Tree invasions into treeless areas: mechanisms and ecosystem processes

Non-native tree invasions occur not only in woodland or forest vegetation, but also into areas with little or no native tree presence. Limiting factors for tree establishment and survival include seasonal or annual drought, low nutrient availability, cold temperature extremes, fire, and other abiotic conditions to which trees are poorly adapted as well as biotic conditions such as herbivory and lack of soil mutualist inoculum. Tree invasions of grasslands and semi-arid riparian areas in particular are now widespread and frequently result in the rapid conversion of these habitats to woodlands or forests. In some cases, these invasions are the result of a change in extrinsic conditions such as climate, fire, and/or grazing that remove what have been previous barriers to tree establishment. However, in other cases, tree species with particular life-history and dispersal traits fill open niches or outcompete native species. Significant examples of tree invasion into treeless areas can be seen with invasions of Pinus species into temperate grasslands and fynbos shrublands, Melaleuca quinquenervia and Triadica sebifera into grassy wetlands, Prosopis and Tamarix species into semi-arid riparian zones, and Acacia and Morella invasions into nutrient-poor shrublands and barrens. The establishment of trees into treeless areas may have strong impacts on ecosystem processes, influencing biogeochemical cycling, carbon sequestration and cycling, and ecohydrology, as well possible edaphic legacies that persist even if trees are removed.     

Neighbourhood interactions and environmental factors influencing old‐pasture succession in the Central Pyrenees

Abstract. The shrub Buxus sempervirens and the trees Abies alba and Fagus sylvatica have recently recolonized old‐pastures in the Central Pyrenees. We mapped all live and dead individuals (> 1.3 m tall) in a large forest plot in Ordesa Valley to examine the importance of density‐dependent processes during recolonization. Biotic interactions were inferred from changes in horizontal structure and the influences of neighbours on tree survival. Buxus differentially influenced establishment and survival of tree species, thereby controlling future canopy composition and spatial structure. The rapidly invading Abies formed denser patches on elevated sites less occupied by Buxus, whereas Fagus preferentially established within shrubs. Abies reached densities which led to intense intraspecific competition and high mortality rates among saplings. Self‐thinning in Abies led to smaller numbers of regularly spaced survivors, and greater relative dominance of Fagus. Disregarding intraspecific effects and abiotic environment, Abies survival was significantly lower under Buxus shrubs, which suggests that the spatial location and abundance of Abies was constrained by the location of Buxus. Fagus survival was not related to Buxus density, but remained significantly lower in denser Abies patches. The higher mortality of Fagus in denser Abies patches, and the resulting spatial segregation of the species, reflects asymmetric interspecific competition. Inhibition from Buxus shrubs and intraspecific competition prevent invading Abies from dominating and may therefore help in maintaining a mixed Abies‐Fagus stand.     

Its all about connections: hubs and invasion in habitat networks

During the early stages of invasion, the interaction between the features of the invaded landscape, notably its spatial structure, and the internal dynamics of an introduced population has a crucial impact on establishment and spread. By approximating introduction areas as networks of patches linked by dispersal, we characterised their spatial structure with specific metrics and tested their impact on two essential steps of the invasion process: establishment and spread. By combining simulations with experimental introductions of Trichogramma chilonis (Hymenoptera: Trichogrammatidae) in artificial laboratory microcosms, we demonstrated that spread was hindered by clusters and accelerated by hubs but was also affected by small‐population mechanisms prevalent for invasions, such as Allee effects. Establishment was also affected by demographic mechanisms, in interaction with network metrics. These results highlight the importance of considering the demography of invaders as well as the structure of the invaded area to predict the outcome of invasions.     

