Tall Shrubs Research Articles

Phytochemical screening and evaluation of biological activity of Calligonum polygonoides L. subsp. comosum

Article history: Received on: 24/07/2015 Revised on: 16/08/2015 Accepted on: 16/09/2015 Available online: 12/11/2015 Calligonum polygonoides L. subsp. comosum; locally known as “arta”, is tall woody shrub, perennial desert plant. TLC screening, estrogenic and antimicrobial activities of different fractions of hydroalcoholic extract of the leaves; n-hexane, CH2Cl2, EtOAc and n-BuOH, were studied in order to find the correlation between the phytoconstituents and the biological activity. Estrogenic activity was studied in immature ovariectomized female Wistar rats by oral administration of 75 and 150 mg extract/kg body weight for seven days using 1 μg estradiol/rat/day as positive control. The antimicrobial activity against Aspergillus fumigatus, Candida albicans, Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, Klebsiella pneumonia was carried out using agar diffusion method with determination of the minimum inhibitory concentration (MIC). The CH2Cl2 fraction showed significant estrogenic and antimicrobial activity by significantly increasing uterine weight and low MIC values for all tested microorganisms ranging from 0.03 and 3.9 mg/mL. The n-hexane fraction showed mainly the presence of sterols and/or triterpenoids. The EtOAc and n-BuOH fractions were the richest in flavonoids while that of methylene chloride was found to contain both classes of chemical compounds. In conclusion; flavonoids, sterols and/or triterpenes exhibit synergistic effect to the antimicrobial and estrogenic activity of Calligonum polygonoides leaves.

Patterns of shrub expansion in Alaskan arctic river corridors suggest phase transition.

Recent increases in deciduous shrub cover are a primary focus of terrestrial Arctic research. This study examined the historic spatial patterns of shrub expansion on the North Slope of Alaska to determine the potential for a phase transition from tundra to shrubland. We examined the historic variability of landscape-scale tall shrub expansion patterns on nine sites within river valleys in the Brooks Range and North Slope uplands (BRNS) between the 1950s and circa 2010 by calculating percent cover (PCTCOV), patch density (PADENS), patch size variability (CVSIZE), mean nearest neighbor distance (MEDIST) and the multi-scale information fractal dimension (dI) to assess spatial homogeneity for shrub cover. We also devised conceptual models for trends in these metrics before, during, and after a phase transition, and compared these to our results. By developing a regression equation between PCTCOV and dI and using universal critical dI values, we derived the PCTCOV required for a phase transition to occur. All nine sites exhibited increases in PCTCOV. Five of the nine sites exhibited an increase in PADENS, seven exhibited an increase in CVSIZE, and five exhibited a decrease in MEDIST. The dI values for each site exceeded the requirements necessary for a phase transition. Although fine-scale heterogeneity is still present, landscape-scale patterns suggest our study areas are either currently in a state of phase transition from tundra to shrubland or are progressing towards spatial homogeneity for shrubland. Our results indicate that the shrub tundra in the river valleys of the north slope of Alaska has reached a tipping point. If climate trends observed in recent decades continue, the shrub tundra will continue towards homogeneity with regard to the cover of tall shrubs.

Effect of grazing on the population of Matteuccia struthiopteris at the southern limit of its distribution in Europe

AbstractWe investigated the impact of deer on an isolated marginal population of Matteuccia struthiopteris and on its plant community in the northern Apennines, where in recent decades the species has decreased dramatically. Our experiment was based on a 6‐year before and after control impact (BACI) design, comparing plots with deer grazing and plots where deer were excluded. Exclusion of grazing interacted with years, increasing the size of existing plants and favoring production of sporophylls. The yearly sequence of increase and decline of ramets and sterile leaves followed the same pattern in the two treatments. This indicated that their annual variation in growth depended on the sequence of more and less favorable years. Few and short plants were associated with grazing, whereas tall plants, shrubs, and tree regeneration were associated with fenced plots. The few plants to persist and dominate on heavily grazed areas were Oxalis acetosella and Cardamine species, while the genus Rubus and tree saplings were grazing‐sensitive species in this forest. Tall herbaceous species increased in size in the fenced areas, however, M. struthiopteris drew more advantage in terms of growth of leaves, showing greater cover than other species. Thus, deer grazing is becoming a threat for the survival of M. struthiopteris in southern Europe where it is already threatened in the long term by climate change.

