Spatio-temporal Trends of Mercury and Stable Isotopes in Lower Food Web of Winam Gulf, Lake Victoria.

Components of the lower food web (mussels, Caridina and Omena) were collected from stations from Winam Gulf, Lake Victoria, Kenya in 2022 and 2023 to analyze for stable isotopes and total mercury (THg). Temporal comparisons were made with data generated for the same species in 1998. Values of δ15N in mussels and Caridina were similar (6.89‰ vs. 6.78 ± 0.13‰), while Omena occupied an elevated trophic position (9.97 ± 0.24‰) with minor shifts in δ15N over time. All species had elevated δ13C values in 2022-2023 versus 1998 supportive of enhanced eutrophication in the Gulf. THg concentrations exhibited modest spatial differences between sites (< 2.6 fold), but not between Caridina and Omena. Larger temporal differences were apparent relative to spatial patterns with THg concentrations decreasing in study species by 2.8 to 4.1-fold between years. An exposure assessment indicated that Omena, commonly found in local markets, can be consumed up to 0.74 kg/month without generating excess THg exposures.

Interaction of livestock grazing and rainfall manipulation enhances herbaceous species diversity and aboveground biomass in a humid savanna.

Understanding of the interaction of livestock grazing and rainfall variability may aid in predicting the patterns of herbaceous species diversity and biomass production. We manipulated the amount of ambient rainfall received in grazed and ungrazed savanna in Lambwe Valley-Kenya. The combined influence of livestock grazing and rainfall on soil moisture, herbaceous species diversity, and aboveground biomass patterns was assessed. We used the number of species (S), Margalef's richness index (Dmg), Shannon index of diversity (H), and Pileou's index of evenness (J) to analyze the herbaceous community structure. S, Dmg, H and J were higher under grazing whereas volumetric soil water contents (VWC) and aboveground biomass (AGB) decreased with grazing. Decreasing (50%) or increasing (150%) the ambient rainfall by 50% lowered species richness and diversity. Seasonality in rainfall influenced the variation in VWC, S, Dmg, H, and AGB but not J (p = 0.43). Overall, Dmg declined with increasing VWC. However, the AGB and Dmg mediated the response of H and J to the changes in VWC. The highest H occurred at AGB range of 400-800 g m-2. We attribute the lower diversity in the ungrazed plots to the dominance (relative abundance > 70%) of Hyparrhenia fillipendulla (Hochst) Stapf. and Brachiaria decumbens Stapf. Grazing exclusion, which controls AGB, hindered the coexistence among species due to the competitive advantage in resource utilization by the more dominant species. Our findings highlight the implication of livestock grazing and rainfall variability in maintaining higher diversity and aboveground biomass production in the herbaceous layer community for sustainable ecosystem management.

Tracing water and energy fluxes and reflectance in an arid ecosystem using the integrated model SCOPE.

Transpiration plays a critical role in the water cycle of terrestrial ecosystems, especially for arid ecosystems in which water availability is typically the main constraint for plant growth. Although remote sensing has provided insights into transpiration across different temporal and spatial scales, its potential has not been fully exploited. This is due to a lack of synchronous observations of fluxes and reflectance. Only a few models have attempted to incorporate both radiative transfer and physiological processes. In this study, we calibrated the newly developed Soil, Canopy Observation, Photochemistry and Energy fluxes (SCOPE) model to trace synchronous fluxes of water, energy and reflectance, and thus, their interplays, in a typical arid ecosystem dominated by Haloxylon ammodendron based on long-term continuous field measurements. An initial global sensitivity analysis is conducted to identify parameters that have the greatest impact on model output before subsequent calibration with field data. The resulting calibrated model gives insight into the interplay between reflectance, energy and water fluxes in an arid ecosystem. The calibrated model is thus a useful tool to understand land surface fluxes and radiative transfer processes theoretically, from which additional reflectance information can be exploited to trace the physiological status of ecosystems.

Tree growth and water-use in hyper-arid Acacia occurs during the hottest and driest season.

