Special feature: measuring components of ecological resilience in long-term ecological datasets

Ecological resilience has become a focal concept in ecosystem management. Palaeoecological records (i.e. the sub-fossil remains preserved in sediments) are useful archives to address ecological resilience since they can be used to reconstruct long-term temporal variations in ecosystem properties. The special feature presented here includes nine new papers from members and associates of the PAGES EcoRe3 community. The papers build on previous work in palaeoecology to investigate, identify and compare components of ecosystem resilience on centennial to millennial timescales. There are four key messages that can be summarized from the findings of papers within the special feature: (i) multi-proxy studies reveal insights into the presence and mechanisms of alternative states; (ii) transitions between alternative states may not necessarily be abrupt; (iii) components of ecological resilience can be identified in long-term ecological data and (iv) the palaeoecological record can also provide insights into factors influencing the resilience of ecosystem functioning. Overall, these papers demonstrate the importance of using long-term ecological records for addressing questions related to the theoretical framework provided by ecological resilience.publishedVersio

Comment on "Dying in the Sun: Direct evidence for elevated UV-B radiation at the end-Permian mass extinction"

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Vegetation- memory effects and their association with vegetation resilience in global drylands

Vegetation memory describes the effect of antecedent environmental and ecological conditions on the present ecosystem state and has been proposed as an important proxy for vegetation resilience. In particular, strong vegetation memory has been identified in dryland regions, but the factors underlying the spatial patterns of vegetation memory remain unknown. We aim to map the components and drivers of vegetation memory in dryland regions using state-of-the-art climate reanalysis data and refined approaches to identify vegetation-memory characteristics across dryland regions worldwide. Using a framework which distinguishes between intrinsic and extrinsic ecological memory, we show that (i) intrinsic memory is a much stronger component than extrinsic memory in the majority of dryland regions and (ii) climate reanalysis datasets change the detection of extrinsic vegetation memory in some global dryland regions. Synthesis. Our study offers a global picture of the vegetation response to two climate variables using satellite data, information which is potentially relevant for mapping components and properties of vegetation responses worldwide. However, the large differences in the spatial patterns in intrinsic vegetation memory in our study compared to previous analyses show the overall sensitivity of this component to the initial choice of extrinsic predictor variables. As a result, we caution against using the oversimplified link between intrinsic vegetation-memory and vegetation recovery rates at large spatial scales.publishedVersio

Long-term ecological responses of a lowland dipterocarp forest to climate changes and nutrient availability

Understanding the long-term impact of projected climate change on tropical rainforests is critical given their central role in the Earth's system. Palaeoecological records can provide a valuable perspective on this problem. Here, we examine the effects of past climatic changes on the dominant forest type of Southeast Asia – lowland dipterocarp forest. We use a range of proxies extracted from a 1400-yr-old lacustrine sedimentary sequence from north-eastern Philippines to determine long-term vegetation responses of lowland dipterocarp forest, including its dominant tree group dipterocarps, to changes in precipitation, fire and nutrient availability over time. Our results show a positive relationship between dipterocarp pollen accumulation rates (PARs) and leaf wax hydrogen isotope values, which suggests a negative effect of drier conditions on dipterocarp abundance. Furthermore, we find a positive relationship between dipterocarp PARs and the proxy for phosphorus availability, which suggests phosphorus controls the productivity of these keystone trees on longer time scales. Other pollen taxa show widely varying relationships with the abiotic factors, demonstrating a high diversity of plant functional responses. Our findings provide novel insights into lowland dipterocarp forest responses to changing climatic conditions in the past and highlight potential impacts of future climate change on this globally important ecosystem.publishedVersio

Abrupt change in tropical Pacific climate mean state during the Little Ice Age

The mean state of the tropical Pacific ocean-atmosphere climate, in particular its east-west asymmetry, has profound consequences for regional climates and for the El Niño/Southern Oscillation variability. Here we present a new high-resolution paleohydrological record using the stable-hydrogen-isotopic composition of terrestrial-lipid biomarkers (δDwax) from a 1,400-year-old lake sedimentary sequence from northern Philippines. Results show a dramatic and abrupt increase in δDwax values around 1630 AD with sustained high values until around 1900 AD. We interpret this change as a shift to significantly drier conditions in the western tropical Pacific during the second half of the Little Ice Age as a result of a change in tropical Pacific mean state tied to zonal sea surface temperature (SST) gradients. Our findings highlight the prominent role of abrupt shifts in zonal SST gradients on multidecadal to multicentennial timescales in shaping the tropical Pacific hydrology of the last millennium, and demonstrate that a marked transition in the tropical Pacific mean state can occur within a period of a few decades.publishedVersio

