Luc De Keersmaeker

Four decades of post-agricultural forest development have caused major redistributions of soil phosphorus fractions.

Fertilisation of agricultural land causes an accumulation of nutrients in the top soil layer, among which phosphorus (P) is particularly persistent. Changing land use from farmland to forest affects soil properties, but changes in P pools have rarely been studied despite their importance to forest ecosystem development. Here, we describe the redistributions of the P pools in a four-decadal chronosequence of post-agricultural common oak (Quercus robur L.) forests in Belgium and Denmark. The aim was to assess whether forest age causes a repartitioning of P throughout the various soil P pools (labile P, slowly cycling P and occluded P); in particular, we addressed the time-related alterations in the inorganic versus organic P fractions. In less than 40xa0years of oak forest development, significant redistributions have occurred between different P fractions. While both the labile and the slowly cycling inorganic P fractions significantly decreased with forest age, the organic fractions significantly increased. The labile P pool (inorganicxa0+xa0organic), which is considered to be the pool of P most likely to contribute to plant-available P, significantly decreased with forest age (from >20 to <10% of total P), except in the 0–5xa0cm of topsoil, where labile P remained persistently high. The shift from inorganic to organic P and the shifts between the different inorganic P fractions are driven by biological processes and also by physicochemical changes related to forest development. It is concluded that the organic labile P fraction, which is readily mineralisable, should be taken into account when studying the bioavailable P pool in forest ecosystems.

Soil acidification along an ammonium deposition gradient in a Corsican Pine stand in northern Belgium

Abstract In a homogeneous Corsican pine ( Pinus nigra ssp laricio ) stand, situated in a region of intensive livestock breeding, effects of different NH x doses on soil acidification were studied. Throughfall collectors were placed every 25 m on a transect of 1.4 km, oriented according to the dominant wind direction. The throughfall water was analysed on NH 4 + . A permanent monitoring plot is situated in the middle of the transect where quantity and chemical composition of different water fractions are monitored fortnightly. Along the transect, soil samples of the upper mineral soil (0–10 cm) were taken and analysed on pHue5f8H 2 O and pHue5f8KCl. The measured deposition of NH 4 + ue5f8N was very high, especially in the forest edge at the prevailing wind direction (55 kg ha −1 year −1 ), where up to twice as much NH 4 + ue5f8N was found as in the monitoring plot. A forest edge zone of 180 m with significantly higher NH 4 + deposition could clearly be delineated. Serious indications were found that all NH 4 + was nitrified but that this process was slowed down. The variation in NH 4 + depositions was clearly reflected in the state of soil acidification: pHue5f8H 2 O and pHue5f8KCl values varied between 3.1 (edge) and 3.8 (centre), and 2.4 (edge) and 2.8 (centre) respectively.

Does Prunus serotina act as an aggressive invader in areas with a low propagule pressure

Since most studies on Prunusxa0serotina in Western Europe focused on heavily invaded areas, we wondered whether P.xa0serotina also acts as an aggressive invader in areas with a low propagule pressure. Based on long-term data for the Liedekerke forest reserve, we found that connectivity to seed sources and light availability were the major drivers of P.xa0serotina presence: long-distance dispersal events and ‘windows of opportunity’ seem to direct P.xa0serotina colonization. In the studied forest, P.xa0serotina could not be considered an aggressive invader since its spread slowed down rather quickly and did not hamper the establishment of native tree species. Furthermore, understory P.xa0serotina showed low growth and seed production, while the high Rubus cover hampered germination and establishment. Nonetheless, calamities opening up the canopy layer in the few areas with high P.xa0serotina sapling density might alter the course of the invasion process.

Landscape factors and regional differences in recovery rates of herb layer richness in Flanders (Belgium)

The recovery of understory plants in recent forests is critical for evaluating the overall capacity of landscapes to maintain plant biodiversity. Here we used a large data set of vegetation plots from the Flemish Forest Inventory in combination with maps of forest history and soil-based Potential Natural Vegetation to evaluate regional differences in the rate of recovery of understory plant diversity in three regions of Flanders, Belgium. We expressed the degree of recovery in recent forests using the species richness of ancient forests as a reference point, and found strong differences among regions in the average level of recovery. These differences appeared to be due to regional variation in average patch connectivity and age (ultimately stemming from differences in land use history) and – to a lesser extent – environmental conditions. We also found an increase in the proportional representation of vertebrate dispersed species and species with short-distance dispersal with increasing levels of recovery. Our results highlight the potential drivers of inter-regional variation in the process of recovery of plant diversity during restoration, and they emphasize the importance of historical and spatial context in the recovery process.

