José A. Carreira

Shrub encroachment can reverse desertification in semi-arid Mediterranean grasslands

The worldwide phenomenon of shrub encroachment in grass-dominated dryland ecosystems is commonly associated with desertification. Studies of the purported desertification effects associated with shrub encroachment are often restricted to relatively few study areas, and document a narrow range of possible impacts upon biota and ecosystem processes. We conducted a study in degraded Mediterranean grasslands dominated by Stipa tenacissima to simultaneously evaluate the effects of shrub encroachment on the structure and composition of multiple biotic community components, and on various indicators of ecosystem function. Shrub encroachment enhanced vascular plant richness, biomass of fungi, actinomycetes and other bacteria, and was linked with greater soil fertility and N mineralization rates. While shrub encroachment may be a widespread phenomenon in drylands, an interpretation that this is an expression of desertification is not universal. Our results suggest that shrub establishment may be an important step in the reversal of desertification processes in the Mediterranean region.

Competition and drought limit the response of water-use efficiency to rising atmospheric carbon dioxide in the Mediterranean fir Abies pinsapo

The gas-exchange and radial growth responses of conifer forests to climatic warming and increasing atmospheric CO2 have been widely studied. However, the modulating effects of variables related to stand structure (e.g., tree-to-tree competition) on those responses are poorly explored. The basal-area increment (BAI) and C isotope discrimination (C stable isotope ratio; δ13C) in the Mediterranean fir Abies pinsapo were investigated to elucidate the influences of stand competition, atmospheric CO2 concentrations and climate on intrinsic water-use efficiency (WUEi). We assessed the variation in δ13C of tree-rings from dominant or co-dominant trees subjected to different degrees of competition. A high- (H) and a low-elevation (L) population with contrasting climatic constraints were studied in southern Spain. Both populations showed an increase in long-term WUEi. However, this increase occurred more slowly at the L site, where a decline of BAI was also observed. Local warming and severe droughts have occurred in the study area over the past 30 years, which have reduced water availability more at lower elevations. Contrastingly, trees from the H site were able to maintain high BAI values at a lower cost in terms of water consumption. In each population, trees subjected to a higher degree of competition by neighboring trees showed lower BAI and WUEi than those subjected to less competition, although the slopes of the temporal trends in WUEi were independent of the competitive micro-environment experienced by the trees. The results are consistent with an increasing drought-induced limitation of BAI and a decreasing rate of WUEi improvement in low-elevation A. pinsapo forests. This relict species might not be able to mitigate the negative effects of a decrease in water availability through a reduction in stomatal conductance, thus leading to a growth decline in the more xeric sites. An intense and poorly asymmetric competitive environment at the stand level may also act as an important constraint on the adaptive capacity of these drought-sensitive forests to climatic warming.

Plastic responses of Abies pinsapo xylogenesis to drought and competition

Radial growth and xylogenesis were studied to investigate the influence of climate variability and intraspecific competition on secondary growth in Abies pinsapo Boiss., a relic Mediterranean fir. We monitored the responses to three thinning treatments (unthinned control -C-, 30% -T30- and 60% -T60- of basal area removed) to test the hypothesis that they may improve the adaptation capacity of tree growth to climatic stress. We also assessed whether xylogenesis was differentially affected by tree-to-tree competition. Secondary growth was assessed using manual band dendrometers from 2005 to 2007. In 2006, xylogenesis (phases of tracheid formation) was also investigated by taking microcores and performing histological analyses. Seasonal dynamics of radial increment were modeled using Gompertz functions and correlations with microclimate and radiation were performed. Histological analyses revealed it as fundamental to calibrate the dendrometer estimates of radial increment and to establish the actual onset and end dates of tracheid production. The lower radial-increment rates and number of produced tracheids were observed in the trees subjected to high competition in the unthinned plots. The growing season differed among the plots, and its duration ranged from an average of 78 days in unthinned plots to 115 days in thinned ones (T60). Variations in the beginning of the growing season (13 April to 22 May) and earlywood-latewood transition (early August) were mainly determined by the temperature pattern, while the onset and the end of the growing season were related to both annual precipitation and tree-to-tree competition. The tracheid-formation phases of radial enlargement and cell-wall thickening showed similar patterns in the trees from thinned and unthinned plots subjected to low and high competition, respectively, but the mean number of tracheids in each phase was always higher in the trees from the thinned plots. The reduction of competition through thinning induced a longer growing season and enhanced the radial growth in A. pinsapo.

Changes in soil phosphatase activity and P transformation rates induced by application of N- and S-containing acid-mist to a forest canopy.