Measuring habitat heterogeneity reveals new insights into bird community composition.

Fine-scale vegetation cover is a common variable used to explain animal occurrence, but we know less about the effects of fine-scale vegetation heterogeneity. Theoretically, fine-scale vegetation heterogeneity is an important driver of biodiversity because it captures the range of resources available in a given area. In this study we investigated how bird species richness and birds grouped by various ecological traits responded to vegetation cover and heterogeneity. We found that both fine-scale vegetation cover (of tall trees, medium-sized trees and shrubs) and heterogeneity (of tall trees, and shrubs) were important predictors of bird richness, but the direction of the response of bird richness to shrub heterogeneity differed between sites with different proportions of tall tree cover. For example, bird richness increased with shrub heterogeneity in sites with high levels of tall tree cover, but declined in sites with low levels of tall tree cover. Our findings indicated that an increase in vegetation heterogeneity will not always result in an increase in resources and niches, and associated higher species richness. We also found birds grouped by traits responded in a predictable way to vegetation heterogeneity. For example, we found small birds benefited from increased shrub heterogeneity supporting the textual discontinuity hypothesis and non-arboreal (ground or shrub) nesting species were associated with high vegetation cover (low heterogeneity). Our results indicated that focusing solely on increasing vegetation cover (e.g. through restoration) may be detrimental to particular animal groups. Findings from this investigation can help guide habitat management for different functional groups of birds.

Cumulative Impacts and Feedbacks of a Gravel Road on Shrub Tundra Ecosystems in the Peel Plateau, Northwest Territories, Canada

Gravel highways in the continuous permafrost zone provide critical transportation links that are increasingly vulnerable to the impacts of climate warming and permafrost thaw. To examine if the physical effects associated with the construction, maintenance, and use of gravel roads alter vegetation and permafrost conditions, we measured vegetation, soils, and near-surface ground temperatures at tall and dwarf shrub tundra sites adjacent to and distant from the Dempster Highway in the Northwest Territories of Canada. We found that alder growth and recruitment were significantly enhanced adjacent to the highway. Where alder shrubs had formed closed canopies, we observed dramatic alterations to plant community composition, soil properties, and ground temperatures. Tall shrub sites adjacent to the road exhibited less understory vegetation, greater litter and organic layer thickness, higher nutrient availability, and thicker snowpack than all other site types. Our results show that in shrub tundra ecosystems the conditions generated by the maintenance and use of a gravel road can drive ecological feedbacks that magnify changes to vegetation communities and soils. We found that where the road facilitated shrub dominance, feedbacks were initiated that enhanced snow accumulation and altered ground temperatures and soil chemistry. In turn, these changes likely promoted enhanced shrub recruitment and growth. Shrub proliferation adjacent to highways is an important consideration for the planning and maintenance of this form of infrastructure. To improve our understanding of the spatial heterogeneity of shrub proliferation, research exploring the relationships between biophysical landscape features and shrub development is also needed.

Why Do Some Evergreen Species Keep Their Leaves for a Second Winter, While Others Lose Them?

