Robust ecological drought projections for drylands in the 21st century

Dryland ecosystems may be especially vulnerable to expected 21st century increases in temperature and aridity because they are tightly controlled by moisture availability. However, climate impact assessments in drylands are difficult because ecological dynamics are dictated by drought conditions that are difficult to define and complex to estimate from climate conditions alone. In addition, precipitation projections vary substantially among climate models, enhancing variation in overall trajectories for aridity. Here, we constrain this uncertainty by utilizing an ecosystem water balance model to quantify drought conditions with recognized ecological importance, and by identifying changes in ecological drought conditions that are robust among climate models, defined here as when >90% of models agree in the direction of change. Despite limited evidence for robust changes in precipitation, changes in ecological drought are robust over large portions of drylands in the United States and Canada. Our results suggest strong regional differences in long‐term drought trajectories, epitomized by chronic drought increases in southern areas, notably the Upper Gila Mountains and South‐Central Semi‐arid Prairies, and decreases in the north, particularly portions of the Temperate and West‐Central Semi‐arid Prairies. However, we also found that exposure to hot‐dry stress is increasing faster than mean annual temperature over most of these drylands, and those increases are greatest in northern areas. Robust shifts in seasonal drought are most apparent during the cool season; when soil water availability is projected to increase in northern regions and decrease in southern regions. The implications of these robust drought trajectories for ecosystems will vary geographically, and these results provide useful insights about the impact of climate change on these dryland ecosystems. More broadly, this approach of identifying robust changes in ecological drought may be useful for other assessments of climate impacts in drylands and provide a more rigorous foundation for making long‐term strategic resource management decisions.     

Grazing effects on plant diversity in the endemic Erica mackayana heathland community of north-west Spain

Background: Large areas of heathland landscapes in Galicia, north-west Spain, have traditionally been extensively grazed by free-ranging cattle and wild ponies. Recently, a large reduction in the number of these larger herbivores has been observed, with unknown consequences for the habitat.

Aims: To evaluate the effects of grazing and herbivore density on plant diversity, community composition and vegetation structure of the endemic wet heathlands dominated by Erica mackayana in Galicia.

Methods: Field sampling of vascular plants, generalised linear models (GLMs), non-metric multidimensional distance scaling (NMDS).

Results: Grazed sites had significantly higher total and rare species richness and diversity than ungrazed sites. Higher densities of cattle resulted in lower numbers of rare species, while wild pony density had no effect on rare species richness. In grazed sites, vegetation was lower with greater variation in height, resulting in greater heterogeneity of the habitat. Precipitation and summer temperatures were related to plant diversity, mainly beta diversity. Soil organic matter negatively correlated with rare species.

Discussion: Grazing, mostly by wild ponies, was demonstrated to be positively related to plant diversity and vegetation structure. Lack of grazing or high cattle densities resulted in a negative effect on total and rare species richness and diversity. Future climate change may negatively affect heathland plant diversity. Galician wild ponies represent a unique case of sustainable management of a wild species and an invaluable cultural heritage. Moreover, they have a significant role in maintaining the endemic E. mackayana heathlands, what would justify specific conservation actions for these large herbivores.     

Use of monocharacteristic growth forms and phenological phases to describe and differentiate plant communities in Mediterranean-type ecosystems

The ecomorphological and phenological study was carried out within aMediterranean vegetation context, in Quercus suberforests,which have been substituted by shrublands of Cistus sppwithin two Natural Parks in the south of the Iberian Peninsula. Theecomorphological characters that show meaningful differences between both typesof vegetation are: location of renewal buds, spinescence, stratification,maximum height of the vegetation, organs periodically shed, leaf consistency,leaf tomentosity, leaf size, and life duration of leaves, plant duration,vegetative regeneration after fire, main vegetative growth season, mainflowering season and fruit type. The phenological phases also help to discernbetween forest and shrubland, specially flower bud formation, fruiting, seeddispersal, and the existence of brachyblast vegetative growth and brachyblastleaf shedding. We propose three new indexes based on phenological phases:active period of the species (APS), active period of thecommunity (APC) and reproductive/vegetative activity of thespecies (RVA). The results of their application, in combination with theecomorphological characters, have proved promising in describing vegetation andin clearly differentiating communities. The results also show the existence ofdifferent ecomorphological groups of plants at community level, with consequentecological, historical, phytocoenological and adaptive implications     

