Grass-dominated Areas Research Articles

Short and long-term fire effects on soil C and N in an African savanna

Fires have been an important ecosystem driver in the Earth system for millions of years. It drives ecosystem composition, diversity and structure, thus rendering numerous terrestrial ecosystems dependent on fires for maintaining ecosystem health and functioning. As an important ecological driver in many ecosystems, fires have been shown to lead to substantial impacts on soil carbon (C) and nitrogen (N). Although many global studies have looked at either short-term or long-term fire effects on soil C and N, they have contradicting outcomes. This study aimed at investigating the effect of both short- and long-term fire effects on soil C and N, C:N ratios and soil C stocks, as well as how fire-induced changes to vegetation may influence C and N distribution. Soil samples (0–5 cm) were collected under tree and shrub canopies and open, grassy areas from Annual Burn and No Burn (fire exclusion) plots at a 68-year-old long-term fire experiment. Samples were collected pre-burn, 1 m and 1 yr post-fire to determine short-term fire impacts, and from the No burn plot to determine long-term fire impacts (compared to the Annual Burn). Soils were analysed for total C, total N, C:N ratios and C stocks. Our results found that fires led to significantly higher soil C and N within the first month post-fire (p < 0.05) and remained significantly higher even up to a year later. Fires also altered soil total C and N in the long-term which saw decadal increases in C and N due to fire exclusion (p < 0.001). C:N ratios improved shortly after burning to similar C:N ratios seen in the long-term No Burn plot. Even though soil C stocks were greatest in the No Burn plot, there was also an increase in soil C stocks on the Annual Burn plot a month after the fire (p < 0.05). Lastly, trees and shrub were found to increase soil C and N below their canopies compared to open, grass-dominated areas, suggesting further indirect effects of fires on soil C and N by means of changing woody cover. These findings have implications for fire management in African savannas when considering fire suppression policies, bush encroachment and global tree-planting initiatives.

An alternative method of developing landscape strategies for urban cooling: A threshold-based perspective

Increasing the amount of blue-green spaces has been recognized as an effective strategy to mitigate the urban heat island (UHI) effect. Specific quantitative and actionable landscape mitigation strategies for different land-use and land-cover (LULC) types to cool down cities, have been rarely addressed. In this paper, the absolute threshold of cooling (ToCabs) —the specific threshold of a certain influencing factor to ensure the effective cooling of a particular patch type, was proposed and defined for the first time to facilitate the research. The effects of size, shape and neighboring greenspace percentage (NGP) of different types of LULC patches on land surface temperature (LST) were examined and their corresponding ToCabs values were determined if existed, in the metropolitan areas of Beijing, Shanghai and Tianjin using satellite images. Results demonstrate that larger-sized and complex-shaped water bodies and forested areas produce greater cooling effect. The size is irrelevant to the cooling intensity of grass-dominated patches. ToCabs of patch size exist and were identified for water bodies (Beijing, 5 ha; Shanghai, 10 ha; and Tianjin, 5 ha) and forested areas (Beijing, 20 ha; Shanghai, 50 ha; and Tianjin, 20 ha). Besides, NGP is negatively correlated with patch LST for all LULC types, indicating that increasing the amount of neighboring vegetation cover can effectively lower the nearby patch LST. ToCabs of NGP exist and were identified for water bodies (Beijing, 30.3%; Shanghai, 61.6%; and Tianjin, 20%) and grass-dominated areas (Beijing, 89.1%; Shanghai, 76.9%; and Tianjin, 81.5%). Among the three influencing factors, the size accounts for greatest variability in patch LST for forested areas and impervious surfaces, and neighboring greenspace proportion accounts for the most variability in LST for water bodies and grass-dominated areas. The methodology and results of this study could help to orientate specific and actionable landscape strategies of urban cooling.

