Forest certification is a voluntary conservation tool that aims to promote sustainable forest management. While research on forest certification has increased recently, there remains a significant gap in understanding how and to what extent certification can promote forest conservation. Mediterranean cork oak open woodlands are ecosystems of high conservation and socio-economic value. However, these ecosystems are threatened by increased adult oak mortality and regeneration failure, often due to inadequate management and the rise of pests and diseases, aggravated by climate change. Forest certification prescribes management practices intended to enhance tree regeneration and maintain stand health conditions. Therefore, it is anticipated that forest certification could mitigate the observed decline of oak trees in Mediterranean regions. Here, we investigate whether forest certification contributes to the ecological sustainability of Mediterranean cork oak open woodlands in Portugal. We compare the stand biometrics of noncertified and certified cork oak stands before and after certification implementation, using both National Forest Inventory data and field sampling from 2005 and 2020. Our findings indicate that the density of adult oak trees decreased by 16% in certified estates and 28 % in noncertified estates between 2005 and 2020. Similarly, cork oak cover declined by 6 % tree cover in certified plots and 19 % in non-certified plots during the same period. Consequently, by 2020, tree density was 20 % higher in certified stands than in the non-certified ones, and tree cover was 36 % higher in certified stands. Tree diameter and height increased at similar rates in both certified and non-certified stands from 2005 to 2020.The age structure of the stands also remained consistent, showing a bell-shaped distribution of tree diameters in both years. However, results on oak regeneration were inconclusive. Our results suggest that cork oak decline, measured by the changes in density and cover of adult trees from 2005 to 2020, is slower in certified cork oak woodlands. Nonetheless, the increase in tree diameter and the age structure shape indicate potential regeneration issues in both certified and non-certified stands, needing further measures to address the aging of cork oak open woodlands.

Understanding and predicting responses of plant community diversity and ecosystem functioning to disturbance is essential to achieving forest conservation and management goals. In recent decades, the abundance and geographic distribution of wild ungulates have expanded in many parts of Europe due to, among other factors, land-use changes, hunting regulations and lack of predators. The study aims to explore the effects of deer browsing and grazing intensity, estimated through a browsing index on woody and semi-woody plants, on understorey vegetation composition, analysing both taxonomic and functional diversity. Specifically, we aim to test the intermediate disturbance hypothesis (IDH), which states that plant biodiversity peaks at intermediate levels of browsing intensity regarding plant species richness and functional groups. We also aim to identify species revealing different levels of browsing intensity, accounting for plant functional traits. Our results revealed that intermediate levels of browsing intensity, through species replacement, strongly and positively affected total species richness. This result is consistent with the IDH prediction, but distinct patterns varied across plant functional types. Moreover, increasing browsing intensity favoured disturbance-tolerant species by replacing functional traits. These species were characterised by ruderal traits, including high specific leaf area, low leaf dry matter content, small height and seed mass, annual and short lifespans. However, although browsing intensity increased functional richness and decreased functional redundancy, indicator species analysis revealed that high browsing intensity favoured highly competitive, browsing-tolerant perennial species. These results suggest that annual species may fail to colonise the understorey even if they are favoured by deer browsing, thereby affecting the functioning and the stability of ecosystem, with a potential homogenisation of understorey vegetation. Although confounding effects cannot be ruled out (e.g., local vegetation structure and diversity), our study highlights the need to consider functional diversity when assessing deer effects on vegetation to draw a complete picture of plant-large herbivore interactions.

