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

Climate 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.     

Spatial and temporal variation in soil CO2 efflux in an old-growth neotropical rain forest, La Selva, Costa Rica

Our objectives were to quantify and compare soil CO₂ efflux of two dominant soil types in an old-growth neotropical rain forest in the Atlantic zone of Costa Rica, and to evaluate the control of environmental factors on CO₂ release. We measured soil CO₂ efflux from eight permanent soil chambers on six Oxisol sites. Three sites were developed on old river terraces (old alluvium) and the other three were developed on old lava flows (residual). At the same time we measured soil CO₂ concentrations, soil water content and soil temperature at various depths in 6 soil shafts (3 m deep). Between old alluvium sites, the two-year average CO₂ flux rates ranged from 117.3 to 128.9 mg C m^–2 h^–1. Significantly higher soil CO₂ flux occurred on the residual sites (141.1 to 184.2 mg C m^–2 h^–1). Spatial differences in CO₂ efflux were related to fine root biomass, soil carbon and phosphorus concentration but also to soil water content. Spatial variability in CO₂ storage was high and the amount of CO₂ stored in the upper and lower soil profile was different between old alluvial and residual sites. The major factor identified for explaining temporal variations in soil CO₂ efflux was soil water content. During periods of high soil water content CO₂ emission decreased, probably due to lower diffusion and CO₂ production rates. During the 2-year study period inter-annual variation in soil CO₂ efflux was not detected.     

Spatial and temporal variability in a butterfly population

Summary The dynamics of a butterfly (Plebejus argus) population were analysed at two levels, (i) the population as a whole and (ii) sections within the population. Some sections of the population fluctuated out of synchrony with others, such that the variability [SD Log(Density+1)] shown by the population as a whole was less than the variability shown by each part of the population — overall temporal variability was dampened by spatial asynchrony. Since observed population variability depends on the spatial scale that is sampled, comparisons of population variability among taxa should be carried out only with caution. Implications for island biogeography and conservation biology are discussed.     

Carbon dioxide efflux and pCO2 in soils of threeQuercus ilex montane forests

Soil CO₂ efflux and pCO₂ in the soil atmosphere were measured during one year at three montane sites of Mediterranean sclerophyllous forests in NE Spain. Two sites were located in the upper and lower slopes of a small catchment in the Prades mountains (mean precipitation 550 mm year^–1), and a third site was located on a lower slope in the Montseny mountains (mean precipitation 900 mm year^–1). The three sites were similar in bedrock and vegetation, but differed in soil characteristics and water availability. Seasonal variation of CO₂ efflux and soil pCO₂ were affected by soil temperature and, to a lesser extent, by soil moisture. Annual mean soil CO₂ efflux (considered as soil respiration) was similar at Montseny and at the comparably located site at Prades (83 ± 18 S.E. vs. 75 ± 9 mg CO₂ m^–2 hour^–1 , respectively), and was highest at the Prades upper slope site (122 ± 22 mg C0₂ m^–2 hour^–1 ). Despite those relatively similar CO₂ effluxes, mean soil pCO₂ was much higher at both Prades sites than at Montseny. Soil pCO₂ always increased with depth at Prades while maxima pCO₂ at Montseny were often at 20–30 cm depth. A model based on gas diffusion theory was able to explain why soil pCO₂ was much higher at Prades than at Montseny, and to reproduce the shape of the vertical profile of pCO₂ at the Prades soils. Nevertheless, the model failed to simulate the soil pCO₂ maximum found at 20–30 cm depth at the Montseny site. Model simulations using a time-variable CO₂ production rate suggested that pCO₂ maxima at intermediate depth could be the result of a transient situation instead of an equilibrium one.     