Estimation of Groundwater Evapotranspiration of Different Dominant Phreatophytes in the Mu Us Sandy Region

Groundwater evapotranspiration (ETG) estimation is an important issue in semiarid areas for groundwater resources management and environmental protection. It is widely estimated by diurnal water table fluctuations. In this study, the ETG at four sites with different plants was estimated using both diurnal water table and soil moisture fluctuations in the northeastern Mu Us sandy region, in order to identify the groundwater utilization strategy by different dominant phreatophytes. Groundwater level was monitored by ventilatory pressure transducers (Solinst LevelVent, Solinst Canada Ltd.; accuracy ±3 mm), while soil moisture was monitored using EM50 loggers (Decagon Devices Inc., Pullman, USA) in K1 and K14 and simulated by Hydrus-1D in other observation wells. A significant spatial variation of ETG was found within a limited area, indicating a poor representativeness of site ETG for regional estimation. The mean values of ETG are 4.01 mm/d, 6.03 mm/d, 8.96 mm/d, and 12.26 mm/d at the Achnatherum splendens site, Carex stenophylla site, Salix psammophila site and Populus alba site, respectively, for the whole growing season. ETG is more sensitive to depth to water table (DWT) in the Carex stenophylla site than in the Achnatherum splendens site for grass-dominated areas and more sensitive to DWT in the Populus alba site than in Salix psammophila site for tree-dominated areas. Groundwater extinction depths are estimated at 4.1 m, 2.4 m, 7.1 m, and 2.9 m in the Achnatherum splendens site, Carex stenophylla site, Salix psammophila site and Populus alba site, respectively.

Reduced urban green cover and daytime cooling capacity during the 2012–2016 California drought

Urban vegetation mitigates elevated temperatures in cities. Drought presents an important challenge to urban heat mitigation as prolonged dry periods cause reduced evapotranspiration and losses of green vegetation cover. To measure drought impacts on the urban environment and climate, we used visible short-wave infrared satellite imagery acquired throughout the 2012–16 drought in California to quantify effects on fractional vegetation cover in the Los Angeles urbanized region. We then used satellite thermal imagery with repeat high-resolution airborne thermal imagery to measure how drought-induced reductions of green vegetation cover affected urban land surface temperature (LST) and the cooling effect of urban vegetation. Green vegetation cover declined from 29.3% in 2011 to 24.6% in 2016 in urban Los Angeles County. Over the drought, the annual mean daytime LST increased from 34.4 °C in 2010 to 37.1 °C in 2014. Despite the large-scale homogenization of LST, tree- and grass-dominated areas differed in how they were affected by the drought, with larger losses of vegetation fractional cover and stronger reductions in cooling effect for grass compared to tree-covered areas due to different plant physiological responses and irrigation changes. Results suggest that drought presents a challenge to urban resiliency, as vegetation is often a main component of heat mitigation plans.

Modeling the distribution and abundance of eastern tur (Capra cylindricornis) in the Caucasus

Our research attempts 1) to link the distribution and abundance of eastern tur (Capra cylindricornis) to environmental variables varying in space and time, and 2) to identify environmental constraints that may have caused the geographic segregation of eastern tur from western tur (Capra caucasica). Our results suggest that 1) eastern tur are heat-sensitive but prefer snow-free, grass-dominated areas near escape cliffs, and in terrain that is difficult for humans and livestock to reach; and 2) present-day genetic and phenotypic differences between the Capra species are largely related to discontinuity of and impeded migration between Capra populations during glacial periods, and insufficient time in the postglacial period for now-adjoining Capra populations to overcome behavioral mechanisms impeding hybridization in the wild so as to erase these differences.

Climate, CO2, and the history of North American grasses since the Last Glacial Maximum.

