Aim Treelines occur globally within a narrow range of mean growing season temperatures, suggesting that low-temperature growth limitation determines the position of the treeline. However, treelines also exhibit features that indicate that other mechanisms, such as biomass loss not resulting in mortality (dieback) and mortality, determine treeline position and dynamics. Debate regarding the mechanisms controlling treeline position and dynamics may be resolved by identifying the mechanisms controlling prominent treeline spatial patterns (or 'form') such as the spatial structure of the transition from closed forest to the tree limit. Recent treeline studies world-wide have confirmed a close link between form and dynamics.Location The concepts presented refer to alpine treelines globally. MethodsIn this review, we describe how varying dominance of three general 'first-level' mechanisms (tree performance: growth limitation, seedling mortality and dieback) result in different treeline forms, what 'second-level' mechanisms (stresses, e.g. freezing damage, photoinhibition) may underlie these general mechanisms, and how they are modulated by interactions with neighbours ('third-level' mechanisms). This hierarchy of mechanisms should facilitate discussions about treeline formation and dynamics. ResultsWe distinguish four primary treeline forms: diffuse, abrupt, island and krummholz. Growth limitation is dominant only at the diffuse treeline, which is the form that has most frequently responded as expected to growing-season warming, whereas the other forms are controlled by dieback and seedling mortality and are relatively unresponsive. Main conclusionsTreeline form provides a means for explaining the current variability in treeline position and dynamics and for exploring the general mechanisms controlling the responses of treelines to climatic change. Form indicates the relative dependence of tree performance on various aspects of the external climate (especially summer warmth versus winter stressors) and on internal feedbacks, thus allowing inferences on the type as well as strength of climate-change responses.

Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and meta-analyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging from -9 to +26 °C MAT and from 60 to 3113 mm MAP) across different ecosystems. In this study we tested the effect of climate (temperature and moisture), litter type and land-use on early stage decomposition (3 months) across nine biomes. We show that litter quality was the predominant controlling factor in early stage litter decomposition, which explained about 65% of the variability in litter decomposition at a global scale. The effect of climate, on the other hand, was not litter specific and explained <0.5% of the variation for Green tea and 5% for Rooibos tea, and was of significance only under unfavorable decomposition conditions (i.e. xeric versus mesic environments). When the data were aggregated at the biome scale, climate played a significant role on decomposition of both litter types (explaining 64% of the variation for Green tea and 72% for Rooibos tea). No significant effect of land-use on early stage litter decomposition was noted within the temperate biome. Our results indicate that multiple drivers are affecting early stage litter mass loss with litter quality being dominant. In order to be able to quantify the relative importance of the different drivers over time, long-term studies combined with experimental trials are needed.

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The Chilean Coastal Cordillera features a spectacular climate and vegetation gradient, ranging from arid and unvegetated areas in the north to humid and forested areas in the south. The EarthShape project ("Earth Surface Shaping by Biota") uses this natural gradient to investigate how climate and biological processes shape the Earth's surface. We explored the Critical Zone, the Earth's uppermost layer, in four key sites located in desert, semidesert, mediterranean, and temperate climate zones of the Coastal Cordillera, with the focus on weathering of granitic rock. Here, we present first results from 16 approximately 2 m-deep regolith profiles to document: (1) architecture of weathering zone; (2) degree and rate of rock weathering, thus the release of mineral-derived nutrients to the terrestrial ecosystems; (3) denudation rates; and (4) microbial abundances of bacteria and archaea in the saprolite. From north to south, denudation rates from cosmogenic nuclides are ~10 t km-2 yr-1 at the arid Pan de Azúcar site, ~20 t km-2 yr-1 at the semi-arid site of Santa Gracia, ~60 t km-2 yr-1 at the mediterranean climate site of La Campana, and ~30 t km-2 yr-1 at the humid site of Nahuelbuta. A and B horizons increase in thickness and elemental depletion or enrichment increases from north (~26 °S) to south (~38 °S) in these horizons. Differences in the degree of chemical weathering, quantified by the chemical depletion fraction (CDF), are significant only between the arid and sparsely vegetated site and the other three sites. Differences in the CDF between the sites, and elemental depletion within the sites are sometimes smaller than the variations induced by the bedrock heterogeneity. Microbial abundances (bacteria and archaea) in saprolite substantially increase from the arid to the semi-arid sites. With this study, we provide a comprehensive dataset characterizing the Critical Zone geochemistry in the Chilean Coastal Cordillera. This dataset confirms climatic controls on 4 weathering and denudation rates and provides prerequisites to quantify the role of biota in future studies.

