Fire effects on the soil seed bank and post‐fire resilience of a semi‐arid shrubland in central Argentina

Soil seed bank is an important source of resilience of plant communities who suffered disturbances. We analysed the effect of an intense fire in the soil seed bank of a semi‐arid shrubland of Córdoba Argentina. We asked if the fire affected seed abundance, floristic and functional composition of the soil seed bank at two different layers (0–5 cm and 5–10 cm), and if fire could compromise the role of the soil seed bank as a source of resilience for the vegetation. We collected soil samples from a burned site and from a control site that had not burned. Samples were installed in a greenhouse under controlled conditions. During 12 months, we recorded all germinated seedlings. We compare soil seed bank with pre‐fire vegetation in terms of floristic and functional composition. The high‐intensity fire deeply affected the abundance of seeds in the soil, but it did not affect its floristic or functional composition. Floristic and functional composition of soil seed banks – at burned and unburned sites‐ differed markedly from that of the pre‐fire vegetation, although a previous study at the same site indicated high resilience after fire of this plant community. Our results indicate that resilience of this system is not strongly dependent on direct germination from seeds buried in the soil. Other sources of resilience, like colonization from neighbouring vegetation patches and resprouting from underground organs appear to gain relevance after an intense fire.     

Methods for tracking sagebrush‐steppe community trajectories and quantifying resilience in relation to disturbance and restoration

Understanding how plant community dynamics are impacted by altered disturbance regimes is a pressing challenge for restoration ecology. Most assessments of community dynamics involve computationally intensive statistical techniques, while management often defers to derived, qualitative “state‐and‐transition” models. Here, we demonstrate an intermediate approach to track and predict community resilience, diversifying the tools available to assess ecosystem change. First, we develop indices of sagebrush‐steppe community structure in permanent monitoring plots based on plant functional types and our conceptual understanding of the ecosystem. The indices define a bivariate space within which the trajectories of permanent monitoring plots can be tracked. Second, we quantify two metrics of community resilience: resistance (overall change during the time period) and stability (average amount of movement per monitoring period). Plots dominated by obligate seeder shrubs displayed low resilience relative to those dominated by grasses and forbs or resprouting shrubs. Resilience was strongly related to initial plant functional type composition and elevation. Our results suggest restoration objectives should consider how plant traits control ecosystem responses to disturbance. We suggest that the approach developed here can help assess longer‐term resilience, evaluate restoration success, and identify communities at risk of state transitions.     

Context‐dependent interactions between adult shrubs and seedlings in a semi‐arid shrubland

Abstract. Question: In semi‐arid systems, rainfall gradients can cause plant‐plant interactions to shift from negative to positive or vice versa. However, the importance of a second major abiotic factor, soil nutrients, has rarely been considered. We consider different combinations of both factors and ask: do net adult‐seedling interactions become less competitive and more facilitative with increasing overall abiotic harshness? Location: Succulent Karoo, Western Cape, South Africa. Methods: We examined the interactions between seedlings and adult shrubs at two sites. Sites differ in rainfall, and each contain two habitats: Nutrient‐rich mounds associated with underground termitaria and a relatively nutrient‐poor matrix. We carried out a spatial pattern analysis of community‐wide seedling‐adult associations. We then conducted field and greenhouse experiments to test the effects of soil and the presence of neighbouring shrubs on the growth and survival of six seedling species. Results: At the higher rainfall site, both competitive and facilitative effects of adults on seedlings were found but did not differ by habitat, despite the more benign conditions in the mound habitat. At the lower rainfall site, adult shrubs generally had neutral effects on seedlings in the matrix habitat. In the nutrient‐rich mound habitat, however, adult shrubs had strong and consistently competitive effects on seedlings. Conclusion: Seedling‐adult interactions could not be predicted by a simple overall gradient of abiotic harshness, demonstrating the need for more complex, mechanistic models to predict plant‐plant interactions. We suggest that rainfall and soil nutrients affect seedling‐adult relations through their interactive effects on the life‐history attributes of the species involved.     

