Xeric Sites Research Articles

Evapotranspiration deficit controls net primary production and growth of silver fir: Implications for Circum-Mediterranean forests under forecasted warmer and drier conditions

Warming-induced drought stress has been hypothesized as a major driver of forest net primary production (NPP) reduction, but we lack reliable field data to assess if higher temperatures lead to forest NPP reduction, particularly in humid sites and at basin to landscape spatial scales. The use of a landscape approach would allow considering the feedbacks operating between climate, topography, soil vegetation and water resources. Here we follow that approach by simulating NPP using the regional hydro-ecologic simulation system (RHESSys) model and by comparing the results with radial growth data (tree-ring widths and intrinsic water-use efficiency – iWUE). We evaluate the relationships between climate, growth, NPP, atmospheric CO2 concentrations (ca) and iWUE in xeric and mesic silver fir forests subjected to contrasting water balances. The growth data successfully validated the 11-month NPP cumulated until spring. The main negative climatic driver of growth and NPP was the summer evapotranspiration deficit, which shows a negative association with tree-ring width indices. Sensitivity analyses indicate that rising ca do not compensate the severe NPP reduction associated to warmer and drier conditions. The positive effect of rising ca on NPP is mediated by climatic site conditions being detected only in mesic sites, whereas the negative effects of drought on NPP override any ca-related enhancement of NPP in xeric sites. Future warmer and drier conditions causing a higher evaporative demand by the atmosphere could lead to a NPP decline in temperate conifer forests subjected to episodic droughts.

Differences in photosynthetic activity might explain the large-scale shifts in pine recruitment in favour of oaks in continental Mediterranean climates

Forest trees in Mediterranean continental areas undergo both summer and winter stress. Recent large-scale studies of regeneration patterns of pine and oak species have reported the expansion of Quercus ilex to the detriment of pine species. To asses this trend from a physiological point of view, we compared the effect of summer and winter stress on the photochemical apparatus of the co-occurring species Pinus pinaster and Q. ilex ssp. ballota. We measured P. pinaster net photosynthetic rates and photosystem II (PS II) photochemical efficiency after summer and compared them with previous results obtained for P. pinaster after winter and for Q. ilex after summer and winter. Pinus pinaster and Q. ilex displayed contrasting photochemical efficiencies and heat dissipation after winter and summer. Pinus pinaster showed a lower fraction of photochemically active PS II reaction centres and lower effective quantum yield of the PS II photochemistry, accompanied by a higher dissipation of absorbed light energy as heat in the PS II antenna complexes. Quercus ilex showed the opposite, which was reflected in its higher photosynthetic rates. Our results support the advance of Q. ilex to the detriment of P. pinaster under the climate change scenario in Mediterranean areas. We discuss the results in the context of forest succession and climate change. The second objective of this study was to evaluate the effectiveness of the chlorophyll fluorescence technique to detect intraspecific differences in P. pinaster photochemical efficiency after summer stress. PS II photochemical parameters discriminated among families and populations, especially under limiting conditions (xeric site). Most of the genetic variation was explained by the family and its interaction with the environment and was higher in the xeric site. The significant heritabilities and differences among families within populations in actual photochemical efficiency (ΦPSII) and non-photochemical quenching (NPQ) and their interaction with the environment suggest that photochemical responses to summer stress are genetically inherited and that adaptive variation is maintained within populations due to local selection. Consequently, these parameters could be used for the selection of more tolerant genotypes to summer photoinhibition.

Do climatic and habitat conditions affect the reproductive success of an invasive tree species? An assessment of the phenology of Acacia longifolia in Portugal

