Håkan Hytteborn

The return of the giants: ecological effects of an increasing elephant population.

Abstract Northern Botswana and adjacent areas, have the worlds largest population of African elephant (Loxodonta africana). However, a 100 years ago elephants were rare following excessive hunting. Simultaneously, ungulate populations were severely reduced by decease. The ecological effects of the reduction in large herbivores must have been substantial, but are little known. Today, however, ecosystem changes following the increase in elephant numbers cause considerable concern in Botswana. This was the background for the “BONIC” project, investigating the interactions between the increasing elephant population and other ecosystem components and processes. Results confirm that the ecosystem is changing following the increase in elephant and ungulate populations, and, presumably, developing towards a situation resembling that before the reduction of large herbivores. We see no ecological reasons to artificially change elephant numbers. There are, however, economic and social reasons to control elephants, and their range in northern Botswana may have to be artificially restricted.

Tree population dynamics, stand structure and species composition in the montane virgin forest of Vallibäcken, northern Sweden

The population dynamics of Betula pubescens and Picea abies in a boreal forest near Kvikkjokk, northern Sweden, are governed by a process of storm gap regeneration similar to the gap regeneration described for boreo-nemoral forests. Cumulative age distribution curves, interpreted as static survivorship curves, lead to a simple theory of differential survival based on properties of the species, i.e. shade tolerance and relative growth rate. The theory is sustained by diameter and height distributions and by the spatial distributions of logs and of trees in different life-phases. Species of the field and ground layers respond differentially to gap formation and the ensuing successional stages. Browsing by moose (Alces alces) may prevent tree species, mainly Sorbus aucuparia, Betula pubescens and Pinus sylvestris, from developing into a tree layer. The regeneration ability for tree species growing in a stand at 460 m a.s.l. is limited compared with the regeneration at 330 m a.s.l., and typical storm gap formation involving more than one tree seems to occur rarely if at all, while overthrown trees with exposed rootplates are uncommon. Spruce at 460 m a.s.l. shows also a lower growth rate and a lower height/diameter ratio compared to the lower situated stands.

The separation of fluctuation and long-term change in vegetation dynamics of a rising seashore*

Field layer vegetation in four transects across a rising seashore of the island Skabbholmen (59°47′N, 19°12′E) in the Stockholm archipelago, eastern central Sweden, was sampled at the beginning and end of a six-year period. The data were analyzed by canonical correspondence analysis (CCA) with two external predictor variables, year of sampling and elevation (a proxy for the longer-term trend of change). During the sampling period, the vegetation changed in the direction of the longer-term trend at a rate consistent with the known rate of land uplift. However, a major part of the observed change was in a different direction, reflecting a shorter-term response to disturbance. The analysis effectively separated different types of floristic dynamics related to processes with different time-scales. A comparison with (unconstrained) correspondence analysis (DCA) illustrated the interpretive advantages of multivariate direct gradient analysis over conventional floristic ordination.

DECAY RATE OF PICEA ABIES LOGS AND THE STORM GAP THEORY: A RE-EXAMINATION OF SERNANDER PLOT III, FIBY URSKOG, CENTRAL SWEDEN

Summary One of the sites (Plot III) used by Sernander (1936) in developing the ‘storm-gap theory’ of cyclic change in Picea ablest forests on unstable soils has been located in Fiby urskog, near Uppsala. Re-mapping has facilitated calculation of the time elapsing between the fall of the trunk and its complete disintegration, which varies according to individual circumstances: the original estimate of 90 years proves something of an over-estimate. The results confirm the validity of the storm-gap theory; indicating that the size of gaps is often increased by further falls in subsequent years and that dwarf trees play an important role in regrowth. Small spruce saplings are concentrated on boulders and logs, where they are less vulnerable to competition from the field layer. Few occur where the shade is deep.

Effects of warming on shrub abundance and chemistry drive ecosystem-level changes in a forest-tundra ecotone

Tundra vegetation is responding rapidly to on-going climate warming. The changes in plant abundance and chemistry might have cascading effects on tundra food webs, but an integrated understanding of how the responses vary between habitats and across environmental gradients is lacking. We assessed responses in plant abundance and plant chemistry to warmer climate, both at species and community levels, in two different habitats. We used a long-term and multisite warming (OTC) experiment in the Scandinavian forest–tundra ecotone to investigate (i) changes in plant community composition and (ii) responses in foliar nitrogen, phosphorus, and carbon-based secondary compound concentrations in two dominant evergreen dwarf-shrubs (Empetrum hermaphroditum and Vaccinium vitis-idaea) and two deciduous shrubs (Vaccinium myrtillus and Betula nana). We found that initial plant community composition, and the functional traits of these plants, will determine the responsiveness of the community composition, and thus community traits, to experimental warming. Although changes in plant chemistry within species were minor, alterations in plant community composition drive changes in community-level nutrient concentrations. In view of projected climate change, our results suggest that plant abundance will increase in the future, but nutrient concentrations in the tundra field layer vegetation will decrease. These effects are large enough to have knock-on consequences for major ecosystem processes like herbivory and nutrient cycling. The reduced food quality could lead to weaker trophic cascades and weaker top down control of plant community biomass and composition in the future. However, the opposite effects in forest indicate that these changes might be obscured by advancing treeline forests.