Invasion of Pinus halepensis from plantations into adjacent natural habitats

Questions: Do invasions of P. halepensis (Aleppo pine) from plantations into adjacent natural communities occur in the Mediterranean region, where the species is native? What are the spatio‐temporal processes involved in pine invasions in contrasting Mediterranean and semi‐arid climatic regions? Location: Mediterranean and semi‐arid regions of Israel. Methods: The density of invading Pinus was measured in relation to the distance from the plantation edge. Plants were categorized by age, height, basal stem girth and developmental stage, their spatial distribution was also recorded. Results: Analysis of plant age distribution indicates that the invasion process started when the plantations were 20–25 years old. Most invading plants were found within 20 m from the plantation edge, but a few individuals reached distances up to 100 m and became new invasion foci. Plant density declined sharply with distance from adult trees, data showing a better fit to a power model than to a negative exponential model. Invading Pinus began to produce cones earlier in the semi‐arid than in the Mediterranean region (9 vs 12 years to 50% reproductive plants). In both regions, higher densities of invading plants were found on the west side of the plantation, the opposite direction to the hot winds that prevail during seed release. Conclusion: The frontal advance of P. halepensis from plantations is relatively slow, but the populations also expand by a saltation process, creating spreading ‘islands’ of pine trees in the natural vegetation. Spatial pattern of recruits with distance from the source population was remarkably similar to the pattern of seed dispersal in the same region (Nathan et al. 1999). This implies that the probability of a dispersed seed developing into a plant is independent of the distance from the forest edge.     

Pine invasions: climate predicts invasion success; something else predicts failure

Aim Explaining why some invasions fail while others succeed is a prevailing question in invasion biology. Different factors have been proposed to explain the success or failure of exotics. Evidence suggests that climate similarities may be crucial. We tested this using 12 species of the genus Pinus that have been widely planted and shown to be highly invasive. Pinus is among the best‐studied group of exotic species and one that has been widely introduced world‐wide, so we were able to obtain data on invasive and non‐invasive introductions (i.e. unsuccessful invasions; areas where after many decades of self‐sowing seeds there is no invasion). Location World‐wide. Methods We developed species distribution models for native ranges using a maximum entropy algorithm and projected them across the globe. We tested whether climate‐based models were able to predict both invasive and non‐invasive introductions. Results Appropriate climatic conditions seem to be required for these long‐lived species to invade because climates accurately predicted invasions. However, climate matching is necessary, but not sufficient to predict the fate of an introduction because most non‐invasive introductions were predicted to have triggered an invasion. Main conclusions Other factors, possibly including biotic components, may be the key to explaining why some introductions do not become invasions, because many areas where Pinus is not invading were predicted to be suitable for invasion based solely on climate.     

Co-invading ectomycorrhizal fungal succession in pine-invaded mountain grasslands

Ectomycorrhizal (EM) fungal communities that associate with invading pines (Pinus spp.) are expected to be poor in species diversity. However, long-term successional trajectories and the persistence of dispersal limitations of EM fungi in the exotic range are not well understood. We sampled the roots and surrounding soil of Pinus elliottii and P. taeda trees invading mountain grasslands of Argentina. We also sampled the EM fungal spore bank in grassland soil near (∼150 m) and far (∼850 m) from the original pine plantations. We found 86 different co-invasive EM fungal OTUs. Differential dispersal capacities among EM fungi were detected in the spore bank of grassland soil, but not under mature pines. After thirty years of invasion, the age, but not the degree of spatial isolation of pine individuals affected the EM fungal composition. We showed how EM fungal succession occurs during pine invasions, which may have clear consequences for ecosystem functioning of co-invaded sites.     

Spatial pattern dynamics over 10 years in a conifer/broadleaved forest,northern Japan

The present study investigated stand dynamics during 10-year period in a conifer/broadleaved mixed forest in Hokkaido, northern Japan, focusing on spatially dependent recruitment, mortality and growth of two growth-form groups, deciduous broadleaved species and the dominant evergreen conifer Abies sachalinensis. The stand-level basal area was maintained over the 10-year period, while a compositional equilibrium at the individual species level was not confirmed. Univariate and bivariate spatial analyses revealed clustering of many of the constituent species. The absence of single-species patches suggested an ambiguous mosaic formed by co-occurrence of Abies and broadleaved trees. The trend towards an aggregated distribution of Abies and broadleaved trees was caused by spatially dependent recruitment rather than mortality. New recruits of broadleaved species were spatially associated with surviving broadleaved trees, while this was not the case for Abies. The degree of competitive effects on growth was not consistent over the 10-year period. Abies showed between-groups competition, but not within-group competition. In contrast, we found asymmetric competitions between the broadleaved trees. Our results suggest that Abies is not sufficiently competitively dominant to exclude broadleaved trees, and that the co-occurrence of the two growth-form groups might be maintained.     

