Causes Of Temporal Variability Research Articles

Variations of soil quality in the southern Qinghai-Tibet Plateau during 1980s to 2020s.

Climate warming and thawing of permafrost in the Qinghai-Tibet Plateau have resulted in soil erosion and the decline of soil quality. Determining the decadal variation of soil quality in the Qinghai-Tibet Plateau is the basis for scientific understanding of soil resources and the key to vegetation restoration and ecological reconstruction. In this study, we used eight indicators (including soil organic matter, total nitrogen, and total phosphorus) to eva-luate soil quality of montane coniferous forest zone (Tibet's natural geographical division zone Ⅱ) and montane shrubby steppe zone (zone Ⅳ) by calculating soil quality index (SQI) in the southern Qinghai-Tibet Plateau in the 1980s and 2020s. Variation partitioning (VPA) was used to examine the drivers for the heterogeneity of the spatial-temporal distribution of soil quality. The results showed that soil quality in each natural zone showed a downward trend in the past 40 years, with SQI of zone Ⅱ decreasing from 0.505 to 0.484 and that of zone Ⅳ decreasing from 0.458 to 0.425. The spatial distribution of soil nutrients and quality was heterogeneous, while soil nutrient conditions and quality in zone Ⅱ were better than those in zone Ⅳ in different periods. The VPA results indicated that the interaction of climate change, land degradation, and vegetation differences was the major cause of temporal variation in soil quality. Differences in climate and vegetation could better explain the spatial variation of SQI.

Disentangling the causes of temporal variation in the opportunity for sexual selection

In principle, temporal fluctuations in the potential for sexual selection can be estimated as changes in intrasexual variance in reproductive success (i.e. the opportunity for selection). However, we know little about how opportunity measures vary over time, and the extent to which such dynamics are affected by stochasticity. We use published mating data from multiple species to investigate temporal variation in the opportunity for sexual selection. First, we show that the opportunity for precopulatory sexual selection typically declines over successive days in both sexes and shorter sampling periods lead to substantial overestimates. Second, by utilising randomised null models, we also find that these dynamics are largely explained by an accumulation of random matings, but that intrasexual competition may slow temporal declines. Third, using data from a red junglefowl (Gallus gallus) population, we show that declines in precopulatory measures over a breeding period were mirrored by declines in the opportunity for both postcopulatory and total sexual selection. Collectively, we show that variance-based metrics of selection change rapidly, are highly sensitive to sampling durations, and likely lead to substantial misinterpretation if used as indicators of sexual selection. However, simulations can begin to disentangle stochastic variation from biological mechanisms.

Temporal interpolation of groundwater level hydrographs for regional drought analysis using mixed models

Large-scale studies of the spatial and temporal variation of groundwater drought status require complete inventories of groundwater levels on regular time steps from many sites so that a standardised drought index can be calculated for each site. However, groundwater levels are often measured sporadically, and inventories include missing or erroneous data. A flexible and efficient modelling framework is developed to fill gaps and regularise data in such inventories. It uses linear mixed models to account for seasonal variation, long-term trends and responses to precipitation and temperature over different temporal scales. The only data required to estimate the models are the groundwater level measurements and freely available gridded weather products. The contribution of each of the four types of trends at a site can be determined and thus the causes of temporal variation of groundwater levels can be interpreted. Validation reveals that the models explain a substantial proportion of groundwater level variation and that the uncertainty of the predictions is accurately quantified. The computation for each site takes less than 130 s and requires little supervision. Hence, the approach is suitable to be upscaled to represent the variation of groundwater levels in large datasets consisting of thousands of boreholes.

