Abstract:In semi-arid regions, afforestation with fast-growing species cultured with low irrigation can be an effective approach for environmental protection. An experiment was conducted to evaluate the stem biomass production of Paulownia in a semi-arid climate and clay soils under contrasting low-irrigation and fertilization treatments. The stem biomass at the stand level was estimated by applying allometric equations fitted in sample resprouts and inventory data. The results show that biomass production improved when either irrigation or fertilizer was added, but the combination of a higher dose of irrigation and fertilization did not lead to the highest biomass production; thus water availability was the main factor controlling biomass production. Under the higher dose of irrigation, the absence of a fertilizer effect would be due in part to the fertile soil, which could supply sufficient nutrients for Paulownia growth at the higher level of soil moisture. The stem OPEN ACCESSForests 2014, 5 2506 biomass estimated ranged from 2.14 to 4.50 t⋅ha −1 (lower irrigation dose without fertilization, and higher irrigation with fertilization). The greater production was similar to other studies in the Mediterranean area receiving more irrigation. Thus, this study permitted us to understand the potential of Paulownia to provide biomass in semi-arid environments with low irrigation due to water use restrictions.

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There are still considerable uncertainties about soil functioning in semi‐arid Mediterranean ecosystems. Hence, studies that have focused on Juniperus species in thermophilic Mediterranean woodlands are practically non‐existent. This study provides a survey of the effects of (i) woodland maturity (mature and young woodlands), (ii) vegetation cover (Juniperus thurifera L. trees, Rosmarinus officinalis L. shrubs and semi‐wooded grasslands) and (iii) growing season (spring, summer and autumn) on soil enzymatic activity (urease, alkaline phosphatase and β‐glucosidase) and microbiological characteristics (microbial biomass C (MBC), basal respiration (Br), and microbial and metabolic quotients) in thermophilic Spanish Juniper woodlands. The results showed that mature woodland maintained a greater enzymatic and microbiological activity than the more productive young woodland. The advanced communities of Juniperus trees and Rosmarinus shrubs exhibited greater enzymatic and microbial activity than grassland areas. Soil biological characteristics were strongly affected by the season. Significant β‐glucosidase activity and Br were observed in the summer, as a consequence of the greater organic matter availability for microbes, despite the reduced soil moisture availability. Thus, foliar abscission in trees and shrubs and withering in grasslands seem to control soil biological activity. Hence, we hypothesize that microbial communities that are adapted to drought exist. In contrast, the presence of greater N and P content in summer seems to reduce the urease and phosphatase activities. In addition, the more advanced communities (Juniperus and Rosmarinus covers, especially in mature woodland) showed an inherent trend towards efficiency in C utilization throughout the growing seasons, thus increasing the MBC at the end of the growing season.