Shrub succession and invasibility in a New Zealand montane grassland

Abstract Two successive shrub invasions of a short tussock grassland induced by grazing and burning were examined in montane South Island, New Zealand. The first invasion was by a native shrub, matagouri (Discaria toumatou Raoul). The second invasion was by an exotic shrub, Scotch broom (Cytisus scoparius (L.) Link), which invaded the matagouri shrubland that had developed over the grassland. The invasions were investigated using analysis of spatial patterns of both shrubs and tussocks, and age, growth rates and size structure of the shrubs. Competition between the two shrub species was examined using spatial patterns and comparing allometric relationships. After initial invasion by matagouri of the grasslands, stand density increased by consolidation about its initial colonization points. Current matagouri distribution is often negatively associated with tussocks. Scotch broom occurs most frequently in a dense sward of introduced grasses and occasionally in tussocks in interstices among matagouri shrubs. Despite the palatability of Scotch broom to sheep that graze the site, there was no evidence that the spiny matagouri facilitates invasion by protecting Scotch broom seedlings; rather there was negative association between the shrub species. The two species probably compete for above-ground space. However, diameter and height growth rates of Scotch broom far exceed those of matagouri so Scotch broom is likely to increase in biomass rapidly at the site. The autogenic organization and disturbance history of the resident plant communities have rendered each vulnerable to successive invasions.     

Tree invasion of a montane meadow complex: temporal trends,spatial patterns,and biotic interactions

Questions: Do spatial and temporal patterns of encroachment of Pinus contorta and Abies grandis in a montane meadow suggest strong biotic controls on the invasion process? Location: Forest–meadow mosaic, 1350 m a.s.l., Cascade Range, Oregon, US. Methods: We combined spatial point pattern analysis, population age structures, and a time‐series of stem maps to quantify spatial and temporal patterns of conifer invasion over a 200‐yr period in three plots totaling 4 ha. Results: Trees established during two broad, but distinct periods (late 1800s, then at much greater density in the mid‐1900s). Recent invasion was not correlated with climatic variation. Abies grandis dominated both periods; P. contorta established at lower density, peaking before A. grandis. Spatially, older (≥90 yr) P. contorta were randomly distributed, but older A. grandis were strongly clumped (0.2‐20 m). Younger (<90 yr) stems were positively associated at small distances (both within and between species), but were spatially displaced from older A. grandis, suggesting a temporal shift from facilitation to competition. Establishment during the 1800s resulted in widely scattered P. contorta and clumps of A. grandis that placed most areas of meadow close to seed sources permitting more rapid invasion during the mid‐1900s. Rapid conversion to forest occurred via colonization of larger meadow openings – first by shade‐intolerant P. contorta, then by shade‐tolerant A. grandis– and by direct infilling of smaller openings by A. grandis. Conclusions: In combination, spatial and temporal patterns of establishment suggest an invasion process shaped by biotic interactions, with facilitation promoting expansion of trees into meadows and competition influencing subsequent forest development. Once invasion is initiated, tree species with different life histories and functional traits can interact synergistically to promote rapid conversion of meadow to forest under a broad range of climatic conditions.     

