High- and low-latitude climate interactions: evidence for enhanced aridity of Asian monsoon dust source areas after 2.4 MYR from ODP Leg 117 magnetic susceptibility data

EXTRACT (SEE PDF FOR FULL ABSTRACT): Whole-core magnetic susceptibility can sometimes be used as a rapid and sensitive indicator of variations in the concentration of terrigenous material. We apply this approach to study the evolution of Plio-Pleistocene climatic cycles of terrigenous sedimentation at Ocean Drilling Program Site 721, on the Owen Ridge in the Arabian Sea.

Reconstruction of aridity for the Sierra de la Laguna, Baja California Sur, Mexico

A well-documented history of past climatic conditions is needed to understand and resolve some ecological problems, but the existing climatological records are too short to detect long-term climatic variability and changes. Some trees, such as pines, produce annual tree rings with different widths depending on prevailing environmental conditions, such as climate. Tree-ring analysis of long-lived trees can be used to estimate past variations in climate. The principal aim of this study is to reconstruct aridity for the southern portion of the Baja California Peninsula, by means of dendroclimatologic techniques.

History and variability of Asian interior aridity recorded by eolian flux in the Chinese Loess Plateau during the past 7 Ma

Eolian flux in the Chinese Loess Plateau was reconstructed by measuring the dry bulk density and CaCO3 content of the late Cenozoic loess-paleosol-red clay sequences in the Lingtai profile. Comparison of eolian flux variation between the Lingtai profile and the ODP sites 885/886 in the North Pacific shows a significant wet-dry variability in addition to a gradual drying trend in the dust source regions in interior Asia. Especially, the increase of eolian fluxes from both continental and pelagic eolian sediments indicates a sharp drying of the dust source regions between 3.6 and 2.6 MaBP, which might be attributed to the tectonic uplift of the Tibetan Plateau, which cut down the moisture input to the interior Asia. The average value and variability of eolian flux are higher after 2.6 MaBP than before, which may be related to the Quaternary climatic fluctuations on the glacial-interglacial timescale after the commencement of major Northern Hemisphere Glaciations. The eolian fluxes of the Lingtai profile and Core V21-146 in northwest Pacific show a synchronous variation on the 10(4)-10(5) a timescale, indicating that the flux variations from both continental and marine records are closely correlated to the Quaternary climatic fluctuation forced by the ice volume changes on a global scale.

Evidence of a universal scaling relationship for leaf CO2 drawdown along an aridity gradient

The leaf carbon isotope ratio (δ13C) of C3 plants is inversely related to the drawdown of CO2 concentration during photosynthesis, which increases towards drier environments. We aimed to discriminate between the hypothesis of universal scaling, which predicts between-species responses of δ13C to aridity similar to within-species responses, and biotic homoeostasis, which predicts offsets in the δ13C of species occupying adjacent ranges. The Northeast China Transect spans 130–900 mm annual precipitation within a narrow latitude and temperature range. Leaves of 171 species were sampled at 33 sites along the transect (18 at ≥ 5 sites) for dry matter, carbon (C) and nitrogen (N) content, specific leaf area (SLA) and δ13C. The δ13C of species generally followed a common relationship with the climatic moisture index (MI). Offsets between adjacent species were not observed. Trees and forbs diverged slightly at high MI. In C3 plants, δ13C predicted N per unit leaf area (Narea) better than MI. The δ13C of C4 plants was invariant with MI. SLA declined and Narea increased towards low MI in both C3 and C4 plants. The data are consistent with optimal stomatal regulation with respect to atmospheric dryness. They provide evidence for universal scaling of CO2 drawdown with aridity in C3 plants.

