Rain Pulses Research Articles

Carbon sink strength of a Mediterranean cork oak understorey: how do semi‐deciduous and evergreen shrubs face summer drought?

AbstractQuestionsHow do semi‐deciduous and evergreen shrubs exploit environmental resources during summer drought? What is the contribution of the understorey shrubby layer to ecosystem carbon assimilation? To what extent are carbon balance and transpiration impacted by a rain pulse?LocationCork oak open woodland in the Mediterranean region.MethodsWe used closed dynamic light and dark chambers to measure gas exchange (CO2 and H2O) in the dominant shrub understorey species Cistus salviifolius, Cistus crispus (semi‐deciduous) and Ulex airensis (evergreen), together with plant physiological and morphological measurements during summer drought and autumn recovery. A hyperbolic light response model constrained by vapour pressure deficits was fitted for up‐scaling shrub photosynthesis to the ecosystem level. The data were compared, on a daily and daytime basis, with gross primary productivity estimates from ecosystem eddy‐covariance flux measurements.ResultsThe onset of summer drought led to a significant leaf area reduction in semi‐deciduous species. A general decrease in photosynthesis in all species was observed, while evapotranspiration and above‐ground respiration fluxes contrasted among species during summer progression and autumn recovery. The shallow‐rooted C. salviifolius was able to use light more efficiently than the other two species, although with poor stomatal control over water loss and consistently higher above‐ground respiration rates, leading to lower water and carbon use efficiencies when compared with C. crispus. The deep‐rooted shrub U. airensis maintained higher leaf water potentials and very low photosynthetic rates while decreasing transpiration rates throughout the summer drought. A summer rain pulse showed that shallow‐rooted shrubs use water in an opportunistic way, with immediate leaf rehydration and concomitant photosynthesis increments. Conversely, deep‐rooted shrubs (U. airensis) were unresponsive, only recovering photosynthesis with high soil water content. An opportunistic growth response may be disadvantageous to shallow‐rooted shrubs in a future climate with extended dry summers and higher probability of rain pulse events. The prominent increase in transpiration rates and plant respiration costs observed during the dry conditions that followed the rain pulse, led to a reduced plant ability to recover after autumn rains.ConclusionsThe shrubs that naturaly colonized this montado understorey showed contrasting strategies to overcome summer drought, suggesting an efficient mosaic exploitation of seasonal environmental resources. The contribution of these shrubs to total ecosystem CO2 uptake during summer and autumn recovery was 17%. This high contribution implies that shrub density management decisions should consider a carbon balance perspective.

Carbon and water fluxes in an arid-zone Acacia savanna woodland: An analyses of seasonal patterns and responses to rainfall events

The study of landscape gas exchange in arid and semi-arid regions is less common than those of more mesic environments, despite their large geographical extent, their importance to regional climate, their socioeconomic values and the carbon and water balances of such regions. In this study we used eddy covariance measurements to examine net ecosystem exchange and water fluxes of a landscape dominated by a N-fixing tree (Acacia aneura; Mulga) as a function of soil moisture content, vapour pressure deficit, leaf area index and pulses of rain. Seasonal budgets of carbon and water, ecosystem-scale water-use-efficiency (the ratio of net ecosystem exchange to evapotranspiration) and inherent water-use-efficiency (ecosystem water-use-efficiency×vapour pressure deficit) were also examined. Across the 12 month study, the landscape was a net sink for carbon, despite prolonged periods of zero rain.Changes in both net ecosystem exchange and evapotranspiration were tightly coupled to changes in the moisture content of the upper (10cm) soil profile, but not the deeper profile and both responded rapidly to changes in soil moisture content. As vapour pressure deficit increased over the course of several consecutive days in the wet season there was no significant response of ecosystem water-use-efficiency. In contrast, in the dry season, as vapour pressure deficit increased ecosystem water-use-efficiency declined curvilinearly. However, in both wet and dry seasons, ecosystem water-use-efficiency declined with increasing soil moisture content. Daily inherent water-use-efficiency increased gradually following each rainfall event. As daily mean vapour pressure deficit increased between rain events, inherent water-use-efficiency increased in both the wet and dry seasons but with a steeper slope in the wet season. However, inherent water-use-efficiency decreased with increasing soil moisture in both seasons, and the slope of a semi-log plot of inherent water-use-efficiency versus soil moisture content decreased faster in the dry season than in the wet season. Similarly, the marginal carbon cost of water was smaller (0.3) in the wet than dry season (0.6). Variations in ecosystem leaf area index were correlated with the under storey component, which was highest in the wet season and lowest in the dry season. We therefore conclude that changes in under storey leaf area index were significant drivers of seasonal changes in canopy gas exchange. Mulga, despite maintaining leaf area index through the dry season in a semi-arid environment, supports little dry season evapotranspiration and relies, to a very large extent, on soil moisture in the upper soil profile rather than deeper stores of water.