Seed rain and environmental controls on invasion of <Emphasis Type="Italic">Picea abies</Emphasis> into grassland

Although changes in land-use, climate, and the spread of introduced tree species have increased the global importance of tree invasions into grasslands, our ability to predict any particular invasion is limited. To elucidate mechanisms driving tree invasions of grasslands, we studied in detail how seed dispersal and fine-scale environment control the expansion of an introduced Picea abies Karst. (Norway spruce) population into Western Carpathian grassland. We mapped invading trees and measured tree size, fecundity, seed rain, seedling density, plant community composition, and light and soil environment within a 200 × 60 m belt across the invasion front. Maximum likelihood estimates of dispersal kernels suggested peak seed deposition directly underneath tree crowns where germination was poor, but mean dispersal distances were sufficiently large to generate overlapping seed shadows from multiple trees that saturated the invasion front with seeds further away from seed-dispersing trees. Partial Mantel tests indicated that germinant density was affected considerably less by seed rain than by moss cover (r = 0.54), overstory tree influence (r = −0.32), soil moisture (r = 0.21), grass cover (r = −0.15), and diffuse radiation (r = 0.13). However, these variables were not independent but formed complex multivariate gradients within the invasion front. Moss cover and soil moisture were negatively correlated with overstory tree influence and the resulting gradient clearly affected germinant density (partial Mantel r = 0.45). In contrast, positively correlated light and grass cover defined a gradient related weakly to germinant density (partial Mantel r = 0.05) as it integrated opposing effects of these variables on germinants. Seedlings had similar environmental associations, but except for the lasting positive effects of moss these tended to weaken with seedling size. Although a few seedlings may establish and survive in the more adverse environment of the outer edges of the invasion front, a significant population expansion may require a gradual build-up of the critical density of invading trees to reduce grass cover and facilitate germination on moist mossy seedbeds within uncolonized areas. Thus, Picea abies appears more likely to spread within temperate grasslands by gradual expansion of its population frontier rather than by advanced groups.     

<Emphasis Type="Italic">Pinus contorta</Emphasis> invasion in the Chilean Patagonia: local patterns in a global context

Alien conifer invasions are affecting ecosystems across the globe, but until recently, reports of such invasions in South America were scarce. Pinus contorta was first established in Chilean Patagonia for erosion control caused by historical fires and cattle farming. Recently, the species has been planted over large areas for commercial purposes. It is well adapted to local conditions and is now spreading into natural areas. This study analyzes natural regeneration of Pinus contorta around Coyhaique city, Chile, to determine the spatial patterns of invasion. Five study sites were selected, four with grasslands dominated by exotic species and one site in the steppe. In each site, the plantation (seed source) was characterized using morphological attributes and density. Regeneration, density, height and age at different distances from the seed source were recorded, and ground cover was measured as an environmental factor influencing the invasion process. A comparative analysis was also conducted between the situation in Chile and other countries affected by P. contorta invasion. In Chile, P. contorta regeneration is significantly influenced by distance from the seed source. Higher densities are found close to the parent stand (up to 13,222 trees ha⁻¹), decreasing as distance from the seed source increases. Age and height structure indicate that the invasion process is at an early stage, and this offers a unique opportunity to study the process of invasion and to monitor it over time. In order to preserve the distinctiveness of Patagonian ecosystem, decisive action is required to control invasive conifers, with P. contorta as the number one priority. Relative to control, there is much that can be learnt from the experiences of other countries, such as New Zealand.     

Tree invasion into a mountain-top meadow in the Oregon Coast Range,USA

Abstract. The grassy meadow on the top of Marys Peak, in the Oregon Coast Range, is being invaded, primarily along the margins, by Abies procera. To examine vegetation and environmental changes across the forest-meadow transition and to evaluate factors affecting tree invasion, belt transects were established at 10 sites. Correlations of environmental variables with ordination axes from Detrended Correspondence Analysis suggest that the sites were distributed along a moisture gradient. Although the sites varied considerably in species composition and environment, tree invasion of the meadow was occurring at all sites. Reduction in abundance of the dense herbaceous vegetation of the meadow is required before the small seedlings of Abies procera can become established. Almost all tree invasion of the meadow occurs in the narrow forest-meadow ecocline because trees at the edge of the forest reduce the cover of herbaceous plants in the adjacent meadow. On some sites, above average snow accumulations further reduce vigor of meadow vegetation and increase tree establishment. On the driest sites wet summers facilitate tree establishment. Infrequent fires, which remove the fire sensitive Abies procera, are probably required to reverse the slow but persistent trend of forest expansion and ultimately maintain the meadow.     

