Tree species dynamics and disturbance in three Swedish boreal forest stands during the last two thousand years

Abstract. Data from three forest stands for the past 2000 yr show how the shade-intolerant species Pinus sylvestris and Betula pubescens maintain significant populations in the Swedish boreal landscape. Age structure data from a northern stand close to the range limits of Picea abies and Pinus complement a local pollen diagram, and reveal cyclic population fluctuations which can be related to periods of climatic stress and fire. Pollen data from two southern stands show that high fire frequencies in the past prevented the expansion of Picea populations. Reduction of the fire frequency during the last 200 yr has favoured Picea. A long time perspective reveals the population dynamics of long-lived species and indicates the controlling factors. Such knowledge permits assessment of the current status and likely future of forest stands.     

Long-term dynamics of vegetation and disturbance of a southern boreal spruce swamp forest

Abstract. Analysis of pollen, charcoal and loss-on-ignition in peat cores from a Picea aèies-dominated swamp forest in central Sweden show the vegetation changes and disturbance patterns over 9500 yr. Six major sequences of local vegetation development are identified: (A) Pinus period, ca. 9500–7000 cal. BP; (B) Open mire period (ca. 7000–4500 cal. BP; (C) Betula period, ca. 4500–2300 cal. BP; (D) Picea period (ca. 2300–1000 cal. BP; (E) Human impact period (ca. 1000–100 cal. BP); and (F) Period of human abandonment during the last ca. 100 yr. The swamp forest has been highly dynamic in response to various natural and anthropogenic disturbance agencies. Several fires have heavily influenced the vegetation development. During the last ca. 900 yr human influence has been important, initially from grazing and trampling by domesticated animals (ca. 1000–500 cal. BP), and subsequently small-scale cereal growing (ca. 400–100 BP). Cutting, burning and animal browsing influenced the structure and dynamics of the swamp forest by creating a more open stand and suppressing tree regeneration. Recent cessation of human impact has led to increased tree regeneration and a denser swamp forest stand. The present high biodiversity, and subsequent conservation interest does not result from long-term stability or absence of fire and human impact. However, in spite of repeated disturbances, a continuity of old and senescent trees produced a forest type with abundant dead wood. With the relatively minor importance of fire over long periods of time, the swamp forest developed a structure maintaining a high biological diversity. An important issue for maintaining long-term biodiversity in the boreal landscape must be to create a mosaic where different forest types are present, with a variety of structures, substrates and processes, to provide a certain degree of freedom for species to move around in the landscape.     

Holocene land uplift and its influence on fire history and ecosystem development in boreal Sweden

Lake Uddjaur in northern Sweden was formed as a consequence of non‐uniform glacio‐isostatic uplift in which a forested valley was gradually flooded and high elevation areas became islands. We hypothesized that small islands in Lake Uddjaur burnt through lightning strike more frequently when they were part of a large forested area compared to when they became true islands, and that this reduction in fire impact has enhanced the domination of late successional species and humus accumulation. Fire history and vegetation dynamics were studied by analysis of macroscopic charcoal (> 0.5 mm) and pollen in humus profiles from two islands. According to a model of isostatic uplift, the islands became gradually isolated from the mainland between ca. 2000 to 1000 BP, i.e. during the same time that fire impact declined. Prior to that, both islands were part of a Pinus‐Betula forest landscape affected by fires from ca. 5800 to ca. 1500 BP. Thereafter fire influence ceased and the islands became more strongly characterized by late successional species, e.g. Picea. This change was associated with substantial humus accumulation. The decreased fire influence on these islands contrasts with the regional increase in fire influence during the last 1000 yr. Long‐term influence of wildfire is important in vegetation dynamics and humus accumulation and, thus, post‐glacial isostatic land uplift can indirectly have a substantial influence on ecosystem development. Consequently, this effect should be further considered in long‐time ecosystem studies of areas with large, non‐uniform land uplift such as those found in northern Fennoscandia and eastern Canada.     