Do vegetative and reproductive phenophases of deciduous tropical species respond similarly to rainfall pulses?

Deciduous trees with high-density wood that occur in dry seasonal tropical regions respond to rainfall seasonality with synchrony in phenophases. However, they may exhibit interannual differences in synchrony and intensity of phenophases, as strategy for large variations in duration and intensity of rainfall pulses. Nevertheless, it remains unknown how phenophases of deciduous trees of the Brazilian semi-arid region respond to rainfall variations. The phenology of Cordia oncocalyx was monitored in deciduous thorny woodland (Caatinga), from April 2009 to March 2011, and was correlated with rainfall, soil humidity, temperature, and photoperiod. The rainy years 2009 and 2011 exhibited higher duration of rainfall pulses and lower frequency of interpulses, but in 2010 pulse duration and total rainfall were lower. Circular statistics showed leaf flush followed by flowering and fruiting in the rainy season, and leaf fall and seed dispersal in the dry season. Both the vegetative and reproductive phenophases respond similarly to variations in rain pulses, with adjustments in time, duration, and intensity, which were correlated with variations in rainfall and soil humidity, excluding photoperiod as a trigger. Total defoliation occurred in the driest months of each year, November to December 2009 and August to October 2010. A sporadic leaf flush in November 2010 was triggered by occasional rains. Vegetative and reproductive synchronies were high in rainy years, but in 2010 synchrony was low, flowering was delayed and reduced, and, despite the low fruiting, we recorded high density in the seed bank. Lower synchrony, temporal separation of phenophases, and storage of fruits on the ground were risk-spreading strategies used by the population in the dry year, which suggest intrapopulation variability in responses to hydric stress. We believe that this plasticity contributes to high species density in the Caatinga.

Refining the ‘pulse‐reserve’ model for arid central Australia: Seasonal rainfall, soil moisture and plant productivity in sand ridge and stony plain habitats of the Simpson Desert

AbstractIn dry ecosystems, water supply is highly variable in time and space, occurring either as short pulses of shallow soil water or long phases of deep soil water. There may be correspondence between hierarchical soil water pulses and hierarchical biological responses, but this has not been directly tested for arid Australia. We compared plant community responses to an isolated short rain pulse (about 130 mm, November 2008) versus the start, mid and later stages of an extended high rainfall phase (about 540 mm, February 2010 to March 2011) at a site on the western edge of the Simpson Desert. We sampled across contrasting habitats: tall longitudinal sand ridges and adjacent stony clay plains (gibber). Thus, we compared responses to spatially variable soil moisture availability (sand vs. clay soil; run‐off/on relief) and temporally variable rainfall supply. Our results supported the hierarchical resource pulse‐biological response concept for dry ecosystems, with the short and long rainfall phases modulating distinct responses among functional types. Shallow‐rooted annual grasses showed an equivalent positive response to the isolated pulse as to the first pulse of the high rain phase; whereas short‐lived forbs, longer‐lived (2–3 years) grasses and persistent forbs had peak abundance during the high rainfall phase. Perennial grass and shrub productivity did not change over time, though there was a distinct trend towards heightened sexual reproduction at the mid‐late stages of the high rain phase. We also found that species seasonal growth tendencies were pronounced, and that spatial aspects of moisture availability resulted in different composition patterns at the habitat scale with the gibber and sand ridge sites showing strong and weak correspondence with temporal rainfall patterning respectively. Thus, we confirm that plant productivity models for this region should include the effects of rainfall connectivity and soil texture as well as species seasonal growth constraints and rooting attributes.