Characteristics of soil CO2 efflux variability in an aseasonal tropical rainforest in Borneo Island

Although soil carbon dioxide (CO₂) efflux from tropical forests may play an important role in global carbon (C) balance, our knowledge of the fluctuations and factors controlling soil CO₂ efflux in the Asian tropics is still poor. This study characterizes the temporal and spatial variability in soil CO₂ efflux in relation to temperature/moisture content and estimates annual efflux from the forest floor in an aseasonal intact tropical rainforest in Sarawak, Malaysia. Soil CO₂ efflux varied widely in space; the range of variation averaged 17.4 μmol m⁻² s⁻¹ in total. While most CO₂ flux rates were under 10 μmol m⁻² s⁻¹, exceptionally high fluxes were observed sporadically at several sampling points. Semivariogram analysis revealed little spatial dependence in soil CO₂ efflux. Temperature explained nearly half of the spatial heterogeneity, but the effect varied with time. Seasonal variation in CO₂ efflux had no fixed pattern, but was significantly correlated with soil moisture content. The correlation coefficient with soil moisture content (SMC) at 30 and 60 cm depth was higher than at 10 cm depths. The annual soil CO₂ efflux, estimated from the relationship between CO₂ efflux and SMC at 30 cm depth, was 165 mol m⁻² year⁻¹ (1,986 g C m⁻² year⁻¹). As this area is known to suffer severe drought every 4–5 years caused by the El Nino-Southern Oscillation, the results suggest that an unpredictable dry period might affect soil CO₂ efflux, leading an annual variation in soil C balance.     

Spatial heterogeneity reduces temporal variability in stream insect communities

Although all natural systems are heterogeneous, the direct influence of spatial heterogeneity on most ecological variables is unknown. In many systems, spatial heterogeneity is positively correlated with both microhabitat refugia and species richness. Both an increased number of microhabitat refugia and the effects of statistical averaging via increased species richness should lead to an inverse relationship between spatial heterogeneity and variability in community composition. To test this prediction, I measured diversity and temporal variability of invertebrate communities in a northern New Hampshire stream along a natural gradient of spatial heterogeneity formed by variation in stream substrates. On average, there was a 42% decrease in community variability along a gradient of increasing heterogeneity. This pattern was robust to changes in metrics of both heterogeneity and community variability. There was also a significant positive relationship between taxon richness and spatial heterogeneity with predicted taxon richness increasing c. 1.5× along the heterogeneity gradient. By resampling community abundance data, I estimated that statistical averaging accounted for only 4% of the observed decrease in community variability in this study. I concluded that the remaining decrease was very likely explained by a greater number of refugia from predation and/or flooding in high‐heterogeneity habitats. The results of this study suggest that maximizing heterogeneity in ecological restoration programmes may promote temporally stable and diverse communities and may aid in responsible management of aquatic resources.     

Spatial and temporal changes in bird assemblages in forest fragments in an eastern Amazonian savannah

We investigated the effects of forest fragmentation on bird assemblages in an Amazonian savannah landscape with forest fragments that have been isolated for more than 100 years. The study was conducted in areas surrounding the village of Alter do Chão (2°31′S, 55°00′W), Santarém, Brazil. Bird surveys and measurements of tree density were undertaken in 25 areas, with 19 plots in forest fragments of different sizes and six in an area of continuous forest. Data on forest‐fragment size, perimeter, and isolation were obtained from a georeferenced satellite image. Variation in number of bird species recorded per plot was not related to vegetation structure (tree density). The number of bird species recorded per plot increased significantly only with fragment area, but was not influenced by fragment shape or degree of isolation, even when considering species from the savannah matrix in the analysis. Fragments had fewer rare species. Multivariate ordination analyses (multiple dimensional scaling, [MDS]) indicated that bird species composition changed along a gradient from small to large forest fragments and continuous‐forest areas. In the Amazonian savannah landscapes of Alter do Chão, the organization and composition of bird assemblages in forest fragments are affected by local long‐term forest‐fragmentation processes. Differences in the number of bird species recorded per plot and assemblage composition between forest fragments and continuous forest were not influenced by forest structure, suggesting that the observed patterns in species composition result from the effects of fragmentation per se rather than from preexisting differences in vegetation structure between sites. Nevertheless, despite their long history of isolation, the forest fragments still preserve a large proportion (on average 80%) of the avifauna found in continuous‐forest areas. The fragments at Alter do Chão are surrounded by natural (rather than planted) grassland, with many trees in the savannah matrix and the landscape has vast areas covered by forest, which may have helped to ameliorate the influences of forest fragmentation.     