Regional Synthesis of Habitat Relationships in Shrubland Birds

Abstract: Shrubland birds are declining throughout the eastern United States. To manage scrub-shrub habitats for birds, managers need information on avian habitat relationships. Past studies have produced contradictory results in some cases and may be of limited generality because of site- and habitat-specific factors. We studied shrubland birds across 6 habitats in 3 New England states to provide more general information on habitat relationships than has been possible in past studies. Our study sites included all major scrub-shrub habitats in New England: wildlife openings, regenerating clear-cuts, beaver ponds, utility rights-of-way, pitch pine (Pinus rigida) woodlands, and scrub oak (Quercus ilicifolia) barrens and ranged from Connecticut to northern New Hampshire, with research conducted from 2002 to 2007. Using N-mixture models of repeated point counts, we found that 6 of 12 shrubland birds preferred areas with greater shrub cover. An additional 4 species appeared to prefer areas with lower-stature vegetation and greater forb cover. Eight of 10 bird species showed relationships with cover of individual plant species, with Spiraea spp., willows (Salix spp.), alders (Alnus spp.), and invasive exotics being the most important. We recommend that shrubland management for birds focus on providing 2 distinct habitats: 1) areas of tall (>1.5 m) vegetation with abundant shrub cover and 2) areas of lower (<1.5 m) vegetation with abundant forb cover but fewer shrubs.     

Anatomical features, monomer lignin composition and accumulation of phenolics in 1-year-old branches of the Mediterranean Cistus ladanifer L.

Mediterranean shrub species are described as having phenology, habitus , reproductive biology and anatomical alterations in certain tissues, allowing their survival during the dry season and protecting them from herbivory. Anatomical and chemical analyses were conducted in 1-year-old branches of Cistus ladanifer L. in order to investigate the role played by shoot structure in the adaptive strategies of this species in the Mediterranean environment. Results showed that both xylem and pith underwent lignification. Pith parenchyma cells had thickened walls, higher lignin content than xylem and different monomer composition. Xylem presented features aiding safe water transport. A large accumulation of phenolic substances was found in xylem, pith and cortical parenchyma. Observations reported in this paper suggest the occurrence of adaptive strategies in 1-year-old branches of C. ladanifer whose structural features: (1) allow mechanical reinforcement of tissues to withstand drought without suffering permanent damage; (2) favour safety rather than efficiency in water transport; (3) defend the plants from animal predation and pathogens by accumulating phenolics in various tissues, and (4) protect inner tissues against UV-B radiation through deposition of phenolic compounds in cortical layers.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 155 , 361–371.     

Role of changing environmental parameters in leaf gas exchange of <Emphasis Type="Italic">Arbutus unedo</Emphasis> L. assessed by field and laboratory measurements

In the frame of the foreseen climate global changes we analysed the physiological responses of Arbutus unedo L. to the variations of carbon dioxide concentration, leaf temperature, and irradiance by measurements of leaf gas exchange and leaf water potential performed both in field and in the laboratory. Stomatal conductance was not affected by increase of leaf temperature. The growth conditions of potted plants likely made stomata more sensitive to the variation of external parameters than naturally growing plants. The interaction between high CO₂ concentration and temperature involved important down-regulation mechanisms in the metabolic pathway of the carbon fixation. From an ecological point of view, the ability of A. unedo to adapt to the field stress makes it highly competitive in the Mediterranean plant community.     

