Northern European Forests Research Articles

Ecophysiological Behavior of Fagus sylvatica L. Growing at Its Southern Distribution Limit: Insights for Understanding the Fate of the European Beech under Warmer and Dryer Growth Conditions

In the last 20 years, a significant mortality of Fagus sylvatica L. (European beech) has been documented in central and northern European forests. Surprisingly, no beech die off occurred at the southern limit. This fact leads us to hypothesize that European beech populations growing at the southern limit of the distribution might have a significant phenotypic plasticity to better cope with low water availability and rising temperatures. To check this hypothesis, we evaluated the ecophysiological behavior of F. sylvatica growing along an altitudinal transect in Calabria (Italy). We selected three study sites (750 m a.s.l., 976 m a.sl. m a.s.l., 1450 m a.s.l.) showing narrow ranges of temperature, rainfall and air humidity. Trees growing at 976 m a.s.l. showed the highest stomatal conductance values during the entire experimental period. The lowest gas exchange and highest leaf mass area were recorded in plants growing at 750 m a.s.l. In the European beech growing at 1450 m a.s.l., higher vessel density, lower mean vessel diameter and higher vessel grouping index values were recorded. Overall, our results highlighted that the measured populations show a considerable phenotypic plasticity leading them to adjust anatomical and physiological traits in response to narrow ranges of environmental parameters. Despite that, the distribution of F. sylvatica seems to be limited to areas with a growing season rainfall of at least 400 mm and vapor pressure deficit (VPD) values < 3 kPa, which may represent the main environmental thresholds which strongly limit the beech growth and, therefore, influence the ability of this species to cope with future environmental conditions.

Detection and quantification of Cronartium pini from Scots pine bark and wood with Cronartium spp.‐specific quantitative PCR

AbstractThe forest disease, Scots pine blister rust, is caused by the rust fungus Cronartium pini. This pathogen was previously known as the host‐alternating type C. flaccidum and non‐host‐alternating type Peridermium pini. Recent epidemics of this disease in Northern European forests, especially young Scots pine forests in Sweden, caused significant economic and ecological losses. Cronartium pini can be identified based on the typical orange blister‐like aecia in Scots pine in summer, but any molecular identification and quantification method has not been available for Cronartium spp. This study developed qPCR primers that are specific to Cronartium spp. and evaluated DNA extraction protocols from pine bark and wood to enable robust qPCR assays. As little as three Cronartium ITS copies can be detected with the protocol. Since only C. pini is known to infect Scots pine in Northern Europe, the protocols were applied to detect C. pini from Scots pine samples without typical symptoms and investigate the C. pini colonization in Scots pine branches from the forest. These results will aid the detection and quantification of C. pini in asymptomatic or symptomatic samples and monitoring Scots pine blister rust in the forest in northern Europe.

Stem growth of Norway spruce in south Sweden in relation to soil moisture, nitrogen deposition, ozone exposure and meteorological variables

Associations between the annual stem basal area increment growth and soil moisture, nitrogen deposition, ground level ozone exposure, air temperatures and the timing of the start of the growing season have been investigated for a twenty four-year period, 1990–2013, based on tree-ring width measurements from seventeen monitoring sites with Norway spruce (Picea abies) forests in southern Sweden. The stem growth-environment associations were analyzed using a fixed effect regression model, with annual stem basal area increment (BAI) as the dependent variable and annual values for a soil moisture index, ozone exposure estimated as AOT30, bulk deposition of nitrogen, summed air temperatures above a threshold and the timing of the start of the growing season as explanatory variables. The statistical analysis was made with and without taking clustering of the sampled trees into account, i.e. that several different tree observations were made at the same monitoring site. The annual number of days with soil moisture below a threshold was the only explanatory variable that could be demonstrated to be negatively associated with changes in BAI, regardless of statistical approach. Positive associations between temperature sums as well as nitrogen deposition with changes in BAI were indicated by low p values using standard p-values, but not when clustering was taken into consideration. Associations between ozone exposure as well as the start date of the growing season with changes in BAI could not be demonstrated since the estimated p values were high regardless of statistical approach. The results show that soil water deficit may considerably limit forest growth in northern European forests.

