Browsing‐mediated shrub canopy changes drive composition and species richness in forest‐tundra ecosystems

Changes in climate and in browsing pressure are expected to alter the abundance of tundra shrubs thereby influencing the composition and species richness of plant communities. We investigated the associations between browsing, tundra shrub canopies and their understory vegetation by utilizing a long‐term (10–13 seasons) experiment controlling reindeer and ptarmigan herbivory in the subarctic forest tundra ecotone in northwestern Fennoscandia. In this area, there has also been a consistent increase in the yearly thermal sum and precipitation during the study period. The cover of shrubs increased 2.8–7.8 fold in exclosures and these contrasted with browsed control areas creating a sharp gradient of canopy cover of tundra shrubs across a variety of vegetation types. Browsing exclusions caused significant shifts in more productive vegetation types, whereas little or no shift occurred in low‐productive tundra communities. The increased deciduous shrub cover was associated with significant losses of understory plant species and shifts in functional composition, the latter being clearest in the most productive plant community types. The total cover of understory vegetation decreased along with increasing shrub cover, while the cover of litter showed the opposite response. The cover of cryptogams decreased along with increasing shrub cover, while the cover of forbs was favoured by a shrub cover. Increasing shrub cover decreased species richness of understory vegetation, which was mainly due to the decrease in the cryptogam species. The effects were consistent across different types of forest tundra vegetation indicating that shrub increase may have broad impacts on arctic vegetation diversity. Deciduous shrub cover is strongly regulated by reindeer browsing pressure and altered browsing pressure may result in a profound shrub expansion over the next one or two decades. Results suggest that the impact of an increase in shrubs on tundra plant richness is strong and browsing pressure effectively counteracts the effects of climate warming‐driven shrub expansion and hence maintains species richness.     

Understory vegetation response to thinning disturbance of varying complexity in coniferous stands

Question: Can augmented forest stand complexity increase understory vegetation richness and cover and accelerate the development of late‐successional features? Does within‐stand understory vegetation variability increase after imposing treatments that increase stand structural complexity of the overstory? What is the relative contribution of individual stand structural components (i.e. forest matrix, gaps, and leave island reserves) to changes in understory vegetation richness? Location: Seven study sites in the Coastal Range and Cascades regions of Oregon, USA. Methods: We examined the effects of thinning six years after harvest on understory plant vascular richness and cover in 40‐ to 60‐year‐old forest stands dominated by Douglas‐fir (Pseudotsuga menziesii). At each site, one unthinned control was preserved and three thinning treatments were implemented: low complexity (LC, 300 trees ha^?1), moderate complexity (MC, 200 trees ha^?1), and high complexity (HC, variable densities from 100 to 300 trees ha^?1). Gaps openings and leave island reserves were established in MC and HC. Results: Richness of all herbs, forest herbs, early seral herbs and shrubs, and introduced species increased in all thinning treatments, although early seral herbs and introduced species remained a small component. Only cover of early seral herbs and shrubs increased in all thinning treatments whereas forest shrub cover increased in MC and HC. In the understory, we found 284 vascular plant species. After accounting for site‐level differences, the richness of understory communities in thinned stands differed from those in control stands. Within‐treatment variability of herb and shrub richness was reduced by thinning. Matrix areas and gap openings in thinned treatments appeared to contribute to the recruitment of early seral herbs and shrubs. Conclusions: Understory vegetation richness increased 6 years after imposing treatments, with increasing stand complexity mainly because of the recruitment of early seral and forest herbs, and both low and tall shrubs. Changes in stand density did not likely lead to competitive species exclusion. The abundance of potentially invasive introduced species was much lower compared to other plant groups. Post‐thinning reductions in within‐treatment variability was caused by greater abundance of early seral herbs and shrubs in thinned stands compared with the control. Gaps and low‐density forest matrix areas created as part of spatially variably thinning had greater overall species richness. Increased overstory variability encouraged development of multiple layers of understory vegetation.     

