Herbivores change plant communities and biogeochemical cycles by changing species composition and soil organic matter contents through different mechanisms. Our objective was to evaluate the effects of livestock removal on plant species composition and soil organic matter stocks in savannas of the Río de la Plata grasslands.
A subtropical savanna in Río Negro, Uruguay.
The experiment consisted of a paired grazing exclusion randomized block design with five sites, where livestock was excluded for 29 years. We recorded differences in plant species composition in different strata and extracted soil cores to analyze soil carbon and nitrogen stocks, and δ13 carbon contents in two different soil organic matter fractions.
Long-term livestock removal reduced plant species richness and diversity, altered the floristic composition and facilitated native-shrub encroachment. Grazing exclusion modified the relative frequency of plant functional types and reduced summer-growing grasses. We recorded 45 species common to both grazing and exclosure treatments, while 19 were present only in exclosure sites and 39 only in grazed sites. Livestock exclusion had a differential effect on soil organic matter formation, increased soil organic carbon and nitrogen stocks in the mineral-associated organic matter (MAOM) but tended to slightly reduce them in the particulate organic matter fraction (POM). Grazing removal modified the C3/C4 species cover and these differences were reflected in the δ13C contents of both POM and MAOM fractions.
Removal of domestic herbivores increased shrub abundance, decreased plant species richness and diversity but increased total carbon and nitrogen contents in soil organic matter. Our results suggest that grazed and exclosure areas should be combined mosaically in the landscape to maximize plant species diversity and soil organic carbon sequestration.
High species richness is observed in certain shrublands on infertile substrates. Mineral nutrients are likely to be the primary limiting resources in these ecosystems, and below-ground plant interactions may be crucial to understanding their diversity. Using ionomics, we investigated whether there were nutritional variations between plant species that coexist in a shrubland located in an edaphically extreme environment.
New Caledonia.
We set up a 20 m × 20 m plot in a diverse shrubland (“maquis”) on ultramafic (infertile) substrate, in which we sampled all 475 plants taller than 1 m and characterized their ionome (22 elements).
In our study, 37 species were identified in the plot, representing all major forms of mycorrhizal symbioses, as well as nitrogen-fixing plants, cluster rooted and parasitic plants. Notably, both nickel hyperaccumulating and manganese hyperaccumulating species were present. Hypervolume approaches were used to assess ionome overlap among the nine most abundant species, with the results revealing limited overlap. Moreover, it was observed that the rarest species in the plot also had the most functionally distinct features.
Different nutritional strategies were present in the plot, as demonstrated by the variety of root symbioses and leaf ionomes. Our findings indicate coexistence of multiple species within this infertile shrubland may be achieved by species partitioning into different highly specialized biogeochemical niches.
How does the grass layer affect seedlings across large environmental gradients in savannas?
Savanna sites in Argentina, Tanzania, and South Africa.
We carried out a joint analysis of three grass removal experiments in which seedlings of various Fabaceae species were transplanted into plots with native grass and companion plots where grass had been removed. First, we estimated the effect of grasses on tree seedling mortality and seedling growth rate at each site. Then, we used the resulting coefficient estimates from site-level models to examine the impact of two climate (monthly precipitation and aridity index) and two soil (soil organic carbon content and clay content) variables on the direction and magnitude of the grass effects.
Grasses increased the risk of mortality, but there was no evidence for a global effect of grasses on tree seedling rate of height growth. The best model fit indicated a high mortality risk of tree seedlings in response to grasses at intermediate aridity index values. No other climate or soil variable influenced tree seedling survival or growth (monthly precipitation, soil organic carbon content and clay content).
Our results support the notion that the grass layer consistently creates a bottleneck to tree seedling establishment in African and South American savannas beyond climate and soil conditions, mainly by affecting tree seedling survival. The negative effect of grasses on seedling survival was lower in dry conditions compared to intermediate aridity levels. These results suggest that grass–seedling interaction is less intense in drier conditions, possibly due to reduced total grass biomass, which leads to decreased site evapotranspiration and improved soil water retention capacity.
Changes in precipitation patterns, such as the predicted increases in the frequency of climatic extremes, are likely to alter plant communities, but whether responses to drought or to wetter conditions respectively cause consistent, opposite responses is debated. Here, we assessed the response in biomass production and species diversity of water-limited plant communities to the direction (increase or decrease) and magnitude (micro- and macro-climatic effects) of changes in soil moisture.
