Detection and removal of disturbance trends in tree-ring series for dendroclimatology

The authors thank The Carnegie Trust for the Universities of Scotland for providing funding for Miloš Rydval’s Ph.D. The Scottish pine network expansion has been an ongoing task since 2006, and funding must be acknowledged to the following projects: EU project “Millennium” (017008-2), Leverhulme Trust project “RELiC: Reconstructing 8000 years of Environmental and Landscape change in the Cairngorms (F/00 268/BG)”, and the NERC project “SCOT2K: Reconstructing 2000 years of Scottish climate from tree rings (NE/K003097/1)”. Kevin J. Anchukaitis was also supported by a grant for the US National Science Foundation (ARC-0902051).Nonclimatic disturbance events are an integral element in the history of forests. Although the identification of the occurrence and duration of such events may help to understand environmental history and landscape change, from a dendroclimatic perspective, disturbance can obscure the climate signal in tree rings. However, existing detrending methods are unable to remove disturbance trends without affecting the retention of long-term climate trends. Here, we address this issue by using a novel method for the detection and removal of disturbance events in tree-ring width data to assess their spatiotemporal occurrence in a network of Scots pine (Pinus sylvestris L.) trees from Scotland. Disturbance trends “superimposed” on the tree-ring record are removed before detrending and the climate signals in the precorrection and postcorrection chronologies are evaluated using regional climate data, proxy system model simulations, and maximum latewood density (MXD) data. Analysis of subregional chronologies from the West Highlands and the Cairngorms in the east reveals a higher intensity and more systematic disturbance history in the western subregion, likely a result of extensive timber exploitation. The method improves the climate signal in the two subregional chronologies, particularly in the more disturbed western sites. Our application of this method demonstrates that it is possible to minimise the effects of disturbance in tree-ring width chronologies to enhance the climate signal.PostprintPeer reviewe

A comparison of times series approaches for dendroecological reconstructions of past canopy disturbance events

Time series analysis can identify outliers in tree-ring widths that may not only indicate past disturbances, but may also estimate the subsequent effects of these disturbances on tree growth. Finding a way to isolate these disturbance signals from tree-ring time series could have broad applications in forest ecology and management. Time series outliers may be expressed as pulse, step, or trend interventions, but few dendroecological studies have explored how well these different types of interventions express the response of tree-ring widths to a canopy disturbance resulting in a release event. This study addresses that question by comparing two different time series approaches for detecting release events: a pulse intervention approach and a new combined step and trend (CST) intervention approach. These methods are tested against tree-ring collections with known historical canopy disturbance events: northern red oaks (Quercus rubra) in New York released by the chestnut blight during the early 1920s, eastern hemlocks (Tsuga canadensis) in Pennsylvania released by adjacent selective logging in 1910, and northern red oaks and chestnut oaks (Quercus montana) in West Virginia released through an experimental thinning in 1982. Clusters of CST interventions, but not pulse interventions, were detected for all three collections during and immediately after the known disturbance events, showing that a CST intervention approach consistently reconstructs these release events. In addition, a CST intervention approach isolated canopy disturbance signals from tree-ring widths as disturbance-growth indices. Detrending disturbances from tree-ring widths provides an alternative approach to reconstruct climate in closed-canopy forests; however, just as importantly, disturbance-growth indices created through this method can also reconstruct changes in tree growth rates, biomass, or carbon resulting from a past disturbance event or forest thinning

Assessing Temperate Forest Growth and Climate Sensitivity in Response to a Long-Term Whole-Watershed Acidification Experiment

Acid deposition is a major biogeochemical driver in forest ecosystems, but the impacts of long-term changes in deposition on forest productivity remain unclear. Using a combination of tree ring and forest inventory data, we examined tree growth and climate sensitivity in response to 26 years of whole-watershed ammonium sulfate ((NH4)2SO4) additions at the Fernow Experimental Forest (West Virginia, USA). Linear mixed effects models revealed species-specific responses to both treatment and hydroclimate variables. When controlling for environmental covariates, growth of northern red oak (Quercus rubra), red maple (Acer rubrum), and tulip poplar (Liriodendron tulipifera) was greater (40%, 52%, and 42%, respectively) in the control watershed compared to the treated watershed, but there was no difference in black cherry (Prunus serotina). Stem growth was generally positively associated with growing season water availability and spring temperature and negatively associated with vapor pressure deficit. Sensitivity of northern red oak, red maple, and tulip poplar growth to water availability was greater in the control watershed, suggesting that acidification treatment has altered tree response to climate. Results indicate that chronic acid deposition may reduce both forest growth and climate sensitivity, with potentially significant implications for forest carbon and water cycling in deposition-affected regions

Spatial pattern and process in forest stands within the Virginia piedmont

Question: Underlying ecological processes have often been inferred from the analysis of spatial patterns in ecosystems. Using an individual-based model, we evaluate whether basic assumptions of species' life-history, drought-susceptibility, and shade tolerance generate dynamics that replicate patterns between and within forest stands. Location: Virginia piedmont, USA. Method: Model verification examines the transition in forest composition and stand structure between mesic, intermediate and xeric sites. At each site, tree location, diameter, and status were recorded in square plots ranging from 0.25 to 1.0 ha. Model validation examines the simulated spatial pattern of individual trees at scales of 1-25 m within each forest site using a univariate Ripley's K function. Results: 7512 live and dead trees were surveyed across all sites. All sites exhibit a consistent, significant shift in pattern for live trees by size, progressing from a clumped understorey (trees ≥ 0.1 m in diameter) to a uniform overstorey (trees > 0.25 m). Simulation results reflect not only the general shift in pattern of trees at appropriate scales within sites, but also the general transition in species composition and stand structure between sites. Conclusions: This shift has been observed in other forest ecosystems and interpreted as a result of competition; however, this hypothesis has seldom been evaluated using simulation models. These results support the hypothesis that forest pattern in the Virginia piedmont results from competition involving species' life-history attributes driven by soil moisture availability between sites and light availability within sites