The effect of fire interval on post‐fire understorey communities in Yellowstone National Park

Questions: How does the time interval between subsequent stand‐replacing fire events affect post‐fire understorey cover and composition following the recent event? How important is fire interval relative to broad‐ or local‐scale environmental variability in structuring post‐fire understorey communities? Location: Subalpine plateaus of Yellowstone National Park (USA) that burned in 1988. Methods: In 2000, we sampled understorey cover and Pinus contorta density in pairs of 12–yr old stands at 25 locations. In each pair, the previous fire interval was either short (7–100 yr) or long (100–395 yr). We analysed variation in understorey species richness, total cover, and cover of functional groups both between site pairs (using paired t‐tests) and across sites that experienced the short fire intervals (using regression and ordination). We regressed three principal components to assess the relative importance of disturbance and broad or local environmental variability on post‐fire understorey cover and richness. Results: Between paired plots, annuals were less abundant and fire‐intolerant species (mostly slow‐growing shrubs) were more abundant following long intervals between prior fires. However, mean total cover and richness did not vary between paired interval classes. Across a gradient of fire intervals ranging from 7–100 yr, total cover, species richness, and the cover of annuals and nitrogen‐fixing species all declined while the abundance of shrubs and fire‐intolerant species increased. The few exotics showed no response to fire interval. Across all sites, broad‐scale variability related to elevation influenced total cover and richness more than fire interval. Conclusions: Significant variation in fire intervals had only minor effects on post‐fire understorey communities following the 1988 fires in Yellowstone National Park.     

Within‐stand spatial structure and relation of boreal canopy and understorey vegetation

Question: How do spatial patterns and associations of canopy and understorey vegetation vary with spatial scale along a gradient of canopy composition in boreal mixed‐wood forests, from younger Aspen stands dominated by Populus tremuloides and P. balsamifera to older Mixed and Conifer stands dominated by Picea glauca? Do canopy evergreen conifers and broad‐leaved deciduous trees differ in their spatial relationships with understorey vegetation? Location: EMEND experimental site, Alberta, Canada. Methods: Canopy and understorey vegetation were sampled in 28 transects of 100 contiguous 0.5 m × 0.5 m quadrats in three forest stand types. Vegetation spatial patterns and relationships were analysed using wavelets. Results: Boreal mixed‐wood canopy and understorey vegetation are patchily distributed at a range of small spatial scales. The scale of canopy and understorey spatial patterns generally increased with increasing conifer presence in the canopy. Associations between canopy and understorey were highly variable among stand types, transects and spatial scales. Understorey vascular plant cover was generally positively associated with canopy deciduous tree cover and negatively associated with canopy conifer tree cover at spatial scales from 5–15 m. Understorey non‐vascular plant cover and community composition were more variable in their relationships with canopy cover, showing both positive and negative associations at a range of spatial scales. Conclusions: The spatial structure and relation of boreal mixed‐wood canopy and understorey vegetation varied with spatial scale. Differences in understorey spatial structure among stand types were consistent with a nucleation model of patch dynamics during succession in boreal mixed‐wood forests.     

Shrubs as ecosystem engineers in a coastal dune: influences on plant populations,communities and ecosystems