Spatial and temporal variability of dissolved sulfur compounds in European estuaries

The spatial and temporal distribution of Dimethylsulfide (DMS),Dimethylsulfoniopropionate (DMSP) – its precursor –,Dimethylsulfoxyde (DMSO) – one of its oxidation products –Carbonyl sulfide (COS) and Carbonyl disulfide (CS₂) wereinvestigated during nine oceanographic cruises carried out in six majortidal European estuaries between July 1996 and May 1998. Low levels ofDMS were recorded (mean of 0.6 nM, standard deviation () 0.3 nM)during these 9 cruises. Positive correlations between DMS and salinitywere frequently observed, with the highest DMS concentrations in theplume of estuaries, which could be explained by change in phytoplanktonspeciation from estuarine to shelf waters. Strong correlation betweenDMSP and DMS was reported for most of the estuaries denoting a simpleconservative mixing of riverine and marine waters controlled by tide. Incontrast to DMS, significant levels of COS and CS₂ with meanconcentrations of 220 ± 150 (pM; pM =10^–12 M) and 25 ± 6 pM respectively wererecorded in four estuaries, indicating that estuaries could be asignificant source of these compounds.     

Spatial and temporal variability in propagule limitation of California native grasses

Community composition and diversity arise from limitations in propagule supply (i.e. propagule or seed limitation) and propagule establishment after arriving at a site (i.e. establishment or microsite limitation). Recent meta‐analyses suggest that the degree of propagule limitation depends on local abiotic and biotic conditions, which in turn are likely to vary spatially and temporally. Nevertheless, seed addition studies testing propagule limitation are rarely replicated in multiple locations and years and often lack experimental manipulations of critical determinants of propagule limitation, such as the density and species richness of the propagule pool and disturbance and resource supply of recipient site. The invasion of California (USA) grasslands by exotic annual species from the Mediterranean region is unique in its scope (over 9 million ha) and persistence (more than 150 years). This invasion provides an exciting context in which to test the role of spatial and temporal variability in mediating propagule limitation and ultimately the potential for restoration. Here I present the results of native‐grass, seed‐addition trials conducted along a 500 km gradient in California in three consecutive years spanning a wide range of environmental conditions and initial conditions (e.g. rainfall, seeding density and disturbance). While native grasses were able to establish at many locations, per capita seed survival was low suggesting that the fate of these species is governed by interplay between propagule and establishment limitation. Establishment primarily varied spatially, with the most successful establishment occurring at fertile locations with low resident species richness. While recruitment was highly variable among years initially, there was no difference among seedling trials after three years.     

Spatial and temporal variability of antibiotic resistance in freshwater bacterial assemblages

Abstract Spatial and temporal variability in antibiotic resistance was examined in bacterial assemblages from streams and ponds on the US Department of Energy's Savannah River Site (SRS) in South Carolina. Sites sampled have been impacted to varying degrees by contamination with organic compounds, heavy metals, and radioactive materials because of production of nuclear materials on the site. Antibiotic resistance in the culturable portion of the bacterial assemblage was determined from coloby formation on media containing antibiotics. Eight antibiotics, chloramphenicol, cycloserine, kanamycin, neomycin, novobiocin, rifampicin, streptomycin, and tetracycline, were used at concentrations of 50 and 200 μg ml⁻¹. Statistically significant differences in frequency of antibiotic resistance were observed among sites and among dates at a single site. Bacterial densities (total and culturable), dissolved organic carbon (DOC) concentration, and human impact also varied among sites but bore no overall relationship to resistance frequency. SRS operations did not have a detectable impact on antibiotic resistance.     