Groundwater drawdown drives ecophysiological adjustments of woody vegetation in a semi‐arid coastal ecosystem

Predicted droughts and anthropogenic water use will increase groundwater lowering rates and intensify groundwater limitation, particularly for Mediterranean semi‐arid ecosystems. These hydrological changes may be expected to elicit differential functional responses of vegetation either belowground or aboveground. Yet, our ability to predict the impacts of groundwater changes on these ecosystems is still poor. Thus, we sought to better understand the impact of falling water table on the physiology of woody vegetation. We specifically ask (a) how is woody vegetation ecophysiological performance affected by water table depth during the dry season? and (b) does the vegetation response to increasing depth to groundwater differ among water‐use functional types? We examined a suite of physiological parameters and water‐uptake depths of the dominant, functionally distinct woody vegetation along a water‐table depth gradient in a Mediterranean semi‐arid coastal ecosystem that is currently experiencing anthropogenic groundwater extraction pressure. We found that groundwater drawdown did negatively affect the ecophysiological performance of the woody vegetation. Across all studied environmental factors, depth to groundwater was the most important driver of ecophysiological adjustments. Plant functional types, independent of groundwater dependence, showed consistent declines in water content and generally reduced C and N acquisition with increasing depths to groundwater. Functional types showed distinct operating physiological ranges, but common physiological sensitivity to greater water table depth. Thus, although differences in water‐source use exist, a physiological convergence appeared to happen among different functional types. These results strongly suggest that hydrological drought has an important impact on fundamental physiological processes, constraining the performance of woody vegetation under semi‐arid conditions. By disentangling the functional responses and vulnerability of woody vegetation to groundwater limitation, our study establishes the basis for predicting the physiological responses of woody vegetation in semi‐arid coastal ecosystems to groundwater drawdown.     

Time‐since‐fire and climate interact to affect the structural recovery of an Australian semi‐arid plant community

How does time‐since‐fire influence the structural recovery of semi‐arid, eucalypt‐dominated Murray‐Mallee shrublands after fire, and is recovery affected by spatial variation in climate? We assessed the structure and dynamics of a hummock grass, Triodia scariosa N.T. Burb, and mallee eucalypts – two key structural components of mallee shrublands – using a >100 year time‐since‐fire chronosequence. The relative influence of climatic variables, both individually and combined with time‐since‐fire, was modelled to account for spatial variation in the recovery of vegetation structural components. Time‐since‐fire was the primary determinant of the structural recovery of T. scariosa and eucalypts. However, climate, notably mean annual rainfall and rainfall variability, also influenced the recovery of the eucalypt overstorey, T. scariosa cover and mean hummock height. We observed that (i) the mean number of live eucalypt stems per individual decreased while mean individual basal area increased, (ii) cover of T. scariosa peaked at ~30 years post‐fire and gradually decreased thereafter, and (iii) the ‘hummock’ form of T. scariosa occurred throughout the chronosequence, whereas the ‘ring’ form tended not to occur until ~30 years post‐fire. Time‐since‐fire was the key determinant of the structural recovery of eucalypt‐dominated mallee shrublands, but there is geographical variation in recovery related to rainfall and its variability. Fire regimes are likely to have different effects across the geographic range of mallee shrublands.     

Drought and its legacy modulate the post‐fire recovery of soil functionality and microbial community structure in a Mediterranean shrubland

The effects of drought on soil dynamics after fire are poorly known, particularly its long‐term (i.e., years) legacy effects once rainfall returns to normal. Understanding this is particularly important for nutrient‐poor soils in semi‐arid regions affected by fire, in which rainfall is projected to decrease with climate change. Here, we studied the effects of post‐fire drought and its legacy on soil microbial community structure and functionality in a Cistus‐Erica shrubland (Spain). Rainfall total and patterns were experimentally modified to produce an unburned control (natural rainfall) and four burned treatments: control (natural rainfall), historical control (long‐term average rainfall), moderate drought (percentile 8 historical rainfall, 5 months of drought per year), and severe drought (percentile 2, 7 months of drought). Soil nutrients and microbial community composition (ester‐linked fatty acid approach) and functionality (enzyme activities and C mineralization rate) were monitored during the first 4 years after fire under rainfall treatments, plus two additional ones without them (six post‐fire years). We found that the recovery of burned soils was lower under drought. Post‐fire drought increased nitrate in the short term and reduced available phosphorus, exchangeable potassium, soil organic matter, enzyme activities, and carbon mineralization rate. Moreover, drought decreased soil total microbial biomass and fungi, with bacteria becoming relatively more abundant. Two years after discontinuing the drought treatments, the drought legacy was significant for available phosphorus and enzyme activities. Although microbial biomass did not show any drought legacy effect, the proportion of fungi and bacteria (mainly gram‐positive) did, being lower and higher, respectively, in former drought‐treated plots. We show that drought has an important impact on soil processes, and that some of its effects persist for at least 2 years after the drought ended. Therefore, drought and its legacy effects can be important for modeling biogeochemical processes in burned soils under future climate change.     