A two thousand year history of a northern Swedish boreal forest stand

Abstract. Successional processes within northern Swedish boreal forest are investigated for the last 2000 years by analysis of pollen, charcoal fragments and insect remains preserved in a deep mor humus layer on a small island in a large lake. Frequent disturbances by fire, blow-down, insect attack and flood have been the chief influence on vegetation dynamics. The gradual climatically induced replacement of Betula pubescens by Picea abies is superimposed on the disturbance dynamics. Picea first became established during the Little Ice Age, which was the longest period without fire in the record. A thin layer of volcanic ash from Iceland confirms the chronology at the onset of the Little Ice Age. The present importance of Pinus sylvestris and Ericaceae on the island is a product of a recent increase in disturbance frequency. The disturbance regime ensures that the communities stay in equilibrium with prevailing climate.     

Earthquake impacts in old‐growth Nothofagus forests in New Zealand

Abstract. Six stands located on different land forms in mixed old‐growth Nothofagus forests in the Matiri Valley (northwest of South Island, New Zealand) were sampled to examine the effects of two recent large earthquakes on tree establishment and tree‐ring growth, and how these varied across land forms. 50 trees were cored in each stand to determine age structure and the cores were cross‐dated to precisely date unusual periods of radial growth. The 1968 earthquake (M = 7.1, epicentre 35 km from the study area) had no discernible impact on the sampled stands. The impact of the 1929 earthquake (M = 7.7, epicentre 20 km from the study area) varied between stands, depending on whether or not they had been damaged by soil or rock movement. In all stands, the age structures showed a pulse of N. fusca establishment following the 1929 earthquake, with this species dominating establishment in large gaps created by landslides. Smaller gaps, created by branch or tree death, were closed by both N. fusca and N. menziesii. The long period of releases (1929–1945) indicates that direct earthquake damage was not the only cause of tree death, and that many trees died subsequently most likely of pathogen attack or a drought in the early 1930s. The impacts of the 1929 earthquake are compared to a storm in 1905 and a drought in 1974–1978 which also affected forests in the region. Our results confirm that earthquakes are an important factor driving forest dynamics in this tectonically active region, and that the diversity of earthquake impacts is a major source of heterogeneity in forest structure and regeneration.     

Local colonization and extinction of field layer plants in a deciduous forest and their dependence upon life history features

Abstract. In this study we used species inventory data collected in 1970 and 1993 from 132 plots in a 14.5ha deciduous forest to examine local extinction and colonization processes among 45 field layer species. Local colonization rate was positively related to both seed size and seed dispersal features. Local extinction rate was negatively related to seed size. Growth form (clonal vs. non-clonal) and presence of a seed bank were not found to be associated with local dynamics. Despite an overall constancy in species composition during this period, plants exhibited a considerable mobility among the 132 plots. This pattern conformed to a suggested ‘carousel model’ of species mobility in grasslands. A tentative suggestion is that this mobility acts on a comparatively broader spatio-temporal scale in deciduous forests as compared with grasslands. Additional data are presented indicating that species abundance (frequency) among field layer plants in deciduous forest communities is consistent among forest fragments, and when comparing local and regional scales. The main conclusion is that life history features of the seed dispersal/recruitment phase, particularly seed size, are causally associated with abundance patterns at least at a local scale, but possibly also on a broader regional scale.     

Stand dynamics,regeneration patterns and long-term continuity in boreal old-growth Picea abies swamp-forests