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Many tropical alpine treelines lie below their climatic potential, because of natural or anthropogenic causes. Forest extension above the treeline depends on the ability of trees to establish in the alpine environment. This ability may be limited by different factors, such as low temperatures, excess solar radiation, competition, soil properties, dispersal ability, and fires. In this paper we address the following two questions: Do trees regenerate above the present treeline, and what are the inhibiting factors for tree establishment? To answer these questions we described the spatial pattern of recent tree establishment below and above the present treeline in northern Ecuador. Also, we experimentally transplanted seedlings into the alpine vegetation (pá ramo) and the forest, and investigated the effect of shade, neighboring plants, and substrate on their survival. The number of naturally occurring tree sprouts (seedlings, saplings and ramets) was highest just outside the forest, and decreased with distance to the forest edge. However, only two species that were radiation-tolerant made up these high numbers, while other species were rare or absent in the pá ramo. In the forest, the species diversity of sprouts was high and the abundance per species was relatively low. The transplanted seedlings survived least in experimental plots without artificial shade where neighboring plants were removed. Seedling survival was highest in artificially shaded plots and in the forest. This shade-dependence of most tree species can strongly slow down forest expansion toward the potential climatic treeline. Due to the presence of radiation-tolerant species, the complete lack of forest expansion probably needs to be ascribed to fire. However, our results show that natural processes can also explain both the low position and the abruptness of tropical treelines.

Current analyses and predictions of spatially explicit patterns and processes in ecology most often rely on climate data interpolated from standardized weather stations. This interpolated climate data represents long-term average thermal conditions at coarse spatial resolutions only. Hence, many climate-forcing factors that operate at fine spatiotemporal resolutions are overlooked. This is particularly important in relation to effects of observation height (e.g. vegetation, snow and soil characteristics) and in habitats varying in their exposure to radiation, moisture and wind (e.g. topography, radiative forcing or cold-air pooling). Since organisms living close to the ground relate more strongly to these microclimatic conditions than to free-air temperatures, microclimatic ground and near-surface data are needed to provide realistic forecasts of the fate of such organisms under anthropogenic climate change, as well as of the functioning of the ecosystems they live in. To fill this critical gap, we highlight a call for temperature time series submissions to SoilTemp, a geospatial database initiative compiling soil and near-surface temperature data from all over the world. Currently, this database contains time series from 7,538 temperature sensors from 51 countries

Alpine treeline ecotones can be gradual transitions, abrupt boundaries, or patchy mosaics, and these different patterns may indicate important processes and dynamic properties. We present observed spatial patterns of a wide range of tropical treelines and try to explain these patterns. Treelines were studied at seven sites in the tropical and subtropical Andes (Argentina, Bolivia, Ecuador, and Venezuela) and on a Hawaiian volcano (Haleakala, Maui). Treeline vegetation structure was described using transects perpendicular to the treeline, and air and soil temperatures were measured above and below the forest boundary. Temperature fluctuations were much larger and the average temperature was higher in alpine vegetation than in forest. Most treelines were abrupt, with surprisingly similar patterns across a wide geographical range. This abruptness could result from positive feedback processes mediated by the differences in microclimate between forest and pá ramo. Our data is not conclusive about the relative importance of microclimate as opposed to fire in mediating such feedbacks. However, our extensive set of comparable data from different sites in a large geographical region is an important step toward a better understanding of the nature and dynamics of tropical alpine treelines.