Plant phenological events are some of the most sensitive indicators of how plant species respond to favourable or stressful conditions. The evaluation of the flowering phenology of invasive plant species is particularly relevant, mainly due to its crucial importance in determining plant reproductive success and the outcome of invasion. We studied the phenology of Acacia longifolia, an aggressive, invasive plant species in the Mediterranean basin. We measured its vegetative growth and reproductive traits, specifically flowering phenophases and fruit production, under different climatic conditions (mesic and xeric Mediterranean climates), and in two different habitats (pine forest and open area). All the measured phenological phases began earlier at the xeric site than at the mesic site; this was particularly evident when comparing reproductive phenophases. Flowering dates were significantly associated with air temperature, with early peak flowering dates linked to increases in air temperature. The number of fruiting flowers per inflorescence in A. longifolia trees was higher at the mesic site, mainly in the pine forest plot, and the number of aborted fruits was notably lower than in the xeric plots. The presence of a pine forest at the mesic site strongly influenced the flowering phenology of A. longifolia and resulted in the highest reproductive success and the lowest branch growth rate. Our results demonstrate that a combination of climate and forest structure can cause pronounced differences in phenology and reproductive success of A. longifolia. These data can help to understand the variations in invasive rates of A. longifolia across the Mediterranean basin.

Age effects on the climatic signal in Araucaria araucana from xeric sites in Patagonia, Argentina

Background: At different cambial ages, trees experience changes in their structure and interactions with environmental conditions. Reciprocal mechanisms between tree age and physical resources, photosynthetic rates, and xylem production may influence hydraulic resistance and plant water stress. However, it is yet uncertain how these mechanisms are associated with changes in growth sensitivity to biophysical drivers, especially climate.Aim: To establish age-associated climate – growth relationships in growth rings of Araucaria araucana trees from the temperate xeric zones of northern Patagonia, Argentina.Methods: We analysed the growth in 211 A. araucana trees from four sampling sites, in three age classes: young (≤120 years), mature (121–275 years), and old (≥276 years). We explored the correlations between the signal strength of tree growth and climate, based on comparisons between each age-class chronology and monthly mean surface air temperature, total precipitation, and the Southern Annular Mode (SAM) index.Results: The young trees showed higher correlations when their growth was compared with precipitation, air temperature, and the SAM index during austral spring and summer months of the same year. In contrast, growth in mature and old trees showed higher correlations with summer temperatures of the previous growing season.Conclusions: The sensitivity of the radial-growth response of A. araucana to climate varies with age and is strongest in the rings of young trees.

Spatial diversity of recent trends in Mediterranean tree growth

Increasing temperatures and shifting precipitation regimes define the Mediterranean Basin (MB) as one of the world’s most sensitive climate change hot spots. Among various ecological effects, disruptions in the growth-climate response of Mediterranean forest species have been recently observed. Complex topographical and climatological factors, however, may result in contrasting patterns of recent tree growth, with their biotic and abiotic drivers often remaining debatable. Here, we compile dendrochronological evidence from 878 cases at 645 sites reported in 66 peer-reviewed publications to assess modern (post-1970) growth trends across the MB (30°–46° N and 10° W–40° E). This collection mainly reveals positive trends in temperate environments across the northwestern part of the Basin, whereas negative trends were often found at xeric sites in the southwestern and eastern regions. Although these response patterns are indicative for both beneficial as well as detrimental effects of climate change on pan-Mediterranean forest ecosystem function and productivity, our review is slightly biased by selective sampling efforts that focused on higher elevations and older trees within a few countries in the northwest. This imbalance emphasizes the need for more evenly distributed study sites and age classes that better reflect ecological rather than political and methodological criteria.

Time series analysis of forest carbon dynamics: recovery of Pinus palustris physiology following a prescribed fire

