Katherine J. Willis

QUATERNARY REFUGIA OF NORTH EUROPEAN TREES

An attempt is made to investigate the nature of cold-stage distributions for those forest trees which today extend to northem Europe. Evidence is taken from the pollen record of the present and earlier interglacials, a model of past climate, modem tree distributions, and physi- ography of southem Europe. The trees occupied mid- altitude sites in the mountains of southem Europe, espe- cially in the westem Balkans and Italy during the last cold stage. These areas would have had a suitable climate, and it is argued that the trees could easily have survived there at densities low enough to escape detection in the pollen record. Most taxa which spread north at the beginning of an interglacial become extinct in the northern part of their ranges, and do not retreat south at the end of the interglacial. The survival of these trees in southern Europe through a warm stage may be at least as important for long-term Qua- ternary survival in Europe as survival during a cold stage.

What is natural? The need for a long-term perspective in biodiversity conservation.

Ecosystems change in response to factors such as climate variability, invasions, and wildfires. Most records used to assess such change are based on short-term ecological data or satellite imagery spanning only a few decades. In many instances it is impossible to disentangle natural variability from other, potentially significant trends in these records, partly because of their short time scale. We summarize recent studies that show how paleoecological records can be used to provide a longer temporal perspective to address specific conservation issues relating to biological invasions, wildfires, climate change, and determination of natural variability. The use of such records can reduce much of the uncertainty surrounding the question of what is “natural” and thereby start to provide important guidance for long-term management and conservation.

Alpines, trees, and refugia in Europe

Refugia were critically important for species survival in both glacial and interglacial stages of the Quaternary. The classical view of glacial stages is that alpine and arctic plants were widespread in the lowlands of central Europe and around the margins of the continental and alpine ice-sheets, whereas trees were restricted to localised refugial areas in southern Europe and the Mediterranean basin. New palaeobotanical evidence in Europe suggests, however, that this classical view is incomplete and that tree distributional ranges during the glacial stages were more extensive and included many local areas of small populations in central and eastern Europe growing in so-called ‘cryptic’ refugia. We argue that this concept of ‘cryptic’ refugia is also applicable to arctic and alpine plants during temperate interglacial stages where small localised populations grow in naturally open habitats that are not beyond or above the forest limit. Determination of the whereabouts of these cold- and warm-stage ‘cryptic’ refugia is very important in our understanding of the spatial patterns of present day genetic diversity and the possible rates of spread of trees in response to future climate change.

Biodiversity and Climate Change

Efforts to elucidate the effect of climate change on biodiversity with detailed data sets and refined models reach novel conclusions. Over the past decade, several models have been developed to predict the impact of climate change on biodiversity. Results from these models have suggested some alarming consequences of climate change for biodiversity, predicting, for example, that in the next century many plants and animals will go extinct (1) and there could be a large-scale dieback of tropical rainforests (2). However, caution may be required in interpreting results from these models, not least because their coarse spatial scales fail to capture topography or “microclimatic buffering” and they often do not consider the full acclimation capacity of plants and animals (3). Several recent studies indicate that taking these factors into consideration can seriously alter the model predictions (4–7).

The vegetational history of the Balkans

Abstract The vegetational history of the Balkans from the last glacial through to present features a dynamic system not subject to immigration of additional taxa. Many temperate taxa appear to have survived in this region during the last glacial in low but persistent populations. Evidence suggests that a greater diversity of taxa existed in the mid to high altitude sites probably where the climate was more humid. At the lateglacial/Holocene transition many tree taxa expanded simultaneously with no evidence of the time-transgressive appearance of taxa which is taken to be indicative of differential immigration in northern Europe. Expansion of temperate woodland was at least 1000 years earlier than in northern Europe suggesting that this was the minimum time for migration of temperate trees into northern Europe. Changes in the composition of the early Holocene woodland included expansion of Pistacia between 9000 and 8000 14 C BP, a change in the forest dominants between 8000 and 7000 14 C BP, and the appearance and increase of Carpinus orient./Ostrya, Abies. Carpinus betulus and Fagus between 7500 and 5000 14 C BP. Since all types were present in the region from the lateglacial, it is suggested that factors other than migration are responsible for their population increase. These factors are considered and include climate, soil development, establishment time and anthropogenic disturbance. Development of the present day landscape started at approximately 4500 14 C BP with the onset of anthropogenic disturbance. Clearance resulted in the increase of open ground herbaceous types with grasses, Cerealia -type and Plantago lanceolata . New trees also to become established included Juglans, Olea, Castanea and Platanus . At least two of these types ( Juglans and Castanea ) were present in the Balkans during the lateglacial suggesting that although some types were imported by the Greeks/Romans others may have expanded in situ as a result of a favourable niche being created by anthropogenic disturbance.

Biodiversity baselines, thresholds and resilience: testing predictions and assumptions using palaeoecological data

Fossil records are replete with examples of long-term biotic responses to past climate change. One particularly useful set of records are those preserved in lake and marine sediments, recording both climate changes and corresponding biotic responses. Recently there has been increasing focus on the need for conservation of ecological and evolutionary processes in the face of climate change. We review key areas where palaeoecological archives contribute to this conservation goal, namely: (i) determination of rates and nature of biodiversity response to climate change; (ii) climate processes responsible for ecological thresholds; (iii) identification of ecological resilience to climate change; and (iv) management of novel ecosystems. We stress the importance of long-term palaeoecological data in fully understanding contemporary and future biotic responses.