In subtropical montane semi-moist forest in SW China (SMSF), a large majority of evergreen tree and tall shrub species was found to have only one cohort of old leaves in early spring. In contrast, almost all species of evergreen tree and tall shrub in warm temperate rain forest (WTRF) in Japan and sclerophylls in Mediterranean-climate forest (MSF) of the Mediterranean Basin have two or more cohorts of old leaves in early spring; they drop their oldest cohort during or soon after leaf outgrowth in spring. Japanese WTRF has no dry season and MSF a dry summer. SMSF has a dry winter. On four evergreen Rhododendron species from SW China with only one cohort of old leaves in spring when in cultivation in Scotland, the majority of leaves in the senescing cohort fell by the end of December. We hypothesize that with dry winters, there is an advantage to dropping older leaves in autumn, because there is a low chance of appreciable positive assimilation in winter and a high chance of desiccation, reducing the resorption of dry mass and mineral nutrients from ageing leaves. Our hypothesis may be extended to cover evergreens at high altitude or high latitude that experience cold soils in winter.

A new species of <i>Berkheya</i> (Asteraceae, Arctotideae) from the Northern Cape, South Africa

Background: Berkheya is a large, mainly southern African genus of approximately 75 species, several of which are poorly known and under-collected. Since revision in 1959, only a few new species have been described. Publication of new taxa facilitates conservation objectives and contributes to a better understanding of the southern African flora.Objectives: The objectives of this study were to describe a new species of Berkheya, consider its taxonomic position within the genus and assess its conservation status.Methods: Macromorphology and micromorphology of the new species were compared with known species.Results: Berkheya dumicola N.G.Bergh Helme was described from two subpopulations from the northern Bokkeveld escarpment, Northern Cape Province, South Africa. The species is a tall shrub with radiate flowerheads, toothed receptacle alveole margins, a uniseriate pappus of small, rounded scales and very short twin-hairs on the cypsela.Conclusion: Berkheya dumicola is a new species with a unique combination of features. Based on morphological characteristics, its closest relative within the genus is likely to be the recently describedBerkheya chrysanthemoides J.C.Manning Goldblatt. The limited geographic extent and small population size of B. dumicola warrant an International Union for Conservation of Nature (IUCN) status of ‘Endangered’.

Vegetation structure and composition determine snowshoe hare (Lepus americanus) activity at arctic tree line

Snowshoe hares (Lepus americanus Erxleben, 1777) are keystone herbivores supporting many boreal-forest predators. Understanding habitat use of hares can help predict how hares and their predators will be affected by disturbances such as climate change, which will have a particularly strong effect at tree line. We examined hare activity at arctic tree line near Churchill, Manitoba, using fecal pellet transects established in August 2012. We counted all hare fecal pellets at two plots per transect and measured plant abundance and vegetation structure surrounding the plots, then used model selection to determine the combinations of habitat features or plant species that best explained hare activity or presence. Hare pellets occurred at a higher density where tall shrub cover was high. Pellet density also increased with increasing canopy cover, tree size, tree density, visibility, and small shrub cover, but was negatively correlated with medium-sized shrub and herb cover. Hares preferred areas with willow (genus Salix L.) and avoided areas with unpalatable black spruce (Picea mariana (Mill.) Britton, Sterns & Poggenb.). With climate warming, tree line is expected to advance northward and the increased tall shrub and willow cover predicted with increasing temperatures should benefit hares by providing greater predator protection and food.

Tall Deciduous Shrubs Offset Delayed Start of Growing Season Through Rapid Leaf Development in the Alaskan Arctic Tundra

