Changes In Soil Water Availability Research Articles

Decoupling between soil moisture and biomass drives seasonal variations in live fuel moisture across co-occurring plant functional types

BackgroundWildfires are important global disturbances influencing ecosystem structure and composition. The moisture content of living and senescent plant components are key determinants of wildfire activity, yet our understanding of how seasonal fluctuations in water availability and biomass control live foliar moisture content (LFMC) across co-occurring plant functional types is limited in diverse forested landscapes.ResultsWe recorded root-zone volumetric water content (VWC) and sampled leaf mass area (LMA) and LFMC of three co-occurring plant functional types across six field sites. We used a linear mixed effects model to quantify the drivers of LFMC and understand whether LFMC dynamics were coupled or decoupled from site conditions. Both LMA and VWC were significant predictors of LFMC variability (p < 0.001), although the strength and direction of these relationships varied across functional types. LFMC dynamics of understorey plant functional types were strongly coupled to site conditions, where the site random effect explained 44.2% (shrub) and 74.8% (herb) of the variability in LFMC across the season respectively. In contrast, overstorey plants were decoupled from site conditions, which explained only 8.1% of the variability in LFMC.ConclusionsLFMC of understorey plants responded to changes in soil water availability (VWC), while overstorey trees responded to biomass fluctuations (LMA). We present a conceptual model describing the influence of these factors on LFMC, which aligns with our findings and draws on the broader literature. This knowledge and conceptual approach can be used to improve our ability to characterize seasonal LFMC variation across different plant functional types, in turn improving our capacity to predict wildfire risk.

The other side of tropical forest drought: do shallow water table regions of Amazonia act as large-scale hydrological refugia from drought?

Tropical forest function is of global significance to climate change responses, and critically determined by water availability patterns. Groundwater is tightly related to soil water through the water table depth (WT), but historically neglected in ecological studies. Shallow WT forests (WT < 5 m) are underrepresented in forest research networks and absent in eddy flux measurements, although they represent c. 50% of the Amazon and are expected to respond differently to global-change-related droughts. We review WT patterns and consequences for plants, emerging results, and advance a conceptual model integrating environment and trait distributions to predict climate change effects. Shallow WT forests have a distinct species composition, with more resource-acquisitive and hydrologically vulnerable trees, shorter canopies and lower biomass than deep WT forests. During 'normal' climatic years, shallow WT forests have higher mortality and lower productivity than deep WT forests, but during moderate droughts mortality is buffered and productivity increases. However, during severe drought, shallow WT forests may be more sensitive due to shallow roots and drought-intolerant traits. Our evidence supports the hypothesis of neglected shallow WT forests being resilient to moderate drought, challenging the prevailing view of widespread negative effects of climate change on Amazonian forests that ignores WT gradients, but predicts they could collapse under very strong droughts.

Temporal variation and controlling factors of tree water consumption in the thornbush savanna

For a typical encroacher tree of the thornbush savanna, we studied the responses of water consumption to changes in soil water availability within 80 cm depth, vapor pressure deficit (VPD), and global radiation. Therefore, we monitored the sap velocities of Senegalia mellifera trees over 2 years in a maximum of 20 stems (6–15 plant individuals) per measurement period on a site in central Namibia. For this water-restricted ecosystem we aimed to understand the role of an encroacher tree on soil water dynamics and potential groundwater recharge.At the day-to-day scale, soil water was the primary driver of cumulative daily sap velocities (Qn-day). In the dry season, Qn-day decreased with increasing soil drought. Rainy seasons triggered multiple sapflow phases, initiated by soil-wetting rain events and associated with increase of soil water. Maximum attainable Qn-day increased with VPD and radiation. We hypothesize a 3-stage response of tree water use to soil water tension (SWT): (1) SWT < pF 2.6–2.7: soil water is optimal for transpiration; (2) pF 2.6–2.7 < SWT < pF 3.2–3.7: transpiration decreases with soil drought; (3) SWT > pF 3.2–3.7: trees minimize transpiration. Our findings contribute to the understanding of tree–grass coexistence in savannas. The responses of tree water use to soil water within 80 cm depth indicate that S. mellifera exploits the same depths as grasses, in contrast to the classic hypothesis of vertical niche differentiation.