Pine invasion decreases density and changes native tree communities in woodland Cerrado

ABSTRACT

Background: Invasive plants can negatively impact native communities, but the majority of the effects of these invasions have been demonstrated only for temperate ecosystems. Tropical ecosystems, including the Cerrado, a biodiversity hotspot, are known to be invaded by numerous non-native species, but studies of their impacts are largely lacking.

Aims: Our research aimed at quantifying how Pinus spp. presence and density affected Cerrado plant communities.

Methods: We sampled areas invaded and non-invaded by Pinus spp. to determine if pine invasion affected native tree richness, diversity, evenness, and density. We also evaluated if community composition differed between invaded and non-invaded sites.

Results: We found invaded plots had lower native tree densities than non-invaded plots and that Pinus spp. invasions changed native tree communities by reducing native species abundances.

Conclusion: Invasive pines had negative impacts on the native Cerrado tree community by reducing native plant density and changing species abundances. Reduced density and abundance at early invasion stages can result in reduction in biodiversity in the long term.     

Araucaria Forest conservation: mechanisms providing resistance to invasion by exotic timber trees

Since only 12.6% of the Brazilian Araucaria Forest remains and timber tree monocultures are expanding, biological invasion is a potential threat to the conservation of natural forest remnants. Here, we test (1) the susceptibility of Araucaria Forest to invasion by Pinus taeda and Eucalyptus saligna, (2) the efficiency of different mechanisms controlling the early establishment of these two exotic timber tree species, and (3) the potential of the native timber tree Araucaria angustifolia to establish successfully in ecologically-managed monocultures of Araucaria, Pinus and Eucalyptus. In Araucaria Forest, more than a thousand Pinus seeds landed annually in a hectare; however, experimentally exposed seeds were 100% removed in only 6 days. Furthermore, all experimentally transplanted seedlings of Pinus taeda and Eucalyptus saligna died in less than a year in Araucaria Forest, but not in the monocultures. Correlative evidence suggests that this mortality was associated to plant community richness, plant abundance, and soil fertility. Araucaria angustifolia, in contrast, showed an establishment success in ecologically-managed tree monocultures as high as that exhibited in its natural habitat. The current resistance of Araucaria Forest to invasion by exotic timber trees is good news for the conservation of Araucaria Forest remnants and for its keystone species. The understanding of the mechanisms providing such resistance against invasion points towards management tools for minimizing future threats.     

Twenty‐five years of plant community dynamics and invasion in New Zealand tussock grasslands

Understanding how plant communities respond to plant invasions is important both for understanding community structure and for predicting future ecosystem change. In a system undergoing intense plant invasion for 25 years, we investigated patterns of community change at a regional scale. Specifically, we sought to quantify how tussock grassland plant community structure had changed and whether changes were related to increases in plant invasion. Frequency data for all vascular plants were recorded on 124, permanent transects in tussock grasslands across the lower eastern South Island of New Zealand measured three times over a period of 25 years. Multivariate analyses of species richness were used to describe spatial and temporal patterns in the vegetation. Linear mixed‐effects models were used to relate temporal changes in community structure to the level and rate of invasion of three dominant invasive species in the genus Hieracium while accounting for relationships with other biotic and abiotic variables. There was a strong compositional gradient from exotic‐ to native‐dominated plant communities that correlated with increasing elevation. Over the 25 years, small‐scale species richness significantly decreased and then increased again; however, these changes differed in different plant communities. Exotic species frequency consistently increased on some transects and consistently declined on others. Species richness changes were correlated with the level of Hieracium invasion and abiotic factors, although the relationship with Hieracium changed from negative to positive over time. Compositional changes were not related to measured predictors. Our results suggest that observed broad‐scale fluctuations in species richness and community composition dynamics were not driven by Hieracium invasion. Given the relatively minor changes in community composition over time, we conclude that there is no evidence for widespread degradation of these grasslands over the last 25 years. However, because of continuing weed invasion, particularly at lower elevations, impacts may emerge in the longer term.     