Spatial and temporal variability in the structure of a tropical forest

This study examines spatial and temporal variation in the forest structure of the Kibale National Park, Uganda by contrasting tree density, tree size, and forest composition among four areas each separated by less than 15 km, and by quantifying changes in the composition of one of these forests over a 20‐year period. Densities of some tree species differed markedly between sites, and some species common at one location were absent at others. Monthly phenological monitoring demonstrated that it was not uncommon for phenological patterns to differ between the forests. To examine temporal variation in the tree composition over a 20‐year period, a sampling regime that was carried out in the early 1970s was replicated on the floristic composition of one of these sites, using identical methods in the same sampling areas. While no form of human intervention occurred in this area between the early 1970s and 1992, there were marked changes in the densities of some tree species. Twenty‐seven percent of the identified species increased in abundance, 33% decreased, and 40% remained relatively unchanged. The observed spatial and temporal variation in forest composition could be the result of abiotic factors, such as altitude or rainfall, or biotic factors such as elephant and/or human influences on ecosystem dynamics; the implications of this variation for frugivores are discussed.     

Spatial and seasonal variations in stem CO2 efflux of Pinus canariensis at their upper distribution limit

We calculated stem CO₂ efflux (E _S) of Pinus canariensis at a timberline site in Tenerife, Canary Islands, from March 7, 2008 and February 9, 2009. E _S varied markedly throughout the year. Although E _S generally followed the seasonal trends in temperature, we observed seasonal adjustment of E _S in both E _S normalized to temperature (E _S10) and the temperature sensitivity (Q ₁₀) resulting in lower E _S10 values during the warm and dry season as compared during the cold and wet season; the latter corresponding with the period of stem growth. The Q ₁₀ by contrast, was higher during the warm and dry summer than during the cold and wet season, an observation suggesting that climate extremes such as summer drought in the Mediterranean may override the observation that Q ₁₀ declines with increasing temperature. As concurrent measurements estimating the potential impact of sap flow on E _S revealed no interaction in P. canariensis, the Q ₁₀ values were used along with E _S10 and stem temperature records to predict annual total E _S and for partitioning total E _S into its maintenance (E _m) and a growth (E _g) component.     

Soil CO2 efflux in uneven-aged managed forests: temporal patterns following harvest and effects of edaphic heterogeneity

Increased nutrient availability can have a large impact in Australian woodland ecosystems, many of which are very poor in nutrients, particularly phosphorus. A study was conducted in an urban Banksia woodland remnant in Perth, southwest Western Australia to test the hypothesis that the soil nutrient status in areas in good condition (GC), poor condition invaded by the perennial grass Ehrharta calycina (PCe), and poor condition invaded by the perennial herb Pelargonium capitatum (PCp), is reflected in the nutrient status of the native and introduced species. Leaf concentrations of P, K, N, Na, Ca, Mg, S, B, Cu, Fe, Mn and Zn of six native (Banksia attenuata, Banksia menziesii, Allocasuarina humilis, Melaleuca systena, Macrozamia fraseri and Conostylis aculeata) and four introduced species (Ehrharta calycina, Pelargonium capitatum, Gladiolus caryophyllaceus and Briza maxima), were measured. Soil pH, electrical conductivity, N (total), P (total), available P, K, S and organic C were assessed beneath all species on all sites. Significantly higher concentrations of soil P (total) and P (HCO₃) were found at PCe and PCp sites than GC sites, while PCp sites also had significantly higher soil concentrations for N (total) and S. Principal Components Analysis of the leaf analyses showed (a) individual species have characteristically different nutrient concentrations; (b) the introduced species Ehrharta calycina and Pelargonium capitatum clustered separately from each other and by vegetation condition. Leaf concentrations of P were significantly (P<0.05) higher, and K and Cu were significantly lower in PCe and PCp sites compared with those at GC sites. Introduced species leaf nutrient concentrations were significantly greater than native species for all nutrients except Mn which was significantly lower, with no differences for Mg and B. The results indicate a key role for P in the Banksia woodland, and we conclude that higher levels of available P at invaded sites are having a detrimental impact on the ecosystem. These results provide new knowledge to enhance conservation practices for the management of the key threatening process of invasion within a biodiversity hot spot. Section Editor: T. Kalapos     

