Both engineered hydraulic systems and plant hydraulic systems are protected against failure by resistance, reparability, and redundancy. A basic rule of reliability engineering is that the level of independent redundancy should increase with increasing risk of fatal system failure. Here we show that hydraulic systems of plants function as predicted by this engineering rule. Hydraulic systems of shrubs sampled along two transcontinental aridity gradients changed with increasing aridity from highly integrated to independently redundant modular designs. Shrubs in humid environments tend to be hydraulically integrated, with single, round basal stems, whereas dryland shrubs typically have modular hydraulic systems and multiple, segmented basal stems. Modularity is achieved anatomically at the vessel-network scale or developmentally at the whole-plant scale through asymmetric secondary growth, which results in a semiclonal or clonal shrub growth form that appears to be ubiquitous in global deserts.plant hydraulic systems ͉ wood anatomy ͉ hydraulic redundancy ͉ xylem structure and function I n engineering terms, the hydraulic system of a plant is a negative-pressure flow system. This type of hydraulic system, whether natural or man-made, is prone to fail when air bubbles (emboli) are introduced, because under strong negative pressure a single embolism can lead to breakage of the water column unless the air bubble is isolated in a branch or pipe. Both drought and freezing can cause embolisms in plants (1).Drought-induced embolisms form under negative pressure, when air is pulled into a water-filled conduit from adjacent air-filled spaces or cells, a process known as ''air seeding.'' This common, even daily, event (2-4) can lead to complete failure of the hydraulic system if runaway embolism occurs (5). Two of the three attributes by which plants' negative-pressure flow systems can be protected against failure, resistance and reparability, have been subjects of active research during the last decade (2-4, 6-10). The third attribute, redundancy, has received much less attention as an important drought adaptation but is emerging as a focus of research (11)(12)(13)(14). Attributes of redundancy in hydraulic systems of vessel-bearing angiosperms include the numbers of vessels (14), the vessel network topology (12), the number and sizes of pits between adjacent vessels (13,15,16), and the division of whole plants into independent hydraulic units (17).A basic rule of reliability engineering states that the level of independent redundancy should increase with increasing risk of fatal system failure (18); hydraulic engineers routinely increase the safety of man-made pressure-flow systems by designing them to be redundant (19). Redundancy in hydraulic systems (Fig. 1) can vary from a high degree of inter-connectedness (i.e., integrated redundancy) to complete, independent compartmentation (i.e., modular redundancy). In a negative-pressure flow system, integrated redundancy allows alternate water transport pathways around blockage...

Summary• Plant light interception efficiency is a crucial determinant of carbon uptake by individual plants and by vegetation. Our aim was to identify whole-plant variables that summarize complex crown architecture, which can be used to predict light interception efficiency.• We gathered the largest database of digitized plants to date (1831 plants of 124 species), and estimated a measure of light interception efficiency with a detailed three-dimensional model. Light interception efficiency was defined as the ratio of the hemispherically averaged displayed to total leaf area. A simple model was developed that uses only two variables, crown density (the ratio of leaf area to total crown surface area) and leaf dispersion (a measure of the degree of aggregation of leaves).• The model explained 85% of variation in the observed light interception efficiency across the digitized plants. Both whole-plant variables varied across species, with differences in leaf dispersion related to leaf size. Within species, light interception efficiency decreased with total leaf number. This was a result of changes in leaf dispersion, while crown density remained constant.• These results provide the basis for a more general understanding of the role of plant architecture in determining the efficiency of light harvesting.

Research linking functional traits to competitive ability of coexisting species has largely relied on rectilinear correlations, yielding inconsistent results. Based on concepts borrowed from natural selection theory, we propose that trait–competition relationships can generally correspond to three univariate selection modes: directional (a rectilinear relationship), stabilising (an n‐shaped relationship), and disruptive (a u‐shaped relationship). Moreover, correlational selection occurs when two traits interact in determining competitive ability and lead to an optimum trait combination (i.e., a bivariate nonlinear selection mode). We tested our ideas using two independent datasets, each one characterising a group of species according to (a) their competitive effect on a target species in a pot experiment and (b) species‐level values of well‐known functional traits extracted from existing databases. The first dataset comprised 10 annual plant species frequent in a summer‐rainfall desert in Argentina, while the second consisted of 37 herbaceous species from cool temperate vegetation types in Canada. Both experiments had a replacement design where the identity of neighbours was manipulated holding total plant density in pots constant. We modelled the competitive ability of neighbours (i.e., the log inverse of target plant biomass) as a function of traits using normal multiple regression. Leaf dry matter content (LDMC) was consistently subjected to negative directional selection in both experiments as well as to stabilising selection among temperate species. Leaf size was subjected to stabilising selection among desert species while among temperate species, leaf size underwent correlational selection in combination with specific leaf area (SLA): selection on SLA was negative directional for large‐leaved species, while it was slightly positive for small‐leaved species. Synthesis. Multiple quadratic regression adds functional flexibility to trait‐based community ecology while providing a standardised basis for comparison among traits and environments. Our analyses of two datasets from contrasting environmental conditions indicate (a) that leaf dry matter content can capture an important component of plant competitive ability not accounted for by widely used competitive traits, such as specific leaf area, leaf size, and plant height and (b) that optimum relationships (either univariate or bivariate) between competitive ability and plant traits may be more common than previously realised.

