Vegetation Changes and Timberline Fluctuations in the Central Alps as Indicators of Holocene Climatic Oscillations

Pollen and plant-macrofossil data are presented for two lakes near the timberline in the Italian (Lago Basso, 2250 m) and Swiss Central Alps (Gouille Rion, 2343 m). The reforestation at both sites started at 9700-9500 BP with Pinus cembra, Larbc decidua, and Betula. The timberline reached its highest elevation between 8700 and 5000 BP and retreated after 5000 BP, due to a mid-Holocene climatic change and increasing human impact since about 3500 BP (Bronze Age). The expansion of Picea abies at Lago Basso between ca. 7500 and 6200 BP was probably favored by cold phases accompanied by increased oceanicity, whereas in the area of Gouille Rion, where spruce expanded rather late (between 4500 and 3500 BP), human influence equally might have been important. The mass expansion of Alnus viridis between ca. 5000 and 3500 BP probably can be related to both climatic change and human activity at timberline. During the early and middle Holocene a series of timberline fluctuations is recorded as declines in pollen and macrofossil concentrations of the major tree species, and as increases in nonarboreal pollen in the pollen percentage diagram of Gouille Rion. Most of ·the periods of low timberline can be correlated by radiocarbon dating with climatic changes in the Alps as indicated by glacier ad­ vances in combination with palynological records, solifluction, and dendrocli­ matical data. Lago Basso and Gouille Rion are the only sites in the Alps showing complete palaeobotanical records of cold phases between 10,000 and 2000 BP with very good time control. The altitudinal range of the Holocene treeline fluc­ tuations caused by climate most likely was not more than 100 to 150 m. A possible correlation of a cold period at ca. 7500-6500 BP (Misox oscil­ lation) in the Alps is made with paleoecological data from North America and Scandinavia and a climatic signal in the GRIP ice core from central Greenland 8200 yr ago (ca. 7400 yr uncal. BP).

Herbivores limit the population size of big-leaf mahogany trees in an Amazonian forest

The Janzen–Connell hypothesis proposes that specialized herbivores maintain high numbers of tree species in tropical forests by restricting adult recruitment so that host populations remain at low densities. We tested this prediction for the large timber tree species, Swietenia macrophylla, whose seeds and seedlings are preyed upon by small mammals and a host-specific moth caterpillar Steniscadia poliophaea, respectively. At a primary forest site, experimental seed additions to gaps – canopy-disturbed areas that enhance seedling growth into saplings – over three years revealed lower survival and seedling recruitment closer to conspecific trees and in higher basal area neighborhoods, as well as reduced subsequent seedling survival and height growth. When we included these Janzen–Connell effects in a spatially explicit individual-based population model, the caterpillar's impact was critical to limiting Swietenia's adult tree density, with a > 10-fold reduction estimated at 300 years. Our research demonstrates the crucial but oft-ignored linkage between Janzen–Connell effects on offspring and population-level consequences for a long-lived, potentially dominant tree species.

Reconstruction of Holocene vegetation dynamics at Lac de Bretaye, a high-mountain lake in the Swiss Alps

A deeper understanding of past vegetation dynamics is required to better assess future vegetation responses to global warming in the Alps. Lake sediments from Lac de Bretaye, a small subalpine lake in the Northern Swiss Alps (1780 m a.s.l.), were analysed to reconstruct past vegetation dynamics for the entire Holocene, using pollen, macrofossil and charcoal analyses as main proxies. The results show that timberline reached the lake’s catchment area at around 10,300 cal. BP, supporting the hypothesis of a delayed postglacial afforestation in the Northern Alps. At the same time, thermophilous trees such as Ulmus, Tilia and Acer established in the lowlands and expanded to the altitude of the lake, forming distinctive boreo-nemoral forests with Betula, Pinus cembra and Larix decidua. From about 5000 to 3500 cal. BP, thermophilous trees declined because of increasing human land use, mainly driven by the mass expansion of Picea abies and severe anthropogenic fire activity. From the Bronze Age onwards (c. 4200–2800 cal. BP), grazing indicators and high values for charcoal concentration and influx attest an intensifying human impact, fostering the expansion of Alnus viridis and Picea abies. Hence, biodiversity in alpine meadows increased, whereas forest diversity declined, as can be seen in other regional records. We argue that the anticipated climate change and decreasing human impact in the Alps today will not only lead to an upward movement of timberline with consequent loss of area for grasslands, but also to a disruption of Picea abies forests, which may allow the re-expansion of thermophilous tree species.