Combined effects of multifactor climate change and land-use on decomposition in temperate grassland

Climate change is likely to alter decomposition rates through direct effects on soil biotic activity and indirect effects on litter quality with possible impacts on the global carbon budget and nutrient cycling. Currently, there is a need to study the combined effects of climatic drivers and agricultural practises on decomposition.In an in situ litter bag experiment, we studied the effects of rainfall variability (including drought combined with heavy rain pulses as well as regular irrigation) interacting with winter warming and increased winter precipitation and with changes in cutting frequency, on decomposition in a temperate grassland. Litter bags contained mixed and species-specific litter of all different climate and land-use manipulations and were placed within the plots of litter origin. Moreover, we aimed to disentangle the causes of changes in decomposition by investigating two further approaches: Firstly, we studied the effects of changes in leaf chemicals as a result of the manipulations by removing litter from the experiment that has been pre-exposed to the manipulations before placing it on an untreated standard plot outside the experiment. Secondly, we assessed the effects of changes in soil faunal activity by investigating the decomposition of standard material under differing rainfall variability.As a result, decomposition was reduced when litter bags were exposed to drought for six weeks within an 11 months period. Neither additional winter rain nor winter warming had an effect on decomposition, likely because winter warming reduced snow cover and increased variability of surface temperatures. Climate manipulations did not change litter quality. Furthermore, decomposition on the untreated standard plot was not affected by the climate manipulations that the litter was previously exposed to. Thus, reduced decomposition under extreme rainfall variability and drought may mainly be caused by a decrease in soil biotic activity, as indicated by reduced decomposition of standard material during drought.More frequent cutting strongly stimulated decomposition, however, this stimulating effect was absent under extreme rainfall variability including drought. The stimulation of decomposition under more frequent cutting was attributed to changes in litter quality, namely a decrease in C/N ratio. Accordingly, litter from more frequently cut communities decomposed faster on the untreated control plot outside the experiment.Projected increases in drought frequency and increased rainfall variability under climate change may inhibit decomposition and alter nutrient and carbon cycling along with soil quality. Especially decomposition in frequently cut grassland appears vulnerable towards drought.

Coexistence and evolutionary dynamics mediated by seasonal environmental variation in annual plant communities

It is well established theoretically that competing species may coexist by having different responses to variation over time in the physical environment. Whereas previous theory has focused mostly on year-to-year environmental variation, we investigate how within-year variation can be the basis of species coexistence. We ask also the important but often neglected question of whether the species differences that allow coexistence are compatible with evolutionary processes. We seek the simplest circumstances that permit coexistence based on within-year environmental variation, and then evaluate the robustness of coexistence in the face of evolutionary forces. Our focus is on coexistence of annual plant species living in arid regions. We first consider environmental variation of a very simple structure where a single pulse of rain occurs, and different species have different patterns of growth activity following the rain pulse. We show that coexistence of two species is possible based on the storage effect coexistence mechanism in this simplest of varying environments. We find an exact expression for the magnitude of the storage effect that allows the functioning of the coexistence mechanism to be analyzed. However, in these simplest of circumstances, coexistence in our models is not evolutionarily stable. Increasing the complexity of the environment to two rain pulses leads to evolutionarily stable species coexistence, and a route to diversity via evolutionary branching. This demonstration of the compatibility of a coexistence mechanism with evolutionary processes is an important step in assessing the likely importance of a mechanism in nature.