Explanations of leaf size variation commonly focus on water availability, yet leaf size also varies with latitude and elevation in environments where water is not strongly limiting. We provide the first conclusive test of a prediction of leaf energy balance theory that may explain this pattern: large leaves are more vulnerable to night-time chilling, because their thick boundary layers impede convective exchange with the surrounding air. Seedlings of 15 New Zealand evergreens spanning 12-fold variation in leaf width were exposed to clear night skies, and leaf temperatures were measured with thermocouples. We then used a global dataset to assess several climate variables as predictors of leaf size in forest assemblages. Leaf minus air temperature was strongly correlated with leaf width, ranging from -0.9 to -3.2°C in the smallest- and largest-leaved species, respectively. Mean annual temperature and frost-free period were good predictors of evergreen angiosperm leaf size in forest assemblages, but no climate variable predicted deciduous leaf size. Although winter deciduousness makes large leaves possible in strongly seasonal climates, large-leaved evergreens are largely confined to frost-free climates because of their susceptibility to radiative cooling. Evergreen leaf size data can therefore be used to enhance vegetation models, and to infer palaeotemperatures from fossil leaf assemblages.

Tree establishment can have multiple effects on the production and biodiversity of rangelands. In mixed (C3-C4) grasslands, winter deciduous trees could favor cold-season species in the understory, improving forage availability in the most critical time of the year. Yet, they could also promote local extinctions and invasions, risking native biodiversity. We evaluate the effect of poplar planting on the structure, composition, and diversity of native grasslands in the Flooding Pampas of Argentina using a network of 9 pairs of adjacent nonafforested and afforested stands (age: 23-25 years, density: 625-1 111 plants ha À1) located in different topographic positions. Phytosociological surveys, basal cover measurements, and tree volume were performed at all stands. Live plant cover was 42% lower under poplars (P , 0.05). Litter cover followed an opposite trend leaving bare soil proportions unchanged with afforestation. Afforested stands had a higher proportion of C3 species compared with nonafforested ones. Little evidence of local extinctions or invasions with afforestation was found. Poplar understories had significantly higher nonnative species cover but similar numbers and lower species diversity (Shannon-Weaver index) yet similar species richness when compared with their nonafforested counterparts. Beyond the diversification of ranch outputs, deciduous tree plantations in the Flooding Pampas can offer a good forage source in their understory that complements nonafforested natural grasslands in quality and seasonality. Resumen El establecimiento de á rboles puede tener mú ltiples efectos sobre la producció n y la biodiversidad de pastizales. En praderas mixtas (C3-C4), los á rboles deciduos podrían favorecer a las especies invernales del estrato herbá ceo, mejorando la disponibilidad de forraje en el período má s crítico del añ o. Sin embargo, los á rboles también podrían promover extinciones e invasiones locales, amenazando la biodiversidad del sistema. Evaluamos los efectos de la forestació n con á lamos sobre la estructura, composició n y diversidad florística de pastizales naturales en la Pampa Inundable de Argentina utilizando una red de nueve sitios apareados integrados por stands adyacentes de pastizal no forestado y forestado (edad: 23-25 añ os, densidad: 625-1 111 plantas ha À1) ubicados en diferentes posiciones topográ ficas. Se llevaron a cabo censos fitosocioló gicos, medidas de cobertura basal, y del volumen de á rboles en todos los sitios. La cobertura de plantas vivas fue 42% menor bajo los á lamos (P , 0.05). La cobertura de broza mostró una tendencia opuesta, manteniendo la proporció n de suelo desnudo sin cambios tras la forestació n. Los stands forestados tuvieron una mayor proporció n de especies C3 comparados con los no forestados. Se encontraron pocos indicios de extinciones o invasiones locales asociados a la forestació n. Las comunidades herbá ceas bajo á lamos tuvieron significativamente mayor cobertura de especies no nativas pero igual riqueza de las mismas comparadas con las situaciones...