Drought-induced tree mortality has been recently increasing and is expected to increase further under warming climate. Conversely, tree species that survive under arid conditions might provide vital information on successful drought resistance strategies. Although Acacia (Vachellia) species dominate many of the globe's deserts, little is known about their growth dynamics and water-use in situ. Stem diameter dynamics, leaf phenology, and sap flow were monitored during 3 consecutive years in five Acacia raddiana trees and five Acacia tortilis trees in the Arid Arava Valley, southern Israel (annual precipitation 20-70 mm, restricted to October-May). We hypothesized that stem growth and other tree activities are synchronized with, and limited to single rainfall or flashflood events. Unexpectedly, cambial growth of both Acacia species was arrested during the wet season, and occurred during most of the dry season, coinciding with maximum daily temperatures as high as 45 °C and vapor pressure deficit of up to 9 kPa. Summer growth was correlated with peak sap flow in June, with almost year-round activity and foliage cover. To the best of our knowledge, these are the harshest drought conditions ever documented permitting cambial growth. These findings point to the possibility that summer cambial growth in Acacia under hyper-arid conditions relies on concurrent leaf gas exchange, which is in turn permitted by access to deep soil water. Soil water can support low-density tree populations despite heat and drought, as long as recharge is kept above a minimum threshold.

Increasing mercury bioaccumulation and biomagnification rates of Nile perch (Lates niloticus L.) in Winam Gulf, Lake Victoria, Kenya.

The Nile perch (Lates niloticus L.) commercial fishery for Lake Victoria in East Africa is an important source of revenue and employment. We focused on shifts in food web structure and total mercury (THg) bioaccumulation and biomagnification in Nile perch, and lower food web items collected from Winam Gulf (Kenya) sampled 24 years apart (1998 and 2022). Stable isotope carbon (δ13C) values were higher in all species from 2022 compared to 1998. Stable nitrogen isotope (δ15N) values in baseline organisms were lower in 2022 compared to 1998. In Nile perch, δ15N values were correlated with total length, but the δ15N-length regressions were steeper in 1998 compared to 2022 except for one large (158 cm) Nile perch from 1998 with an uncharacteristically low δ15N value. Total Hg concentrations were lower in lower trophic species from 2022 compared to 1998. However, the THg bioaccumulation rate (as a function of fish length) in Nile perch was greater in 2022 compared to 1998 resulting in 24.2 % to 42.4 % higher wet weight dorsal THg concentrations in 2022 Nile perch for market slot size (50 to 85 cm) fish. The contrasting observations of increased THg bioaccumulation with size in 2022 against decreases in the rate of trophic increase with size and lower THg concentrations of lower food web items imply reduced fish growth and potential bioenergetic stressors on Winam Gulf Nile perch. All samples except 1 large Nile perch (139 cm total length collected in 2022) had THg concentrations below the European Union trade limit (500 ng/g wet weight). However, for more vulnerable individuals (women, children and frequent fish eaters), we recommend a decrease in maximum monthly meal consumption for 55-75 cm Nile perch from 16 meals per month calculated for 1998 to a limit of 8 meals per month calculated for 2022.

Influence of photosynthetic active radiation on sap flow dynamics across forest succession stages in Dinghushan subtropical forest ecosystem.

With the intensification of global change, forests are subjected to varying degrees of drought or high-temperature stress, which has an indelible impact on the growth of trees. However, knowledge on the response of sap flow to environmental changes in different types of forests is still rare, especially in China's subtropical forest ecosystem. Consequently, studying how different tree species regulate their sap flow in response to shifting environmental conditions is essential for understanding forest transpiration, water use efficiency, and drought stress resilience. Therefore, this study aimed to investigate the sap flow dynamics of seven tree species in five forest plots, i.e., pine forest (PF), two types of mixed conifer-broadleaf forests (MF1+MF2), monsoon evergreen broadleaved forest (MEBF), and montane monsoonal evergreen broad-leaf forest (MOBF) at Dinghushan National Reserve in Southern China, using the heat dissipation probe technique and synchronous environmental factor recordings. Results demonstrated a significant influence of photosynthetic active radiation (PAR) on sap flow across all tree species, with mean PAR values ranging from over 1200 to 425 µmol m-2 s-1, establishing it as the principal driving factor. This observation underscores the heightened responsiveness or sensitivity of tree species to variations in PAR as the forest undergoes development and maturation. The correlation between vapor pressure deficit (VPD) and tree sap flow decreased as succession progressed. Moreover, the influence of soil water content (SWC) on sap flow stability against environmental changes increased. Similar patterns were also found between the two MF, with MF-2 displaying ecological characteristics and environmental conditions more closely aligned with those of the late-successional MEBF. The study reveals the intricate relationship between environmental factors and sap flow regulation in tree species within a subtropical forest ecosystem. Addressing a comparative gap in sap flow correlation among dominant tree species at Dinghushan, it establishes a hydro-physiological foundation for understanding tree species substitution during forest succession. The results provide key insights for forest management and climate-related research. Future studies should delve into the long-term implications of observed sap flow dynamics, exploring their impact on tree species adaptability amid ongoing environmental changes.

