Changes in forest understorey vegetation in Norway related to long‐term soil acidification and climatic change

Question: Does the understorey vegetation of Norwegian boreal forests change in relation to broad‐scale, long‐term changes? Location: Norway. Methods: Permanently marked 1‐m² vegetation plots from 17 monitoring reference areas in forests dominated by Picea abies (11 areas, 620 plots) and Betula spp. (six areas, 300 plots) were analysed twice, at the start in 1988–1997 and 5 yr later (1993–2002). Species subplot frequency data were analysed separately for each area by univariate and multivariate statistical methods; 5‐yr changes in single species abundances, species number per plot and species composition were tested. Results: Two distinct patterns of change were found: 1. Abundance of several vascular plant species decreased in SE Norwegian Picea forests, most noticeably of species with a preference for richer soils, such as Oxalis acetosella. 2. Abundance of many bryophyte species as well as bryophyte species number per plot increased in forests of both types over most of Norway. Conclusions: The pattern of vascular plant changes is probably a time‐delayed response of long‐lived, mainly clonal, populations to acidified soils resulting from deposition of long‐distance airborne pollutants. The pattern bryophyte changes, with reference to the close link between climatic conditions for growth and abundance changes for Hylocomium splendens established in previous demographic studies, is related to climatic conditions favourable for bryophyte growth. We conclude that many forest understorey plants are sensitive indicators of environmental change, and that the concept used for intensive monitoring of Norwegian forests enables early detection of changes in vegetation brought about by broad‐scale, regional, impact factors.     

Dynamics of species richness and composition in a limestone grassland restored after tree cutting

Abstract. After abandonment many limestone grasslands have been overgrown by trees and shrubs; as a result, species-rich communities with many regionally rare and endangered species are vanishing. Some studies suggest that, in cases where grassland is being restored, the species composition and rate of change is strongly dependent on the initial conditions, i.e. the earlier presence of grassland species and the opportunity for colonization of new sites by grassland species. These hypotheses were tested in a five-year restoration experiment after the clearing of a 35-yr-old secondary pine wood developed on abandoned grassland. Tree cutting induced rapid changes in the floristic composition and species cover. The number of grassland species from the class Festuco-Brometea increased significantly in the restored grassland, but their cover was much lower than in the old grassland. Canonical Correspondence Analysis showed significant differences in species composition between the grassland restored in former wood gaps and that developed in former closed wood. In wood gap sites the cover of species from the class Molinio-Arrhenatheretea and tufted perennials was much higher, whereas the cover of Festuco-Brometea species was lower. Significantly more shrubs, woodland species, ruderal and nitrophilous species as well as annual and biennial species occurred in the former closed wood site. It was found that richness and composition of the restored grassland depended strongly on the community composition before tree cutting, as well as on the presence of grassland species in the neighbourhood. Periodical tree cutting enables the maintenance of a temporal-spatial mosaic of scrub-grassland communities in isolated habitats and the preservation of local species diversity.     

Weed vegetation of arable land in Central Europe: Gradients of diversity and species composition

Question: What are the main broad‐scale spatial and temporal gradients in species composition of arable weed communities and what are their underlying environmental variables? Location: Czech Republic and Slovakia. Methods: A selection of 2653 geographically stratified relevés sampled between 1954–2003 was analysed with direct and indirect ordination, regression analysis and analysis of beta diversity. Results: Major changes in weed species composition were associated with a complex gradient of increasing altitude and precipitation and decreasing temperature and base status of the soils. The proportion of hemicryptophytes increased, therophytes and alien species decreased, species richness increased and beta diversity decreased with increasing altitude. The second most important gradient of weed species composition was associated with seasonal changes, resulting in striking differences between weed communities developed in spring and summer. In summer, weed communities tended to have more neophytes, higher species richness and higher beta diversity. The third gradient reflected long‐term changes in weed vegetation over past decades. The proportion of hemicryptophytes and neophytes increased, while therophytes and archaeophytes decreased, as did species richness over time. The fourth gradient was due to crop plants. Cultures whose management involves less disturbances, such as cereals, harboured less geophytes and neophytes, and had higher species richness but lower beta diversity than frequently disturbed cultures, such as root crops. Conclusions: Species composition of Central European weed vegetation is mainly influenced by broad‐scale climatic and edaphic factors, but its variations due to seasonal dynamics and long‐term changes in agricultural management are also striking. Crop plants and crop‐specific management affect it to a lesser, but still significant extent.     