Question: How do two shrubs with contrasting life‐history characteristics influence abundance of dominant plant taxa, species richness and aboveground biomass of grasses and forbs, litter accumulation, nitrogen pools and mineralization rates? How are these shrubs – and thus their effects on populations, communities and ecosystems – distributed spatially across the landscape? Location: Coastal hind‐dune system, Bodega Head, northern California. Methods: In each of 4 years, we compared vegetation, leaf litter and soil nitrogen under canopies of two native shrubs –Ericameria ericoides and the nitrogen‐fixing Lupinus chamissonis– with those in adjacent open dunes. Results: At the population level, density and cover of the native forb Claytonia perfoliata and the exotic grass Bromus diandrus were higher under shrubs than in shrub‐free areas, whereas they were lower under shrubs for the exotic grass Vulpia bromoides. In contrast, cover of three native moss species was highest under Ericameria and equally low under Lupinus and shrub‐free areas. At community level, species richness and aboveground biomass of herbaceous dicots was lower beneath shrubs, whereas no pattern emerged for grasses. At ecosystem level, areas beneath shrubs accumulated more leaf litter and had larger pools of soil ammonium and nitrate. Rates of nitrate mineralization were higher under Lupinus, followed by Ericameria and then open dune. At landscape level, the two shrubs – and their distinctive vegetation and soils – frequently had uniform spatial distributions, and the distance separating neighbouring shrubs increased as their combined sizes increased. Conclusions: Collectively, these data suggest that both shrubs serve as ecosystem engineers in this coastal dune, having influences at multiple levels of biological organization. Our data also suggest that intraspecific competition influenced the spatial distributions of these shrubs and thus altered the distribution of their effects throughout the landscape.     

Increase in nonnative understorey vegetation cover after nonnative conifer removal and passive restoration

Nonnative conifers are widespread in the southern hemisphere, where their use as plantation species has led to adverse ecosystem impacts sometimes intensified by invasion. Mechanical removal is a common strategy used to reduce or eliminate the negative impacts of nonnative conifers, and encourage native regeneration. However, a variety of factors may preclude active ecological restoration following removal. As a result, passive restoration – unassisted natural vegetation regeneration – is common following conifer removal. We asked, ‘what is the response of understorey cover to removal of nonnative conifer stands followed by passive restoration?' We sampled understorey cover in three site types: two‐ to ten‐year‐old clearcuts, native forest and current plantations. We then grouped understorey species by origin (native/nonnative) and growth form, and compared proportion and per cent cover of these groups as well as of bare ground and litter between the three site types. For clearcuts, we also analysed the effect of time since clearcut on the studied variables. We found that clearcuts had a significantly higher average proportion of nonnative understorey vegetation cover than native forest sites, where nonnative vegetation was nearly absent. The understorey of clearcut sites also averaged more overall vegetation cover and more nonnative vegetation cover (in particular nonnative shrubs and herbaceous species) than either plantation or native forest sites. Notably, 99% of nonnative shrub cover in clearcuts was the invasive nonnative species Scotch broom (Cytisus scoparius). After ten years of passive recovery since clearcutting, the proportion of understorey vegetation cover that is native has not increased and remains far below the proportion observed in native forest sites. Reduced natural regeneration capacity of the native ecosystem, presence of invasive species in the surrounding landscape and land‐use legacies from plantation forestry may inhibit native vegetation recovery and benefit opportunistic invasives, limiting the effectiveness of passive restoration in this context. Abstract in Spanish is available with online material.     

Does extreme flooding drive vegetation and faunal composition across the Gulf Plains of north‐eastern Australia?

High levels of sub‐speciation in Australian mesic zone taxa have been attributed to the creation of biogeographic barriers by Pleistocene expansion of the arid zone. However, several of these barriers also align with major floodplains. The Carpentarian Barrier in the Gulf Plains (GUP) – one of Australia's most significant biogeographic barriers – experiences extreme floods on a sub‐decadal timescale. These floods rise suddenly, cover thousands of square kilometres to a depth of several metres and can take weeks to subside. We investigated whether these floods have shaped community composition. If this is the case, species that are particularly vulnerable to extreme flooding – understorey animals and woody plants, particularly shrubs – should be under‐represented, and grasslands should be over‐represented on GUP floodplains. We used Akaike selection of logistic models to assess influence of floodplains and other potential drivers on grassland distribution and shrub abundance, and on representation of understorey fauna. We also compared post‐flood faunal records with the bioregional data set to assess influence of the 2009 flood on representation of understorey reptiles. Grasslands and shrub‐free vegetation were significantly over‐represented and understorey fauna significantly under‐represented on floodplains, even when the influences of other factors were taken into account. Understorey Gekkota were absent from – and understorey skinks under‐represented in – recently flooded areas. Hence, floods appear to have shaped community composition on GUP floodplains by selectively displacing and/or destroying woody plants – particularly shrubs – and understorey animals. Our findings demonstrate association rather than causality, but show that further examination of the ecological and biogeographic impacts of extreme flooding is warranted. Influence of floods on the Australian biota should be considered an ongoing ecological and evolutionary driver, and one that is likely to intensify as extreme floods are expected to become more frequent under climate change.     