The spread of C4 grasses in the late Neogene is one of the most important ecological transitions of the Cenozoic, but the primary driver of this global expansion is widely debated. We use the stable carbon isotopic composition (δ(13)C) of bison and mammoth tissues as a proxy for the relative abundance of C3 and C4 vegetation in their grazing habitat to determine climatic and atmospheric CO2 controls on C4 grass distributions from the Last Glacial Maximum (LGM) to the present. We predict the spatial variability of grass δ(13)C in North America using a mean of three different methods of classification and regression tree (CART) machine learning techniques and nine climatic variables. We show that growing season precipitation and temperature are the strongest predictors of all single climate variables. We apply this CART analysis to high-resolution gridded climate data and Coupled Model Intercomparison Project (CMIP5) mean paleoclimate model outputs to produce predictive isotope landscape models ("isoscapes") for the current, mid-Holocene, and LGM average δ(13)C of grass-dominated areas across North America. From the LGM to the present, C4 grass abundances substantially increased in the Great Plains despite concurrent increases in atmospheric CO2. These results suggest that changes in growing season precipitation rather than atmospheric CO2 were critically important in the Neogene expansion of C4 grasses.

Assessing the impact of grass invasion on the population dynamics of a threatened Caribbean dry forest cactus

The negative impact of invasive plants on native species has been well documented, but little is known about the specific role that invaders play on the population decline of native species. Here we used demographic models to evaluate how the alien grass Megathyrsus maximus affects the population dynamics of the native cactus Harrisia portoricensis. Demographic data gathered for H. portoricensis in grass-invaded and non-invaded areas were used to construct matrix projection models. Our demographic analysis includes numerical simulations, life table response experiments, life-cycle parameters and a systematic evaluation of the effects of matrix dimensionality. Results revealed that, while positive λ values can be achieved in grass-free areas dominated by native vegetation, population growth rates of H. portoricensis are reduced in grass-invaded areas (λ=0.88–0.91). Models predicted that H. portoricensis populations in grass-invaded areas might become extinct in the next 14–59years, suggesting that the long-term persistence of this cactus may be linked to the fate of the grass invasion. Overall, our data show that the conversion of native forests into grass-dominated areas is having detrimental impacts on the population dynamics of native species, creating novel regeneration barriers that native species cannot surpass. Under this scenario, management strategies designed for the conservation of H. portoricensis should consider grass-eradication and control policies and long-term monitoring of populations. The information derived from this work may help us understand the mechanisms by which invasive species are gaining dominance over native species, which is crucial for managing and predicting biological invasions.

Low below-ground organic carbon storage in a subarctic Alpine permafrost environment

Abstract. This study investigates the soil organic carbon (SOC) storage in Tarfala Valley, northern Sweden. Field inventories, upscaled based on land cover, show that this alpine permafrost environment does not store large amounts of SOC, with an estimate mean of 0.9 ± 0.2 kg C m−2 for the upper meter of soil. This is 1 to 2 orders of magnitude lower than what has been reported for lowland permafrost terrain. The SOC storage varies for different land cover classes and ranges from 0.05 kg C m−2 for stone-dominated to 8.4 kg C m−2 for grass-dominated areas. No signs of organic matter burial through cryoturbation or slope processes were found, and radiocarbon-dated SOC is generally of recent origin (&lt;2000 cal yr BP). An inventory of permafrost distribution in Tarfala Valley, based on the bottom temperature of snow measurements and a logistic regression model, showed that at an altitude where permafrost is probable the SOC storage is very low. In the high-altitude permafrost zones (above 1500 m), soils store only ca. 0.1 kg C m−2. Under future climate warming, an upward shift of vegetation zones may lead to a net ecosystem C uptake from increased biomass and soil development. As a consequence, alpine permafrost environments could act as a net carbon sink in the future, as there is no loss of older or deeper SOC from thawing permafrost.

