Deciduous Shrub Cover Research Articles

Fire disturbance promotes biodiversity of plants, lichens and birds in the Siberian subarctic tundra.

Fire shapes the world's terrestrial ecosystems and has been influencing biodiversity patterns for millennia. Anthropogenic drivers alter fire regimes. Wildfires can amplify changes in the structure, biodiversity and functioning of the fast-warming tundra ecosystem. However, there is little evidence available, how these fires affect species diversity and community composition of tundra ecosystems over the long term. We studied long-term fire effects on community composition and diversity at different trophic levels of the food web in the subarctic tundra of Western Siberia. In a space-for-time approach we compared three large fire scars (>44, 28 and 12years old) to unburnt controls. We found that diversity (measured as species richness, Shannon index and evenness) of vascular and non-vascular plants and birds was strongly affected by fire, with the greatest species richness of plants and birds for the intermediate-age fire scar (28years). Species composition of plants and birds still differed from that of the control >44years after fire. Increased deciduous shrub cover was related to species richness of all plants in a hump-shaped manner. The proportion of southern (taiga) bird species was highest in the oldest fire scar, which had the highest shrub cover. We conclude that tundra fires have long-term legacies with regard to species diversity and community composition. They may also increase landscape-scale species richness and facilitate range expansions of more southerly distributed species to the subarctic tundra ecosystem.

Shrub shading moderates the effects of weather on arthropod activity in arctic tundra

1. Rapid warming has facilitated an increase in deciduous shrub cover in arctic tundra. Because shrubs create a cooler microclimate during the growing season, shrub cover could modulate the effects of global warming on the phenology and activity of ectotherms, including arthropods. This possibility was explored here using two dominant arthropod groups (flies and wolf spiders) in Alaskan tundra.2. We monitored arthropods with pitfall traps over five summers at four sites that differed in shrub abundance, and used generalised additive mixed models (GAMMs) to separate the two underlying components of pitfall trap catch: the seasonal trend in arthropod density and the effects of short‐term weather variation (air temperature, wind speed, rainfall, solar radiation) on arthropod activity.3. We found that shrub cover significantly altered the seasonal trend in the abundance of flies by reducing early‐season pitfall catch, in line with observed later snowmelt in shrub‐dominated plots at these sites.4. Additionally, shrub cover modulated the effects of many weather variables on arthropod activity: shrub cover shifted wolf spiders' temperature–activity relationship, dampened the positive effect of solar radiation on the activity of arthropods in total, and ameliorated the negative effect of wind on the activity of flies.5. Thus, these results indicate that shrub encroachment will probably be accompanied by altered arthropod responses to warming and other key weather variables. Because the rate of key ecological processes – herbivory, decomposition, predation – are controlled by activity at the organismal level, these effects on arthropods will have long‐term ecosystem‐level consequences.

Potential Arctic tundra vegetation shifts in response to changing temperature, precipitation and permafrost thaw

Abstract. Over the past decades, vegetation and climate have changed significantly in the Arctic. Deciduous shrub cover is often assumed to expand in tundra landscapes, but more frequent abrupt permafrost thaw resulting in formation of thaw ponds could lead to vegetation shifts towards graminoid-dominated wetland. Which factors drive vegetation changes in the tundra ecosystem are still not sufficiently clear. In this study, the dynamic tundra vegetation model, NUCOM-tundra (NUtrient and COMpetition), was used to evaluate the consequences of climate change scenarios of warming and increasing precipitation for future tundra vegetation change. The model includes three plant functional types (moss, graminoids and shrubs), carbon and nitrogen cycling, water and permafrost dynamics and a simple thaw pond module. Climate scenario simulations were performed for 16 combinations of temperature and precipitation increases in five vegetation types representing a gradient from dry shrub-dominated to moist mixed and wet graminoid-dominated sites. Vegetation composition dynamics in currently mixed vegetation sites were dependent on both temperature and precipitation changes, with warming favouring shrub dominance and increased precipitation favouring graminoid abundance. Climate change simulations based on greenhouse gas emission scenarios in which temperature and precipitation increases were combined showed increases in biomass of both graminoids and shrubs, with graminoids increasing in abundance. The simulations suggest that shrub growth can be limited by very wet soil conditions and low nutrient supply, whereas graminoids have the advantage of being able to grow in a wide range of soil moisture conditions and have access to nutrients in deeper soil layers. Abrupt permafrost thaw initiating thaw pond formation led to complete domination of graminoids. However, due to increased drainage, shrubs could profit from such changes in adjacent areas. Both climate and thaw pond formation simulations suggest that a wetter tundra can be responsible for local shrub decline instead of shrub expansion.