Observations of the Low‐Mode Internal Tide and Its Interaction With Mesoscale Flow South of the Azores

AbstractUnderstanding the temporal variability of internal tides plays a crucial role in identifying sources and sinks of energy in the ocean. Using a 10‐month‐long time series from moored instruments inside a tidal beam south of the Azores, the magnitude and the underlying causes of temporal variability in the first two modes of the internal tide energy flux was studied. We analyzed changes of the direction and coherence of the energy flux, its modal structure, and the impact of two eddies. Semidiurnal energy fluxes were further compared with estimates from a 1/10° ocean global circulation model, as well as with fluxes derived from satellite altimetry. All energy fluxes correlate reasonably well in direction, deviations from its fixed phase relation to astronomical forcing, and modal composition while model and satellite underestimate the total energy flux. A pronounced damping of the in situ fluxes coincides with the passing of two eddies. In the presence of a surface‐intensified eddy, the coherent part of the energy flux in the first two modes is lowered by more than 40%, a subsurface eddy coincides with a decrease of the energy flux mainly in the second mode. These observations support the hypothesis that eddy interactions increase the incoherent part of the energy flux and transfer energy from low modes into higher modes, which can lead to increased local dissipation. It remains an open question how much of the energy converted from lower to higher modes results in local dissipation, a crucial part in creating energetically consistent ocean‐climate models.

Variation of Individual Location Radiance in VIIRS DNB Monthly Composite Images

With the growing size and use of night light time series from the Visible Infrared Imaging Radiometer Suite Day/Night Band (DNB), it is important to understand the stability of the dataset. All satellites observe differences in pixel values during repeat observations. In the case of night light data, these changes can be due to both environmental effects and changes in light emission. Here we examine the stability of individual locations of particular large scale light sources (e.g., airports and prisons) in the monthly composites of DNB data from April 2012 to September 2017. The radiances for individual pixels of most large light emitters are approximately normally distributed, with a standard deviation of typically 15–20% of the mean. Greenhouses and flares, however, are not stable sources. We observe geospatial autocorrelation in the monthly variations for nearby sites, while the correlation for sites separated by large distances is small. This suggests that local factors contribute most to the variation in the pixel radiances and furthermore that averaging radiances over large areas will reduce the total variation. A better understanding of the causes of temporal variation would improve the sensitivity of DNB to lighting changes.

Temporal patterns of extra-pair paternity in a population of Grasshopper Sparrows (Ammodramus savannarum) in Maryland

The incidence of extra-pair paternity (EPP) is highly variable across bird taxa. While EPP is known to affect reproductive variance, the causes of temporal variation in rates of EPP are poorly studied. Breeding density has often been proposed as an important factor influencing EPP variation, but it has received mixed support. Over a 5-year period we examined the rate of EPP in a socially monogamous Grasshopper Sparrow (Ammodramus savannarum) population on a restored grassland in Maryland. Of 124 broods, 62.1% had extra-pair young (EPY), and of 448 nestlings, 36.8% were the result of extra-pair fertilizations. Over the 5-year study, percent EPY ranged from 26% to 60%. EPP rates were positively correlated with the local density of territorial males, supporting the density hypothesis, and 81% of the EPY were from neighboring males. Male within-pair mating success was related to extra-pair mating, suggesting an active behavioral tradeoff between within- and extra-pair matings, especially at high densities. By examining males at the individual level, we found males that were not cuckolded also had the most genetic offspring (within-pair and extra-pair), whereas males that were cuckolded also failed to sire many extra-pair offspring. Here we show that density plays an important role in the availability of mates, EPP, and the opportunity for sexual selection. Received 9 April 2016. Accepted 3 August 2017.

Temporal variability in offspring quality and individual reproductive output in a broadcast-spawning marine fish

AbstractThe protracted spawning period of broadcast-spawning marine fishes has potential to generate considerable variability in metrics of individual reproductive output. We undertook a temporally detailed genetic study of larvae produced by Atlantic cod (Gadus morhua) from two spatially proximate populations spawning under controlled semi-natural conditions over 94 days. Based on daily samples of larvae (n = 4489 in total), we document fine-scaled temporal changes in, and correlates of, offspring phenotype and reproductive output (egg batches produced or fertilized). Larval length and standardized yolk-sac volume declined 11 and 49% over the spawning period, respectively. The adaptive significance of these trends is unclear. Longer, heavier females produced longer, better-provisioned larvae. Body size affected the number of egg batches to which an individual contributed genetically but not spawning duration. Males contributed gametes to a greater number of egg batches (19.5 vs. 9.2), and spawned over a longer period of time (48.9 vs. 30.8 days), than females. After accounting for body size and condition, egg batch number and spawning duration differed between adjacent populations separated by < 10 km. Our work highlights the need to understand the environmental and adaptive causes of temporal variability in offspring quality and its consequences to individual fitness and per capita population growth in batch-spawning fishes.