Leveraging research infrastructure co-location to evaluate constraints on terrestrial carbon cycling in northern European forests

Integrated long-term, in-situ observations are needed to document ongoing environmental change, to “ground-truth” remote sensing and model outputs and to predict future Earth system behaviour. The scientific and societal value of in-situ observations increases with site representativeness, temporal duration, number of parameters measured and comparability within and across sites. Research Infrastructures (RIs) can support harmonised, cross-site data collection, curation and publication. Integrating RI networks through site co-location and standardised observation methods can help answers three questions about the terrestrial carbon sink: (i) What are present and future carbon sequestration rates in northern European forests? (ii) How are these rates controlled? (iii) Why do the observed patterns exist? Here, we present a conceptual model for RI co-location and highlight potential insights into the terrestrial carbon sink achievable when long-term in-situ Earth observation sites participate in multiple RI networks (e.g., ICOS and eLTER). Finally, we offer recommendations to promote RI co-location.

Landscape-level determinants of butterfly species richness in northern Europe: A country-wide survey reveals the paramount importance of forest land

Human-induced changes in habitat amount and quality constitute the key driver behind negative trends in global biodiversity. Conservation actions must be based on solid knowledge of the factors affecting species richness in the remaining habitat fragments. Besides local characteristics, it is critical to consider a broader spatial context when evaluating habitat quality. Here we present an analysis of landscape-level determinants of butterfly species richness based on a country-wide survey in Estonia, representing the northern European forest zone. This study compares favourably to previous research in its scope (over 1200 systematically selected sites), and provides a relatively ‘natural’ reference point for the analyses based on areas with higher human population density. The number of butterfly species locally recorded was sought to be explained by variables describing the surroundings of the study sites at four spatial scales. The proportion of forest land was by far the strongest factor (positively) affecting the number of butterfly species recorded, followed by soil alkalinity. Notably, the effects of forest land area at the smallest (in a radius of 250 m) and largest (2000 m) spatial scales were partly independent of each other. The former likely reflects immediate habitat quality while the latter should be related to the colonisation-extinction balance in the metapopulation context. The correlates of anthropogenic pressure (e.g., urbanization, roads, agriculture) had little effect. The results highlight the key role of designing responsible forestry practices in the regional context of nature conservation.

Nitrogen deposition causes eutrophication in bryophyte communities in central and northern European forests

Key messageOur results indicate that nitrogen deposition is likely to adversely affect forest bryophyte communities, having negative impacts in terms of increased dominance of nitrophilic species at the expense of N-sensitive species and a decrease in evenness.ContextElevated atmospheric deposition of nitrogen (N) has long been recognised as a threat to biodiversity and, despite declines in European emission levels, will remain a threat in the future.AimsIt has proven difficult to show clear large-scale impacts of N deposition on vascular forest understorey species, and few studies have looked at impacts on forest bryophytes. Here, we assess the impact of nitrogen deposition on forest bryophyte communities.MethodsWe used data from 187 plots included in European monitoring schemes to analyse the relationship between levels of throughfall nitrogen deposition and bryophyte taxonomic and functional diversity and community nitrogen preference.ResultsWe found that nitrogen deposition is significantly associated with increased bryophyte community nitrogen preference and decreases in species evenness.ConclusionOur results indicate that nitrogen deposition is likely to adversely affect forest bryophyte communities, having negative impacts in terms of increased dominance of nitrophilic species at the expense of N-sensitive species and a decrease in species evenness.

When a threatened species becomes a threat: a key to reading the Habitats Directive based on occurrence and distribution of Cerambyx cerdo L. in Mediterranean urban and peri‐urban areas

Abstract Cerambyx cerdo is a large saproxylic species included in the Annex IV of the European Habitats Directive as a ‘priority species’. Although C. cerdo populations have shown a significant decline in size and distribution in central and northern European forests, in the Mediterranean basin it is often considered as a pest in urban and peri‐urban areas. Based on European legislations currently in use, we propose a simplified decision‐making flowchart that should be followed regarding the adoption of different control measures against severe C. cerdo infestations.

Survival and growth of saprotrophic and mycorrhizal fungi in recalcitrant amine, amide and ammonium containing media.