The indirect effects of acid-misting applied to tree canopies, on soil P availability, phosphatase activity and P mineralisation rates in a Sitka spruce (Picea sitchensis (Bong.) Carr) stand have been investigated. The treated trees, grouped according to five height classes, had for four years previously received acid mist consisting of a mixture of H2SO4 and NH4NO3 (pH 2.5) at concentrations of 3.2 mM H 1 and 1.6 mM each of NH 1 4 ; NO 2 3 and SO 22 4 : These simulate cloudwater composition in uplands affected by acidic deposition. Another group of mixed heightclass trees, received a double dose of acid mist. Control — no acid-misted-trees received only the usual precipitation inputs. The acid mist treatments had previously been found to induce a significant reduction in stem diameter growth, but no canopy visible injury symptoms were observed. Root bioassays had demonstrated the induction of P nutritional stress in acid-misted versus no acid-misted trees. Soil chemical analysis showed that acidifying inputs induced changes in the inorganic P subcycle by: (i) increasing acidity and P sorption capacity of the soils; and (ii) decreasing the concentration of labile inorganic P. We report here on changes in the organic P subcycle (phosphatase activity and gross mineralisation rates). We used a method which applies independent treatments to soil samples (control, HgCl 2-addition, HgCl2addition 1 autoclaving) to separate mineral solubilisation, organic mineralisation and immobilisation of solution P. Soil phosphatase activity was significantly lower under acid-misted trees (263.9mg pNP h 21 g 21 ) than under control trees (382.0 mg pNP h 21 g 21 ). Soils beneath double-dose acid-misted trees showed a 42% reduction in the labile inorganic P pool-size, and a 33% increase in the labile organic P poolsize, compared to soils under control trees. A pattern of decreasing net P solubilisation and gross P mineralisation rates with increasing acidmist dose-level was also found. Thus, the combined effects of acid-misting on the inorganic and organic P subcycles, in addition to the suggested effect of N saturation enhancing demand for P by the trees, may have induced P deficiency and reduced tree growth. The effects described are considered to be important factors in the forest decline syndrome. q 2000 Elsevier Science Ltd. All rights reserved.

Human impacts drive forest structure and diversity. Insights from Mediterranean mountain forest dominated by Abies pinsapo (Boiss.)

Environmental factors and land-use are likely interacting to drive forest structure and species diversity, making it difficult to disentangle their separate impacts. Both components influence the resilience and stability of mountain forests in the Mediterranean Basin, since secular land-use has shaped stands structure and forests dynamics are constrained to the environment. Approaches covering environmental factors and contrasting land-uses are still needed to understand their implications on forest dynamics. We investigated patterns of environmental variables, stand structure and biodiversity in forest dominated by Abies pinsapo for 61 stands over their whole range. In addition, since land-use by local inhabitant could be a key factor, logging intensity was quantified by stumps basal area and recent land-use history was investigated. Partial redundancy analysis (pRDA) was conducted to assess to which degree the structural attributes are related to environmental characteristics and/or the land-use in the main A. pinsapo forests. Environmental characteristics accounted for 13% of the total variance, while the contrasting land-use (Spain vs. Morocco) and the stumps basal area, as indicator of human logging, accounted for 23% of the total variance. Trees species diversity was near 5 times higher in Moroccan stands than in stands from south Spain. According to recent human logging, Spanish stands have been strictly protected, but Moroccan stands showed widespread axe-logging by local inhabitant over the smaller tree sizes. Our results suggest that the preservation of a minor perturbation regimen by local populations enhance and help to preserve the whole biodiversity of this relict habitat.

Stand-structural effects on Heterobasidion abietinum-related mortality following drought events in Abies pinsapo

Climate change may affect tree–pathogen interactions. This possibility has important implications for drought-prone forests, where stand dynamics and disease pathogenicity are especially sensitive to climatic stress. In addition, stand structural attributes including density-dependent tree-to-tree competition may modulate the stands’ resistance to drought events and pathogen outbreaks. To assess the effects of stand structure on root-rot-related mortality after severe droughts, we focused on Heterobasidion abietinum mortality in relict Spanish stands of Abies pinsapo, a drought-sensitive fir. We compared stand attributes and tree spatial patterns in three plots with H. abietinum root-rot disease and three plots without root-rot. Point-pattern analyses were used to investigate the scale and extent of mortality patterns and to test hypotheses related to the spread of the disease. Dendrochronology was used to date the year of death and to assess the association between droughts and growth decline. We applied a structural equation modelling approach to test if tree mortality occurs more rapidly than predicted by a simple distance model when trees are subjected to high tree-to-tree competition and following drought events. Contrary to expectations of drought mortality, the effect of precipitation on the year of death was strong and negative, indicating that a period of high precipitation induced an earlier tree death. Competition intensity, related to the size and density of neighbour trees, also induced an earlier tree death. The effect of distance to the disease focus was negligible except in combination with intensive competition. Our results indicate that infected trees have decreased ability to withstand drought stress, and demonstrate that tree-to-tree competition and fungal infection act as predisposing factors of forest decline and mortality.