Spatial and temporal variability in export fluxes of biogenic matter in Kongsfjorden

The marine ecosystem of Kongsfjorden experiences large variations in primary productivity due to pronounced seasonal variations in sunlight, glacier melt, and ice cover. The objective of this study was to assess spatial and seasonal variability in the downward export of biogenic matter in Kongsfjorden. Short-term sediment traps were deployed for periods ranging from 21 to 52 h at three stations from the inner fjord to the outer fjord in May, August, and October 2012 and at one mid-fjord station in January 2013. Total particulate matter, particulate organic carbon, phytoplankton cells, chlorophyll a, biogenic particulate silica, and zooplankton fecal pellet fluxes were measured to determine the magnitude and composition of the material exported in the fjord. The amount and composition of export fluxes reflected a large phytoplankton bloom grazed upon by zooplankton in May, the melting of glaciers and the intrusion of Atlantic Water in August, the end of the glacier melt period in October, and the polar night in January. Overall, seasonal changes in the phytoplankton community impacted export efficiency in the fjord, directly through phytoplankton sinking and indirectly through zooplankton grazing. Results obtained in this study may reflect the magnitude and composition of export fluxes to expect in coming years in Kongsfjorden, especially under conditions of warmer Atlantic Water and longer glacier melt periods.     

Spatial and temporal variability in a stratified hypersaline microbial mat community

Hypersaline microbial mat communities have recently been shown to be more diverse than once thought. The variability in community composition of hypersaline mats, both in terms of spatial and temporal dimensions, is still poorly understood. Because this information is essential to understanding the complex biotic and abiotic interactions within these communities, terminal restriction fragment analysis and 16S rRNA gene sequencing were used to characterize the near-surface community of a hypersaline microbial mat in Guerrero Negro, Mexico. Core samples were analyzed to assay community variability over large regional scales (centimeter to kilometer) and to track depth-related changes in population distribution at 250-μm intervals over a diel period. Significant changes in total species diversity were observed at increasing distances across the mat surface; however, key species (e.g. Microcoleus sp.) were identified throughout the mat. The vertical position and abundance of >50% of the 60 peaks detected varied dramatically over a diel cycle, including Beggiatoa sp., cyanobacteria, Chloroflexus sp., Halochromatium sp., Bacteroidetes sp. and several as-yet-identified bacteria. Many of these migrations correlated strongly with diel changes in redox conditions within the mat, contributing to strong day–night community structure differences.     

Spatial and temporal variability in cyanobacterial populations controlled by physical processes

The Fitzroy impoundment is a long slender water body (10 m deep)formed by the regulation of the Fitzroy River in tropical Australia.Large, monsoonally driven discharges in late summer flush theimpoundment repeatedly leaving, after 2 months, a longitudinallyuniform, well-mixed water column, rich in dissolved nutrientsand with high turbidity. For the rest of the year flows arenegligible. Paradoxically, two sites with initially identicalnutrient and stratification characteristics, and located only30 km apart, develop quite different patterns of cyanobacterialsuccession. The upstream site is initially dominated by Anabaenacircinalis which appears in early spring and collapses withinthe month. A mixed population of Anabaenopsis elenkinii andAphanizomenon issatschenkoi then develops at both sites. Thisis followed by a mixture of small cyanobacteria (consistingof Cylindrospermopsis, Planktolyngbya and Limnothrix) whichdevelops mainly at the downstream site and persists for 3 monthsuntil flushed away by flood flows. We report on data coveringan 8 month period of investigation of the stratification, lightclimate, temperature and nutrient dynamics at these two sites.We show that large-scale climatic conditions and the local weatherpattern set the physical and chemical conditions which determinethe cyanobacterial response.     