Differential water uptake among plant species in humid alpine meadows

AbstractQuestionDo the three chosen species ( Caltha palustris, Veratrum nigrum and Stellera chamaejasme) in humid alpine meadows take up water from different depths? Do these species differ in their responsiveness to rainfall?Location Mt. Balang, China.MethodsRainwater, soil water and plant water were collected from three plots over a 15‐day period during June–July 2006, and water isotopes analysed. The linear mixed models procedure was used to compare δD and δ 18O values across soil depths, while three‐factor ANOVA was performed to compare δD and δ 18O values among the three species. The contributions of different soil depths to plant water sources were analysed using MixSir. A classic two‐endmember linear mixed model was used to calculate the proportion of rainwater in plant stem water.ResultsSoil water showed substantial temporal shifts in isotopic profile from day to day, as influenced by rainwater input. The three plant species, C. palustris (CP), V. nigrum (VN) and S. chamaejasme (SC), acquired water from different soil depths, with their reliance on deeper soil water increasing in the order of CP < VN < SC. Moreover, CP, VN and SC absorbed 69.18%, 35.50% and 13.46% of their stem water, respectively, from the 4 July rainwater, suggesting that their responsiveness to rain pulses significantly differed in the order of CP > VN > SC.ConclusionsThe isotopic profiles of soil water shifted dramatically in response to rainfall, suggesting rapid movement of soil water. There was separation of the plant water uptake depths among the three species, with SC depending less on surface soil and being associated with drier soil habitats compared with CP and VN. Moreover, CP absorbed rainwater before it percolated below the root zone, while VN and SC did not; the differential responsiveness to water pulses may be related to their different water requirements as a result of adaptation to different soil water conditions. Our study demonstrates inter‐specific differentiation in plant water uptake in humid alpine meadows. Such information is important for species co‐existence and plant community composition.

Influência do pastejo e da precipitação sobre a fenologia de quatro espécies em área de caatinga

Objetivou-se com este experimento avaliar o efeito do pastejo caprino sobre a fenologia do marmeleiro (Croton sonderianus Müll. Arg.), catingueira (Caesalpinia pyramidalis Tull.), malva (Malva sylvestris L.) e pereiro (Aspidosperma pyrifolium Mart.) em área de caatinga no Cariri paraibano. O experimento foi conduzido na fazenda experimental Bacia-Escola pertencente à UFPB, São João do Cariri, PB, no ano de 2008. Foram monitoradas cinco plantas por piquete, com leituras semanais, num total de 28 no período de 05/03/2008 a 19/10/2008, observando-se as seguintes variáveis: número total de folhas, folhas verdes, folhas amarelas, folhas secas, frutos e flores.Os tratamentos consistiram em três taxas de lotação e foram distribuídos em três piquetes: T1 (3,1 an/ha), T2 (1,5 an/ha) e T3 (0 an/ha). Para análise dos dados, agruparam-se as leituras por datas nas respectivas semanas. Foi verificado efeito do pastejo sobre os parâmetros fenológicos observados, notadamente no tratamento com maior taxa de lotação, para as espécies do marmeleiro e da malva, ocasionando redução antecipada do total de folhas, flores e frutos, promovendo alteração na fenologia dessas espécies. Observou-se menor efeito do pastejo sobre a fenologia do pereiro, e não foi verificado efeito sobre a catingueira. Notou-se que os pulsos de precipitação foram suficientes para desencadear os eventos fenológicos nas espécies. A precipitação interferiu no comportamento fenológico das espécies, desencadeando o período de brotamento logo após os primeiros eventos de chuvas, bem como a floração e frutificação, acontecendo em meados do período chuvoso. A queda de folhas ocorre logo após o término do período chuvoso na catingueira, marmeleiro e malva, sendo mais persistente no pereiro.

Phenotypic flexibility in basal metabolic rate is associated with rainfall variability among populations of rufous-collared sparrow

Phenotypic flexibility in metabolic rates allows organisms to reversibly adjust their energy flow to meet challenges imposed by a variable environment. In turn, the food habits hypothesis (FHH) predicts that species or populations adjust their basal metabolic rate (BMR) according to the diet attributes such as food abundance or predictability. Desert ecosystems represent a temporally heterogeneous environment because of low rain pulse predictability, which is also associated with temporal variation in food resources. In the present study, we investigated the relationship between the magnitude of BMR flexibility in response to dietary acclimation and the inter-annual rainfall variability in three populations of rufous-collared sparrows. Specifically we addressed the question of whether birds from a desert environment are more flexible in BMR than those from non-desert habitats. We found a positive trend between BMR flexibility and the inter-annual rainfall variability. In fact, dietary treatments had a significant effect only in desert birds, a result that also supported the FHH. Our study confirms the existence of phenotypic variation in response to environmental conditions among populations, and also highlights the importance of considering the circumstances in which phenotypic flexibility evolves and the specific environmental cues that induce their expression.

