Semi-arid Shrubland Research Articles

Rainfall distribution variability controls surface but not belowground litter decomposition in a semi-arid shrubland

IntroductionRainfall patterns are expected to become increasingly erratic as a result of global climate change, with more intense but less frequent rainfall events leading to an increased occurrence of drought events. This process may lead to significant declines in vegetation cover and subsequent increases in soil erosion, consequently accelerating the bury of detached litter by soil deposition and the mixture of residues from different plant species. Responses of litter decomposition to increasing rainfall variability in distribution and subsequent litter mixing or soil cover have scarcely received attention.MethodsTo fill this gap in our knowledge, we analyzed the influence of rainfall variability, soil cover, and litter mixing on shrub-species litter decomposition in a semi-arid shrubland. We explored the effects of redistributing the frequency and amount of precipitation on surface and belowground decomposition of litter from two separate or mixed predominant shrubs.ResultsDecomposition of belowground litter was consistently higher than that of surface litter over the entire field-incubation process. Mass loss significantly decreased in surface litter but not in belowground litter due to the lower frequency and larger amount of precipitation compared to the control treatment. Furthermore, exclusion of 30% precipitation had no significant effects on decomposition of either surface or belowground litter. We observed stronger synergistic effect for belowground litter mixture relative to surface litter mixture of the two shrubs, especially in the hotter months over the 5-month incubation.DiscussionThese findings support that rainfall variability in terms of distribution pattern rather than in the amount controls the litter decomposition on the soil surface in the semi-arid shrubland. Meanwhile, soil burial or litter mixing have greater effects on litter decomposition, individually or jointly. Together, our results highlight the need to consider rainfall distribution variability and incorporate soil-covering and litter-mixing as driving factors of organic matter turnover in drylands.

Warming and rainfall reduction alter soil microbial diversity and co-occurrence networks and enhance pathogenic fungi in dryland soils

In this 9-year manipulative field experiment, we examined the impacts of experimental warming (2 °C, W), rainfall reduction (30 % decrease in annual rainfall, RR), and their combination (W + RR) on soil microbial communities and native vegetation in a semi-arid shrubland in south-eastern Spain. Warming had strong negative effects on plant performance across five coexisting native shrub species, consistently reducing their aboveground biomass growth and long-term survival. The impacts of rainfall reduction on plant growth and survival were species-specific and more variable. Warming strongly altered the soil microbial community alpha-diversity and changed the co-occurrence network structure. The relative abundance of symbiotic arbuscular mycorrhizal fungi (AMF) increased under W and W + RR, which could help buffer the direct negative impacts of climate change on their host plants nutrition and enhance their resistance to heat and drought stress. Indicator microbial taxa analyses evidenced that the marked sequence abundance of many plant pathogenic fungi, such as Phaeoacremonium, Cyberlindnera, Acremonium, Occultifur, Neodevriesia and Stagonosporopsis, increased significantly in the W and W + RR treatments. Moreover, the relative abundance of fungal animal pathogens and mycoparasites in soil also increased significantly under climate warming. Our findings indicate that warmer and drier conditions sustained over several years can alter the soil microbial community structure, composition, and network topology. The projected warmer and drier climate favours pathogenic fungi, which could offset the benefits of increased AMF abundance under warming and further aggravate the severe detrimental impacts of increased abiotic stress on native vegetation performance and ecosystem services in drylands.

