Abundance Of Shrubs Research Articles

Livestock settlement affects shrub abundance via plant functional diversity but not species richness in arid environments

Recent theoretical and empirical studies have assumed that livestock grazing acts as a biotic filter favouring plant species with functional traits that confer resistance to herbivory. Considering that plant functional traits affect the species abundance and persistence in a community, alterations in functional diversity would result in changes in species richness, evenness or growth form composition. However, empirical tests of the intermediary role of functional diversity among livestock pressure, climate and species diversity and composition remain rare. We evaluated if functional diversity acts as an intermediary between livestock pressure and several components of taxonomical diversity in plant communities located along an aridity gradient in central-western Argentina. We estimated functional diversity using four plant traits that are expected to reflect trade-offs between resource-use strategies and grazing resistance. We found that livestock pressure increased shrub abundance by negatively affecting functional diversity, and this effect was amplified by increases in aridity. On the other hand, aridity promoted declines in species richness by reductions in functional diversity. Overall, our results suggest that functional diversity acts as intermediary component between livestock pressure and growth form composition; and also between aridity and species richness. Finally, our findings highlight that livestock grazing may be considered as a biotic filter affecting growth form composition but not species richness and evenness.

Open karst habitats promote the diversity of ground-dwelling orthopterans and cockroaches (Insecta: Orthoptera, Blattodea) along a temporary river

Temporary rivers are dynamic freshwater ecosystems, supporting unique and often highly diverse terrestrial communities in the riparian habitats during the flowing phase. However, studies investigating the differences in orthopteran and cockroach (Insecta: Orthoptera, Blattodea) assemblages between riparian and adjacent upland habitats are scarce, despite their abundance and functional importance in terrestrial ecosystems. Orthopterans and cockroaches were sampled in 2014 at 72 sites along the temporary Krcic River (SE Croatia) in two habitat types—riparian and karst, using pitfall traps. Abundance, taxa richness, true diversity and conservation value of ground-dwelling orthopterans and cockroaches were significantly higher in the karst than in the riparian habitat. This result can be ascribed to significant differences in environmental factors: soil temperature, air humidity and vegetation structure, namely tree, shrub and herb layer abundances. Most assemblage parameters and spatial distribution of ground-dwelling orthopteran and cockroach taxa were driven by higher soil temperature and less developed tree layer in the karst, both groups preferring open habitats. Karst sites with a well-developed shrub layer and herb layer had the highest conservation value for rare ground-dwelling orthopterans (e.g. Paramogoplistes novaki) and locally rare cockroaches (e.g. Ectobius balcani), respectively. Nevertheless, densely vegetated riparian sites also harboured some locally valuable species (e.g. Troglophilus cavicola). This study highlights the importance of open karst habitats in promoting the diversity of ground-dwelling orthopterans and cockroaches, and contributes to the understanding of key environmental factors driving their habitat choice, as a prerequisite for efficient conservation and monitoring.

The role of distyly in pollen flow of the hummingbird-pollinated Palicourea rigida (Rubiaceae)

Distylous species possess two floral morphs: S-morph with short styles and long stamens, and L-morph with long styles and short stamens. The reciprocal positioning of anthers and stigmas is hypothesized to promote legitimate (between-morph) pollination. Deviations from reciprocity can negatively affect legitimate pollen deposition. The functioning of reciprocal herkogamy occurs if pollinators collect pollen of the L- and S-morph on different parts of the body, and then contact the stigmas of the opposite morph. However, few studies provide experimental evidence that the pollen deposition from the two morphs is stratified on the pollinator's body, especially for hummingbirds. We studied three populations of Palicourea rigida Kunth, an abundant shrub of the Neotropical savanna, to test the function of distyly as a mechanism to promote the differential placement of pollen on different parts of the pollinator's body and facilitate legitimate pollination. For this, we quantified high and low organs reciprocity, pollen dimorphism and pollen flow and developed a method for simulating floral visits by hummingbirds (main pollinators) to test whether pollen deposition occurs in a stratified manner on the bill. We found greater reciprocity in low organs, which explains the greater legitimate pollen deposition on S than L stigmas in two populations. In the other population, there was no difference in legitimate pollen deposition between morphs. Our findings corroborate the evolutionary models which have proposed that distyly promotes pollen deposition on specific parts of the pollinator's body. Pollen from S anthers was more deposited at the base of the bill, while pollen from L anthers appeared on the middle section of the bill. This stratified pollen deposition on the hummingbird's bill corresponded to the anther height of the pollen-donor morph and stigma height of the opposite morph, facilitating the reception of legitimate pollen, thus demonstrating the ecological functioning of distyly.