Assessing broad life cycle impacts of daily onboard decision-making, annual strategic planning, and fisheries management in a northeast Atlantic trawl fishery

Capture fisheries are the only industrial-scale harvesting of a wild resource for food. Temporal variability in environmental performance of fisheries has only recently begun to be explored, but only between years, not within a year. Our aim was to better understand the causes of temporal variability within and between years and to identify improvement options through management at a company level and in fisheries management. We analyzed the variability in broad environmental impacts of a demersal freeze trawler targeting cod, haddock, saithe, and shrimp, mainly in the Norwegian Sea and in the Barents Sea. The analysis was based on daily data for fishing activities between 2011 and 2014 and the functional unit was a kilo of landing from one fishing trip. We used biological indicators in a novel hierarchic approach, depending on data availability, to quantify biotic impacts. Landings were categorized as target (having defined target reference points) or bycatch species (classified as threatened or as data-limited). Indicators for target and bycatch impacts were quantified for each fishing trip, as was the seafloor area swept. No significant difference in fuel use was found between years, but variability was considerable within a year, i.e., between fishing trips. Trips targeting shrimp were more fuel intensive than those targeting fish, due to a lower catch rate. Steaming to and from port was less important for fuel efficiency than steaming between fishing locations. A tradeoff was identified between biotic and abiotic impacts. Landings classified as main target species generally followed the maximum sustainable yield (MSY) framework, and proportions of threatened species were low, while proportions of data-limited bycatch were larger. This improved considerably when reference points were defined for saithe in 2014. The variability between fishing trips shows that there is room for improvement through management. Fuel use per landing was strongly influenced by target species, fishing pattern, and fisheries management. Increased awareness about the importance of onboard decision-making can lead to improved performance. This approach could serve to document performance over time helping fishing companies to better understand the effect of their daily and more long-term decision-making on the environmental performance of their products. Fishing companies should document their resource use and production on a detailed level. Fuel use should be monitored as part of the management system. Managing authorities should ensure that sufficient data is available to evaluate the sustainability of exploitation levels of all harvested species.

Fecal Shedding of Thermophilic Campylobacter in a Dairy Herd Producing Raw Milk for Direct Human Consumption

Factors affecting the fecal shedding of thermophilic Campylobacter in Italian dairy farms were investigated in a 12-month longitudinal study performed on a dairy farm authorized to sell raw milk in Italy. Fifty animals were randomly selected from 140 adult and young animals, and fecal samples were collected six times at 2-month intervals. At each sampling time, three trough water samples and two trough feed samples also were collected for both adult and young animals. Samples were analyzed with real-time PCR assay and culture examination. Overall, 33 samples (9.7%) were positive for thermophilic Campylobacter by real-time PCR: 26 (9.2%) of 280 fecal samples, 6 (16.6%) of 36 water samples, and 1 (4.2%) of 24 feed samples. Campylobacter jejuni was isolated from 6 of 280 samples; no other Campylobacter species was isolated. A higher (but not significantly) number of positive fecal samples were found in younger animals (11.33 versus 6.92% of adult animals), and a significantly higher number of positive water samples were collected from the water troughs of young animals. A distinct temporal trend was observed during the study period for both cows and calves, with two prevalence peaks between November and December and between May and July. Several factors such as calving, housing practices, herd size, management practices forcing together a higher number of animals, and variations in feed or water sources (previously reported as a cause of temporal variation in different farming conditions) were excluded as the cause of the two seasonal peaks in this study. The factors affecting the seasonality of Campylobacter shedding in the dairy herds remain unclear and warrant further investigation. The results of the present study indicate that special attention should be paid to farm hygiene management on farms authorized to produce and sell raw milk, with increased surveillance by the authorities at certain times of the year.

