Recent Mediterranean droughts have highlighted concerns that climate change may be contributing to observed drying trends, but natural climate variability in the region is still poorly understood. We ...analyze 900 years (1100–2012) of Mediterranean drought variability in the Old World Drought Atlas (OWDA), a spatiotemporal tree ring reconstruction of the June‐July‐August self‐calibrating Palmer Drought Severity Index. In the Mediterranean, the OWDA is highly correlated with spring precipitation (April–June), the North Atlantic Oscillation (January–April), the Scandinavian Pattern (January–March), and the East Atlantic Pattern (April–June). Drought variability displays significant east‐west coherence across the basin on multidecadal to centennial timescales and north‐south antiphasing in the eastern Mediterranean, with a tendency for wet anomalies in the Black Sea region (e.g., Greece, Anatolia, and the Balkans) when coastal Libya, the southern Levant, and the Middle East are dry, possibly related to the North Atlantic Oscillation. Recent droughts are centered in the western Mediterranean, Greece, and the Levant. Events of similar magnitude in the western Mediterranean and Greece occur in the OWDA, but the recent 15 year drought in the Levant (1998–2012) is the driest in the record. Estimating uncertainties using a resampling approach, we conclude that there is an 89% likelihood that this drought is drier than any comparable period of the last 900 years and a 98% likelihood that it is drier than the last 500 years. These results confirm the exceptional nature of this drought relative to natural variability in recent centuries, consistent with studies that have found evidence for anthropogenically forced drying in the region.
Key Points
There is large multidecadal drought variability across the Mediterranean over the last 900 years
Droughts tend to be zonally symmetric, but there is strong north‐south antiphasing in eastern basin
There is an 89%/98% likelihood that the recent Levant drought is the worst of the last 900/500 years
•Long time-series of seed production are critical to masting studies.•Dendrochronology may enable the extension of observational records of masting.•Quercus ring-width does not respond strongly to ...masting.•Analysing seasonwood or subsampling super-producers does not enhance the signal.•Quercus tree-rings show low potential for the reconstruction of masting.
Oak (Quercus) are a dominant and important tree genus in the Central Hardwood Region (USA) due to their commercial timber value, and food value of their protein-rich seed – acorns – to wildlife. Acorn production is characteristic of masting, with highly variable crop sizes that are synchronized within populations, but with consideration variation among individual trees. Critical to acorn masting studies are longitudinal datasets, which are often difficult to maintain due to a host of constraints. One possible approach to extending mast datasets involves the use of dendrochronology, where tree-ring-width variability serves as a proxy for annual acorn production. In this study, we addressed the future utility of mast reconstructions by examining relationships between acorn production, climate, and tree-ring data from five common oak species in southern Appalachian hardwood forests (Bent Creek Experimental Forest). We found little evidence to suggest that acorn production influences seasonally resolved tree-ring data, even when we considered early and latewood growth separately, or when we analyzed trees with substantially higher overall investment in acorn production (i.e., super producers). Monthly climate (temperature, precipitation) correlated to acorn production was strongest when lagged, which could indicate weather conditions were more important for phases of flower production and fertilization than acorn maturation. However, relationships were relatively weak, indicating limited potential for the forecasting of acorn crops, or for reconstruction methods that combine observed weather data and tree-ring responses to masting. Overall, our findings provide some support for the role of resource dynamics in regulating masting in oaks, but relationships between radial growth and acorn production are not sufficiently strong, at least in this location, to permit tree-ring based reconstruction of masting in these oak species.
Lake Burgäschi is a small lake on the Swiss Plateau, which has been inhabited since the Mesolithic, but is best known for its Neolithic lakeside settlements. Archaeological research has been ...conducted at Lake Burgäschi for the last 170 years, undergoing all the developments to modern archaeology. Recently, Lake Burgäschi has been re-explored as part of an interdisciplinary research project under the direction of the Institute of Archaeological Sciences at the University of Bern. Excavations were carried out at various sites around Lake Burgäschi, allowing for a new precise chronological classification of already known sites, but also revealing previously unknown settlement sites on the lakeshore. Furthermore, by means of interdisciplinary investigations on the pottery, animal and human bones as well as flint artefacts, previous knowledge on subsistence, mobility and land-use has been substantially extended. The present volume closes a long lasting research gap and combines new results with ancient data to a comprehensive synthesis.