The effect of plant litter on ecosystem properties in a Mediterranean semi‐arid shrubland

Abstract. We studied the distribution of litter in a shrubland of the Negev with a semi‐arid Mediterranean climate of less than 200 mm of rainfall per year. Our focus was on the effects of litter on properties of landscape patches relevant to ecosystem processes (water runoff and soil erosion), annual plant community responses (seedling density, biomass production and species richness), and animal activity (soil disturbance by termites). Three 60‐m transects, extending across a pair of opposing north‐ and south‐facing slopes and their drainage channel, showed that litter accumulates not only under shrubs, but to a lesser extent also on the crusted inter‐shrub open areas. We used 35 experimental units (‘cells’, 0.5m × 1 m), each containing a crust and a shrub patch. Because runoff flows from crusted patches and is intercepted by shrub patches, the latter were in the lower third of the cells. Leaf litter was added in single and double amounts providing ca. 0.5 and 1.0 cm litter depth, to either, both, or none of the patches. Litter addition significantly decreased the amount of runoff, regardless of the location and amount of litter applied. Litter on the crust increased species number and seedling density of species with low abundance. Adding a double litter layer increased annual plant biomass production, while a single amount had no effect. Litter addition to the shrub patch affected neither biomass nor species richness. Litter addition to both patches at both quantities caused a large increase in termite activity. Termites caused disturbance by disrupting the crust, which may contribute to the reduction in runoff amounts. In the open, flat crust patches, annual plant communities are limited in their productivity and species richness, as there are few structures stopping the outflow of water, soil and seeds. Litter adds such structures, but affects the plant communities only when added to litter‐free crust. Litter accumulation and its patchy distribution have large impacts on landscape patch properties affecting resource distribution, plant productivity and diversity, and animal activity. Therefore, understanding litter distribution in relation to the patchy structure of the landscape of semi‐arid shrubland should be viewed as an important component of shrubland management.     

Fire resilience of a rare,freshwater crustacean in a fire‐prone ecosystem and the implications for fire management

Understanding species’ responses to fire regimes, particularly rare or threatened species, is important for land managers tasked with managing for biodiversity. Hickman's Allanaspides (Allanaspides hickmani, Anaspidesidae) is a rare, primitive, shrimp‐like crustacean, with high conservation value. It is restricted to a single catchment in the island state of Tasmania, Australia, where it occurs within moorland pools typically containing crayfish (Ombrastacoides spp.) burrows. Although its moorland habitat has a long history of firing, adverse fire regimes are a potential threat to the species. A large part of its range is subject to planned burning to help manage the detrimental effects of high‐intensity wildfires. The resilience of A. hickmani to low–moderate‐intensity fires was investigated over 13 years using a replicated before‐after‐control‐impact design. The fires resulted in an initial reduction in vegetation cover and surface water and an increase in water temperature. There was no effect of fire on A. hickmani captures 4 months after small‐scale, low‐intensity autumn burns. However, 5 months later, following an unintended larger‐scale, medium‐intensity spring burn, there was an 80–90% reduction in A. hickmani captures and their numbers did not recover until 6–9 years post‐fire. It is not known whether the reduced catch was due to a reduction in the number of A. hickmani or their movement from pools into crayfish burrows. These findings together with evidence of a varied fire history, including high‐intensity wildfires, within their range suggests that A. hickmani and its habitat are resilient to a range of fire frequencies and intensities provided that the fire regime does not degrade or lead to a complete loss of peat. Climate change predictions for warmer and drier summers in western Tasmania will increase the risk of peat loss. Planned burning is likely to be important for the protection of A. hickmani habitat from predicted adverse fire regimes.     