Altitudinal gradient of plant diversity patterns in the Sierra de Quilmes (Monte desert – Argentina)

The Monte Desert is the most arid rangeland in Argentina. Its northern portion, the High Monte ecoregion, is a geomorphologically complex area, characterized by mountain and bolsons landforms. Yet, most studies about plant diversity in the area have disregarded the altitudinal gradient. We examined the patterns of species richness, composition, and diversity along an altitudinal gradient in the Sierra de Quilmes (High Monte ecoregion), through the deconstruction of biodiversity into growth forms. We sampled vegetation from eight sampling sites in each of five altitudinal bands ranging from 1650 to 2550 m a.s.l. We found 123 species belonging to 33 families. Community composition changed with altitude. The abundance of shrubs increased with altitude, while the abundance of trees decreased. Both alpha and beta diversities peaked in the lowest and intermediate altitudes (1700 and 2300 bands). This pattern was particularly consistent for woody plants. The higher beta diversity in the 1700 band might result from geomorphology and soil features variation. The remaining gradient conformed to a hump-shaped pattern with a peak in the 2300 band, which may present more suitable climatic conditions. This study contributes to a broader understanding of the causes determining vegetation diversity patterns in the High Monte Desert.

A new Vegetation-Plot Database for the Coastal Forests of Kenya

Biodiversity data based on standardised sampling designs are key to ecosystem conservation. Data of this sort have been lacking for the Kenyan coastal forests despite being biodiversity hotspots. Here, we introduce the Kenyan Coastal Forests Vegetation-Plot Database (GIVD ID: AF-KE-001), consisting of data from 158 plots, subdivided into 3,160 subplots, across 25 forests. All plots include data on tree identity, diameter and height. Abundance of shrubs is presented for 316 subplots. We recorded 600 taxa belonging to 80 families, 549 of which identified to species and 51 to genus level. Species richness per forest site varied between 43 and 195 species; mean diameter between 13.0 ± 9.8 and 30.7 ± 20.7 cm; and mean tree height between 5.49 ± 3.99 and 12.29 ± 10.61 m. This is the first plot-level database of plant communities across Kenyan coastal forests. It will be highly valuable for analysing biodiversity patterns and assessing future changes in this ecosystem. Taxonomic reference: African Plant Database (African Plant Database version 3.4.0). Abbreviations: DBH = diameter at breast height; GIVD = Global Index of Vegetation-Plot Databases; KECF-VPD = Kenyan Coastal Forests Vegetation Plot Database.

Silvicultural herbicides and forest succession influence understory vegetation and nutritional ecology of black-tailed deer in managed forests

The increasing intensity of forestry practices, especially using herbicides to promote early survival and growth of planted conifers, has the potential to influence the abundance of nutritious, shade-intolerant forbs and shrubs on which black-tailed deer (Odocoileus hemionus columbianus) and other forest ungulates depend. Therefore, we compared summer forage resources and nutrition of female black-tailed deer in stands that received herbicide treatments paired with stands that did not in Douglas-fir/western hemlock forests of western Washington. We investigated how these responses changed as forests aged from early seral to canopy closure (2–20 years post-harvest) in both treatments, and also in unmanaged mid-seral stands (30–90 years old). We measured the biomass and nutritional quality of understory vegetation and the diet composition, diet quality and nutrient intake of tractable black-tailed deer within each paired stand. We found that herbicides did not affect the rate of closure of the overstory canopy, but influenced plant species composition of the understory vegetation and reduced total and non-conifer understory biomass, primarily in the first 9 years before canopy closure. Herbicides also reduced the biomass of two of the six plant functional groups, evergreen shrubs and deciduous shrubs and trees, but did not reduce forb, graminoid, or fern biomass. Herbicides reduced bite size, harvest rate, daily dry matter intake and daily digestible energy (DE) intake of deer, but increased dietary digestible protein (DP) content, and these effects occurred primarily in the first 3 years post-treatment. Biomass of plants suitable for black-tailed deer, and the deers’ bite rate, daily feeding time, dietary DE, and daily digestible DP intake did not respond to herbicides. Our yearling and sub-adult, non-lactating deer met requirements for yearling growth and adult maintenance for dietary DE in 78%, and DE intake in 27% of plots, but only met the higher requirements for lactating females in 5% of plots. All plots where daily DE requirements of lactation or growth were met were <10 years old. Deer were equally likely to obtain diets that satisfied requirements in treated and untreated plots, indicating that although herbicide application reduced understory biomass and DE intake, it did not explain whether deer were able to meet requirements. We also found that the biomass of deciduous shrubs and trees, forbs, and suitable forage was ~25 times greater in 4–9 year old stands than closed canopy stands (14–90 years old), indicating the importance of maintaining early seral habitat for sustaining populations of black-tailed deer.