Temperate-like stand dynamics in relict Mediterranean-fir (Abies pinsapo, Boiss.) forests from southern Spain

Introduction and statement of the research questionsGap dynamics have been widely studied in forests of Abies spp. from temperate and boreal regions. The local microclimate and competition for light have been identified as the main factors controlling changes in species composition and canopy structure, however little is known on dynamics of such forests in Mediterranean.Experimental design and aimsWe studied forest structure and dynamics of Abies pinsapo stands in southern Spain, in contrasting habitats and successional status. In addition past regeneration patterns and their relationship to canopy structure, disturbances and forest-use history were investigated.ResultsStands structure attributes were within the range described for temperate conifer biomes. The age structure revealed two main cohorts comprised of a few > 100 year-old trees and abundant younger trees established in a single recruitment event after the stands were protected in the 1950s. Initial growth-rate analyses indicated that A. pinsapo regenerated mainly in small canopy gaps, while only 15% recruited from the forest understorey. For the last ten years, basal area increment was lower than 10 cm2 y−1 in 91% of studied trees and growth rate differences between trees narrowed.ConclusionStand dynamics in A. pinsapo forests maintain general features of temperate fir forests. Tree establishment over time and current stand structure fit to known changes in forest use. Widespread growth decline trends might be linked to stand stagnation and global warming.RésuméIntroduction et état des questions de rechercheLes dynamiques des trouées ont été largement étudiées dans les forêts d’Abies spp. à partir des régions tempérées et boréales. Le microclimat local et la compétition pour la lumière ont été identifiés comme les principaux facteurs de contrôle des changements dans la composition des espèces et la structure du couvert, mais on sait peu de choses sur la dynamique de ces forêts dans les écosystèmes méditerranéens.Plan expérimental et objectifsNous avons étudié la structure de la forêt et la dynamique de peuplements d’Abies pinsapo dans le sud de l’Espagne, dans des habitats contrastés et dans des états d’évolution successifs. En outre, les modèles passés de régénération et leurs relations avec la structure du couvert, les perturbations et l’histoire de l’utilisation des forêts ont été étudiés.RésultatsLes attributs de structure des peuplements se situent dans la gamme décrite pour les biomes de conifères tempérés. La structure des âges a révélé deux cohortes principales composées de quelques vieux arbres d’un âge supérieur à 100 ans, et de nombreux jeunes arbres établis dans un seul évènement de régénération après que les peuplements aient été protégés dans les années 1950. Les premières analyses des taux de croissance ont indiqué que A. pinsapo s’est régénéré principalement dans les petites trouées, tandis que seulement 15 % des arbres ont été recrutés dans le sous-bois. Au cours des dix dernières années, l’augmentation de la surface terrière a été inférieure à 10 cm2 an−1 chez 91 % des arbres étudiés et les différences entre les taux de croissance des arbres étaient réduites.ConclusionLes dynamique des peuplements d’A. pinsapo conservent les caractéristiques générales des forêts de sapins tempérés. L’installation des arbres et la structure actuelle des peuplements permettent de connaître les changements dans l’utilisation de la forêt. La baisse généralisée des tendances de croissance pourrait être liée à la stagnation du peuplement et au réchauffement de la planète.

Phenological and water-use patterns underlying maximum growing season length at the highest elevations: implications under climate change

Consequences of climate change on tree phenology are readily observable, but little is known about the variations in phenological sensitivity to drought between populations within a species. In this study, we compare the phenological sensitivity to temperature and water availability in Abies pinsapo Boiss., a drought-sensitive Mediterranean fir, across its altitudinal distribution gradient. Twig growth and needle fall were related to temperature, precipitation and plant water status on a daily scale. Stands located at the top edge of the distributional range showed the most favourable water balance, maximum growth rates and little summer defoliation. Towards higher elevations, the observed delay in budburst date due to lower spring temperatures was overcome by a stronger delay in growth cessation date due to the later onset of strong water-deficit conditions in the summer. This explains an extended growing season and the greatest mean growth at the highest elevation. Conversely, lower predawn xylem water potentials and early partial stomatal closure and growth cessation were found in low-elevation A. pinsapo trees. An earlier and higher summer peak of A. pinsapo litterfall was also observed at these water-limited sites. Our results illustrate the ecophysiological background of the ongoing altitudinal shifts reported for this relict tree species under current climatic conditions.

Utilizing Microbial Community Structure and Function to Evaluate the Health of Heavy Metal Polluted Soils

The aim of this chapter is to combine perspectives developed in soil ecology, microbiology and biochemistry into a common framework for evaluating the health of soils polluted with heavy metals.

The stable isotope ecology of terrestrial plant succession

We review the relevance and use of stable isotopes for the study of plant community succession. Stable isotope measurements provide information on the origin of resources acquired by plants, the processes governing resource uptake and transformation, and the physiological and environmental conditions of plant growth. When combined with measurements of the stable isotope ratio values of soil microbial biomass, soil organic matter and plant litter, isotope measurements of plants can indicate effects of successional changes on ecosystem processes. However, their application to questions of plant succession and ecosystem change is limited by the degree to which the underlying assumptions are met in each study, and complementary measures may be required, depending upon the question of interest. First, we discuss the changes that occur in the stable isotope composition of plants and ecosystems with ontogeny and species replacements, as well as their potential evolutionary significance. Second, we discuss the imprints of plant competition and facilitation on leaf and wood tissue, as well as how stable isotopes can provide novel insights on the mechanisms underlying plant interactions. Finally, we discuss the capacity for stable isotope measurements to serve as a proxy record for past disturbances such as fire, logging and cyclones.