Seasonal changes in the water-use strategies of three herbaceous species in a native desert steppe of Ningxia, China

Frequent periods of drought conditions are known to limit plant performance, primary production, and ecosystem stability in arid and semi-arid desert steppe environments. Plants often avoid competition by shifting their water use seasonally, which affects the water-use patterns of dominant species as well as the composition and structure of plant communities. However, the water-use strategies of dominant herbaceous species, which grow under natural field conditions in the desert steppe region of Ningxia Hui Autonomous Region, China, are poorly known. Here, we explored the possible sources of water uptake and water-use efficiency (WUE) of three dominant herbaceous plant species (Stipa breviflora, Agropyron mongolicum, and Glycyrrhiza uralensis) in a native desert steppe in the semi-arid area of Ningxia through an analysis of multiple parameters, including (1) the stable isotopic oxygen and hydrogen (δ18O and δ2H) compositions of precipitation, soil water, and stem water, (2) the carbon isotope (13C) composition of leaves, and (3) the soil water contents, based on field sampling across varying water conditions from June to September, 2017. Frequent small precipitation events replenished shallow soil water, whereas large events only percolated down to the deep soil layers. Changes in soil water availability affected the water-use patterns of plants. Generally, during light precipitation periods, the deep root system of G. uralensis accessed deeper (>80 cm) soil water, whereas S. breviflora and A. mongolicum, which only have shallow roots, primarily absorbed water from the shallow and middle soil layers. As precipitation increased, all three plant species primarily obtained water from the shallow soil layers. Variation in soil water uptake between the dry and wet seasons enabled plants to make better use of existing satoil water. In addition, the δ13C values of G. uralensis and S. breviflora were higher than those of A. mongolicum. The δ13C values of the three plant species were significantly negatively correlated with soil water content. Therefore, G. uralensis and S. breviflora maintained a higher WUE through their conservative and water-saving strategies across the entire growing season. In contrast, A. mongolicum, with a relatively low WUE in the wet season but a high WUE in the dry season, exhibited a more flexible water-use strategy. The different water-use strategies of these dominant plant species demonstrated the mechanisms by which plant communities can respond to drought.

Repeated drying and rewetting differently affect abiotic and biotic soil phosphorus (P) dynamics in a sandy soil: A 33P soil incubation study

Soil drying and rewetting (DRW) events are expected to occur at higher frequencies because of alterations in climate patterns. Readily extractable inorganic and microbial soil phosphorus (P) pools may be affected due to rapid changes in soil water availability. We aimed to determine how soil P dynamics are affected by repeated soil DRW using a sandy grassland soil that regularly experiences DRW. In a laboratory soil incubation study, the soil was exposed to three DRW cycles, with each cycle consisting of a two-day drying phase, a three-day dryness phase and a four-day moist phase after rapid rewetting. The indicators of abiotic processes (P sorption) and biotic processes (respiration, microbial abundance, potential phosphatase enzyme activities) were regularly determined together with water-extractable P, resin-extractable P and microbial P in a 33P-labelled soil.During the first DRW cycle, microbial P was reduced by half and accompanied by a concomitant but not equivalent increase in water-extractable P and a slight as well as delayed increase in resin-extractable P. Thus, increases in water-extractable P were explained by microbial P released during drying but also by microbial P occupying soil P sorption sites, thereby decreasing soil P sorption. Changes in the 33P-isotopic composition of microbial P at the same time suggested that microorganisms did not respond homogenously to the DRW treatment and indicated an increased mineralisation of previously unavailable organic P compounds. However, during the second and third DRW cycles, only water-extractable P, soil P sorption and potential phosphatase activities were affected by the DRW treatment, whereas all other parameters remained similar in values to the constant moist treatment. The effects of DRW on soil P dynamics appeared to affect water-extractable P more long-lastingly, whereas microbial P and most of the biotic indicators quickly adjusted to the DRW treatment. We conclude that the current concepts suggesting an increased mobility of soil P towards other environmental compartments due to soil DRW should consider that abiotic and biotic soil P dynamics are not equally affected in the case of short repetition of DRW incidences.