Vegetation factors and atmospheric dryness regulate the dynamics of ecosystem water use efficiency in a temperate semiarid shrubland

Ecosystem water use efficiency (WUE), a key indicator of the coupling between carbon and water cycle, has been widely used to quantify ecosystem responses to climate change. However, large uncertainties remain regarding the dynamics and driving factors of WUE in temperate semiarid shrublands. Using eddy-covariance measurements, we investigated the role of vegetation and hydrometeorological factors in affecting the temporal dynamics of WUE (GPP/ET, where GPP and ET represent gross primary production and evapotranspiration, respectively) over a semiarid shrubland of northern China during 2012–2016. Daily WUE was lower (<1.5 g C kg−1 H2O) in early-spring and late-autumn, and higher (2.0–7.0 g C kg−1 H2O) in summer months (June–August). Annual (January–December) WUE (1.43–1.78 g C kg−1 H2O) was higher than most values previously reported for semiarid and arid shrubland ecosystems. Vegetation factors such as surface conductance (gs) and normalized difference vegetation index (NDVI) played a primary role in promoting monthly WUE during the warm and moist growing-season (April–October), while high vapor pressure deficit (VPD) and low root-zone soil water content (SWC) were key hydrometeorological factors that imposed constraints on daily WUE. Using both a machine-learning model and a data-binning algorithm, we showed that NDVI and VPD were the most important factors regulating WUE dynamics. Ongoing climate warming and increasingly frequent extreme events (e.g., droughts and heatwaves) are expected to induce high VPD, which in turn could reduce ecosystem WUE and undermine ecosystem carbon sequestration in semiarid shrublands. In addition, increasing vegetation cover in the studied area owing to continued restoration efforts is expected to enhance ecosystem WUE, and may result in stronger vegetation controls on WUE dynamics.

Soil elemental cycles become more coupled in response to increased nitrogen deposition in a semiarid shrubland

Background and aimsIncreased N deposition can break the coupled associations among chemical elements in soil, many of which are essential plant nutrients. We evaluated the effects of four years of N deposition (0, 10, 20, 50 kg N ha−1 yr−1) on the temporal dynamics of the spatial co-variation (i.e., coupling) among ten chemical elements in soils from a semiarid shrubland in central Spain.MethodsSoil element coupling was calculated as the mean of Spearman rank correlation coefficients of all possible pairwise interactions among elemental cycles, in absolute value. We also investigated the role of atomic properties of elements as regulators of coupling.ResultsWhile N deposition impacts on nutrient bioavailability were variable, soil elemental coupling consistently increased in response to N. Coupling responses also varied among elements and N treatments, and four out of ten elemental cycles also responded to N in a season-dependent manner. Atomic properties of elements such as mass, valence orbitals, and electronegativity contributed to explain the spatial coupling of soil elements, most likely due their role on the capacity of elements to interact with one another.ConclusionsThe cumulative effects of N deposition can alter the spatial associations among chemical elements in soils, while not having evident consequences on the bioavailability of single elments. These results indicate that considering how multiple elements co-vary in topsoils may provide a useful framework to better understand the simultaneous response of multiple elemental cycles to global change.

Dry-season length affects the annual ecosystem carbon balance of a temperate semi-arid shrubland

Semi-arid ecosystems have been shown to dominate over tropical forests in determining the trend and interannual variability of land carbon (C) sink. However, the magnitude and variability of ecosystem C balance remain largely uncertain for temperate semi-arid shrublands at the decadal scale. Using eddy-covariance and micro-meteorological measurements, we quantified the interannual variation in net ecosystem production (NEP) and its components, gross primary production (GPP) and ecosystem respiration (Reco, i.e., the sum of autotrophic and heterotrophic respiration), in a semi-arid shrubland of the Mu Us Desert, northern China during 2012–2022. This shrubland was an overall weak C sink over the 11 years (NEP = 12 ± 46 g C m−2 yr−1, mean ± SD). Annual NEP ranged from −66 to 77 g C m−2 yr−1, with the ecosystem frequently switching between being an annual C sink and a C source. GPP was twice as sensitive as Reco to prolonged dry seasons, leading to a close negative relationship between annual NEP and dry-season length (R2 = 0.80, P < 0.01). Annual GPP (R2 = 0.51, P = 0.01) and NEP (R2 = 0.58, P < 0.01) were positively correlated with annual rainfall. Negative annual NEP (the ecosystem being a C source) tended to occur when the dry season exceeded 50 d yr−1 or rainfall dropped below 280 mm yr−1. Increases in dry-season length strengthened the effects of low soil moisture relative to high vapor pressure deficit in constraining NEP. Both GPP and NEP were more closely correlated with C uptake amplitude (annual maximum daily values) than with C uptake period. These findings indicate that dry-season extension under climate change may reduce the long-term C sequestration in semi-arid shrublands. Plant species adapted to prolonged dry seasons should be used in ecosystem restoration in the studied area to enhance ecosystem functions.