Time to better integrate paleoecological research infrastructures with neoecology to improve understanding of biodiversity long-term dynamics and to inform future conservation

Anthropogenic pressures are causing a global decline in biodiversity. Successful attempts at biodiversity conservation requires an understanding of biodiversity patterns as well as the drivers and processes that determine those patterns. To deepen this knowledge, neoecologists have focused on studying present-day or recent historical data, while paleoecologists usually study long-term data through the composition of various biological proxies and environmental indicators. By establishing standard protocols or gathering databases, research infrastructures (RIs) have been instrumental to foster exchange and collaboration among scientists within neoecology (e.g. Global Information Biodiversity Facility or National Ecological Observatory Network) and paleoecology (e.g. Paleobiology Database, Neotoma Paleoecology Database or European Pollen Database). However, these two subdisciplines (and their RIs) have traditionally remained segregated although both provide valuable information that combined can improve our understanding of biodiversity drivers and underlying processes, as well as our predictions of biodiversity responses in the future. For instance, integrative studies between paleo- and neoecology have addressed the global challenge of biodiversity loss by validating climate and ecological models, estimating species fundamental niches, understanding ecological changes and trajectories, or establishing baseline conditions for restoration. Supporting and contributing to research infrastructures from both paleo- and neoecology, as well as their further integration, could boost the amount and improve the quality of such integrative studies. We argue this will enable improved capabilities to anticipate the impacts of global change and biodiversity losses. To boost such integration and illustrate our arguments, we (1) review studies integrating paleo- and neoecology to advance in the light of global changes challenge, (2) describe RIs developed in paleoecology, and (3) discuss opportunities for further integration of RIs from both disciplines (i.e. paleo- and neoecology).publishedVersio

Rate-of-change analysis in palaeoecology revisited: a new approach

Dynamics in the rate of compositional change (rate-of-change; RoC), preserved in paleoecological sequences, are thought to reflect changes due to exogenous (climate and human forcing) or endogenous (local dynamics and biotic interactions) drivers. However, changes in sedimentation rates and sampling strategies can result in an uneven distribution of time intervals and are known to affect RoC estimates. Furthermore, there has been relatively little exploration of the implications of these challenges in quantifying RoC in paleoecology. Here, we introduce R-Ratepol – an easy-to-use R package – that provides a robust numerical technique for detecting and summarizing RoC patterns in complex multivariate time-ordered stratigraphical sequences. First, we compare the performance of common methods of estimating RoC and detecting periods of high RoC (peak-point) using simulated pollen-stratigraphical data with known patterns of compositional change and temporal resolution. In addition, we propose a new method of binning with a moving window, which shows a more than 5-fold increase in the correct detection of peak-points compared to the more traditional way of using individual levels. Next, we apply our new methodology to four representative European pollen sequences and show that our approach also performs well in detecting periods of significant compositional change during known onsets of human activity, early land-use transformation, and changes in fire frequency. Expanding the approach using R-Ratepol to open-access paleoecological datasets in global databases, such as Neotoma, will allow future paleoecological and macroecological studies to quantify major changes in biotic composition or in sets of abiotic variables across broad spatiotemporal scales.publishedVersio

A guide to the processing and standardization of global palaeoecological data for large-scale syntheses using fossil pollen