Increasing temperatures in arctic regions are causing earlier spring snowmelt, leading to earlier plant emergence, which could lengthen the period of carbon uptake. Warming is also leading to a shift from graminoid to deciduous shrub-dominated tundra, and in many areas deciduous shrubs are becoming taller. As taller shrubs become increasingly dominant, arctic landscapes may retain more snow, which could lengthen spring snow cover duration and offset advances in the start of the growing season that are expected as a result of earlier spring snowmelt. As a consequence, deeper snow and later snowmelt in taller shrub tundra could delay plant emergence and shorten the period of carbon uptake. This study tracked leaf development of two abundant deciduous shrubs, Betula nana and Salix pulchra, and compared individuals along a natural shrub height gradient on the North Slope of Alaska. We measured spring snow depth and snow cover duration, bud and developing leaf nitrogen content, as well as the timing of budburst and leaf expansion. Taller deciduous shrubs in shrub-dominated communities had deeper snow surrounding them, and became snow-free 1 to 6 days later, delaying budburst by 2 to 12 days relative to shorter deciduous shrubs in graminoid-dominated communities. However, leaf development of tall shrubs caught up to that of short shrubs; occasionally, tall shrubs reached full leaf expansion 1 to 4 days before short shrubs, indicating more rapid leaf development. This convergence in the timing of later leaf development stages is potentially enabled by approximately 16% to 25% greater nitrogen in buds and developing leaves of taller shrubs compared with shorter shrubs. Our findings suggest that delayed snowmelt in areas dominated by taller shrubs may have a short-lived impact on the timing of leaf development, likely resulting in no difference in duration of peak photosynthetic period between tall and short stature shrubs.

Warming-Induced Shrub Expansion and Lichen Decline in the Western Canadian Arctic

Strong evidence for a pan-Arctic expansion of upright shrubs comes from analysis of satellite imagery, historical photographs, vegetation plots, and growth rings. However, there are still uncertainties related to local-scale patterns of shrub growth, resulting interactions among vegetation functional groups, and the relative roles of disturbance and climate as drivers of observed change. Here, we present evidence that widespread and rapid shrub expansion and lichen declines over a 15,000 km2 area of the western Canadian Arctic have been driven by regional increases in temperature. Using 30 m resolution Landsat satellite imagery and high resolution repeat color-infrared aerial photographs, we show that 85% of the land surface has a positive 1985–2011 trend (P 0.03 NDVI/decade) occurred consistently across all landscape positions and most vegetation types. Comparison of 208, 1:2,000 scale vertical air photo pairs from 1980 and 2013 clearly shows that this greening was driven by increased canopy cover of erect dwarf and tall shrubs, with declines in terricolous lichen cover. Disturbances caused by wildfires, exploratory gas wells, and drained lakes all produced strong, yet localized increases in NDVI due to shrub growth. Our analysis also shows that a 4°C winter temperature increase over the past 30 years, leading to warmer soils and enhanced nutrient mineralization provides the best explanation for observed vegetation change. These observations thus provide early corroboration for modeling studies predicting large-scale vegetation shifts in low-Arctic ecosystems from climate change.

A new species of Malesherbia (Passifloraceae Subfam. Malesherbioideae) from Peru

Malesherbia laraosensis is described as a new species from the Department of Lima, Peru. The species is apparently narrowly endemic. The new species apparently belongs to Malesherbia sect. Malesherbia and is superficially similar to M. splendens and M. scarlatiflora in flower, but differs dramatically in habit—it is a very low-growing shrublet with branches ca. 15 cm tall, versus tall shrubs 1 m tall in the other two species. It is also characterized by obovate, dark green, adaxially subglabrous leaves (versus narrowly ovate-acuminate, densely pubescent and glandular in the other species). The new species brings the species total for Peruvian Malesherbia to 10.

Long‐term experimental warming and nutrient additions increase productivity in tall deciduous shrub tundra

