From coppice to high forest – vegetation change and mesophication in acidic mixed oak forests in the southeastern lowlands of Lower Saxony during 25 years

Die Wiederholung älterer Vegetationsaufnahmen auf gut relokalisierbaren Probeflächen zeigte in mitteleuropäischen Waldökosystemen oft starke Veränderungen der Vegetation als Ergebnis verschiedener Umwelteinwirkungen. Gegenstand dieser Untersuchung sind bodensaure, histo-risch vorwiegend als bäuerliche Niederwälder bewirtschaftete Eichen-Mischwälder des Betulo-Quercetum auf sandigen Partien der Geestplatten südlich des Aller-Urstromtals. Vierzehn eigene, in den Jahren 1990 und 1991 angefertigte Vegetationsaufnahmen in neun Waldgebieten wurden im Jahr 2015 wiederholt. Veränderungen in der Struktur und Bewirtschaftung gaben Anlass, die Nutzungsgeschichte der Bestände seit Ende des 18. Jahrhunderts anhand von historischen Kar-ten und durch Befragung von Lokalhistorikern, Waldbesitzern und Förstern zu dokumentieren. Folgende zwei Hypothesen bilden die Grundlage der Studie: (1) Bestandesschluss nach Aufgabe der historischen Niederwaldnutzung führte zu einem Rückgang lichtliebender Pflanzenarten. (2) Stickstoffdeposition aus der Luft hatte auf den relativ nährstoffarmen, grundwasserfernen Standorten die Zunahme von stickstoffliebenden Pflanzen und mesophilen Waldarten zur Folge. Anhand veränderter Häufigkeiten in den 14 Vegetationsaufnahmen wurden „Gewinner- und Verliererarten“ ermittelt; außerdem wurden Artenzahlen und mittlere Ellenberg-Zeigerwerte der rezenten und historischen Vegetationsaufnahmen verglichen. Im Verlauf von 25 Jahren waren alle Flächen, von denen neun historisch alte Wälder und fünf Aufforstungen oder Vorwälder auf ehemaligen Heide- und Ackerflächen waren, noch Eichen-Mischwälder des Betulo-Quercetum. Fast alle der acht noch 1990/91 niederwaldartigen Bestände hatten diesen Charakter verloren, indem bei Durchforstung nur ein Eichenstamm mehrstämmi-ger Stockausschläge belassen wurde. Insgesamt nahmen Birken als Pioniergehölze ab, und die Strauchschicht wurde u.a. durch die Zunahme der neophytischen Spätblühenden Traubenkirsche dichter. In Übereinstimmung mit Hypothese 1 konnte eine Entwicklung zu schattentoleranterer 120 Vegetation nachgewiesen werden; viele ehemals vorhandene Lichtzeiger – vor allem charakteristische Arten der Birken-Eichenwälder– gingen zurück oder verschwanden. Die mittleren Lichtzahlen der Flächen nahmen ab und die mittleren Feuchtezahlen infolge des ausgeglicheneren Bestandesklimas zu. Auch Hypothese 2 konnte bestätigt werden, denn eine Reihe von Stickstoffzeigern und mesophilen Waldarten nahm zu. Dies brachte eine Zunahme von obligaten Waldarten auf Kosten von auch auf mageren Offenlandstandorten verbreiteten Arten sowie bei den Zeigerwerten eine Erhöhung der mittleren Stickstoffzahlen mit sich. Neben Stickstoffeinträgen dürfte die Regeneration der Standorte durch geringere Biomasseentnahme hierfür eine Rolle spielen. Diese Entwicklungen sind in jüngerer Zeit auch in anderen Regionen Mitteleuropas nachgewiesen worden, doch war der Zeitraum seit der Erstaufnahme dort meist deutlich länger. Die Geschwindigkeit der Veränderungen überrascht daher. Die Ergebnisse unterstreichen, dass die Birken- Eichenwälder der Region nicht der potenziell natürlichen Vegetation entsprechen, sondern kulturgeschichtlich interessante Nutzungsrelikte sind. Eine weitere Bewirtschaftung als Eichenwald und ein beispielhafter Erhalt niederwaldartiger Nutzung ist wünschenswert; eine Erhaltung bzw. Wiederherstellung des früheren Vegetationszustands ist aber angesichts fortwährender Stickstoffeinträge und fehlenden Nährstoffentzugs unrealistisch.Resurveying historical permanent or quasi-permanent vegetation plots in Central European forest ecosystems often revealed strong vegetation changes over the past decades due to different environmental drivers. Subject of this study were acidic mixed oak forests (Betulo-Quercetum) on sandy soils on glacial deposits south of the Aller glacial valley (Lower Saxony, Germany), most of which have been historically used as coppice forests. Fourteen own relevés from the years 1990 and 1991 were resurveyed in 2015. Changes in structure and forest management gave reason to document the land use history of the forest stands since the second half of the 18th century, using historical maps and questioning of local historians, land owners and foresters. The following hypotheses were tested: (1) Canopy closure after abandonment of historical land use led to a decrease of heliophilous plant species. (2) Airborne nitrogen deposition led to an increase of nitrophilous plant species and mesophilous forest species on the relatively nutrientpoor and dry sites. Using frequency and cover changes winner and loser species were identified. Also species numbers as well as mean Ellenberg indicator values for actual and historical relevés were compared. During 25 years all stands, of which nine were ancient forests and five afforestations or pioneer forests on former heathland or farmland, were still mixed oak forests belonging to the Betulo- Quercetum. However, nearly all of the eight stands still recognizable as coppice forest in 1990/91 had lost this character due to conversion to high forest by cutting of side shoots from trunks. Altogether birch as pioneer tree decreased and the shrub canopy became denser, partly due to the spread of the neophyte Prunus serotina. In agreement with existing literature and hypothesis (1) a shift towards more shade-tolerant communities was proved. Light indicators, among them many characteristic plant species of acidic mixed oak forests, decreased or disappeared. This resulted not only in a decrease of mean Ellenberg light values, but also an increase of moisture values of Thilo Heinken: Vom bodensauren Niederwald zum mesophilen Hochwald – Vegetationsveränderungen in Eichenmischwäldern im Flachland südlich der Aller im Verlauf von 25 Jahren 121 the plots due to a more mesic microclimate. Also hypothesis (2) was confirmed, as a number of nitrogen indicators and mesophilous forest species increased, indicating a ‘mesophication’ of the studied forests. This resulted in an increase of strict forest species at the expense of species which also occur on nutrient-poor sites in the open land, and of mean nutrient values of the plots. Besides nitrogen deposition a soil recovery process due to a lower biomass removal may play a role for this effect. All trends have been recently described in other regions of Central Europe, but the time span between the first census and the resurvey was mostly much longer in these studies. The speed of vegetation change is thus surprising. The results emphasize that mixed oak forests do not correspond with the potential natural vegetation, but are interesting relicts of former landuse in cultural heritage terms. In the future a management as oak forest and an exemplary conservation of coppice forest stands is desirable, but a conservation or restoration of the former vegetation state is not realistic, given the continuous nitrogen deposition and missing removal of nutrients