Changes in soil water availability and air-temperature impact biomass allocation and C:N:P stoichiometry in different organs of Stylosanthes capitata Vogel

Temperature and soil water availability play important roles in the biogeochemical cycles of essential elements for plant growth, such as carbon (C), nitrogen (N), and phosphorus (P). In this study, we investigated how drought and warming impact C:N:P stoichiometric ratios of different plant organs (leaves, inflorescences, and stems), and biomass allocation and production of a field-grown pasture of Stylosanthes capitata, a tropical forage legume. We evaluated the effects of elevated temperature (+2 °C above ambient temperature) under two conditions of soil water availability, irrigated, and non-irrigated. In general, we observed that different functional plant organs showed distinct responses to drought and warming demonstrating how important is to evaluate different functional plant organs to unravel crop nutrient dynamics. In addition, interactive effects between warming and drought were observed in many situations, highlighting the importance of multifactorial studies. Our data showed that warming produced plants with more inflorescences, decreasing leaf:inflorescence ratio. However, only warming under well-watered conditions improved biomass production (in 38%). Warmed and irrigated plants showed higher stoichiometric homeostasis compared to other treatments. In an opposite direction, drought decreased P concentration and increased N:P ratios in different organs, reducing the stoichiometric homeostasis under both conditions of temperature. We have concluded that warm and well-watered conditions without restrictions in soil nutrient availability can enhance plant production, presumably due to a higher level of stoichiometric homeostasis.

Identifying water sources used by alpine riparian plants in a restoration zone on the Qinghai-Tibet Plateau: Evidence from stable isotopes

Riparian zones are vulnerable to water regimes, which alter soil water availability and impact the persistence of riparian plants. However, little is known about the water use patterns of alpine riparian shrubs (e.g., Myricaria squamosa) in response to changes in soil water availability on the Qinghai–Tibet Plateau. This study examined the water-use patterns of M. squamosa along a zone of gradual degradation (light, moderate, and severe), located in the lower reaches of the Shaliu River in the Qinghai Lake watershed. Stable water isotopes (δ2H and δ18O) in xylem water, soil water and groundwater, as well as leaf water potential were monitored during the growing seasons from 2012 to 2013, and quantified the water-use proportions via MixSIAR model. Results showed significant differences in the isotopic signatures of M. squamosa from the light, moderate, and severe degradation sites across seasons, suggesting that M. squamosa exploited different water sources. MixSIAR results also revealed that M. squamosa used high proportions of shallow soil water in the light degradation site (35.4%) compared with the severe degradation sites (13.4%). By contrast, M. squamosa exhibited an ability to shift its water sources and to rely more on groundwater in the severe degradation site across seasons. The contrasting water-use patterns of M. squamosa along the gradual degradation zone were closely linked with the distributions of active root zones when competing for water. Higher predawn leaf water potential (Ψpd) of M. squamosa (mean Ψpd = −2.29 ± 0.7 MPa) was found in the light degradation site and lower Ψpd values in the severe degradation site (mean Ψpd = −3.3 ± 0.8 MPa), suggesting that M. squamosa depended on a high degree of flexible plasticity in water use to alleviate water stress along the gradual degradation. These results linked to water-use patterns and ecophysiological characteristics (e.g., Ψpd) of plants responding to changes in available water are important for informing decision-making management strategies designed to prevent ecological degradation.

Silicon uptake by a pasture grass experiencing simulated grazing is greatest under elevated precipitation

BackgroundGrasses are hyper-accumulators of silicon (Si) and often up-regulate Si following herbivory. Positive correlations exist between Si and plant water content, yet the extent to which Si uptake responses can be mediated by changes in soil water availability has rarely been studied and never, to our knowledge, under field conditions. We used field-based rain-exclusion shelters to investigate how simulated grazing (shoot clipping) and altered rainfall patterns (drought and elevated precipitation, representing 50% and 150% of ambient precipitation levels, respectively) affected initial patterns of root- and shoot-Si uptake in a native Australian grass (Microlaena stipoides) in Si-supplemented and untreated soils.ResultsSi supplementation increased soil water retention under ambient and elevated precipitation but not under drought, although this had little effect on Si uptake and growth (tiller numbers or root biomass) of M. stipoides. Changes in rainfall patterns and clipping had strong individual effects on plant growth and Si uptake and storage, whereby clipping increased Si uptake by M. stipoides under all rainfall treatments but to the greatest extent under elevated precipitation. Moreover, above-ground–below-ground Si distribution only changed following elevated precipitation by decreasing the ratio of root:shoot Si concentrations.ConclusionsResults highlight the importance of soil water availability for Si uptake and suggest a role for both active and passive Si transport mechanisms. Such manipulative field studies may provide a more realistic insight into how grasses initially respond to herbivory in terms of Si-based defence under different environmental conditions.

