Heterogeneity of soil and plant N and C associated with individual plants and openings in North American shortgrass steppe

Small-scale spatial heterogeneity of soil organic matter (SOM) associated with patterns of plant cover can strongly influence population and ecosystem dynamics in dry regions but is not well characterized for semiarid grasslands. We evaluated differences in plant and soil N and C between soil from under individual grass plants and from small openings in shortgrass steppe. In samples from 0 to 5 cm depth, root biomass, root N, total and mineralizable soil N, total and respirable organic C, C:N ratio, fraction of organic C respired, and ratio of respiration to N mineralization were significantly greater for soil under plants than soil from openings. These differences, which were consistent for two sites with contrasting soil textures, indicate strong differentiation of surface soil at the scale of individual plants, with relative enrichment of soil under plants in total and active SOM. Between-microsite differences were substantial relative to previously reported differences associated with landscape position and grazing intensity in shortgrass steppe. We conclude that microscale heterogeneity in shortgrass steppe deserves attention in investigation of controls on ecosystem and population processes and when sampling to estimate properties at plot or site scales.     

Significant increase in natural disturbance impacts on European forests since 1950

Over the last decades, the natural disturbance is increasingly putting pressure on European forests. Shifts in disturbance regimes may compromise forest functioning and the continuous provisioning of ecosystem services to society, including their climate change mitigation potential. Although forests are central to many European policies, we lack the long-term empirical data needed for thoroughly understanding disturbance dynamics, modeling them, and developing adaptive management strategies. Here, we present a unique database of >170,000 records of ground-based natural disturbance observations in European forests from 1950 to 2019. Reported data confirm a significant increase in forest disturbance in 34 European countries, causing on an average of 43.8 million m³ of disturbed timber volume per year over the 70-year study period. This value is likely a conservative estimate due to under-reporting, especially of small-scale disturbances. We used machine learning techniques for assessing the magnitude of unreported disturbances, which are estimated to be between 8.6 and 18.3 million m³/year. In the last 20 years, disturbances on average accounted for 16% of the mean annual harvest in Europe. Wind was the most important disturbance agent over the study period (46% of total damage), followed by fire (24%) and bark beetles (17%). Bark beetle disturbance doubled its share of the total damage in the last 20 years. Forest disturbances can profoundly impact ecosystem services (e.g., climate change mitigation), affect regional forest resource provisioning and consequently disrupt long-term management planning objectives and timber markets. We conclude that adaptation to changing disturbance regimes must be placed at the core of the European forest management and policy debate. Furthermore, a coherent and homogeneous monitoring system of natural disturbances is urgently needed in Europe, to better observe and respond to the ongoing changes in forest disturbance regimes.     

Environmental influences on local amphibian diversity: the role of floods on river basins

We tested two hypotheses about boundary units and seven about environmental control of species diversity in order to explain geographical trends in the richness of amphibian species in the Mediterranean watershed of the southern coast of the Iberian Peninsula. The number of amphibian species tends to decrease from west to east. The longitudinal trend in the richness of amphibian species actually occurs on passing from one basin to another, but there is not any longitudinal trend within the basins. Multivariate analyses confirmed that the disturbances of episodic river-basin floodings were the principal factor which controls the richness of amphibian species. They explained 94.8% of the observed variations in the richness of amphibian species in this area, according to the intermediate disturbance hypothesis.We propose hydrographic basins as suitable geographical units for further biogeographical analysis and for considering the role of disturbances produced by floods in the environmental control of species diversity.     

The response of lichens inhabiting exposed wood of spruce logs to post-hurricane disturbances in Western Carpathian forests

We investigated which of the following environmental factors: the number of years since the windthrow of the tree (the age of dead wood), the phytocenosis (the type of forest community), altitude, exposure, wood hardness and the spatial scale of forest disturbances (small gaps with a few fallen spruces vs large-area windthrows) contributed to the diversity and abundance of lichens inhabiting the exposed wood of windthrown spruce trees in Polish Western Carpathian forests. Both Shannon H index and sum of coverage coefficients rose with increasing age of the wood, levelling off after 11–14 y (diversity) and 14–17 y (abundance). This factor appeared to be the most important for this group of lichens, but the significant positive impact of large-area windthrows on the lichen abundance was also demonstrated by using a GLM model. The age of the wood we precisely determined on the basis of data on Norway spruce mortality collected annually in permanent plots of the Gorce National Park since 2000. Using the Shore durometer we linked the course of the wood-inhabiting lichen succession with wood decay more precisely than before. The largest number of species was associated with medium hard wood, i.e., 51 < x ≤ 80 on the Shore scale. Based on the NMDS analysis, we distinguished four age groups of logs, differing in lichen abundance and defined by the dominance of distinctive species. A large number of usually corticolous lichen species used the wood of windthrown spruce logs as an optional habitat to survive large-scale, post-hurricane forest disturbances.     