Acacia tree density strongly affects N and P fluxes in savanna

Studies of nutrient cycling in savanna ecosystems rarely consider how fluxes are affected by local variations in tree density and nutrient redistribution by herbivores. We studied how the density of Acacia zanzibarica trees in a humid savanna ecosystem affected the input of nitrogen (N) through N2-fixation and N and phosphorus (P) outputs through fire and also internal pathways of N and P return through herbivores. We found that N inputs and P outputs both increased with increasing density of N2-fixing trees, the N effect being due to tree density rather than to differences in the rate of N2-fixation. However, total N outputs due to fire did not vary with tree density because losses from the herb layer decreased as losses from the tree layer increased. In contrast, total P outputs did increase with tree density because P losses from the tree layer exceeded those from the herb layer. We suggest that variation in the density of N2-fixing trees coupled with the effects of fire can cause substantial differences in the local N and P balances in savanna vegetation. To some extent, these differences may be evened out by the tendency for browsing herbivores to transfer nutrients from Acacia- to grass-dominated areas. We conclude that encroachment by N2-fixing trees and shrubs has important consequences for ecosystem properties such as N and P dynamics.

Quantifying habitat loss: Assessing tree encroachment into a serpentine savanna using dendroecology and remote sensing

For over a century, increases in the abundance of woody plants in savannas have been occurring worldwide in a process known as encroachment. Encroachment into savannas is a significant management concern because it affects the unique values associated with savanna systems, including high levels of both taxonomic and landscape diversity. Improving methods for reconstructing encroachment patterns should aid savanna management, especially if the methods provide a baseline from which to assess and project ecological change. We reconstructed the encroachment history of a small serpentine Jeffrey pine savanna and forecasted future landscape change using two distinct approaches. First, we used dendroecology to determine encroachment rates, establish historical site reference conditions, and project tree-growth trends. Second, we used historical aerial photographs to construct a spatial model of past tree encroachment and to predict near future encroachment. We found encroachment began ∼1850 and was not related to differences in topography across the landscape. Trees greater than 30cm in diameter have increased from a mean stem density of ∼1.6 trees per ha in 1890 to a mean stem density of ∼13.8 trees per ha in 2009. Concomitant with the increase in tree density and average tree size was the contraction of the grass-dominated areas of the savanna, which represented ∼50% of Little Bald Hills in 1942, but less than 10% in 2009. If current encroachment rates continue, our models suggest that less than 5% of Little Bald Hills will be grassland in 50years. This is not the first study to utilize both historical photo analysis and dendroecology, but it is the first to use these tools to identify explicit locations where encroachment is likely to occur in the near future.

Testing the Amazon savannization hypothesis: fire effects on invasion of a neotropical forest by native cerrado and exotic pasture grasses.

Changes in climate and land use that interact synergistically to increase fire frequencies and intensities in tropical regions are predicted to drive forests to new grass-dominated stable states. To reveal the mechanisms for such a transition, we established 50 ha plots in a transitional forest in the southwestern Brazilian Amazon to different fire treatments (unburned, burned annually (B1yr) or at 3-year intervals (B3yr)). Over an 8-year period since the commencement of these treatments, we documented: (i) the annual rate of pasture and native grass invasion in response to increasing fire frequency; (ii) the establishment of Brachiaria decumbens (an African C4 grass) as a function of decreasing canopy cover and (iii) the effects of grass fine fuel on fire intensity. Grasses invaded approximately 200 m from the edge into the interiors of burned plots (B1yr: 4.31 ha; B3yr: 4.96 ha) but invaded less than 10 m into the unburned plot (0.33 ha). The probability of B. decumbens establishment increased with seed availability and decreased with leaf area index. Fine fuel loads along the forest edge were more than three times higher in grass-dominated areas, which resulted in especially intense fires. Our results indicate that synergies between fires and invasive C4 grasses jeopardize the future of tropical forests.

