Scaling the soil-plant-atmosphere continuum: from physics to ecosystems.

Over half of the annual precipitation falling on land can be channeled back to the atmosphere through plant vascular systems. Discovering the emergent properties of plant water transport at the community and ecosystem level will improve our understanding of the role of plants in the global water cycle, and how this role might change with future land use and climate patterns.A specific example of how plant water transport influences community level dynamics was described by Barbara Bond (Oregon State University, Corvallis, OR, USA). It is well documented that the productivity of forests declines with age but the causes are debatable. Bond showed that this decline is often related specifically to tree height, and in many cases to associated limitations in water transport. The longer the path length, and the taller the tree, the greater the pressure difference required to move the transpiration stream against friction and gravity. To the extent that the operating range of negative pressure available for transport is limited by cavitation or by other forms of dehydrative damage, larger trees will not be able to generate the driving force of a smaller tree. As a result they should suffer reductions in leaf area and sapwood area and/or reduction in stomatal conductances. Either result will tend to reduce productivity. Acting in concert with other size-related limitations, this hydraulic constraint will contribute to declining productivity with size. To a varying extent, the hydraulic limitation can be compensated by increased trunk capacitance, conduit diameter and increased root system conductivity, but these surely must come with their own set of trade-offs and costs that need to be evaluated.Culminating the symposium was Ram Oren's presentation (Duke University, Durham NC, USA) on controls over ecosystem water use. Variation in stand transpiration (EC), a major part of ecosystem water use, is related to the surface area of leaves per unit ground (leaf area index, L) and the extent of coupling between leaf area and atmosphere. Departures from the relationship between EC and L (independent of coupling) can be expected when hydraulic constraints and environmental conditions reduce stomatal conductance. Hydraulic constraints include the size limitations presented by Bond and the restrictions on the water use envelope resulting from soil and xylem properties as discussed by Sperry. For example, pine stands on sand versus loam soils have dramatically different L versus EC relationships because of the limitations on the rate of water uptake from coarse soils. Effects of fertilization and irrigation on water-use patterns were predictable based on analysis of transport limitations. The sensitivity of stand water use with respect to evaporative demand was also influenced by transport considerations, showing a strong convergence across species and sites wherein higher maximum canopy stomatal conductance was associated with greater sensitivity to vapor pressure deficit. As also seen from Rick Meinzer's presentation (USFS, Corvallis, OR, USA) this convergence was a direct consequence of the role of stomata in regulating leaf water status in response to leaf water supply.The overall impression created by the symposium was that there was an exciting sense of synergy and discovery emerging from the integration of work at all scales along the hydraulic continuum. It was clear by the end of the session, that the constraints associated with long-distance transport have an important influence on shaping the form and function of plants, their ecology and their role in ecosystem functioning.