Aspects of population dynamics in Halimione portulacoides communities

Summary Studies on sample plots inHalimione portulacoides communities show that environmental disturbances, either natural or induced by man, start a sequence of partly overlapping density maxima inSuaeda maritima, Aster tripolium andPuccinellia maritima successively, before the originalHalimione community totally recovers. When succession time before recovering is long enough, there are tendencies in redundancy of this sequence stressing the unilinear character of the succession. Minor environmental impacts induce a longer time-lag period of theSuaeda density maximum, suggesting threshold values of these impacts for the species to maintain minimal population densities or to become locally extinct. This sequence of interim species starting after an environmental disturbance, suggests also a gradient character in various biological attributes, for instance in life-time, propagation, nutrient and genetic plasticity strategies. The mechanism described can therefore be interpreted as a complex of mostly well-adapted and well-integrated inherent species strategies capable of absorbing environmental shocks. It is suggested that in the salt-marsh ccosystem the pattern of spatial variation in densities and that of temporal variation in fluctuations of the three species populations under natural conditions reflect corresponding patterns of environmental disturbances in the vegetation taking into account a timelag associated with the magnitude of the impact concerned.Contribution to the Symposium on Plant species and plant communities held at Nijmegen, 11–12 November 1976, on the occasion of the 60th birthday of Professor Victor Westhoff.Nomenclature follows Heukels-van Ooststroom. Flora van Nederland, 18e druk, 1975. Wolters-Noodhoff, Groningen.The authors are greatly indebted to Dr K.F. Vaas (Yerseke) for reviewing the English text.Communication Nr. 160.     

Forecasting intensifying disturbance effects on coral reefs

Anticipating future changes of an ecosystem's dynamics requires knowledge of how its key communities respond to current environmental regimes. The Great Barrier Reef (GBR) is under threat, with rapid changes of its reef‐building hard coral (HC) community structure already evident across broad spatial scales. While several underlying relationships between HC and multiple disturbances have been documented, responses of other benthic communities to disturbances are not well understood. Here we used statistical modelling to explore the effects of broad‐scale climate‐related disturbances on benthic communities to predict their structure under scenarios of increasing disturbance frequency. We parameterized a multivariate model using the composition of benthic communities estimated by 145,000 observations from the northern GBR between 2012 and 2017. During this time, surveyed reefs were variously impacted by two tropical cyclones and two heat stress events that resulted in extensive HC mortality. This unprecedented sequence of disturbances was used to estimate the effects of discrete versus interacting disturbances on the compositional structure of HC, soft corals (SC) and algae. Discrete disturbances increased the prevalence of algae relative to HC while the interaction between cyclones and heat stress was the main driver of the increase in SC relative to algae and HC. Predictions from disturbance scenarios included relative increases in algae versus SC that varied by the frequency and types of disturbance interactions. However, high uncertainty of compositional changes in the presence of several disturbances shows that responses of algae and SC to the decline in HC needs further research. Better understanding of the effects of multiple disturbances on benthic communities as a whole is essential for predicting the future status of coral reefs and managing them in the light of new environmental regimes. The approach we develop here opens new opportunities for reaching this goal.     