Climate change is increasing the frequency of droughts and the risk of severe wildfires, which can interact with shrub encroachment and browsing by wild ungulates. Wild ungulate populations are expanding due, among other factors, to favorable habitat changes resulting from land abandonment or land-use changes. Understanding how ungulate browsing interacts with drought to affect woody plant mortality, plant flammability, and fire hazard is especially relevant in the context of climate change and increasing frequency of wildfires. The aim of this study is to explore the combined effects of cumulative drought, shrub encroachment, and ungulate browsing on the fire hazard of Mediterranean oak woodlands in Portugal. In a long-term (18 years) ungulate fencing exclusion experiment that simulated land abandonment and management neglect, we investigated the population dynamics of the native shrub Cistus ladanifer, which naturally dominates the understory of woodlands and is browsed by ungulates, comparing areas with (no fencing) and without (fencing) wild ungulate browsing. We also modeled fire behavior in browsed and unbrowsed plots considering drought and nondrought scenarios. Specifically, we estimated C. ladanifer population density, biomass, and fuel load characteristics, which were used to model fire behavior in drought and nondrought scenarios. Overall, drought increased the proportion of dead C. ladanifer shrub individuals, which was higher in the browsed plots. Drought decreased the ratio of live to dead shrub plant material, increased total fuel loading, shrub stand flammability, and the modeled fire parameters, that is, rate of surface fire spread, fireline intensity, and flame length. However, total fuel load and fire hazard were lower in browsed than unbrowsed plots, both in drought and nondrought scenarios. Browsing also decreased the population density of living shrubs, halting shrub encroachment. Our study provides long-term experimental evidence showing the role of wild ungulates in mitigating drought effects on fire hazard in shrub-encroached Mediterranean oak woodlands. Our results also emphasize that the long-term effects of land abandonment can interact with climate change drivers, affecting wildfire hazard. This is particularly relevant given the increasing incidence of land abandonment.

Shrub encroachment is a global phenomenon occurring in a wide range of ecosystems, including in tree plantations, which is known to affect ecosystem functioning and productivity. Forest management in tree plantations (e.g. shrub clearing, tree thinning and pruning) often aims to increase wood and non-woody products yield while optimizing other ecosystems services such as carbon sequestration. However, little is known on the effect of shrub clearing on tree growth and tree water status in juvenile tree stands. Under climate change scenarios that predict increased frequency of extreme events such as droughts, management during the early stages of stand plantations may be crucial for its sustainability. This study aimed to explore the combined effect of shrub understorey removal and drought in two cork oak (Quercus suber L.) juvenile plantations, with similar stand age but distinct soil types and forest management, on tree water status, assessed through leaf water potential measurement, and on tree growth. The study was conducted during two consecutive years, including a drought year. Results revealed that the shrub understorey removal positively affected the physiological response of trees in both plantations but only affected tree growth (i.e. total height and leaf area) in the plantation with a higher tree density and shrub biomass. Shrub biomass and species composition most probably had a determinant effect on tree growth due to soil water competition. Our results emphasize the need for site-specific forest management during the early stages of cork oak plantation for maintaining its future sustainability and provision of ecosystem services.

Forest certification is a conservation tool, which aims to promote the sustainable management and conservation of forest ecosystems. Establishing set-aside or lower intervention conservation zones to promote biodiversity conservation is a requisite of forest certification. We tested the effects of conservation zones on the tree biometrics and regeneration, as well as on the taxonomic, functional, and structural diversity of the shrub and herb understory, in Mediterranean oak woodlands. We also assessed how oak biometrics, regeneration and understory diversity varied among conservation zones established 10, 14, and 20 years before our sampling dates. Oak regeneration tended to be higher in conservation zones than in controls, but results varied with the age of conservation zones. For example, the abundance of oak seedlings and saplings was higher in 10-year-old conservation zones than in those established 20 years ago. Abundance of young oak trees was higher in 14-year-old conservation zones than in 10- and 20-year-old conservation zones. The understory vertical diversity was significantly higher in 14- and 20-year-old conservation zones than in controls. Functional diversity differed significantly between conservation zones and controls, with a higher abundance of late-successional shrubs, namely fleshy-fruited species in 20-year-old conservation zones. The plant species composition of the shrub and the herb understory was most dissimilar between older conservation zones and controls. Additionally, the cover and diversity of the understory herb species decreased with the age of conservation zones. Conservation zones implemented under forest certification increase habitat structural complexity of oak woodlands, which may benefit wildlife species, but there will be trade-offs with the cover and diversity of the herb understory. Forest managers must evaluate such trade-offs when establishing conservation zones in cork oak woodlands under forest certification schemes.

