Titanium as an indicator of residual soil on arid-land plants.

Titanium (Ti) has been suggested as a soil contamination indicator for plant samples slated for trace element analysis because it is abundant in soil but not in plants. Based on results from our survey of regional soils and plants for cesium (Cs), we sought to confirm Ti as a valid soil contamination indicator reasoning that there are three sources of Ti associated with plant samples: (i) contamination during laboratory processing, (ii) vascular uptake via roots, and (iii) field soil residue on shoot surfaces. Our experiments showed that: (i) milling increased Ti by 4 mg.kg(-1), and Ti in reagents and on labware added another 5 to 6 mg.kg(-1); (ii) Ti in Crepis acuminata seedling shoots attributable to root uptake averaged 5 mg.kg(-1); (iii) soil-dusted seedlings showed elevenfold and eightfold increases in Ti and Cs, respectively. Further, investigation of shoot washing methods determined that (iv) none of seven washing agents removed all soil from any of two to seven plant species, and (v) Artemisia tridentata and Phlox hoodii specimens washed with water retained particles and displayed elemental signatures consistent with adhering soil. We conclude that Ti is a valid soil contamination indicator for arid-land plant samples, and that trace, soil-borne analytes measured in samples where Ti values are high and, hence, soil contaminated, should be described as plant associated. Furthermore, we give guidance on minimizing Ti contamination of samples during laboratory processing and on use of Ti together with washing to minimize yet gauge soil contamination during trace element analysis.

Interactions among unrelated species: Granivorous rodents, a parasitic fungus, and a shared prey species.

Granivorous rodents and a parasitic fungus in the Sonoran Desert utilize a common prey species, Erodium cicutarium, a desert annual plant. Experimental removal of rodents from field exclosures resulted in significantly higher densities of E. cicutarium. Fungal infection was significantly higher in the absence of rodents, suggesting that, while they do not interact directly, rodents and the fungus affect each other's densities by their use of a common prey species.

Density-dependent germination response by seeds of desert annuals.

Field observations and experiments indicate that the presence of seedlings at high densities inhibits subsequent germination of desert annuals. Since plants growing at high densities face severe competition for limited resources, this response by seeds is interpreted as an adaptation to avoid an unfavorable competitive climate where growth and survivorship are likely to be low.