Abstract. The occurrence of macrofossil charcoal (long axis > 0.5 mm) and Picea abies (Norway spruce) pollen in peat stratigraphies, in combination with size and age data from 2976 P. abies trees were used to analyse ecosystem continuity and stand-structure in ten old-growth swamp-forests in northern Sweden. All stands were dominated by P. abies, a species whose abundance increased westwards in Sweden between 3000 and 2000 yr B.P. In three stands no macrofossil charcoal was found and the maximum age of the peat, determined by ¹⁴C dating, varied from 1800 to 3600 yr B.P. In the other seven stands the number of levels containing charcoal varied from 1 to 23, but only between 1 and 7 levels were found after the appearance of spruce. Here the maximum age of the peat varied from 400 to 7900 yr B.P. The ten stands had an all-sized stand structure and a stand continuity of ca. 300 yr. The shape of the age structure was similar to an inverse J-curve. This indicates a continuous recruitment over time in a self-perpetuating ecosystem. In a short-term perspective (< 300 yr), the swamp-forests are characterized by individual trees continually emerging while others are dying. it is suggested that internal dynamics of continuous small-scale disturbances in combination with local site-specific factors determine the structure of these forests. in a long-term perspective, some of the present spruce swamp-forests within the northern boreal zone have functioned as true fire-free refugia since the establishment of P. abies populations while others have been affected by recurring fires, although not as frequently as forests on surrounding drier sites. The hypothesis that Scandinavian spruce swamp-forests in general have functioned as true longterm fire-free refugia is thus modified by the present results.     

Structure,dynamics and disturbance regime of temperate broad-leaved forests in Japan

Abstract. Structure, diversity and dynamics of five Japanese temperate old-growth forests were compared, differing from each other in, i.a., climate, dominant tree type, topography, disturbance regime. The forests were Aya, Ogawa, Kanumazawa Riparian, Kanumazawa upland and Senju. A permanent plot (1–6 ha) was established in each forest and trees were censused several times at intervals of two years. Mean annual recruitment rates and mortality rates in these forests were both in the range of 0.5 to 4.6 %/yr at the community level. Analyses of the structure and dynamics of populations showed that the underlying process was different among the forests. Some forests experienced compositional shifts in their canopies, others had a constant canopy composition but appeared to lack effective regeneration in recent years. The recruitment rate appeared to be strongly affected by competitive undergrowth vegetation such as dwarf bamboo which has been controlled by natural disturbance or human impact. It is likely that the forests with mostly low recruitment rates had a low species diversity. The current variation in structure, diversity and dynamics of the studied forests might have been determined not only by physical conditions (e.g. climate) but also by chance factors (e.g. disturbance, outbreak of deer population).     

Effects of age and distance on the composition of mixed deciduous forest fragments in an agricultural landscape

Abstract. Forest patches in central Belgium were inventoried twice for the presence or absence of forest plant species to study the effects of age and distance on species composition. All forests in the study area were subdivided based on their land use history. To avoid effects of autocorrelated environmental characteristics on species composition, habitat homogeneity was indirectly investigated using a TWINSPAN classification of the vegetation data. Two major habitats (alluvial and non‐alluvial forests) were distinguished and analysed separately. Patterns of species composition were investigated at the landscape level using Mantel tests. Species composition similarity measures were calculated between all pairs of fragments based on the floristic data. A highly significant correlation was found between species composition similarity and inter‐patch distance. Difference in age, which we used as a measure for habitat quality, was less important in explaining species composition patterns. The effects of spatial configuration became significant when difference in age was accounted for, and a partial correlation was calculated between inter‐patch distance and species composition similarity. Different results were found for alluvial and non‐alluvial forest types. Alluvial forests were more influenced by the spatial configuration than the non‐alluvial. For the non‐alluvial forest type effects measured with the difference in age between forest fragments obscured the effects of inter‐patch distance. Based on our findings we suggest that species composition is not only internally generated, but external processes such as differential colonization caused by varying degrees of isolation may be of overriding importance.     