Biotic homogenization can decrease landscape-scale forest multifunctionality

Many experiments have shown that local biodiversity loss impairs the ability of ecosystems to maintain multiple ecosystem functions at high levels (multifunctionality). In contrast, the role of biodiversity in driving ecosystem multifunctionality at landscape scales remains unresolved. We used a comprehensive pan-European dataset, including 16 ecosystem functions measured in 209 forest plots across six European countries, and performed simulations to investigate how local plot-scale richness of tree species (α-diversity) and their turnover between plots (β-diversity) are related to landscape-scale multifunctionality. After accounting for variation in environmental conditions, we found that relationships between α-diversity and landscape-scale multifunctionality varied from positive to negative depending on the multifunctionality metric used. In contrast, when significant, relationships between β-diversity and landscape-scale multifunctionality were always positive, because a high spatial turnover in species composition was closely related to a high spatial turnover in functions that were supported at high levels. Our findings have major implications for forest management and indicate that biotic homogenization can have previously unrecognized and negative consequences for large-scale ecosystem multifunctionality.

Past forests of Europe

European forests have varied in their composition, structure, and extent over the last 5 million years or more in response to global climate changes. European forests have also undergone very major changes due to the alternating glacial-interglacial cycles of the Quaternary (last 2.6 million years). European forests have greatly changed in their extent and structure in the last 5 000 years due to human activities (the Homo sapiens phase) in the current Holocene interglacial in which we live. Contemporary ecologists and foresters can learn from ‘lessons from the past’ about forest responses and resilience to environmental changes in the past.

Abies – Circum-Mediterranean firs in Europe: distribution, habitat, usage and threats

Most European firs occur predominantly in small to medium-sized populations in the Mediterranean region, sometimes with fragmented and limited distributions, except for silver fir (Abies alba). They all are genetically closely related and can easily hybridise, perhaps as a consequence of late speciation during the late Quaternary. Circum-Mediterranean firs occur principally in mountain areas with medium to high precipitations rates which are mostly concentrated during the winter period. The species are able to tolerate long droughts in summer and tend to form pure stands when in optimal habitats. In the past firs have been extensively logged for construction and fire wood and their stands were replaced by other more disturbance adapted species or converted into rural areas. Nowadays with the exception of silver fir and Caucasian fir (Abies nordmanniana), circum-Mediterranean firs do not have a wide commercial interest. In Turkey they are still exploited for timber wood, while other firs have an ornamental use in gardening. Great importance is given to their preservation, especially to those populations which have very limited areas and specimens, with the creation of protected reserves and conservation programmes. Wild fires, livestock grazing and genetic drift represent actually their main threats.

Betula pendula, Betula pubescens and other birches in Europe: distribution, habitat, usage and threats

Silver birch (Betula pendula Roth) and downy birch (Betula pubescens Ehrh.) are short-lived, relatively small broadleaved trees that occur throughout most of Europe, particularly in northern regions. In southern Europe, birch trees are confined to mountainous areas, as they do not tolerate prolonged summer drought. Birch has a light canopy of small serrated leaves, and characteristic smooth, white to grey bark. In northern regions, birch trees can dominate the landscape up to the tree-line, whereas in the centre of their range they often occur early in secondary succession because of their abundant seed production, low demands on soil quality, and intolerance of shade. Birch trees provide the predominant hard wood source in northern Europe, and some varieties of Betula pendula produce highly priced veneers, while Betula pubescens is mostly used for pulp and fire wood. Other rarer species of birch are endemic to Europe contributing to the continental biodiversity even at high elevations and latitudes.

Castanea sativa in Europe: distribution, habitat, usage and threats

The sweet chestnut (Castanea sativa Mill.) is the only native species of the genus in Europe. The broad diffusion and active management by man resulted in the establishment of the species at the limits of its potential ecological range, which makes it difficult to trace its original natural area. The present distribution ranges from North-Western Africa (e.g. Morocco) to North-Western Europe (southern England, Belgium) and from south-western Asia (e.g. Turkey) to Eastern Europe (e.g. Romania), the Caucasus (Georgia, Armenia) and the Caspian Sea. In Europe the main chestnut forests are concentrated in a few countries such as Italy, France and the Iberian Peninsula. The sweet chestnut has a remarkable multipurpose character, and may be managed for timber production (coppice and high forest) as well as for fruit production (traditional orchards), including a broad range of secondary products and ecosystem services.

