The 2010 ASHS Annual Conference

To reserve potable water for appropriate beneficial uses, reclaimed water is provided for irrigation of parks, golf courses, and other public landscapes within the Santa Clara Valley Water District in California.  Unfortunately, the use of this water to irrigate landscapes appears to have led to the decline or death of coast redwoods within those sites.  Since the reclaimed water used to irrigate these landscapes was found to be particularly high in sodium and chloride, an experiment was designed and implemented to determine the tolerance of this species to Na⁺ and Cl^-. 

S. sempervirens ‘Aptos Blue’ is widely used in landscapes and is known to be sensitive to salinity in irrigation water.  Selecting a cultivar may also reduce variability in the data collected.  Trees were grown in containers in a greenhouse and irrigated with a nutrient solution having an electrical conductivity (EC) of 0.5 dS/m.  This solution also served as the control treatment.  Four salt types: NaCl, Na₂SO₄, CaCl₂, and an equimolar combination of NaCl and CaCl₂, were added to the 0.5 dS/m nutrient solution at concentrations yielding total ECs of 1.0, 3.0, 4.5, and 6.0 dS/m, to create 16 salinity treatments.  Irrigation occurred once daily with excessive volumes of treatment solution so as to reduce effects of water deficit stress and salt accumulation.  The experiment was carried out over a period of 16 months.

Trunk diameter was measured every two weeks.  At 1.0 dS/m, there were no differences in trunk diameter.  However, increasing treatment concentration above 1.0 dS/m, despite salt type, caused an increasing reduction in trunk diameter change.  Plots of meq (Na + Ca)/L versus trunk diameter yielded the same linear regression results despite salt type.  Tree height and width were also measured, but results from these data were less clear due to high variability.  Tissue samples of the distal and proximal portions of leaves were also taken at about 4 month intervals and analyzed for Na and Cl.  An extensive collection of digital branch and leaf images were produced to record the development of toxicity symptoms.