Growth trends and dynamics in sub‐alpine forest stands in the Varaita Valley (Piedmont,Italy) and their relationships with human activities and global change

Abstract. A study of the forest lines, tree lines and the structures of the sub‐alpine forest was performed in Vallone Vallanta and in Alevé forest in the Varaita Valley (Cottian Alps, Piedmont, Italy). Forest‐ and tree lines were analysed over 1728 ha while forest structures were studied on six 3000‐m² plots located at the tree line (2), at the forest line (2) and inside the sub‐alpine forest (2). Dendro‐ecological analysis of living plants and stumps showed that Larix decidua was more abundant in the past than today and that Pinus cembra has expanded, both upwards and within sub‐alpine forests. Age structure analysis revealed that the current sub‐alpine forest stands were established 200–220 yr ago, probably following a clearcut. At the forest lines the tree density decreases, and some trees are more than 500 yr old, whereas at the tree lines most of the trees (almost exclusively Pinus cembra) are younger than 100 yr. Growth dynamics were investigated both by observing Basal Area Increment (BAI) in the old and dominant trees, and by comparing the BAIs of classes of trees with a given cambial age range in different time periods. The results showed that the growth rates of mature Pinus cembra and Larix decidua had increased. These increments are more substantial for Pinus than for Larix. The growth rate of young trees (< 100 yr) of both species has decreased over recent decades. This could be due to competition caused by increased tree densities that have resulted from a decrease in grazing.     

Forest history and the development of old‐growth characteristics in fragmented boreal forests

Questions: Can small and isolated high‐conservation value forests (e.g. designated woodland key habitats) maintain old‐growth forest characteristics and functionality in fragmented landscapes? To what extent have past disturbances (natural and anthropogenic) influenced the development of old‐growth characteristics of these forests? How long does it take for selectively cut stands to attain conditions resembling old‐growth forests? Location: Southern boreal zone of central Sweden. Methods: We linked multiple lines of evidence from historical records, biological archives, and analyses of current forest structure to reconstruct the forest history of a boreal landscape, with special emphasis on six remaining core localities of high‐conservation value forest stands. Results: Our reconstructions revealed that several of these stands experienced wildfires up to the 1890s; all had been selectively harvested in the late 1800s; and all underwent substantial structural and compositional reorganization over the following 100‐150 years. This time interval was sufficient to recover considerable amounts of standing and downed dead wood (mean 60.3 m³ ha^?1), a range of tree ages and sizes (mean basal area 32.6 m² ha^?1), and dominance of shade‐tolerant spruce. It was insufficient to obtain clearly uneven tree age structures and large (>45 cm diameter) living and dead trees. Thus, these forests contain some, but not all, important compositional and structural attributes of old‐growth forests, their abundance being dependent on the timing and magnitude of past natural and anthropogenic disturbances. Our landscape‐level analysis showed marked compositional and structural differences between the historical forest landscape and the present landscape, with the latter having a greater proportion of young forests, introduction of non‐native species, and lack of large trees and dead wood. Conclusions: The remnant high‐conservation value stands were not true representatives of the pre‐industrial forests, but represent the last vestige of forests that have regenerated naturally and maintained a continuous tree cover. These traits, coupled with their capacity for old‐growth recovery, make them valuable focal areas for conservation.     

Spatio‐temporal colonization patterns of forest plant species in a mixed deciduous forest

Abstract. An integrated analysis of the colonization patterns of forest plant species was carried out in a 34‐ha, mixed deciduous forest in northern Belgium. First, we sought to describe the relationships between land use history and environmental conditions. Land use history and soil type were related and negative correlations between pH and secondary forest age were found. The density of the shrub layer increases with secondary forest age. Litter quantity and cover of Urtica dioica were mainly indirectly influenced by land use history. Litter starts accumulating at low pH values and high shrub density and Urtica dioica grows vigorously on nutrient enriched soils where much light can reach the ground. Next, the importance of these human‐altered environmental conditions for the colonization of forest plant species was assessed relative to the importance of dispersal limitation. Therefore, the distribution of 16 forest species was mapped and species‐specific spatio‐temporal isolation measures were calculated. The analysis revealed that the colonization patterns of the slowly colonizing species (i.e. ‘ancient forest plant species’) are best explained by a combination of spatio‐temporal isolation, soil type, pH and the (non‐)cover of Urtica dioica. By contrast, spatio‐temporal isolation was never a limiting factor for good colonizing forest species. Our results suggest that colonization of ‘ancient forest plant species’ is hampered by a combination of dispersal‐ and recruitment limitation and that the relative importance of both factors is species‐specific.     