Frequency and intensity of fire determines the structure and regulates the function of savanna ecosystems worldwide, yet our understanding of prescribed fire impacts on carbon in these systems is rudimentary. We combined eddy covariance (EC) techniques and fuel consumption plots to examine the short-term response of longleaf pine forest carbon dynamics to one prescribed fire at the ends of an edaphic gradient (mesic and xeric sites). We also introduce novel (to the EC research community) statistical time-series approaches to quantify the drivers of carbon dynamics in these systems. We determined that our mesic site was a moderate sink of carbon (−157.7 ± 25.1 g C m−2 year−1), while the xeric site was carbon neutral (5.9 ± 32.8 g C m−2 year−1) during the study. The fire released 408 and 153 g C m−2 year−1 for the mesic and xeric sites, respectively. When loss associated with fire was combined with net ecosystem exchange rates, both sites became moderate carbon sources for the year. Analyses of assimilation and respiration parameters (e.g., maximum photosynthesis, quantum efficiency, and daytime ecosystem respiration) showed a positive trend over time pre-fire and a negative trend over time post-fire for maximum ecosystem CO2 uptake rates, and the opposite relationship for daytime ecosystem respiration rates. Within 30 days following fire, ecosystem physiological activity was statistically similar to pre-fire and appeared to be driven by the pine canopy. Our results suggest that prescribed fire (low intensity, high frequency) maintains the existing structure and function (in this case, carbon flux rates) because longleaf pine ecosystems have evolved with fire. This study, 1 year in length, provides a foundational understanding of the complex interaction between fire and carbon dynamics for longleaf pine ecosystems. Moreover, it provides a case study for applying time series analysis methods to EC data where there are complex relationships between ecosystem physiological activity and environmental drivers. However, to elicit a broader understanding of the complex interaction occurring between fire and carbon dynamics long- term studies are needed.

Site and Age Condition the Growth Responses to Climate and Drought of RelictPinus nigraSubsp.salzmanniiPopulations in Southern Spain

To assess if tree age may modulate the main climatic drivers of radial growth, two relict Pinus nigra subsp. salzmannii populations (Maria, most xeric site; Magina, least xeric site) were sampled in southern Spain near the limits of the species range. Tree-ring width residual chronologies for two age groups (mature trees, age ≤ 100 years (minimum 40 years); old trees, age > 100 years) were built to evaluate their responses to climate by relating them to monthly precipitation and temperature and a drought index (DRI) using correlation and response functions. We found that drought is the main driver of growth of relict P. nigra populations, but differences between sites and age classes were also observed. First, growth in the most xeric site depends on the drought severity during the previous autumn and the spring of the year of tree-ring formation, whereas in the relatively more mesic site growth is mainly enhanced by warm and wet conditions in spring. Second, growth of mature trees responded more to drought severity than that of old trees. Our findings indicate that drought severity will mainly affect growth of relict P. nigra populations dominated by mature trees in xeric sites. This conclusion may also apply to similar mountain Mediterranean conifer relicts.

Impacts of extreme precipitation and seasonal changes in precipitation on plants

Abstract. The global hydrological cycle is predicted to become more intense in future climates, with both larger precipitation events and longer times between events in some regions. Redistribution of precipitation may occur both within and across seasons, and the resulting wide fluctuations in soil water content (SWC) may dramatically affect plants. Though these responses remain poorly understood, recent research in this emerging field suggests the effects of redistributed precipitation may differ from predictions based on previous drought studies. We review available studies on both extreme precipitation (redistribution within seasons) and seasonal changes in precipitation (redistribution across seasons) on grasslands and forests. Extreme precipitation differentially affected above-ground net primary productivity (ANPP), depending on whether extreme precipitation led to increased or decreased SWC, which differed based on the current precipitation and aridity index of the site. Specifically, studies to date reported that extreme precipitation decreased ANPP in mesic sites, but, conversely, increased ANPP in xeric sites, suggesting that plant-available water is a key factor driving responses to extreme precipitation. Similarly, the effects of seasonal changes in precipitation on ANPP, phenology, and leaf and fruit development varied with the effect on SWC. Reductions in spring or summer generally had negative effects on plants, associated with reduced SWC, while subsequent reductions in autumn or winter had little effect on SWC or plants. Similarly, increased summer precipitation had a more dramatic impact on plants than winter increases in precipitation. The patterns of response suggest xeric biomes may respond positively to extreme precipitation, while comparatively mesic biomes may be more likely to be negatively affected. Moreover, seasonal changes in precipitation during warm or dry seasons may have larger effects than changes during cool or wet seasons. Accordingly, responses to redistributed precipitation will involve a complex interplay between plant-available water, plant functional type and resultant influences on plant phenology, growth and water relations. These results highlight the need for experiments across a range of soil types and plant functional types, critical for predicting future vegetation responses to future climates.