Effects of food availability on the trophic niche of the hazel dormouse Muscardinus avellanarius

The scale at which variations in food availability affect the foraging habits of individual animals can determine how the distribution of food resources affects populations. For species of conservation concern, these factors can have important implications for the management of habitats, as spatial and temporal variations in resource availability influence the trophic ecology of both individuals and populations. The hazel dormouse Muscardinus avellanarius is a species with seasonal dietary shifts and limited ranging, and whose populations in Great Britain are exhibiting marked decline, despite conservation measures. We compared resource availability and variation in dormouse traits with their trophic characteristics, determined by stable isotope analysis of dormouse hair and of their putative food items.The trophic levels of individual dormice were associated with the abundance of invertebrates in the surrounding habitat and in the woodland as a whole. Assessment of dormouse dietary composition suggests that the proportions of invertebrates and of tree seeds and flowers in dormouse diets are affected by the abundance of food plants in the local habitat. This suggests that dormice can exploit both invertebrates and plants in proportion to their availability, and are variable in their predatory habits, in response to both the availability of invertebrates and plants.Dormouse populations exhibit a broader trophic niche in autumn than in spring, most likely a consequence of their consumption of foods derived from a wider variety of tree and shrub species. We found no relationship between isotope signatures or food availability and the body mass or torpor of individuals, or the status of populations. This may be because, on the sites we studied, dormice could adapt to different food availabilities without discernible individual and population effects.Dormice are opportunistic feeders, rather than specialists, making use of abundant food resources at a local scale. Habitat conservation for dormice, therefore, could benefit from establishing and maintaining habitats that increase the overall abundance and uniform distribution of both flower and fruit-bearing shrubs and trees and invertebrate populations, at a fine spatial scale.

Long-term influence of season of grazing and rainfall on vegetation in the eastern Karoo, South Africa

In the Nama-Karoo, South Africa, rainfall and grazing by livestock are two important drivers of botanical composition. Summer rains are advantageous particularly to the grass layer, whereas the characteristic dwarf shrubs survive better than grasses during drought, and benefit from rainfall during cool months. Heavy grazing by livestock during summer reduces the abundance of grasses, whereas winter grazing increases it. This study examined the influences of season of grazing (summer vs winter) and rainfall on botanical composition at two long-term grazing trials at Grootfontein in the eastern Karoo from 1949 to 1967. Severe summer grazing strongly reduced grass abundance and slightly increased the abundance of both palatable and unpalatable dwarf shrubs. High previous-season rainfall strongly promoted short-lived grasses. Dwarf shrubs and grasses declined in response to a decline in annual rainfall over the study period, except for the creeping grass Tragus koelerioides that increased in abundance, possibly in response to decreased competition and an increase in available gaps in the vegetation layer. Results indicate that grazing systems that exclude repeated summer-only grazing maintain ecological health, and that rainfall is the main driver of composition unless extreme grazing systems are employed.

