Responses of tree-type and shrub-type Prosopis (mimosaceae) taxa to water and nitrogen availabilities

The objective of this work was to compare the response of two tree-type Prosopis (P. alba and Prosopis flexuosa-tree) and two shrub-type taxa (P. flexuosa-shrub and Prosopis strombulifera) to water and nitrogen availabilities. Our hypothesis was that treetype taxa, native to high-resource environments, would be able to take advantage of water and nitrogen supplies, allocating resources mainly to growth, while shrubs, native to low-resource environments, would divert resources to functions related to survival, such as storage of carbohydrates. Two water availabilities (low-water availability: WO and high-water availability: W I) and four nitrogen treatments (0, 50, 100 and 200 mg N per seedling) were applied in a factorial greenhouse experiment. The addition of N did not affect the shoot:root (S:R) ratio, the rate of leaf appearance (RLA), specific leaf area, CO2 uptake (A), transpiration, water-use efficiency, C:N ratio or the amount of glucose used in total non-structural carbohydrates, nitrogen or fiber. Prosopis alba was the only species in which the addition of N resulted in a higher biomass accumulation under Wl. Water limitation induced a decrease in growth in every taxa, except in P. strombulifera. The RLA and SLA were unresponsive to water availability, but differed among species: P. strombulifera showed the lowest values for both variables. P. alba under Wl had a greater A than the other species. No significant correlation was found between SLA and A. Under both water availabilities P strombulifera allocated glucose preferentially to reserves, and P flexuosa (tree- and shrub-type) to growth (fiber). P strombulifera had a greater C:N ratio than the other species. In conclusion, only P alba was able to take advantage of increased water and nitrogen supplies and only P strombulifera diverted resources to storage of carbohydrates while P flextiosa exhibited an intermediate response pattern of allocation. (C) 2003 Elsevier B.V. All rights reserved.