We reciprocally translocated soils containing seed banks from two climates (semi-arid and mediterranean) at a micro-climatic (opposite slopes) and a macro-climatic scale (between climates) in Chile.
Biomass production for the soils that were translocated from wetter to drier climates was unrelated to the available soil moisture. The lowest biomass was produced in the wettest climate on the wet slope. Biomass production increased after a translocation to the drier climate (representing the largest change in climate). Nonetheless, the highest overall biomass for the wet to dry translocation was produced on the mediterranean dry slope with intermediate soil moisture. However, on the same mediterranean dry slope, biomass was almost zero for soil translocated the other way round (from drier to wetter). Diversity after 24 months was unaffected by micro-climatic change, but soils transplanted toward the drier climate yielded a plant community with increased diversity.
Our results showed direction and magnitude of climate change but also the response factor that is studied matters to detect direction-dependent responses; i.e., species richness had a linear and reversible response. However, the response of biomass depended on the origin of the transplanted material (soil and plant community), indicating history dependence (hysteresis). This emphasizes that responses to unidirectional climate manipulation experiments may not be able to capture the entire nature of the response of plant communities to climate change.
Analysing how multiple facets of biodiversity vary across space and time can help to predict the vulnerability of mountaintop floras to future environmental changes. Here we addressed the following questions: (a) Are elevational patterns of mountaintop plant diversity consistent across taxonomy, function and phylogeny? (b) How have the taxonomic, functional and phylogenetic dimensions of mountaintop plant communities changed over the past two decades? (c) Is the magnitude of these temporal trends dependent on elevation?
Dovrefjell, central Norway.
The floristic composition of four mountaintops, spread across an elevational gradient from the tree line to the uppermost margins of vascular plant life, was surveyed every 7 years between 2001 and 2022. Six metrics of taxonomic, functional and phylogenetic richness and differentiation were calculated for each mountaintop and survey. Using these data, we assessed how richness and differentiation metrics varied over space (across the elevational gradient) and over time (between surveys).
All diversity metrics decreased towards higher elevations, except phylogenetic differentiation which increased significantly by 7% per 100 m elevational gain. Taxonomic richness remained virtually stable between 2001 and 2022, whereas phylogenetic richness increased by 7.5% per decade. Functional richness also increased, but mainly on the lowest mountaintop, by 17% per decade. No significant temporal trends in taxonomic, functional and phylogenetic differentiation were detected.
Our findings underpin rearrangements in the functional and phylogenetic structure of mountain plant communities over the past two decades that cannot be predicted from trends in taxonomic richness alone. This highlights the necessity to look beyond species richness and consider multiple facets of biodiversity when studying environmental change impacts on mountain biodiversity and ecosystem functioning.
Plant cover values in vegetation plot data are bounded between 0 and 1, and cover is typically recorded in discrete classes with non-equal intervals. Consequently, cover data are skewed and heteroskedastic, which hampers the application of conventional regression methods. Recently developed ordinal beta regression models consider these statistical difficulties. Our primary question is whether we can detect species trends in vegetation plot time series data with this modelling approach. A second question is whether trends in cover have additional value compared to trends in occurrence, which are easier to assess for practitioners.
The Netherlands, Western Europe.
We used vegetation plot data collected from 10,000 fixed plots which were surveyed once every four years during 1999–2022. We used the ordinal zero-augmented beta regression (OZAB) model, a hierarchical model consisting of a logistic regression for presence and an ordinal beta regression for cover. We adapted the OZAB model for longitudinal data and produced estimates of cover and occurrence for each four-year period. Thereafter we assessed trends in cover and in occurrence across all periods.
We found evidence of a trend in cover in 318 out of the 721 species (44%) with sufficient data. Most species showed similar directional trends in occurrence and percent cover. No trend in occurrence was detected for 64 species that had evidence of a trend in cover. Declining species had stronger relative changes in cover than in occurrence.
Our model enables researchers to detect trends in cover using longitudinal vegetation plot data. Cover trends often corroborated trends in occurrence, but we also regularly found trends in cover even in the absence of evidence for trends in occurrence. Our approach thus contributes to a more complete picture of (changes in) vegetation composition based on large monitoring data sets.
Following a significant disturbance, vegetation development may, or may not reach the desired target. Here, we examine which relevant global environmental factors have a substantial impact on the course of spontaneous vegetation succession, and what their relative relevance is in achieving the desired outcome.
