The Earth’s tallest trees tend to grow in geographic regions where water is abundant. An abundance of water in the environment, however, does not by itself enable trees to overcome several inherent challenges with being tall. First, water must be transported over very long distances through tiny conduits, and it adheres to their inside walls. Thus, the progress of water flow is impeded by hydraulic resistance that may accumulate with longer transport distances. Second, gravity pulling down on the water imparts a tension that increases with height in tree at a rate of ‒0.01 MPa m-1. Therefore, water inside foliage and branches at the top of a 100 m tall tree is under far more tension than foliage emerging from the base, yet normal physiological processes still occur. Third, environmental conditions become increasingly desiccating with height in forest canopy. Relative humidity decreases while sunlight, wind speed, and vapor pressure deficit increase along this vertical gradient. The strategies trees use to overcome these three challenges in part enable them to grow so tall. In order to improve our understanding of the limits to tree height growth, I studied the mechanisms by which tall trees cope with these height-related constraints, using the conifers coast redwood (Sequoia sempervirens (D. Don) Endlicher) and giant sequoia (Sequoiadendron giganteum (Lindley) J. Buchholz) as well as the angiosperm mountain ash (Eucalyptus regnans F. von Mueller).

In Chapter 1, I explored two mechanisms that trees use to compensate for the accumulation of hydraulic resistance with height growth. The first mechanism is varying the diameters of the conduits with height in tree according to a theoretical model, wherein a series of conduits from tree top to base widens at a specific minimum rate that is maintained throughout height growth. The premise is based in fluid dynamics. An elongating series of cylindrical conduits with constant diameter is hydraulically less efficient than the same elongating series that also widens. I tested this model in exceptionally tall individuals of coast redwood, giant sequoia, and mountain ash. Data matched the theoretical predictions. However, a decelerating rate of conduit widening was observed in the bottom one-third of the trees suggesting a limit to this hydraulic compensation mechanism that I interpreted as an optimization of carbon investment for a given hydraulic benefit. The second mechanism is a whole-tree increase in the amount of sapwood that provisions the leaves. Across a large range of tree sizes, I quantified the rate of accumulation of sapwood relative to leaf area using data available from the literature. Sapwood accumulated at a faster rate than leaf area in the conifers but the rates were equivalent in mountain ash. Sapwood and leaves serve water supply and demand roles, but they are also carbon sinks and sources, respectively. I therefore interpreted these results as hydraulic compensation in the conifers that may limit height growth due to carbon balance constraints, whereas the mountain ash appeared to have additional height growth potential. Limits to these two compensation mechanisms may thus be imposed by carbon balance constraints that limit tree height growth.

Chapter 2 investigates water relations of the foliage of giant sequoia, to determine how foliage remains adequately hydrated against height-related constraints. Together with coauthors Rikke Reese Næsborg and Todd E. Dawson, I generated pressure-volume curves on foliage collected crown-wide from 12 large giant sequoia trees up to 95 m tall, to identify the tissue-level drivers responsible for maintenance of sufficient turgor pressures and water contents both with height tree and over time as the dry season progressed. Hydraulic capacitance was about twice as large as reported for other tree species. Maintenance of turgor pressure in all parts of the trees was accomplished by increases in tissue osmotica with height that depressed the turgor loss point at a rate equivalent to the gravitational potential gradient. High relative water contents were sustained with height by building structurally stiffer tissues as well as carrying an increased proportion of water in the symplasm versus apoplasm. Seasonal increases in the fraction of apoplastic water were important for maintaining physiological function when water in the environment may have been more limiting. This suite of foliar water relations traits permits minimum midday water potentials to operate close to the turgor loss point and may also enable the Earth’s largest tree species to survive short-term drought.

In Chapter 3, I quantify the importance of stem water storage in tall giant sequoia. Water storage in trees is known to contribute substantially to daily transpiration to extend physiological function, but does gravity dampen the dynamics of water storage with height? In the top 5 to 6 m of tall giant sequoia trees I collected detailed architectural information and installed automated sensors that monitored diurnal fluxes in sap flow, stem diameter, and water potential. To provide context for water use at the tree tops, I also installed sap flow gauges at the tree bases. Unsurprisingly, larger stems released larger volumes of stored water. However, tree top water storage contributed a tiny fraction of daily transpired water, and hydraulic capacitance was similarly low, supporting the hypothesis that chronically low water potentials dampen the water storage dynamics with height in tall trees. Lag times among the sensors indicated that a large portion of the stored water was expressed from live secondary phloem tissues of the inner bark. Despite the reliance on seemingly small volumes of stored water, whole-tree sap flow exceeded 3000 L d-1, which is the highest daily water budget reported for any tree on Earth.

These studies together underscore how water use and management in giant trees is truly dynamic over space and time. Over a tree’s lifetime, modifications to sapwood anatomy and volume foster efficient water movement along the entire flow path. Seasonal shifts in the compartments where foliar water is held may extend physiological processes during dry periods. The physiological consequences of daily swings in atmospheric conditions are controlled by tissue osmotic and elastic properties, as well as the release of stored water to the transpiration stream. Each of these dynamics varies along a tree’s vertical profile to compensate for height-related constraints and enables the tallest trees to support functional leaves well beyond 100 m above the ground.