The elimination of hazardous compounds in chemical wastes can be a complex and technically demanding task. In the search for environmental-friendly technologies, fungal mediated remediation and removal procedures are of concern. In this study, we investigated whether there are fungal species that can survive and grow on solely amine-containing compounds. One compound containing a primary amine group; 2-diethylaminoethanol, one compound with a primary amide group; 2,6-dichlorobenzamide (BAM), and a third compound containing a quaternary ammonium group; N3-trimethyl(2-oxiranyl)methanaminium chloride, were selected. The choice of these compounds was motivated by their excessive use in large scale manufacturing of protein separation media (2-diethylaminoethanol and the quaternary amine). 2,6-dichlorobenzamide, the degradation product of the herbicide 2,6-dichlorobenzonitrile (dichlobenil), was chosen since it is an extremely recalcitrant compound. Utilising part of the large fungal diversity in Northern European forests, a screening study using 48 fungal isolates from 42 fungal species, including saprotrophic and mycorrhizal fungi, was performed to test for growth responses to the chosen compounds. The ericoid (ERM) mycorrhizal fungus Rhizoscyphus ericae showed the best overall growth on 2-diethylaminoethanol and BAM in the 1-20 g L-1 concentration range, with a 35-fold and 4.5-fold increase in biomass, respectively. For N3-trimethyl(2-oxiranyl)methanaminium chloride, the peak growth occurred at 1 g L-1. In a second experiment, including three of the most promising fungi (Laccaria laccata, Hygrophorus camarophyllus and Rhizoscyphus ericae) from the screening experiment, a simulated process water containing 1.9% (w/v) 2-diethylaminoethanol and 0.8% (w/v) N3-trimethyl(2-oxiranyl)methanaminium chloride was used. Laccaria laccata showed the best biomass increase (380%) relative to a control, while the accumulation for Rhizoscyphus ericae and Hygrophorus camarophyllus were 292% and 136% respectively, indicating that mycorrhizal fungi can use amine- and amide-containing substrates as nutrients. These results show the potential of certain fungal species to be used in alternative green wastewater treatment procedures.

Seasonal Dynamics of Fungi Associated with Healthy and Diseased Pinus sylvestris Needles in Northern Europe.

The relationship between the ecological success of needle pathogens of forest trees and species richness of co-inhabiting endophytic fungi is poorly understood. One of the most dangerous foliar pathogens of pine is Dothistroma septosporum, which is a widely spread threat to northern European forests. We sampled two Pinus sylvestris sites in Estonia and two in Norway in order to analyse the relations between the abundance of D. septosporum and overall fungal richness, specific fungal species composition, time of season, needle age and position in the canopy. In both countries, the overall species richness of fungi was highest in autumn, showing a trend of increase with needle age. The overall species richness in the second-year needles in Estonia and third-year needles in Norway was similar, suggesting that a critical colonization threshold for needle shed in P. sylvestris is breached earlier in Estonia than in Norway. The fungal species richness in P. sylvestris needles was largely affected by Lophodermium conigenum. Especially in older needles, the relative abundance of L. conigenum was significantly higher in spring compared to summer or autumn. The timing of recruitment and colonization mechanisms of different foliage endophytes are shortly discussed.

Dynamics and Vertical Distribution of Roots in European Beech Forests and Douglas Fir Plantations in Bulgaria

Identifying patterns in roots spatial distribution and dynamics, and quantifying the root stocks, annual production and turnover rates at species level is essential for understanding plant ecological responses to local environmental factors and climate change. We studied selected root traits in four different stands, two European beech (Fagus sylvatica L.) forests and two Douglas fir (Pseudotsuga menziezii Mirb. Franco) plantations. Root system vertical distribution and dynamics were studied using sequential coring method and characterised into three root diameter size classes (0–2, 2–5 and 5–10 mm) sampled at three different soil depths (0–15, 15–30, 30–45 cm). Root annual production and turnover rates were analysed and quantified using Decision Matrix and Maximum-Minimum estimation approaches. The overall root mass (<10 mm diameter up to 0–45 cm soil depth) was higher in the beech forests than in the Douglas fir plantations. Some root traits, e.g., the overall root mass, the fine (0–2 mm) and small (2–5 mm) roots mass, differed significantly between the sampling plots rather than between the forest types. The root system revealed a tree species specific vertical distribution pattern. More than half of the fine and small roots biomass of the Douglas fir stands were allocated in the uppermost soil layer and decreased significantly with depths, while in the beech forests the biomass was more uniformly distributed and decreased gradually with increasing soil depth. Although both tree species belong to two different plant functional types and the stands were situated in two distantly located regions with different climatic and soil characteristics, we revealed similar trends in the root biomass and necromass dynamics, and close values for the annual production and turnover rates. The mean turnover rates for all studied stands obtained by sequential coring and Decision Matrix were 1.11 yr−1 and 0.76 yr−1 based on mean and maximum biomass data, respectively. They were similar to the averaged values suggested for Central and Northern European forests but higher compared to those reported from Southern Europe.