The analysis of spatio-temporal forest changes (1775–2000) in Flanders (northern Belgium) indicates habitat-specific levels of fragmentation and area loss

IntroductionSpatio-temporal forest changes can have a progressive negative impact on the habitat of species that need forest continuity, i.e. the continuous presence of forest. Long-term species data that demonstrate such an impact are often not available. Instead we applied a spatial analysis on maps of the historical and present-day forests, by calculating landscape indices that explain forest plant species diversity.MethodsWe digitized for this purpose, forests in Flanders (northern Belgium, ~13,500 km2) at four time slices (1775, 1850, 1904–1931, 2000) and created a map of forest continuity in 2000. The ecological relevance of the analysis was further enhanced by a site classification, using a map of potential forest habitat types based on soil–vegetation relationships.ResultsOur results indicated that, between 1775 and 2000, forests occupied 9.7–12.2 % of the total study area. If continuity was not taken into consideration, forest fragmentation slightly increased since 1775. However, only 16 % of the forest area in 2000 remained continuously present at least since 1775 and is therefore called ancient forest (AF). Moreover, connectivity of forest that originated after 1775, called recent forest, was low and only 14 % was in physical contact with AF. The results were habitat-specific as forest on sites that are potentially suitable for a high number of slow-colonizing species, e.g. ancient forest plants, were affected most.ConclusionWe discuss that a GIS analysis of this kind is essential to provide statistics for forest biodiversity conservation and restoration, in landscapes with a dynamic and heterogeneous forest cover.

Can tree species choice influence recruitment of ancient forest species in post-agricultural forest?

Germination and establishment of ancient forest species are bottlenecks in forest habitat restoration. We hypothesised that tree species can influence these processes on acidification sensitive soils through their effects on the soil. Seeds of seven ancient forest species were sown in soil collected in a post-agricultural forest under four different tree species, notably Acer pseudoplatanus, Alnus glutinosa, Fagus sylvatica and Populus x euramericana. Germination of four species was affected by tree species, while only Scilla non-scripta was indifferent and germination of Anemone nemorosa and Paris quadrifolia was unsuccessful. The acid intolerant Primula elatior only germinated in the soil of P. x euramericana but also the acid tolerant Lonicera periclymenum and Stellaria holostea performed best in the soil of P.x euramericana and worst in the soil of A. glutinosa. Lamium galeobdolon preferred the soil of A. pseudoplatanus. Germination of competitive seed bank species was much more successful in the soil of P. x euramericana than in soil of other tree species. From our results we deduce that tree species selection can affect germination of ancient forest species. Forest managers should be aware of the effects of tree species on their colonisation chances and favour trees with good decomposable litter especially on acidification sensitive soils.

Application of the Ancient Forest Concept to Potential Natural Vegetation Mapping in Flanders, A Strongly Altered Landscape in Northern Belgium

Construction of potential natural vegetation (PNV) poses particular challenges in landscapes heavily altered by human activity and must be based on transparent, repeatable methods. We integrated the concept of ancient forest (AF) and ancient forest species (AFS) into a four-step procedure of PNV mapping: 1) classification of forest vegetation relevés; 2) selection of those vegetation types that can serve as PNV units, based on AF and AFS; 3) merging of selected vegetation types into five PNV units that can be predicted from a digital morphogenetic soil map; 4) mapping of three additional PNV units based on additional environmental data. The second step, concerning the selection of reference forest vegetation, is of particular interest for PNV construction in Flanders (northern Belgium), where forest cover has been subject to temporal disruption and spatial fragmentation. Among the variety of extant forest recovery states, we chose as PNV units those vegetation types for which a high proportion of relevés had been located in AF and that contained many AFS. As the frequency of AFS depends on site conditions, we only compared and selected vegetation types that are found on similar sites according to average Ellenberg indicator values. While succession is irrelevant for the definition of PNV, colonization rates of AFS can be used to estimate the time required for PNV to be restored in a site.

Can soil acidity and light help to explain tree species effects on forest herb layer performance in post-agricultural forests?

AimsTree species affect herb layer species through their effects on soil quality and light regime but their relative importance and interactions are insufficiently known.MethodsPot experiment with soil taken from stands planted with tree species with contrasting effects on soil acidification, two light regimes and six forest perennials.ResultsThe survival or growth of Mercurialis perennis, Lamium galeobdolon, Anemone nemorosa and Primula elatior was lower in the acid Alnus soils than in the less acid Fraxinus soils. By contrast, the acid tolerant Convallaria majalis and Dryopteris dilatata were barely affected by tree species. Light conditions had less impact than soil chemistry and did not compensate for unfavourable soil conditions. Ca and P concentrations increased in plants grown in Fraxinus soils. The Mg and Al shoot/root ratios of respectively one and two of the acid tolerant species was elevated in the most acid soil.ConclusionsTree species effects on forest perennials are mainly explained by increased Al concentrations under acidifying species. Changed plant concentrations and allocation are likely associated to Al antagonism. We found no light compensation for the soil effect on the studied species. However, light alters the plant nutrient concentrations and allocation which may suggest an indirect effect.

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. nMethods: 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. nResults: 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. nConclusions: 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.