Tight Coupling Between Shoot Level Foliar N and P, Leaf Area, and Shoot Growth in Arctic Dwarf Shrubs Under Simulated Climate Change

Nutrient availability limits productivity of arctic ecosystems, and this constraint means that the amount of nitrogen (N) in plant canopies is an exceptionally strong predictor of vegetation productivity. However, climate change is predicted to increase nutrient availability leading to increases in carbon sequestration and shifts in community structure to more productive species. Despite tight coupling of productivity with canopy nutrients at the vegetation scale, it remains unknown how species/shoot level foliar nutrients couple to growth, or how climate change may influence foliar nutrients–productivity relationships to drive changes in ecosystem carbon gain and community structure. We investigated the influence of climate change on arctic plant growth relationships to shoot level foliar N and phosphorus (P) in three dominant subarctic dwarf shrubs using an 18-year warming and nutrient addition experiment. We found a tight coupling between total leaf N and P per shoot, leaf area and shoot extension. Furthermore, a steeper shoot length-leaf N relationship in deciduous species (Vaccinium myrtillus and Vaccinium uliginosum) under warming manipulations suggests a greater capacity for nitrogen to stimulate growth under warmer conditions in these species. This mechanism may help drive the considerable increases in deciduous shrub cover observed already in some arctic regions. Overall, our work provides the first evidence at the shoot level of tight coupling between foliar N and P, leaf area and growth i.e. consistent across species, and provides mechanistic insight into how interspecific differences in alleviation of nutrient limitation will alter community structure and primary productivity in a warmer Arctic.

Trends in NDVI and Tundra Community Composition in the Arctic of NE Alaska Between 1984 and 2009

As Arctic ecosystems experience increases in surface air temperatures, plot-level analyses of tundra vegetation composition suggest that there are important changes occurring in tundra communities that are typified by increases in shrubs and declines in non-vascular species. At the same time analyses of NDVI indicate that the Arctic tundra is greening. Few studies have combined plot-level trends in species composition and cover with remote sensing measurements to understand the linkages between tundra vegetation dynamics and NDVI over time. This study reports on trends in species composition for field plots in the Arctic National Wildlife Refuge in NE Alaska from 1984 to 2009 and links these trends to the trends in NDVI at fine and coarse scales. Over this time frame there were few changes in plant community composition. None of the five tundra types that were measured had increases in total vegetative cover, and deciduous shrub cover did not show the large increases reported elsewhere. Surface-(plot) measured NDVI was positively correlated to deciduous and evergreen shrub composition suggesting that these functional groups had a strong influence on NDVI values. Modeled values of NDVI, derived from measures of deciduous and evergreen shrub composition over time, decreased slightly for tussock tundra but did not change for other tundra types. This result suggests that surface NDVI did not change over time on these tundra types. Fine-scale (30-m pixels) Landsat NDVI also did not show any changes for the pixels located at the permanent plots (1992–2009). However, coarse-scale (8-km pixels) AVHRR NDVI across the study area did increase (1988–2007). Furthermore, aggregate values of Landsat pixels matching the same area as AVHRR pixels also did not show significant changes over time. Although Landsat NDVI was consistent with surface-measured NDVI, AVHRR NDVI was not. AVHRR NDVI values showed increases that were in neither the field nor Landsat data. This result suggests that AVHRR may be demonstrating increasing trends in NDVI that are not occurring on the ground in some Arctic tundra ecosystems. These results highlight the need to combine remote sensing with on-the-ground measurements of plant community composition and NDVI in the analysis of the responses of Arctic tundra ecosystems to climate change.

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.

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.

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.

English

This paper looks at two methods for visualizing and mapping vegetation change on a large surface mining complex in southern West Virginia. Two Quickbird image sets, acquired in 2003 and 2007, were used to identify vegetation trends and to map significant change events manifested during the four year interval. Vegetation trends were visualized using Normalized Difference Vegetation Index (NDVI) difference images, which proved to be a simple and effective means of identifying vegetation change events for further investigation. The study then evaluated Feature Analyst—a commercial analysis package—for its ability to map and quantify two of the most significant change events identified in the study area. A field investigation used handheld GPS receivers equipped with ArcPad software and GPS-equipped cameras to verify the cause of the two events—1) the defoliation of stands of black locust, and 2) a significant increase in area dominated by deciduous shrub vegetation, caused by rapid growth in autumn olive. Feature Analyst was able to delineate the extent of black locust defoliation, estimated at over 152 acres on several reclaimed permits. In the second case, the analysis estimated that deciduous shrub cover expanded from 32 acres to over 81 acres on one permit, representing an increase from 6.5% to 16.5% of the total permit area. Feature Analyst showed significant promise for extracting vegetation features from the source images, including individual trees. Feature Analyst's ability to effectively utilize panchromatic and multispectral image sets suggests it is an effective tool for use with increasingly high resolution data available from commercial vendors.

