Microhabitat heterogeneity promotes soil fertility and ground-dwelling arthropod diversity in Mediterranean wood-pastures

P. 192-201Mediterranean wood-pastures are extensive agroforestry systems that hold great ecological, social and cultural values, which consist of mixtures of grassland, scattered trees and shrubs, primarily used for livestock grazing. For centuries, low-intensity management in these landscapes has resulted in multiple microhabitats that favour biodiversity and are considered key elements for the long-term persistence of wood-pastures. However, the relative contribution of each microhabitat type to wood-pasture biodiversity and functioning remains poorly studied. We investigated the impact that different wood-pasture microhabitats have on soil chemical fertility and the decomposition food web. We analysed the two main microhabitats that make wood-pastures up – open grasslands and isolated trees – and three additional “litter-trapping” microhabitats – shrubs scattered in the grassland matrix, canopied shrubs and piles of pruning debris—in terms of soil chemical properties (organic matter content, total N, C:N ratio, available P, and exchangeable base cations), collembolan and dipteran (mostly detritivores) abundance, and carabid (seed-eaters and predators) and staphylinid (mostly predators) beetle abundance, body size, biomass, species richness and composition. Grasslands were the most different microhabitats, with the lowest soil nutrient content and particular carabid and staphylinid species composition. Trees had the highest soil fertility levels and abundance of Diptera and staphylinids, and held unique staphylinid assemblages. “Litter-trapping” microhabitats had medium to high soil nutrient values and shared a distinct staphylinid assemblage compared to grasslands and trees. Besides, scattered shrubs provided shelter for large-sized carabid and staphylinid predators, while canopied shrubs held the highest carabid abundance and biomass. “Litter-trapping” microhabitats retained tree leaves that would otherwise be lost to the wood-pasture, recovering nutrients to the system, and provided new habitat, shelter and food for detritivores and unique predator assemblages. “Litter-trapping” microhabitats thus enhanced soil fertility and the decomposition process, at the same time as increased the abundance and diversity of the communities of ground-dwelling detritivores and predators inhabiting wood-pastures. These findings confirmed that microhabitat heterogeneity resulting from low-intensity management is essential to maintain both the primary production and the biodiversity conservation value of wood-pastures.S

Disruption of trophic interactions involving the heather beetle by atmospheric nitrogen deposition

P. 436-445Elevated nitrogen (N) deposition impacts the structure and functioning of heathland ecosystems across Europe. Calluna plants under high N-inputs are very sensitive to secondary stress factors, including defoliation attacks by the heather beetle. These attacks result in serious damage or death of Calluna, its rapid replacement by grasses, and the subsequent loss of heathland. We know very little about the mechanisms that control the populations and trigger outbreaks of the heather beetle, impeding proper management measures to mitigate the damage. We investigated the effects of N deposition on the relationships between the heather beetle, its host plant, and two arthropod predators at building (rejuvenated through fire) and mature heathlands. The study combines field manipulation experiments simulating a range of N deposition rates (0, 1, 2, 5 g N m−2 year−1 for 2 years, and 5.6 g N m−2 year−1 for 10 years), and food-choice laboratory experiments testing the preferences of adults and larvae of the heather beetle for N-treated Calluna plants, and the preferences of predators for larvae grown on plants with different N-content. The larvae of the heather beetle achieved the highest abundances after the long-term (10-year) addition of N at mature Calluna plots in the field. Contrary to the adults, the larvae foraged preferentially on the most N-rich Calluna shoots under laboratory conditions. Predators showed no aggregative numerical responses to the accumulation of heather beetle larvae at high N-input experimental plots. During the feeding trials, predators consumed a small number of larvae, both in total and per individual, and systematically avoided eating the larvae reared on high-N Calluna shoots. Our study showed that the most severe defoliation damage by the heather beetle is inflicted at the larval stage under prolonged availability of high-N inputs, and that arthropod predators might not act as effective regulators of the beetle's populations.S

Provenance and seed mass determine seed tolerance 5 to high temperatures associated to forest fires in Pinus pinaster