Changes in soil water availability in vineyards can be traced by the carbon and nitrogen isotope composition of dried wines

The grapevine is one of the most important edible fruit plants cultivated worldwide, and it is highly sensitive to changes in the soil water content. We studied the total carbon and nitrogen contents and stable isotope compositions (C/NWSR, δ13CWSR and δ15NWSR values) of the solid residues obtained by freeze-drying wines produced from two white grapevine cultivars (Vitis vinifera L. cv Chasselas and Petite Arvine) field grown under different soil water regimes while maintaining other climatic and ecopedological conditions identical. These experiments simulated the more frequent and extended climate change-induced periods of soil water shortage. The wines were from the 2009–2014 vintages, produced using the same vinification procedure. The plant water status, reflecting soil water availability, was assessed by the predawn leaf water potential (Ψpd), monitored in the field during the growing seasons. For both wine varieties, the δ13CWSR values are highly correlated with Ψpd values and record the soil water availability set by soil water holding capacity, rainfall and irrigation water supply. These relationships were the same as those observed for the carbon isotope composition of fruit sugars (i.e., must sugars) and plant water status. In Chasselas wines, the nitrogen content and δ15NWSR values decreased with soil water deficit, indicating control of the flux of soil-water soluble nutrients into plants by soil water availability. Such a correlation was not found for Petite Arvine, probably due to different N-metabolism processes in this genetically atypical cultivar. The results presented in this study confirm and generalize what was previously found for red wine (Pinot noir); the carbon isotope composition of wine solid residues is a reliable indicator of the soil and the plant water status and thus can be used to trace back local climatic conditions in the vineyard's region. In most wines (except Petite Arvine) the C/NWSR and δ15NWSR values are indicators of the origin of the nitrogen supplied to the plant's fruit (grape) that can be used to assess the N dynamics in the soil-water-plant system.

Manipulative experiments demonstrate how long-term soil moisture changes alter controls of plant water use

Tree transpiration depends on biotic and abiotic factors that might change in the future, including precipitation and soil moisture status. Although short-term sap flux responses to soil moisture and evaporative demand have been the subject of attention before, the relative sensitivity of sap flux to these two factors under long-term changes in soil moisture conditions has rarely been determined experimentally. We tested how long-term artificial change in soil moisture affects the sensitivity of tree-level sap flux to daily atmospheric vapor pressure deficit (VPD) and soil moisture variations, and the generality of these effects across forest types and environments using four manipulative sites in mature forests. Exposure to relatively long-term (two to six years) soil moisture reduction decreases tree sap flux sensitivity to daily VPD and relative extractable water (REW) variations, leading to lower sap flux even under high soil moisture and optimal VPD. Inversely, trees subjected to long-term irrigation showed a significant increase in their sensitivity to daily VPD and REW, but only at the most water-limited site. The ratio between the relative change in soil moisture manipulation and the relative change in sap flux sensitivity to VPD and REW variations was similar across sites suggesting common adjustment mechanisms to long-term soil moisture status across environments for evergreen tree species. Overall, our results show that long-term changes in soil water availability, and subsequent adjustments to these novel conditions, could play a critical and increasingly important role in controlling forest water use in the future.

Does water availability regulate biomass partitioning between trunk and branches?

