Summary: Root competition is defined as a reduction in the availability of a soil resource to roots that is caused by other roots. Resource availability to competitors can be affected through resource depletion (scramble competition) and by mechanisms that inhibit access of other roots to resources (contest competition, such as allelopathy). It has been proposed that soil heterogeneity can cause size‐asymmetric root competition. Support for this hypothesis is limited and contradictory, possibly because resource uptake is affected more by the amount and spatial distribution of resource‐acquiring organs, relative to the spatial distribution of resources, than by root system sizeper se. Root competition intensity between individual plants generally decreases as resource availability (but not necessarily habitat productivity) increases, but the importance of root competition relative to other factors that structure communities may increase with resource availability. Soil organisms play important, and often species‐specific, roles in root interactions. The findings that some roots can detect other roots, or inert objects, before they are contacted and can distinguish between self and non‐self roots create experimental challenges for those attempting to untangle the effects of self/non‐self root recognition, self‐inhibition and root segregation or proliferation in response to competition. Recent studies suggesting that root competition may represent a ‘tragedy‐of‐the‐commons’ may have failed to account for this complexity. Theories about potential effects of root competition on plant diversity (and vice versa) appear to be ahead of the experimental evidence, with only one study documenting different effects of root competition on plant diversity under different levels of resource availability. Roots can interact with their biotic and abiotic environments using a large variety of often species‐specific mechanisms, far beyond the traditional view that plants interact mainly through resource depletion. Research on root interactions between exotic invasives and native species holds great promise for a better understanding of the way in which root competition may affect community structure and plant diversity, and may create new insights into coevolution of plants, their competitors and the soil community.

Pit membranes in bordered pits between neighbouring vessels play a major role in the entry of air-water menisci from an embolised vessel into a water-filled vessel (i.e., air-seeding). Here, we investigate intervessel pit membrane thickness (T_PM) and embolism resistance (P₅₀, i.e., the water potential corresponding to 50% loss of hydraulic conductivity) across a broad range of woody angiosperm species. Data on T_PM and double intervessel wall thickness (T_VW) were compiled based on electron and light microscopy. Fresh material that was directly fixated for transmission electron microscopy (TEM) was investigated for 71 species, while non-fresh samples were frozen, stored in alcohol, or air dried prior to TEM preparation for an additional 60 species. T_PM and P₅₀ were based on novel observations and literature. A strong correlation between TPM and P50 was found for measurements based on freshly fixated material (r = 0.78, P >0.01, n = 37), and between T_PM and T_VW (r = 0.79, P >0.01, n = 59), while a slightly weaker relationship occurred between TVW and P50 (r = 0.40, P >0.01, n = 34). However, non-fresh samples showed no correlation between T_PM and P₅₀, and between T_PM and T_VW. Intervessel pit membranes in non-fresh samples were c.28% thinner and more electron dense than fresh samples. Our findings demonstrate that T_PM measured on freshly fixated material provides one of the strongest wood anatomical correlates of droughtinduced embolism resistance in angiosperms. Assuming that cellulose microfibrils show an equal spatial density, T_PM is suggested to affect the length and the shape of intervessel pit membrane pores, but not the actual pore size. Moreover, the shrinking effect observed for T_PM after dehydration and frost is associated with an increase in microfibril density and porosity, which may provide a functional explanation for embolism fatigue.

Summary: Although the above and belowground sizes and shapes of plants strongly influence plant competition, community structure, and plant–environment interactions, plant sizes and shapes remain poorly characterized across climate regimes. We investigated relationships among shoot and root system size and climate. We assembled and analyzed, to our knowledge, the largest global database describing the maximum rooting depth, lateral spread, and shoot size of terrestrial plants – more than doubling the Root Systems of Individual Plants database to 5647 observations. Water availability and growth form greatly influence shoot size, and rooting depth is primarily influenced by temperature seasonality. Shoot size is the strongest predictor of lateral spread, with root system diameter being two times wider than shoot width on average for woody plants. Shoot size covaries strongly with rooting system size; however, the geometries of plants differ considerably across climates, with woody plants in more arid climates having shorter shoots, but deeper, narrower root systems. Additionally, estimates of the depth and lateral spread of plant root systems are likely underestimated at the global scale.