P. 381-391We show that Pinus pinaster provenance affects both seed germination and seedling recruitment after fire. This information is crucial for managers to select the best seed-provisioning populations when implementing reforestation programs to assist the natural post-fire regeneration of the speciesS

Plant and vegetation functional responses to cumulative high nitrogen deposition in rear-edge heathlands

P. 980-990Elevated atmospheric nitrogen (N) deposition is a major driver of change, altering the structure/functioning of nutrient-poor Calluna vulgaris-heathlands over Europe. These effects amply proven for north-western/central heathlands may, however, vary across the ecosystem's distribution, especially at the range limits, as heathlands are highly vulnerable to land-use changes combined with present climate change. This is an often overlooked and greatly understudied aspect of the ecology of heathlands facing global change. We investigated the effects of five N-fertilisation treatments simulating a range of N deposition rates (0, 10, 20, and 50 kg N ha−1 yr−1 for 1 year; and 56 kg N ha−1 yr−1 for 9 years) on the Calluna-plants, the plant functional groups, species composition and richness of two life-cycle stages (building/young- and mature-phase) of Calluna-heathlands at their rear-edge limit. Our findings revealed a dose-related response of the shoot length and number of flowers of young and mature Calluna-plants to the addition of N, adhering to the findings from other heathland locations. However, cumulative high-N loading reduced the annual growth and flowering of young plants, showing early signs of N saturation. The different plant functional groups showed contrasting responses to the cumulative addition of N: annual/perennial forbs and annual graminoids increased with quite low values; perennial graminoids were rather abundant in young heathlands but only slightly augmented in mature ones; while bryophytes and lichens strongly declined at the two heathland life-cycle stages. Meanwhile there were no significant N-driven changes in plant species composition and richness. Our results demonstrated that Calluna-heathlands at their low-latitude distribution limit are moderately resistant to cumulative high-N loading. As north-western/central European heathlands under high-N inputs broadly experienced the loss of plant diversity and pronounced changes in plant species dominance, rear-edge locations may be of critical importance to unravel the mechanisms of heathland resilience to future global change.S

Interactions between large high-severity fires and salvage logging on a short return interval reduce the regrowth of fire-prone serotinous forests

P. 54-63New fire disturbance regimes under accelerating global environmental change can have unprecedented consequences for ecosystem resilience, lessening ecosystem natural regeneration. In the Mediterranean Basin, fire-dependent obligate seeder forests that are prone to increasingly frequent stand-replacing fires and then salvaged logged repeatedly can be vulnerable to additional disturbances for decades. In this study, we investigated, for the first time, the cumulative and interactive effects of two large high-severity fires at a short (<15-year) return interval and the subsequent burned timber harvesting with biomass removal on the post-disturbance recovery of such forests. We further assessed the type and amount of the material legacies (deadwood) that persisted through the different post-disturbance successional trajectories, as well as the influence of these legacies on forest regeneration. The early recovery of the studied forests after two consecutive large fires and post-fire logging was, in the first place, driven by fire repetition, which led to reduced seedling recruitment and enhanced regrowth of resprouter shrubs. Despite no interactive effects between fire and logging were detected after a single large fire event, two repeated fires followed by salvage harvesting had a greater negative impact than two fires alone (synergistic effects) on seedling establishment; while a lower positive impact (subadditive effects) on the recovery of resprouter shrubs. There was also an interaction modification effect in which fire repetition worsened the per-unit impact of salvage logging on forest regeneration. Nonetheless, the residual legacies, i.e., fine and coarse woody debris (unburned needles, downed branches, pieces of deadwood, and burned pine cones) that remained after the manual harvesting of the burned trees, aided seedling re-establishment and hindered the regrowth of the shrubby understorey. These findings indicate that high-intensity salvage logging after two large high-severity fires at a short return interval is inadvisable in fire-prone serotinous pine forests, unless it explicitly retains the key material legacies that help tree natural regeneration and enhance ecosystem resilience to the next disturbance.S

Time- and age-related effects of experimentally simulated nitrogen deposition on the functioning of montane heathland ecosystems