Reproductive strategy of a small endemic cyprinid, the Yarqon bleak (Acanthobrama telavivensis), in a mediterranean-type stream

A mediterranean-type climate exists in five widely separated regions; the Mediterranean basin, parts of western North America, parts of western and southern Australia, southwestern South Africa and parts of central Chile. Streams in these regions feature seasonal disturbances of contrasting hydrology with high predictability of the timing of flooding and drying but low constancy. We would expect fish living in these streams to avoid scouring flow and breed after cessation of the flood period. The aim of the present study was to examine the adaptation of the Yarqon bleak, Acanthobrama telavivensis, an endemic cyprinid in the coastal streams of Israel, to mediterranean-type stream (mediterranean—written with a small m, is used in connection with climate or ecological region and is distinguished from Mediterranean that is used in a geographical context, referring to the Mediterranean basin.) conditions. For that we studied its reproductive strategy (age at maturity and life span, gonad activity, oocyte maturation, spawning activity and habitats, appearance of juveniles), in a major costal stream (Yarqon). Our findings show that the Yarqon bleak exhibits life history traits attuned with a mediterranean-climate hydrologic regime. It breeds in late winter and early spring, a window of opportunity between flash floods and habitat desiccation. Being a multiple spawner allows the fish to compensate for the potential loss of part of its reproductive output due to scouring flows of late floods. The ability of the Yarqon bleak to spawn on different substrate-types enables it to take advantage of different stream conditions that pertain in different years. The fish attains pre-adult size (ca. 33–42 mm) within the first year, prior to drying out of most stream reaches, and matures by the beginning of the second year (males >41; females >42 mm). The cost of these tactics is a short life span (4–5 age groups). The reproductive strategy of the Yarqon bleak falls into the category of in-channel breeding but, unlike the case suggested by a low flow recruitment model, the fish breed during the period of flood cessation, a transitional time between high and low flows, rather than at the time of low flow. Breeding at this time of the year in mediterranean-type streams puts early stages somewhat at risk of being washed away by late floods, but gains them a longer period of growth under favorable conditions. We suggest an additional positive tradeoff that should be investigated: the reduced competition with age 0 of other fish that breed later in the season. This suggested model of recruitment during the period of flood cessation seems appropriate for fish in streams with seasonal contrasting flows of high predictability but low constancy.     

Fire severity and seedling establishment in Pinus halepensis woodlands, eastern Iberian Peninsula

Given the observed heterogeneity in fire severity produced within wildfires, we asked to what extent this heterogeneity might affect post-fire regeneration. For this purpose, we studied the post-fire dynamics of Pinus halepensis (Aleppo pine) in the eastern Iberian Peninsula. Sampling was stratified on the basis of fire severity. We defined three fire severity classes based on the degree of consumption of the pine canopy. The results suggested that there is no clear relationship between seedling density and fire severity; however, mortality was lower and growth (height, shoot biomass and root biomass) was higher in the high severity class. These results can be explained by soil processes: Sites in the high fire severity class may have sustained higher fire intensities, resulting in higher soil organic matter mineralisation and higher ash deposition, and thus in higher post-fire soil fertility. This higher fertility would produce faster growth in pine seedlings. Independent of the severity class, seedling mortality was higher in quadrats (50 × 50 cm) with higher cover of the perennial grass Brachypodium retusum (Poaceae), suggesting a possible competitive effect. For all plots in all 3 severity classes, spatial analysis suggests an aggregate seedlings pattern, but with independence from the position of the adult (source) trees. This revised version was published online in June 2006 with corrections to the Cover Date.     