Understanding causes of tree growth response to gap formation: ∆13C-values in tree rings reveal a predominant effect of light

Carbon isotope ratios in growth rings of a tropical tree species show that treefall gaps stimulate diameter growth mainly through changes in the availability of light and not water. The formation of treefall gaps in closed canopy forests usually entails considerable increases in light and nutrient availability for remaining trees, as well as altered plant water availability, and is considered to play a key role in tree demography. The effects of gaps on tree growth are highly variable and while usually stimulatory they may also include growth reductions. In most studies, the causes of changes in tree growth rates after gap formation remain unknown. We used changes in carbon isotope 13C discrimination (Δ13C) in annual growth rings to understand growth responses after gap formation of Peltogyne cf. heterophylla, in a moist forest of Northern Bolivia. We compared growth and Δ13C of the 7 years before and after gap formation. Forty-two trees of different sizes were studied, half of which grew close (<10 m) to single treefall gaps (gap trees), the other half more than 40 m away from gaps (controls). We found variable responses among gap trees in growth and Δ13C. Increased growth was mainly associated with decreased Δ13C, suggesting that the growth response was driven by increased light availability, possibly in combination with improved nutrient availability. Most trees showing zero or negative growth change after gap formation had increased Δ13C, suggesting that increased water stress did not play a role, but rather that light conditions had not changed much or nutrient availability was insufficient to support increased growth. Combining growth rates with Δ13C proved to be a valuable tool to identify the causes of temporal variation in tree growth.

Identifying sources of temporal variability in hydrological extremes of the upper Blue Nile basin

Summary It is known that changes in catchment runoff variability are a function of changes in climate as well as catchment behavior. For proper management of a certain watershed it is important to have a good understanding of the main causes of variability. Specifically, changes in extreme conditions of water resources are imperative as their consequences are far reaching. This paper attempts to identify the cause of hydrological extremes variability in the upper Blue Nile basin of Ethiopia. A method is proposed to utilize conceptual hydrological models to simulate long term (41 years) hydro-meteorological data and analyse the outputs using the Quantile Perturbation Method (QPM) specially designed for investigation of the temporal variability of extreme values in time series over multi-annual to (multi-)decadal time scales. Two conceptual hydrological models were calibrated and evaluated for their performance to simulate extreme high flows and changes in these flows for corresponding changes in rainfall conditions. The temporal variability results show similar patterns for simulated and observed extreme flows. This indicates the major influence of climate variability in extreme flows as demonstrated by the rainfall input in the models. There is no discernible change in the catchment response, e.g. quick runoff coefficient as a function of soil saturation state, between periods of the 1960–1970s, the 1980s and the 1990–2000s, which are attributed to land policy changes. This shows the influence of changes in catchment characteristics is minimal. (Multi-)decadal climate variability is identified as the main cause of temporal variation in hydrological extremes of the Blue Nile basin.

Meteoric ion layers in the ionospheres of venus and mars: Early observations and consideration of the role of meteor showers

Layers of metal ions produced by meteoroid ablation have been known in Earth’s ionosphere for decades, but have only recently been discovered at Venus and Mars. Here we report the results of a search for meteoric layers in earlier datasets from Venus and Mars. We find 13 candidates at Venus in Mariner 10, Venera 9/10, and Pioneer Venus Orbiter data that augment the 18 previously identified in Venus Express data. We find 8 candidates at Mars in Mariner 7 and Mariner 9 data that augment the 71 and 10 previously identified in Mars Global Surveyor and Mars Express data, respectively. These new findings extend the ranges of conditions under which meteoric layers have been observed, support studies of the temporal variability of meteoric layers, and (for Venus) independently confirm the existence of meteoric layers. One of the proposed causes of temporal variations in the occurrence rate of meteoric layers is meteor showers. This possibility is controversial, since meteor showers have minimal observed effect on meteoric layers in Earth’s ionosphere. In order to aid progress towards a resolution of this issue, we present a series of tests for this hypothesis.

Sexual selection and temporal phenotypic variation in a damselfly population

Temporal variation in selection can be generated by temporal variation in either the fitness surface or phenotypic distributions around a static fitness surface, or both concurrently. Here, we use within- and between-generation sampling of fitness surfaces and phenotypic distributions over 2 years to investigate the causes of temporal variation in the form of sexual selection on body size in the damselfly Enallagma aspersum. Within a year, when the average female body size differed substantially from the average male body size, male body size experienced directional selection. In contrast, when male and female size distributions overlapped, male body size experienced stabilizing selection when variances in body size were large, but no appreciable selection when the variances in body size were small. The causes of temporal variation in the form of selection can only be inferred by accounting for changes in both the fitness surface and changes in the distribution of phenotypes.