Vegetation and ecosystem carbon recovery following shifting cultivation in Mizoram-Manipur-Kachin rainforest eco-region, Southern Asia

BackgroundShifting cultivation (locally known as “jhum”) is a major driver of deforestation and loss of ecosystem services in rainforests. For developing any effective conservation of biodiversity and carbon service program, an in-depth understanding to the recovery of vegetation and carbon after abandonment of jhum is essential. We estimated species richness, abundance and composition of trees, shrubs and herbs, carbon distribution in aboveground and belowground components along a chronosequence of jhum fallow in northeast India, and elucidated the factors affecting the recovery processes of jhum fallows.MethodsSpecies composition and other plant community attributes, carbon storage in different pools were studied in 5 jhum fallows (< 5, 5–10, 11–15, 16–20, 21–25 years old) and an old-growth forest. The data were subjected to linear mixed effect modeling using R-package “nlme” for identifying the important factors contributing to the recovery of vegetation and carbon.ResultsSpecies composition varied significantly (P < 0.05) between jhum fallows and old-growth forest. Tree density varied from 28 stems ha−1 in 5 years old jhum fallow to 163 stems ha−1 in old-growth forest. Both biomass carbon in all components and soil organic carbon were significantly (P = 0.01) lower in jhum fallows than in the old-growth forest except living non-woody biomass component. The recovery of aboveground biomass carbon was faster during early successive years than the mid-successive jhum fallows. Total ecosystem carbon and soil organic carbon stock in the oldest jhum fallow was 33% and 62% of those in the old-growth forest, respectively. The fallow age was found to be the most important explanatory factor in the recovery process of vegetation and carbon stock in re-growing fallows.ConclusionThe shifting cultivation fallows gradually recovered both vegetation and carbon and are potential repository sites for biodiversity conservation, which may take much longer time to reach up to old-growth forest in northeast India.

Reindeer trampling promotes vegetation changes in tundra heathlands: Results from a simulation experiment

AbstractQuestionHerbivores exert strong influences on vegetation through activities such as trampling, defoliation, and fertilization. The combined effect of these activities on plant performance may cause dramatic vegetation shifts. Because herbivore pressures and the relative importance of their different activities are not equally distributed across the landscape, it is important to understand their isolated effect. One example of an herbivore‐induced vegetation shift is the reindeer‐driven transition from a subarctic tundra vegetation dominated by dwarf shrubs into a more productive, graminoid‐dominated state. Here, we asked how each of the grazing activities by reindeer separately and combined shape vegetation composition.LocationNordreisa, Norway.MethodsWe used a field experiment over six summers to study the separate and interacting effects of reindeer trampling, defoliation, addition of faeces and removal of moss on tundra heath vegetation, and to identify which of these factors were most important in driving the plant community towards a graminoid‐dominated state.ResultsThe combination of all treatments resulted in the strongest changes in vegetation, but trampling was the single most important factor altering the vegetation composition by reducing the abundance of both evergreen and deciduous dwarf shrubs. In contrast to what was expected, none of our treatments, separate or combined, resulted in an increased abundance of graminoids in 5 years, although such rapid vegetation changes have been observed in the field in similar environmental conditions.ConclusionsTrampling is the key process by which reindeer influence the abundance of functional groups, but only many processes combined result in strong changes in community composition. Moreover, additional factors not included in this experiment, such as urine, may be important in causing a state shift to a graminoid‐dominated community.

Lo esencial es invisible a los ojos (humanos): patrones UV explican la mayor tasa de visita de los polinizadores a las flores amarillas del arbusto Cytisus scoparius

