The impact of land use on alien species incidence and number in local assemblages worldwide

While the regional distribution of non-native species is increasingly well documented for some taxa, global analyses of non-native species in local assemblages are still missing. Here, we use a worldwide collection of assemblages from five taxa - ants, birds, mammals, spiders and vascular plants - to assess whether the incidence, frequency and proportions of naturalised non-native species depend on type and intensity of land use. In plants, assemblages of primary vegetation are least invaded. In the other taxa, primary vegetation is among the least invaded land-use types, but one or several other types have equally low levels of occurrence, frequency and proportions of non-native species. High land use intensity is associated with higher non-native incidence and frequency in primary vegetation, while intensity effects are inconsistent for other land-use types. These findings highlight the potential dual role of unused primary vegetation in preserving native biodiversity and in conferring resistance against biological invasions

Numerical reconstruction of the spatial component in the source term of a time-fractional diffusion equation

In this article, we are concerned with the analysis on the numerical reconstruction of the spatial component in the source term of a time-fractional diffusion equation. This ill-posed problem is solved through a stabilized nonlinear minimization system by an appropriately selected Tikhonov regularization. The existence and the stability of the optimization system are demonstrated. The nonlinear optimization problem is approximated by a fully discrete scheme, whose convergence is established under a novel result verified in this study that the $H^1$-norm of the solution to the discrete forward system is uniformly bounded. The iterative thresholding algorithm is proposed to solve the discrete minimization, and several numerical experiments are presented to show the efficiency and the accuracy of the algorithm.Comment: 17 pages, 2 figures, 2 table

Stem mortality and forest Dieback in a 20-years experimental Drought in a Mediterranean holm oak forest

Climatic warming is predicted to increase the intensity and duration of extreme weather such as droughts and heat waves. Climate change could therefore increase stem mortality and forest dieback in many ecosystems around the world, especially in semi-arid forests. We investigated the influence of climatic conditions and a throughfall displacement experimental treatment (15% decrease in the amount of soil moisture) on the intensity of stem mortality in a Mediterranean forest. We also investigated the use of remotely sensed data from MODIS as a tool to estimate the intensity of stem mortality and a possible strategy of forest management to mitigate this forest dieback induced by climate change. Stem mortality was higher when mean annual temperature was higher and rainfall was lower, especially during spring and summer. Mortality was higher and more affected by the drought treatment in the holm oak, Quercus ilex, than in co-occurring species of tall shrubs better adapted to drier conditions. Two spectral MODIS indices (NDVI and EVI) were good predictors of stem mortality, but others (GPP, PsnNet, and NPP specifically calculated to predict forest health and development) were not clearly correlated with stem mortality. Selective stem thinning strongly reduced stem mortality (especially in Q. ilex) by buffering the effects of climate change on forest structure. Finally, the possible future substitution of the current dominant species of this forest, Q. ilex, by species of tall shrubs better adapted to drought is also discusse

Efectes de l'estassada en un alzinar amb baixa disponibilitat d'aigua

El document és una contribució a les "V Jornades sobre el Bosc de Poblet i les Muntanyes de Prades" que van tenir lloc el 16 i 17 de novembre de 2018 al Museu de la Vida Rural (l'Espluga de Francolí, Tarragona)". Podeu consultar-ne les actes, publicades sota una llicència CC BY-NC-ND, a http://parcsnaturals.gencat.cat/web/.content/home/poblet/coneix-nos/centre_de_documentacio/fons_documental/5a_jornades/V-Jornades_Poblet_Prades.pdfEn un alzinar natural s'ha realitzat una estassada selectiva per a estudiar el seu ús com a eina per a reduir la mortalitat d'arbres, l'augment de la productivitat forestal, i invertir la regressió recent que està patint l'espècie dominant (l'alzina, Quercus ilex) degut al canvi climàtic. Durant els 4 anys estudiats, l'estassada ha comportat un augment de la productivitat i una disminució de la mortalitat de troncs en aquest alzinar, contribuint al manteniment del funcionament de l'ecosistema sota les condicions climàtiques canviants. Q. ilex ha estat l'única espècie que ha experimentat un major creixement dels troncs, independentment del seu tamany, i una disminució de la mortalitat degut a l'estassada selectiva. En canvi, el creixement i la mortalitat de troncs no ha variat significativament en les altres espècies arbustives, més resistents a la sequera que Q. ilex. Els nostres resultats posen de manifest la utilitat de l'estassada selectiva com a eina per a augmentar la productivitat forestal i contribuir a la conservació d'aquest bosc mediterrani. Altres beneficis de l'aclarida selectiva, com la disminució del risc d'incendi forestal i l'increment de la disponibilitat d'aigua per a la població, també han estat discutits en aquest trebal

Effects of thinning in a water-limited holm oak forest

A natural holm oak forest was selectively thinned to test thinning as a tool to reduce tree mortality, increase productivity, and reverse the recent regression of the dominant species (Quercus ilex) induced by climate change. Thinning increased aboveground productivity and reduced stem mortality in this Mediterranean forest during four years just after thinning, contributing to the maintenance of forest functioning under changing climatic conditions. Q. ilex was the only species positively affected by the thinning: stem growth increased for all stem sizes, and mortality was significantly lower in thinned plots. On the contrary, mortality rates of Phillyrea latifolia and Arbutus unedo were not significantly lower. Stem growth increased for P. latifolia only in the smallest stem-size class. Our results highlight the suitability of selective thinning for improving the forest productivity and ensuring the conservation of Mediterranean coppices. Other benefits of selective thinning, such as a decrease in the risk of fire dispersion and an increase in the water supply for human populations, are also discussed

