Jörg Brunet

Microclimate moderates plant responses to macroclimate warming

Significance Around the globe, climate warming is increasing the dominance of warm-adapted species—a process described as “thermophilization.” However, thermophilization often lags behind warming of the climate itself, with some recent studies showing no response at all. Using a unique database of more than 1,400 resurveyed vegetation plots in forests across Europe and North America, we document significant thermophilization of understory vegetation. However, the response to macroclimate warming was attenuated in forests whose canopies have become denser. This microclimatic effect likely reflects cooler forest-floor temperatures via increased shading during the growing season in denser forests. Because standing stocks of trees have increased in many temperate forests in recent decades, microclimate may commonly buffer understory plant responses to macroclimate warming. Recent global warming is acting across marine, freshwater, and terrestrial ecosystems to favor species adapted to warmer conditions and/or reduce the abundance of cold-adapted organisms (i.e., “thermophilization” of communities). Lack of community responses to increased temperature, however, has also been reported for several taxa and regions, suggesting that “climatic lags” may be frequent. Here we show that microclimatic effects brought about by forest canopy closure can buffer biotic responses to macroclimate warming, thus explaining an apparent climatic lag. Using data from 1,409 vegetation plots in European and North American temperate forests, each surveyed at least twice over an interval of 12–67 y, we document significant thermophilization of ground-layer plant communities. These changes reflect concurrent declines in species adapted to cooler conditions and increases in species adapted to warmer conditions. However, thermophilization, particularly the increase of warm-adapted species, is attenuated in forests whose canopies have become denser, probably reflecting cooler growing-season ground temperatures via increased shading. As standing stocks of trees have increased in many temperate forests in recent decades, local microclimatic effects may commonly be moderating the impacts of macroclimate warming on forest understories. Conversely, increases in harvesting woody biomass—e.g., for bioenergy—may open forest canopies and accelerate thermophilization of temperate forest biodiversity.

Herb layer vegetation of south Swedish beech and oak forests—effects of management and soil acidity during one decade

Abstract Effects of forest management and soil acidity on herb layer vegetation were studied after 10 years on 190 permanent plots in south Swedish beech (Fagus sylvatica) and oak (Quercus robur) forests. Species richness generally increased with management intensity, mainly due to establishment of ruderal species from the seed bank. Species richness of the typical forest flora was unaffected by management. Moderate management of oak stands favoured several species which are commonly found in semi-natural pastures. Classification and ordination of the data showed that the main floristic gradient within Swedish beech and oak forest vegetation is related to soil acidity. Species richness of the typical forest flora was strongly positively correlated with soil pH in beech forests, but this correlation was weaker in oak forests. The number of herbaceous plants with a broader habitat range increased with pH only in the oak forest plots. Long term changes in the forest environment, which may affect the vegetation, are the decline of grazing 150-50 years ago and soil acidification mainly caused by atmospheric pollutants during the last 50 years. In the one-decade perspective of this study, however, we did not find a general trend towards a more acid-tolerant flora. Neither could we find a general decrease of pasture species in currently ungrazed oak stands. The results indicate that most typical forest plants are well adapted to and partly depend on occurrence of canopy gaps and soil disturbance. If canopy thinning is followed by periods of canopy closure the characteristic shade tolerant flora of Swedish beech and oak forests may be able to persist as long as soil chemical limits of existence are not exceeded.

Current Near‐to‐Nature Forest Management Effects on Functional Trait Composition of Saproxylic Beetles in Beech Forests

With the aim of wood production with negligible negative effects on biodiversity and ecosystem processes, a silvicultural practice of selective logging with natural regeneration has been implemented in European beech forests (Fagus sylvatica) during the last decades. Despite this near-to-nature strategy, species richness of various taxa is lower in these forests than in unmanaged forests. To develop guidelines to minimize the fundamental weaknesses in the current practice, we linked functional traits of saproxylic beetle species to ecosystem characteristics. We used continental-scale data from 8 European countries and regional-scale data from a large forest in southern Germany and forest-stand variables that represented a gradient of intensity of forest use to evaluate the effect of current near-to-nature management strategies on the functional diversity of saproxylic beetles. Forest-stand variables did not have a statistically significant effect on overall functional diversity, but they did significantly affect community mean and diversity of single functional traits. As the amount of dead wood increased the composition of assemblages shifted toward dominance of larger species and species preferring dead wood of large diameter and in advanced stages of decay. The mean amount of dead wood across plots in which most species occurred was from 20 to 60 m(3) /ha. Species occurring in plots with mean dead wood >60 m(3) /ha were consistently those inhabiting dead wood of large diameter and in advanced stages of decay. On the basis of our results, to make current wood-production practices in beech forests throughout Europe more conservation oriented (i.e., promoting biodiversity and ecosystem functioning), we recommend increasing the amount of dead wood to >20 m(3) /ha; not removing dead wood of large diameter (50 cm) and allowing more dead wood in advanced stages of decomposition to develop; and designating strict forest reserves, with their exceptionally high amounts of dead wood, that would serve as refuges for and sources of saproxylic habitat specialists.