A menudo, los polinizadores emplean los colores de las flores como senales indirectas del status de la recompensa. Sin embargo, los visitantes florales pueden percibir patrones del espectro cromatico invisibles para el ojo humano. Cytisus scoparius (retama) es un arbusto abundante en bosques perturbados del NO de la Patagonia que presenta variacion intra-especifica en el color de sus flores. Nosotros evaluamos si la mayor tasa de visita de los polinizadores a las flores amarillas, previamente documentada, es consecuencia de un patron UV mas atractivo en este morfo. Contamos cada 4 dias a lo largo de tres semanas el numero de visitas a flores amarillas y rojas rociadas con bloqueador solar (bloqueando el espectro UV), flores sin bloqueador (control) y flores con bloqueador en su pedicelo (para controlar posibles efectos indeseados del bloqueador, como el olor). El efecto del bloqueador solar en los petalos de las flores amarillas redujo la tasa de visita desde la primera medicion. Al final del muestreo, las flores amarillas con bloqueador en sus petalos fueron visitadas 7 veces menos que las flores control. Sin embargo, el efecto del bloqueador sobre los petalos de las flores rojas recien fue evidente al final del muestreo, reduciendo la tasa de visita solo 3 veces. Estos resultados sugieren que ambos morfos florales poseen patrones UV y que estos patrones atraen polinizadores, pero que su presencia estimula mas visitas en las flores amarillas que en las rojas. Esto puede deberse a que el espectro UV es mas contrastante en fondos amarillos que rojos. Finalmente, las flores amarillas y rojas con bloqueador solar en sus petalos tuvieron una tasa de visita similar, sugiriendo que los polinizadores se ven mas atraidos hacia las flores amarillas debido a la existencia de patrones UV y no por el color amarillo per se. Este trabajo resalta la importancia de analizar los patrones cromaticos invisibles al ojo humano para comprender mejor la relacion entre las senales florales y las tasas de visita de los polinizadores. https://doi.org/10.25260/EA.20.30.1.0.911

Fire regimes and structural changes in oak-pine forests of the Mogollon Highlands ecoregion: Implications for ecological restoration

Ponderosa pine (Pinus ponderosa) forests occur at their warmer, drier environmental limits in the Mogollon Highlands ecoregion (MHE) of the Southwestern United States, and are commonly found in stringers or discrete stands that form ecotones with interior chaparral. These “rear edge” forests are likely to be highly vulnerable to rapid changes in structure and composition with climate warming, drought, and wildfire. There is increasing interest in understanding historical conditions, ecosystem changes, and restoration needs for MHE forests. However, comprehensive reconstruction analysis of fire regimes and stand structure has not been done for these systems, which differ from many montane ponderosa pine forests by having an abundance of understory shrubs. In this study we used demographic data from field plots, fire scar samples, and dendroecology to reconstruct historical fire regimes and landscape structure at ponderosa pine-dominated sites that spanned a range of environmental conditions on the Prescott and Tonto National Forests. We found strong evidence of historical surface fire regimes with mean fire intervals ranging 1.3–15.6 years across the five MHE sites during the period 1700–1879. We found very little evidence of historical high-severity fire at any study site. Historical forest structure was open with tree densities ranging 84.7–136.4 trees ha−1 and stand basal area (BA) ranging 4.5–8.4 m2 ha−1. Historical composition showed codominance of ponderosa pine, Arizona white oak (Quercus arizonica), Emory oak (Q. emoryi), and Gambel oak (Q. gambelii). Thus, oak species and likely other hardwoods were important historical components of these ecosystems. Contemporary forests are greater in stand density and BA by 359–703% and 285–502%, respectively, compared to historical estimates. In addition, we observed contemporary shifts in species composition. Changes related to disruption of historical fire regimes have increased susceptibility of ponderosa pine forests in the MHE to rapid shifts in structure and composition that may come about with climate change and high-intensity wildfire. Meeting fuels reduction and ecological restoration goals will be challenging for land managers due to vigorous regeneration responses of shrubs to tree thinning, prescribed burning, or other management activities. Managers will be required to balance attention to historical reference conditions, conservation of biological diversity, and needs for fuels management.

Living in a world of fire: the population dynamics of Mulinum spinosum in Northwestern Patagonia grasslands

How do fire frequency and fire size affect the long-term population dynamics of Mulinum spinosum? Which demographic parameters contribute most to the overall effect of fire on population growth? What is the relative importance of resprouting in sustaining population increase? Grass-shrub northwestern Patagonian steppe, Argentina. We monitored five permanent plots excluded from grazing for 6 years. We measured shrub abundance, dimensions, reproductive status, seedling emergence, and the size structure of M. spinosum, a resprouting native shrub inhabiting the northwest Patagonian steppe. Data were used to parametrize a stochastic matrix model developed to explore the influence of fire frequency and fire size on long-term population dynamics. We analyzed hypothetical scenarios that included fire frequencies ranging from one per year up to exclusion. Except for annual fires, projections show growing populations regardless of fire frequency. For fire return intervals greater than 50 years, the population becomes independent of fires, with an annual population growth rate of 5.6%. The results suggest two relevant aspects of the population dynamics of this species: M. spinosum is well adapted to the current fire frequency and its resprouting capability will allow M. spinosum to survive and persist in the community, even under frequent fires. Climate change models forecast an increase in summer temperature in NW Patagonia and, consequently, an enhanced fire frequency. Fire is a driver of M. spinosum encroachment that gets worse in overgrazed grasslands. Mulinum spinosum encroachment derivate in a relative replacement of palatable grasses by shrubs changes the ecosystem functionality and reduces productivity. Controlling this process is highly difficult and we suggest a change in the land use for the areas already deteriorated.