Historical fire regimes, reconstructed from land‐survey data, led to complexity and fluctuation in sagebrush landscapes

Sagebrush landscapes provide habitat for Sage-Grouse and other sagebrush obligates, yet historical fire regimes and the structure of historical sagebrush landscapes are poorly known, hampering ecological restoration and management. To remedy this, General Land Office Survey (GLO) survey notes were used to reconstruct over two million hectares of historical vegetation for four sagebrush-dominated (Artemisia spp.) study areas in the western United States. Reconstructed vegetation was analyzed for fire indicators used to identify historical fires and reconstruct historical fire regimes. Historical fire-size distributions were inverse-J shaped, and one fire > 100 000 ha was identified. Historical fire rotations were estimated at 171-342 years for Wyoming big sagebrush (A. tridentata ssp. wyomingensis) and 137-217 years for mountain big sagebrush (A. tridentata ssp. vaseyana). Historical fire and patch sizes were significantly larger in Wyoming big sagebrush than mountain big sagebrush, and historical fire rotations were significantly longer in Wyoming big sagebrush than mountain big sagebrush. Historical fire rotations in Wyoming were longer than those in other study areas. Fine-scale mosaics of burned and unburned area and larger unburned inclusions within fire perimeters were less common than in modern fires. Historical sagebrush landscapes were dominated by large, contiguous areas of sagebrush, though large grass-dominated areas and finer-scale mosaics of grass and sagebrush were also present in smaller amounts. Variation in sagebrush density was a common source of patchiness, and areas classified as "dense" made up 24.5% of total sagebrush area, compared to 16.3% for "scattered" sagebrush. Results suggest significant differences in historical and modern fire regimes. Modern fire rotations in Wyoming big sagebrush are shorter than historical fire rotations. Results also suggest that historical sagebrush landscapes would have fluctuated, because of infrequent episodes of large fires and long periods of recovery and maturity. Due to fragmentation of sagebrush landscapes, the large, contiguous expanses of sagebrush that dominated historically are most at risk and in need of conservation, including both dense and scattered sagebrush. Fire suppression in Wyoming big sagebrush may also be advisable, as modern fire rotations are shorter than their historical counterparts.

Spatial patterns of grasses and shrubs in an arid grassland environment

In the Chihuahuan Desert of Mexico and New Mexico, shrub invasion is a common problem, and once-abundant grassland ecosystems are being replaced by shrub-dominated habitat. The spatial arrangement of grasses and shrubs in these arid grasslands can provide better insight into community dynamics and can provide information on grass shrub interactions. To better understand the dynamics of the Chihuahuan Desert ecosystem and to provide information regarding the interactions between grasses and shrubs, we examined the spatial patterns of grasses and shrubs in remaining grass-dominated areas, interspersed with some shrubs. We developed 18, 10 × 20 m vegetation distribution plots by mapping the location of all grasses and shrubs on each and repeating the measurements three years later. Spatial patterns were then assessed for each plot using a second-order spatial statistic, Ripley's K-function, as well as any observed changes in the spatial patterns over a three-year period. We observed clumped grass distributions, indicating a lack of competition among grasses; random shrub distributions; and even grass distribution with respect to shrub locations, indicating competition between grasses and shrubs. We also observed a tendency for grass distributions to become more even over time, and grasses to become less even with respect to shrub locations over time. These changes occurred during a period of greater than average rainfall, indicating that greater water availability may lead to increased competition among grasses and decreased competition between grasses and shrubs.

Fire and vegetation type effects on soil hydrophobicity and infiltration in the sagebrush-steppe: I. Field analysis

Soil water repellency, infiltration, and fire severity were evaluated at 235 locations within a sagebrush-steppe ecosystem in southern Idaho, USA, after a 78,000ha wildfire. Fire frequency in the sagebrush-steppe has increased dramatically in the last century due to the proliferation of introduced annual weeds. Wildfire in shrub-steppe can increase runoff and erosion by decreasing vegetation cover and increasing soil hydrophobicity. Soil water repellency was measured with the water drop penetration time (WDPT) test and relative infiltration rates were measured by the mini-disc infiltrometer (MDI). Shrub-dominated locations showed highest water repellency in areas of moderate fire severity and infiltration rate was inversely related to distance from individual sagebrush plants. Highest water repellency in grass-dominated areas was also associated with moderate fire severity. Likewise, the lowest relative mean infiltration rates occurred in shrub-dominated or grass-dominated areas of moderate fire severity. These findings indicate that land management strategies should be targeted towards moderate fire severity areas.