The extent to which a vertical trunk is differentiated from its branches is a key trait for the architecture of trees and may affect interspecific relationships. In this study, we analysed the effect of soil water availability on biomass partitioning for Nothofagus pumilio by means of a nursery experiment. Juvenile trees were subject to three irrigation conditions: no irrigation, intermediate irrigation and high irrigation. Irrigation conditions emulated the mean precipitation of the most representative environments inhabited by N. pumilio. Changes in soil water availability modified the biomass partitioning patterns of trees. In comparison to the other two conditions, high irrigation caused: (i) a higher ratio of biomass partitioning to stems than roots; (ii) more trunk growth in relation to its branches; and (iii) more photosynthetic organs relative to the aboveground biomass. Trunk size relative to that of its most recent branches was not increased by water availability. Water availability may play a significant role in the capacity of N. pumilio for space occupation due to the effects on axis differentiation.

Plasticity in gas-exchange physiology of mature Scots pine and European larch drive short- and long-term adjustments to changes in water availability.

Adjustment mechanisms of trees to changes in soil-water availability over long periods are poorly understood, but crucial to improve estimates of forest development in a changing climate. We compared mature trees of Scots pine (Pinus sylvestris) and European larch (Larix decidua) growing along water-permeable channels (irrigated) and under natural conditions (control) at three sites in inner-Alpine dry valleys. At two sites, the irrigation had been stopped in the 1980s. We combined measurements of basal area increment (BAI), tree height and gas-exchange physiology (Δ13 C) for the period 1970-2009. At one site, the Δ13 C of irrigated pine trees was higher than that of the control in all years, while at the other sites, it differed in pine and larch only in years with dry climatic conditions. During the first decade after the sudden change in water availability, the BAI and Δ13 C of originally irrigated pine and larch trees decreased instantly, but subsequently reached higher levels than those of the control by 2009 (15years afterwards). We found a high plasticity in the gas-exchange physiology of pine and larch and site-specific responses to changes in water availability. Our study highlights the ability of trees to adjust to new conditions, thus showing high resilience.

Drought response transcriptomes are altered in poplar with reduced tonoplast sucrose transporter expression.

Transgenic Populus tremula x alba (717-1B4) plants with reduced expression of a tonoplast sucrose efflux transporter, PtaSUT4, exhibit reduced shoot growth compared to wild type (WT) under sustained mild drought. The present study was undertaken to determine whether SUT4-RNAi directly or indirectly altered poplar predisposition and/or response to changes in soil water availability. While sucrose and hexose levels were constitutively elevated in shoot organs, expression responses to drought were most altered in the root tips of SUT4-RNAi plants. Prior to any drought treatment, constitutively elevated transcript levels of abscisic acid biosynthetic genes and bark/vegetative storage proteins suggested altered metabolism in root tips of RNAi plants. Stronger drought-stimulation of stress-inducible genes encoding late-embryogenesis-abundant proteins in transgenic roots was consistent with increased vulnerability to soil drying. Transcript evidence suggested an RNAi effect on intercellular water trafficking by aquaporins in stem xylem during soil drying and recovery. Co-expression network analysis predicted altered integration of abscisic acid sensing/signaling with ethylene and jasmonate sensing/signaling in RNAi compared to WT roots. The overall conclusion is that steepened shoot-root sugar gradient in RNAi plants increased sensitivity of root tips to decreasing soil water availability.

Stoichiometric homeostasis predicts plant species dominance, temporal stability, and responses to global change.

Why some species are consistently more abundant than others, and predicting how species will respond to global change, are fundamental questions in ecology. Long-term observations indicate that plant species with high stoichiometric homeostasis for nitrogen (HN), i.e., the ability to decouple foliar N levels from variation in soil N availability, were more common and stable through time than low-HN species in a central U.S. grassland. However, with nine years of nitrogen addition, species with high H(N) decreased in abundance, while those with low H(N) increased in abundance. In contrast, in climate change experiments simulating a range of forecast hydrologic changes, e.g., extreme drought (two years), increased rainfall variability (14 years), and chronic increases in rainfall (21 years), plant species with the highest H(N) were least responsive to changes in soil water availability. These results suggest that H(N) may be predictive of plant species success and stability, and how plant species and ecosystems will respond to global-change-driven alterations in resource availability.