Spatial and temporal variability across life's hierarchies in the terrestrial Antarctic

Antarctica and its surrounding islands lie at one extreme of global variation in diversity. Typically, these regions are characterized as being species poor and having simple food webs. Here, we show that terrestrial systems in the region are nonetheless characterized by substantial spatial and temporal variations at virtually all of the levels of the genealogical and ecological hierarchies which have been thoroughly investigated. Spatial variation at the individual and population levels has been documented in a variety of genetic studies, and in mosses it appears that UV-B radiation might be responsible for within-clump mutagenesis. At the species level, modern molecular methods have revealed considerable endemism of the Antarctic biota, questioning ideas that small organisms are likely to be ubiquitous and the taxa to which they belong species poor. At the biogeographic level, much of the relatively small ice-free area of Antarctica remains unsurveyed making analyses difficult. Nonetheless, it is clear that a major biogeographic discontinuity separates the Antarctic Peninsula and continental Antarctica, here named the 'Gressitt Line'. Across the Southern Ocean islands, patterns are clearer, and energy availability is an important correlate of indigenous and exotic species richness, while human visitor numbers explain much of the variation in the latter too. Temporal variation at the individual level has much to do with phenotypic plasticity, and considerable life-history and physiological plasticity seems to be a characteristic of Antarctic terrestrial species. Environmental unpredictability is an important driver of this trait and has significantly influenced life histories across the region and probably throughout much of the temperate Southern Hemisphere. Rapid climate change-related alterations in the range and abundance of several Antarctic and sub-Antarctic populations have taken place over the past several decades. In many sub-Antarctic locations, these have been exacerbated by direct and indirect effects of invasive alien species. Interactions between climate change and invasion seem set to become one of the most significant conservation problems in the Antarctic. We conclude that despite the substantial body of work on the terrestrial biodiversity of the Antarctic, investigations of interactions between hierarchical levels remain scarce. Moreover, little of the available information is being integrated into terrestrial conservation planning, which lags far behind in this region by comparison with most others.     

Spatial and temporal simulation of soil CO2 concentrations in a small forested catchment in Virginia

The question of how to extrapolate point measurements of soil CO₂ processes to coarser scales remains unanswered because we know little about the spatial and temporal variability in the CO₂ concentration of soil air. In this work, we describe a series of simple physically-based models that simulate soil temperature, soil tension, and soil CO₂ processes. We apply these models to simulate the spatial and temporal dynamics of soil CO₂ concentrations throughout a small catchment in the Virginia Blue Ridge. Output from the simulations is compared with field measurements. We find that despite some model deficiencies, we are able to simulate the gross patterns through space and time of soil air CO₂ concentration. During the growing season when soil temperature is high, we find that soil water status is the limiting control on soil respiration and CO₂ concentration. We also find that soil CO₂ concentration can be high despite low respiration values due to decreased soil diffusivity as moisture fills pore spaces.     

Spatial and temporal variability of canopy structure in a tropical moist forest

Tree fall gaps are widely considered to play a prominent role in the maintenance of species diversity, while the spatiotemporal variability of canopy structure within closed forest stands is largely ignored. In this study we examined the vertical and horizontal components of canopy structure and its seasonal variability in a tropical wet semideciduous rainforest in Panama. Leaf area indices (LAI) were derived from measurements of diffuse radiation and empirically-based leaf angle distribution by mathematical inversion of a light interception model. Vertical distribution of LAI was non-homogeneous with 50% of the leaf area being concentrated in the uppermost 5 m of the canopy. In the wet season, when foliage is most abundant, the horizontal distribution of LAI in a 2100 m² plot ranged widely from 3 to 8, with a mean of 5.41. Changes in mean LAI between wet and dry seasons were small but highly significant. While ca 40% of the area was not affected by local changes in LAI, sizeable small scale changes in LAI did occur between wet and dry season in some locations. Local changes in LAI ranged from –2.3 to 2.4. These changes resulted in a 50% or more increase in light reaching the forest floor at 29% of the measuring locations, and a doubling or more at 13% of the location. Our results imply that structural heterogeneity by simple tree fall gaps do not adequately describe the dynamics of forest canopies.