Evaluation and prediction of shrub cover in coastal Oregon forests (USA)

We used data from regional forest inventories and research programs, coupled with mapped climatic and topographic information, to explore relationships and develop multiple linear regression (MLR) and regression tree models for total and deciduous shrub cover in the Oregon coastal province. Results from both types of models indicate that forest structure variables were most important for explaining both total and deciduous shrub cover. Four relationships were noted: (1) shrub cover was negatively associated with Tsuga heterophyllabasal area and density of shade-tolerant trees; (2) shrub cover was negatively associated with variables that characteristically peak during stem exclusion and mid-succession; (3) shrub cover was positively associated with variables that characterize later successional stages; and (4) higher total and especially deciduous shrub cover were positively associated with hardwood stands. Environmental variables were more important for explaining deciduous shrub cover compared to total shrub cover, but they have an indirect effect on total shrub cover by influencing tree composition. However, because of land ownership patterns, it was difficult to decouple environmental from disturbance factors associated with management strategies across multiple ownerships. Tree models performed similarly (PRD = 0.17–0.27) or better compared to MLR models (PRD = 0.17–0.23) although they contained more (2) predictor variables. Our results indicate that response variable transformation can greatly improve regression tree model performance. While interpretation of MLR and tree models were somewhat similar, the tree models allowed a more explicit understanding of relationships and provided thresholds for anticipating shifts in shrub cover. Such thresholds are useful to forest managers who are monitoring and evaluating critical amounts of shrub cover necessary for different ecosystem components such as bird habitat. Lack of strong predictive power in both types of models may be because many common shrubs can persist and maintain consistent cover under a variety of stand and environmental conditions or there may be a lag time between disturbance events and shrub response. The stochastic nature of disturbance and their interactions with site conditions also makes prediction at this scale in this highly managed landscape inherently problematic. Yet, our models provide both a predictive and conceptual tool for understanding shrub cover patterns across the region. © 2004 Elsevier Ltd. All rights reserved.

Differences in arctic tundra vegetation type and phenology as seen using bidirectional radiometry in the early growing season

Ground-based nadir and off-nadir radiometric measurements were collected on the North Slope of Alaska in early June to determine their utility in 1) differentiating arctic tundra vegetation community types through remote sensing, and 2) examining differences in structure and phenology of arctic tundra vegetation communities. We employed a PS-II (personal spectrometer II) portable field spectroradiometer to measure narrow-band spectra at nadir and the PARABOLA (portable apparatus for rapid acquisition of bidirectional observations of the land and atmosphere) three-channel radiometer to gather bidirectional radiometric data over almost the complete ground hemisphere. Measurements were taken at a lowland wet sedge site and at two tussock tundra sites containing different amounts of woody deciduous shrub cover. PS-II nadir measurements alone allowed spectral differentiation between the highly senescent wet sedge site and the two relatively greener tussock tundra sites but did not allow clear discrimination between tussock tundra sites. Off-nadir measurements, however, permitted good discrimination between the two tussock tundra sites. For example, the reflectance-based normalized difference vegetation index (NDVI p) difference between the tussock tundra sites at a 75° forward-scatter view angle averaged 26% over a range of solar zenith angles ( Θ s S). Similarly, NDVIp calculated from PARABOLA spectral hemispheric data averaged 6% higher in woody tussock tundra than in nonwoody tussock tundra. Differences between the tussock sites calculated by using NDVI based on hemispheric radiance (NDVI L) were greater than those calculated by using hemispheric NDVIp by approximately a factor of two regardless of Θ s Measurements taken when Θ s =75° generally exhibited the largest differences between NDVI at the two tussock tundra sites. Although nadir measurements exhibited minimal differences between tussock tundra types, bidirectional and hemispheric-based NDVI values revealed differences in tussock tundra woody vegetation cover because large view zenith angles were sensitive to the woody shrub foliage that extends above the hummocky tundra surface in early spring. These results may be useful for detecting changes in tundra woody cover through time.