Sediment budgets and source determinations using fallout Cesium-137 in a semiarid rangeland watershed, Arizona, USA

Analysis of soil redistribution and sediment sources in semiarid and arid watersheds provides information for implementing management practices to improve rangeland conditions and reduce sediment loads to streams. The purpose of this research was to develop sediment budgets and identify potential sediment sources using 137Cs and other soil properties in a series of small semiarid subwatersheds on the USDA ARS Walnut Gulch Experimental Watershed near Tombstone, Arizona, USA. Soils were sampled in a grid pattern on two small subwatersheds and along transects associated with soils and geomorphology on six larger subwatersheds. Soil samples were analyzed for 137Cs and selected physical and chemical properties (i.e., bulk density, rocks, particle size, soil organic carbon). Suspended sediment samples collected at measuring flume sites on the Walnut Gulch Experimental Watershed were also analyzed for these properties. Soil redistribution measured using 137Cs inventories for a small shrub-dominated subwatershed and a small grass-dominated subwatershed found eroding areas in these subwatersheds were losing −5.6 and −3.2 t ha −1 yr −1, respectively; however, a sediment budget for each of these subwatersheds, including depositional areas, found net soil loss to be −4.3 t ha −1 yr −1 from the shrub-dominated subwatershed and −0.1 t ha −1 yr −1 from the grass-dominated subwatershed. Generally, the suspended sediment collected at the flumes of the six other subwatersheds was enriched in silt and clay. Using a mixing model to determine sediment source indicated that shrub-dominated subwatersheds were contributing most of the suspended sediment that was measured at the outlet flume of the Walnut Gulch Experimental Watershed. The two methodologies (sediment budgets and sediment source analyses) indicate that shrub-dominated systems provide more suspended sediment to the stream systems. The sediment budget studies also suggest that sediment yields measured at the outlet of a watershed may be a poor indicator of actual soil redistribution rates within these semiarid watersheds. Management of these semiarid rangelands must consider techniques that will protect grass-dominated areas from shrub invasion to improve rangeland conditions.

Fuels Management at the Landscape Scale

important factor affecting burn severity. Grazing effects increased in grassland systems and decreased as shrub cover increased. While post-fi re determinations of grazing impacts were diffi cult at best, additional large-scale research on grazing/fi re behavior is needed. “Managing the Invasive Fine Fuels at the Landscape Scale” was presented by Ronald Clementsen, BLM, who reported on the Mojave Desert Initiative (MDI). The MDI was established as a forum for government agencies (BLM, National Park Service [NPS], US Fish and Wildlife Service [USFWS], Department of Defense, Nevada Department of Wildlife, Utah Department of Wildlife Resources, and Arizona Game and Fish Department) and other partners to collaboratively address wildfi re and invasive species issues within a defi ned eco-region of the northeastern Mojave Desert in Arizona, Nevada, and Utah. Their goals are to 1) protect remaining unburned Mojave Desert vegetation and reduce reburning; 2) restore strategically located islands, key habitat areas, and corridors; 3) improve communication, collaboration, and coordination; and 4) maximize leveraged funding. They developed regional priorities and guidance for incident commanders to minimize or avoid further habitat loss to fi re. They also completed regional assessment to identify priority work areas and to develop project selection criteria. High-density, unburned desert tortoise critical habitat is their fi rst priority for protection and restoration. It is followed by unburned critical habitat for all listed species (not included in priority one) within BLM National Conservation Areas, BLM and NPS monuments, NPS recreation areas, and USFWS refuges. They will identify, prioritize, and develop potential projects across the MDIdefi ned–eco-region, including fuel breaks, green-strips, seedings, plantings, applications of preemergent herbicide, etc. Robin Tausch, US Forest Service (USFS) Rocky Mountain Research Station, addressed “Fuels Management Strategies for Woody Natives to Avoid Promoting Invasive Species.” Using data from research projects in Nevada, Utah, Idaho, and Oregon, Robin showed the remarkable increase in the area, cover, size, and density of pinyon pine and juniper. These trees are changing plant communities in many areas of the western United States. Once established, Introductory Focus by Featured Speakers Workshop IV was moderated by Brad Washa, Bureau of Land Management (BLM). Jason Davison of the University of Nevada Cooperative Extension spoke about a “Case Study of the Murphy Complex Fire.” A team of scientists and managers1 had been charged to 1) provide preliminary observations and recommendations on how plant communities and livestock grazing affected fi re behavior, 2) recommend long-term research or studies needed to address issues regarding the use of livestock to reduce fuels while maintaining post-fi re resource values, and 3) discuss the potential application of the fi ndings from the Murphy Complex wildfi res in Idaho and Nevada to other locations. After identifying many areas with high discontinuity and contrast, the team evaluated each area using 10 factors: 1) actual use by livestock, 2) distance from water, 3) anomaly performance (related to pasture actual grazing pressures in grassland areas), 4) fi re history, 5) location of seedings, 6) vegetation type, 7) shrub cover, 8) cheatgrass cover, 9) prefi re plant biomass, and 10) fi re behavior/suppression. They ranked burn severity factors across all focus areas and across grass-dominated areas. Vegetation (fuel loads) was the most