Permafrost warming and vegetation changes in continental Antarctica

Continental Antarctica represents the last pristine environment on Earth and is one of the most suitable contexts to analyze the relations between climate, active layer and vegetation. In 2000 we started long-term monitoring of the climate, permafrost, active layer and vegetation in Victoria Land, continental Antarctica. Our data confirm the stability of mean annual and summer air temperature, of snow cover, and an increasing trend of summer incoming short wave radiation. The active layer thickness is increasing at a rate of 0.3 cm y−1. The active layer is characterized by large annual and spatial differences. The latter are due to scarce vegetation, a patchy and very thin organic layer and large spatial differences in snow accumulation. The active layer thickening, probably due to the increase of incoming short wave radiation, produced a general decrease of the ground water content due to the better drainage of the ground. The resultant drying may be responsible for the decline of mosses in xeric sites, while it provided better conditions for mosses in hydric sites, following the species-specific water requirements. An increase of lichen vegetation was observed where the climate drying occurred. This evidence emphasizes that the Antarctic continent is experiencing changes that are in total contrast to the changes reported from maritime Antarctica.

Increased water‐use efficiency does not lead to enhanced tree growth under xeric and mesic conditions

Higher atmospheric CO2 concentrations (c(a)) can under certain conditions increase tree growth by enhancing photosynthesis, resulting in an increase of intrinsic water-use efficiency (i WUE) in trees. However, the magnitude of these effects and their interactions with changing climatic conditions are still poorly understood under xeric and mesic conditions. We combined radial growth analysis with intra- and interannual δ(13)C and δ(18)O measurements to investigate growth and physiological responses of Larix decidua, Picea abies, Pinus sylvestris, Pinus nigra and Pseudotsuga menziesii in relation to rising c(a) and changing climate at a xeric site in the dry inner Alps and at a mesic site in the Swiss lowlands. (i)WUE increased significantly over the last 50 yr by 8-29% and varied depending on species, site water availability, and seasons. Regardless of species and increased (i)WUE, radial growth has significantly declined under xeric conditions, whereas growth has not increased as expected under mesic conditions. Overall, drought-induced stomatal closure has reduced transpiration at the cost of reduced carbon uptake and growth. Our results indicate that, even under mesic conditions, the temperature-induced drought stress has overridden the potential CO2 'fertilization' on tree growth, hence challenging today's predictions of improved forest productivity of temperate forests.

Minimum wood density of Juniperus thurifera is a robust proxy of spring water availability in a continental Mediterranean climate

AbstractAimThe Mediterranean Basin is considered to be a climate‐change hotspot, for which rising temperatures and associated aridification have been forecast. Such trends could affect the performance and growth of conifers in these drought‐prone areas. We evaluated whether radial growth and wood density can act as proxy measures of precipitation and drought in a Mediterranean conifer.LocationIberian Juniperus thurifera forests in northern and eastern Spain.MethodsWe sampled 10 stands encompassing J. thurifera's distributional area. We related four annually resolved tree‐ring variables (earlywood and latewood width, and maximum and minimum wood density) to climatic factors (temperature, precipitation and drought index) for the period 1951–2000. We then analysed the spatial and temporal patterns of those associations.ResultsOf all the four tree‐ring variables analysed, minimum wood density presented the strongest response to climate. Higher values of minimum wood density were related to drier spring conditions, particularly in the most xeric sites, confirming that a drought‐induced reduction in the radial expansion of tracheids increases earlywood density.Main conclusionsMinimum wood density is a biogeographically meaningful proxy of spring water availability for the Mediterranean conifer J. thurifera.

Differential effects of extreme drought on production and respiration: synthesis and modeling analysis