Ungulates influence ecosystem services in important ways, including by altering the amount of standing plant biomass and species composition. Browsing, for example, removes biomass and, in consequence, can decrease the risk of wildfires. The influence of ungulates on carbon storage is more complicated. Browsers reduce carbon stocks directly by consuming biomass, but if browsing reduces fine fuel loads, then long-term carbon storage may increase. We investigated how wild ungulates mediate trade-offs between carbon stocks and wildfire hazard in a Mediterranean oak woodland. We assessed the effects of deer (Cervus elaphus and Dama dama) on the colonization by gum cistus (Cistus ladanifer), a highly flammable Mediterranean shrub, through a long-term ungulate exclusion experiment. We established fenced (unbrowsed) and open (browsed) plots in areas where shrubs were previously cleared, and compared shrub density, height, biomass, and above-ground carbon stocks 6 and 14 years after fencing. There were no significant differences in shrub density between treatments after 6 years of browsing exclusion but, by this point, height was already significantly greater in fenced plots. At the end of the experiment, shrub density and height were both significantly greater in fenced plots. Biomass and carbon stocks of gum cistus increased and reached a plateau in fenced plots, while in open plots biomass and carbon stocks increased initially but then decreased. Ungulates also prevented accumulation of litter carbon stocks, which was significantly higher in fenced plots after 14 years of browsing exclusion. We also modelled fire behaviour in fenced and open plots. Browsing reduced fine fuel load, an important contributor to fire spread, by 80%. In browsed plots, modelled wildfire rate of spread, flame length, and fireline intensity decreased by 50%, 65%, and 90%, respectively, which also decreased the probability of canopy fire and of oak mortality. Synthesis and applications. By decreasing fire hazard and the probability of canopy fire, ungulates may ultimately increase ecosystem carbon stocks through improved adult oak survival. These critical, indirect trade-offs need to be assessed when addressing the effects of ungulates on ecosystems and the management of their populations.

Herbivory, plant facilitation, and competition have complex impacts on tree regeneration which are seldom investigated together. Grazing exclosure experiments have allowed quantification of the effects of large herbivores on tree regeneration dynamics but have often ignored the effect of herbivorous insects. We experimentally tested how folivory (percentage of leaf damaged by insects) and microenvironment (tree canopy cover and herbs) jointly alter performance (growth and survival) of seedlings of two dominant Mediterranean oak-species within ungulate exclosures in a 3-year field study. An agroforestry system dominated by cork oak (Quercus suber) and holm oak (Q. rotundifolia) was assessed in south-east Portugal. We aimed also to determine whether the two oak species differed in the interdependences between folivory, microenvironment, covaring factors, and seedling performance. Unexpectedly, under the low–moderate insect defoliation, growth and survival of cork and holm oak seedlings were positively associated with herbivore damage. Herb removal increased oak folivory by 1.4 times. Herb removal was also positively associated with growth, directly and indirectly through its negative effect on oak folivory. Tree canopy favored insect folivory upon cork oak seedlings directly and upon holm oak indirectly via decreasing light availability. Folivory was threefold greater upon cork than upon holm oak-seedlings. Our study shows that tree canopy, herbs, and covarying factors can affect cork and holm oak-seedling performances through complex pathways, which markedly differ for the two species. The combined effect of insect herbivory and positive and negative plant-plant interactions need to be integrated into future tree regeneration efforts toward tackling the regeneration crisis of oak agroforestry systems of the Mediterranean.

The sum of fruit and seed predation by multiple species may strongly affect plant reproduction and population dynamics. We evaluated the combined effects of ungulates, seed-eating rodents and insect pre-dispersal seed predators on the reproductive success of the Mediterranean gum cistus shrub (Cistus ladanifer), over two consecutive years within a long-term ungulate-exclusion experiment. We compared fruiting success in shrubs exposed and protected from ungulates by examining fruit abortion and fruit production. We also investigated the effect of insect predation on seed production (i.e. proportion of depredated fruit and seed loss) and measured fruit weight, seed number per fruit, and seed weight of unpredated fruits. Ungulate browsing directly removed 42.3% of the plant reproductive structures, early in the reproductive season and insect predation reduced mature seeds by over 40%. Results also emphasize the additive effects of ungulate browsing on pre-dispersal insect predation and fruit abortion which increased by 74.7% and 60.9%, respectively. Rodents, which only occurred in ungulate-excluded plots, had a limited and later effect on seed production with 6% of mature fruit loss. Fruit weight, seed weight and number were higher in shrubs protected from ungulates. Our study indicated that seed predation by mice was irrelevant, but ungulate and invertebrate seed predation interacted to strongly limit the reproductive success of C ladanifer, potentially affecting plant population dynamics in the long-term.