The land use history (1278–1990) of a mixed hardwood forest in western Belgium and its relationship with chemical soil characteristics

Aim During the last decades, an increasing number of studies have stressed the importance of historical human influence on the ecology of forests and on the characteristics of forest soils. Therefore, the objectives of this study are (1) the quantification of the land use history of Ename Wood from 1278 to 1990 and (2) to find out whether the former land use of the forest has long-lasting effects on present-day chemical soil properties. Location The 62-ha present-day Ename Wood is situated in western Belgium and is the remainder of the 145-ha historical Ename Wood. Methods We disposed of eighteen land-use maps for the period between 1278 and 1990 which were digitized using a geographic information system (GIS). Transition between the different land uses and Shannon–Wiener diversity indices were calculated to quantify the history of changing land use. Mixed soil samples were taken in lots delimited on the basis of the historical data. Next, the soil properties were combined with the land-use variables using redundancy analysis and ANOVA . Results The quantification of the land use changes showed that the present Ename Wood is the result of several forest regression and progression phases, with a complete clearance in the nineteenth century. Diversity in land use was maximal between the fourteenth and the sixteenth century due a variety of transitional forms between forest and pasture. A positive correlation between the duration of arable land use since the 19th century clearance and soil pH, calcium and phosphate content was observed and a negative correlation was found with the carbon content, the total nitrogen content and the C:N ratio. These correlations are probably caused by a combination of acidification processes and the accumulation of organic matter under forest in combination with manuring practices in the twentieth century. Present-day forest lots which have been pastured for some time between 1278 and nineteenth-century clearance still had a significantly lower pH and degree of base saturation, which is probably caused by the export of nutrient rich plant material at that time. Discussion and conclusions The results demonstrate that the developed methodology is successful and confirm that historical land use, even in the distant past, can still influence present-day soil characteristics. For this reason, long-term historical land use should always be considered in forest ecological research.     

Vegetation patterns in heterogeneous landscapes: The importance of history and environment

Abstract. Throughout the eastern United States, plant species distributions and community patterns have developed in response to heterogeneous environmental conditions and a wide range of historical factors, including complex histories of natural and anthropogenic disturbance. Despite increased recognition of the importance of disturbance in determining forest composition and structure, few studies have assessed the relative influence of current environment and historical factors on modern vegetation, in part because detailed knowledge of prior disturbance is often lacking. In the present study, we investigate modern and historical factors that control vegetation patterns at Harvard Forest in central Massachusetts, USA. Similar to the forested uplands throughout the northeastern United States, the site is physiographically heterogeneous and has a long and complex history of natural and anthropogenic disturbance. However, data on forest composition and disturbance history collected over the past > 90 years allow us to evaluate the importance of historical factors rigorously, which is rarely possible on other sites. Soil analyses and historical sources document four categories of historical land use on areas that are all forested today: cultivated fields, improved pastures/mowings, unimproved pastures, and continuously forested woodlots. Ordination and logistic regressions indicate that although species have responded individualistically to a wide range of environmental and disturbance factors, many species are influenced by three factors: soil drainage, land use history, and C:N ratios. Few species vary in accordance with ionic gradients, damage from the 1938 hurricane, or a 1957 fire. Contrary to our expectation that the effects of disturbance will diminish over time, historical land use predicts 1992 vegetation composition better than 1937 composition, perhaps because historical woodlots have become increasingly differentiated from post-agricultural stands through the 20th century. Interpretations of modern vegetation must consider the importance of historical factors in addition to current environmental conditions. However, because disturbances such as land use practices and wind damage are complex, it is often difficult to detect disturbance effects using multivariate approaches, even when the broad history of disturbance is known.     