Plant-plant interactions influence post-fire recovery depending on fire history and nurse growth form

BackgroundsPlant-plant interactions are among the most important factors affecting the natural recovery of vegetation. While the impacts of nurse plants on species composition and biodiversity are well documented, the effects of different nurse’s growth forms on all biodiversity components including taxonomic, functional, and phylogenetic diversity have been less studied and compared, especially for their effects on different times after fire disturbance. This research was focused on comparing the effects of a perennial grass (Elymus hispidens), a perennial herb (Phlomis cancellata), and a high shrub species (Lonicera nummulariifolia) on species composition and the biodiversity components, and how these impacts change across five sites with short-term (1 and 4 years sites), long-term (10 and 20 years sites) times since last fire and a control site where no fire was known in recorded history in semi-arid shrublands of Fereizi Chenaran located in Northeast of Iran. The changes of species composition and taxonomic, functional, and phylogenetic diversity were calculated with respect to the presence/absence of nurse’s growth forms, fire history, and their interactions.ResultsNurse shrubs affected species composition and all biodiversity components, whereas all indices were reduced when considering Elymus grass as nurse plant. On the other hand, the herb Phlomis enhanced species composition and taxonomic diversity, while it had a negative effect on functional and phylogenetic diversity. Such specific effects of nurse types were mostly observed under long timescales (i.e., 10- and 20-year sites). Interestingly, the relative importance of nurse types and time since the last fire largely explained the variation of species composition and biodiversity components, with larger effects of nurse types on all biodiversity components. However, we found a significant contribution of fire explaining variation of species composition and phylogenetic diversity.ConclusionsThese results indicated nurse plants can affect the post-fire recovery of vegetation by providing specific mechanisms controlling beneficiary relatedness depending on their growth forms and time scales since the last fire. Therefore, these findings suggest perennial plants in the form of nurse species as a useful factor to develop techniques of active restoration in burned ecosystems.

Drivers of chaparral photosynthetic rate reduction under modern drought conditions

Terrestrial vegetation communities are experiencing rapid and novel changes to photosynthetic rates under the changing climate. Chaparral, a semi-arid shrubland ecosystem of the Southwestern United States and Northern Mexico, is projected to experience substantial increases in aridity and stochastic precipitation. This study identifies the primary meteorological drivers of photosynthesis for three widespread chaparral shrub species—Adenostoma sparsifolium, Adenostoma fasciculatum, and Ceanothus perplexans—from 2019 to 2021. Monthly leaf-level carbon exchange rates, water potentials (WPs), and meteorological conditions were collected for each species. Average monthly primary productivity (n = 25) demonstrated vapor pressure deficit (VPD) as a significant limit to photosynthetic rates for A. sparsifolium and A. fasciculatum. VPD was also the most influential predictor of WP for all three species. These results suggest increasing atmospheric dryness as a key predictor for reduction in chaparral primary productivity, particularly for deeply-rooted, resprouting species. There are additional indications that VPD could exacerbate drought-related mortality for C. perplexans and A. sparsifolium by pushing WP to novel extremes. This study concludes that atmospheric dryness, across 3 years of differing soil water stress levels, was consistently a substantial physiological limitation for three common, chaparral species. Although this experiment occurred over a limited window and cannot assess climatic response trends, acute increases in air temperature and VPD within the region would exacerbate photosynthetic limitation for these species and may contribute to declining primary productivity in broader chaparral ecosystems.