Plant-animal interactions imply costs and benefits with net balance depending on interacting species and ecological context. Ungulates, in particular, confer costs (e.g., plant leaf consumption, flower bud predation) and benefits (e.g., plant overcompensation, seed dispersal) to plants. Magnitude of costs and benefits may be altered by habitat management or ecological conditions favoring high density ungulate populations. Little is known however on whether plant costs or benefits predominate over the years, or the long-term outcomes of plant-animal interactions in habitat types sustaining high density ungulate populations. We investigated how high density ungulate populations alter plant costs and benefits by quantifying ungulate long-term effects on the shrub Cistus ladanifer (Cistaceae) individual size, seed weight and number, seed bank, and population density, through a 12-year ungulate exclusion experiment in a Mediterranean scrubland. We monitored plant size and flower buds in plants exposed or protected from ungulates and number of developed capsules and seeds consumed (potential seed dispersal) by ungulates during three reproductive seasons. We found that ungulates negatively affected shrub size and led to a dramatically decline of shrub reproductive structures and seed production, affecting the plant reproductive cycle. Number of buds was 27 times higher and number of developed seed 5 times higher in ungulate-excluded as compared to ungulate-exposed plots. After 9 years of ungulate exclusion, the C. ladanifer seed bank was 2.6 times higher in ungulate-excluded plots. The population density of C. ladanifer was 4 times higher in ungulate-excluded plots. Our long-term experiment showed that high density ungulate populations can alter plant-animal interactions by reducing plant benefits and increasing plant costs.

Extreme drought events and plant invasions are major drivers of global change that can critically affect ecosystem functioning and alter ecosystem-atmosphere exchange. Invaders are expanding worldwide and extreme drought events are projected to increase in frequency and intensity. However, very little is known on how these drivers may interact to affect the functioning and resilience of ecosystems to extreme events. Using a manipulative shrub removal experiment and the co-occurrence of an extreme drought event (2011/2012) in a Mediterranean woodland, we show that native shrub invasion and extreme drought synergistically reduced ecosystem transpiration and the resilience of key-stone oak tree species. Ecosystem transpiration was dominated by the water use of the invasive shrub Cistus ladanifer, which further increased after the extreme drought event. Meanwhile, the transpiration of key-stone tree species decreased, indicating a competitive advantage in favour of the invader. Our results suggest that in Mediterranean-type climates the invasion of water spending species and projected recurrent extreme drought events may synergistically cause critical drought tolerance thresholds of key-stone tree species to be surpassed, corroborating observed higher tree mortality in the invaded ecosystems. Ultimately, this may shift seasonally water limited ecosystems into less desirable alternative states dominated by water spending invasive shrubs.