Fraxinus excelsior in Europe: distribution, habitat, usage and threats

Common ash (Fraxinus excelsior L.) is a medium-sized deciduous tree with large compound leaves that develop relatively late in spring. It flowers before leaf-buds burst and trees can carry male, female, or hermaphrodite flowers, or different combinations of the flower types. It grows throughout the European temperate zone, but is absent from the driest Mediterranean areas because it does not tolerate extended summer drought, and from the northern boreal regions, with its seedlings in particular being vulnerable to late spring frost. Soils exert a strong control on common ash distribution locally. The species grows best on fertile soils where soil pH exceeds 5.5. It rarely forms pure stands, more often it is found in small groups in mixed stands. Ash trees produce high quality timber that combines light weight, strength, and flexibility. Before the mass use of steel, it was used for a wide range of purposes, from agricultural implements to construction of boat and car frames. Today

Larix decidua and other larches in Europe: distribution, habitat, usage and threats

The European larch (Larix decidua Mill.) is a pioneer, very long-lived, fast-growing coniferous tree, which occurs in the central and eastern mountains of Europe, forming open forests or pasture woods at the upper tree limits. Larch is the only deciduous conifer in Europe as an adaptation to continental alpine climates. In fact, it is able to tolerate very cold temperatures during winter and, by losing its needles, avoids foliage desiccation. It is a transitional species, colonising open terrain after natural disturbances. It forms pure stands but more often it is found with other alpine tree species, which tend to replace it if no other disturbances occur. Thanks to its adaptability and the durability of its wood, the European larch represents an important silvicultural tree species in the alpine regions, planted even outside its natural ranges. Its wood is largely used for carpentry, furniture and pulp for paper. In lower altitudes or with high precipitation rates, larch is more susceptible to fungal diseases. Outbreaks of insect defoliators, principally caused by the larch bud moth (Zeiraphera diniana), can limit tree development, with economic losses in plantations, but they rarely lead to the death of the trees.

Picea abies in Europe: distribution, habitat, usage and threats

Among the coniferous species, Norway spruce (Picea abies (L.) Karst.) is one of the most important trees in Europe both for economic and ecological aspects, with a long tradition of cultivation. It can be a big tree, reaching 50-60 m in height with a straight and regular trunk, particularly used for timber constructions, pulpwood for paper and furniture. This widespread species dominates the Boreal forests in Northern Europe and the subalpine areas of the Alps and Carpathian Mountains. Thanks to its high performances in different site conditions, it can also be found outside its natural distribution on lower elevations in more temperate forests. Norway spruce has been massively planted up to its niche limits, where it is particularly susceptible to heat and drought, due to its shallow root system. For this reason it is expected to be severely affected under global warming conditions. Disturbed and weakened plants can be easily attacked by rot fungi such as Heterobasidion annosum and Armillaria, or by the bark beetles Ips typographus, one of the most destructive spruce forest pests.

Juglans regia in Europe: distribution, habitat, usage and threats

Juglans regia L., commonly known as common, English or Persian walnut, is an economically very important tree species, prized both for its nuts and for its attractive high-quality timber. It is the most widespread nut tree worldwide.

Litter nutrients and retranslocation in a central African rain forest dominated by ectomycorrhizal trees

In the strongly seasonal, but annually very wet, parts of the tropics, low-water availability in the short dry season leads to a semi-deciduous forest, one which is also highly susceptible to nutrient loss from leaching in the long wet season. Patterns in litterfall were compared between forest with low (LEM) and high (HEM) abundances of ectomycorrhizal trees in Korup National Park, Cameroon, over 26 months in 1990–92. Leaf litter was sorted into 26 abundant species which included six ectomycorrhizal species, and of these three were the large grove-forming trees Microberlinia bisulcata, Tetraberlinia bifoliolata and Tetraberlinia moreliana. Larger-tree species shed their leaves with pronounced peaks in the dry season, whereas other species had either weaker dependence, showed several peaks per year, or were wet-season shedders. Although total annual litterfall differed little between forest types, in the HEM forest (dominated by M. bisulcata) the dry-season peak was more pronounced and earlier than that in the LEMforest. Species differed greatly in their mean leaf litterfall nutrient concentrations, with an approx. twofold range for nitrogen and phosphorus, and 2.5–3.5-fold for potassium, magnesium and calcium. In the dry season, LEM and HEM litter showed similar declines in P and N concentration, and increases in K and Mg; some species, especially M. bisculcata, showed strong dry-wet season differences. The concentration of P (but not N) was higher in the leaf litter of ectomycorrhizal than nonectomycorrhizal species. Retranslocation of N and P was lower among the ectomycorrhizal than nonectomycorrhizal species by approx. twofold. It is suggested that, within ectomycorrhizal groves on this soil low in P, a fast decomposition rate with minimal loss of mineralized P is possible due to the relatively high litter P not limiting the cycle at this stage, combined with an efficient recapture of released P by the surface organic layer of ectomycorrhizas and fine roots. This points to a feedback between two essential controlling steps (retranslocation and mineralization) in a tropical rain forest ecosystem dominated by ectomycorrhizal trees.

