Shrub Expansion Research Articles

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.

On the influence of shrub height and expansion on northern high latitude climate

There is a growing body of empirical evidence documenting the expansion of shrub vegetation in the circumpolar Arctic in response to climate change. Here, we conduct a series of idealized experiments with the Community Climate System Model to analyze the potential impact on boreal climate of a large-scale tundra-to-shrub conversion. The model responds to an increase in shrub abundance with substantial atmospheric heating arising from two seasonal land–atmosphere feedbacks: a decrease in surface albedo and an evapotranspiration-induced increase in atmospheric moisture content. We demonstrate that the strength and timing of these feedbacks are sensitive to shrub height and the time at which branches and leaves protrude above the snow. Taller and aerodynamically rougher shrubs lower the albedo earlier in the spring and transpire more efficiently than shorter shrubs. These mechanisms increase, in turn, the strength of the indirect sea-ice albedo and ocean evaporation feedbacks contributing to additional regional warming. Finally, we find that an invasion of tall shrubs tends to systematically warm the soil, deepen the active layer, and destabilize the permafrost (with increased formation of taliks under a future scenario) more substantially than an invasion of short shrubs.

Shrub encroachment alters sensitivity of soil respiration to temperature and moisture

A greater abundance of shrubs in semiarid grasslands affects the spatial patterns of soil temperature, moisture, and litter, resulting in fertile islands with potentially enhanced soil metabolic activity. The goal of this study was to quantify the microsite specificity of soil respiration in a semiarid riparian ecosystem experiencing shrub encroachment. We quantified the response of soil respiration to different microsite conditions created by big mesquite shrubs (near the trunk and the canopy edge), medium‐sized mesquite, sacaton bunchgrasses, and open spaces. We hypothesized that soil respiration would be more temperature sensitive and less moisture sensitive and have a greater magnitude in shrub microsites compared with grass and open microsites. Field and incubation soil respiration data were simultaneously analyzed in a Bayesian framework to quantify the microsite‐specific temperature and moisture sensitivities and magnitude of respiration. The analysis showed that shrub expansion increases the heterogeneity of respiration. Respiration has greater temperature sensitivity near the shrub canopy edge, and respiration rates are higher overall under big mesquite compared with those of the other microsites. Respiration in the microsites beneath medium‐sized mesquites does not behave like a downscaled version of big mesquite microsites. The grass microsites show more similarity to big mesquite microsites than medium‐sized shrubs. This study shows there can be a great deal of fine‐scale spatial heterogeneity that accompanies shifts in vegetation structure. Such complexity presents a challenge in scaling soil respiration fluxes to the landscape for systems experiencing shrub encroachment, but quantifying this complexity is significantly important in determining overall ecosystem metabolic behavior.

Shrub expansion at the forest–tundra ecotone: spatial heterogeneity linked to local topography

Recent densification of shrub cover is now documented in many Arctic regions. However, most studies focus on global scale responses, yielding very little information on the local patterns. This research aims to quantify shrub cover increase at northern treeline (Québec, Canada) in two important types of environment, sandy terraces and hilltops (which cover about 70% of the landscape), and to identify the species involved. The comparison of a mosaic of two aerial photographs from 1957 (137 km2) and one satellite image taken in 2008 (151 km2) revealed that both hilltops and terraces recorded an increase in shrub cover. However, the increase was significantly greater on terraces than on hilltops (21.6% versus 11.6%). According to ground truthing, the shrub cover densification is associated mainly with an increase of Betula glandulosa Michx. The numerous seedlings observed during the ground truthing suggest that shrub densification should continue in the future.

Transformation of Forest Vegetation after 40 Years of Protection in the Tomczyce Nature Reserve (Central Poland)

The Tomczyce nature reserve is characterized by a degenerated forest vegetation. We assume that the regeneration process was possible to launch after the establishment of the nature reserve in 1968. The vegetation of the Tomczyce forest complex was characterized for the first time by Jakubowska-Gabara (1976) whose studies were taken as a basis of our research. The phytocenoses after 40. years of protection have a greater participation of species with higher trophic and moisture requirements. The regeneration process in communities with pine trees is caused by an expansion of broad-leaved trees and shrubs. Transformation of forest vegetation causes a decrease in cover of thermophilous and heliophilous species. On the other hand, species of oak-hornbeam forests as well as anthropophytes are in expansion. Planning of the protection activities in the nature reserve needs to include these dynamic tendencies of the vegetation.

Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra ecosystems. Here, we (1) synthesize these findings, (2) present a conceptual framework that identifies mechanisms and constraints on shrub increase, (3) explore causes, feedbacks and implications of the increased shrub cover in tundra ecosystems, and (4) address potential lines of investigation for future research. Satellite observations from around the circumpolar Arctic, showing increased productivity, measured as changes in ‘greenness’, have coincided with a general rise in high-latitude air temperatures and have been partly attributed to increases in shrub cover. Studies indicate that warming temperatures, changes in snow cover, altered disturbance regimes as a result of permafrost thaw, tundra fires, and anthropogenic activities or changes in herbivory intensity are all contributing to observed changes in shrub abundance. A large-scale increase in shrub cover will change the structure of tundra ecosystems and alter energy fluxes, regional climate, soil–atmosphere exchange of water, carbon and nutrients, and ecological interactions between species. In order to project future rates of shrub expansion and understand the feedbacks to ecosystem and climate processes, future research should investigate the species or trait-specific responses of shrubs to climate change including: (1) the temperature sensitivity of shrub growth, (2) factors controlling the recruitment of new individuals, and (3) the relative influence of the positive and negative feedbacks involved in shrub expansion.

Twentieth century erosion in Arctic Alaska foothills: The influence of shrubs, runoff, and permafrost

[1] Recent changes in the climate of Arctic Alaska, including warmer summers and a lengthened growing season, have increased vegetation productivity and permafrost temperatures. In this study, we use (1) time series imagery to examine the landscape pattern of tall shrub distribution and expansion in Arctic Alaska and (2) lake sediments from watersheds where shrub expansion is occurring to compare twentieth century temporal trends between shrub cover, erosion, and runoff. Landsat thematic mapper data from 1986 and 2009 were used to evaluate the expansion of tall shrubs across three regional subscenes in the Arctic foothills in northeast Alaska. We found that tall shrubs occupied floodplains and streams in 1986 and have been expanding their coverage along these corridors. The interaction between shrub expansion and erosion was examined by reconstructing the last 60–100 years of erosion from sediment cores in four lakes with shrub expansion in the surrounding watersheds. Three of the four lake cores show a steadily increasing or fluctuating erosion rate until 1980, after which these cores show a synchronous decline. We postulate that the increase in shrubs since 1980 in landscape positions prone to erosion has contributed to the decline in erosion. A decrease in the magnitude and frequency of runoff events has likely also contributed to the decline in erosion. Our results indicate a general decline in erosion since 1980 that is contemporaneous with shrub expansion and peak runoff decline, punctuated by episodic erosion events in one of four catchments.

Celebrating the ecosystem’s three-quarter century: Introduction to a Virtual Special Issue on Sir Arthur Tansley’s ecosystem concept

Celebrating the ecosystem’s three-quarter century: Introduction to a Virtual Special Issue on Sir Arthur Tansley’s ecosystem concept

Tree and Shrub Expansion Over the Past 34 Years at the Tree-Line Near Abisko, Sweden

Shrubs and trees are expected to expand in the sub-Arctic due to global warming. Our study was conducted in Abisko, sub-arctic Sweden. We recorded the change in coverage of shrub and tree species over a 32- to 34-year period, in three 50 x 50 m plots; in the alpine-tree-line ecotone. The cover of shrubs and trees (<3.5 cm diameter at breast height) were estimated during 2009-2010 and compared with historical documentation from 1976 to 1977. Similarly, all tree stems (> or =3.5 cm) were noted and positions determined. There has been a substantial increase of cover of shrubs and trees, particularly dwarf birch (Betula nana), and mountain birch (Betula pubescens ssp. czerepanovii), and an establishment of aspen (Populus tremula). The other species willows (Salix spp.), juniper (Juniperus communis), and rowan (Sorbus aucuparia) revealed inconsistent changes among the plots. Although this study was unable to identify the causes for the change in shrubs and small trees, they are consistent with anticipated changes due to climate change and reduced herbivory.