Warming Arctic temperatures can drive changes in vegetation structure and function directly by stimulating plant growth or indirectly by stimulating microbial decomposition of organic matter and releasing more nutrients for plant uptake and growth. The arctic biome is currently increasing in deciduous shrub cover and this increase is expected to continue with climate warming. However, little is known how current deciduous shrub communities will respond to future climate induced warming and nutrient increase. We examined the plant and ecosystem response to a long‐term (18 years) nutrient addition and warming experiment in an Alaskan arctic tall deciduous shrub tundra ecosystem to understand controls over plant productivity and carbon (C) and nitrogen (N) storage in shrub tundra ecosystems. In addition, we used a meta‐analysis approach to compare the treatment effect size for aboveground biomass among seven long‐term studies conducted across multiple plant community types within the Arctic. We found that biomass, productivity, and aboveground N pools increased with nutrient additions and warming, while species diversity decreased. Both nutrient additions and warming caused the dominant functional group, deciduous shrubs, to increase biomass and proportional C and N allocation to aboveground stems but decreased allocation to belowground stems. For all response variables except soil C and N pools, effects of nutrients plus warming were largest. Soil C and N pools were highly variable and we could not detect any response to the treatments. The biomass response to warming and fertilization in tall deciduous shrub tundra was greater than moist acidic and moist non‐acidic tundra and more similar to the biomass response of wet sedge tundra. Our data suggest that in a warmer and more nutrient‐rich Arctic, tall deciduous shrub tundra will have greater total deciduous shrub biomass and a higher proportion of woody tissue that has a longer residence time, with a lower proportion of C and N allocated to belowground stems.

Ecological and genetic analysis of Gentiana lutea L. populations from Ukrainian Carpatians

Analysis of five natural (Lemska, Gutyn Tomnatyk, Sheshul-Pavlyk, Krachunyeska, Troyaska-Tataruka) and one man-made (Pozhyzhevska) populations of G. lutea of the Ukrainian Carpathians based on the parameters of genetic diversity (polimorphy, heterozygosis, the division of the total genetic variation for intra- and interpopulation) and ecological characteristics (density, age structure, the ability to self-sustaining) as well. Total (Dt) and effective (De) population density and number of indices: regeneration (RI), substitution (SI), aging (AI), age (Δ) and efficiency (ω) were determined. Vitality analysis of populations was performed by the method of Yu. A. Zlobin, while the type of strategy determined by Y. V. Tsaryk approach. Character of self-sustaining was determined by the ratio between the number of individuals of generative and vegetative origin. Analysis of genetic diversity of populations was performed by labeling inter-satellite sequences (ISSR-PCR) and arbitrary DNA ones (RAPD-PCR). Comprehensive analysis of the population states by their ecological and genetic parameters were performed similarly to the 5-point system of N. M. Beltyukova (2010). For the first time an integrated ecological and genetic approach to assessing the current state of the six populations of G. lutea of Ukrainian Carpathians was used. The research results can be used for stabilizing the number of violations and restoring the endangered natural populations. It was established that the indices of genetic polymorphism were similar for different investigated populations. It was found significant genetic divergence, which is due to their geographic isolation and differences in growth conditions. By the results of complex ecological and genetic analysis of G. lutea populations were classified into three groups: stable, relatively stable, unstable. The stability of populations (Lemska, Sheshul-Pavlyk), to our mind, is defined as their environmental and genetic characteristics and location in the conservation area. All this together provides the ability of populations to survive and adapt to changing environmental conditions. As relatively stable was classified population from Gutyn Tomnatyk mount, which, despite the small number of individuals showed high levels of genetic polymorphism. This conservation of the genetic diversity by scanty isolated populations indicates the fact that G. lutea species so far has genetic resources that are capable of restoring the population in case of creation the favorable conditions. Troyaska-Tataruka and Krachunyeska populations was classified as unstable, which is obviously due to positioning of these localities in conditions of heavy anthropogenic pressure and the inability of their individuals to compete with tall shrubs in phytocenoses. The division of localities into separate loci may alter the overall genetic structure of the population; deteriorating of growth conditions may lead to disruption of population structures and, as s result, reduce their stability. Man-made population on Pozhyzhevska by the results of ecological and genetic analysis was stable. The ability of this population to withstand external influences, to compete with other types of groups and explore new territory was determined. Stability of agropopulation from Pozhyzhevska is evidence that artificial plantations of G. lutea in the highlands of Ukrainian Carpathians can serve as a way to preserve the gene pool of this species.