How to tell a shrub from a tree: A life‐history perspective from a South African savanna

The ecological differences between ‘shrubs’ and ‘trees’ are surprisingly poorly understood and clear ecological definitions of these two constructs do not exist. It is not clear whether a shrub is simply a small tree or whether shrubs represent a distinct life‐history strategy. This question is of special interest in African savannas, where shrubs and trees often co‐dominate, but are often treated uniformly as ‘woody plants’ even though the tree to shrub ratio is an important determinant of ecosystem functioning. In this study we use data from a long‐term fire experiment, together with a trait‐based approach to test (i) if woody species usually classified as shrubs or trees in African savanna differ in key traits related to disturbance and resource use; and (ii) if these differences justify the interpretation of the two growth forms as distinct life‐history strategies. We measured for 22 of the most common woody plant species of a South African savanna 27 plant traits related to plant architecture, life‐history, leaf characteristics, photosynthesis and resprouting capacity. Furthermore we evaluated their performance during a long‐term fire experiment. We found that woody plants authors call (i) shrubs; (ii) shrubs sometimes small trees; and (3) trees responded differently to long‐term fire treatments. We additionally found significant differences in architecture, diameter‐height‐allometry, foliage density, resprouting vigour after fire, minimum fruiting height and foliar δ¹³C between these three woody plant types. We interpret these findings as evidence for at least two different life‐history‐strategies: an avoidance/adaptation strategy for shrubs (early reproduction + adaptation to minor disturbance) and an escape strategy for trees (promoted investment in height growth + delayed reproduction).     

Functional groups of woody species in semi‐arid regions at low latitudes

In seasonally dry environments, woody species have different survival strategies. However, little is known about how environmental variables affect the phenology and water dynamics of these species. We aim to understand which variables initiate the vegetative phenophases of species in a tropical semiarid climate at 3°S latitude, where variation in photoperiod is minimal and rainfall is seasonal. We hypothesize that groups of species with similar vegetative phenologies, under similar conditions, are functionally similar in terms of water storage and use. We analyse the relationship between functional characteristics related to the acquisition and utilization of water, such as wood density, water storage capacity, water potential and vegetative phenology. The attributes were ordered by multidimensional scaling, and a multiple response permutation procedure was used to test consistency of the groups. Canonical correspondence analysis and Mantel tests were used to evaluate the phenophase response to environmental variables. We found four functional groups: (i) deciduous low wood density, which lose 75% of their leaves one month before the end of the rains; (ii) evergreen high wood density; (iii) early deciduous high wood density, which lose 75% of their leaves one month after the end of the rains; and (iv) late deciduous high wood density, which lose 75% of their leaves two months after the end of the rains. As expected, the vegetative phenodynamics of the deciduous high wood density group were mainly influenced by water availability. The evergreens did not show a correlation with rainfall. Only leaf shedding of the late deciduous, and the vegetative phenophases of the evergreens, responded to an increase in temperature and photoperiod. Bud‐break responded to increased photoperiod and soil humidity in the deciduous low wood density group. The foliar periodicity groups can be explained by the presence of species that differ mainly in their mechanisms of water acquisition and use.     

Response of a rare herb (Trioncinia retroflexa) from semi‐arid tropical grassland to occasional fire and grazing

Trioncinia retroflexa (Asteraceae) is a rare perennial herb endemic to Dichanthium‐dominated grasslands on basaltic soils in central Queensland. Trioncinia lives for approximately 5 years and some seed remains viable in the soil for at least 18 months. Population densities increased sixfold over 4 years, coinciding with recovery during a period of above‐average rainfall after a 4‐year drought preceding this study. The species’ lifespan, which can exceed 5 years, coupled with its extended seed‐bank viability ensures that populations can endure drought despite low levels of seedling recruitment. The restricted occurrence of the species to spasmodically grazed stock routes provides evidence that Trioncinia is sensitive to the continuous grazing that typically occurs in paddocks. The hypothesis that the species requires occasional disturbance for survival was experimentally tested. Recruitment was depressed with burning and inflorescence production enhanced with both burning and simulated intense spasmodic grazing after the first application of treatments. However, these effects were not evident after a second application of the treatments and there were no significant effects of treatment on population densities at the completion of the 4‐year experiment. Although not requisite, sporadic dry‐season grazing and burning of these areas would appear to be compatible with the survival of Trioncinia. This project highlights the value of stock routes for rare plant conservation.     