Interactive effects of fuels reduction and large herbivores on shrub assemblages in dry conifer forests of the interior west, USA

Deciduous shrubs are widely distributed throughout temperate and boreal conifer forests and influence a wide range of ecological processes and forest resources. In the interior western U.S., many deciduous shrubs are highly preferred forage by wild (elk, Cervus canadensis; deer, Odocoileus spp.) and domestic (cattle) ungulates which can influence shrub abundance, composition, structural characteristics, and related ecological processes and interactions. Stand disturbances and silvicultural practices can also affect shrub assemblages and managers in the interior western U.S. are increasingly implementing fuels reduction treatments such as stand thinning and prescribed fire to reduce fuel loads caused by more than a century of fire suppression. We evaluated the effects of ungulate herbivory and fuels reduction, alone and in concert, on deciduous shrub assemblages in coniferous dry forests of the interior west. We measured shrub richness, diversity, height, abundance and community composition in forest stands that underwent fuels reduction 15–17 years earlier, compared to untreated stands where no silvicultural treatments have occurred in over 50 years. Within each stand type, we also measured shrub assemblages in stands with and without ungulate herbivory. Shrub richness, diversity, frequency and height all declined in stands subjected to either fuels reduction treatments or herbivory; effects were most pronounced under the combined effect of fuels reduction and herbivory. Fuels reduction and herbivory also resulted in significant differences in shrub abundance and assemblage composition. Fuels reduction in dry forests with abundant ungulates may contribute to suppressed, more homogenous shrub communities. These effects may result in unintended impacts or alterations to important ecosystem processes and forest resources. Our results highlight the importance of considering responses of forest resources with low economic value, such as shrubs, in forest management activities.

Pathways of tundra encroachment by trees and tall shrubs in the western Brooks Range of Alaska

Climate change is expected to increase woody vegetation abundance in the Arctic, yet the magnitude, spatial pattern and pathways of change remain uncertain. We compared historical orthophotos photos (1952 and 1979) with high‐resolution satellite imagery (2015) to examine six decades of change in abundance of white spruce Picea glauca and tall shrubs (Salix spp., Alnus spp.) near the Agashashok River in northwest Alaska. We established ~3000 random points within our ~5500 ha study area for classification into nine cover types. To examine physiographic controls on tree abundance, we fit multinomial log‐linear models with predictors derived from a digital elevation model and with arctic tundra, alpine tundra and ‘tree’ as levels of a categorical response variable. Between 1952 and 2015, points classified as arctic and alpine tundra decreased by 31% and 15%, respectively. Meanwhile, tall shrubs increased by 86%, trees mixed with tall shrubs increased by 385% and forest increased by 84%. Tundra with tall shrubs rarely transitioned to forest. The best multinomial model explained 71% of variation in cover and included elevation, slope and an interaction between slope and ‘northness’. Treeline was defined as the elevation where the probability of tree presence equaled that of tundra. Mean treeline elevation in 2015 was 202 m, corresponding with a June–August mean air temperature &gt; 11°C, which is &gt; 4°C warmer than the 6–7°C isotherm associated with global treeline elevations. Our results show dramatic increases in the abundance of trees and tall shrubs, question the universality of air temperature as a predictor of treeline elevation and suggest two mutually exclusive pathways of vegetation change, because tundra that gained tall shrubs rarely transitioned to forest. Conversion of tundra to tall shrubs and forest has important and potentially contrasting implications for carbon cycling, surface energy exchange and wildlife habitat in the Arctic.