Detection and removal of disturbance trends in tree-ring series for dendroclimatology

Non-climatic disturbance events are an integral element in the history of forests. While the identification of the occurrence and duration of such events may help to understand environmental history and landscape change, from a dendroclimatic perspective, disturbance can obscure the climate signal in tree rings. However, existing detrending methods are unable to remove disturbance trends without affecting the retention of long-term climate trends. Here we address this issue by using a novel method for the detection and removal of disturbance events in tree-ring width data to assess their spatiotemporal occurrence in a network of Scots pine (Pinus sylvestris L.) trees from Scotland. Disturbance trends ‘superimposed’ on the tree-ring record are removed before detrending and the climate signals in the pre-correction and post-correction chronologies are evaluated using regional climate data, proxy system model simulations, and maximum latewood density (MXD) data. Analysis of sub-regional chronologies from the West Highlands and the Cairngorms in the east reveals a higher intensity and more systematic disturbance history in the western sub-region, likely a result of extensive timber exploitation. The method improves the climate signal in the two sub-regional chronologies, particularly in the more disturbed western sites. Our application of this method demonstrates that it is possible to minimise the effects of disturbance in tree-ring width chronologies in order to enhance the climate signal.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Intensified vegetation water use under acid deposition

Despite the important role vegetation plays in the global water cycle, the exact controls of vegetation water use, especially the role of soil biogeochemistry, remain elusive. In this study, we reveal a new mechanism of soil biogeochemical control of large-scale vegetation water use. Nitrate and sulfate deposition from fossil fuel burning have caused substantial soil acidification, leading to the leaching of soil base cations. Of these, calcium has a unique role in plant cells by regulating stomatal aperture, thus affecting vegetation water use. We hypothesized that the leaching of the soil calcium supply, induced by acid deposition, would increase large-scale vegetation water use. We present evidence from a long-term whole watershed acidification experiment demonstrating that the alteration of the soil calcium supply by acid deposition can significantly intensify vegetation water use (~10% increase in evapotranspiration) and deplete available soil water. These results are critical to understanding future water availability, biogeochemical cycles, and surface energy flux and to help reduce uncertainties in terrestrial biosphere models

Testing the efficacy of tree-ring methods for detecting past disturbances

The retrospective study of abrupt and sustained increases in the radial growth of trees (hereinafter 'releases') by tree-ring analysis is an approach widely used for reconstructing past forest disturbances. Despite the range of dendrochronological methods used for release-detection, a lack of in-depth comparison between them can lead researchers to question which method to use and, potentially, increases the uncertainties of disturbance histories derived with different methods. Here, we investigate the efficacy and sensitivity of four widely used release detection methods using tree-ring width series and complete long-term inventories of forest stands with known disturbances. We used support vector machine (SVM) analysis trained on long-term forest census data to estimate the likelihood that Acer rubrum trees experiencing reductions in competition show releases in their tree-ring widths. We compare methods performance at the tree and stand level, followed by evaluation of method sensitivity to changes in their parameters and settings. Disturbance detection methods agreed with 60-76% of the SVM-identified growth releases under high canopy disturbance and 80-94% in a forest with canopy disturbance of low severity and frequency. The median competition index change (CIC) of trees identified as being released differed more than twofold between methods, from −0.33 (radial-growth averaging) to −0.68 (time-series). False positives (type I error) were more common in forests with low severity disturbance, whereas false negatives (type II error) occurred more often in forests with high severity disturbance. Sensitivity analysis indicated that reductions of the detection threshold and the length of the time window significantly increased detected stand-level disturbance severity across all methods. Radial-growth averaging and absolute-increase methods had lower levels of type I and II error in detecting disturbance events with our datasets. Parameter settings play a key role in the accuracy of reconstructing disturbance history regardless of the method. Time-series and radial-growth averaging methods require the least amount of a priori information, but only the time-series method quantified the subsequent growth increment related to a reduction in competition. Finally, we recommend yearly binning of releases using a kernel density estimation function to identify local maxima indicating disturbance. Kernel density estimation improves reconstructions of forest history and, thus, will further our understanding of past forest dynamics

Carpathians_-_SI – Supplemental material for Influence of sampling and disturbance history on climatic sensitivity of temperature-limited conifers

<p>Supplemental material, Carpathians_-_SI for Influence of sampling and disturbance history on climatic sensitivity of temperature-limited conifers by Miloš Rydval, Daniel L Druckenbrod, Miroslav Svoboda, Volodymyr Trotsiuk, Pavel Janda, Martin Mikoláš, Vojtěch Čada, Radek Bače, Marius Teodosiu and Rob Wilson in The Holocene</p

Carpathians_-_SI_-_v105pn_chron.m – Supplemental material for Influence of sampling and disturbance history on climatic sensitivity of temperature-limited conifers

<p>Supplemental material, Carpathians_-_SI_-_v105pn_chron.m for Influence of sampling and disturbance history on climatic sensitivity of temperature-limited conifers by Miloš Rydval, Daniel L Druckenbrod, Miroslav Svoboda, Volodymyr Trotsiuk, Pavel Janda, Martin Mikoláš, Vojtěch Čada, Radek Bače, Marius Teodosiu and Rob Wilson in The Holocene</p