The effects of habitat loss and fragmentation on the relative abundance and conservation of Ludwig’s Bustard Neotis ludwigii in South Africa

Ludwig’s Bustard Neotis ludwigii is near-endemic to the semi-arid shrublands of southwestern southern Africa and is listed as Endangered. The primary threat to this species is collisions with overhead powerlines; however, loss and fragmentation of its habitat may contribute to its population decline. The 1990 and 2020 land-use land-cover (LULC) maps prepared for South Africa were used to determine the LULC categories that best describe suitable habitat for Ludwig’s Bustard using beta regressions and data on exact localities, as recorded in the real-time BirdLasser logging app. Optimized Hot Spot Analysis was used to determine the priority areas for conserving Ludwig’s Bustard and its habitat. Beta regression models were compiled using the reporting rate for this bird and the total surface area of suitable habitat, and three other landscape metrics, calculated using the per pentad LULC categories considered suitable habitat for the bird. These analyses revealed that, between the first and second Southern African Bird Atlas Projects, the distribution range of Ludwig’s Bustard increased by 153 quarter-degree grid cells, whereas the reporting rate for the species declined by 5.6%, along with a small net decline in its quarterdegree-grid-cell-based area of occupancy, its pentad-based area of occupancy and habitat. The habitat of Ludwig’s Bustard is dominated by shrubland with limited grassland, in primarily the Nama Karoo and Succulent Karoo biomes, and the Grassy Karoo Biome in the eastern part of its distribution. In the relative abundance hot spot of Ludwig’s Bustard, there does not currently appear to be any significant loss or fragmentation (distance between patches) of its habitat. A landscape-based approach to conserving Ludwig’s Bustard can be achieved by expanding the current network of protected areas and conserving critical biodiversity areas and ecosystem support areas, especially in the bird’s relative abundance hot spots in the Nama Karoo and Succulent Karoo biomes.

Soil biological quality as affected by vegetation types in shrublands of a semi-arid montane environment

Mountain shrublands in semi-arid zones have a relevant effect in protecting water and soil, and conserving biodiversity. Nutrient availability in vegetation layers of semi-arid shrublands, may increase biomass production and litter accumulation rates that may affect the soil nutrient cycle through the positive feedback. However, the effect of semi-arid mountainous shrublands on litter quality and soil properties is poorly studied. To fill these gaps, we studied different vegetation types (i.e., Ulmus carpinifolia Gled., Berberis integerrima Bunge, Malus orientalis Ugl., Pistacia atlantica Desf., Amygdalus lycioides Spach., Rhus coriaria L., Cotoneaster kotschyi Klotz. and Juniperus excelsa M. Bieb.), to assess litter quality, soil properties, and C and N mineralization in Iran. Totally, 240 litter or organic horizon and topsoil (0–10 cm depth) samples were taken under the studied vegetation types. Based on analysis of variance and Tukey test, soil fauna (earthworms, acarina, collembola, nematode, protozoa), microflora (bacteria and fungi), microbial (basal and substrate-induced respirations, microbial biomass) and enzyme (urease, acid phosphatase, arylsulfatase and invertase) properties, as well as C and N mineralization were highest under deciduous species, especially U. carpinifolia vegetation type. Conversely, J. excelsa, as an evergreen species, decreased soil quality bio-indicators by decreasing litter quality and soil fertility in the high mountain vulnerable semi-arid sites. Based of heat plots of principal component analysis, soil biological and biochemical properties, especially enzyme indicators, created hot spots of soil quality under the studied vegetation types. As a conclusion, deciduous species are the hotspot's bioindicator in the mountainous ecosystems, indicating their potential role in enriching soil nutrient cycles.