Forest Landscape Heterogeneity Increases Shrub Diversity at the Expense of Tree Seedling Diversity in Temperate Mixedwood Forests

Partial disturbances enhance spatial heterogeneity through the diversification of forest structure, which contributes to niche partitioning and consequently to species diversity. However, this heterogeneity–diversity relationship may differ between groups of species, and is potentially modified by biotic interactions at the community level. We propose that shrub diversity will be greater in heterogeneous landscapes, while tree diversity will be lower in those same landscapes, due to the biotic interactions of shrub competition. We conducted field sampling in the balsam fir/yellow birch bioclimatic domain in western Québec, a forested ecosystem disturbed by natural and anthropogenic partial disturbances. We selected 12 forested landscapes (1 km2), four in each of three classes of landscape heterogeneity (heterogeneous, moderately heterogeneous, homogenous). Shrub and tree species regeneration abundance was measured in three different size classes of canopy gaps and the forest understory. Gap proportions were assessed in each landscape using aerial LiDAR data. Tree and shrub alpha-diversity significantly responded to landscape heterogeneity, shrubs being more diverse while tree seedlings were less diverse in heterogeneous landscapes. Heterogeneous landscapes showed highest species accumulation rates for shrubs in medium-sized gaps. For tree seedlings, species accumulation rates were highest in heterogeneous landscapes in the forest understory. Our study thus supports the heterogeneity–diversity relationship with shrubs having higher alpha and beta diversity in heterogeneous landscapes whereas local-scale tree diversity was higher in homogenous landscapes.

Positive interaction facilitates landscape homogenization by shrub expansion in the forest–tundra ecotone

AbstractQuestionsChanges in vegetation structure, including shrub expansion, occur in forest–tundra ecotones in sub‐arctic regions. However, the community‐level processes driving vegetation change are poorly understood. We aimed to evaluate factors mediating the assembly processes for community initiators of vegetation change and determine the ecological processes driving vegetation changes.LocationKuujjuarapik/Whapmagoostui (55.31 N, 77.75 W), Quebec, Canada.MethodsVegetation was surveyed in eight 100‐m line transects with 16 quadrats (1 m × 1 m) per transect established in both forest and tundra habitats. To elucidate the assembly processes of the initiators of vegetation change and other community components of the forest–tundra ecotone, we evaluated β‐diversity among and within habitats and its causal factors (i.e. spatial and environmental factors). Interspecies interactions were estimated based on the patterns of co‐occurrence between all pairs of species.ResultsAlthough environmental and spatial factors significantly affected the community structure of the specific and shared species in the forest habitat, the variation in community structure within and among tundra habitats was not explained by environmental or spatial factors for any community components. Therefore, community assemblages of shared species in tundra habitat were independent from local conditions determined by environment and spatial location. However, we found a positive co‐occurrence pattern among the dominant shared shrubs to be a characteristic of the tundra habitat, and this relationship explained the patterns of community structure within tundra habitats.ConclusionsCommunity assemblages of initiators of vegetation change in tundra habitats are unrestricted by any external factors, such as dispersal limitation or environmental filtering. However, positive relationships in the abundance or occurrence of dominant shrubs may indicate a positive feedback loop between colonizing species and environmental modifications. This positive feedback loop may be a driving process in the expansion of shared species and vegetation change.

Locally high, but regionally low: the impact of the 2014–2016 drought on the trees of semi-arid savannas, South Africa

Drought is thought to be a factor limiting the abundance of shrubs and trees in savannas, but this has rarely been tested empirically. The severe drought of 2014–2016 provided an opportunity to document the impact of drought on the mortality of woody plants in semi-arid savannas of South Africa. Annual data from long-term tree monitoring plots were combined with opportunistic sampling in areas of high mortality to determine the extent and severity of mortality across an area of approximately 10 000 km2. Although mortality was not extensive, certain landscapes suffered mortality similar to the highest values reported for semi-arid ecosystems. Rainfall deficit was identified as a key determinant of spatial variation in mortality at the regional scale, which varied from 3% to 49% over two years. Spatial variation at the landscape scale was also high, and affected primarily by species composition and height of the woody plants, with higher mortality for taller individuals. An effect of topographic position and unknown edaphic factors was also evident. Species that suffered high mortality included a common bush encroachment species, but also many non-encroachers. Additional study is needed to confirm that the observed mortality will cause a long-term reduction in woody plant density.

