Arid ecosystems cover about 40% of Earth's land area and are home to over two billion people, yet they remain vulnerable to climate change and human actions. Using numerical simulations, and data from Mediterranean ecosystems in Spain, Morocco and Greece, Kefi et al. show that patch-size distribution of vegetation follows a power law. As grazing pressure increases, patch size deviates from the power law close to the transition to desert conditions. So patch-size distribution may be a useful early warning of desertification. The cover shows an arid landscape (top) in the El Planeron nature reserve in Belchite, Spain, and the lower panels show degradation in this landscape. In a separate paper, Scanlon et al. use satellite imagery to show that the size distribution of tree clusters in the Kalahari basin also follows a scale-free power law. This can be explained by positive feedback associated with preferential environments near existing trees. In News & Views Ricard Sole discusses both papers. COVER IMAGE Sonia & Michael Kefi/ Yolanda Pueyo/ Santiago Begueria Portugues This paper describes and models the effect of grazing on vegetation patchiness in three arid Mediterranean ecosystems. The patch size distribution of the vegetation in these ecosystems follows a power law, which can be explained by invoking local positive interactions among plants. Deviations from power laws occur when grazing pressure is high, and may be a harbinger of imminent desertification. Humans and climate affect ecosystems and their services1, which may involve continuous and discontinuous transitions from one stable state to another2. Discontinuous transitions are abrupt, irreversible and among the most catastrophic changes of ecosystems identified1. For terrestrial ecosystems, it has been hypothesized that vegetation patchiness could be used as a signature of imminent transitions3,4. Here, we analyse how vegetation patchiness changes in arid ecosystems with different grazing pressures, using both field data and a modelling approach. In the modelling approach, we extrapolated our analysis to even higher grazing pressures to investigate the vegetation patchiness when desertification is imminent. In three arid Mediterranean ecosystems in Spain, Greece and Morocco, we found that the patch-size distribution of the vegetation follows a power law. Using a stochastic cellular automaton model, we show that local positive interactions among plants can explain such power-law distributions. Furthermore, with increasing grazing pressure, the field data revealed consistent deviations from power laws. Increased grazing pressure leads to similar deviations in the model. When grazing was further increased in the model, we found that these deviations always and only occurred close to transition to desert, independent of the type of transition, and regardless of the vegetation cover. Therefore, we propose that patch-size distributions may be a warning signal for the onset of desertification.

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Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems

Plant traits-the morphological, anatomical, physiological, biochemical and phenological characteristics of plants-determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait-based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits-almost complete coverage for 'plant growth form'. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait-environmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives.

/pdf/try-plant-trait-database-enhanced-coverage-and-open-access-25uv128bwj.pdf

TRY plant trait database : Enhanced coverage and open access

Self-Organized Criticality

Leaf size varies by over a 100,000-fold among species worldwide. Although 19th-century plant geographers noted that the wet tropics harbor plants with exceptionally large leaves, the latitudinal gradient of leaf size has not been well quantified nor the key climatic drivers convincingly identified. Here, we characterize worldwide patterns in leaf size. Large-leaved species predominate in wet, hot, sunny environments; small-leaved species typify hot, sunny environments only in arid conditions; small leaves are also found in high latitudes and elevations. By modeling the balance of leaf energy inputs and outputs, we show that daytime and nighttime leaf-to-air temperature differences are key to geographic gradients in leaf size. This knowledge can enrich “next-generation” vegetation models in which leaf temperature and water use during photosynthesis play key roles.

http://science.sciencemag.org/content/sci/357/6354/917.full.pdf

Global climatic drivers of leaf size.

Ecosystem resilience depends on functional redundancy (the number of species contributing similarly to an ecosystem function) and response diversity (how functionally similar species respond differently to disturbance). Here, we explore how land-use change impacts these attributes in plant communities, using data from 18 land-use intensity gradients that represent five biomes and > 2800 species. We identify functional groups using multivariate analysis of plant traits which influence ecosystem processes. Functional redundancy is calculated as the species richness within each group, and response diversity as the multivariate within-group dispersion in response trait space, using traits that influence responses to disturbances. Meta-analysis across all datasets showed that land-use intensification significantly reduced both functional redundancy and response diversity, although specific relationships varied considerably among the different land-use gradients. These results indicate that intensified management of ecosystems for resource extraction can increase their vulnerability to future disturbances.