Number of source populations as a potential driver of pine invasions in Brazil

To understand current patterns of Pinus invasion in an Araucaria forest in southern Brazil, we quantified invasion at the local scale and compared it with habitat characteristics, propagule size, and number of source populations, using generalized linear models. We also compared observed and expected invasive species status based on a previously developed model (Z scores) using Chi square and correlation tests to evaluate the predictability of species status based on their traits. Of the 16 Pinus species currently present in the site, three are invasive (P. elliottii, P. glabra, and P. taeda), three are naturalized (P. clausa, P. oocarpa, and P. pseudostrobus), and ten are present only as the originally planted individuals. While P. taeda spread the farthest, P. glabra had greater overall density, but none of the invasive species has spread more than 250 m in 45 years. Invasive Pinus plants were found where forest tree density was below 805 trees ha^?1, and invasive Pinus density decreased log-linearly with an increase in native tree density. Number of individuals introduced and number of source populations were strong predictors of naturalization, thus both propagule size and propagule diversity can potentially be driving invasion success. Z scores based on species traits did not predict which species would invade in Rio Negro. Our findings suggest that Araucaria forests might not resist invasion by Pinus as recently suggested and support the hypothesis that propagule pressure is a fundamental driver of invasions with propagule diversity being a possible component of this mechanism.     

Canopy and age structures of some old sub-boreal Picea stands in British Columbia

Abstract. 14 old, unlogged, Picea-dominated stands in the moist cool Sub-Boreal Spruce biogeoclimatic subzone of central British Columbia, Canada, were sampled to describe canopy heterogeneity, regeneration patterns and tree population age structures. These stands are composed of Picea engelmannii × glauca hybrids, Abies lasiocarpa and lesser amounts of Pinus contorta and Populus tremuloides, and had survived 124–343 yr since the last stand-destroying wildfire. Canopy cover was patchy and highly variable (ranging from 30.5 % to 86.4 %) but was not significantly related to stand age. Vertical canopy structure was less variable, reflecting the shade-tolerance and live crown ratios (length of live canopy expressed relative to tree height) of component species: 18.8 % for Populus, 20.2 % for Pinus, 46.7 % for Picea and 51.4 % for Abies. Individual stands varied considerably in their population structures and in their stand development trajectories, yet some patterns are evident. Survivors of the initial post-disturbance cohort of trees took 51 to 118 yr (mean = 80, s.d. = 20) to establish. Some stands had all tree species present during stand initiation, while other stands indicated early successional roles for Populus and Pinus, or a late successional role for Abies. Abies recruitment, while often slow in the beginning, occurs uniformly throughout the history of most stands, reflecting the high shade-tolerance of this species. Picea is often recruited in high densities early in stand development, and then (after long periods of exclusion) may be displaced by Abies in some stands but maintains itself in others. Minor, single-tree disturbances (due to bark beetles, root rot, and windthrow) were important in accelerating the reinitiation of Picea in the understory. Results thus suggest that stands from this region can be self-perpetuating in the absence of fire. Yet, post-fire tree populations still clearly dominate these spruce-fir forests, for only the oldest stand had greater basal area in the replacement cohort than in the initial cohort.     

青藏高原东缘林线杜鹃-岷江冷杉原始林的空间格局

为深入理解高山林线原始林中优势树种的空间结构特征和种间关系,以青藏高原东缘林线杜鹃-岷江冷杉原始林1 hm²样地调查数据为基础,采用成对相关函数g(r)函数,分析了优势种岷江冷杉(Abies faxoniana)和凝毛杜鹃(Rhododendron agglutinatum)各径级的空间分布格局和各径级间的空间关联性。结果表明:岷江冷杉和凝毛杜鹃径级分布连续,呈倒"J"型,均为进展种群。小尺度上,对于岷江冷杉活立木(小树+中树+大树)、小树、中树为显著聚集分布,大树为均匀分布,死树则为随机分布;凝毛杜鹃活立木(小树+中树+大树)、小树、中树、大树、死树均为显著聚集分布。小尺度上随径级增大岷江冷杉的分布格局从聚集分布趋于均匀分布。岷江冷杉大树与其小树的空间关联验证了Janzen-Connell假说中的距离制约效应。通过验证接受了随机死亡假说,发现种内的密度制约效应不明显。两个树种的枯立木对凝毛杜鹃小树均表现为排斥关系,而对岷江冷杉小树表现为相互独立的关系。两个优势种总体表现为相互独立,但种间不同径级间的空间关联性各异。     