On the brink of change: plant responses to climate on the Colorado Plateau

The intensification of aridity due to anthropogenic climate change in the southwestern U.S. is likely to have a large impact on the growth and survival of plant species that may already be vulnerable to water stress. To make accurate predictions of plant responses to climate change, it is essential to determine the long-term dynamics of plant species associated with past climate conditions. Here we show how the plant species and functional types across a wide range of environmental conditions in Colorado Plateau national parks have changed with climate variability over the last twenty years. During this time, regional mean annual temperature increased by 0.18°C per year from 1989–1995, 0.06°C per year from 1995–2003, declined by 0.14°C from 2003–2008, and there was high interannual variability in precipitation. Non-metric multidimensional scaling of plant species at long-term monitoring sites indicated five distinct plant communities. In many of the communities, canopy cover of perennial plants was sensitive to mean annual temperature occurring in the previous year, whereas canopy cover of annual plants responded to cool season precipitation. In the perennial grasslands, there was an overall decline of C3 perennial grasses, no change of C4 perennial grasses, and an increase of shrubs with increasing temperature. In the shrublands, shrubs generally showed no change or slightly increased with increasing temperature. However, certain shrub species declined where soil and physical characteristics of a site limited water availability. In the higher elevation woodlands, Juniperus osteosperma and shrub canopy cover increased with increasing temperature, while Pinus edulis at the highest elevation sites was unresponsive to interannual temperature variability. These results from well-protected national parks highlight the importance of temperature to plant responses in a water-limited region and suggest that projected increases in aridity are likely to promote grass loss and shrub expansion on the Colorado Plateau.

What are the main climate drivers for shrub growth in Northeastern Siberian tundra?

Abstract. Deciduous shrubs are expected to rapidly expand in the Arctic during the coming decades due to climate warming. A transition towards more shrub-dominated tundra may have large implications for the regional surface energy balance, permafrost stability and carbon storage capacity, with consequences for the global climate system. However, little information is available on the natural long-term shrub growth response to climatic variability. Our aim was to determine the climate factor and time period that are most important to annual shrub growth in our research site in NE-Siberia. Therefore, we determined annual radial growth rates in Salix pulchra and Betula nana shrubs by measuring ring widths. We constructed shrub ring width chronologies and compared growth rates to regional climate and remotely sensed greenness data. Early summer temperature was the most important factor influencing ring width of S. pulchra (Pearson's r = 0.73, p &lt; 0.001) and B. nana (Pearson's r = 0.46, p &lt; 0.001). No effect of winter precipitation on shrub growth was observed. In contrast, summer precipitation of the previous year correlated positively with B. nana ring width (Pearson's r = 0.42, p &lt; 0.01), suggesting that wet summers facilitate shrub growth in the following growing season. S. pulchra ring width correlated positively with peak summer NDVI, despite the small coverage of S. pulchra shrubs (&lt; 5 % surface cover) in our research area. We provide the first climate-growth study on shrubs for Northeast Siberia, the largest tundra region in the world. We show that two deciduous shrub species with markedly different growth forms have a similar growth response to changes in climate. The obtained shrub growth response to climate variability in the past increases our understanding of the mechanisms underlying current shrub expansion, which is required to predict future climate-driven tundra vegetation shifts.

Patterns and processes of global shrub expansion

Shrub expansion is a global phenomenon that is occurring on savannas, rangelands, and grasslands. In addition, this is an increasingly documented occurrence in the Arctic. Numerous recent studies have strived to pinpoint the drivers of this phenomenon, quantify the changes, and understand their implications for regional and global land use, disturbance regimes, and nutrient cycling. Inquiry into these topics has been facilitated by recent technological developments in satellite remote sensing, aerial photograph analysis, and computer simulation modeling. We provide a new review that accounts for more recent studies in these regions, Arctic shrub expansion, and technological and analytical developments. This four-part discussion focuses on observed patterns of shrub expansion in three rangeland types (desert grasslands, mesic grasslands, savannas) and the Arctic tundra, the primary causes of this expansion, critical comparisons and contrasts between these land types, and recommendations for future avenues of research. These new avenues can inform the development of future land management policies, as well as ongoing investigations to understand and mitigate the effects of climate change.

Maggengo meadow patches enclosed by forests in the Italian Alps: evidence of landscape legacy on plant diversity

The maggenghi are mid slope meadows typical of all the southern and of great parts of the northern European Alps, for centuries managed with traditional and low intensity techniques. Usually, they are scattered patches in surrounding forests. The spontaneous expansion of trees and shrubs, favored by the recent decline of mountain agriculture, lead the maggengo patches patterns and shapes to change. Our objective was to analyze the effect of this change on current plant diversity of the remnant patches, as the adaptive response could be slow and possibly related more to historical than to current landscape patterns. We analyzed the trend of the size, shape, elongation, fractal dimension and connectivity of maggengo patches of a Central-Eastern Italian Alpine district, in four time steps, from 1973 to 2006, and in 1859, when mountain agriculture was still widespread. Then, we studied the relationships between those landscape metrics and two current patch-level plant diversity measures: interior species richness and species density. Aerial photographs were used to investigate that trend, while a historical cadastral map was used to assess the landscape metrics in 1859. As expected, in the last 30 years, the total size of maggenghi has been reduced by 57% while their shapes have been progressively simplified. Interior species richness was positively related to size, both in 2006 and over the past 30 years, but not to any 1859 measures. Conversely, species density was positively correlated only with 1859 size, shape index and connectivity. We conclude that the historical shape, size and connectivity are some of the key variables affecting the plant species density of maggengo patches, but not of their interior plant species richness.