Response of plant species and life form diversity to variable fire histories and biomass in the jarrah forest of south‐west Australia

Frequent fires reduce the abundance of woody plant species and favour herbaceous species. Plant species richness also tends to increase with decreasing vegetation biomass and cover due to reduced competition for light. We assessed the influence of variable fire histories and site biomass on the following diversity measures: woody and herbaceous species richness, overall species richness and evenness, and life form evenness (i.e. the relative abundance or dominance among six herbaceous and six woody plant life forms), across 16 mixed jarrah (Eucalyptus marginata) and marri (Corymbia calophylla) forest stands in south‐west Australia. Fire frequency was defined as the total number of fires over a 30‐year period. Overall species richness and species evenness did not vary with fire frequency or biomass. However, there were more herbaceous species (particularly rushes, geophytes and herbs) where there were fewer shrubs and low biomass, suggesting that more herbaceous species coexist where dominance by shrubs is low. Frequently burnt plots also had lower number and abundance of shrub species. Life form evenness was also higher at both high fire frequency and low biomass sites. These results suggest that the impact of fire frequency and biomass on vegetation composition is mediated by local interactions among different life forms rather than among individual species. Our results demonstrate that measuring the variation in the relative diversity of different woody and herbaceous life forms is crucial to understanding the compositional response of forests and other structurally complex vegetation communities to changes in disturbance regime such as increased fire frequency.     

Arctic arthropod assemblages in habitats of differing shrub dominance

Recent climate warming in the Arctic has caused advancement in the timing of snowmelt and expansion of shrubs into open tundra. Such an altered climate may directly and indirectly (via effects on vegetation) affect arctic arthropod abundance, diversity and assemblage taxonomic composition. To allow better predictions about how climate changes may affect these organisms, we compared arthropod assemblages between open and shrub‐dominated tundra at three field sites in northern Alaska that encompass a range of shrub communities. Over ten weeks of sampling in 2011, pitfall traps captured significantly more arthropods in shrub plots than open tundra plots at two of the three sites. Furthermore, taxonomic richness and diversity were significantly greater in shrub plots than open tundra plots, although this pattern was site‐specific as well. Patterns of abundance within the five most abundant arthropod orders differed, with spiders (Order: Araneae) more abundant in open tundra habitats and true bugs (Order: Hemiptera), flies (Order: Diptera), and wasps and bees (Order: Hymenoptera) more abundant in shrub‐dominated habitats. Few strong relationships were found between vegetation and environmental variables and arthropod abundance; however, lichen cover seemed to be important for the overall abundance of arthropods. Some arthropod orders showed significant relationships with other vegetation variables, including maximum shrub height (Coleoptera) and foliar canopy cover (Diptera). As climate warming continues over the coming decades, and with further shrub expansion likely to occur, changes in arthropod abundance, richness, and diversity associated with shrub‐dominated habitat may have important ecological effects on arctic food webs since arthropods play important ecological roles in the tundra, including in decomposition and trophic interactions.     