Slab and Mantle Controls on the Sr–Nd–Pb–Hf Isotope Evolution of the Post 42 Ma Izu–Bonin Volcanic Arc

The arc outflux is a hybrid of material fluxes from slab, mantle and crustal sources, whose proportions and composition may vary through time. Here we investigate the causes of temporal variability in the post 42Ma eruptive products of the Izu^Bonin arc in the NW Pacific Ocean basin using Sr^Nd^Pb^Hf isotopes in combination with major and trace element systematics in mafic to evolved arc magmas. Following arc initiation at 50Ma, Izu^Bonin arc formation was continuous except for a short period of quiescence ( 20^23Ma) during back-arc basin formation in the early Miocene.The Sr^Nd^Pb^Hf isotope trends do not co-vary through time. An increasing trend of Pb isotope ratios with time reflects a change in the trench sediment input as it becomes richer in radiogenic Pb. Sr isotopes show no unidirectional trend with time owing to the dominant contribution of unradiogenic Sr from the subducting igneous crust ( 66% of arc Sr) and mantle ( 31%) that buffer the minor amount of radiogenic Sr contributed from subducting sediment or altered oceanic crust. Nd and Hf isotope compositions are both controlled by the depleted sub-arc mantle. Nd isotope ratios increase significantly with time, whereas any temporal increase in Hf isotope ratios remains negligible relative to the inherent data variability. This temporal divergence is due to the much higher parent/daughter ratios of the sub-arc mantle source (Sm/Nd1⁄4 0·24; Lu/Hf1⁄4 0·04), which result in significant ingrowth in Nd but a negligible ingrowth of Hf. Overall, the isotopic evolution is unaffected by along-strike arc rifting and back-arc basin formation, or by the evolution of the siliceous Izu^Bonin arc crust. Compared with the greater isotopic variability of the broader Izu^Bonin^Mariana arc system, the post 42Ma Izu^Bonin arc exemplifies the long-term evolution of a simple arc system with comparatively uniform input from slab and mantle sources. Its isotopic evolution is hence not representative of that of other arcs, and this may reflect the different subduction geometry and components involved.

Plausible causes of temporal variation in suspended sediment concentration in the upper Changjiang River and major tributaries during the second half of the 20th century

In the second half of the 20th century, suspended sediment concentration (SSC) varied in different ways in the upper Changjiang River and major tributaries due to the changing human activity and precipitation. The variation in annual precipitation ( P m) can explained only 16%–37% of the variation in suspended sediment concentration, and thus variation in precipitation is not the main cause for the variation in suspended concentration. Using the analysis of double-mass curve, the effect of human activities on the variation in suspended concentration has been studied. The suspended concentration of the lower Jinshajiang River increased from 1983 due to large-scale mining and road and urban construction. The double-mass curve for the Minjiang shows three periods where the relationship between SSC and P m changes because of reservoir construction and reforestation. The SSC of the Jialingjiang River decreased from 1984 because marked sediment-reduction effect has been achieved by soil and water conservation measures. The SSC of the Wujiang River increased from 1968 and then decreased from 1983, the former can be related to the vegetation destruction that enhanced soil erosion and the latter to the construction of reservoirs that trapped sediment. Compared to the 4 tributaries, the suspended sediment of the main stem of the upper Changjiang River shows smaller variation in SSC. The results of a multiple regression analysis show that the contribution percentages of SSC of the Jinshajiang, Minjiang, Jialingjiang and Wujiang Rivers to that of the upper Changjiang are 28%, 13%, 44% and 15%, respectively.