Viable seeds in deposits of the former lakes Kvismaren and Hornborgasjön, Sweden☆

Abstract The viable seed content in sediments from two shallow lowland lakes, Lake Kvismaren and Lake Hornborgasjon in south Sweden, was investigated and compared. The number of seeds per species and their depth distribution (0–50 cm) in the sediment were examined through germination trials in a greenhouse. Samples of sediments were given either an exposed moist treatment (Kvismaren and Hornborgasjon) or a submerged treatment (Hornborgasjon). The species composition of viable seeds differed considerably between the two lakes. The sediments in Kvismaren were dominated by Alisma plantago-aquatica L. and Carex spp. and in Hornborgasjon by Carex acuta L., other Carex spp. and Juncus spp. From the two samples sites in Kvismaren, 481 000 seeds m −2 and 248 000 seeds m −2 germinated. In Hornborgasjon an average of 48 300 seeds m −2 germinated. Exposed moist treatments resulted in the germination of 30 species. Submerging the sediment samples gave only 23. Depth distributions of seeds in the sediments differed strikingly between the lakes. Large numbers of seeds were found in a distinct horizon ∼ 25 cm below the sediment surface in Kvismaren. This horizon is interpreted as being the remains of an earlier vegetation stage. In Hornborgasjon the greatest number of seeds was found near the sediment surface.

Fine-scale dynamics and community stability in boreal peatlands: revisiting a fen and a bog in Sweden after 50 years

Multi-decadal studies of community and ecosystem dynamics are rare; however, this time frame is most relevant for assessing the impact of anthropogenic influences and climate change on ecosystems. For this reason, we investigated changes in vegetation and microtopography over 52 years in two contrasting mire ecosystems, one ombrotrophic (bog) and one minerotrophic (fen), representing different successional stages and contrasting hydrological settings. In both peatlands, floristic composition was recorded in the same permanent plots (n = 55-56, 0.25 m2) in both 1960 and 2012 and microtopography was mapped over a large area (ca. 2500 m2) that encompassed these same plots. We quantified and compared the community-level changes and internal spatial dynamics, tested associations between pH/microtopography and community/species change, and examined how the area and location of hummock microforms had changed over time. The bog exhibited little site level change in vegetation, where few species changed significantly in cover and plot frequency. However, detailed analyses revealed some large within-plot changes over time in the bog, illustrating that bogs can be highly dynamic systems at a fine scale. In contrast, the rich fen experienced a clear directional change; specifically, bryophyte abundance decreased by 70% and brown mosses were almost extinct. Although pH had decreased over time at the rich fen, this decrease at the plot-level was not associated with the decline of brown moss abundance. The microtopographic structure did not change substantially at the bog where ∼70% was covered by lawn/hummocks; however, in the rich fen hummocks expanded (from 10% to 16% cover) and moved or expanded down slope. Our study suggests, that at the site-level, the bog ecosystem was more resistant to environmental changes over time compared to the rich fen, as evidenced by shifts in vegetation and microtopography. The contrasting scales of vegetation dynamics observed within a bog (i.e., within-plot changes vs. site-level) indicate that plant-environment feedbacks contribute to the peatland level stability. While in rich fens, internal feedbacks may be weaker and the ecosystems vegetation and microtopographic structure are vulnerable to shifting hydrological fluxes.

Viable seeds in sediments in Lake Hjälmaren

Abstract The contents of viable seeds in sediments of Lake Hjalmaren, south Sweden, were investigated. Only a few seeds, mainly Phragmites australis (Cav.) Trin. ex Steud. and Typha sp. were recovered, and most were found at sites where fine material accumulates. Although their density was low, seeds were present in sediments at distances of 100–500 m from the nearest vegetation.

Swedish beech forests and the storm gap theory

The storm gap theory of forest dynamics was developed by Sernander largely as a result of his investigations of the Norway spruce Picea abies forests of Fibyurskog and Granskär following the great storms of 1931 and 1932.This theory is reviewed in the light of subsequent investigations and its relevance to forests of common beech, Fagus sylvatica, which Watt had also investigated in southern England; developing his theory of pattern and process in the plant community as a result. Ancient unmanaged Swedish beech forests are extremely important as they enable us to ascertain how such forests develop in a state of nature. They are often important in terms of biodiversity and the veteran trees they contain are especially valuable in the conservation of rare epiphytic lichens and bryophytes. The history of Fagus sylvatica and variations in its extent in Swedish forests during the Holocene is illustrated by a review of articles concerning a number of important forest areas.

Century-long tree population dynamics in a deciduous forest stand in central Sweden

Question We quantify tree dynamics over a century of free development in a small broadleaved forest dominated by Fraxinus excelsior and Ulmus glabra. What are the internal and external factors driving the changes, and how predictable are they? What were the time scale and effects of the spread of Dutch elm disease (DED)? Location Vardsatra, eastern central Sweden. Methods The survival, growth and recruitment of all trees (≥ 12 cm in girth) were monitored in 1912, 1967, 1988 and 2013 (more often for a part of the forest). Woody species in the field and shrub layers were surveyed in permanent plots in 1976 and 2012. We used transition matrix models to project changes in population sizes and species composition within the century and for 2050. Results The results indicate that the forest was in a successional development during the first period. The species composition had stabilised by 1967, except for an expansion of Acer platanoides and the drastic effect of DED that struck the forest around 2000. It took only a decade to kill virtually all large elms in the forest, leading to strong decrease in stem density and basal area. The evidence for effects of DED is still weak, but there has been an increase in saplings, notably of Fraxinus, Prunus padus, Ulmus, and of shoots of Corylus avellana. Several species that are abundant in the vicinity and as seeds fail to establish (Picea abies, Betula spp., Quercus robur, Populus tremula). Projections for 2050 based on the third period (1988-2013) are probably unrealistic since also Fraxinus may disappear because of the recent arrival of the ash dieback. Conclusions Slow dynamics in forests that could follow from climate change will locally probably be overruled by unforeseen catastrophes, such as invasions by forest pathogens. These initiate changes with long lag phases difficult to quantify. Still, a dense deciduous forest can resist invasion of colonist species and of regionally dominant conifers; the reason being unfavourable conditions for establishment rather than dispersal limitation This article is protected by copyright. All rights reserved.