, where the most natural forests contain certain species, typical from either biogeographic region, growing ...under limiting conditions due to their marginal location. In order to identify the main climatic factors controlling growth, and thus better understand how they impact wood formation processes of the key tree species in these ecosystems, we analyzed tree rings of two somehow contrasting oaks (Quercus robur, Atlantic; and Q. pyrenaica, sub-Mediterranean) at their distribution boundary towards the Mediterranean region. For this, two nearby sites with slightly different regime of water availability were selected for each species. We developed chronologies of radial increment (expressed as latewood width) and functional anatomical traits (size and number of earlywood vessels) for the last decades, and also monitored wood formation along two growing seasons. Our results suggest that the combination of anatomical traits and radial growth constitute a useful tool to understand the behavior of these species in boundary distribution areas. We found some differences between sites, especially for Q. pyrenaica, but the main factors controlling growth were clearly identified at all sites. Earlywood characteristics were mainly related to temperature, whereas latewood width responded to precipitation, regardless of the species. However, vessels of low-elevation Q. robur seemed to be controlled by factors affecting carbohydrate balance, while those of high-elevation Q. pyrenaica were associated to spring conditions for growth resumption. Summer water availability was linked to a wider latewood for both species. In addition, the analysis of xylogenesis carried out at all sites was essential to interpret climate responses by providing evidences for the existence of a cause-effect relationship.
Groundwater-dependent ecosystems occur in arid and semi-arid areas worldwide and are sensitive to changes in groundwater availability. Prosopis tamarugo Phil, endemic to the Atacama Desert, is ...threatened by groundwater overexploitation due to mining and urban consumption. The effect of groundwater depletion on two representative sites (low -and high-depletion) was studied using a multi-scale approach, combining remote sensing based estimations of canopy growth and water condition, and tree-ring based analysis of stem growth. On the stand level two NDVI (Normalized Difference Vegetation Index) -derived parameters: NDVI in winter and the difference between NDVI in summer and winter showed significant negative trends in the high-depletion site, indicating drought stress. Radial stem growth of viable P. tamarugo trees was 48% lower in the high-depletion site. At the tree level, the Green Canopy Fraction (GCF) also indicated drought stress since a larger percentage of trees fell within lower GCF classes. Groundwater depletion of 3 m, reaching a groundwater depth of >10 m, increased drought stress, and led to reduced growth in viable trees. Viable trees may be able to adapt to the drop in groundwater levels by increasing root growth, whereas for non-viable trees, the effects might be detrimental.
•Analysis of groundwater depletion in the Atacama Desert; the effects on tree growth.•Remote sensing metrics indicated reduced crown viability and growth.•Groundwater depletion resulted in a decrease of 48% radial stem growth.
On November 10th 2021, Dieter Eckstein passed away at age 82. Born and raised as a forester’s child, his entire life was connected to trees and wood. He grew up to become a dedicated scientist and ...teacher. His legacy includes both his own considerable research accomplishments as well as his founding of a growing network of tree biologists and wood scientists. From his doctoral degree onwards, the concepts and applications of dendrochronology were his passion, motivated by great curiosity in environmental influences on tree growth. He proved that dendroarchaeology can be accurate and precise, even for timber grown in the mild European maritime climate. He pioneered both techniques and concepts of xylogenesis and quantitative wood anatomy and advanced the potential for tropical dendrochronology. In all of these accomplishments, Dieter collaborated with students and colleagues from all over the world. His Dendrochronological Laboratory at the University of Hamburg hosted both young and experienced scientists from many countries. The European Working Group on Dendrochronology, which he founded in the early 1990s, was his natural habitat and playground to invent and present new research activities. We and the entire dendrochronology community have lost an inspiring colleague and visionary.
•B. neglecta provides diverse ecosystem services including frankincense production.•Growth and leaf phenology of B. neglecta are controlled by water availability.•B. neglecta performs well under ...varying rainfall conditions.•B. neglecta is able to regenerate after disturbance.•B. neglecta is a suitable species for restoration of dry woodlands in Ethiopia.
Boswellia neglecta S. Moore is a frankincense-producing tree species dominantly found in the dry woodlands of southeastern Ethiopia. Currently, the population of this socio-economically and ecologically important species is threatened by complex anthropogenic and climate change related factors. Evaluation of tree age and its radial growth dynamics in relation to climate variables helps to understand the response of the species to climate change. It is also crucial for sustainable forest resource management and utilization. Dendrochronological and remote-sensing techniques were used to study periodicity of wood formation and leaf phenology and to assess the growth dynamics of B. neglecta. The results show that B. neglecta forms two growth rings per year in the study area. The growth ring structure is characterized by larger vessels at the beginning of each growing season and smaller vessels formed later in the growing season, suggesting adaptation to decreasing soil moisture deficits at the end of the growing season. Seasonality in cambial activity matches with a bimodal leaf phenological pattern. The mean annual radial growth rate of B. neglecta trees is 2.5mm. Tree age varied between 16 and 28years, with an average age of 22years. The young age of these trees indicates recent colonization of B. neglecta in the study region. The growth rate and seasonal canopy greenness (expressed by Normalized Difference Vegetation Index – NDVI) were positively correlated with rainfall, suggesting that rainfall is the main climatic factor controlling growth of B. neglecta. The observed temporal changes in leaf phenology and vessel size across the growth rings indicate that the species is drought tolerant. Therefore, it can be regarded as a key tree species for restoration of moisture-related limited areas across the Horn of Africa.