Water use by a warm-temperate deciduous forest under the influence of the Asian monsoon: contributions of the overstory and understory to forest water use.

The warm temperate deciduous forests in Asia have a relatively dense understory, hence, it is imperative that we understand the dynamics of transpiration in both the overstory (E O) and understory (E U) of forest stands under the influence of the Asian monsoon in order to improve the accuracy of forest water use budgeting and to identify key factors controlling forest water use under climate change. In this study, E O and E U of a temperate deciduous forest stand located in South Korea were measured during the growing season of 2008 using sap flow methods. The objectives of this study were (1) to quantify the total transpiration of the forest stand, i.e., overstory and understory, (2) to determine their relative contribution to ecosystem evapotranspiration (E eco), and (3) to identify factors controlling the transpiration of each layer. E O and E U were 174 and 22 mm, respectively. Total transpiration accounted for 55 % of the total E eco, revealing the importance of unaccounted contributions to E eco (i.e., soil evaporation and wet canopy evaporation). During the monsoon period, there was a strong reduction in the total transpiration, likely because of reductions in photosynthetic active radiation, vapor pressure deficit and plant area index. The ratio of E U to E O declined during the same period, indicating an effect of monsoon on the partitioning of E eco in its two components. The seasonal pattern of E O was synchronized with the overstory canopy development, which equally had a strong regulatory influence on E U.

Manipulative lowering of the water table during summer does not affect CO2 emissions and uptake in a fen in Germany.

We simulated the effect of prolonged dry summer periods by lowering the water table on three manipulation plots (D(1-3)) in a minerotrophic fen in southeastern Germany in three years (2006-2008). The water table at this site was lowered by drainage and by excluding precipitation; three nonmanipulated control plots (C(1-3)) served as a reference. We found no significant differences in soil respiration (R(Soil)), gross primary production (GPP), or aboveground respiration (R(AG)) between the C(1-3) and D(1-3) plots in any of the measurement years. The water table on the control plots was naturally low, with a median water table (2006-2008) of 8 cm below the surface, and even lower during summer when respiratory activity was highest, with median values (C(1-3)) between 11 and 19 cm below the surface. If it is assumed that oxygen availability in the uppermost 10 cm was not limited by the location of the water table, manipulative lowering of the water table most likely increased oxygen availability only in deeper peat layers where we expect R(Soil) to be limited by poor substrate quality rather than anoxia. This could explain the lack of a manipulation effect. In a second approach, we estimated the influence of the water table on R(Soil) irrespective of treatment. The results showed a significant correlation between R(Soil) and water table, but with R(Soil) decreasing at lower water tables rather than increasing. We thus conclude that decomposition in the litter layer is not limited by waterlogging in summer, and deeper peat layers bear no significant decomposition potential due to poor substrate quality. Consequently, we do not expect enhanced C losses from this site due to increasing frequency of dry summers. Assimilation and respiration of aboveground vegetation were not affected by water table fluctuations between 10 and >60 cm depth, indicating the lack of stress resulting from either anoxia (high water table) or drought (low water table).

Interactive influence of rainfall manipulation and livestock grazing on species diversity of the herbaceous layer community in a humid savannah in Kenya.