Spatial variability of CO 2 exchange for riparian and open grasslands within a first-order agricultural basin in Southern Ontario

Agriculture is one of the most widespread landuse types in Ontario, Canada, where cultivated lands are often located within small first-order catchments with extensive grass-dominated riparian areas. These grass-dominated areas may be significant to the overall catchment carbon functioning as they are often high in soil organic matter and elevated biological productivity due to the availability of agricultural nutrients. As such, this study examines midday CO 2 exchange between the atmosphere and two grass-dominated riparian areas (one adjacent to a cultivated disk-tilled cornfield, one adjacent to a no-till grassland) and an open area grassland (no-till field) in a predominantly agricultural basin in Southern Ontario using daytime dynamic closed-chamber measurements from May 7 to October 27, 2003. Soils in this catchment are generally comprised of 50% sand, 35% silt and 15% clay, with riparian areas containing an organic layer approximately 0.20–0.40 m deep. The influence of nearby agriculture on riparian areas is apparent, as larger rates of CO 2 exchange were correlated with elevated soil nutrients as well as above ground biomass and canopy height, which were larger in the riparian zone influenced by intensive agriculture (Rip-Corn) than the grassland-influenced riparian area (Rip-Grass) and the adjacent grassland field (Grass). Spatial patterns in soil and vegetation respiration ( R Tot), net ecosystem CO 2 exchange (NEE) and gross ecosystem productivity (GEP) for all sites showed strong positive relationships with soil characteristics such as total nitrogen, carbon–nitrogen ratio and above-ground biomass. Our results show that grass-dominated riparian and non-riparian areas, with similar vegetation that appear to be homogenous, located approximately 250–300 m from one another, may exhibit very different CO 2 fluxes. In general, CO 2 exchange is much greater in riparian areas than in the adjacent upland grasslands, and this is more exaggerated where those upland sites are tilled and receive inorganic fertilizers. Furthermore, our results indicate that the effects of land-use (i.e. agriculture) over-ride the effects of microclimate in controlling spatial patterns in CO 2 exchange in this watershed. This highlights the need to better assess CO 2 fluxes from heterogeneous agricultural landscapes, and emphasizes that estimates based on data grouped solely on soil or vegetation units can be rather conservative and may not capture the inherent spatial variability and small scale processes that drive CO 2 exchange.