Succession after stand replacing disturbances by fire,wind throw,and insects in the dark Taiga of Central Siberia

The dark taiga of Siberia is a boreal vegetation dominated by Picea obovata, Abies sibirica, and Pinus sibirica during the late succession. This paper investigates the population and age structure of 18 stands representing different stages after fire, wind throw, and insect damage. To our knowledge, this is the first time that the forest dynamics of the Siberian dark taiga is described quantitatively in terms of succession, and age after disturbance, stand density, and basal area. The basis for the curve–linear age/diameter relation of trees is being analyzed. (1) After a stand-replacing fire Betula dominates (4,000 trees) for about 70 years. Although tree density of Betula decreases rapidly, basal area (BA) reached >30 m²/ha after 40 years. (2) After fire, Abies, Picea, and Pinus establish at the same time as Betula, but grow slower, continue to gain height and eventually replace Betula. Abies has the highest seedling number (about 1,000 trees/ha) and the highest mortality. Picea establishes with 100–400 trees/ha, it has less mortality, but reached the highest age (>350 years, DBH 51 cm). Picea is the most important indicator for successional age after disturbance. Pinus sibirica is an accompanying species. The widely distributed “mixed boreal forest” is a stage about 120 years after fire reaching a BA of >40 m²/ha. (3) Wind throw and insect damage occur in old conifer stands. Betula does not establish. Abies initially dominates (2,000–6,000 trees/ha), but Picea becomes dominant after 150–200 years since Abies is shorter lived. (4) Without disturbance the forest develops into a pure coniferous canopy (BA 40–50 m²/ha) with a self-regenerating density of 1,000 coniferous canopy trees/ha. There is no collapse of old-growth stands. The dark taiga may serve as an example in which a limited set to tree species may gain dominance under certain disturbance conditions without ever getting monotypic.     

Invasion of woody species into temperate grasslands: Relationship with abiotic and biotic soil resource heterogeneity

Question: Invasion of woody species into grasslands is a global phenomenon. This is also topical in semi‐natural temperate grasslands that are no longer profitable for agricultural management. Trees and grasses interact through harsh root competition, but below‐ground processes have been neglected in the dynamics of semi‐natural grasslands. Trees are thought to have a competitive advantage in resource‐rich and heterogeneous soils. We tested whether soil resource quantity and heterogeneity differ between paired temperate semi‐natural grasslands and forests (former grasslands), and whether this was caused abiotically by varying soil depth or biotically by fine roots. Location: Thin‐soil calcareous alvar grasslands with overgrown parts (young Pinus sylvestris forests) in W. Estonia. Methods: The quantity and spatial heterogeneity of soil resources (moisture and nutrients), soil depth, and root parameters (mass, length and specific length) were measured in 1‐m transects of 11 samples in 26 paired grasslands and forests. The quantity and heterogeneity of soil resources were compared between vegetation types and related to soil depth and root parameters. Results: Soil resources were lower and more heterogeneous in forests than in grasslands. The invasion of woody species was enhanced abiotically by deeper soil. Root mass was larger in the forests, but root length was longer in the grasslands. Both root mass and specific root length were more heterogeneous in the forests. Forest root length was negatively correlated with transient soil moisture patches and positively correlated with more persistent nutrient‐rich patches. No such relationship was found in grasslands. Conclusions: Abiotic soil heterogeneity (local deep‐soil patches) supports woody species invasion, but the trees themselves also biotically make soils more heterogeneous, which further enhances woody species invasion. Large trees use soil resources patchily, making soils biotically poorer and more heterogeneous in resources. The dynamics of temperate semi‐natural grasslands are strongly linked to below‐ground ecological processes, and high soil heterogeneity can be both the cause and the outcome of woody species invasion.