Land-use intensification reduces functionalredundancy and response diversity in plantcommunities

The micromorphology of trichomes of 56 Teucrium L. species belonging to the 9 sections of the genus in the Mediterranean area was surveyed by scanning electron microscopy (SEM) of leaves, calyx, corolla and nutlets. 25 trichome types are described, 12 of them are new. Thin walled hairs are the exclusive type found in the corolla and are the most widespread type on the abaxial side of the leaves. Subsessile glandular hairs, 2-4-celled, are found on the nutlet and leaves of the semi-shrubby and paleoendemic species. The presence of short or elongated, generally adpressed simple thick-walled slighuy conical hairs provides an additional character to clarify the boundaries between sect. Chamedrys (Mill.) Schreb, and sect. Polium (Mill.) Schreb. Branched non-glandular hairs are confined to sect. Polium subsect Polium , except for the rare branched hair conical and thick-walled type found in Teucrium barbarum Jahand. & Maire (sect. Chamaedrys ) and T. heterophyllum L`Her. from sect. Teucrium . This last section is well defined by the absence of simple slighuy conical thick-walled hairs and the glandular hairs in the corolla. Sect. Teucriopsis Benth, is of particular interest for the exclusive presence of branched and peltate glandular hairs on the nutlets surface. Sect. Chamedrys is a homogeneous group, distinguished from the other sections by an indumentum formed only by trichomes types evolved from the simple slighuy conical thick-walled hairs. This section overlaps, in the presence of glandular sub-sessile hair on the nutlets surface, with sects. Isotriodon Boiss, and sect. Polium subsect. Rotundifolia Cohen ex Valdes Berm. & Sanchez Crespo. The trichomes type of the calyx teeth, abaxial side of the leaf and latero-posterior corolla lobes can be used as a distinctive taxonomic character at specific and infra-specific level. This study supports Bentham's delimitauon of sections with the addiuons subequently made by Boissier.

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Trichome morphology in Teucrium L. (Labiatae). A taxonomic review

https://estudogeral.sib.uc.pt/bitstream/10316/3919/1/file9fbbf778057e4a5da49173f43c173704.pdf

Change in plant spatial patterns and diversity along the successional gradient of Mediterranean grazing ecosystems

Changes in plant functional types in response to goat and sheep grazing in two semi-arid shrublands of SE Spain

http://www.biblioteca.uma.es/bbldoc/tesisuma/16643094.pdf

Quantitative characterization of the regressive ecological succession by fractal analysis of plant spatial patterns

Teucrium section Polium species growing in the Iberian Peninsula and Balearic Islands are revised. 48 species and 7 subspecies placed in 4 subsections and 11 nototaxa are listed. 38 are the endemic species of the Iberian peninsula. For each taxa a description, distribution, chromosomic number, biological type and ecology are given, and a key for their identification is included. Locotypic indication, type and ilustration are indicated in every taxa. In the present work for the Iberian peninsula, in order to select the sections, the inflorescence is considered a basic character. In the case of subsections the shape of the corolle, indument of the leaves and the tiges and the morphology of the leaves and the calix have been used as differential characters, and lastly for the species, the calix teeth, leaves and ciliated or non ciliated corolle lobes and inflorescence type have been selected.

/pdf/revision-del-genero-teucrium-l-seccion-polium-mill-schreb-4r9xtolshz.pdf

Teresa Navarro

Papers

Trichome morphology in Teucrium L. (Labiatae). A taxonomic review

Change in plant spatial patterns and diversity along the successional gradient of Mediterranean grazing ecosystems

Changes in plant functional types in response to goat and sheep grazing in two semi-arid shrublands of SE Spain

Quantitative characterization of the regressive ecological succession by fractal analysis of plant spatial patterns

Revisión del género Teucrium L., sección Polium (Mill.) Schreb., (Lamiaceae) en la Península Ibérica y Baleares