The response of CO2 flux to rain pulses at a saline desert

AbstractAs the substantial component of the ecosystem respiration, soil CO2 flux is strongly influenced by infrequent and unpredictable precipitation in arid region. In the current study, we investigated the response of soil CO2 flux to rain pulses at a saline desert in western China. Soil CO2 flux was measured continuously during the whole growing season of 2009 at six sites. We found that there were remarkable changes in amplitude or diurnal patterns of soil CO2 flux induced by rainfall events: from bimodal before rain to a single peak after that. Further analysis indicated that there is a significant linear relationship (P < 0.001) between soil CO2 flux and soil temperature (Tsoil). However, a hysteresis between the waveform of diurnal course of CO2 flux and Tsoil was observed: with soil CO2 flux always peaked earlier than Tsoil. Furthermore, a double exponential decay function was fitted to the soil CO2 flux after rainfall, and total carbon (C) releases were estimated by numerical integration for rainfall events. The relative enhancement and total C release, in association with the rain pulses, was linearly related to the amount of precipitation. According to the size and frequency of rainfall events, the total amount of C release induced by rain pulses was computed as much as 7.88 g C·m–2 in 2009, equivalent to 10.25% of gross primary production. These results indicated that rain pulses played a significant role in the carbon budget of this saline desert ecosystem, and the size of them was a good indicator of rain‐induced flux enhancement. Copyright © 2012 John Wiley & Sons, Ltd.

Precipitation variability and fire influence the temporal dynamics of soil CO2 efflux in an arid grassland

AbstractClimate models suggest that extreme rainfall events will become more common with increased atmospheric warming. Consequently, changes in the size and frequency of rainfall will influence biophysical drivers that regulate the strength and timing of soil CO2 efflux – a major source of terrestrial carbon flux. We used a rainfall manipulation experiment during the summer monsoon season (July–September) to vary both the size and frequency of precipitation in an arid grassland 2 years before and 2 years after a lightning‐caused wildfire. Soil CO2 efflux rates were always higher under increased rainfall event size than under increased rainfall event frequency, or ambient precipitation. Although fire reduced soil CO2 efflux rates by nearly 70%, the overall responses to rainfall variability were consistent before and after the fire. The overall sensitivity of soil CO2 efflux to temperature (Q10) converged to 1.4, but this value differed somewhat among treatments especially before the fire. Changes in rainfall patterns resulted in differences in the periodicity of soil CO2 efflux with strong signals at 1, 8, and 30 days. Increased rainfall event size enhanced the synchrony between photosynthetically active radiation and soil CO2 efflux over the growing season before and after fire, suggesting a change in the temporal availability of substrate pools that regulate the temporal dynamics and magnitude of soil CO2 efflux. We conclude that arid grasslands are capable of rapidly increasing and maintaining high soil CO2 efflux rates in response to increased rainfall event size more than increased rainfall event frequency both before and after a fire. Therefore, the amount and pattern of multiple rain pulses over the growing season are crucial for understanding CO2 dynamics in burned and unburned water‐limited ecosystems.