Comparison of forest above‐ground biomass from dynamic global vegetation models with spatially explicit remotely sensed observation‐based estimates

Gaps in our current understanding and quantification of biomass carbon stocks, particularly in tropics, lead to large uncertainty in future projections of the terrestrial carbon balance. We use the recently published GlobBiomass data set of forest above‐ground biomass (AGB) density for the year 2010, obtained from multiple remote sensing and in situ observations at 100 m spatial resolution to evaluate AGB estimated by nine dynamic global vegetation models (DGVMs). The global total forest AGB of the nine DGVMs is 365 ± 66 Pg C, the spread corresponding to the standard deviation between models, compared to 275 Pg C with an uncertainty of ~13.5% from GlobBiomass. Model‐data discrepancy in total forest AGB can be attributed to their discrepancies in the AGB density and/or forest area. While DGVMs represent the global spatial gradients of AGB density reasonably well, they only have modest ability to reproduce the regional spatial gradients of AGB density at scales below 1000 km. The 95th percentile of AGB density (AGB₉₅) in tropics can be considered as the potential maximum of AGB density which can be reached for a given annual precipitation. GlobBiomass data show local deficits of AGB density compared to the AGB₉₅, particularly in transitional and/or wet regions in tropics. We hypothesize that local human disturbances cause more AGB density deficits from GlobBiomass than from DGVMs, which rarely represent human disturbances. We then analyse empirical relationships between AGB density deficits and forest cover changes, population density, burned areas and livestock density. Regression analysis indicated that more than 40% of the spatial variance of AGB density deficits in South America and Africa can be explained; in Southeast Asia, these factors explain only ~25%. This result suggests TRENDY v6 DGVMs tend to underestimate biomass loss from diverse and widespread anthropogenic disturbances, and as a result overestimate turnover time in AGB.     

Species- and clone-specific responses to environmental stimuli in the cladocerans Bosmina cornuta and B. pellucida - a comparison with Daphnia

We studied the variation of small-scale swimming behaviour in eight Bosmina cornuta and ten B. pellucida clones in response to key environmental factors to test whether swimming behaviour and genotypes are linked in non-Daphnia cladocerans. We quantified (1) the short-term responses to changes in temperature, light intensity and pH, (2) the response to long-term temperature acclimation, and (3) the pH-related survival rates. Vertical swimming activity S was quantified in cuvette experiments as crossings of a line at 2 cm height per individual an hour. S differed significantly among species and conspecific clones. At any temperature, light intensity and pH tested, B. cornuta (clone variation: 40-58 crossings/ind.- h) showed a higher vertical swimming activity than B. pellucida (clone variation: 25-48 crossings/ind.- h). A short-term change of water temperature (range tested: 10-25C) only affected S of B. cornuta, whereas that of B. pellucida remained unaltered. In contrast, S increased with rising temperature following long-term temperature acclimation (range tested: 10-20C) in both species. Swimming activity was inversely related to the light intensity (range tested: 60-60,000 lux), but decrease of activity was stronger in B. pellucida (44′ 12 crossings/ind - h) than in B. cornuta (50′ 40 crossings/ind.- h). Short-term changes of pH (range tested: 4-6) did not influence swimming activity in any species, although a prolonged exposure (24 h) to pH 4 was lethal. Thus, Bosmina showed behavioural responses which permit to distinguish between the species and which are related to their seasonal succession and distribution pattern.     

How to efficiently determine the size at maturity of small-sized tropical fishes: A case study based on 144 species identified via DNA barcoding from southwestern Madagascar

In order to provide biological evidence of the real impact of mosquito seine nets in southwestern Madagascar, an efficient procedure for determining the size at maturity of small-sized tropical fishes was developed. The fishes caught by two small-scale fishermen were studied between October 2017 and April 2018. One catch per day was analyzed three days per month during the full-moon period. In the laboratory, fishes were all sorted by morphospecies, photographed and measured. One individual per morphospecies was selected for being identified using CO1 DNA barcoding. A total of 34,051 individual fishes belonging to 144 DNA bacoded species from 48 families was obtained from 42 samples, 467 individuals from 22 morphospecies that had not been successfully barcoded were excluded from the analyses. The macroscopic observations of 8,143 individuals between 0.7 and 10 cm SL indicated the proportion of individuals with clearly observable gonads was 15% only.Among the 144 species identified via DNA barcoding, 83 consisted of individuals that were all without clearly observable gonads, seven of individuals that were all with clearly observable gonads and 54 included of individuals with and without clearly observable gonads. As the determination of L₅₀ using logistic general linear models failed for most species, the minimum size at maturity was retained to determine the proportion of juveniles and adults for these 54 species. Compared to the data available in FishBase, the minimum size at maturity appears more adequate to discrimine juvenile from adult fish of small-sized tropical species.