Decadal-scale variation in dune erosion and accretion rates: An investigation of the significance of changing storm tide frequency and magnitude on the Sefton coast, UK

Monitoring of frontal dune erosion and accretion on the Sefton coast in northwest England over the past 50 years has revealed significant spatial and temporal variations. Previous work has shown that the spatial variations primarily reflect longshore differences in beach and nearshore morphology, energy regime and sediment budget, but the causes of temporal variations have not previously been studied in detail. This paper presents the results of work carried out to test the hypothesis that a major cause of temporal variation is changes in the frequency and magnitude of storms, surges and resulting high tides. Dune toe erosion/accretion records dating from 1958 have been compared with tide gauge records at Liverpool and Heysham. Relatively high dune erosion rates at Formby Point 1958–1968 were associated with a relatively large number of storm tides. Slower erosion at Formby, and relatively rapid accretion in areas to the north and south, occurred during the 1970's and 1980's when there were relatively few major storm tides. After 1990 rates of dune erosion at Formby increased again, and dunes to the north and south experienced slower accretion. During this period high storm tides have been more frequent, and the annual number of hours with water levels above the critical level for dune erosion has increased significantly. An increase in the rate of mean sea-level rise at both Liverpool and Heysham is evident since 1990, but we conclude that this factor is of less importance than the occurrence of extreme high tides and wave action associated with storms. The incidence of extreme high tides shows an identifiable relationship with the lunar nodal tidal cycle, but the evidence indicates that meteorological forcing has also had a significant effect. Storms and surges in the eastern Irish Sea are associated with Atlantic depressions whose direction and rate of movement have a strong influence on wind speeds, wave energy and the height of surge tides. However, preliminary analysis has indicated only a modest relationship between dune erosion/accretion rates and the North Atlantic Oscillation index.

Temporal variations in carbon dioxide and methane concentrations under urban conditions

Temporal variations in the surface concentrations of two greenhouse gases (carbon dioxide and methane) in the atmosphere over a large city are studied on the basis of the data obtained during the 2003–2005 observations at a Moscow station for environmental monitoring. This station is based on the TROICA mobile observatory and located at the meteorological station of the Faculty of Geography, Moscow State University, on Vorob’evy gory. The methods of isolating the background concentrations of greenhouse gases under urban conditions are proposed, and the excess concentrations of CO2 and CH4 over their background values are estimated for different seasons and times of day. The CO2 and CH4 concentrations are shown to have more pronounced diurnal cycles in summer than in winter. The main causes of temporal variations in the surface concentrations of CO2 and CH4 under urban conditions and the differences between the mean concentrations of these greenhouse gases in Moscow and other areas of Russia are analyzed. It is shown that variations in the surface concentrations of carbon dioxide and methane on different time scales are caused by different atmospheric processes (global circulation, mesoscale gravity waves, surface temperature inversions, etc.)

Effects of fire on sward structure and grazing in western Serengeti, Tanzania

Fire is one of the major ecological factors that determines structure and function of grassland and savanna ecosystems through its effects on soil nutrient pools, floristic composition and primary production, and foraging behaviour and distribution of populations of wildlife species. Therefore, some wildlife protected area authorities in African savannas have a fire management scheme to cater for a variety of purposes such as, to prevent bush encroachment into grassland, to increase production of quality forage, to control wild fires from outside the protected areas and their spread, and to keep animals in tourist areas for visitors’ enjoyment. The responses to fire of ecosystem processes can be quantified but the outcomes are generally complex depending on the nature of the fire regime, primarily the frequency and timing of the fire events. For example, while ash after fire may have a fertilising effect that will facilitate seedling establishment and increase the growth and production, there is evidence that in East Africa, fire may lead to the dominance of a few species over several other species. Fire may also lead to loss of nutrients, for example nitrogen, through volatilisation. Such variaty in responses to fire makes the use of fire as a tool in the management of wildlife and protected areas usually controversial. Therefore, the main challenge of management is to strike a fire management regime as close as possible to a natural fire regime in this ecosystem.In the Serengeti ecosystem, fire has been regarded to have a critical role on herbivores and their forage resources. Thus, burning has been a management tool in protected areas and a common practice of range management by pastoralists. Herdsmen use fire to improve the pasture quality for cattle. Removal of old or dead grass material by fire enhances sprouting of plants, which improves, at least for a short-term, the quality of the sward. In the Serengeti National Park, early dry-season burns had been executed by the Serengeti Ecological Monitoring Program Department from end of May to mid August without clear understanding about grassland responses to the prescribed burns.The perceived importance of fire to the management and conservation of Serengeti ecosystem has driven this thesis to focus on the effects of fire on large herbivores and their forage resources in the Serengeti National Park. To accomplish that, the thesis examines the relationships between burning and 1) above ground net primary productivity in relation to sward structure and precipitation (Paper I), 2) forage quality, i.e. the concentration of macronutrients (N, P, K, Na, Mg and Ca), in vitro organic matter digestibility and levels of acid detergent fibre per phytomass component (Paper II), and 3) the temporal and spatial patterns of herbivores’ consumption (Paper III), and 4) the patch selection by ungulate species, individually and/or grouped in diet groups (Paper IV).The result of the early dry season burns is a significantly higher daily above ground net primary production (ANPP) on burnt plots at early post-fire stages, coinciding with the dry season (July-September) and during the short rainy period (October-December). Though not significantly different, it was also high on burnt plots at the end of the main rain period (March – May), but higher on non-burnt plots in December-February, and in early dry season, June- July of the subsequent year.Results from this area with high diversity of large herbivore species show that grazing herbivores switch between burnt and non-burnt patches by trading off between forage quantity and quality along growing season. The quality on burnt areas appears to be governed by enhanced concentrations of macronutrients, increased digestibility and reduced concentration of acid detergent fibres. The above quality variables are linked to increased ratio between live and total phytomass. Quality of forage explains the preference by grazing animals for burnt areas during some periods, and in non-burnt areas due to high phytomass in other periods.Selection for non-burnt or burnt patches or against patches burnt more than once in a period of three years by some species of ungulates and/or diet groups are also indicative that quality and/or quantity of forage in the respective patch underlay patch choice. The cause of temporal variation in selectivity of the patches needs further investigation. However, it is unequivocal that maintenance of mosaic of burnt and non-burnt areas with adequate provision of forage amount and quality all year round is of paramount importance.