Range expansion of the small spruce bark beetle Ips amitinus: a newcomer in northern Europe

AbstractIps amitinus arrived in Northern Europe at the beginning of 1900s, although its recent expansions to the northernmost conifers have been rapid. Analyses of recent records, MaxEnt models and regional population size estimates are used to discuss its peculiar range shifts and potential as a forest pest in Northern Europe. Ips amitinus was probably absent in northern glacial refugia for Norway spruce in the Russian plain and northward expansions from its glacial refugia in the Central European mountains may have been slowed down by: (i) ecological barriers of post‐glacial dry plains and bogs in Central Europe; (ii) heavy utilization of conifers; and (iii) Allee effects as a result of fragmented forests and an unfavourable climate for a cold‐adapted species in the continental lowlands. MaxEnt models predict that I. amitinus may become widespread in the Northern European forests, whereas its populations in the southernmost mountain ranges of Europe may decline in the future. The population levels of I. amitinus in recently invaded northern areas are still lower than those in core areas of Central Europe, although the population development in Central Europe indicates that future bark beetle outbreak periods may boost the I. amitinus populations in Northern Europe as well.

Spruce and beech as local determinants of forest fungal community structure in litter, humus and mineral soil

Beech forests reaches its native distribution limit in SE Norway, but is expected to expand substantially northwards due to climate warming. This may potentially result in a fundamental transformation of contemporary Northern European forests, with tentative effects on the associated belowground fungi. Fungal communities mediate vital ecosystem processes such as ecosystem productivity and carbon sequestration in boreal forests. To investigate how soil fungi is affected by the vegetation transition from spruce to beech forest, we sampled litter, humus and mineral soil in a forest landscape dominated by beech, spruce or a mixture of these. The fungal communities in the soil samples were analyzed by DNA metabarcoding of the rDNA ITS2 region. Although soil layers were the most important structuring gradient, we found clear differences in fungal species composition between spruce and beech plots. The differences in fungal community composition were most evident in the litter and least in the mineral soil. Decomposers, most notably Mycena, dominated the litter layer while various mycorrhizal fungi dominated the humus and mineral layers. Some ectomycorrhizal taxa, such as Cenoccocum and Russula, were more abundant in spruce forests. Differences in fungal community composition between forest types can potentially have large impacts on carbon sequestration rates.

Increasing large scale windstorm damage in Western, Central and Northern European forests, 1951\u20132010

Using reports of forest losses caused directly by large scale windstorms (or primary damage, PD) from the European forest institute database (comprising 276 PD reports from 1951–2010), total growing stock (TGS) statistics of European forests and the daily North Atlantic Oscillation (NAO) index, we identify a statistically significant change in storm intensity in Western, Central and Northern Europe (17 countries). Using the validated set of storms, we found that the year 1990 represents a change-point at which the average intensity of the most destructive storms indicated by PD/TGS > 0.08% increased by more than a factor of three. A likelihood ratio test provides strong evidence that the change-point represents a real shift in the statistical behaviour of the time series. All but one of the seven catastrophic storms (PD/TGS > 0.2%) occurred since 1990. Additionally, we detected a related decrease in September–November PD/TGS and an increase in December–February PD/TGS. Our analyses point to the possibility that the impact of climate change on the North Atlantic storms hitting Europe has started during the last two and half decades.