Abstract. Extremes in climate may severely impact ecosystem structure and function, with both the magnitude and rate of response differing among ecosystem types and processes. We conducted a modeling analysis of the effects of extreme drought on two key ecosystem processes, production and respiration, and, to provide a broader context, we complemented this with a synthesis of published results that cover a wide variety of ecosystems. The synthesis indicated that across a broad range of biomes, gross primary production (GPP) was generally more sensitive to extreme drought (defined as proportional reduction relative to average rainfall periods) than was ecosystem respiration (ER). Furthermore, this differential sensitivity between production and respiration increased as drought severity increased; it occurred only in grassland ecosystems, and not in evergreen needle-leaf and broad-leaf forests or woody savannahs. The modeling analysis was designed to enable a better understanding of the mechanisms underlying this pattern, and focused on four grassland sites arrayed across the Great Plains, USA. Model results consistently showed that net primary productivity (NPP) was reduced more than heterotrophic respiration (Rh) by extreme drought (i.e., 67% reduction in annual ambient rainfall) at all four study sites. The sensitivity of NPP to drought was directly attributable to rainfall amount, whereas the sensitivity of Rh to drought was driven by soil drying, reduced carbon (C) input and a drought-induced reduction in soil C content – a much slower process. However, differences in reductions in NPP and Rh diminished as extreme drought continued, due to a gradual decline in the soil C pool leading to further reductions in Rh. We also varied the way in which drought was imposed in the modeling analysis; it was either imposed by simulating reductions in rainfall event size (ESR) or by reducing rainfall event number (REN). Modeled NPP and Rh decreased more by ESR than REN at the two relatively mesic sites but less so at the two xeric sites. Our findings suggest that responses of production and respiration differ in magnitude, occur on different timescales, and are affected by different mechanisms under extreme, prolonged drought.

The history of widespread decrease in oak dominance exemplified in a grassland–forest landscape

Regionally-distinctive open oak forest ecosystems have been replaced either by intensive agriculture and grazing fields or by denser forests throughout eastern North America and Europe. To quantify changes in tree communities and density in the Missouri Plains, a grassland–forest landscape, we used historical surveys from 1815 to 1864 and current surveys from 2004 to 2008. To estimate density for historical communities, we used the Morisita plotless density estimator and applied corrections for surveyor bias. To estimate density for current forests, we used Random Forests, an ensemble regression tree method, to predict densities from known values at plots using terrain and soil predictors. Oak species decreased from 62% of historical composition to 30% of current composition and black and white oaks historically were dominant species across 93% of the landscape and currently were dominant species across 42% of the landscape. Current forest density was approximately two times greater than historical densities, demonstrating loss of savanna and woodlands and transition to dense forest structure. Average tree diameters were smaller than in the past, but mean basal area and stocking remained similar over time because of the increase in density in current forests. Nevertheless, there were spatial differences; basal area and stocking decreased along rivers and increased away from rivers. Oak species are being replaced by other species in the Missouri Plains, similar to replacement throughout the range of Quercus. Long-term commitment to combinations of prescribed burning and silvicultural prescriptions in more xeric sites may be necessary for oak recruitment. Restoration of open oak ecosystems is a time-sensitive issue because restoration will become increasingly costly as oaks are lost from the overstory and the surrounding matrix becomes dominated by non-oak species.

Developmental instability as an index of adaptation to drought stress in a Mediterranean oak

An increase in temperature and water deficits caused by the ongoing climate change might lead to a decline growth rates and threaten the persistence of tree species in drought-prone areas within the Mediterranean Basin. Developmental instability (the error in development caused by stress) may provide an index of the adaptability of woody plants to withstand climatic stressors such as water shortage. This study evaluated the effects of drought stress on growth variables in three stands of a Mediterranean oak (Quercus faginea) exposed to differing climatic conditions (xeric, mesic and cooler) along an altitudinal gradient in northeastern Spain, in two climatically contrasting years (wet and dry years). Two indices of developmental instability, fluctuating and translational asymmetries, which reflect environmental stress, were measured in leaves and current-year shoots, respectively. We also measured branch biomass and fractal complexity of branches as indicators of the species’ performance. After a period of drought the individuals’ at the most xeric site presented lower developmental instability and less branch biomass than did the individuals from the mesic and cooler sites. We interpret that difference as an adaptive response to drought which reflects a trade-off between maintenance of homeostasis and growth when water is scarce. The study demonstrated that developmental instability constitutes a useful index to assess the degree of adaptation to stressful environmental conditions. The assessment of developmental instability in sites and years with contrasting climatic conditions provides a means of quantifying the capacity of plants to develop plastic adaptive responses to climatic stress.