Effects of Precipitation Variation on Annual and Winter Soil Respiration in a Semiarid Mountain Shrubland in Northern China

In response to global climate change, future precipitation changes are expected to profoundly influence soil respiration in arid and semiarid areas. However, few studies focus on CO2 emissions from soils undergoing precipitation changes in semiarid mountain shrublands in winter. A precipitation-manipulation experiment with three levels of precipitation (30% decreased precipitation (DP), ambient precipitation (AP), and 30% increased precipitation (IP)) was performed to examine the effects of variable precipitation on soil respiration (SR) and wintertime contributions to annual SR emissions in Vitex negundo var. heterophylla shrub ecosystems located on the Middle Taihang Mountain in Hebei Province, northern China. The results showed that the average annual SR rates and winter SR rates ranged from 1.37 to 1.67 μmol m−2 s−1 and 0.42 to 0.59 μmol m−2 s−1 among the different precipitation treatments. The model based on soil moisture better represented the soil-respiration rates, suggesting that the variable precipitation extended the water’s limitation of the soil’s CO2 emissions. The cumulative annual soil CO2 emissions were 523, 578, and 634 g C m−2 in response to the DP, AP, and IP treatments, respectively. The ratio of the soil CO2 emissions in winter to the annual CO2 emissions varied from 7.6 to 8.8% in response to the different precipitation treatments. Therefore, ignoring the soil CO2 emissions in winter leads to the underestimation of the carbon losses in semiarid shrublands. Our results highlight that variable precipitation significantly influences soil-respiration rates, and soil CO2 emissions in winter must not be ignored when predicting the future feedback between SR and climate change in semiarid regions.

Warming reduces both photosynthetic nutrient use efficiency and water use efficiency in Mediterranean shrubsWarming reduces nutrient use efficiency

The ratio between net photosynthetic rates and the foliar contents of essential plant macronutrients (N, P, K) is termed photosynthetic nutrient use efficiency (PNutUE). A universal trade-off exists whereby plants cannot maximize their PNutUE and their intrinsic water use efficiency (WUEi, carbon gain per unit water spent) simultaneously, because any increase in intercellular CO2 concentration (ci) through a greater stomatal opening would increase PNutE but also enhances transpiration and therefore decreases WUEi. Rising temperatures associated with climate change can result in large decreases in WUEi in semiarid shrubs through photosynthetic machinery impairment and enhanced stomatal conductance and transpiration, but we know remarkably little about the influence of warming and drought on PNutUE and its interplay with WUEi in dryland vegetation. Using a 6-year (2011–2017) manipulative field experiment, we examined the effects of warming (2.5ºC, W), rainfall reduction (30%, RR), and their combination (W+RR) on the photosynthetic use efficiency of three essential nutrients (PNUE, PPUE and PKUE) and on WUEi in three shrub species growing at two semiarid shrublands for the years 2015–2017. Across species, warming (W and W+RR) reduced PNUE by 42.9%, PPUE by 43.8% and PKUE by 41.5% on average relative to shrubs growing under ambient temperatures, whereas RR did not significantly affect their PNutUE. These drastic reductions in PNutUE with warming were mainly driven by non-stomatal and largely non-nutritional decreases in net photosynthetic rates, which were almost halved in warmed shrubs. The photosynthetic use efficiencies of N, P and K were inversely related to foliar δ13C, a proxy for time integrated WUEi, in both ambient (control and RR) and warmed (W and W+RR) shrubs, but with significantly smaller slopes and intercepts for warmed shrubs. Thus, plants achieve a smaller gain in PNutUE for any given increase in stomatal conductance (and reduction in WUEi) under warmer climatic conditions. The strong negative impact of warming on PNutE, along with the warming-induced shift in the trade-off between PNutUE and WUEi, could be indicative of an increasing inability of native plants to cope with warmer conditions, with dire implications for dryland vegetation productivity and survival under climate change.