•Excoecaria agallocha forms distinct annual rings that characterized by a band of radially flattened fibres.•Radial growth of this species is mainly driven by salinity that is modulated by freshwater ...inputs through precipitation during the monsoon.•Using a combination of ring width and vessel features allows better understanding of hydrology-growth relationships in E. agallocha.
The Bangladesh Sundarbans is the largest continuous mangrove in the world that providing crucial environmental services, particularly related to coastal protection and livelihoods of millions of people. However, anthropogenic disturbances, diseases infestation and environmental changes including sea level rise (SLR) and fresh-water flux into the delta are threatening the Sundarbans and other mangrove ecosystems worldwide. Protection of mangrove ecosystems requires knowledge on factors that mainly drive growth and vitality of tree species to evaluate which consequences can be expected from, mainly hydrology-related, environmental changes. In this study, we assessed the nature and periodicity of tree rings in Excoecaria agallocha, a wide spread mangrove species in the Bangladesh Sundarbans. We also analysed the influence of climatic factors, such as precipitation, temperature and vapor pressure deficit (VPD), and river discharge, as a proxy of salinity on ring width (RW) and vessel features, such as mean vessel area (MVA) and mean vessel density (MVD). E. agallocha forms distinct tree-ring boundary that characterized by a narrow (2–4 cells wide) band of radially flattened fibres. The RW as well as the MVA and MVD are crossdatable. The RW is mainly driven by salinity which is influenced by freshwater inputs through precipitation during monsoon along with river discharge January to April. The MVA and MVD responded to similar seasons and months as RW, but mostly with opposite signs in MVD. The results suggest that fresh water inputs through precipitation and river discharge positively influence the radial growth of E. agallocha in the Sundarbans. The RW and vessel features can be used as proxies to explore the growth dynamics of this species, especially in relation to global environmental changes.
•Coastal Douglas-fir provenances are most productive at the northern range limits.•Drought tolerances generally increases towards south.•Productive AND drought tolerant provenances originate from the ...Olympic Peninsula.•Latewood/earlywood ratio will be strongly reduced in the expected future climate.•Winter photosynthesis might compensate for summer limitations in some provenances.
Forests of the future should be resistant to exacerbating climatic conditions, especially to increasing drought, but at the same time provide a sufficient amount and quality of timber. In this context coastal Douglas-fir (Pseudotsuga menziesii (Mirb.)) is a promising species since it remains productive even under chronic drought. By choosing suitable provenances within the range of Douglas-fir (P. menziesii (Mirb.)) for a given site we can further optimise tree fitness under dry conditions or even increase timber yield.
Eighteen coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) var. menziesii) provenances were tested for seedling survival, yield, wood quality, and drought tolerance by taking advantage of a Dutch provenance trial, established in 1971 within the framework of the 1966/1967 IUFRO seed collection program. The site of the Dutch trial is representative for many sites in Central Europe and is characterised by a moderate precipitation and temperature regime. Measurements on height and diameter growth were combined with a dendrochronological study on growth response to drought years.
We found a clear latitudinal trend indicating that Douglas-fir provenances from the northern part of the species-distribution range are generally more productive than provenances from the south. In contrast, drought tolerance increased towards the south. This suggests that it is impossible to identify provenances combining maximum productivity with lowest susceptibility towards drought. However, based on the results from the trial we can give recommendations on suitable provenances that are expected to perform best under future conditions in Central Europe. On sites where severe drought events are unlikely to occur in future, fast growing provenances from the north, like Nimkish, should be planted. These provenances respond plastically to drought years, but the strong reduction of tree growth in the drought year itself indicates that these provenances will be harmed by an increasing frequency of drought events. However, on sites where water availability is likely to decrease, provenances from the Olympic Peninsula like Forks and Matlock are very promising since they showed still relatively high yield in combination with a high potential to cope with drought.
If summer drought increases in frequency and severity as expected, the latewood/earlywood ratio will be drastically reduced with negative consequences for wood quality and cavitation resistance. However, some provenances, like Marblemount or Matlock, might compensate for the negative effect of summer drought on latewood/earlywood ratio by the contribution of photosynthesis in winter to whole-year carbon stock.
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