The seasonal dynamic of ant‐flower networks in a semi‐arid tropical environment

1. Several studies have recently focused on the structure of ecological networks involving ants and plants with extrafloral nectaries; however, little is known about the effects of temporal variation in resource abundance on the structure of ant–plant networks mediated by floral nectar. 2. In this study, it was evaluated how strong seasonality in resource availability in a semi‐arid tropical environment affects the structure of ant–flower networks. We recorded ants collecting floral nectar during two seasons (from December 2009 to January 2013): dry and green seasons. Then, we built interaction networks for flower‐visiting ants in the Brazilian Caatinga separately for each combination of transect and season. 3. In general, strong seasonality directly influenced patterns of ant–flower interactions and the overall complexity of these ecological networks. During the dry season, networks were more connected, less modular, and exhibited greater niche overlap of flower‐visiting ants than during the green season. Moreover, resource utilisation by ants during the dry season tended to be more aggregated. These findings indicate that during the dry season, ant species tended to share many resource bases, probably owing to lower overall resource availability during this season. Species composition of the ant network component was highly season specific; however, a central core of highly generalised ants was present during both seasons. 4. The stability of this central core between seasons could strongly affect the ecological and evolutionary dynamics of these interaction networks. This study contributes to the understanding of the structure and dynamics of ant‐flower interactions in extremely seasonal environments.     

Effects of long‐term fire exclusion and frequent fire on plant community composition: A case study from semi‐arid shrublands

Time since last fire and fire frequency are strong determinants of plant community composition in fire‐prone landscapes. Our study aimed to establish the influence of time since last fire and fire frequency on plant community composition and diversity of a south‐west Australian semi‐arid shrubland. We employed a space‐for‐time approach using four fire age classes: ‘young’, 8–15 years since last fire; ‘medium’, 16–34; ‘old’, 35–50; and ‘very old’, 51–100; and three fire frequency classes: burnt once, twice and three times within the last 50 years. Species diversity was compared using one‐way ANOVA and species composition using PERMANOVA. Soil and climatic variables were included as covariables to partition underlying environmental drivers. We found that time since last fire influenced species richness, diversity and composition. Specifically, we recorded a late successional transition from woody seeders to long‐lived, arid‐zone, resprouting shrub species. Fire frequency did not influence species richness and diversity but did influence species composition via a reduction in cover of longer‐lived resprouter species – presumably because of a reduced ability to replenish epicormic buds and/or sufficient starch stores. The distinct floristic composition of old and very old habitat, and the vulnerability of these areas to wildfires, indicate that these areas are ecologically important and management should seek to preserve them.     

Shift in community structure in an early‐successional Mediterranean shrubland driven by long‐term experimental warming and drought and natural extreme droughts

Global warming and recurring drought are expected to accelerate water limitation for plant communities in semiarid Mediterranean ecosystems and produce directional shifts in structure and composition that are not easily detected, and supporting evidence is scarce. We conducted a long‐term (17 years) nocturnal‐warming (+0.6°C) and drought (?40% rainfall) experiments in an early‐successional Mediterranean shrubland to study the changes in community structure and composition, contrasting functional groups and dominant species, and the superimposed effects of natural extreme drought. Species richness decreased in both the warming and drought treatments. Responses to the moderate warming were associated with decreases in herb abundance, and responses to the drought were associated with decreases in both herb and shrub abundances. The drought also significantly decreased community diversity and evenness. Changes in abundance differed between herbs (decreases) and shrubs (increases or no changes). Both warming and drought, especially drought, increased the relative species richness and abundance of shrubs, favoring the establishment of shrubs. Both warming and drought produced significant shifts in plant community composition. Experimental warming shifted the community composition from Erica multiflora toward Rosmarinus officinalis, and drought consistently shifted the composition toward Globularia alypum. The responses in biodiversity (e.g., community biodiversity, changes of functional groups and compositional shifts) were also strongly correlated with atmospheric drought (SPEI) in winter–spring and/or summer, indicating sensitivity to water limitation in this early‐successional Mediterranean ecosystem, especially to natural extreme droughts. Our results suggest that the shifts in species assembles and community diversity and composition are accelerated by the long‐term nocturnal‐warming and drought, combined with natural severe droughts, and that the magnitude of the impacts of climate change is also correlated with the successional status of ecosystem. The results thus highlight the necessity for assessing the impacts on ecosystemic functioning and services and developing effective measures for conserving biodiversity.