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Water and carbon cycles are strongly coordinated and water availability is a primary limiting factor in many terrestrial ecosystems. Photosynthesis requires sufficient water supply to leaves and constraints on delivery at any point in the hydraulic continuum can lead to stomatal closure and reduced photosynthesis. Thus, maximizing water transport enhances assimilation and can provide plants with a competitive advantage. Unregulated water transport, however, can lead to excessive gradients in xylem tension that result in the development of air or vapor bubbles (i.e. embolisms) that block xylem water transport, potentially leading to permanent loss of function of the xylem. As such there can be a tradeoff between maximizing water transport and minimizing the risk of xylem embolism. This tradeoff has led to the development of a variety of hydraulic mechanisms to maximize efficiency and reduce vulnerability. Although several of these were first described centuries ago (such as stomatal control of transpiration), research in this field continues to uncover previously unrecognized processes employed by plants for maintaining hydraulic safety and/or efficiency. The hydraulic traits described in this chapter include xylem structural characteristics that enhance resistance to embolism such as pit and cell wall architecture; a continuum of strategies for constraining xylem tension to avoid embolism including isohydric and anisohydric control of leaf water potential; and safety and recovery mechanisms such as the capacitive discharge of stored water, hydraulic “circuit breakers” and the ability to repair xylem embolisms. Each of these will be discussed in terms of the variation in their use by contrasting tree types, their variability across organs and species, and their plasticity across environmental gradients. Beyond providing information about the means by which trees currently compete and survive, understanding the hydraulic mechanisms described in this chapter may provide insight into ways that trees are affected by, and the degree to which they may acclimate to rapidly changing climatic conditions.

Carbon (C), nutrients and water (H2O) have been known for five decades to flow between plants through ectomycorrhizal (EM) networks. This flux has the potential to affect plant and fungal performance and resource distribution within communities. We asked two questions: (1) What are the pathways and mechanisms for C, nutrient and H2O fluxes between plants through EM networks? (2) What are the magnitude, fate and importance of C, nutrient and H2O transfer among EM plants? Mycorrhizal networks provide a distinct pathway for resource fluxes among plants and mycorrhizal fungi, partitioning them away from other competing soil microbes and plant roots in the soil matrix, and potentially providing a competitive advantage (or disadvantage) for some individuals involved in the network. Carbon and nutrients flow symplastically and apoplastically through mycorrhizal symbionts, hyphae and rhizomorphs along source-sink gradients across the networking mycelia and plant community. EM networks can also facilitate the hydraulic redistribution of soil or plant water following water potential gradients. Carbon fluxes through EM networks have been shown to supply 0–10 % of autotrophic, up to 85 % of partial myco-heterotrophic (MH), and 100 % of fully MH plant C. This C supply has been loosely associated with the increased survival and growth of autotrophic plants, but has been shown to be essential for the survival of MH plants. Network-mediated nitrogen (N) fluxes between N2-fixing and non-N2-fixing plants have supplied up to 40 % of receiver N, and this has been associated with increased plant productivity. Fluxes between non-N2-fixing plants have supplied

There is an increasing demand for data on tree structural and functional attributes that can be collected simultaneously at multiple sites and integrated across landscapes. Here, we present several examples of approaches applicable for measuring tree and stand structures and for characterizing spatial and temporal variation in important functions of the above- and belowground parts of both large and small trees. Special attention is given to explanations of the theoretical basis for several sap flow techniques and to the types of information that can be gleaned from carefully planned measurements of sap flow in stems and roots. A variety of approaches for characterizing the structure and function of root systems: the hidden half of trees, are also described.