Composition of soil seed banks in southern California coastal sage scrub and adjacent exotic grassland

Soil seed banks are important to many plant communities and are recognized as an important component of management plans. Understanding seed bank composition and density is especially important when communities have been invaded by exotic species and must be managed to promote desirable species. We examined germinable soil seed banks in southern California coastal sage scrub (CSS) that is heavily invaded by exotic grasses and in adjacent exotic grassland. Soils from both communities had similar seed banks, dominated by high densities of exotic grass and forb species. Up to 4,000 exotic grass seeds and at least 400 exotic forb seeds/m2 were found in most soils, regardless of aboveground vegetation type. Native forbs averaged 400 seeds/m2 in grass-dominated areas and about 800 in shrub-dominated soils. Shrub seed density was <1 and <10 seeds/m2 in grass- and shrub-dominated areas, respectively, indicating that the shrub seed bank is not persistent compared to annuals. We also compared pre- and post-burn soil seed banks from one location that burned in October 2003. Late-season burning in both grass- and CSS-dominated areas disproportionately reduced exotic grass seed densities relative to native seed densities. The similarity of the seed banks in adjacent grass and shrub communities suggests that without intervention, areas currently dominated by CSS may become more similar to grass-dominated areas in terms of aboveground vegetation. In such areas, the first growing season following a wildfire is a window of opportunity for increasing native diversity at a time when density of exotic grass seeds is low.

Breeding success of the endemic mara Dolichotis patagonum in relation to habitat selection: Conservation implications

Although maras ( Dolichotis patagonum) are among the most representative mammals of South America, knowledge on the ecological processes affecting their conservation is scarce. In particular, the study of habitat requirements and its relationship with breeding success is necessary to identify possible threats and develop conservation action for this endemic mammal. I investigated habitat selection patterns by maras and their relationship with breeding success in Península Valdés, Argentine Patagonia. Maras bred from mid-August to late December, and they tended to build the breeding warrens in open, grass-dominated habitats more than expected while avoided closed habitats dominated by taller shrubs. Mean number of breeding adults per warren was 4.26, while mean number of pups born per warren was 4.46. Overall, 30 (45%) of 67 pups survived until the 6th week of life, but pups born in warrens located in open habitats survived significantly better than pups born in warrens located in closed habitats (50% vs. 30% of the pups born). As grass-dominated areas uphold the highest densities of domestic sheep and habitat modification due to overgrazing is a widespread process across arid Patagonia, effects on the availability of suitable breeding sites for maras are likely to occur and require further investigation.

Vegetation Patterns in Relation to Bird Nesting Preferences on West Island, South Australia

SummaryWest Island near Victor Harbor, SA is an important near-shore nesting site for several bird species, but nesting behaviour in shorebirds is often affected by changes in rookery vegetation. Because the island is under increasing threat from invasion and expansion of weeds, its vegetation patterns were determined and preference analysis undertaken for nesting by Silver Gulls, (Larus novaehollandiae) and Little Penguins (Eudyptula minor), as studies in WA suggest they are affected by vegetation changes. The results indicated that there were four broad types of vegetation on the island: grass-dominated, shrub-dominated, mallow/weedy forb-dominated and native forb-dominated. These were correlated with soil and exposure features, the grasses and shrubs being most abundant on more sheltered alkaline and less saline soils, while shrubs dominated in rocky, leeward areas. The native forbs and low shrubs occurred in more exposed saline and alkaline areas, weedy forbs in more sheltered, less saline places. Nesting preference analysis showed that both the Silver Gulls and Little Penguins significantly preferred open, grass-dominated areas. Penguins were significantly less abundant in quadrats with high levels of exposed surface rock and shrubs or weedy annual forbs (although they did still nest under rocks in these areas), whereas the gulls significantly avoided native, perennial forb-dominated areas, although this may also be a reflection of the degree of exposure of the latter. As the weedy forbs are known to invade areas of increasing fertility resulting from nesting, this highlights the need for successful weed control in offshore rookeries if successful nesting is to continue in the long term.