Growth response to climate and drought change along an aridity gradient in the southernmost Pinus nigra relict forests

Tree populations at the rear edge of species distribution are sensitive to climate stress and drought. However, growth responses of these tree populations to those stressors may vary along climatic gradients. To analyze growth responses to climate and drought using dendrochronology in rear-edge Pinus nigra populations located along an aridity gradient. Tree-ring width chronologies were built for the twentieth century and related to monthly climatic variables, a drought index (Standardized Precipitation-Evapotranspiration Index), and two atmospheric circulation patterns (North Atlantic and Western Mediterranean Oscillations). Growth was enhanced by wet and cold previous autumns and warm late winters before tree-ring formation. The influence of the previous year conditions on growth increased during the past century. Growth was significantly related to North Atlantic and Western Mediterranean Oscillations in two out of five sites. The strongest responses of growth to the drought index were observed in the most xeric sites. Dry conditions before tree-ring formation constrain growth in rear-edge P. nigra populations. The comparisons of climate-growth responses along aridity gradients allow characterizing the sensitivity of relict stands to climate warming.

Minimum carbon payment along an aridity gradient for dryland afforestation in non annex I countries with stochastic weather and prices

Recent findings demonstrate that trees in deserts are efficient carbon sinks. It remains however unknown whether the Clean Development Mechanism will accelerate the planting of trees in Non Annex I dryland countries. We estimated the price of carbon at which a farmer would be indifferent between his customary activity and the planting of trees to trade carbon credits, along an aridity gradient. Carbon yields were simulated by means of the CO2FIX v3.1 model for Pinus halepensis with its respective yield classes along the gradient (Arid – 100mm to Dry Sub Humid conditions – 900mm). Wheat and pasture yields were predicted on somewhat similar nitrogen-based quadratic models, using 30 years of weather data to simulate moisture stress. Stochastic production, input and output prices were afterwards simulated on a Monte Carlo matrix. Results show that, despite the high levels of carbon uptake, carbon trading by afforesting is unprofitable anywhere along the gradient. Indeed, the price of carbon would have to raise unrealistically high, and the certification costs would have to drop significantly, to make the Clean Development Mechanism worthwhile for non annex I dryland countries farmers. From a government agency's point of view the Clean Development Mechanism is attractive. However, such agencies will find it difficult to demonstrate “additionality”, even if the rule may be somewhat flexible. Based on these findings, we will further discuss why the Clean Development Mechanism, a supposedly pro-poor instrument, fails to assist farmers in Non Annex I dryland countries living at minimum subsistence level.

Human impacts and aridity differentially alter soil N availability in drylands worldwide

Aims Climate and human impacts are changing the nitrogen (N) inputs and losses in terrestrial ecosystems. However, it is largely unknown how these two major drivers of global change will simultaneously influence the N cycle in drylands, the largest terrestrial biome on the planet. We conducted a global observational study to evaluate how aridity and human impacts, together with biotic and abiotic factors, affect key soil variables of the N cycle. Location Two hundred and twenty-four dryland sites from all continents except Antarctica widely differing in their environmental conditions and human influence. Methods Using a standardized field survey, we measured aridity, human impacts (i.e. proxies of land uses and air pollution), key biophysical variables (i.e. soil pH and texture and total plant cover) and six important variables related to N cycling in soils: total N, organic N, ammonium, nitrate, dissolved organic:inorganic N and N mineralization rates. We used structural equation modelling to assess the direct and indirect effects of aridity, human impacts and key biophysical variables on the N cycle. Results Human impacts increased the concentration of total N, while aridity reduced it. The effects of aridity and human impacts on the N cycle were spatially disconnected, which may favour scarcity of N in the most arid areas and promote its accumulation in the least arid areas. Main conclusions We found that increasing aridity and anthropogenic pressure are spatially disconnected in drylands. This implies that while places with low aridity and high human impact accumulate N, most arid sites with the lowest human impacts lose N. Our analyses also provide evidence that both increasing aridity and human impacts may enhance the relative dominance of inorganic N in dryland soils, having a negative impact on key functions and services provided by these ecosystems.