Vegetation-Permafrost Relations within the Forest-Tundra Ecotone near Old Crow, Northern Yukon, Canada

Old Crow Flats is a 5600 km2 glaciolacustrine plain that straddles the forest-tundra ecotone in northern Yukon. Continuous taiga corridors occur in the entrenched river valleys, where annual mean ground temperatures (Tg) at the depth of zero annual amplitude at two locations were −3.1 and −4.0oC in 2013. On the Flats, the vegetation cover is patchy, and Tg varied between −5.1 and −2.6oC. Annual mean near-surface permafrost temperatures (Tps) measured on the Flats between 2008 and 2011 in patches of taiga, tall shrubs and low shrubs were correlated with local snow depth. Snow depth was controlled by vegetation height if the snow supply was not limited, for example, where low shrubs and large lakes dominate the landscape. In this setting, snow depths and, hence, Tps in taiga patches were higher (−2.6 to −2.9oC) than in the surrounding shrub vegetation (−3.5 to −5.5oC). Where taiga patches were more extensive, redistributed snow was trapped at the patch edge and Tps in taiga was lower (−4.1 to −4.3oC) than in the surrounding shrub patches (−3.2 to −3.6oC). The permafrost temperature field is heterogeneous under patchy vegetation in the forest-tundra ecotone. Our data suggest that it is governed by both the snow-holding capacity of local vegetation patches and the spatial configuration of vegetation in the surrounding landscape, which controls snow supply. Copyright © 2014 John Wiley & Sons, Ltd.

Tall shrub and tree expansion in Siberian tundra ecotones since the 1960s

Circumpolar expansion of tall shrubs and trees into Arctic tundra is widely thought to be occurring as a result of recent climate warming, but little quantitative evidence exists for northern Siberia, which encompasses the world's largest forest-tundra ecotonal belt. We quantified changes in tall shrub and tree canopy cover in 11, widely distributed Siberian ecotonal landscapes by comparing very high-resolution photography from the Cold War-era 'Gambit' and 'Corona' satellite surveillance systems (1965-1969) with modern imagery. We also analyzed within-landscape patterns of vegetation change to evaluate the susceptibility of different landscape components to tall shrub and tree increase. The total cover of tall shrubs and trees increased in nine of 11 ecotones. In northwest Siberia, alder (Alnus) shrubland cover increased 5.3-25.9% in five ecotones. In Taymyr and Yakutia, larch (Larix) cover increased 3.0-6.7% within three ecotones, but declined 16.8% at a fourth ecotone due to thaw of ice-rich permafrost. In Chukotka, the total cover of alder and dwarf pine (Pinus) increased 6.1% within one ecotone and was little changed at a second ecotone. Within most landscapes, shrub and tree increase was linked to specific geomorphic settings, especially those with active disturbance regimes such as permafrost patterned-ground, floodplains, and colluvial hillslopes. Mean summer temperatures increased at most ecotones since the mid-1960s, but rates of shrub and tree canopy cover expansion were not strongly correlated with temperature trends and were better correlated with mean annual precipitation. We conclude that shrub and tree cover is increasing in tundra ecotones across most of northern Siberia, but rates of increase vary widely regionally and at the landscape scale. Our results indicate that extensive changes can occur within decades in moist, shrub-dominated ecotones, as in northwest Siberia, while changes are likely to occur much more slowly in the highly continental, larch-dominated ecotones of central and eastern Siberia.

Spatial and temporal functional changes in alpine summit vegetation are driven by increases in shrubs and graminoids.