Canopy greenness, atmospheric aridity, and large rain events jointly regulate evapotranspiration partitioning in a temperate semiarid shrubland

Canopy greenness, atmospheric aridity, and large rain events jointly regulate evapotranspiration partitioning in a temperate semiarid shrubland

Post-Fire Recovery of Plant Biodiversity Changes Depending on Time Intervals since Last Fire in Semiarid Shrublands

Fire is a key disturbance affecting plant biodiversity patterns and evolution. Although a wide range of studies have shown important impacts of fire on vegetation, most have focused on taxonomic diversity, with less emphasis on other aspects of biodiversity, such as functional and phylogenetic diversity. Therefore, we assessed the recovery of biodiversity facets across different times since the last fire in semiarid shrublands in Northeast Iran. We quantified changes in plant biodiversity facets, including taxonomic, functional, and phylogenetic diversity, and the diversity of seven functional traits in five ecologically comparable sites that have experienced wildfire disturbances at short-term (1 and 4 year sites) and long-term (10 and 20 year sites) intervals, in com- parison to an unburnt site. Our results showed significant changes in all biodiversity facets related to the year since the last fire, with a significant increase in biodiversity and diversity of functional traits under long-term rather than short-term conditions, and in comparison to the unburned site. We conclude that wildfire influences the presence of plant species with distant functional and evolutionary relatedness and causes an increase in plant species and diversity of functional traits de- pending on time intervals. Therefore, wildfire can promote positive effects on the recovery of bio- diversity aspects and the evolution of vegetation in semiarid shrublands.

Interspecific facilitation favors rare species establishment and reduces performance disparities among adults

AbstractQuestionsA variety of mechanisms sustain diversity in natural communities as a result of ecological interactions between organisms. Competition has been studied extensively in the context of species maintenance, but facilitation is often conceptualized as simply reducing competition between functionally different species, which tends to decline throughout the plants' life span. Here we explore how interspecific facilitation may sustain diversity throughout the species' life by avoiding the extinction of locally rare species at juvenile stages and reducing performance disparities between neighbors of differing species at mature stages.MethodsTo do so, we measured whether rarer species relied more on facilitation than abundant ones in semiarid shrubland in southeast Spain. A mechanistic explanation of this relationship was subsequently tested by correlating rarity with the species' affinity to a particularly edaphic stressful environment. Finally, we assessed whether growing associated with neighbors in vegetation patches shaped by facilitation could balance performance disparities between species when they become adults.ResultsWe show that facilitation (i) favors the rare species, which, in addition, tend to be those with low affinity to the stressful environment, and (ii) reduces the performance dissimilarities among plants growing associated within multispecific vegetation patches compared to plants growing alone.ConclusionsThese facilitative effects, beyond the reduction of competition between functionally similar species, might ensure positive and long‐lasting effects of biotic interactions, implying a more critical role for facilitation in preserving biodiversity than previously thought.

Fuel Connectivity, Burn Severity, and Seedbank Survivorship Drive Ecosystem Transformation in a Semiarid Shrubland

Fig. 1. One day, we arrived at what was supposed to be the first day of a week of field measurements for a study on long-term fire effects (Mahood & Balch 2019). To our dismay, a huge fire burned almost the entire study area that had just been contained. Photo credit: Adam Mahood. Fig. 2. Our original plan was not going to work. But we had just driven 15 hours to northern Nevada, so we had to do something. We decided to do a seed bank study, where we collected soil cores on either side of the fire perimeter, along a gradient of burn severity. (a) is a soil core, (b) is the greenhouse table where we germinated the seeds. (c) It was very obvious to both see where the edge of the fire was as well as visually assess the burn severity (pictured, C. Nick Whittemore). Photo credit: Adam Mahood. Fig. 3. We went back the following May to measure initial recovery (pictured, Dylan Murphy). Photo credit: Adam Mahood. Fig. 4. Many places had diverse flora in the spring following the fire. Photo credit: Adam Mahood. Fig. 5. We returned 3 years after the fire to measure recovery once the community stabilized. By then, many of the plots were dominated by cheatgrass. Photo credit: Adam Mahood. These photographs illustrate the article “Fuel connectivity, burn severity, and seedbank survivorship drive ecosystem transformation in a semiarid shrubland” by Adam L. Mahood, Michael J. Koontz, and Jennifer K. Balch published in Ecology. https://doi.org/10.1002/ecy.3968

Fuel connectivity, burn severity, and seed bank survivorship drive ecosystem transformation in a semiarid shrubland.