Elevated atmospheric CO2 and humidity delay leaf fall in Betula pendula, but not in Alnus glutinosa or Populus tremula × tremuloides

Anthropogenic activity has increased the level of atmospheric CO2, which is driving an increase of global temperatures and associated changes in precipitation patterns. At Northern latitudes, one of the likely consequences of global warming is increased precipitation and air humidity. In this work, the effects of both elevated atmospheric CO2 and increased air humidity on trees commonly growing in northern European forests were assessed. The work was carried out under field conditions by using Free Air Carbon dioxide Enrichment (FACE) and Free Air Humidity Manipulation (FAHM) systems. Leaf litter fall was measured over 4 years (FACE) or 5 years (FAHM) to determine the effects of FACE and FAHM on leaf phenology. Increasing air humidity delayed leaf litter fall in Betula pendula, but not in Populus tremula × tremuloides. Similarly, under elevated atmospheric CO2, leaf litter fall was delayed in B. pendula, but not in Alnus glutinosa. Increased CO2 appeared to interact with periods of low precipitation in summer and high ozone levels during these periods to effect leaf fall. This work shows that increased CO2 and humidity delay leaf fall, but this effect is species-specific.

Land-mollusc forest communities along an altitudinal transect in northern Italy

While central and northern European land-mollusc forest communities have received considerable research effort, southern Europe has enjoyed much less attention. Despite the extent of Italian forests in terms of altitude and latitude, there has been only one previous quantitative study of their molluscan faunas. This paper describes the molluscan communities of six forest habitats in northern Italy along an altitudinal transect. Species richness had its highest values at low elevations, especially in maritime woodland and in riverine poplar woodland. Species abundance decreased from lowland to montane habitats, possibly as a consequence of a transition from eutrophic to oligotrophic habitats and from calcareous to acidic soils. Compared with the central and northern European forests, the Italian forests were poorer as far as species richness is concerned, but showed greater diversity among locations as the Jaccard presence/absence similarity index among habitats is equal or lower than 0.5. Considering abundances, all molluscan communities were significantly different in pairwise comparisons, with the single exception of the montane mixed-deciduous and beech forests. The low faunal similarity among sites and the high percentage of species found in only one habitat support the need for a network of diverse protected areas to preserve land-mollusc biodiversity.

LICHEN DIVERSITY OF OLD-GROWTH FORESTS IN MOLISE (CENTRAL-SOUTHERN ITALY)

The role of lichens as indicators of environmental continuity is widely reported in central and northern European forests. Management plans consider them e.g. to predict the presence of valuable species and for the selection of key woodland habitats. Nevertheless, effects of management to many forest-associated lichens in Mediterranean environments are poorly known and threats are only recently investigated. Aim of this work is to contribute to the identification of lichen communities colonizing old-growth forests and variables influencing their diversity. The study was carried out in two large forested areas in the high Molise region, belonging to the network identified under the PRIN 2007 “Metodi innovativi per la identificazione, caratterizzazione e gestione dei boschi vetusti in ambito Mediterraneo”. Abeti Soprani (Pescopennataro, Isernia) and Collemeluccio (Pescolanciano, Isernia) are the selected stands because of their historical value and phytogeographical interest. These stands represent two different habitat types: Abeti Soprani is a Southern Apennine silver fir forest and Collemeluccio a is mixed forest of silver fir and turkey oak. The Lichen Diversity Values (LDV) was releved according to a randomized stratified sampling design within a plot of one hectare in each study area. The percentage contributions to the total lichen diversity of each growth form, photobiont and reproductive strategy were taken into account in order to assess the differences among the plots. The paper reports the results obtained during the first year of survey as a starting point for planning a long-term monitoring. The LDV seems to be very high in these Mediterranean old-growth forests confirming their important role in lichen diversity conservation.

On the relationship of canopy LAI and photon recollision probability in boreal forests

The theory of spectral invariants, or ‘ p-theory’, states that the canopy scattering coefficient at any wavelength can be related to the leaf scattering coefficient at the same wavelength through a spectrally invariant canopy structural parameter — the photon recollision probability p. The p-theory has recently gained interest in the vegetation reflectance modeling community as an efficient tool for characterizing scattering in clumped foliage structures. In this short communication paper, we report empirical data of the relationship of canopy leaf area index (LAI), diffuse non-interceptance and photon recollision probability for 1032 coniferous and broadleaved forest plots measured in Finland. Our results indicate that the relationship of canopy LAI and diffuse non-interceptance is near-universal in boreal stands i.e. it does not depend on stand age, tree species or growth conditions. This allows improving parameterizations used by canopy reflectance models which utilize the photon recollision probability concept. Our results also suggest that establishing species-specific p-LAI functions for northern European forests requires more research on the influence of micro- and macroscale foliage grouping on photon recollision probability.