P. 149-159Ecosystems adapted to low nitrogen (N) conditions such as Calluna-heathlands are especially sensitive to enhanced atmospheric N deposition that affects many aspects of ecosystem functioning like nutrient cycling, soil properties and plant-microbial-enzyme relationships. We investigated the effects of five levels of experimentally- simulated N deposition rates (i.e., N fertilization treatments: 0, 10, 20 and 50 kg N ha− 1 yr− 1 for 3 years, and 56 kg N ha− 1 yr− 1 for 10 years) on: plant, litter, microbial biomass and soil nutrient contents, soil extracellular enzymatic activities, and plant root ericoid mycorrhizal colonization. The study was conducted in marginal montane Calluna-heathlands at different developmental stages resulting from management (young/building-phase and mature-phase). Our findings revealed that many soil properties did not show a statistically significant response to the experimental addition of N, including: total N, organic carbon (C), C:N ratio, extractable N-NO3 −, available phosphorus (P), urease and β-glucosidase enzyme activities, and microbial biomass C and N. Our results also evidenced a considerable positive impact of chronic (10-year) high-N loading on soil extractable N-NH4 +, acid phosphatase enzyme activity, Calluna root mycorrhizal colonization by ericoid fungi, Calluna shoot N and P contents, and litter N content and N:P ratio. The age of heathland vegetation influenced the effects of N addition on ericoid mycorrhizal colonization, resulting in higher colonized roots in young heathlands at the control, low and medium N-input rates; and in mature ones at the high and chronically high N rates. Also, young heathlands exhibited greater soil extractable N-NO3 −, available P, microbial biomass N, Calluna shoot N and P contents, and litter N content, compared to mature ones. Our results highlighted that accounting for the N-input load and duration, as well as the developmental stage of the vegetation, is important for assessing the effects of added N, particularly at the heathlands' southern distribution limit.S

Remote Sensing Applied to the Study of Fire Regime Attributes and Their Influence on Post-Fire Greenness Recovery in Pine Ecosystems

P. 1-18We aimed to analyze the relationship between fire regime attributes and the post-fire greenness recovery of fire-prone pine ecosystems over the short (2-year) and medium (5-year) term after a large wildfire, using both a single and a combined fire regime attribute approach. We characterized the spatial (fire size), temporal (number of fires, fire recurrence, and return interval), and magnitude (burn severity of the last fire) fire regime attributes throughout a 40-year period with a long-time series of Landsat imagery and ancillary data. The burn severity of the last fire was measured by the dNBR (difference of the Normalized Burn Ratio) spectral index, and classified according to the ground reference values of the CBI (Composite Burn Index). Post-fire greenness recovery was obtained through the difference of the NDVI (Normalized Difference Vegetation Index) between pre- and post-fire Landsat scenes. The relationship between fire regime attributes (single attributes: fire recurrence, fire return interval, and burn severity; combined attributes: fire recurrence-burn severity and fire return interval-burn severity) and post-fire greenness recovery was evaluated using linear models. The results indicated that all the single and combined attributes significantly affected greenness recovery. The single attribute approach showed that high recurrence, short return interval and low severity situations had the highest vegetation greenness recovery. The combined attribute approach allowed us to identify a wider variety of post-fire greenness recovery situations than the single attribute one. Over the short term, high recurrence as well as short return interval scenarios showed the best post-fire greenness recovery independently of burn severity, while over the medium term, high recurrence combined with low severity was the most recovered scenario. This novel combined attribute approach (temporal plus magnitude) could be of great value to forest managers in the development of post-fire restoration strategies to promote vegetation recovery in fire-prone pine ecosystems in the Mediterranean Basin under complex fire regime scenariosS

Impact of burn severity on soil properties in a Pinus pinaster ecosystem immediately after fire