Habitat‐specific demography across dune fixation stages in a semi‐arid sandland: understanding the expansion, stabilization and decline of a dominant shrub

Summary 1. Maintaining viable populations in different habitats requires physiological, morphological and demographic adaptations of plants. In sandland environments, plants experience substantial variation in growing conditions during the dune fixation process, with high sand mobility in early stages and denser vegetation cover in later stages. 2. We studied the changes in demography of a dominant shrub, Artemisia ordosica, at three stages of dune fixation: semi‐fixed dunes, fixed dunes and fixed dunes covered with microbiotic crust. Demographic data from three annual censuses were used to parameterize integral projection models (IPMs) to conduct comparative demographic analyses. 3. Plant growth and reproduction decreased strongly as dunes became more fixed. Shrinkage in plant height occurred very frequently, particularly in the fixed dunes with microbiotic crust. Population growth rate (λ) declined substantially with dune fixation: from rapid expansion in semi‐fixed dunes (λ = 1.35–1.09) to moderate decline in fixed dunes with microbiotic crust (λ = 0.94–0.89). 4. Elasticity analysis revealed that survival was a key vital rate for population growth in all habitats. Growth and fecundity were of higher importance in the semi‐fixed habitat than in the other two habitats where shrinkage became an important factor determining λ. Seedlings and small plants were critical for population growth in semi‐fixed dunes, whereas moderate to large‐sized plants were most important in the other habitats. 5. Results of life table response experiments showed that the observed strong decrease in λ during dune fixation was mainly caused by reduction in fecundity, but with additional and considerable contributions from reduced plant growth and increased occurrence of shrinkage. Thus, populations in semi‐fixed dunes are able to expand rapidly due to a much higher fecundity compared to those in other habitats. 6. Synthesis. Artemisia ordosica adopts different life history strategies along the dune fixation process. Fast expansion in semi‐fixed dunes is enabled by high seed production and effective recruitment, while populations at later dune fixation stages are maintained through frequent plant shrinkage. Integral projection models are highly appropriate tools for analysing such life history changes as they are based on statistical comparisons of vital rates across habitats.

Rhus glabra response to season and intensity of fire in tallgrass prairie

Altered fire regimes play a key role in shrub expansion in grasslands worldwide. We assessed how season and type or intensity of fire affected the growth and demography of Rhus glabra, a common woody invader in North American mesic grasslands. Fire during any season killed 99% of ramets but stimulated new ramet recruitment from belowground buds, resulting in a near-complete turnover of ramet populations. During the first 2 years following fire, populations on spring-burned sites had the greatest post-fire ramet densities and population growth rates, and winter- and spring-burned populations showed the highest resprouting rates. However, after 10 years, R. glabra cover on summer-burned sites was 3.5 times greater than on autumn- or winter-burned treatments. Thus, short-term post-fire responses may not be good predictors of long-term changes in abundance. Low-intensity spring backfires resulted in the highest ramet population growth rates, whereas high-intensity headfires in any season resulted in slower growth, and populations burned with low-intensity winter fires declined. In addition, season of fire influenced browsing pressure, suggesting that plant responses may be partially a result of indirect effects of fire on rates of herbivory. Overall, our results demonstrate that the application of frequent autumn or winter backfires is an effective management tool for limiting R. glabra expansion in grasslands, and that long-term data are critical for management decision-making, particularly in systems characterised by high interannual climate variability.

Clonal Diversity in an Expanding Community of Arctic Salix spp. and a Model for Recruitment Modes of Arctic Plants

Rapid climate change in arctic environments is leading to a widespread expansion in woody deciduous shrub populations. However, little is known about the reproductive, dispersal, and establishment mechanisms associated with shrub expansion. It is assumed that harsh environmental conditions impose limitations on plant sexual reproduction in the Arctic, such that population survival and expansion is predominately a function of clonal recruitment. We present contrary evidence from microsatellite genetic data suggesting the prevalence of recruitment by seed. Further, we present a conceptual model describing modes of recruitment in relation to the abiotic environment. Climate change may be alleviating abiotic stress so that resources are available for more frequent recruitment by seed. Such changes have widespread implications for ecosystem structure and functioning, including species composition, wildlife habitat, biogeochemical cycling, and surface energy balance.

