We review observational, experimental, and model results on how plants respond to extreme climatic conditions induced by changing climatic variability. Distinguishing between impacts of changing mean climatic conditions and changing climatic variability on terrestrial ecosystems is generally underrated in current studies. The goals of our review are thus (1) to identify plant processes that are vulnerable to changes in the variability of climatic variables rather than to changes in their mean, and (2) to depict/evaluate available study designs to quantify responses of plants to changing climatic variability. We find that phenology is largely affected by changing mean climate but also that impacts of climatic variability are much less studied, although potentially damaging. We note that plant water relations seem to be very vulnerable to extremes driven by changes in temperature and precipitation and that heat-waves and flooding have stronger impacts on physiological processes than changing mean climate. Moreover, interacting phenological and physiological processes are likely to further complicate plant responses to changing climatic variability. Phenological and physiological processes and their interactions culminate in even more sophisticated responses to changing mean climate and climatic variability at the species and community level. Generally, observational studies are well suited to study plant responses to changing mean climate, but less suitable to gain a mechanistic understanding of plant responses to climatic variability. Experiments seem best suited to simulate extreme events. In models, temporal resolution and model structure are crucial to capture plant responses to changing climatic variability. We highlight that a combination of experimental, observational, and/or modeling studies have the potential to overcome important caveats of the respective individual approaches.

Aim Despite their importance for predicting fluxes to and from terrestrial ecosystems, dynamic global vegetation models have insufficient realism because of their use of plant functional types (PFTs) with constant attributes. Based on recent advances in community ecology, we explore the merits of a traits-based vegetation model to deal with current shortcomings. Location Global.Methods A research review of current concepts and information, providing a new perspective, supported by quantitative analysis of a global traits database.Results Continuous and process-based trait-environment relations are central to a traits-based approach and allow us to directly calculate fluxes based on functional characteristics. By quantifying community assembly concepts, it is possible to predict trait values from environmental drivers, although these relations are still imperfect. Through the quantification of these relations, effects of adaptation and species replacement upon environmental changes are implicitly accounted for. Such functional links also allow direct calculation of fluxes, including those related to feedbacks through the nitrogen and water cycle. Finally, a traits-based model allows the prediction of new trait combinations and no-analogue ecosystem functions projected to arise in the near future, which is not feasible in current vegetation models. A separate calculation of ecosystem fluxes and PFT occurrences in traitsbased models allows for flexible vegetation classifications.Main conclusions Given the advantages described above, we argue that traitsbased modelling deserves consideration (although it will not be easy) if one is to aim for better climate projections.

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In ecology, strategy schemes based on propositions about the selection of plant attributes are common, but quantification of such schemes in relation to nutrient and water supply is lacking. Through structural equation modeling, we tested whether plant strategies related to nutrient and water/oxygen supply are reflected in a coordination of traits in natural communities. Structural equation models, based on accepted ecological concepts, were tested with measured plant traits of 105 different species across 50 sites in mesic to wet plant communities in the Netherlands. For each site, nutrient and water supply were measured and modeled. Hypothesized multivariate strategy models only partly reflected current theoretical schemes. Alternative models were consistent, showing that lack of consistency of the original models was because of (i) strong correlations among traits that supposedly belong to different strategy components; (ii) poor understanding of mechanisms determining the covariation of plant maximum height, leaf size, and stem density; and (iii) lack of integrative and long-term measures of nutrient supply needed to predict coordinated plant trait responses. Our main conclusion is that a combination of trade-offs (partly) across different plant organs and diverging effects of resource supply ultimately determines the coordination of plant traits needed to "make a living."

The large variation in the relationships between environmental factors and plant traits observed in natural communities exemplifies the alternative solutions that plants have developed in response to the same environmental limitations. Qualitative attributes, such as growth form, woodiness, and leaf habit can be used to approximate these alternative solutions. Here, we quantified the extent to which these attributes affect leaf trait values at a given resource supply level, using measured plant traits from 105 different species (254 observations) distributed across 50 sites in mesic to wet plant communities in The Netherlands. For each site, soil total N, soil total P, and water supply estimates were obtained by field measurements and modeling. Effects of growth forms, woodiness, and leaf habit on relations between leaf traits (SLA, specific leaf area; LNC, leaf nitrogen concentration; and LPC, leaf phosphorus concentration) vs. nutrient and water supply were quantified using maximum-likelihood methods and Bonferroni post hoc tests. The qualitative attributes explained 8-23% of the variance within sites in leaf traits vs. soil fertility relationships, and therefore they can potentially be used to make better predictions of global patterns of leaf traits in relation to nutrient supply. However, at a given soil fertility, the strength of the effect of each qualitative attribute was not the same for all leaf traits. These differences may imply a differential regulation of the leaf economy traits at a given nutrient supply, in which SLA and LPC seem to be regulated in accordance to changes in plant size and architecture while LNC seems to be primarily regulated at the leaf level by factors related to leaf longevity.