Seed Bank and Understorey Species Composition in a Semi-arid Environment: The Effect of Shrub Age and Rainfall

Understorey vegetation in patches of Retama sphaerocarpa shrubsin semi-arid environments is dependent on the overstorey shrublife history. Community structure changes with shrub age asa result of physical amelioration of environmental conditionsby the canopy and organic matter accumulation in the soil. Weinvestigated the effect of the canopy on understorey speciesdiversity in the field and its relationships with the soil seedbank under 50 shrubs from 5 to 25+ years old, and compared speciescomposition in the field in a wet and a dry year. Species compositionof the soil seed bank under R. sphaerocarpa shrubs did not differsignificantly with shrub age, but seed density increased asthe shrubs aged. In the field, community composition changedwith shrub age, increasing species richness in a process thatdepended on the amount of spring rainfall. Our results suggestthat the soil seed bank is rather uniform and that the shrubcanopy strongly selects which species appear in the understorey.There were seeds of many species present under both young andold shrubs but which only established under old shrubs. Thisshowed dispersal was not limiting species abundance and suggestedthat the canopy was an important sorting factor for speciespresent in the understorey. Less frequent species contributedthe most to patch diversity, and rainfall effectively controlledspecies emergence. Understorey community composition dependedon multiple interspecific interactions, such as facilitationby the shrub and competition from neighbours, as well as ondispersal processes. Facilitation in this environment is a keyfeature in the structuring of plant communities and in governingecosystem functioning. Copyright 2000 Annals of Botany Company Community structure, competition, dispersal, facilitation, species composition, rainfall variability, Retama sphaerocarpa, seed bank, semi-arid environments     

Effects on bird abundance and species richness of edge restructuring to include shrubs at the interface between conifer plantations and moorland

Capsule?Bird species richness and (for most species) abundance were positively related to the extent of shrub cover at the interface between conifer plantations and moorland, but it appears that responses to shrub development vary between different bird guilds.

Aims?To assess the bird assemblages in both winter and breeding seasons at the interface between managed conifer plantations and open moorland, where that interface had been restructured to include a mosaic of shrubs and open ground.

Methods?Timed point counts were used to sample the birds at restructured plantation – moorland interface areas and also in neighbouring plantations (post- and pre-thicket age classes) and neighbouring moorland. Associations between species richness and abundances with measures of shrub cover and composition were assessed using GLMMs.

Results?A total of 60 bird species were recorded including 29 on lists of conservation concern, most of which were associated with shrub interface habitats. Species richness and, for most species, abundance were positively related to the extent of shrub cover. Positive relationships between shrub cover in interface areas and the abundance of some species in neighbouring plantations and open moorland suggested a resource subsidy to birds in neighbouring habitats. In contrast, some birds tended to be less abundant in plantations next to areas with more shrub cover. These species were more abundant in the shrub itself, suggesting redistribution by species with a preference for early successional shrub habitats.

Conclusions?The long-term management of shrub, especially with regard to successional development, is a challenging aspect of forest and landscape management that deserves further study.     

The importance of shrub cover for four cactus species differing in growth form in an Andean semi‐desert

Question: Factors influencing seedling establishment are known to vary between open sites and those protected by plant cover. In many desert regions, protected microhabitats below shrubs are essential for establishment of many cactus species. Very little is known about these factors for Andean cacti and how the importance of vegetation cover varies with cactus species. Are Andean cacti associated more frequently to vegetation cover than to open ground? Are they associated to certain shrub species? Is the distributional pattern in relation to cover similar for different cactus species? In what microhabitat (below or away from shrubs) are cactus seeds more abundant? These questions are addressed for the case of an Andean semi‐desert. Location: Semi‐arid tropical Andes, La Paz department, Bolivia. Methods: We examined 132 isolated shrubs = 50 cm along a line across two microhabitats: areas below and away from shrubs/trees. Shrub crown size was measured. The among‐shrub samples were taken from open spaces contiguous to each of the sampled shrubs. In both microhabitats, all cactus species were recorded. The cardinal direction of the cacti was also registered. Correlation between canopy diameter and number of beneficiaries was evaluated for Prosopis flexuosa. The cactus seed bank in each microhabitat was also studied. Results and Conclusions: The four cactus species found behaved differently in relation to shrub canopies. These distributional differences could be due to differences in growth form. Columnar cacti apparently need the shade of shrubs. Only the columnar species is able to grow near the base of the tallest nurse species. The opuntioid cacti studied seem more facultative: although apparently preferring shrub un‐der‐canopies, they are able to establish in open ground. The globose cactus is the most indifferent to the presence of plant cover. These patterns parallel others found in North America. The capacity of different cacti to appear in open spaces could be related to vegetative propagation, and not necessarily to seedling tolerance of heat.     