•The relationship between NEP and tree biomass increment was assessed.•We summarized NEP from previous-year fall to current-year fall.•Tree biomass increment lagged one year behind NEP for half of ...the time-series.•In recent years tree biomass increment became synchronized with current-year NEP.•A shift to synchrony may reflect changes in stand-level carbon allocation dynamics.
Eddy covariance methodologies have greatly improved our understanding of the forest carbon cycle, including controls over year-to-year variability in productivity (measured as net ecosystem productivity, NEP, where NEP is the difference between the mass of carbon fixed by photosynthesis and that lost by ecosystem respiration). However, establishing and maintaining eddy covariance towers requires sizeable financial and logistical investments. Tree-ring methods, which can produce annual estimates of tree biomass increment from individual trees, provide an alternative approach for assessing forest productivity. Attempts to link these measures of productivity (i.e., NEP and tree biomass increment) have produced inconsistent results, in part because NEP time series are typically too short to provide robust comparisons. We here use a relatively long (20-year) NEP time series together with annual tree biomass increment (derived from tree-ring data) from the same site to determine to what extent the two productivity measures relate to each other. We conducted this study at the Howland Research Forest, central Maine USA, which supports a mature, mixed-species conifer forest. We expressed stand-level tree biomass increment on a per-area basis, which allowed direct comparisons with NEP data. Our results revealed a strong relationship between tree biomass increment and annual NEP measurements when the latter are summarized from previous-year fall to current-year fall, a marked improvement over more typical calendar-year summaries. Further, our results suggest tree biomass increment lagged one year behind NEP (i.e., assimilated carbon was not allocated to wood formation until the following year) for roughly the first half of the time-series, but later became synchronized with current-year NEP. This shift to synchrony may reflect a change in stand-level carbon allocation and growth dynamics. The apparent shift in carbon allocation from storage into current-year wood formation is most evident in two recent years with above-average spring temperatures. Although our results demonstrate a link between annual tree biomass increment and NEP, they also point to complexities that may confound our interpretation of these productivity measures.
Aim
The International Tree‐Ring Data Bank (ITRDB) is the most comprehensive database of tree growth. To evaluate its usefulness and improve its accessibility to the broad scientific community, we ...aimed to: (a) quantify its biases, (b) assess how well it represents global forests, (c) develop tools to identify priority areas to improve its representativity, and d) make available the corrected database.
Location
Worldwide.
Time period
Contributed datasets between 1974 and 2017.
Major taxa studied
Trees.
Methods
We identified and corrected formatting issues in all individual datasets of the ITRDB. We then calculated the representativity of the ITRDB with respect to species, spatial coverage, climatic regions, elevations, need for data update, climatic limitations on growth, vascular plant diversity, and associated animal diversity. We combined these metrics into a global Priority Sampling Index (PSI) to highlight ways to improve ITRDB representativity.
Results
Our refined dataset provides access to a network of >52 million growth data points worldwide. We found, however, that the database is dominated by trees from forests with low diversity, in semi‐arid climates, coniferous species, and in western North America. Conifers represented 81% of the ITRDB and even in well‐sampled areas, broadleaves were poorly represented. Our PSI stressed the need to increase the database diversity in terms of broadleaf species and identified poorly represented regions that require scientific attention. Great gains will be made by increasing research and data sharing in African, Asian, and South American forests.
Main conclusions
The extensive data and coverage of the ITRDB show great promise to address macroecological questions. To achieve this, however, we have to overcome the significant gaps in the representativity of the ITRDB. A strategic and organized group effort is required, and we hope the tools and data provided here can guide the efforts to improve this invaluable database.
Summary
As climate change drives increased drought in many forested regions, mechanistic understanding of the factors conferring drought tolerance in trees is increasingly important. The ...dendrochronological record provides a window through which we can understand how tree size and traits shape growth responses to droughts.
We analyzed tree‐ring records for 12 species in a broadleaf deciduous forest in Virginia (USA) to test hypotheses for how tree height, microenvironment characteristics, and species’ traits shaped drought responses across the three strongest regional droughts over a 60‐yr period.
Drought tolerance (resistance, recovery, and resilience) decreased with tree height, which was strongly correlated with exposure to higher solar radiation and evaporative demand. The potentially greater rooting volume of larger trees did not confer a resistance advantage, but marginally increased recovery and resilience, in sites with low topographic wetness index. Drought tolerance was greater among species whose leaves lost turgor (wilted) at more negative water potentials and experienced less shrinkage upon desiccation.