Local temperatures inferred from plant communities suggest strong spatial buffering of climate warming across Northern Europe

Recent studies from mountainous areas of small spatial extent (<2500 km(2) ) suggest that fine-grained thermal variability over tens or hundreds of metres exceeds much of the climate warming expected for the coming decades. Such variability in temperature provides buffering to mitigate climate-change impacts. Is this local spatial buffering restricted to topographically complex terrains? To answer this, we here study fine-grained thermal variability across a 2500-km wide latitudinal gradient in Northern Europe encompassing a large array of topographic complexities. We first combined plant community data, Ellenberg temperature indicator values, locally measured temperatures (LmT) and globally interpolated temperatures (GiT) in a modelling framework to infer biologically relevant temperature conditions from plant assemblages within <1000-m(2) units (community-inferred temperatures: CiT). We then assessed: (1) CiT range (thermal variability) within 1-km(2) units; (2) the relationship between CiT range and topographically and geographically derived predictors at 1-km resolution; and (3) whether spatial turnover in CiT is greater than spatial turnover in GiT within 100-km(2) units. Ellenberg temperature indicator values in combination with plant assemblages explained 46-72% of variation in LmT and 92-96% of variation in GiT during the growing season (June, July, August). Growing-season CiT range within 1-km(2) units peaked at 60-65°N and increased with terrain roughness, averaging 1.97 °C (SD = 0.84 °C) and 2.68 °C (SD = 1.26 °C) within the flattest and roughest units respectively. Complex interactions between topography-related variables and latitude explained 35% of variation in growing-season CiT range when accounting for sampling effort and residual spatial autocorrelation. Spatial turnover in growing-season CiT within 100-km(2) units was, on average, 1.8 times greater (0.32 °C km(-1) ) than spatial turnover in growing-season GiT (0.18 °C km(-1) ). We conclude that thermal variability within 1-km(2) units strongly increases local spatial buffering of future climate warming across Northern Europe, even in the flattest terrains.

Factors influencing vegetation gradients across ancient‐recent woodland borderlines in southern Sweden

. We studied gradients in field layer vegetation across ecotone-type borderlines between 12 ancient woodlands and adjacent secondary deciduous woodlands on former arable land. The aim of the study was to determine how distance from the borderline influences species distributions as compared with soil factors and degree of canopy closure. Correspondence Analysis showed that distance from the borderline is closely related to the first ordination axis at all study sites. Canonical Correspondence Analysis with variation partitioning revealed that distance from the borderline was the single most important factor in explaining vegetation variation. In general, the results suggest the following order of decreasing importance: Distance from the borderline < Soil reaction < Soil nitrogen < Soil moisture < Canopy cover. However, the sum of soil variables, as estimated by weighted averages of Ellenberg indicator values for moisture, reaction and nitrogen, accounted for as much as ca. 50–70% of the total variation explained by environmental variables. Important gradients in field layer vegetation are due to a decrease in typical woodland species and an increase in other species with increasing distance from the ancient woodland. The results suggest dispersal limitation of woodland species as an important determinant of secondary forest succession. However, the importance of distance to species distributions decreases with increasing stand age as most woodland species gradually colonize the recent woodlands. After 70 yr, ca. 50 % of the woodland species present at a site showed complete colonization within 50 m from the ancient woodland border.

Regional differences in floristic change in South Swedish oak forests as related to soil chemistry and land use

Abstract. Changes of the ground layer vegetation between1983 and 1993 were studied on permanent oak forest plots inSkane and eastern Smaland, two regions of South Sweden richin oak but with different climatic conditions. 79 of the 123more common species differed considerably in frequencybetween Skane and Smaland. The oak forests in Skane con-tained more typical forest species, whereas the number oflight-demanding grassland species was higher in Smaland.Almost half of the more common species showed significantchanges in frequency between 1983 and 1993. Several speciesof clearings and some light-demanding grassland species in-creased their frequency in Skane. They were probably fa-voured by canopy thinning which occurred in 65 out of 95permanent plots. Only two species decreased in frequency inSkane, but 14 species showed a decrease in Smaland. All weretypical grassland species and their decline may be regarded asa long-term response to ceased grazing and tree canopy clo-sure as almost no tree cutting occurred during the study periodin Smaland. Total species richness increased with manage-ment intensity in Skane. This was due to decreasing specieslosses and increasing establishment of new species at intensemanagement. A small general increase in species numberswas recorded in Smaland. Species richness was more closelyrelated to soil pH in Skane than in Smaland. The floristicvariability between the oak forest plots was closely related tosoil pH in both Skane and Smaland. We suggest that changesof light conditions as a result of changes in forest managementare important for current secondary succession in South Swed-ish oak forests, whereas the general floristic pattern is to alarge extent controlled by soil acidity.Keywords: Detrended Correspondence Analysis; Forest man-agement; Ground layer vegetation; Secondary succession;Soil acidity; Species richness.Nomenclature: Lid (1987).