Vegetation shift from deciduous to evergreen dwarf shrubs in response to selective herbivory offsets carbon losses: evidence from 19 years of warming and simulated herbivory in the subarctic tundra

Selective herbivory of palatable plant species provides a competitive advantage for unpalatable plant species, which often have slow growth rates and produce slowly decomposable litter. We hypothesized that through a shift in the vegetation community from palatable, deciduous dwarf shrubs to unpalatable, evergreen dwarf shrubs, selective herbivory may counteract the increased shrub abundance that is otherwise found in tundra ecosystems, in turn interacting with the responses of ecosystem carbon (C) stocks and CO₂ balance to climatic warming. We tested this hypothesis in a 19‐year field experiment with factorial treatments of warming and simulated herbivory on the dominant deciduous dwarf shrub Vaccinium myrtillus. Warming was associated with a significantly increased vegetation abundance, with the strongest effect on deciduous dwarf shrubs, resulting in greater rates of both gross ecosystem production (GEP) and ecosystem respiration (ER) as well as increased C stocks. Simulated herbivory increased the abundance of evergreen dwarf shrubs, most importantly Empetrum nigrum ssp. hermaphroditum, which led to a recent shift in the dominant vegetation from deciduous to evergreen dwarf shrubs. Simulated herbivory caused no effect on GEP and ER or the total ecosystem C stocks, indicating that the vegetation shift counteracted the herbivore‐induced C loss from the system. A larger proportion of the total ecosystem C stock was found aboveground, rather than belowground, in plots treated with simulated herbivory. We conclude that by providing a competitive advantage to unpalatable plant species with slow growth rates and long life spans, selective herbivory may promote aboveground C stocks in a warming tundra ecosystem and, through this mechanism, counteract C losses that result from plant biomass consumption.     

Effects of experimental rainfall manipulations on Chihuahuan Desert grassland and shrubland plant communities

Aridland ecosystems are predicted to be responsive to both increases and decreases in precipitation. In addition, chronic droughts may contribute to encroachment of native C₃ shrubs into C₄-dominated grasslands. We conducted a long-term rainfall manipulation experiment in native grassland, shrubland and the grass–shrub ecotone in the northern Chihuahuan Desert, USA. We evaluated the effects of 5 years of experimental drought and 4 years of water addition on plant community structure and dynamics. We assessed the effects of altered rainfall regimes on the abundance of dominant species as well as on species richness and subdominant grasses, forbs and shrubs. Nonmetric multidimensional scaling and MANOVA were used to quantify changes in species composition in response to chronic addition or reduction of rainfall. We found that drought consistently and strongly decreased cover of Bouteloua eriopoda, the dominant C₄ grass in this system, whereas water addition slightly increased cover, with little variation between years. In contrast, neither chronic drought nor increased rainfall had consistent effects on the cover of Larrea tridentata, the dominant C₃ shrub. Species richness declined in shrub-dominated vegetation in response to drought whereas richness increased or was unaffected by water addition or drought in mixed- and grass-dominated vegetation. Cover of subdominant shrubs, grasses and forbs changed significantly over time, primarily in response to interannual rainfall variability more so than to our experimental rainfall treatments. Nevertheless, drought and water addition shifted the species composition of plant communities in all three vegetation types. Overall, we found that B. eriopoda responded strongly to drought and less so to irrigation, whereas L. tridentata showed limited response to either treatment. The strong decline in grass cover and the resistance of shrub cover to rainfall reduction suggest that chronic drought may be a key factor promoting shrub dominance during encroachment into desert grassland.     