Interpreting post-drought rewetting effects on soil and ecosystem carbon dynamics in a Mediterranean oak savannah

A transient increase in soil carbon dioxide efflux after rewetting of previously dry soils, termed the Birch effect, can significantly influence the ecosystem carbon balance. This has generally been related to increased soil microbial respiration in response to a temporal increase in labile soil carbon. In order to quantify ecosystem carbon losses by the Birch effect and to trace the underlying biogeochemical processes, we monitored the effects of first natural rain pulses on soil and ecosystem carbon dioxide fluxes and their isotopic composition (δ 13C) after an extended summer drought in a Mediterranean oak woodland. While GPP was affected negatively, rain largely enhanced soil respiratory processes, which increased their relative contribution to NEE by up to 95%, resulting in a significant net carbon loss from the ecosystem. This was reflected by high correlation ( r 0.91) between the two fluxes. Further, isotopic composition of soil respiration (δ 13C S) explained on average 71% of the isotopic composition of ecosystem respiration (δ 13C R). We found a strong relationship between soil moisture and the increase in soil respiration ( R S) and NEE, indicating that the Birch effect observed during this study did not result from a transient increase in labile soil carbon but from a gradual (several days) moisture response of size and/or activity of the soil microbial community. We also observed large variation in δ 13C of soil and ecosystem respired CO 2 that corresponded to the rain pulses with enrichment of up to 8 and 6‰, respectively, and a subsequent depletion to initial values during the following dry days, which might be explained by increased relative contribution of soil microbial communities from deeper soil layers to overall soil respiration, a switch in the respired carbon source (e.g. anaplerotic carbon) and changes in apparent fractionation during the mineralization of soil organic matter after the depletion of labile carbon pools.

Winter establishment of the alien annual Schismus barbatus is not affected by insect herbivory in Northern-Central Monte Desert

We studied the effect of ant herbivory on the establishment and survival of the annual plant Schismus barbatus. We hypothesized that ants may control this species when biomass of native plants is generally low during their vegetative period. We predicted that ant herbivory will decrease plant survival and reproduction. We tested our prediction with an insect exclosure experiment in a sandy desert of Northern-Central Monte. We found more than 12 000 established seedlings per square meter on early May, after two consecutive rain pulses of ca. 20 mm each. Overall, we found that almost 75% of recruited plants survived by the end of the cool season (September), and that 22–24% of the initially established plants survived as mature reproductive plants by the end of the growing season (December). Contrary to our expectation, insect herbivory did not affect plant establishment, plant survival or the proportion of flowering and fruiting individuals of S. barbatus. The large number of seedlings reported, the ability to exploit a temporal window free of plant competitors and enemies, and the availability of microsites where this species can succeed, all suggest that S. barbatus may have the potential to become an important plant invader in the Monte Desert.

Rain pulse response of soil CO2exchange by biological soil crusts and grasslands of the semiarid Colorado Plateau, United States

[1] Biological activity in arid grasslands is strongly dependent on moisture. We examined gas exchange of biological soil crusts (biocrusts), the underlying soil biotic community, and the belowground respiratory activity of C3 and C4 grasses over 2 years in southeast Utah, USA. We used soil surface CO2 flux and the amount and carbon isotope composition (δ13C) of soil CO2 as indicators of belowground and soil surface activity. Soil respiration was always below 2 μmol m−2 s−1 and highly responsive to soil moisture. When moisture was available, warm spring and summer temperature was associated with higher fluxes. Moisture pulses led to enhanced soil respiration lasting for a week or more. Biological response to rain was not simply dependent on the amount of rain, but also depended on antecedent conditions (prior moisture pulses). The short-term temperature sensitivity of respiration was very dynamic, showing enhancement within 1–2 days of rain, and diminishing each day afterward. Carbon uptake occurred by cyanobacterially dominated biocrusts following moisture pulses in fall and winter, with a maximal net carbon uptake of 0.5 μmol m−2 s−1, although typically the biocrusts were a net carbon source. No difference was detected in the seasonal activity of C3 and C4 grasses, contrasting with studies from other arid regions (where warm- versus cool-season activity is important), and highlighting the unique biophysical environment of this cold desert. Contrary to other studies, the δ13C of belowground respiration in the rooting zone of each photosynthetic type did not reflect the δ13C of C3 and C4 physiology.