Long-term Responses of Mountain Ecosystems to Environmental Changes: Resilience, Adjustment, and Vulnerability

The steep environmental gradients of mountain ecosystems over short distances reflect large gradients of several climatic parameters and hence provide excellent possibilities for ecological research on the effects of environmental change. To gain a better understanding of the dynamics of abiotic and biotic parameters of mountain ecosystems, long-term records are required since permanent plots in mountain regions cover in the best case about 50 - 70 years. In order to extend investigations of ecological dynamics beyond these temporal limitations of permanent plots, paleoecological approaches can be used if the sampling resolution can be adapted to ecological research questions, e.g. a sample every 10 years. Paleoecological studies in mountain ecosystems can provide new ecological insights through the combination of different spatial and temporal scales. [f we thus improve our understanding of processes across both steep environmental gradients and different time scales, we may be able to better estimate ecosystem responses to current and future environmental change (Ammann et al. 1993; Lotter et al. 1997). The complexity of ecological interactions in mountain regions forces us to concentrate on a number of sub-systems - without losing sight of the wider context. Here, we summarize a few case studies on the effects of Holocene climate change and disturbance on the vegetation of the Western Alps. To categorize the main response modes of vegetation to climatic change and disturbance in the Alps we use three classes of ecological behaviour: "resilience", "adjustment", and "vulnerability", We assume a resilient (or elastic) behaviour if vegetation is able to recover to its former state, regaining important ecosystem characteristics, such as floristic composition, biodiversity, species abundances, and biomass (e.g. Küttel 1990; Aber and Melillo 199 1). Conversely, vegetation displacements may occur in response to climatic change and/or disturbance. In some cases, this may culminate in irreversible large-scale processes such as species and/or community extinctions. Such drastic developments indicate high ecosystem vulnerability (or inelasticity or instability, for detailed definitions see Küttel 1990; Aber and Melillo 199 1) to climatic change and/or disturbance. In this sense, the "vulnerability" (or instability) of an ecosystem is expressed by the degree of failure to recover to the original state before disturbance and/or climatic change. Between these two extremes (resilience vs. vulnerability), ecosystem adjustments to climatic change and/or disturbance may occur, including the appearance of new and/or the disappearance of old species. The term "adjustment" is hence used to indicate the response of vegetational communities, which adapted to new environmental conditions without losing their main character. For forest ecosystems, we assume vegetational adjustments (rather than vulnerability) if the dominant (or co-dominant) tree species are not outnumbered or replaced by formerly unimportant plant species or new invaders. Adaptation as a genetic process is not discussed here and will require additional pbylogeographical studies (that incorporate the analysis of ancient DNA) in order to fully understand the distributions of ecotypes.

Wie rasch ändert sich die Waldvegetation als Folge von raschen Klimaänderungen?

How rapidly does forest vegetation change due to rapid climatic change? Current predictions of future climates show a global increase of mean temperatures of 1.4 to 5.8 °C. How rapidly can forest vegetation adapt to such predicted large changes, and in which way? We looked for answers in three different disciplines: ecological modelling, palaeoecology and succession theory. We found that changes of forest vegetation after rapid climatic changes can be continuous or abrupt. Rapid or abrupt changes may result within years to decades, among others, from marked drought as a direct effect of climate warming, limiting tree growth in the driest parts of Switzerland within a few years or decades. Indirectly, climate warming affects forest vegetation by forest fires, windstorms and, consequently, insect outbreaks. Questions relevant to forestry arise from these considerations: What is the most suitable combination of tree species for which management should aim in the future, and how do we adequately manage protection forests so that they can resist or adapt to the climatic change?