Initial response of understorey vegetation to fire severity and salvage‐logging in the southern boreal forest of Québec

Abstract. In this study we compared the effects of fire on understorey vegetation in the Québec southern boreal forest with effects of salvage‐logging (clear‐cutting after fire). All 61 400‐m² sampling sites were controlled for overstorey composition (Deciduous, Mixed and Coniferous) and disturbance type, which consisted of three fire impact severity (FIS) classes (Light, Moderate and Extreme) and two harvesting techniques (Stem‐only and Whole‐tree Harvesting). Percent‐cover data of vegetation and post‐disturbance environmental characteristics were recorded in the field during the first two years after fire as well as soil texture. Ordination of fire alone demonstrated that, on Coniferous sites, fire initiates a succession whereby the understorey Coniferous sites approaches that of Deciduous‐Mixed sites, due to the release of the understorey from Sphagnum spp. dominance, this pattern being a function of FIS. On Deciduous‐Mixed stands, increased FIS resulted in a transition from herb to shrub dominance. Ordination of all five disturbance types showed that the impact of salvage‐logging on understorey composition was within the range of fire, but marginalized to the extreme end of the FIS spectrum. Variance partitioning demonstrated that overstorey and soil texture were the most important explanatory variables of fire alone, while disturbance type explained the largest independent fraction of understorey variation when salvage‐logging was introduced. Salvage‐logging also results in significant reductions in understorey abundance, richness and diversity, while indicator species analysis suggests that it favours mesoxerophytic to xeric species. Results are interpreted in light of shade‐tolerance dynamics, forest floor disturbance and soil moisture regimes. Implications for sustainable forest management are discussed.     

Exotic shrub invasion in an undisturbed wetland has little community-level effect over a 15-year period

In this long-term study, we examined the invasion by the exotic shrub glossy buckthorn (Rhamnus frangula L.) and the response of co-occurring plants in a large, undisturbed wetland. We first sampled the vegetation in 1991 and repeated the sample 15 years later using the same, permanently located sample units (n = 165). Despite dramatic increases in the abundance of buckthorn, the invasion elicited little apparent response by the resident plant community. Species richness and cover in the herbaceous plant stratum had no apparent relationship with change in buckthorn cover. The number of shrub species other than buckthorn showed no relationship with change in buckthorn cover, but the cover of other shrubs decreased as buckthorn cover increased. Species composition changed independently of changes in buckthorn cover. These results show that dramatic increases in the abundance of an invasive species do not necessarily cause large changes in the native plant community and suggest disturbance history influences community response to invasion.     

Interactive effects of grazing and shrubs on the annual plant community in semi‐arid Mediterranean shrublands

Question: We studied the interactive effects of grazing and dwarf shrub cover on the structure of a highly diverse annual plant community. Location: Mediterranean, semi‐arid shrubland in the Northern Negev desert, Israel. Methods: Variation in the biomass and plant density of annual species in the shrub and open patches was monitored during four years, inside and outside exclosures protected from sheep grazing, in two contrasting topographic sites: north and south‐facing slopes that differed in their dominant dwarf shrubs species: Sarcopoterium spinosus and Corydothymus capitatus, respectively. Results: Above‐ground biomass, density and richness of annual species were lower under the canopy of both shrub species compared to the adjacent open patches in the absence of grazing. Grazing reduced the biomass of annuals in open patches of both topographic sites, but not in the shrub patches. On the north‐facing slope, grazing also reduced plant density and richness in the open patches, but increased plant density in the shrub patches. At the species level, various response patterns to the combined effects of grazing and patch type were exhibited by different annuals. Protection against the direct impacts of grazing by shrub cover as well as species‐specific interactions between shrubs and annuals were observed. A conceptual mechanistic model explaining these interactions is proposed. Conclusion: In semi‐arid Mediterranean shrublands grazing and dwarf shrub cover interact in shaping the structure of the annual plant community through (1) direct impacts of grazing restricted to the open patches, (2) species‐specific facilitation/ interference occurring in the shrub patches and (3) subsequent further processes occurring among the interconnected shrub and open patches mediated through variation in seed flows between patches.     