Drought and Ecological Site Interaction on Plant Composition of a Semi-Arid Rangeland

Fluctuating climatic patterns are increasing the frequency and severity of drought, a concern for native plant communities on grazed semi-arid rangelands. Vegetation successional models have focused on the impact of management and have failed to quantify the effects of extreme drought. From 2001 to 2011, plant community composition was sampled on ecological sites in a semi-arid rangeland managed with conservative grazing and frequent fire since 1937. Ordination and classification were used to assess the interactive effects of ecological site and extreme drought on plant species composition, holding all other external drivers constant. Deeper soil clay loam sites had 4x greater beta diversity than shallower and rockier low stony hill sites, an indication of greater species turnover and instability in response to extreme drought. Cumulative effects of drought years explained similarity between sites and species composition. Response to extreme drought varied by species; no response (Bouteloua curtipendula), decreased (Nassella leucotricha), and increased (Bouteloua rigidiseta and Eriochloa sericea). Annual C3 plant responses were explained by short-term drought and perennial C3 and C4 plant responses were explained by long-term drought. Clay loam sites had maximum species richness and diversity values during neutral periods with quadratic declines associated with climatic extremes (dry or wet) compared to the more xeric sites which had minimum species richness and diversity during neutral periods with quadratic increases during climatic extremes. The interaction between site and drought, holding all other external drivers constant, can enhance our understanding of plant community dynamics and secondary plant succession of degraded semi-arid rangelands.

Indirect effects of an invasive annual grass on seed fates of two native perennial grass species

Invasive plants exhibit both direct and indirect negative effects on recruitment of natives following invasion. We examined indirect effects of the invader Bromus tectorum (cheatgrass) on seed fates of two native grass species, Elymus elymoides and Pseudoroegneria spicata, by removing B. tectorum and by adding inoculum of the shared seed pathogen Pyrenophora semeniperda in factorial experiments at xeric and mesic field sites. We also included a supplemental watering treatment to increase emergence and also the potential for pathogen escape. We recorded emergence and survival of native seedlings and also determined the fate of unemerged seeds. At the xeric site, Pyrenophora-caused mortality was high (34%), and effects of other pathogens and failed emergence of germinants were smaller. Cheatgrass removal negatively affected both emergence (35 vs. 25%) and spring survival (69 vs. 42%). Pyrenophora-caused seed mortality increased with inoculum augmentation for both species (22 vs. 47% overall), but emergence was negatively impacted only for P. spicata (20 vs. 34%). At the mesic site, Pyrenophora-caused mortality was low (6%). Cheatgrass removal doubled emergence (26 vs. 14%). Seed mortality increased significantly with inoculum augmentation for P. spicata (12 vs. 5%) but not E. elymoides, while emergence was not significantly affected in either species. A large fraction of seeds produced germinants that failed to emerge (37%), while another large fraction (35%) was killed by other pathogens. We conclude that facilitation by cheatgrass at the xeric site but interference at the mesic site was probably mediated through litter effects that could be ameliorative or suppressive. Apparent competition between cheatgrass and native grasses could occur through Pyrenophora, especially in a xeric environment, but effects were weak or absent at emergence. This was probably because Pyrenophora attacks the same slow-germinating fraction that is subject to pre-emergence mortality from other causes, including attack by other pathogens such as Fusarium.

Modelling soil moisture, water partitioning, and plant water stress under irrigated conditions in desert urban areas

ABSTRACTThis study conducts a comparative analysis of the effects of irrigation scheduling at two urban residential sites in the Phoenix, Arizona, metropolitan area: a xeriscaped site (gravel base with low water use plants) and a mesiscaped site (turf grass and shade trees). A model of soil moisture dynamics was calibrated to observed soil moisture data from irrigated and non‐irrigated sensors at the xeric site and an irrigated sensor at the mesic site. Model simulations were conducted at both irrigated sites to investigate effects of irrigation scheduling; plant stress parameters; and precipitation variability on soil moisture dynamics, water balance partitioning, and plant water stress. Results indicated a substantial difference in soil water storage capacity at the two sites. At the xeric site, seasonal variation of irrigation input was shown to be critical in avoiding losses from the rooting zone, while reducing plant water stress. At the mesic site, sensitivity to irrigation scenarios was lower, though small amounts of water savings were achieved with seasonal applications. The model was then used to determine the minimum annual irrigation required to achieve specified levels of plant stress using long‐term meteorological records. A bimodal schedule consisting of low winter and summer irrigation was identified as a means to conserve water while keeping moderate levels of dynamic water stress. Results from the quantitative model can potentially assist water and landscape managers in desert urban areas by identifying opportunities for water savings, while providing a deeper understanding of the ecohydrological differences between the two types of landscape designs. Copyright © 2014 John Wiley & Sons, Ltd.