Classical approaches to investigating temporal and spatial changes in community composition offer only partial insight into the ecology that drives species distribution, community patterns and processes, whereas a functional approach can help to determine many of the underlying mechanisms that drive such patterns. Here, we aim to bring these two approaches together to understand such drivers, using an elevation gradient of sites, a repeat species survey and species functional traits. We used data from a repeat vegetation survey on five alpine summits and measured plant height, leaf area, leaf dry matter content and specific leaf area (SLA) for every species recorded in the surveys. We combined species abundances with trait values to produce a community trait-weighted mean (CTWM) for each trait, and then combined survey results with the CTWMs. Across the gradient of summits, more favourable conditions for plant growth (warmer, longer growing season) occurred at the lower elevations. Vegetation composition changes between 2004 and 2011 (according to non-metric multi-dimensional scaling ordination) were strongly affected by the high and increasing abundance of species with high SLA at high elevations. Species life-form categories strongly affected compositional changes and functional composition, with increasing dominance of tall shrubs and graminoids at the lower-elevation summits, and an overall increase in graminoids across the gradient. The CTWM for plant height and leaf dry matter content significantly decreased with elevation, whereas for leaf area and SLA it significantly increased. The significant relationships between CTWM and elevation may suggest specific ecological processes, namely plant competition and local productivity, influencing vegetation preferentially across the elevation gradient, with the dominance of shrubs and graminoids driving the patterns in the CTWMs.

The Importance of Large-Diameter Trees to Forest Structural Heterogeneity

Large-diameter trees dominate the structure, dynamics and function of many temperate and tropical forests. However, their attendant contributions to forest heterogeneity are rarely addressed. We established the Wind River Forest Dynamics Plot, a 25.6 ha permanent plot within which we tagged and mapped all 30,973 woody stems ≥1 cm dbh, all 1,966 snags ≥10 cm dbh, and all shrub patches ≥2 m2. Basal area of the 26 woody species was 62.18 m2/ha, of which 61.60 m2/ha was trees and 0.58 m2/ha was tall shrubs. Large-diameter trees (≥100 cm dbh) comprised 1.5% of stems, 31.8% of basal area, and 17.6% of the heterogeneity of basal area, with basal area dominated by Tsuga heterophylla and Pseudotsuga menziesii. Small-diameter subpopulations of Pseudotsuga menziesii, Tsuga heterophylla and Thuja plicata, as well as all tree species combined, exhibited significant aggregation relative to the null model of complete spatial randomness (CSR) up to 9 m (P≤0.001). Patterns of large-diameter trees were either not different from CSR (Tsuga heterophylla), or exhibited slight aggregation (Pseudotsuga menziesii and Thuja plicata). Significant spatial repulsion between large-diameter and small-diameter Tsuga heterophylla suggests that large-diameter Tsuga heterophylla function as organizers of tree demography over decadal timescales through competitive interactions. Comparison among two forest dynamics plots suggests that forest structural diversity responds to intermediate-scale environmental heterogeneity and disturbances, similar to hypotheses about patterns of species richness, and richness- ecosystem function. Large mapped plots with detailed within-plot environmental spatial covariates will be required to test these hypotheses.

Nitrogen isotope tracer acquisition in low and tall birch tundra plant communities: a 2 year test of the snow–shrub hypothesis

Deciduous shrub density and landcover are increasing across many areas of the Arctic. Shrub growth may be promoted by a snow-shrub feedback whereby relatively tall shrubs accumulate deeper snow, raising winter soil temperature minima, increas- ing microbial activity, and enhancing soil solution nitrogen (N). Although there is good evidence for the above components of the hypothesis, it has not yet been determined if shrubs can access the elevated N pool generated by deepened snow. We added isotopic N tracer ( 15 N) in late summer to control and snowfenced low birch hummock tundra to test the influence of deepened snow on N cycling. Further- more, tracer was added to tall birch tundra to compare N cycling in low and tall shrub ecosystems that have the same species composition. Experimentally deep- ened snow in low birch tundra did not significantly affect 15 N uptake by shrubs or any other species 2 years after the tracer addition. However, there were strong differences between the low and tall birch ecosystems, with the deciduous shrubs and graminoids accumulating more 15 N than the evergreen shrubs in the relatively productive tall shrub site, and vice versa in the relatively infertile low birch site. The greater 15 N acquisition by birch in the more fertile site, together with the absence of a deepened snow effect on 15 N acquisition by any species in the low birch hummock ecosystem, suggest that climate-change induced increases in birch shrub growth and expansion across the landscape will tend to occur most rapidly in and around existing tall birch shrub patches.