Long-term experimental drought combined with natural extremes accelerate vegetation shift in a Mediterranean holm oak forest

Increasing drought combined with natural extremes are expected to accelerate forest die-off and shifts in vegetation in the Mediterranean Basin. However, fewer studies have explored these climate-driven changes in forest ecosystems. A long-term (17-year) experimental drought (−30% precipitation) was established in a Mediterranean holm oak forest with high (H) and low (L) canopies to determine the changes in stem mortality, recruitment and composition shifts. Experimental drought increased annual stem mortality rate at the community level for both H- and L-canopies. Natural drought amplified the effects of experimental drought on stem mortality at the community level and of Q. ilex for H- and L-canopies. The timescales of natural drought, however, varied substantially with canopy types and species, with shorter timescales in L- than H-canopy and for Q. ilex than P. latifolia. Furthermore, experimental drought combined with natural extremes amplified the increases in stem mortality and decreases in growth for L-canopy. Contrasting responses between Q. ilex and P. latifolia for the relative in abundance and growth were observed in L-canopy and drought treatment reinforced the vegetation shift favoring P. latifolia. These findings suggest continuous drought regimes accelerated a vegetation shift, implying potential consequences for the functions and services for water-limited forest ecosystems

Delayed and altered post-fire recovery pathways of Mediterranean shrubland under 20-year drought manipulation

Increasing water deficits and severe droughts are expected to alter the dynamics of vegetation post-disturbance recovery by decreasing new recruitment and limiting growth in semi-arid Mediterranean ecosystems in future. However, which vegetation metrics will be shifted and how they respond over time are not clear, and the experimental evidence is still limited. Here we assessed the impacts of a long-term (20 years) experimental drought (-30% rainfall) on the pathways of vegetation metrics related to species richness, community composition and abundance dynamics for an early-successional Mediterranean shrubland. The results indicate that the pathways of vegetation metrics were differently affected by experimental drought. The abundance of Globularia alypum follows pathway 1 (altered mature state). Simpson diversity and abundance of Erica multiflora follow pathway 2 (delayed succession) while species richness, community abundance and shrub abundance follow pathway 3 (alternative stable state). There were no significances for the resilience to extremely dry years (the ratio between the performance after and before severe events) between control and drought treatment for all vegetation metric. But, their resilience for the metrics (except Simpson diversity) to extremely dry years in 2016-17 were significantly lower than that of 2001 and of 2006-07, possibly caused by the severe water deficits in 2016-17 at mature successional stage. Principal component analysis (PCA) shows that the first two principal components explained 72.3 % of the variance in vegetation metrics. The first axis was mainly related to the changes in community abundance, shrub abundance and species richness while the second axis was related to Simpson diversity and abundance of G. alypum and E. multiflora. Principal component scores along PC1 between control and drought treatment were significantly decreased by long-term experimental drought, but the scores along PC2 were not affected. Further research should focus on successional pathways in more water-deficit conditions in Mediterranean ecosystems and the consequences of changes in vegetation recovery pathways on ecosystem functions such as biomass accumulation and soil properties.Peer reviewe

Changes in plant species abundance alter the multifunctionality and functional space of heathland ecosystems

Though it is well established that species composition affects ecosystem function, the way in which species combine to control overall ecosystem functioning is still debated. In experimental mesocosms, we planted three functionally distinct dry-heath species in varying proportions and measured multiple ecosystem properties related to nutrient cycling and carbon storage (hereafter functions). Overall ecosystem functioning was described as the main axes of variation in ecosystem functioning (functional space) and the proportion of ecosystem functions at high levels; for example, fast carbon and nutrient cycling (cluster-based multifunctionality). The first functional space axis, related to nitrogen availability, was driven by plant species abundance, particularly that of legumes, which strongly affected many individual functions. The second, related to total plant biomass and woodiness, was mostly driven by the abundance of dwarf shrubs. Similarly, cluster-based multifunctionality was related to the initial abundance of all species, but particularly the legume. Interactions between species also affected ecosystem multifunctionality, but these effects were smaller in magnitude. These results indicate that species interactions could play a secondary role to species abundance and identity in driving the overall ecosystem functioning of heathlands, but also that axes of variation in functional space are clearly linked to plant functional composition

Seasonal and diurnal variations of plant isoprenoid emissions from two dominant species in Mediterranean shrubland and forest submitted to experimental drought

We tested the effect of increasing drought conditions in the Mediterranean Basin on isoprenoid emissions for the coming decades by analyzing their effect experimentally on the dominant Mediterranean species Erica multiflora in a Garraf shrubland and Quercus ilex in a Prades forest in Catalonia (Spain). Drought was simulated in Garraf using automatically sliding curtains to decrease the amount of soil moisture by 5% and in Prades by partial rainfall exclusion and runoff exclusion for a 25% decrease. We measured photosynthetic rates (A), stomatal conductance (gs) and rates of isoprenoid emission in the morning and at midday for four seasons and determined the relationship of emission rates with environmental conditions. Terpenes were emitted by both species, but only E. multiflora emitted isoprene. α-Pinene and limonene were the most abundant terpenes. Isoprenoid emissions increased with air temperature and generally decreased as the amount of soil moisture increased. The results of this study suggest that higher isoprenoid emissions can be expected in the warmer and drier conditions predicted for the coming decades in the Mediterranean region