Refining the cheatgrass–fire cycle in the Great Basin: Precipitation timing and fine fuel composition predict wildfire trends

Larger, more frequent wildfires in arid and semi‐arid ecosystems have been associated with invasion by non‐native annual grasses, yet a complete understanding of fine fuel development and subsequent wildfire trends is lacking. We investigated the complex relationships among weather, fine fuels, and fire in the Great Basin, USA. We first modeled the annual and time‐lagged effects of precipitation and temperature on herbaceous vegetation cover and litter accumulation over a 26‐year period in the northern Great Basin. We then modeled how these fine fuels and weather patterns influence subsequent wildfires. We found that cheatgrass cover increased in years with higher precipitation and especially when one of the previous 3 years also was particularly wet. Cover of non‐native forbs and native herbs also increased in wet years, but only after several dry years. The area burned by wildfire in a given year was mostly associated with native herb and non‐native forb cover, whereas cheatgrass mainly influenced area burned in the form of litter derived from previous years’ growth. Consequently, multiyear weather patterns, including precipitation in the previous 1–3 years, was a strong predictor of wildfire in a given year because of the time needed to develop these fine fuel loads. The strong relationship between precipitation and wildfire allowed us to expand our inference to 10,162 wildfires across the entire Great Basin over a 35‐year period from 1980 to 2014. Our results suggest that the region's precipitation pattern of consecutive wet years followed by consecutive dry years results in a cycle of fuel accumulation followed by weather conditions that increase the probability of wildfire events in the year when the cycle transitions from wet to dry. These patterns varied regionally but were strong enough to allow us to model annual wildfire risk across the Great Basin based on precipitation alone.     

Impact of the non-indigenous shrub species Spartium junceum (Fabaceae) on native vegetation in central Spain

Aims The introduction of potentially invasive species through ornamental cultivation or for rehabilitation purposes is a serious environmental problem. They cause damage to biodiversity through loss, increased mortality or ' in situ ' selection phenomena in natural flora. Spartium junceum is a Mediterranean shrub that is not native in most areas of the Iberian Peninsula, although it is extensively grown for the rehabilitation of roadsides. We have investigated the effect on the native vegetation of an old S. junceum (Fabaceae) plantation in a conservation area in the centre of the Iberian Peninsula: the Cuenca Alta del Manzanares nature reserve in Madrid.Methods We compared S. junceum stands with the native nanophanerophytic Cistus ladanifer community at different ecosystem properties: soil properties, temporal soil seed bank contents, standing vegetation and net primary production of annual grasslands growing in these shrublands.Important findings The results highlighted S. junceum 's ability to become established in the new environment (marginal areas of the nature reserve) and ensure its successful growth. This is more apparent in northern and eastern exposures where this formation contacts with the core of the best conserved native vegetation in the nature reserve. Soils under Spartium showed a higher nitrogen content, indicating its capacity—in common with other legumes—to fix N, and conferring an advantage over Cistus, which is N-limited. Other soil nutrients such as phosphorus, magnesium and calcium and water availability are also higher in Spartium soils than in Cistus. Phosphorus is usually a constraint for N-fixers, but in our study, it is the most significant soil variable in both shrub formations and is important to the success of Spartium. Water availability is a key factor for Mediterranean vegetation, and particularly in autumn when soils are recharged. The Spartium formation is able to retain water as its growth produces a closer canopy than Cistus, thereby preventing water evaporation and contributing to the success of this species. Perennials are more frequent in the standing vegetation than in the seed bank, whereas therophytes are similar. The standing vegetation has therophytes and chamaephytes as the predominant growth forms in Spartium sites, and hemicryptophytes and phanerophytes in Cistus. Therophytes are dominant in Spartium and Cistus seed banks, although the first formation has more species. Spartium has a higher number of hemicryptophytes and Cistus is higher in phanerophytes. Northern and eastern aspects show significant differences in richness—with a predominance of annual weed species in Spartium —and in above-ground net primary production, probably as a consequence of the nutrients present in the soils. Ruderal annual species under Spartium (Bromus tectorum, Chenopodium album) have higher germination rates in the greenhouse than in the standing vegetation, suggesting they are at potential risk if environmental conditions change.