Reproduction as one of the main causes of temporal variability in the elemental composition of zooplankton

With the aim to determine the contribution of development and reproduction to the variability in the elemental composition of zooplankton, we measured the carbon (C), hydrogen (H), nitrogen (N), and phosphorus (P) elemental composition of the planktonic crustacean assemblage in an alpine lake for a whole seasonal cycle. The species included three distinct living modes: a cladoceran, Daphnia pulicaria; a cyclopoid copepod, Cyclops abyssorum; and a calanoid copepod, Diaptomus cyaneus. For the three species, reproduction was the main cause of elemental variability. Adult females of the three species lost from 32% to 48% of their initial absolute C and H content during reproduction, which corresponded to a similar decrease in their lipid and carbohydrate content. The N content did not change in any of the three species, nor did the protein and chitin content. Daphnia and Diaptomus lost 35% and 56%, respectively, of their initial absolute P content during reproduction, whereas the P content of Cyclops did not change. The three species stored energy compounds under unfavorable conditions for later use in offspring production, but only Diaptomus and Daphnia mobilized stored P. Corresponding stoichiometric changes with reproduction included a decrease in C : N ratio for the three species; an increase in N : P ratio for Daphnia adult females and adults of Diaptomus; and a C : P ratio increase in Diaptomus females and decline in Cyclops females. Differences in C : P ratio changes corresponded with differences in allocation to their respective reproductive tissues. Diaptomus males and Daphnia females did not change their C : P ratio with reproduction.

Factors affecting interannual variation in growth of western Canadian aspen forests during 1951-2000

Trembling aspen (Populus tremuloides Michx.) is the most important deciduous tree in the North American boreal forest and is also the dominant tree in the aspen parkland zone along the northern edge of the Canadian prairies. Since the 1990s, observations of dieback and reduced growth of aspen forests have led to concerns about the potential impacts of climate change. To address these concerns, a regional-scale study (CIPHA) was established in 2000 that includes annual monitoring of forest health and productivity of 72 aspen stands across the western Canadian interior. Tree-ring analysis was conducted to determine the magnitude and cause of temporal variation in stand growth of aspen at the scale (1800 km × 500 km area) encompassed by this study. The results showed that during 1951–2000 the region's aspen forests underwent several cycles of reduced growth, notably between 1976 and 1981, when mean stand basal area increment decreased by about 50%. Most of the growth variation was explained by interannual variation in a climate moisture index in combination with insect defoliation. The results of the analysis indicate that a major collapse in aspen productivity likely occurred during the severe drought that affected much of the region during 2001–2003.