P. 1-11We analyse the effects of burn severity on individual soil properties and soil quotients in Mediterranean fire-prone pine forests immediately after a wildfire. Burn severity was measured in the field through the substrate stratum of the Composite Burn Index and soil samples were taken 7–9 days after a wildfire occurred in a Pinus pinaster Ait. ecosystem. In each soil sample, we analysed physical (size of soil aggregates), chemical (pH, organic C, total N and available P) and biological (microbial biomass C, b-glucosidase, urease and acid phosphatase activities) properties. Size of aggregates decreased in the areas affected by high burn severity. Additionally, moderate and high severities were associated with increases in pH and available P concentration and with decreases in organic C concentration. Microbial biomass C showed similar patterns to organic C along the burn severity gradient. The enzymatic activities of phosphatase and b-glucosidase showed the highest sensitivity to burn severity, as they strongly decreased from the low-severity scenarios. Among the studied soil quotients, the C : N ratio, microbial quotient and b-glucosidase : microbial biomass C quotient decreased with burn severity. This work provides valuable information on the impact of burn severity on the functioning of sandy siliceous soils in fire-prone pine ecosystems.S

Fire recurrence and emergency post-fire management influence seedling recruitment and growth by altering plant interactions in fire-prone ecosystems

P. 63-75Projections of future wildfire regimes forecast an increased frequency of large high-severity fires that create very harsh environmental conditions and constitute a challenge to post-fire ecosystem regeneration. Under these new circumstances, better knowledge of the plant interaction mechanisms underlying post-fire seedling establishment success would aid restoration management to achieve the intended targets. We evaluated the combined effect of recurrent large stand-replacing fires and conventional post-fire restoration activities (salvage logging after a single large fire, and direct seeding and linear subsoiling plus seedling planting after two subsequent large fires) on tree seedling recruitment and performance (development, annual growth, and biomass) in the early stages of succession in fire-prone maritime pine (Pinus pinaster Ait.) ecosystems. We quantified plant facilitative/competitive interactions between naturally recruited pine seedlings, neighbouring seedlings and potential nurse shrubs with different post-fire regeneration strategies (obligate seeders vs resprouters), by computing the relative interaction index (RII). The results evidenced that fire recurrence altered plant species composition and conditioned initial pine seedling recruitment and establishment, prevailing over the expected negative impact of salvage logging and positive impact of seeding. Seedling recruitment was sufficient to ensure natural tree regeneration after a single fire event and undermined by repeated fires. Both delaying burned timber removal during salvage logging operations and retaining immature dead trees without commercial value onsite in subsoiled stands enhanced seedling recruitment via facilitative interactions. Higher seedling growth and height under shrubs than in open ground resulted in lower aerial and root biomass, indicating elongation in response to shade, and net competition for resources. Inter-specific competition between naturally regenerated seedlings and shrubs was aggravated by intra-specific competition with neighbouring seedlings and by mechanical site preparation in subsoiled stands. All in all, post-burn increased soil fertility most likely counterbalanced the environmental stress created by fire, shifting the net outcome of plant interactions from positive (facilitation) to negative (competition). We recommend alternative post-fire management actions that decrease plant competition and take advantage of facilitation by residual burned wood, to ultimately accelerate ecosystem recovery after large stand-replacing fires.S

Assessment of the influence of biophysical properties related to fuel conditions on fire severity using remote sensing techniques: a case study on a large fire in NW Spain

P. 512-520This study analyses the suitability of remote sensing data from different sources (Landsat 7 ETM+, MODIS and Meteosat) in evaluating the effect of fuel conditions on fire severity, using a megafire (11 891 ha) that occurred in a Mediterranean pine forest ecosystem (NW Spain) between 19 and 22 August 2012. Fire severity was measured via the delta Normalized Burn Ratio index. Fuel conditions were evaluated through biophysical variables of: (i) the Visible Atmospherically Resistant Index and mean actual evapotranspiration, as proxies of potential live fuel amount; and (ii) Land Surface Temperature and water deficit, as proxies of fuel moisture content. Relationships between fuel conditions and fire severity were evaluated using Random Forest models. Biophysical variables explained 40% of the variance. The Visible Atmospherically Resistant Index was the most important predictor, being positively associated with fire severity. Evapotranspiration also positively influenced severity, although its importance was conditioned by the data source. Live fuel amount, rather than fuel moisture content, primarily affected fire severity. Nevertheless, an increase in water deficit and land surface temperature was generally associated with greater fire severity. This study highlights that fuel conditions largely determine fire severity, providing useful information for defining pre-fire actions aimed at reducing fire effects