Worldwide.
The outcome of vegetation changes in 528 studies describing spontaneous succession worldwide was classified at a simple, semi-quantitative scale: fully-successful, partly-successful, and unsuccessful, considering 10 different types of disturbances. Latitude, climatic factors (mean annual temperature, annual temperature range, mean annual precipitation, seasonality of precipitation), and biological factors (number of vascular plant species, and number of invasive alien species) were considered as explanatory variables. The ordination method (principal coordinate analysis) was used to visualize relationships among variables and their relationships to succession outcomes. For a detailed insight into the importance of the particular variables, we applied machine learning techniques, specifically one called “conditional random forest”. In addition, the effect of different types of initial disturbance was assessed using Generalized Linear Models.
Globally, disturbance type emerged as the most influential factor in determining succession outcomes. The most successful were results from recovering vegetation after fire, whereas the most unsuccessful were those after volcano eruptions. For climatic factors, the success of succession decreased with a low annual temperature range and high temperature mean. Biological factors such as the number of invasive alien species and species richness had the least but significant influence on the succession success.
The most relevant factor determining the outcome of spontaneous succession was disturbance type, followed by temperature variables. Notably, latitude emerged as a practical proxy for many ecologically relevant factors. Therefore, we conclude that latitude may be a valuable predictor of the success of succession and, consequently, of the success of ecological restoration projects that are based on spontaneous succession.
The alpine vegetation of the Alps is particularly vulnerable to climate change, as the temperature increase in this region is twice the global average and the available area for new colonisations decreases with increasing elevation. While numerous studies have investigated the response of vascular plants to a warming climate in the alpine belt, only a handful have investigated that of cryptogams in the European Alps. Based on a 21-year monitoring project, we assessed the effects of climate change on cryptogams along elevation, from the treeline to the subnival belt.
Four GLORIA summits in Valais (Switzerland).
Between 2001 and 2022, terricolous lichens and bryophytes (from 2008) were inventoried in 52 1-m2 plots distributed across four summits: 2360 m a.s.l. (treeline), 2550 m (lower alpine), 2990 m (upper alpine) and 3210 m (subnival). Changes in species cover and richness were analysed using generalised linear mixed-effects model (GLMMs).
For bryophytes, total cover remained stable overall. However, six species declined significantly between 2008 and 2022, and the species richness decreased after 2015. For terricolous lichens, total cover significantly increased on the lower alpine summit, while species richness increased on the upper alpine and subnival summits.
Bryophytes have probably suffered from the increasingly dry conditions, with a succession of very warm and dry summers over the last decades. Terricolous lichens have taken advantage of the warmer conditions to increase their cover on the lower alpine summit, and new species have colonised the upper summits. However, as they compete with vascular plants for soil and light, they may suffer from shrub and tree encroachment in the future and will be limited upwards by the rarity of developed soils. The large topo-climatic gradient (850 m) and the length of the time series suggest that similar trends are likely to be more widespread across the Alps.
Many species and ecosystems that diversified and adapted under consumer control in prehistoric times are nowadays highly threatened. Nature protection areas (PAs) form a major conservation strategy to avoid their losses. We argue that many PAs across Earth are in disequilibrium with current climatic conditions. At the same time, the main consumers of woody vegetation keeping these systems in climatic disequilibrium, that is, large-bodied herbivores and/or fire, have strongly declined or changed in occurrence in (pre-)historic times. Without active intervention, this lack of consumer control will cause the systems to approach climate equilibrium with major implications for baseline-focused approaches in species and nature protection and restoration. In a global analysis we quantified the prevalence of climatic disequilibrium in PAs for all terrestrial biomes. We calculated climatic disequilibrium in PAs as the difference between actual tree cover and the potential tree cover under current climatic conditions (i.e., mean annual temperature and annual precipitation sum). We show that climatic disequilibrium conditions in tree cover are a widespread phenomenon in PAs across all biomes with highest values for the temperate grassland, tundra and taiga biomes. We argue that trophic rewilding, notably the restoration of functionally diverse large-herbivore assemblages, would not only help maintain climatic disequilibrium states, but also reduce labour and costs for management.
京公网安备 11010802042870号
京ICP备2023020795号-1
扫码关注我们
求助内容：
应助结果提醒方式：