Resource pulse use by desert plants

Water becomes available to plants in principally two ways: as short-term pulses of shallow soil moisture and as continually available soil moisture in deeper soil layers. Using this simplified two-layer model of water availability, we asked what physiological and morphological plant characteristics would maximize photosynthetic carbon gain under different patterns of soil moisture availability. We found that there is a strong morphological tradeoff associated with the use of shallow and deeper water sources. During pulse periods, low root/shoot ratios and an all-shallow root system maximize photosynthetic carbon gain, but between pulses, when only deeper soil water is available, higher root/shoot ratios and a predominantly deep root system is more favorable. In a pulse-driven environment, intermediate morphologies are optimal and depending on pulse frequency and the availability of deeper soil moisture, optimally adapted plants lean more towards the shallow-rooted herbeceous extreme or the deep-rooted xeromorphic extreme. Stem succulence for water storage is an optimal solution only when deeper soil moisture is unavailable.

By optimizing plant water use under a range of soil moisture conditions, we generated a array of optimal phenotypes, which resemble the plant functional types recognized in arid ecosystems. Thus we show that certain character combinations in desert plants may be interpreted as an optimal adaptation to a specific pattern of water availability. Implicit in this modeling result is that a) precipitation patterns, particularly the seasonal distribution of precipitation, can determine which functional types dominate in a region. Furthermore, based on this approach it may be possible to predict changes in functional types composiyion, whith changing precipitation patterns.

Related Publications

  • Ehleringer, J.R., Schwinning, S. & Gebauer, R.L.E. 1999. Water use in arid land ecosystems. IN: Advances in Plant Physiological Ecology, Press, M.C. (ed.), Blackwell Science, Oxford, pp. 347-365.

  • Schwinning, S. and Ehleringer, J.R. 2001. Water use tradeoffs and optimal adaptations to pulse-driven arid ecosystems. Journal of Ecology 89: 464-480. pdf file