Nitrogen mineralisation in deciduous forest soils in south Sweden in gradients of soil acidity and deposition

Abstract Net nitrogen mineralisation was studied in 600 deciduous forests in four geographically separate regions of southern Sweden. The total nitrogen deposition in the forests varied between 7 and 17 kg ha −1 year −1 . The most frequent canopy-dominant tree species was Quercus robur , others being Acer platanoides, Carpinus betulus, Fraxinus excelsior, Tilia cordata and Ulmus glabra . Soil pH (0.2 M KCl), varied between 3.1 and 7.7 and the C/N ratio between 8 and 50 in soil sampled 5 cm below the litter layer. The potential net mineralisation rate, measured in a 15-week laboratory incubation experiment, could be explained to 20% by soil pH and to 4.5. The most striking finding was that rates of net nitrogen mineralisation and nitrification were by far the highest in the southernmost region, especially in the most acid soils, rates about twice as high as in the other regions. Sites that differed in the dominant tree species did not differ in net nitrogen mineralisation. Nitrogen deposition explained 30% of the variation in mineralisation rate. It is suggested that the high nitrogen deposition in recent decades has favoured ammonification through increasing the nitrogen pool in the soil and altering the quality of the organic matter, the microbial community adapting to the acidified soils and the increased nitrification.

Effects of nitrogen deposition on field layer vegetation in south Swedish oak forests

The influence of atmospheric nitrogen deposition on mixed-oak forest vegetation along a deposition gradient in south Sweden was studied using multivariate methods. The main vegetation gradients were largely controlled by soil acidity, nitrogen supply, canopy composition and the location of the sample plots. The results suggest that nitrogen deposition has affected the field layer vegetation directly by increased nitrogen availability and indirectly by accelerating soil acidification. A close correlation between nitrogen deposition and longitude/latitude does not allow a sharp separation between effects of deposition and other geographically controlled factors. However, according to time series studies, 20 of the 30 field layer species that are most closely associated with high nitrogen deposition in our study have increased in frequency in areas with high deposition during the past decades. This group of field layer species includes many species generally considered as nitrophilous, but also several acid-tolerant species.

Environmental and historical factors limiting the distribution of rare forest grasses in south Sweden

Abstract High soil acidity is often the most important limiting factor for common woodland species. Rare species, however, may be additionally restricted by forest history and management. The distribution of the rare forest grasses Hordelymus europaeus, Festuca altissima, Bromus benekenii and Bromus ramosus in Skane, south Sweden, is studied in respect to both soil acidity and factors related to former land use. Bromus ramosus seems to be limited primarily by a low soil pH. However, the rarity of the grasses compared with other forest species with a similar range in respect to soil pH may be attributed to former land use. Historical studies show that the rare forest grasses are restricted to estate beech forests with long continuity. This could be due to a combination of low colonising ability and sensitivity to coppice/wooded meadow management or severe grazing, common management features until the early 20th century in most other forest types. With a low colonising ability of the rare grasses, the existing populations probably are relics of an originally wider distribution before humans changed the forest environment. Conservation of the existing pattern of deciduous forest and management without large clear fellings may be necessary to ensure the survival of the rare grasses and other forest plants with similar ecology.

Interacting effects of tree characteristics on the occurrence of rare epiphytes in a Swedish beech forest area

Abstract Many epiphytes in Swedish beech forests are associated with old and damaged trees. In this study we examined the impact of bark, soil and stemflow pH, water-holding capacity and bark structures on this association. We also analyzed whether the influence of these factors differed between species of conservation concern (red-listed and indicator species of woodland key habitats) and species not of conservation concern. One hundred and one age-determined living beech trees (range 58–277 yrs) in 13 beech-dominated stands were surveyed in a forest landscape in southern Sweden. We recorded 119 species in total (76 lichens, 43 bryophytes) of which 21 were red-listed and 17 indicator species (26 lichens, 12 bryophytes). NMS ordination showed that the species composition of lichens changed primarily with tree age and along the gradient of smooth bark versus moss cover. Bark pH and tree vitality were the most important gradients for bryophyte composition. The combination of old damaged beech trees and high bark pH resulted in the highest mean number of species of conservation concern for both lichens and bryophytes. The link between these factors is partly explained by a positive effect of tree age on the stemflow pH. Species number of bryophytes not of conservation concern increased with bark pH, whereas the corresponding group of lichens was favored primarily by increased light availability. Neither the water-holding capacity of bark nor soil pH affected patterns of species number. The results from this study show that old beech trees infected by fungi, with a higher bark pH, are the most valuable trees for epiphytes of conservation concern.