Summary1. Our understanding of the generality of plant functional responses to water availability is limited; current field studies use either very rough approximations of water and oxygen availability or only focus on water-stressed ecosystems. Studies that relate species' responses to a surplus of water are limited to controlled experiments. 2. The aim of this study was to investigate how traits are selected along a gradient of soil moisture, ranging from oxygen-stressed to drought-stressed. We tested 15 traits: eight leaf traits, two root traits, two seed traits and three allometry traits and related their community means to process-based measures of drought stress and oxygen stress for 171 plots in the Netherlands. Because the trait values had been taken from a large database, an independent field survey was carried out to validate the relationships thus derived. 3. We show that root porosity and seed floating capacity are mostly strongly related, although still moderately, to oxygen and drought stress (R 2 = 27% and 42%, respectively). Leaf traits responded weakly to either of the stressors. The field survey yielded similar relationships. Trait combinations were much more closely related to oxygen or drought stress than individual traits, suggesting that there are multiple trait solutions at a given level of water and oxygen stress. 4. The relatively weak relationships found between traits and water-related stressors contrast with the strong control of other environmental drivers (disturbance, nutrients) on traits and suggest that these strong constraints imposed by other environmental drivers necessitate varied solutions to cope with water availability.

. SWAP version 4; Theory description and user manual. Wageningen, Wageningen Environmental Research, Report 2780. 244 pp.; 57 fig.; 17 tab.; 312 ref. SWAP 4 simulates transport of water, solutes and heat in the vadose zone. It describes a domain from the top of canopy into the groundwater which may be in interaction with a surface water system. The program has been developed by Wageningen Environmental Research and Wageningen University, and is designed to simulate transport processes at field scale and during entire growing seasons. This is a new release with recent developments on atmosphere, soil water and crop growth interactions.This manual describes the theoretical background, model use, input requirements and output tables. • Acquisition, duplication and transmission of this publication is permitted with clear acknowledgement of the source.• Acquisition, duplication and transmission is not permitted for commercial purposes and/or monetary gain.• Acquisition, duplication and transmission is not permitted of any parts of this publication for which the copyrights clearly rest with other parties and/or are reserved.Wageningen Environmental Research assumes no liability for any losses resulting from the use of the research results or recommendations in this report. Wageningen Environmental Research Report 2780 | ISSN 1566-7197Photo cover: The picture on the front cover shows SWAP's core processes in the soil below a grass vegetation positioned in a rural area with different land uses. ContentsPreface 7 and hence the dynamics of light interception. During crop development a part of the living biomass dies due to senescence (Chapter 7).Grass growth is special: it is perennial, very sensitive to nitrogen, and grass is either grazed or mowed. Therefore SWAP includes a separate WOFOST module for grass, which simulates these special grass features (Chapter 7).SWAP simulates transport of salts, pesticides and other solutes that can be described with basic physical relations: convection, diffusion, dispersion, root uptake, Freundlich adsorption and first order decomposition. In case of advanced pesticide transport, including volatilization and kinetic adsorption, SWAP can be used in combination with PEARL. In case of advanced transport of nitrogen and phosphorus, SWAP can be used in combination with ANIMO or Soil-N (Chapter 8).SWAP may simulate soil temperature analytically, using an input sine function at the soil surface and the soil thermal diffusivity. In the numerical approach, SWAP takes into account the influence of soil moisture on soil heat capacity and soil thermal conductivity. The top boundary condition may include air temperatures or soil surface temperatures (Chapter 9).The snow module calculates the accumulation and melting of a snowpack when the air temperature is below a threshold value. The water balance of the snow pack includes storage, incoming snow and rain and outgoing melting and sublimation. Melting may occur due to air temperature rise or heat release from rainfall. When a snowpack is p...

Evaporation from mosses and lichens can form a major component of the water balance, especially in ecosystems where mosses and lichens often grow abundantly, such as tundra, deserts and bogs. To facilitate moss representation in hydrological models, we parameterized the unsaturated hydraulic properties of mosses and lichens such that the capillary water flow through moss and lichen material during evaporation could be assessed. We derived the Mualem‐van Genuchten parameters of the drying retention and the hydraulic conductivity functions of four xerophilous moss species and one lichen species. The shape parameters of the retention functions (2.17 < n < 2.35 and 0.08 < α < 0.13 cm−1) ranged between values that are typical for sandy loam and loamy sand. The shapes of the hydraulic conductivity functions of moss and lichen species diverged from those of mineral soils, because of strong negative pore‐connectivity parameters (−2.840 < l < −2.175) and low hydraulic conductivities at slightly negative pressure heads (0.016 < K0 < 0.280 cm/d). These K0 values are surprisingly low, considering that mosses are very porous. However, during evaporation, large pores and voids were air filled and did not participate in capillary water flow. Small K0 values cause mosses and lichens to be conservative with water during wet conditions, thus tempering evaporation compared to mineral soils. On the other hand, under dry conditions, mosses and lichens are able to maintain a moisture supply from the soil, leading to a higher evaporation rate than mineral soils. Hence, the modulating effect of mosses on evaporation possibly differs between wet and dry climates. Copyright © 2013 John Wiley & Sons, Ltd.