The role of interannual, seasonal, and synoptic climate on the carbon isotope ratio of ecosystem respiration at a semiarid woodland

The terrestrial carbon cycle is influenced by environmental variability at scales ranging from diurnal to interannual. Here, we present 5-years of growing season (day 131–275) observations of the carbon isotope ratio of ecosystem respiration (δ13CR) from a semiarid woodland. This ecosystem has a large necromass component resulting from 97%Pinus edulis mortality in 2002, is dominated by drought-tolerant Juniperus monosperma trees, and experiences large variability in the timing and intensity of seasonal and synoptic water availability. Mean growing season δ13CR was remarkably invariant (−23.57±0.4‰), with the exception of particularly enriched δ13CR in 2006 following a winter with anomalously low snowfall. δ13CR was strongly coupled to climate during premonsoon periods (∼May to June), including fast (≤2 days) responses to changes in crown-level stomatal conductance (Gc) and vapor pressure deficit (vpd) following rain pulses. In contrast, δ13CR was relatively decoupled from Gc and environmental drivers during monsoon and postmonsoon periods (July–August and September, respectively), exhibiting only infrequent couplings of δ13CR to vpd and soil water content (SWC) with longer lags (∼8 days) and variable response slopes (both positive and negative). Notably, δ13CR exhibited consistent dynamics after rainfall events, with depleted δ13CR occurring within 1 h, progressive hourly δ13CR enrichment over the remainder of the night, and net δ13CR depletions over the multiple nights postevent in monsoon and postmonsoon periods. Overall this ecosystem demonstrated strong dependence of δ13CR on precipitation, with an apparent dominance by the autotrophic δ13C signal in premonsoon periods when deep soil moisture is abundant and surface soil moisture is low, and weaker coupling during monsoonal periods consistent with increasing heterotrophic dominance when deep soil moisture has declined and surface moisture is variable.

Variations in water and CO2 fluxes over a saline desert in western China

AbstractWe studied the variations in water and carbon dioxide fluxes of a saline desert in western China using the eddy covariance technique. In addition, bare‐soil evaporation was measured using the micro‐lysimeter method. The following three inquiries were made: (1) the magnitude of evapotranspiration (ET) and net ecosystem carbon exchange (NEE) and how these components vary seasonally; (2) how NEE and ET vary following rain pulses of different magnitudes; and (3) how ET divides into evaporation and transpiration and its relationship to carbon dioxide exchange. The saline desert acted as a net C sink of − 49 g C m−2 year−1, with a gross ecosystem productivity (GEP) of 345 g C m−2 year−1 and an ecosystem respiration (Reco) of 296 g C m−2 year−1. Rainfall pulses exerted strong control over ecosystem‐scale water and CO2 fluxes. There was a 1 to 2 days lag in maximal ET despite the magnitude of rainfall pulses, while NEE took 4–5 days to reach its peak if the rainfall was large enough to increase the photosynthetic activity of vascular plants. Overall, the ratio of total transpiration to ET was 38%, but it was about 92% during the months when the vascular plants were active. The NEE was increasingly more negative as the growing season progressed, indicating a greater net uptake of CO2 and a greater water use efficiency. Copyright © 2011 John Wiley & Sons, Ltd.

Short-term effects of rain on soil respiration in two New England forests

To gain new insights into the underlying mechanisms responsible for wetting-induced soil respiration, rain simulation field experiments were carried out in two temperate mixed-hardwood forests in New England (Great Mountain Forest and Harvard Forest). The rain-induced CO2 pulses were observed in both xeric and mesic soils. The pulse intensity was negatively correlated with the site moisture level and the pre-rain soil CO2 flux. At both forests, plots without O horizon responded to wetting with limited or even negative enhancement, confirming previous finding that the rain pulse was likely due to enhanced microbial consumption on substrates mainly of microbial origin. Our results show that the flux rain pulse was a reproducible phenomenon not limited to dry soils.

Functional changes in the control of carbon fluxes after 3 years of increased drought in a Mediterranean evergreen forest?