Climate response of the soil nitrogen cycle in three forest types of a headwater Mediterranean catchment

Future changes in climate may affect soil nitrogen (N) transformations, and consequently, plant nutrition and N losses from terrestrial to stream ecosystems. We investigated the response of soil N cycling to changes in soil moisture, soil temperature, and precipitation across three Mediterranean forest types (evergreen oak, beech, and riparian) by fusing a simple process-based model (which included climate modifiers for key soil N processes) with measurements of soil organic N content, mineralization, nitrification, and concentration of ammonium and nitrate. The model describes sources (atmospheric deposition and net N mineralization) and sinks (plant uptake and hydrological losses) of inorganic N from and to the 0–10 cm soil pool as well as net nitrification. For the three forest types, the model successfully recreated the magnitude and temporal pattern of soil N processes and N concentrations (Nash-Sutcliffe coefficient = 0.49–0.96). Changes in soil water availability drove net N mineralization and net nitrification at the oak and beech forests, while temperature and precipitation were the strongest climatic factors for riparian soil N processes. In most cases, net N mineralization and net nitrification showed a different sensitivity to climatic drivers (temperature, soil moisture, and precipitation). Our model suggests that future climate change may have a minimal effect on the soil N cycle of these forests (<10% change in mean annual rates) because positive warming and negative drying effects on the soil N cycle may counterbalance each other.

Soil water availability strongly alters the community composition of soil protists

Abstract Drought and heavy rainfall are contrasting conditions expected to result from increasingly extreme weather during climate change; and both scenarios will strongly affect the functioning of soil systems. However, little is known about the specific responses of soil microorganisms, whose functioning is intimately tied to the magnitude of the water-filled pore space in soil. Soil heterotrophic protists, being important aquatic soil organisms are considered as key-regulators of microbial nutrient turnover. We investigated the responses of distinct protist taxa to changes in soil water availability (SWA) using a modified enumeration technique that enabled quantification of protist taxa up to genus level. Our study revealed a non-linear shift of protist abundance with decreasing SWA and this became apparent at a maximum water-filled pore size of ≤40 μm. Generally, taxa containing large specimen were more severely affected by drought, but responses to either drought or rewetting of soils were not uniform among taxa. Changes in water availability may thus affect the functioning of key taxa and soil ecosystems long before aboveground “drought” effects become apparent.

Rapid and long-term effects of water deficit on gas exchange and hydraulic conductance of silver birch trees grown under varying atmospheric humidity

BackgroundEffects of water deficit on plant water status, gas exchange and hydraulic conductance were investigated in Betula pendula under artificially manipulated air humidity in Eastern Estonia. The study was aimed to broaden an understanding of the ability of trees to acclimate with the increasing atmospheric humidity predicted for northern Europe. Rapidly-induced water deficit was imposed by dehydrating cut branches in open-air conditions; long-term water deficit was generated by seasonal drought.ResultsThe rapid water deficit quantified by leaf (ΨL) and branch water potentials (ΨB) had a significant (P < 0.001) effect on gas exchange parameters, while inclusion of ΨB in models resulted in a considerably better fit than those including ΨL, which supports the idea that stomatal openness is regulated to prevent stem rather than leaf xylem dysfunction. Under moderate water deficit (ΨL≥-1.55 MPa), leaf conductance to water vapour (gL), transpiration rate and leaf hydraulic conductance (KL) were higher (P < 0.05) and leaf temperature lower in trees grown in elevated air humidity (H treatment) than in control trees (C treatment). Under severe water deficit (ΨL<-1.55 MPa), the treatments showed no difference. The humidification manipulation influenced most of the studied characteristics, while the effect was to a great extent realized through changes in soil water availability, i.e. due to higher soil water potential in H treatment. Two functional characteristics (gL, KL) exhibited higher (P < 0.05) sensitivity to water deficit in trees grown under increased air humidity.ConclusionsThe experiment supported the hypothesis that physiological traits in trees acclimated to higher air humidity exhibit higher sensitivity to rapid water deficit with respect to two characteristics - leaf conductance to water vapour and leaf hydraulic conductance. Disproportionate changes in sensitivity of stomatal versus leaf hydraulic conductance to water deficit will impose greater risk of desiccation-induced hydraulic dysfunction on the plants, grown under high atmospheric humidity, in case of sudden weather fluctuations, and might represent a potential threat in hemiboreal forest ecosystems. There is no trade-off between plant hydraulic capacity and photosynthetic water-use efficiency on short time scale.