Disturbance history and tree establishment in old-growth Pinus koraiensis-hardwood forests in the Russian Far East

Abstract. Dendro-ecological studies were undertaken to document the disturbance history in two old-growth mixed Pinus koraiensis-hardwood forests in the southern part of the Sikhote-Alin mountains in the Russian Far East. Establishment of four common canopy conifers, Abies holophylla, A. nephrolepis, P. koraiensis and Picea ajanensis, and three common canopy hardwoods, Acer mono, Betula costata and Tilia amurensis, were also inferred from population age structures and spatial dispersion patterns. Growth releases on increment cores suggested that peak periods of growth releases indicating partial canopy disturbances have repeatedly occurred over the past 230 yr at intervals from ca. 35 to 100 yr. Slight releases and suppressions other than the peak releases occurred in many years of both histories, suggesting the formation of smaller-scale canopy gaps. Despite the predominance of anthropogenic fires in Primorskiy Kray at present, destructive fires had not affected either forest. Wind disturbances and low intensity fires are likely factors controlling the dynamics of the forests. Under the disturbance regime, P. koraiensis has maintained its populations through its dependence on canopy gaps for establishment. Age distribution and gap dependence of P. ajanensis, A. nephrolepis and A. mono suggested continuous establishment of these species under a closed canopy, whereas occasional establishment of T. amurensis was derived largely from vegetative reproduction. Restricted establishment of A. holophylla and B. costata suggested a variety in kinds of disturbance throughout the histories. Continuous habitation of the study area by P. koraiensis is likely under the disturbance regime without destructive fires.     

Influence of historical,geographical and environmental variables on understorey composition and richness in Danish forests

Question: What is the relative influence of forest continuity, environmental differences and geographical context on vegetation and species richness in ancient and recent forests? Location: Himmerland and Hornsherred in Denmark. Methods: Lists of forest species from deciduous forests were subjected to CCA with variation partitioning to quantify the relative amount of variation in species composition attributable to historical, present geographical and environmental variables. GLM was used to estimate the importance of the variables to species richness. Results: The importance of temporal forest continuity in one region was negligible but was considerable in the other. The variation in species composition explained by geographical, environmental and historical variables showed little overlap in both regions, particularly at the fine scale. Conclusions: This paper does not support the idea that differences in the flora between ancient and recent forests is mainly caused by environmental differences. Furthermore, species richness seemed unaffected by isolation and forest connectivity.     

Vegetation establishment in post-fire Adenostoma chaparral in relation to fine-scale pattern in fire intensity and soil nutrients

At fine spatial scales (0.1–10m), chaparral communities have been shown to be strongly influenced by canopy-gap patterns, leading to periodicities in vegetation at 4–5 m spatial scales. Fine-scale variations in fire behavior and post-fire erosion can lead to changes in the patterning of viable seeds and nutrients and may alter the spatial patterning of post-fire chaparral communities. This study deals with the relationship among fire behavior, post-fire nutrient availabilities and vegetation patterns in a 1-yr old, post-fire Adenostoma fasciculatum chaparral community in the Sierra Nevada Mountains, California, USA. Variations in mineral soil exchangeable cations (Ca, Mg) and extractable phosphorus (P0₄-P) were correlated with ash distribution. Cations and measures of ammonium and nitrate were also correlated with fire intensity, measured by the diameter of the smallest remaining A. fasciculatum twigs following fire. Fire intensity was correlated with the pattern of post-fire vegetation establishment based on first axis DC A scores. However, ash PO4-P was more highly correlated with sample DCA scores, local species richness and total cover (p < 0.01), suggesting that small-scale variations in PO4-P which correlate with ash distributions may be important in structuring this community. Two- and three-term local variance analysis revealed a maximum of pattern intensity in DCA first axis scores at 4–5 m intervals that likely corresponds to pre-fire canopy-gap patterns. However, total cover showed pattern at spatial scales of 8–10 m, and was correlated at this scale with patterns of ash distribution and fire intensity. Microtopographic patterns also occur at similar spatial scales. Microtopographic patterns appear important in determining post-fire plant nutrient and water distributions and, thereby, patterns of plant establishment. Thus, the scale and intensity of post-fire vegetation pattern may differ considerably from pre-fire conditions.