AbstractOur objective was to test how a long‐term increased water limitation affects structural and functional properties of a Mediterranean ecosystem, and how these changes modify the response of the main carbon fluxes to climatic controls. In 2003, a 27% throughfall exclusion experiment was installed in a Quercus ilex L. forest in France. Gross primary production (GPP), ecosystem respiration (RECO) and net ecosystem exchange (NEE) were estimated in a control and a dry treatment. Decreasing throughfall decreased GPP by 14% and had a smaller effect on RECO (−12%), especially soil respiration RS (−11%). Interannual variability of GPP (29%) was higher than for RECO (12%). Error propagation was used to estimates uncertainties in the NEE fluxes, which ranged from 3% to 10% in the control treatment but up to 167% for NEE in the dry treatment because more steps and data types were involved in the scaling. After 3 years of throughfall exclusion, we found no acclimation of RS to climatic drivers. Functional properties of the response of RS to soil water, temperature and rain pulse remained similar in the control and the dry treatments. A diurnal clockwise hysteresis in RS was probably controlled by canopy photosynthesis with a 3 h lag. The proportion of diurnal variation of respiration due to photosynthesis was similar in all treatments (4–5%). Because of the characteristic of rain in Mediterranean climates, a continuous decrease of water input in these environments have an effect on topsoil water and consequently on RS only during short periods when rainfall is characterized by infrequent and small events that does not allow the topsoil to reach field capacity and does not allow to dry completely. However, in the longer term, we expect a stronger decrease in RS in the dry treatment driven by the decrease in GPP.

Multiscale analysis of temporal variability of soil CO2 production as influenced by weather and vegetation

AbstractEcosystem processes are influenced by weather and climatic perturbations at multiple temporal scales with a large range of amplitudes and phases. Technological advances of automated biometeorological measurements provide the opportunity to apply spectral methods on continuous time series to identify differences in amplitudes and phases and relationships with weather variation. Here we used wavelet coherence analysis to study the temporal covariance between soil CO2 production and soil temperature, soil moisture, and photosynthetically active radiation (PAR). Continuous (hourly average) data were acquired over 2 years among three vegetation types in a semiarid mixed temperate forest. We showed that soil temperature and soil moisture influence soil CO2 production differently at multiple periods (e.g. hours, days, weeks, months, years), especially after rain pulse events. Our results provide information about the periodicity of soil CO2 production among vegetation types, and provide insights about processes controlling CO2 production through the study of phase relationships between two time series (e.g. soil CO2 production and PAR). We tested the performance of empirical models of soil CO2 production using the continuous wavelet transform. These models, built around soil temperature and moisture, failed at multiple periods across the measured dates, suggesting that empirical models should include other factors that regulate soil CO2 production at different temporal scales. Our results add a new dimension for the analysis of continuous time series of biometeorological measurements and model testing, which will prove useful for analysis of increasing sensor data obtained by environmental networks.

Temporal and shrub adaptation effect on soil microbial functional diversity in a desert system

SummaryThe Negev Desert is characterized by spatial and temporal patterns of resource distribution, in which soil biota are considered to be among the most sensitive biological characteristics, easily influenced by changes related to soil and abiotic factors. Soil water availability and organic matter are among the most important factors, acting as triggers that determine the length of the period of activity. The main source of organic matter in this xeric environment is input from annual and perennial shrubs. In order to persist and propagate in this xeric environment, the plants have developed different ecophysiological adaptations (e.g. the excretion of salt (Reaumuria negevensis) and chemical compounds (Artemisia sieberi) via the leaves). We found that the values of soil moisture obtained for soil samples collected in the vicinity of R. negevensis were larger than for samples collected in the vicinity of Noaea mucronata and A. sieberi and in the open area. The maximum values of CO2 evolution, microbial biomass and Shannon index (H′) were obtained for the samples collected from the vicinity of N. mucronata. Therefore, we assume that the vicinity of N. mucronata afforded the best conditions for the soil bacterial community. In the Negev Desert, we also found that water availability and pulses of rain compared with frequent rainfall influenced CO2 evolution, microbial biomass, qCO2 and the Shannon index (H′). The differences in water amount and availability between the two rainy seasons caused larger values in most of the properties during the first four seasons (December 2005–November 2006) compared with the last four seasons (December 2006–November 2007) for most of the samples.