Assessment of ecosystem CO2 efflux and its components in a Pinus canariensis forest at the treeline

We estimated respiratory fluxes in a treeline-associated Pinus canariensis forest in Tenerife, Canary Islands, an ecotone with strong seasonal changes in soil water availability. CO2 efflux rates from the foliage, above ground woody tissue and the soil were measured by chamber techniques. Site-specific models obtained from these chamber measurements were then combined with half-hourly measurements of canopy, stem and soil temperature as well as with soil water potential, leaf area and stem sapwood volume data for scaling up component-specific CO2 efflux to ecosystem respiration (R ECO). Integrated over an entire year R ECO was 550 g C m−2 ground surface area (average of 2008 and 2009) and comprised the following component fluxes: 57 % from the soil, 10 % from above ground woody tissue and 33 % from the foliage. Between year differences in R ECO and its components were <3 %. R ECO varied markedly throughout an entire year generally following the seasonal trends in temperature during most of the year. During the dry summer, however, R ECO was significantly reduced due to limited soil water availability in the rooting horizon. Thus, in treeline-associated forests under predicted scenarios of increasing aridity in Mediterranean regions, it is likely that there will be a shift in the contribution of CO2 efflux from the soil (R S), the foliage (R F) and above ground woody tissue (R W) to R ECO from predominately below ground to increasingly above ground. Such changes should be taken into account for predicting the response of treeline forests to a changing climate at their upper distribution limit.

Seasonal changes in fructan accumulation in the underground organs of Gomphrena marginata Seub. (Amaranthaceae) under rock-field conditions

Rock field plant species present several adaptations to the climatic characteristics and seasonality of this habitat. In this study, we evaluate under field conditions the seasonal changes in non-structural carbohydrate accumulation in tuberous organs of Gomphrena marginata. Tuber fragments of plants of G. marginata were sampled monthly for a year (August 2010 to July 2011) in a rock-field area in southeastern Brazil. Quantitative analyses of total fructose, fructo-oligosaccharides, fructo-polysaccharides, reducing sugars and free proline were carried out. Qualitative analyses of carbohydrates were also performed by thin layer chromatography (TLC) and high performance anion exchange chromatography (HPAEC/PAD). Soil moisture was evaluated by gravimetry and plant water status was evaluated by measuring the relative water content (RWC) and water potential (Ψw) of underground organs. Throughout the analyzed period, a well-marked seasonality of fructan accumulation was observed in tuberous roots of G. marginata correlating significantly with seasonal changes in soil water availability. In the rainy period (October to February), the content of fructo-polysaccharides decrease while during the dry and quiescent growth period (April to September) a high accumulation of fructo-polysaccharides was observed without changes in the RWC of tuberous roots, suggesting that cell osmoregulation contributed to water status maintenance in G. marginata. Our study presents a second species of Amaranthaceae that accumulates fructans as its main reserve carbohydrate, which can be an important strategy for survival in an environment characterized by a long dry period.

Response of ammonia oxidizing archaea and bacteria to changing water filled pore space

Abstract This study examined the effect of water filled pore space (WFPS) on gross N fluxes and community structure and abundance of ammonia oxidizing archaea and bacteria in a semi-arid soil. Different WFPS altered the community structure of both AOA and AOB. Ammonia oxidizer communities (for both archaea and bacteria) from ‘wet’ soils (95, 85 and 75% WFPS) and ‘dry’ soils (25, 45 and 55% WFPS) were distinctly different from one another. Additionally there was a significant relationship between community structure and gross rates of nitrification. There was also a significant relationship between WFPS and bacterial amoA abundance but not archaeal amoA abundance suggesting that bacterial ammonia oxidizers are more responsive to changes in soil water availability. These results are in agreement with other studies suggesting that both groups of ammonia oxidizers have distinct physiological characteristics and ecological niches with consequences for nitrification in response to WFPS. Overall findings from this study indicate that nitrification, both in terms of process rates and populations responsible for nitrification activity, is highly responsive to soil water availability.