Changes in rainfall regime and grazing pressure affect vegetation composition and diversity with ecological implications for savannahs. The savannah in East Africa has experienced increased livestock grazing and rainfall variability but the impacts associated with those changes on the herbaceous layer have rarely been documented. We investigated the effect of livestock grazing, rainfall manipulation and their interaction on the composition and diversity of the herbaceous community in the savannah for two years in Lambwe, Kenya. Rainfall manipulation plots were set up for vegetation sampling; these plots received either 50% more or 50% less rainfall than control plots. Simpson's diversity and Berger-Parker indices were used to determine diversity changes and dominance respectively. The frequency of species was used to compute their abundance and their life forms as determined from the literature. Grazing significantly increased species diversity through suppression of dominant species. Rainfall manipulation had no significant impact on plant diversity in fenced plots, but rainfall reduction significantly reduced diversity in grazed plots. In contrast, rainfall manipulation had no impact on dominance in either fenced or grazed plots. The interaction of grazing and rainfall manipulation is complex and will require additional survey campaigns to create a complete picture of the implications for savannah structure and composition.

An ethnobotanical study of medicinal plants used by the Nandi people in Kenya.

ETHNOPHARMACOLOGICAL RELEVANCE: The study of local knowledge about natural resources is becoming increasingly important in defining strategies and actions for conservation or recuperation of residual forests. AIMS OF THE STUDY: This study therefore sought to collect information from local populations concerning the use of Nandi Forest medicinal plants; verify the sources of medicinal plants used and determine the relative importance of the species surveyed. MATERIALS AND METHODS: Data was obtained using semi-structured forms to record the interviewee's personal information and topics related to the medicinal use of specific plants. A total of 40 medicinal plants used locally for the treatment and/or control of human ailments were collected through interviews conducted with selected traditional doctors and professional healers. RESULTS: This study demonstrated that local people tend to agree with each other in terms of the plants use and that leaf material form the major component of plant parts exploited. The other harvested materials consist of stem bark, the roots and the whole plant, though at a lower intensity for making liquid concoctions from different plants. Majority of the remedies were prepared from a single species. In most cases, the mode of administration was oral. In the forest, some of the plants collected were scarce. This scarcity was attributed to indiscriminate logging, overexploitation, poor harvesting methods and current agricultural trends. CONCLUSION: Conservation procedures and creation of awareness were identified as the main remedies to the current situation.

Higher assimilation than respiration sensitivity to drought for a desert ecosystem in Central Asia.

Responses of ecosystem assimilation and respiration to global climate change vary considerably among terrestrial ecosystems constrained by both biotic and abiotic factors. In this study, net CO2 exchange between ecosystem and atmosphere (NEE) was measured over a 4-year period (2013-2016) using eddy covariance technology in a desert ecosystem in Central Asia. Ecosystem assimilation (gross primary production, GPP) and respiration (Reco) were derived from NEE by fitting light response curves to NEE data based on day- and nighttime data, and their responses to soil water content (SWC) and evaporative fraction (EF) were assessed during the growing season. Results indicated that both GPP and Reco linearly decreased with declining SWC, with the sensitivity of GPP to SWC being 3.8 times higher than that of Reco during the entire growing season. As a result, ecosystem CO2 sequestration capacity decreased from 4.00µmolm-2s-1 to 1.00µmolm-2s-1, with increasing soil drought. On a seasonal scale, significant correlation between GPP and SWC was only found in spring while that between Reco and SWC was found in all growing seasons with the sensitivity increasing steadily from spring to autumn. EF had a low correlation with SWC, GPP and Reco (R2=0.03, 0.02, 0.05, respectively), indicating that EF was not a good proxy for soil drought and energy partitioning was not tightly coupled to ecosystem carbon exchanges in this desert ecosystem. The study deepens our knowledge of ecosystem carbon exchange and its response to drought as well as its coupling with ecosystem energy partitioning in an extreme dry desert. The information is critical for better assessing carbon sequestration capacity in dryland, and for understanding its feedback to climate change.

Soil water availability and capacity of nitrogen accumulation influence variations of intrinsic water use efficiency in rice.