Complementary impacts of small rodents and semi‐domesticated ungulates limit tall shrub expansion in the tundra

Summary While shrubs appear to be expanding in Arctic tundra due to climatic warming, patches of tall shrubs in riparian habitats are most likely to colonize new areas. Shrub recruits outside established patches represent the forefront of area expansion, but their dynamics may be sensitive to the action of several herbivore species. The empirical evidence for how different‐sized herbivores affect recruits of tall shrubs is lacking. Moreover, although management and natural population dynamics of herbivores happens at landscape and regional scales, field research on herbivore impacts on shrubs seldom covers these scales. Reindeer management and different rodent population dynamics result in regional variation in herbivore abundances in arctic Norway. We conducted an herbivore exclusion experiment, covering three low‐arctic river catchments with contrasting herbivore abundances. We assessed the impacts of small rodents and reindeer on growth, and survival of willow Salix spp. recruits after 3 years of herbivore exclusion. As expected, the Salix recruits increased in sizes and had lower mortality when released from herbivores. Both types of herbivores had strong impacts on size and survival of Salix recruits. Spatially contrasting results were consistent with regional differences in the abundance of reindeer and rodents; herbivore impacts on shrubs were found when at least one type of herbivore was abundant. However, the impact was not independent of herbivore species. While both browsing from reindeer and rodents pruned the recruits and prevented them from escaping the field layer, the rodents also inflicted substantial mortality and thus thinned the stand of recruits. Synthesis and applications. Sympatric populations of rodents and reindeer have strongly complementary impacts on shrub recruits and may limit the expansion potential of tall shrubs even in the most productive habitats of arctic tundra. The spatial correspondence between shrub recruits performance and herbivore abundances, found after a short time period, suggests that the extent of tall shrub expansion in tundra is contingent on current variation and future trends in herbivore populations. In areas where humans control large herbivore populations, management may opt to counteract climate‐driven shrub expansion also in habitats that are most prone to such expansion.

Online appendix 1:Vegetation change (1988–2010) in Camdeboo National Park (South Africa), using fixed-point photo monitoring: The role of herbivory and climate

Fixed-point photo monitoring supplemented by animal census data and climate monitoring potential has never been explored as a long-term monitoring tool for studying vegetation change in the arid and semi-arid national parks of South Africa. The long-term (1988–2010), fixed-point monitoring dataset developed for the Camdeboo National Park, therefore, provides an important opportunity to do this. Using a quantitative estimate of the change in vegetation and growth form cover in 1152 fixed-point photographs, as well as series of step-point vegetation surveys at each photo monitoring site, this study documented the extent of vegetation change in the park in response to key climate drivers, such as rainfall, as well as land use drivers such as herbivory by indigenous ungulates. We demonstrated the varied response of vegetation cover within three main growth forms (grasses, dwarf shrubs [< 1 m] and tall shrubs [> 1 m]) in three different vegetation units and landforms (slopes, plains, rivers) within the Camdeboo National Park since 1988. Sites within Albany Thicket and Dwarf Shrublands showed the least change in vegetation cover, whilst Azonal vegetation and Grassy Dwarf Shrublands were more dynamic. Abiotic factors such as drought and flooding, total annual rainfall and rainfall seasonality appeared to have the greatest influence on growth form cover as assessed from the fixed-point photographs. Herbivory appeared not to have had a noticeable impact on the vegetation of the Camdeboo National Park as far as could be determined from the rather coarse approach used in this analysis and herbivore densities remained relatively low over the study duration.Conservation implications: We provided an historical assessment of the pattern of vegetation and climatic trends that can help evaluate many of South African National Parks’ biodiversity monitoring programmes, especially relating to habitat change. It will help arid parks in assessing the trajectories of vegetation in response to herbivory, climate and management interventions.