We studied six species of Quercus, which can be regarded as components of two different functional and ecological groups within the Spanish flora. The first group consists of evergreen species (Q. coccifera, Q. ilex subsp ballota, Q. ilex and Q. suber) that occupy typical Mediterranean ecosystems in the Iberian Peninsula. The second group consists of deciduous species (Q. faginea and Q. pubescens), inhabiting wetter and richer soil conditions.
Relative growth rates (RGR) were measured in these two groups and were compared to other parameters commonly presumed and widely used to predict RGR, i.e., leaf area ratio, leaf weight ratio, specific leaf mass, leaf nitrogen content, and plant carbon allocation. The values of the other parameters did not accurately fit with our RGR estimations for the six species.
The possible relationship between RGR and the xylem physiology of higher plants is not well documented at present. We hypothesize there is a physiological basis for a mechanistic relationship, i.e., a fast-growing plant must ensure a high rate of resource supply (water and nutrients) to the productive organs. High resource supply can be achieved only by a highly conductive vascular system, allowing the leaves to be well supplied without being stressed.
Several traits related to water movement through the plant were measured, e.g., mean maximum vessel diameter, whole plant conductivity (for the liquid pathway) and stem conductivity. Among these, stem specific conductivity seems to be a very accurate predictor of RGR, supporting our hypothesis. The theoretical implication of such a relationship is discussed.
Key words: Quercus, hydraulic conductivity, relative growth rate.
Correspondence: Eustaquio Gil-Pelegrin, Forest Resources Unit, Servicio de Investigación Agraria-DGA, Apdo. 727, 50080 Zaragoza, Spain
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