A key challenge in ecology is understanding how multiple drivers interact to precipitate persistent vegetation state changes. These state changes may be both precipitated and maintained by disturbances, but predicting whether the state change will be fleeting or persistent requires an understanding of the mechanisms by which disturbance affects the alternative communities. In the sagebrush shrublands of the western United States, widespread annual grass invasion has increased fuel connectivity, which increases the size and spatial contiguity of fires, leading to postfire monocultures of introduced annual grasses (IAG). The novel grassland state can be persistent and is more likely to promote large fires than the shrubland it replaced. But the mechanisms by which prefire invasion and fire occurrence are linked to higher postfire flammability are not fully understood. A natural experiment to explore these interactions presented itself when we arrived in northern Nevada immediately after a 50,000 ha wildfire was extinguished. We hypothesized that the novel grassland state is maintained via a reinforcing feedback where higher fuel connectivity increases burn severity, which subsequently increases postfire IAG dispersal, seed survivorship, and fuel connectivity. We used a Bayesian joint species distribution model and structural equation model framework to assess the strength of the support for each element in this feedback pathway. We found that prefire fuel connectivity increased burn severity and that higher burn severity had mostly positive effects on the occurrence of IAG and another nonnative species and mostly negative or neutral relationships with all other species. Finally, we found that the abundance of IAG seeds in the seed bank immediately after a fire had a positive effect on the fuel connectivity 3 years after the fire, completing a positive feedback promoting IAG. These results demonstrate that the strength of the positive feedback is controlled by measurable characteristics of ecosystem structure, composition, and disturbance. Further, each node in the loop is affected independently by multiple global change drivers. It is possible that these characteristics can be modeled to predict threshold behavior and inform management actions to mitigate or slow the establishment of the grass-fire cycle, perhaps via targeted restoration applications or prefire fuel treatments.

An artificial intelligence framework for predicting fire spread sustainability in semiarid shrublands

Background Fire behaviour simulation and prediction play a key role in supporting wildfire management and suppression activities. Aims Using machine-learning methods, the aim of this study was to predict the onset of fire propagation (go vs no-go) and type of fire behaviour (surface vs crown fire) in southern Australian semiarid shrublands. Methods Several machine-learning (ML) approaches were tested, including Support Vector Machine, Multinomial Naive Bayes and Multilayered Neural Networks, as was the use of augmented datasets developed with Generative Adversarial Networks (GAN) in classification of fire type. Key results Support Vector Machine was determined as the optimum machine learning classifier based on model overall accuracy against an independent evaluation dataset. This classifier correctly predicted fire spread sustainability and active crown fire propagation in 70 and 79% of the cases, respectively. The application of synthetically generated datasets in the Support Vector Machine model fitting process resulted in an improvement of model accuracy by 20% for the fire sustainability classification and 4% for the crown fire occurrence. Conclusions The selected ML modelling approach was shown to produce better results than logistic regression models when tested on independent datasets. Implications Artificial intelligence frameworks have a role in the development of predictive models of fire behaviour.