Effects of cattle and rabbit grazing on clonal expansion of spiny shrubs in wood-pastures

Spiny shrubs protect non-defended plants against herbivores. Therefore, they play a role for the diversity in grazed ecosystems. While the importance of these keystone nurse shrubs is presently recognized, little is known about the factors controlling them. This knowledge is required to understand the functioning of grazed ecosystems and for sustainable management. We studied effects of cattle and rabbits on clonal expansion of Prunus spinosa in two ancient wood-pastures in the Netherlands. At each site we set up five blocks in grassland perpendicular to the edges of mature Prunus thickets, each block containing three herbivore treatments: (1) open-to-cattle-and-rabbits, (2) open to rabbits, cattle excluded, (3) cattle and rabbits excluded. We monitored the number and volume of Prunus ramets from 1998 to 2000 and again in 2003, 3 years after exclosure-removal to restore grazing. For 1998–2000 ramet volume, but not ramet number, differed between treatments. Ramet volume was highest when both cattle and rabbits were excluded. Ramet volume did not differ between grazing by rabbits or cattle and rabbits combined, indicating that rabbits alone may be as effective in inhibiting clonal expansion as cattle and rabbits combined. Three years after exclosure-removal ramet number and volume had increased in all treatments. Number of ramets remained unaffected by (former) treatments. Ramet volume remained highest in the former cattle-plus-rabbits exclusion treatment, differing significantly from the ‘open-to-cattle-and-rabbits’ treatment. So, once successfully established during herbivore absence, further expansion is not prevented by cattle and rabbit grazing. This study shows that vertebrate herbivory controls the keystone nurse-shrub in wood-pastures: combined cattle and rabbit grazing, and notably rabbits alone, inhibit expansion. Temporary herbivore absence allows expansion of ramets, which persists after herbivore-reappearance. Sustainable management of wood-pastures should allow spatial-temporal fluctuations of herbivore densities, leading to increased vegetation structure and associated biodiversity.

Long-term experimental manipulation of climate alters the ectomycorrhizal community of Betula nana in Arctic tundra

Climate warming is leading to shrub expansion in Arctic tundra. Shrubs form ectomycorrhizal (ECM) associations with soil fungi that are central to ecosystem carbon balance as determinants of plant community structure and as decomposers of soil organic matter. To assess potential climate change impacts on ECM communities, we analysed fungal internal transcribed spacer sequences from ECM root tips of the dominant tundra shrub Betula nana growing in treatments plots that had received long-term warming by greenhouses and/or fertilization as part of the Arctic Long-Term Ecological Research experiment at Toolik Lake Alaska, USA. We demonstrate opposing effects of long-term warming and fertilization treatments on ECM fungal diversity; with warming increasing and fertilization reducing the diversity of ECM communities. We show that warming leads to a significant increase in high biomass fungi with proteolytic capacity, especially Cortinarius spp., and a reduction of fungi with high affinities for labile N, especially Russula spp. In contrast, fertilization treatments led to relatively small changes in the composition of the ECM community, but increased the abundance of saprotrophs. Our data suggest that warming profoundly alters nutrient cycling in tundra, and may facilitate the expansion of B. nana through the formation of mycorrhizal networks of larger size.

Environmental change without climate change?

Environmental change without climate change?

Dynamics of alpine plant litter decomposition in a changing climate

Climatic changes resulting from anthropogenic activities over the passed century are repeatedly reported to alter the functioning of pristine ecosystems worldwide, and especially those in cold biomes. Available literature on the process of plant leaf litter decomposition in the temperate Alpine zone is reviewed here, with emphasis on both direct and indirect effects of climate change phenomena on rates of litter decay. Weighing the impact of biotic and abiotic processes governing litter mass loss, it appears that an immediate intensification of decomposition rates due to temperature rise can be retarded by decreased soil moisture, insufficient snow cover insulation, and shrub expansion in the Alpine zone. This tentative conclusion, remains speculative unless empirically tested, but it has profound implications for understanding the biogeochemical cycling in the Alpine vegetation belt, and its potential role as a buffering mechanism to climate change.