<list list-type="1" id="jpe12189-list-0001"> Tree recruitment in Mediterranean ecosystems is strongly limited at the seedling stage by drought. Increasing evidence shows the critical positive role of the canopy nurse effect on seedling survival which results from direct and indirect, positive and negative interactions between species. Most studies, however, have only focused on the effects of tree canopy on water and light, ignoring other critical factors affecting seedling regeneration, such as canopy effects on high temperatures and the competing herb biomass. Here, we evaluate how tree canopy cover and removal of herbs affect the survival and growth of seedlings of two dominant Mediterranean Quercus species during a 3-year study. We use an integrated model that combines several data sets to quantify and predict regeneration dynamics along environmental gradients of soil moisture, temperature and light. Low soil moisture, increased soil temperature and herb biomass negatively affected seedling survival of both Quercus species. Seedling growth was positively associated with increasing soil moisture and light. Although tree canopy cover directly facilitated seedling survival in both Quercus species, it also negatively affected herb biomass and thus indirectly facilitated the survival of Quercus suber, but not of Quercus ilex seedlings at low levels of soil moisture. Overall, tree canopies increased seedling survival but not growth during the establishment phase, mainly by ameliorating the effects of low soil moisture and high temperatures. Tree canopy indirectly facilitated survival of Q.suber seedlings by negatively affecting the competing herb layer.Synthesis and applications. To improve tree recruitment and conserve Mediterranean Quercus woodlands, the removal of herbs should be integrated into management plans for dry habitats. Interactions between abiotic and biotic factors may also effect the regeneration of these tree species. In particular, a healthy tree canopy will become important for providing conditions to facilitate seedling establishment if these habitats become drier and warmer, as predicted by some climate change scenarios.To improve tree recruitment and conserve Mediterranean Quercus woodlands, the removal of herbs should be integrated into management plans for dry habitats. Interactions between abiotic and biotic factors may also effect the regeneration of these tree species. In particular, a healthy tree canopy will become important for providing conditions to facilitate seedling establishment if these habitats become drier and warmer, as predicted by some climate change scenarios.

Climate change scenarios for the Iberian Peninsula predict increasing temperatures and increasingly variable precipitation regimes, which will challenge the sustainability and biodiversity of Mediterranean ecosystems such as the semi-natural evergreen oak woodlands.To assess the effects of precipitation variability on productivity, species composition and vegetation gas exchange of the understorey vegetation in a typical managed cork oak woodland, a large-scale rainfall manipulation experiment was established. We studied the impacts of a change in the timing of precipitation events on this ecosystem, without altering total annual precipitation inputs. The two water manipulation treatments were: 'weekly watering treatment', where natural conditions were simulated with a normal dry period of 7 days, and '3-weekly watering treatment', with the normal dry period increased three-fold to 21 days.Our experimental precipitation patterns resulted in significant differences in temporal soil moisture dynamics between the two treatments. Average soil water content (SWC) at 3 cm depth during the growing season was 16.1 +/- 0.17% and 15.8 +/- 0.18% in the weekly and 3-weekly watering treatments, respectively, with a mere 5% increase in the variability of SWC when extending the dry period from one to three weeks. Water infiltration into deeper soil layers (>50 cm) was significantly higher in the 3-weekly watering treatment as compared to the weekly watering treatment. This might be beneficial to Quercus suber, the tree component in this ecosystem, as its extensive tree root system enables water acquisition from deeper soil layers.However, manipulation of the within-season precipitation variability, with a shift to fewer, but larger rain events, without change in total precipitation amount, had no significant effect on aboveground net primary productivity (ANPP), belowground net primary productivity (BNPP) and species composition, with average values of peak biomass of 385 g m(-2) and 222 g m(-2) for ANPP and BNPP, respectively.The experimental precipitation patterns did not result in significant differences in the vegetation gas exchange between the two watering treatments. The CO2 and H2O exchange parameters correlated well with air temperature. In addition, evapotranspiration showed a good correlation with SWC.Incorporating the data of SWC in the conceptual 'bucket model' showed that, independently of the watering regime, soil water availability during the life-cycle of these annual plants did not reach severe water stress conditions, which can explain the lack of a significant treatment effect in our study. In addition, our results showed that the annual plant community in these Mediterranean ecosystems is well adapted to short-term drought, through their phenological patterns and physiological adaptations. (C) 2012 Elsevier B.V. All rights reserved,

Future climate scenarios for the Mediterranean imply increasing precipitation variability. This study presents a large-scale water manipulation experiment simulating changes in precipitation variability, aiming at a better understanding of the effects of rainfall patterns on soil C and N cycling and understorey productivity in a Mediterranean oak woodland.