Vegetation Cover Estimation in Semi-Arid Shrublands after Prescribed Burning: Field-Ground and Drone Image Comparison

The use of drones for vegetation monitoring allows the acquisition of large amounts of high spatial resolution data in a simple and fast way. In this study, we evaluated the accuracy of vegetation cover estimation by drones in Mediterranean semi-arid shrublands (Sierra de Filabres; Almería; southern Spain) after prescribed burns (2 years). We compared drone-based vegetation cover estimates with those based on traditional vegetation sampling in ninety-six 1 m2 plots. We explored how this accuracy varies in different types of coverage (low-, moderate- and high-cover shrublands, and high-cover alfa grass steppe); as well as with diversity, plant richness, and topographic slope. The coverage estimated using a drone was strongly correlated with that obtained by vegetation sampling (R2 = 0.81). This estimate varied between cover classes, with the error rate being higher in low-cover shrublands, and lower in high-cover alfa grass steppe (normalized RMSE 33% vs. 9%). Diversity and slope did not affect the accuracy of the cover estimates, while errors were larger in plots with greater richness. These results suggest that in semi-arid environments, the drone might underestimate vegetation cover in low-cover shrublands.

Seasonal controlling factors of CO2 exchange in a semiarid shrubland in the Chihuahuan Desert, Mexico

The still significant uncertainties associated with the future capacity of terrestrial systems to mitigate climate change are linked to the lack of knowledge of the biotic and abiotic processes that regulate CO2 net ecosystem exchange (NEE) in space/time. Mainly, rates and controls of CO2 exchange from arid ecosystems, despite dominating the global trends in interannual variability of the terrestrial CO2 sink capacity, are probably the most poorly understood of all. We present a study on rates and controls of CO2 exchange measured with the eddy covariance (EC) technique in the Chihuahuan Desert in the Northeast of Mexico, to understand how the environmental controls of the NEE switch throughout the year using a multilevel approach. Since this is a water-limited ecosystem, the hydroecological year, based on the last precipitation and the decay of air temperature, was used to compare the wet (from May 16 to October 30, 2019) and dry (November 1, 2019 to May 15, 2020) seasons' controlling mechanisms, both at diurnal and nocturnal times. Annual NEE was −303.5 g C m−2, with a cumulative Reco of 537.7 g C m−2 and GPP of 841.3 g C m−2. NEE showed radiation, temperature, and soil moisture sensitivity along the day, however, shifts in these controls along the year and between seasons were identified. The winter precipitations during the dry season led to fast C release followed by lagged C uptake. Despite this flux pulse, the ecosystem was a net sink throughout most of the year because the local vegetation is well adapted to grow and uptake C under these arid conditions, even during the dry season. Understanding the controls of the sink-source shifts is relevant since the predictions for future climate include changes in the precipitation patterns.

Comparing climatic suitability and niche distances to explain populations responses to extreme climatic events

Habitat suitability calculated from species distribution models (SDMs) has been used to assess population performance, but empirical studies have provided weak or inconclusive support to this approach. Novel approaches measuring population distances to niche centroid and margin in environmental space have been recently proposed to explain population performance, particularly when populations experience exceptional environmental conditions that may place them outside of the species niche. Here, we use data of co‐occurring species' decay, gathered after an extreme drought event occurring in the southeast of the Iberian Peninsula which highly affected rich semiarid shrubland communities, to compare the relationship between population decay (mortality and remaining green canopy) and 1) distances between populations' location and species niche margin and centroid in the environmental space, and 2) climatic suitability estimated from frequently used SDMs (here MaxEnt) considering both the extreme climatic episode and the average reference climatic period before this. We found that both SDMs‐derived suitability and distances to species niche properly predict populations performance when considering the reference climatic period; but climatic suitability failed to predict performance considering the extreme climate period. In addition, while distance to niche margins accurately predict both mortality and remaining green canopy responses, centroid distances failed to explain mortality, suggesting that indexes containing information about the position to niche margin (inside or outside) are better to predict binary responses. We conclude that the location of populations in the environmental space is consistent with performance responses to extreme drought. Niche distances appear to be a more efficient approach than the use of climate suitability indices derived from more frequently used SDMs to explain population performance when dealing with environmental conditions that are located outside the species environmental niche. The use of this alternative metrics may be particularly useful when designing conservation measures to mitigate impacts of shifting environmental conditions.