Juvenile–adult tree associations in a continental Mediterranean ecosystem: no evidence for sustained and general facilitation at increased aridity

Question: How do tree species identity, microhabitat and water availability affect inter- and intra-specific interactions between juvenile and adult woody plants? Location: Continental Mediterranean forests in Alto Tajo Natural Park, Guadalajara, Spain. Methods: A total of 2066 juveniles and adults of four co-occurring tree species were mapped in 17 plots. The frequency of juveniles at different microhabitats and water availability levels was analysed using log-linear models. We used nearest-neighbour contingency table analysis of spatial segregation and J-functions to describe the spatial patterns. Results: We found a complex spatial pattern that varied according to species identity and microhabitat. Recruitment was more frequent in gaps for Quercus ilex, while the other three species recruited preferentially under shrubs or trees depending on the water availability level. Juveniles were not spatially associated to conspecific adults, experiencing segregation from them inmany cases. Spatial associations, both positive and negative, were more common at higher water availability levels. Conclusions: Our results do not agree with expectations from the stressgradient hypothesis, suggesting that positive interactions do not increase in importance with increasing aridity in the study ecosystem. Regeneration patterns are species-specific and depend on microhabitat characteristics and dispersal strategies. In general, juveniles do not look for conspecific adult protection. This work contributes to the understanding of species co-existence, proving the importance of considering a multispecies approach at several plots to overcome limitations of simple pair-wise comparisons in a limited number of sites.

Aridity modulates N availability in arid and semiarid Mediterranean grasslands

While much is known about the factors that control each component of the terrestrial nitrogen (N) cycle, it is less clear how these factors affect total N availability, the sum of organic and inorganic forms potentially available to microorganisms and plants. This is particularly true for N-poor ecosystems such as drylands, which are highly sensitive to climate change and desertification processes that can lead to the loss of soil nutrients such as N. We evaluated how different climatic, abiotic, plant and nutrient related factors correlate with N availability in semiarid Stipa tenacissima grasslands along a broad aridity gradient from Spain to Tunisia. Aridity had the strongest relationship with N availability, suggesting the importance of abiotic controls on the N cycle in drylands. Aridity appeared to modulate the effects of pH, plant cover and organic C (OC) on N availability. Our results suggest that N transformation rates, which are largely driven by variations in soil moisture, are not the direct drivers of N availability in the studied grasslands. Rather, the strong relationship between aridity and N availability could be driven by indirect effects that operate over long time scales (decades to millennia), including both biotic (e.g. plant cover) and abiotic (e.g. soil OC and pH). If these factors are in fact more important than short-term effects of precipitation on N transformation rates, then we might expect to observe a lagged decrease in N availability in response to increasing aridity. Nevertheless, our results suggest that the increase in aridity predicted with ongoing climate change will reduce N availability in the Mediterranean basin, impacting plant nutrient uptake and net primary production in semiarid grasslands throughout this region.

Decoupling of soil nutrient cycles as a function of aridity in global drylands

The biogeochemical cycles of carbon (C), nitrogen (N) and phosphorus (P) are interlinked by primary production, respiration and decomposition in terrestrial ecosystems. It has been suggested that the C, N and P cycles could become uncoupled under rapid climate change because of the different degrees of control exerted on the supply of these elements by biological and geochemical processes. Climatic controls on biogeochemical cycles are particularly relevant in arid, semi-arid and dry sub-humid ecosystems (drylands) because their biological activity is mainly driven by water availability. The increase in aridity predicted for the twenty-first century in many drylands worldwide may therefore threaten the balance between these cycles, differentially affecting the availability of essential nutrients. Here we evaluate how aridity affects the balance between C, N and P in soils collected from 224 dryland sites from all continents except Antarctica. We find a negative effect of aridity on the concentration of soil organic C and total N, but a positive effect on the concentration of inorganic P. Aridity is negatively related to plant cover, which may favour the dominance of physical processes such as rock weathering, a major source of P to ecosystems, over biological processes that provide more C and N, such as litter decomposition. Our findings suggest that any predicted increase in aridity with climate change will probably reduce the concentrations of N and C in global drylands, but increase that of P. These changes would uncouple the C, N and P cycles in drylands and could negatively affect the provision of key services provided by these ecosystems.