Variation in species composition and vegetation structure of succulent scrub on Tenerife in relation to environmental variation

Abstract. On Tenerife, the occurrence of environmental gradients over short distances provides a unique opportunity to investigate the relationship between vegetation and environmental factors. In the semi‐arid coastal region of Tenerife, floristic composition, species richness and vegetation structure of perennial plants have been studied in 67 locations covering the existing precipitation gradient. On the island as a whole, variation in species composition could be best explained by mean annual precipitation; at coastal sites, substrate age and soil characteristics also played a significant role. On the other hand, substrate chemistry and the type of eruptive material explained little of the floristic variation. Stand biomass was strongly correlated with mean annual precipitation and was, on the youngest lava flows studied, also affected by substrate age. The native stem succulent species made up the bulk of total biomass along the whole precipitation gradient. Disturbed and undisturbed sites differed significantly in stand biomass and cover. Species richness was correlated with precipitation and substrate age. Distribution of plant functional types was also related to the precipitation gradient. The relative abundance of hemicryptophytes and shrubs with non‐hairy leaves increased with increasing precipitation whereas the ratio of shrubs with hairy/non‐hairy leaves and succulent plants decreased. Some alien plants were quite frequent at disturbed sites but, on the whole, they contributed little to the species spectrum and to the stand biomass. Undisturbed sites remained almost free of introduced species not considering annuals.     

Effects of selective logging on tree diversity,composition and plant functional type patterns in a Bornean rain forest

Abstract. The effects of selective logging on tree diversity, changes in tree species composition and plant functional types were studied with the use of seven permanent plots in virgin and in logged forest. All plots were located in a lowland dipterocarp rain forest in East Kalimantan on the island of Borneo. Just after logging and during the following 20 yr tree diversity measured as Fishers’α was not significantly affected in logged forest plots. Temporal shifts in tree species composition were analysed with Principal Component Analysis (PCA). Logged forest plots had much larger changes over time than virgin forest plots. In the smallest diameter class, some logged forest plots showed a distinct trajectory in PCA space compared to virgin forest plots, while in larger diameter classes movement of logged plots in PCA space was random. This suggests that there is no predetermined community to which logged forest plots tend to shift when recovering from logging. We found a significant negative correlation between diameter increments and the species‐specific wood densities of tree species. Species‐specific wood density and potential tree height were used to assign species to five PFTs. As expected, logging increased the fraction of softwood stems in small diameter classes. In the largest diameter classes (≥ 50 cm DBH) a strong decrease of softwood emergent stems was found in logged forest plots. After more than 20 yr no recruitment was found of softwood emergent stems in selectively logged forest.     

Subalpine vegetation pattern three decades after stand‐replacing fire: effects of landscape context and topography on plant community composition,tree regeneration,and diversity

Objective: Our purpose was to characterize vegetation compositional patterns, tree regeneration, and plant diversity, and their relationships to landscape context, topography, and light availability across the margins of four stand‐replacing subalpine burns. Location: Four 1977 to 1978 burns east of the Continental Divide in Colorado: the Ouzel burn, a burn near Kenosha Pass, the Badger Mountain burn, and the Maes Creek burn. Methods: Vegetation and environmental factors were sampled in 200 0.01‐ha plots on transects crossing burn edges, and stratified by elevation. We utilized dissimilarity indices, mixed‐effects models, and randomization tests to assess relationships between vegetation and environment. Results: Three decades after wildfire, plant communities exhibited pronounced compositional shifts across burn edges. Tree regeneration decreased with increasing elevation and distance into burn interiors; concomitant increases in forbs and graminoids were linked to greater light availability. Richness was roughly doubled in high‐severity burn interiors due to the persistence of a suite of native species occurring primarily in this habitat. Richness rose with distance into burns, but declined with increasing elevation. Only three of 188 plant species were non‐native; these were widespread, naturalized species that comprised <1% total cover. Conclusions: These subalpine wildfires generated considerable, persistent increases in plant species richness at local and landscape scales, and a diversity of plant communities. The findings suggest that fire suppression in such systems must lead to reduced diversity. Concerns about post‐fire invasion by exotic plants appear unwarranted in high‐elevation wilderness settings.