Dielectric soil moisture sensors have the potential for nondestructive and real-time monitoring of the stem water content (q st ) of living trees. This study was conducted to investigate the water use characteristics of trees in drylands through monitoring of q st using newly developed capacitance sensors (GS3). The plants used for data collection were Prosopis juliflora (Sw.) DC. (mesquite, invasive) in Sudan and Tamarix ramosissima Ledeb. (tamarisk, invasive) and Prosopis pubescens Benth.(screwbean mesquite, native) in the United States. The GS3 probes were installed into the trunks of two trees for each species. Stem-specific calibration equations and temperature calibration equations were derived through laboratory experiments and analysis of field observation data. The temperature calibration equations reduced inappropriate variations of q st caused by daily fluctuations in stem temperature, suggesting that these are essential for correct interpretation of monitoring data of q st in arid environments. The q st of the mesquite trees in Sudan clearly increased after heavy rainfall events and started decreasing when the soil water content became close to the wilting point. These findings indicate that mesquite trees use soil water in rainy seasons, even though they are generally considered to use groundwater through deep tap roots. The q st of neither species in the United States responded clearly to rainfall events, indicating that they depend on shallow saline groundwater. The q st of the tamarisk decreased monotonically throughout the monitoring period, apparently in response to feeding damage caused by the tamarisk leaf beetle (Diohabda sp.), which had been released for biological control of tamarisk.

Prosopis juliflora is a useful ''multi-purpose'' tree, but it invades rapidly in arid and semi-arid environments, causing livelihood degradation of local communities. A rain pulse under drought conditions has been reported to be an important factor to promote plant invasion. The purpose of this research was to evaluate the effect of a rain pulse on the invading process of Prosopis juliflora and to propose a feasible plan to control the expansion of this species in Sudan. First, seed germination was examined under eight irrigation conditions, corresponding to 4-32 mm of rainfall. Most seeds imbibed in all the irrigation conditions, about half or more of the imbibed seeds germinated in 12 mm or more of irrigation. Twenty mm or more of irrigation induced seed emergence. Second, initial growth of germinated seeds was examined under ten irrigation conditions (1-32 mm rainfall) over 54 hours or 138 hours of cultivation. Radicle elongated most vigorously in 24 mm irrigation, and extended over 20 cm in length. Allocation to radicle was dominant, and its elongation was promoted in higher soil moisture condition. Based on these results, we propose that seedlings of P. juliflora should be eradicated within a few weeks after a single large rainfall, which might be convenient and effective to control additional expansion of this species in Sudan.

Loss of apical dominance, resulting in seedlings with more than one leader, generally referred to as multiple leaders (ML), has been a major problem in forest tree plantation in the tropics. A study to examine the effects of site preparation by burning on growth performance and multiple leaders (ML) formation, and its variation in eight-genotypes of Acacia mangium and Acacia auriculiformis was conducted. The design used for this field trial is a randomized complete block design with four replications at two sites (burnt and unburned) and eight genotypes (four from each species), and the effects were monitored for 24 months. There were significant differences at P < 0.05 between sites and genotypes for the number of ML trees/plot, basal diameter and survival. Differences among the genotypes, in terms of height, were also significant, but not between sites. The site × genotype interaction was significant only for ML trees per plot. Generally, the number and the size of ML trees per plot in the burnt site were higher and bigger than those in the unburned site. The number of ML trees per plot formed increased exponentially with time, reaching a peak in about four months after their first occurrence and then leveled off to an almost steady state thereafter at both sites. The average number of multiples (leaders) per tree ranged from 2 to 5 at the burnt site whilst at the unburned site, all the ML trees were of two leaders. A. mangium provenances were more responsive to burning than A. auriculiformis, whereas at the unburned site, the variation between them was not significant. Based on the above variation, it can be concluded that burning causes ML formation and can affect the growth of these selected Acacia genotypes.

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