Responses of competitive understorey species to spatial environmental gradients inaccurately explain temporal changes

Understorey plant communities play a key role in the functioning of forest ecosystems. Under favourable environmental conditions, competitive understorey species may develop high abundances and influence important ecosystem processes such as tree regeneration. Thus, understanding and predicting the response of competitive understorey species as a function of changing environmental conditions is important for forest managers. In the absence of sufficient temporal data to quantify actual vegetation changes, space-for-time (SFT) substitution is often used, i.e. studies that use environmental gradients across space to infer vegetation responses to environmental change over time. Here we assess the validity of such SFT approaches and analysed 36 resurvey studies from ancient forests with low levels of recent disturbances across temperate Europe to assess how six competitive understorey plant species respond to gradients of overstorey cover, soil conditions, atmospheric N deposition and climatic conditions over space and time. The combination of historical and contemporary surveys allows (i) to test if observed contemporary patterns across space are consistent at the time of the historical survey, and, crucially, (ii) to assess whether changes in abundance over time given recorded environmental change match expectations from patterns recorded along environmental gradients in space. We found consistent spatial relationships at the two periods: local variation in soil variables and overstorey cover were the best predictors of individual species’ cover while interregional variation in coarse-scale variables, i.e. N deposition and climate, was less important. However, we found that our SFT approach could not accurately explain the large variation in abundance changes over time. We thus recommend to be cautious when using SFT substitution to infer species responses to temporal changes.</p