Soil respiration in drought prone regions is highly dependent on the precipitation regime and soil moisture conditions, which are expected to change in a global warming context. In the present study we used an extensive collection of field chamber measurements of soil respiration (R-s) from forest and grassland sites of centre and south of Portugal distributed over a 10 year period. This data were summarized and analysed with the objective to describe seasonal variability of R-s as affected by soil moisture (H-s) and soil temperature (T-s). A Bayesian framework was used to test the effectiveness of soil bioclimatic models in estimating R-s on a daily and monthly time step. R-s seasonality was similar between sites, reaching a maximum in spring and autumn and a minimum in the dry season (July-September). No differences were observed for R-s between sites with different standing biomass or soil carbon stocks either on an annual or seasonal timescale. H-s, and not T-s, was the driving factor of R-s during most of the year. T-s drove R-s response only above certain H-s limits: 10% for forest sites and 15% for grassland sites leading to a Q(10) of 2.01, 1.61 and 1.31 for closed forests, open forests and grasslands, respectively. The Bayesian analysis showed that models using H-s as an independent variable performed better than models driven by T-s alone. Monthly estimates of R-s in grasslands can be predicted by simple climatic models based on H-s but none of them was suitable for forest ecosystems, stressing the need for a process-based approach. This study adds to the evidence that H-s controls R-s fluxes for Mediterranean ecosystems and should always be taken into account for extrapolation purposes. (C) 2012 Elsevier B.V. All rights reserved.

Grazing is a global, dominant land use affecting biodiversity and ecosystem processes. In Mediterranean ecosystems grazing is a major ecological and evolutionary driver but, surprisingly, there is little information on the use of grazing as a tool to manage biodiversity in these ecosystems. We conducted an experiment to assess if the coexistence of grazing and grazing-excluded patches would increase plant and invertebrate diversity in a Mediterranean evergreen oak woodland. Plant community traits were different between treatments. Plant and litter biomass was higher, and the proportion of bare ground was lower, in grazing-excluded plots. Grazing affected functional diversity with legumes, invertebrate detritivores and sup sucking insects being more abundant in ungrazed plots. There were no differences between treatments in the number of species but there were plant species and invertebrate taxa recorded in grazed or grazing excluded plots, only. Ant communities were functionally different between treatments. Honeydew ant eaters were associated with ungrazed and higher plant biomass plots, and seed-eater as well as aggressive predator ant species were associated with grazed, more open habitat, plots. Management practices aiming to maintain grazing and grazing-excluded patches can increase habitat heterogeneity and promote diversity at the estate level.

Cork oak savannas are composed by a sparse tree canopy (30-70 trees/ha) and a grassland understory predominantly composed of C3 annuals that survive the hot and dry Mediterranean summers as seeds in the soil. Microbial communities can be more or less efficient at converting organic substrates into microbial biomass carbon depending on the quantity and quality of organic matter inputs. The cork oak savannas have two distinct types of plant litter that can affect soil microbial biomass and activity differently: herbaceous litter and the more recalcitrant woody plant litter resulting from the trees. Spatial variability of soil microbial biomass and activity due to the tree-grassland component of cork oak savannas were evaluated in order to better understand the soil carbon dynamics of these systems.To quantify changes in soil microbial biomass and activity, measurements were performed in a Cork oak savanna in Southern Portugal. At this site 8 plots were randomly established under mature cork oak trees and paired with 8 open grassland plots. During one year soil cores (0-10 cm) were monthly collected at each site for measuring soil microbial biomass C and other eco-physiology parameters. Results/Conclusion Soil microbial biomass carbon (Cmic) and nitrogen (Nmic) were always higher under the tree canopy than in the open grasslands. Organic carbon (Corg) was also higher under the tree canopies. The Cmic/Corg ratio relates to the microbial activity and its potential to mineralize organic substances. The Cmic/Corg ratio was lower under the tree canopies than in the open grasslands. Less microbial biomass was supported per unit of Corg. Basal activity was always higher under the canopy than in the open grassland.Trees scattered in the savanna function as islands inducing larger soil microbial communities and higher basal activity under the canopies. Lower Cmic/Corg ratio under the tree canopies suggests a more recalcitrant nature of the litter and a decrease in relative availability of organic matter under the trees.