Aim: Palaeoecological data are crucial for comprehending large-scale biodiversity patterns and the natural and anthropogenic drivers that influence them over time. Over the last decade, the availability of open-access research databases of palaeoecological proxies has substantially increased. These databases open the door to research questions needing advanced numerical analyses and modelling based on big-data compilations. However, compiling and analysing palaeoecological data pose unique challenges that require a guide for producing standardized and reproducible compilations. Innovation: We present a step-by-step guide of how to process fossil pollen data into a standardized dataset compilation ready for macroecological and palaeoecological analyses. We describe successive criteria that will enhance the quality of the compilations. Though these criteria are project and research question-dependent, we discuss the most important assumptions that should be considered and adjusted accordingly. Our guide is accompanied by an R-workflow—called FOSSILPOL—and corresponding R-package—called R-Fossilpol—that provide a detailed protocol ready for interdisciplinary users. We illustrate the workflow by sourcing and processing Scandinavian fossil pollen datasets and show the reproducibility of continental-scale data processing. Main Conclusions: The study of biodiversity and macroecological patterns through time and space requires large-scale syntheses of palaeoecological datasets. The data preparation for such syntheses must be transparent and reproducible. With our FOSSILPOL workflow and R-package, we provide a protocol for optimal handling of large compilations of fossil pollen datasets and workflow reproducibility. Our workflow is also relevant for the compilation and synthesis of other palaeoecological proxies and as such offers a guide for synthetic and cross-disciplinary analyses with macroecological, biogeographical and palaeoecological perspectives. However, we emphasize that expertise and informed decisions based on palaeoecological knowledge remain crucial for high-quality data syntheses and should be strongly embedded in studies that rely on the increasing amount of open-access palaeoecological data.publishedVersio

Exploring spatio-temporal patterns of palynological changes in Asia during the Holocene

Historical legacies influence present-day ecosystem composition and dynamics. It is therefore important to understand the long-term dynamics of ecosystems and their properties. Analysis of ecosystem properties during the Holocene using fossil pollen assemblages provides valuable insights into past ecosystem dynamics by summarising so-called pollen-assemblage properties (PAPs). Using 205 fossil pollen data-sets (records), we quantify eight PAPs [pollen-taxonomic richness, diversity, evenness, pollen-compositional turnover, pollen-compositional change, and rate of pollen-compositional change (RoC)] for the Asian continent at different spatial scales (in individual records, within and across climate-zones, and within the continent) and time (temporal patterns over the past 12,000 years). Regression tree (RT) partitioning of the PAP-estimates using sample-age as a sole predictor revealed the “change-point(s)” (time or sample-age of major change in a PAP). We estimated the density of RT and multivariate regression tree (MRT) change-points in 1,000-year time bins during the Holocene. Pollen-compositional turnover (range of sample scores along the first DCCA axis) and change (number of MRT partitions) in each record reveal gradual spatial variation across latitude and a decline with longitude eastward. Temporally, compositional turnover declines linearly throughout the Holocene at all spatial scales. Other PAPs are heterogeneous across and within spatial scales, being more detectable at coarser scales. RT and MRT change-point density is broadly consistent in climate-zones and the continent, increasing from the early- to mid-Holocene, and mostly decrease from the mid-Holocene to the present for all PAPs. The heterogenous patterns in PAPs across the scales of study most likely reflect responses to variations in regional environmental conditions, anthropogenic land-use, and their interactions over space and time. Patterns at the climate-zone and continental scales indicate a gradual but congruent decline in major PAPs such as compositional turnover, rate of compositional change, and major temporal compositional changes (MRT) during the Holocene, especially during recent millennia, suggesting that vegetation in Asia has become progressively more homogenous. Data properties (e.g., spatial distribution of the records, distribution of samples within the records, and data-standardisation and analytical approaches) may also have partly influenced the results. It is critically important to evaluate the data properties and the approaches to data standardisation and summarisation.publishedVersio

Approaches to pollen taxonomic harmonisation in Quaternary palynology

Pollen taxonomic harmonisation involves the standardisation of the nomenclature of pollen and fern spores with similar morphotypes at the determination level that is common to all grains or spores with that morphotype within the pollen sequence(s) of interest. Such harmonisation is required prior to subsequent investigations such as numerical analysis, comparing, mapping, synthesis, and environmental reconstruction involving several pollen sequences. Here we present two approaches to harmonisation. These are a ‘top-down’ and a ‘bottom-up’ approach. The bottom-up approach is preferred. It is based on the concept of the regional pollen flora for the sequence(s) in the spatial area(s) of study. We present bottom-up harmonisation tables for the continental or sub-continental scales developed for the Humans on Planet Earth (HOPE) project. The tables are for North America, Latin America, Europe, Asia (three parts), and Indo-Pacific. These harmonisations are project-specific and sequence-specific, relating to the geographical area and to the sequences in the area under consideration, both of which are linked to the research questions being addressed. A new bottom-up harmonisation with a consistent taxonomic level and nomenclature is needed when additional sequences or areas are added. However, the HOPE tables can serve as a starting point for further research involving multi-sequence analyses or syntheses.publishedVersio