Leaf intrinsic water use efficiency (WUEi) coupling maximum assimilation rate (Amax) and transpirable water lost via stomatal conductance (gsc) has been gaining increasing concern in sustainable crop production. Factors that influence leaf Amax and WUEi in rice (Oryza sativa L. cv Unkang) at flooding and rainfed conditions were evaluated. Positive correlations for leaf nitrogen content (Nm) and maximum carboxylation rate (Vcmax), for nitrogen allocation in Rubisco enzymes and mesophyll conductance (gm) were evident independent of cropping cultures. Rainfed rice exhibited enriched canopy leaf average Nm resulting in higher Amax, partially supporting improved leaf WUEi. Maximum WUEi (up to 0.14 µmol mmol(-1)) recorded in rainfed rice under drought conditions resulted from increasing gm/gsc ratio while at cost of significant decline in Amax due to hydraulically constrained gsc. Amax sensitivity related to gsc which was regulated by plant hydraulic conductance. WUEi was tightly correlated to Vcmax/gsc and gm/gsc ratios across the paddy and rainfed not to light environment, morphological and physiological traits, highlighting enhance capacity of Nm accumulation in rainfed rice with gsc at moderately high level similar to paddy rice facilitate optimization in Amax and WUEi while, is challenged by drought-vulnerable plant hydraulic conductance.

Changes in forest soil properties in different successional stages in lower tropical China.

BACKGROUND: Natural forest succession often affects soil physical and chemical properties. Selected physical and chemical soil properties were studied in an old-growth forest across a forest successional series in Dinghushan Nature Reserve, Southern China. METHODOLOGY/PRINCIPAL FINDINGS: The aim was to assess the effects of forest succession change on soil properties. Soil samples (0-20 cm depth) were collected from three forest types at different succession stages, namely pine (Pinus massoniana) forest (PMF), mixed pine and broadleaf forest (PBMF) and monsoon evergreen broadleaf forest (MEBF), representing early, middle and advanced successional stages respectively. The soil samples were analyzed for soil water storage (SWS), soil organic matter (SOM), soil microbial biomass carbon (SMBC), pH, NH4(+)-N, available potassium (K), available phosphorus (P) and microelements (available copper (Cu), available zinc (Zn), available iron (Fe) and available boron (B)) between 1999 and 2009. The results showed that SWS, SOM, SMBC, Cu, Zn, Fe and B concentrations were higher in the advanced successional stage (MEBF stage). Conversely, P and pH were lower in the MEBF but higher in the PMF (early successional stage). pH, NH4(+)-N, P and K declined while SOM, Zn, Cu, Fe and B increased with increasing forest age. Soil pH was lower than 4.5 in the three forest types, indicating that the surface soil was acidic, a stable trend in Dinghushan. CONCLUSION/SIGNIFICANCE: These findings demonstrated significant impacts of natural succession in an old-growth forest on the surface soil nutrient properties and organic matter. Changes in soil properties along the forest succession gradient may be a useful index for evaluating the successional stages of the subtropical forests. We caution that our inferences are drawn from a pseudo-replicated chronosequence, as true replicates were difficult to find. Further studies are needed to draw rigorous conclusions regarding on nutrient dynamics in different successional stages of forest.

[Characteristics of dominant tree species stem sap flow and their relationships with environmental factors in a mixed conifer-broadleaf forest in Dinghushan, Guangdong Province of South China].

By the method of Granier' s thermal dissipation probe, the stem sap flow density of four dominant tree species (Pinus massoniana, Castanopsis chinensis, Schima superba, and Machilus kwangtungensis) in a mixed conifer-broadleaf forest in Dinghushan Reserve of South China was continuously measured in the dry season (November) and wet season (July) in 2010, and the environmental factors including air temperature, relative humidity, and photosynthetically active radiation (PAR) were measured synchronically, aimed to study the characteristics of the stem sap flow of the tree species in response to environmental factors. During the dry and wet seasons, the diurnal changes of the stem sap flow velocity of the tree species all presented a typical single-peak curve, with high values in the daytime and low values in the nighttime. The average and maximum sap flow velocities and the daily sap flow flux of broad-leaved trees (C. chinensis, S. superba, and M. kwangtungensis) were significantly higher than those of coniferous tree (P. massoniana), and the maximum sap flow velocity of P. massoniana, C. valueschinensis, S. superba, and M. kwangtungensis was 29.48, 38.54, 51.67 and 58.32 g H2O x m(-2) x s(-1), respectively. A time lag was observed between the sap flow velocity and the diurnal variations of PAR, vapor pressure deficiency, and air temperature, and there existed significant positive correlations between the sap flow velocity and the three environmental factors. The PAR in wet season and the air temperature in dry season were the leading factors affecting the stem sap flow velocity of the dominant tree species.