The European Forest Plant Species List (EuForPlant): Concept and applications

Question When evaluating forests in terms of their biodiversity, distinctiveness and naturalness, the affinity of the constituent species to forests is a crucial parameter. Here we ask to what extent are vascular plant species associated with forests, and does species’ affinity to forests vary between European regions? Location Temperate and boreal forest biome of Northwestern and Central Europe. Methods We compiled EuForPlant, a new extensive list of forest vascular plant species in 24 regions spread across 13 European countries using vegetation databases and expert knowledge. Species were region-specifically classified into four categories reflecting the degree of their affinity to forest habitats: 1.1, species of forest interiors; 1.2, species of forest edges and forest openings; 2.1, species that can be found in forest as well as open vegetation; and 2.2, species that can be found partly in forest, but mainly in open vegetation. An additional “O” category was distinguished, covering species typical for non-forest vegetation. Results EuForPlant comprises 1,726 species, including 1,437 herb-layer species, 159 shrubs, 107 trees, 19 lianas and 4 epiphytic parasites. Across regions, generalist forest species (with 450 and 777 species classified as 2.1 and 2.2, respectively) significantly outnumbered specialist forest species (with 250 and 137 species classified as 1.1 and 1.2, respectively). Even though the degree of shifting between the categories of forest affinity among regions was relatively low (on average, 17.5%), about one-third of the forest species (especially 1.2 and 2.2) swapped categories in at least one of the study regions. Conclusions The proposed list can be used widely in vegetation science and global change ecology related to forest biodiversity and community dynamics. Shifting of forest affinity among regions emphasizes the importance of a continental-scale forest plant species list with regional specificity.publishedVersio

The European Forest Plant Species List (EuForPlant): Concept and applications

Question When evaluating forests in terms of their biodiversity, distinctiveness and naturalness, the affinity of the constituent species to forests is a crucial parameter. Here we ask to what extent are vascular plant species associated with forests, and does species' affinity to forests vary between European regions? Location Temperate and boreal forest biome of Northwestern and Central Europe. Methods We compiled EuForPlant, a new extensive list of forest vascular plant species in 24 regions spread across 13 European countries using vegetation databases and expert knowledge. Species were region-specifically classified into four categories reflecting the degree of their affinity to forest habitats: 1.1, species of forest interiors; 1.2, species of forest edges and forest openings; 2.1, species that can be found in forest as well as open vegetation; and 2.2, species that can be found partly in forest, but mainly in open vegetation. An additional "O" category was distinguished, covering species typical for non-forest vegetation. Results EuForPlant comprises 1,726 species, including 1,437 herb-layer species, 159 shrubs, 107 trees, 19 lianas and 4 epiphytic parasites. Across regions, generalist forest species (with 450 and 777 species classified as 2.1 and 2.2, respectively) significantly outnumbered specialist forest species (with 250 and 137 species classified as 1.1 and 1.2, respectively). Even though the degree of shifting between the categories of forest affinity among regions was relatively low (on average, 17.5%), about one-third of the forest species (especially 1.2 and 2.2) swapped categories in at least one of the study regions. Conclusions The proposed list can be used widely in vegetation science and global change ecology related to forest biodiversity and community dynamics. Shifting of forest affinity among regions emphasizes the importance of a continental-scale forest plant species list with regional specificity

Forest microclimate dynamics drive plant responses to warming

Climate warming is causing a shift in biological communities in favor of warm-affinity species (i.e., thermophilization). Species responses often lag behind climate warming, but the reasons for such lags remain largely unknown. Here, we analyzed multidecadal understory microclimate dynamics in European forests and show that thermophilization and the climatic lag in forest plant communities are primarily controlled by microclimate. Increasing tree canopy cover reduces warming rates inside forests, but loss of canopy cover leads to increased local heat that exacerbates the disequilibrium between community responses and climate change. Reciprocal effects between plants and microclimates are key to understanding the response of forest biodiversity and functioning to climate and land-use changes

Combining biodiversity resurveys across regions to advance global change research