Rangelands as Carbon Sinks to Mitigate Climate Change: A Review

Rangelands cover large areas in the United States and across the world and are natural carbon sinks which if properly managed and maintained can sequester substantial amounts of atmospheric carbon dioxide in the form of soil organic carbon and mitigate climate change. Varied climatic conditions impact carbon sequestration at the arid and semi-arid ecological sites on rangelands. Best management practices, site-specific policies and technological options are important approaches to manage these ecosystems to mitigate the impact of current climate variations. There are a number of co-benefits associated with management of rangelands for carbon storage, these include; improve soil quality and resilience, agronomic productivity, advancing global food security and restoring ecosystem diversity. Also, the non-equilibrium model is proposed for use on these xeric sites of rangelands since it better reflects the ecological dynamics that impact carbon sequestration and as such policies should be embodied this understanding. More studies are needed on the ecological dynamics, and factors that affect soil carbon sequestration process and mechanisms on arid and semi-arid environments as well as the soil organic carbon residence time so as to better our understanding of these regions. The voluntary carbon market was a good approach for stimulating carbon sequestration on rangelands, however the causes of failure should be revisited, addressed and necessary amendment to policies made that will be the driving force towards environment restoration and conservation. Some of the obvious challenges and opportunities with regards to efficient use of rangelands as carbon sinks to mitigate as well as adapt to climate change are discussed in this paper.

Contrasting water strategies of two Mediterranean shrubs of limited distribution: uncertain future under a drier climate

Plants have evolved different strategies to cope with drought, involving alternative ecophysiologies and different levels of plasticity. These strategies are critical for species of limited distribution, which are especially vulnerable to the current rates of rapid environmental change. The aim of this study was to assess the water strategy of two species with limited distribution, Cneorum tricoccon L. and Rhamnus ludovici-salvatoris Chodat., and evaluate their interpopulation variability along an aridity gradient to estimate their vulnerability to a drier climate. We measured different ecophysiological traits influenced by drought--stomatal conductance, maximum photochemical efficiency of photosynthesis II, carbon isotope ratio and chlorophyll concentration--in two climatically contrasting years, before and during summer drought. Both species were vulnerable to drought at the aridity limit of the gradient, but showed contrasting water strategies: while C. tricoccon was consistent in its water conservation strategy across the aridity gradient, R. ludovici-salvatoris was not, displaying higher and more variable stomatal conductances and being able to increase water-use efficiency at the most xeric sites. Changes in length and intensity of drought events may favor one species' strategy to the detriment of the other: C. tricoccon is more vulnerable to chronic and prolonged droughts, whereas short but acute droughts might have a stronger effect on R. ludovici-salvatoris. In those communities where these two species coexist, such different strategies might lead to changes in community structure under climate change scenarios, with unknown cascade effects on ecosystem functioning.

Secondary dormancy of seeds in relation to the Bromus tectorum-Pyrenophora semeniperda pathosystem

Bromus tectorum is a highly invasive annual grass. The fungal pathogen Pyrenophora semeniperda can kill a large fraction of B. tectorum seeds. Outcomes in this pathosystem are often determined by the speed of seed germination. In this paper we extend previous efforts to describe the pathosystem by characterising secondary dormancy acquisition of B. tectorum. In the laboratory approximately 80% of seeds incubated at –1.0 MPa became dormant. In the field, seeds were placed in the seed bank in late autumn, retrieved monthly and dormancy status determined. The field study confirmed the laboratory results; ungerminated seeds became increasingly dormant. Our data suggest that secondary dormancy is much more likely to occur at xeric sites.