Holocene biomass burning and global dynamics of the carbon cycle

Fire regimes have changed during the Holocene due to changes in climate, vegetation, and in human practices. Here, we hypothesise that changes in fire regime may have affected the global CO2 concentration in the atmosphere through the Holocene. Our data are based on quantitative reconstructions of biomass burning deduced from stratified charcoal records from Europe, and South-, Central- and North America, and Oceania to test the fire-carbon release hypothesis. In Europe the significant increase of fire activity is dated approximately 6000 cal. yr ago. In north-eastern North America burning activity was greatest before 7500 years ago, very low between 7500-3000 years, and has been increasing since 3000 years ago. In tropical America, the pattern is more complex and apparently latitudinally zonal. Maximum burning occurred in the southern Amazon basin and in Central America during the middle Holocene, and during the last 2000 years in the northern Amazon basin. In Oceania, biomass burning has decreased since a maximum 5000 years ago. Biomass burning has broadly increased in the Northern and Southern hemispheres throughout the second half of the Holocene associated with changes in climate and human practices. Global fire indices parallel the increase of atmospheric CO2 concentration recorded in Antarctic ice cores. Future issues on carbon dynamics relatively to biomass burning are discussed to improve the quantitative reconstructions.

How can a knowledge of the past help to conserve the future? Biodiversity conservation and the relevance of long-term ecological studies

This paper evaluates how long-term records could and should be utilized in conservation policy and practice. Traditionally, there has been an extremely limited use of long-term ecological records (greater than 50 years) in biodiversity conservation. There are a number of reasons why such records tend to be discounted, including a perception of poor scale of resolution in both time and space, and the lack of accessibility of long temporal records to non-specialists. Probably more important, however, is the perception that even if suitable temporal records are available, their roles are purely descriptive, simply demonstrating what has occurred before in Earths history, and are of little use in the actual practice of conservation. This paper asks why this is the case and whether there is a place for the temporal record in conservation management. Key conservation initiatives related to extinctions, identification of regions of greatest diversity/threat, climate change and biological invasions are addressed. Examples of how a temporal record can add information that is of direct practicable applicability to these issues are highlighted. These include (i) the identification of species at the end of their evolutionary lifespan and therefore most at risk from extinction, (ii) the setting of realistic goals and targets for conservation ‘hotspots’, and (iii) the identification of various management tools for the maintenance/restoration of a desired biological state. For climate change conservation strategies, the use of long-term ecological records in testing the predictive power of species envelope models is highlighted, along with the potential of fossil records to examine the impact of sea-level rise. It is also argued that a long-term perspective is essential for the management of biological invasions, not least in determining when an invasive is not an invasive. The paper concludes that often inclusion of a long-term ecological perspective can provide a more scientifically defensible basis for conservation decisions than the one based only on contemporary records. The pivotal issue of this paper is not whether long-term records are of interest to conservation biologists, but how they can actually be utilized in conservation practice and policy.

Sensitivity of global terrestrial ecosystems to climate variability

The identification of properties that contribute to the persistence and resilience of ecosystems despite climate change constitutes a research priority of global relevance. Here we present a novel, empirical approach to assess the relative sensitivity of ecosystems to climate variability, one property of resilience that builds on theoretical modelling work recognizing that systems closer to critical thresholds respond more sensitively to external perturbations. We develop a new metric, the vegetation sensitivity index, that identifies areas sensitive to climate variability over the past 14 years. The metric uses time series data derived from the moderate-resolution imaging spectroradiometer (MODIS) enhanced vegetation index, and three climatic variables that drive vegetation productivity (air temperature, water availability and cloud cover). Underlying the analysis is an autoregressive modelling approach used to identify climate drivers of vegetation productivity on monthly timescales, in addition to regions with memory effects and reduced response rates to external forcing. We find ecologically sensitive regions with amplified responses to climate variability in the Arctic tundra, parts of the boreal forest belt, the tropical rainforest, alpine regions worldwide, steppe and prairie regions of central Asia and North and South America, the Caatinga deciduous forest in eastern South America, and eastern areas of Australia. Our study provides a quantitative methodology for assessing the relative response rate of ecosystems—be they natural or with a strong anthropogenic signature—to environmental variability, which is the first step towards addressing why some regions appear to be more sensitive than others, and what impact this has on the resilience of ecosystem service provision and human well-being.

DOES SOIL CHANGE CAUSE VEGETATION CHANGE OR VICE VERSA? A TEMPORAL PERSPECTIVE FROM HUNGARY

The long-term relationship between major climatic change, vegetation change, and soil development is complex and poorly understood. In northeastern Hungary, for example, geochemical and pollen studies from a lake sedimentary sequence indicate that in the early postglacial, vegetation changed from a coniferous to deciduous forest, and soils from a podzol to brown earth. But which changed first? Did climatic warming result in a transformation from one soil type to another, which in turn resulted in a change in forest composition, or did the vegetation change first and subsequently alter the soil? How long did these soil transformation processes take? And what mechanisms were involved in the development of a brown-earth soil from a podzol? This paper presents the results of a study addressing some of these questions using palaeoecological analyses of a sedimentary sequence from lake Kis-Mohos To in northeastern Hungary. A proposed model for the process by which a podzol becomes transformed into a brown earth is presented, and possible triggering mechanisms are discussed. Results suggest that in northeastern Hungary the postglacial increase in deciduous populations was not consequent on soil type; rather, deciduous trees increased on podzolic soils, and this increase was one of the triggering mechanisms responsible for the development of brown-earth soils.