More and more ecologists have started to resurvey communities sampled in earlier decades to determine long-term shifts in community composition and infer the likely drivers of the ecological changes observed. However, to assess the relative importance of and interactions among multiple drivers, joint analyses of resurvey data from many regions spanning large environmental gradients are needed. In this article, we illustrate how combining resurvey data from multiple regions can increase the likelihood of driver orthogonality within the design and show that repeatedly surveying across multiple regions provides higher representativeness and comprehensiveness, allowing us to answer more completely a broader range of questions. We provide general guidelines to aid the implementation of multiregion resurvey databases. In so doing, we aim to encourage resurvey database development across other community types and biomes to advance global environmental change research

Replacements of small- by large-ranged species scale up to diversity loss in Europe’s temperate forest biome

The loss of biodiversity at the global scale has been difficult to reconcile with observations of no net loss at local scales. Vegetation surveys across European temperate forests show that this may be explained by the replacement of small-ranged species with large-ranged ones, driven by nitrogen deposition. Biodiversity time series reveal global losses and accelerated redistributions of species, but no net loss in local species richness. To better understand how these patterns are linked, we quantify how individual species trajectories scale up to diversity changes using data from 68 vegetation resurvey studies of seminatural forests in Europe. Herb-layer species with small geographic ranges are being replaced by more widely distributed species, and our results suggest that this is due less to species abundances than to species nitrogen niches. Nitrogen deposition accelerates the extinctions of small-ranged, nitrogen-efficient plants and colonization by broadly distributed, nitrogen-demanding plants (including non-natives). Despite no net change in species richness at the spatial scale of a study site, the losses of small-ranged species reduce biome-scale (gamma) diversity. These results provide one mechanism to explain the directional replacement of small-ranged species within sites and thus explain patterns of biodiversity change across spatial scales

Observer and relocation errors matter in resurveys of historical vegetation plots

Aim: Revisits of non-permanent, relocatable plots first surveyed several decades ago offer a direct way to observe vegetation change and form a unique and increasingly used source of information for global change research. Despite the important insights that can be obtained from resurveying these quasi-permanent vegetation plots, their use is prone to both observer and relocation errors. Studying the combined effects of both error types is important since they will play out together in practice and it is yet unknown to what extent observed vegetation changes are influenced by these errors. Methods: We designed a study that mimicked all steps in a resurvey study and that allowed determination of the magnitude of observer errors only vs the joint observer and relocation errors. Communities of vascular plants growing in the understorey of temperate forests were selected as study system. Ten regions in Europe were covered to explore generality across contexts and 50 observers were involved, which deliberately differed in their experience in making vegetation records. Results: The mean geographic distance between plots in the observer+relocation error data set was 24m. The mean relative difference in species richness in the observer error and the observer+relocation data set was 15% and 21%, respectively. The mean pseudo-turnover between the five records at a quasi-permanent plot location was on average 0.21 and 0.35 for the observer error and observer+relocation error data sets, respectively. More detailed analyses of the compositional variation showed that the nestedness and turnover components were of equal importance in the observer data set, whereas turnover was much more important than nestedness in the observer+relocation data set. Interestingly, the differences between the observer and the observer+relocation data sets largely disappeared when looking at temporal change: both the changes in species richness and species composition over time were very similar in these data sets. Conclusions: Our results demonstrate that observer and relocation errors are non-negligible when resurveying quasi-permanent plots. A careful interpretation of the results of resurvey studies is warranted, especially when changes are assessed based on a low number of plots. We conclude by listing measures that should be taken to maximally increase the precision and the strength of the inferences drawn from vegetation resurveys

ReSurveyGermany: Vegetation-plot time-series over the past hundred years in Germany

Vegetation-plot resurvey data are a main source of information on terrestrial biodiversity change, with records reaching back more than one century. Although more and more data from re-sampled plots have been published, there is not yet a comprehensive open-access dataset available for analysis. Here, we compiled and harmonised vegetation-plot resurvey data from Germany covering almost 100 years. We show the distribution of the plot data in space, time and across habitat types of the European Nature Information System (EUNIS). In addition, we include metadata on geographic location, plot size and vegetation structure. The data allow temporal biodiversity change to be assessed at the community scale, reaching back further into the past than most comparable data yet available. They also enable tracking changes in the incidence and distribution of individual species across Germany. In summary, the data come at a level of detail that holds promise for broadening our understanding of the mechanisms and drivers behind plant diversity change over the last century