The tree‐ring record reveals that tree height and leaf drought tolerance traits influenced growth responses during and after significant droughts in the meteorological record. As climate change‐induced droughts intensify, tall trees with drought‐sensitive leaves will be most vulnerable to immediate and longer‐term growth reductions.
•Intra-annual growth patterns differ between cold and dry sites.•Growth bimodality is typical for the dry continental site.•Growth bimodality is the response to autumn soil moisture ...availability.•Growth at the cold site exhibits a right-skewed unimodal distribution.•High spring growth rates can compensate for growth cessation during dry summers.
Mediterranean trees and shrubs adjust physiological processes to ensure their functioning under shifting dry conditions. However, little is known about spatial and between-species differences in cambial dynamics and how their temporal variability determines climatic adaptability. We used the Vaganov‒Shashkin process-based model to simulate five decades of daily cambial dynamics of coexisting junipers (shrubs/short-stature trees) and pines (tall trees) at two climatically contrasting continental sites in northeastern Spain. We hypothesized that different sites and growth forms may shift from unimodal to bimodal growth patterns, which in turn determines their ability to cope with climate stress. The model produced simulations coherent with ring-width chronologies, xylogenesis data and field monitoring of soil moisture. The model successfully captured differences in growth patterns between sites, but it was unable to highlight differences between coexisting species. The cambial dynamics of all the species were controlled by soil moisture in summer, resulting in bimodal or right-skewed unimodal growth patterns. Simulated cambial dynamics were strongly bimodal, with two distinct peaks in spring and autumn and frequent growth cessation during summer at the dry site with Juniperus thurifera and Pinus halepensis. The growth dynamics of both species at the cold site (Juniperus communis, Pinus uncinata) were rather right-skewed unimodal, suggesting reduced but continuous summer cambial activity. Species at the cold site responded to increasing moisture limitation over the last decades by earlier onset of cambial activity and increased spring cambial kinetics. The shifting of spring growth phenology and kinetics was less effective at the dry site, where both species reduced their growth during summer significantly. Our results suggest that growth adaptability on intra-annual scale determines the performance of woody plants under ongoing climate change in drought-prone regions. High responsiveness of cambial activity corroborates other physiological adaptations which jointly determine the climate-change resistance of junipers and pines.
•The correlation between tree-ring δ13C and ecosystem GPP was assessed in a forest.•Early-wood and late-wood δ13C were analyzed separately.•Tree-ring δ13C positively correlated with forest GPP of the ...current-year.•Early- and late-wood δ13C traced GPP seasonally and annually.
Tree-ring carbon (C) isotope composition (δ13C) varies depending on photosynthetic capacity and stomatal conductance, and it is therefore intrinsically associated with vegetation productivity. Still, very little is known about the relationship between tree-ring δ13C values and forest gross primary productivity (GPP). Here, we investigated relationships between tree-ring δ13C, tree-ring width index (RWI) and ecosystem-level GPP in a Korean pine (Pinus koraiensis)-dominated old-growth temperate forest in northeastern China. Specifically, we generated chronologies of RWI and δ13C from early-wood (δ13CEW) and late-wood (δ13CLW) of Korean pines, and we built a 17-year continuous ecosystem-level GPP series using eddy covariance measurements, in which a GPP model was used for gap filling. RWI showed vague relationships with tree-ring δ13C and whole-forest GPP, and it was insensitive to interannual variations in climate at the study site. By contrast, both δ13CEW and δ13CLW showed significant relationships with climate variables and GPP. This suggests that isotopic signals of photosynthetic C uptake were imprinted on the formation of tree rings and that tree-ring δ13C of Korean pine is a better proxy for forest GPP than RWI. In addition, tree-ring δ13C of Korean pines was positively correlated with GPP, with an especially close relationship between δ13CEW and GPP of the current year. This implies that photosynthetic activity, rather than stomatal conductance, was the main driver of tree-ring δ13C signals of Korean pine and it was mainly influenced by the climate conditions of the current year, probably because of a limited use of previously fixed C during early-wood formation. Our findings demonstrate the great potential of tree-ring δ13C for reconstructing GPP timeseries at the centennial scale for forest ecosystems, and they could help to constrain parameters in terrestrial C cycle models to improve predictions of C fluxes.
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