The effects of fire and stand age on seedling establishment ofPulsatilla patensin a pine-dominated boreal forest

Successful management of disturbance-related endangered plant species requires thorough knowledge concerning their recruitment ecology. Pulsatilla patens (L.) Mill. is found in northern European forests that in the past have been subjected to a periodic wildfire regime and, thus, is supposedly adapted to corresponding changes in forest light conditions and thickness of the moss and litter layers. We tested whether this is true in a field experiment where seeds of P. patens were sown in experimentally burned and control boreal Scots pine forest sites of different successional age (clear-cut, 30 years old, 45 years old). The germination and early establishment of P. patens was enhanced in the experimentally burned conditions and was more successful in younger successional stages. On the other hand, in sites with a tree layer, the germination of P. patens was better in relatively darker areas, suggesting that the negative effect of successional age is probably not a result of changing light conditions, but rather a result of thicker moss and litter layers in the later successional stages.Key words: boreal forest, germination, forest fire, litter layer, Pulsatilla patens, seed sowing.

Fine‐Scale Diversity and Rarity Hotspots in Northern Forests

Abstract: To evaluate the efficiency by which fine‐scale diversity hotspots capture threatened species in northern forests, we investigated the spatial distribution of vascular plants, bryophytes, macrolichens, and polypore fungi with 1‐ha resolution (in 0.25‐ha plots) within six approximately 2‐km2 forest landscapes in Norway. Of 1295 recorded species (118,000 records), 62 species (842 records) were on the Norwegian Red List, of which 7 species (12 records) were new to Norway. We studied the degree of clustering of different red‐listed species among plots by comparing observed co‐occurrence of red‐listed species to a null model of random co‐occurrence. Red‐listed species were moderately clustered within study areas but less so within each forest type. Clustering was similar for areas with high and low densities of red‐listed species. However, the proportion of records captured in rarity hotspots (plots with the highest number of red‐listed species) depended on both the total number of records of red‐listed species per area and the degree of clustering of species among plots. The 5% highest‐ranking rarity hotspots captured, on average, 59% of red‐listed species and 23% of all records of red‐listed species in study areas, compared with 56% and 18% captured by hotspots selected from a random distribution of species, respectively. Hotspots based on the most species‐rich plots (richness hotspots) were significantly less effective than rarity hotspots in capturing red‐listed species and their records. Based on the observed pattern of species co‐occurrence and the high proportion of area seemingly occupied by red‐listed species in mature forest, we hypothesize that 20–25% of the populations of red‐listed species may be located within rarity hotspots constituting 5% at the scale of 1 ha. We conclude that limited spatial overlap of species constitutes an important constraint on the efficiency of a fine‐scale hotspot conservation strategy, such as the “woodland key habitats” (0.1–10 ha) currently mapped and protected in northern European forests.

Calculating the long-term stable nitrogen sink in northern European forests

Abstract Nitrogen accumulation rates in a boreal Scots pine monoculture in central Sweden and three groups of mixed Scots pine/Norway spruce/silver birch forests in northern Sweden are estimated by using measured and calculated litter fall data, calculated limit values for litter decomposition, and the nitrogen concentration at the limit value (Nlimit). Humus has been accumulating in these forests for 120–3000 years. The Nlimit value is determined by extrapolating a linear relationship between accumulated litter mass loss and the increasing litter N concentration. Values for foliar litter fall and Nlimit are used to calculate the quantity of an hypothesized stable N fraction remaining after accumulated mass loss has reached an asymptotic plateau (the limit value). This stable remaining amount of nitrogen is compared to measured amounts of N in the humus layers of the forests. Measured rates of N accumulation in humus range from 0.147 to 0.255 g N m–2 year–1 and differ from estimated rates of N accumulation in humus by 1.8–30.4%, the higher value possibly due to a slow succession from N-poor pine to N-rich birch, not accounted for in the model. Relating the nitrogen accumulated in these forests at maturity to a nitrogen budget based on estimated rates of nitrogen input, N2 fixation, denitrification and leaching to the mineral soil gives a good agreement of about 0.30–0.35 g N m–2 year–1 stored in humus and vegetation at forest maturity. In these forests, this low rate of N accumulation and storage can apparently continue for thousands of years until the system is “reset” by forest fire. We conclude that in undisturbed boreal forests, once N is bound in recalcitrant fractions it is stable, and that the humus is thus a long-term nitrogen sink.