Browsing and fire decreases dominance of a resprouting shrub in woody encroached grassland

North American grasslands have experienced increased relative abundance of shrubs and trees over the last 150yr. Alterations in herbivore composition, abundance, and grazing pressure along with changes in fire frequency are drivers that can regulate the transition from grassland to shrubland or woodland (a process known as woody encroachment). Historically, North American grasslands had a suite of large herbivores that grazed and/or browsed (i.e., bison, elk, pronghorn, deer), as well as frequent and intense fires. In the tallgrass prairie, many large native ungulates were extirpated by the 1860s, corresponding with increased homesteading (which led to decreased fire frequencies and intensities). Changes in the frequency and intensity of these two drivers (browsing and fire) have coincided with woody encroachment in tallgrass prairie. Within tallgrass prairie, woody encroachment can be categorized in to two groups: non-resprouting species that can be killed with fire and resprouting species that cannot be killed with fire. Resprouting species require additional active management strategies to decrease abundance and eventually be removed from the ecosystem. In this study, we investigated plant cover, ramet density, and physiological effects of continuous simulated browsing and prescribed fire on Cornus drummondii C.A. Mey, a resprouting clonal native shrub species. Browsing reduced C.drummondii canopy cover and increased grass cover. We also observed decreased ramet density, which allowed for more infilling of grasses. Photosynthetic rates between browsed and unbrowsed control shrubs did not increase in 2015 or 2016. In 2017, photosynthetic rates for browsed shrubs were higher in the unburned site than the unbrowsed control shrubs at the end of the growing season. Additionally, after the prescribed fire, browsed shrubs had ~90% decreased cover, ~50% reduced ramet density, and grass cover increased by ~80%. In the roots of browsed shrubs after the prescribed fire, nonstructural carbohydrates (NSC) experienced a twofold reduction in glucose and a threefold reduction in both sucrose and starch. The combined effects of browsing and fire show strong potential as a successful management tool to decrease the abundance of clonal-resprouting woody plants in mesic grasslands and illustrate the potential significance of browsers as a key driver in this ecosystem.

Arctic soil carbon turnover controlled by experimental snow addition, summer warming and shrub removal

Northern latitude tundra heaths have accumulated large amounts of organic carbon (C) in the soil. Changes in climatic conditions such as temperature and winter precipitation might affect the C balance and potentially change these tundra ecosystems from being C sinks to sources of CO2 emitted to the atmosphere. However, studies on C fluxes with single and combined winter snow and summer warming effects are scarce. This study investigates gross ecosystem production (GEP), ecosystem respiration (ER), net ecosystem production (NEP) and carbon isotopic composition of CO2 emitted from a dry heath in arctic Greenland one and two years following field manipulations of summer temperature, shrub abundance and winter snow depth. Our aims were to quantify climatic change effects on CO2 fluxes and the growing season carbon balance of the ecosystem and to investigate shifts in δ13C of emitted CO2 potentially due changes in emission from old soil C versus recently fixed carbon. Ecosystem CO2 fluxes and δ13C-CO2 were measured using closed chambers, and soil CO2 concentrations and δ13C were measured depth-specifically using gas probes.We found a significant increase of CO2 emissions in all treatments during both years. Growing season NEP increased by 38 and 73% with 1 m enhanced winter snow depth, by 113 and 144% with summer warming and by 61 and 320% with total shrub removal in 2013 and 2014, respectively. The snow effect can be explained by the delay in the onset of growth as indicated by early season reduced vegetation greenness. The effect of warming was a result of an increase of ER by 39 and 32%, and the effect of shrub removal was mainly due to a reduction in GEP by 34 and 48%, in 2013 and 2014, respectively. Furthermore, the δ13C of the carbon source of CO2 emitted from warmed plots increased significantly two years after the experiment was initiated. This might indicate increased decomposition of 13C enriched soil organic matter and hence increased mineralization of the old carbon stock in the soil under warmed conditions. The increase of NEP, the additive response of all treatments, and the indications of increased emission of carbon from old stocks due to warming (or warming-induced drying), demonstrate the risk of a relatively fast feedback to climate warming during the snow-free season.