Spatial association of Aspidosperma quebracho‐blanco juveniles with shrubs and conspecific adults in the Arid Chaco,Argentina

Abstract The survival of Aspidosperma quebracho‐blanco juveniles in the Arid Chaco is facilitated under the canopy of nurse plants. The possible effects of nurse plants were studied at intra‐ and interspecific levels by analysing the spatial distribution of juveniles and adults of A. quebracho‐blanco, of the main shrubs Larrea divaricata and Mimozyganthus carinatus, and of the group of deciduous and evergreen shrub species, and their pair associations. Data were analysed using the SADIE (Spatial Analysis by Distance Indices) software. A. quebracho‐blanco seedling abundance followed the distribution pattern of the main shade‐providing species: an aggregated spatial distribution pattern in most of the categories studied. The seedling bank of A. quebracho‐blanco also showed an aggregated pattern and was spatially associated with shrubs and adults of its own species. The intensity of the association depended on the functional types: deciduous Fabaceae, deciduous non‐Fabaceae, evergreen and conspecific adults, each of which provides a different canopy structure and therefore different amounts of shade. The spatial association was significant with the evergreen group, and less significant with the deciduous Fabaceae group. There was no positive association with deciduous non‐Fabaceae, or with gaps (open sky). The differences generated by canopy cover may influence the nurse effect, as observed in the intensity of association of A. quebracho‐blanco with shrubs and conspecific adults.     

Vegetation responses in Alaskan arctic tundra after 8 years of a summer warming and winter snow manipulation experiment

We used snow fences and small (1 m²) open‐topped fiberglass chambers (OTCs) to study the effects of changes in winter snow cover and summer air temperatures on arctic tundra. In 1994, two 60 m long, 2.8 m high snow fences, one in moist and the other in dry tundra, were erected at Toolik Lake, Alaska. OTCs paired with unwarmed plots, were placed along each experimental snow gradient and in control areas adjacent to the snowdrifts. After 8 years, the vegetation of the two sites, including that in control plots, had changed significantly. At both sites, the cover of shrubs, live vegetation, and litter, together with canopy height, had all increased, while lichen cover and diversity had decreased. At the moist site, bryophytes decreased in cover, while an increase in graminoids was almost entirely because of the response of the sedge Eriophorum vaginatum. These community changes were consistent with results found in studies of responses to warming and increased nutrient availability in the Arctic. However, during the time period of the experiment, summer temperature did not increase, but summer precipitation increased by 28%. The snow addition treatment affected species abundance, canopy height, and diversity, whereas the summer warming treatment had few measurable effects on vegetation. The interannual temperature fluctuation was considerably larger than the temperature increases within OTCs (<2°C), however. Snow addition also had a greater effect on microclimate by insulating vegetation from winter wind and temperature extremes, modifying winter soil temperatures, and increasing spring run‐off. Most increases in shrub cover and canopy height occurred in the medium snow‐depth zone (0.5–2 m) of the moist site, and the medium to deep snow‐depth zone (2–3 m) of the dry site. At the moist tundra site, deciduous shrubs, particularly Betula nana, increased in cover, while evergreen shrubs decreased. These differential responses were likely because of the larger production to biomass ratio in deciduous shrubs, combined with their more flexible growth response under changing environmental conditions. At the dry site, where deciduous shrubs were a minor part of the vegetation, evergreen shrubs increased in both cover and canopy height. These changes in abundance of functional groups are expected to affect most ecological processes, particularly the rate of litter decomposition, nutrient cycling, and both soil carbon and nitrogen pools. Also, changes in canopy structure, associated with increases in shrub abundance, are expected to alter the summer energy balance by increasing net radiation and evapotranspiration, thus altering soil moisture regimes.