Thesis Abstract

<p></p>                  <b>ABSTARCT&nbsp;</b><div>Recycled wastewater is a popular alternative water resource. Recycled water typically has higher salinity than potable water and therefore may not be an appropriate water source for landscapes planted with salt-intolerant plant species. Coast redwoods (Sequoia sempervirens) are an important agricultural, horticultural and ecological species assumed to be salt intolerant. However, no studies have analysed how salinity impacts coast redwood growth. To determine salt-related growth limitations, as well as susceptibility to particular salt ions, we divided 102 S. sempervirens ‘Aptos Blue’ saplings evenly into 17 salinity treatments a control and four different salts (sodium chloride, calcium chloride, sodium chloride combined with calcium chloride, and sodium sulfate). Each salt type was applied at four different concentrations 1.0, 3.0, 4.5 and 6.0 dS m21 . Trees were measured for relative growth, and leaves were analysed for ion accumulation. Results showed that the relative stem diameter growth was inversely proportional to the increase in salinity (electrical conductivity), with R2 values ranging from 0.72 to 0.82 for different salts. Analysis of variance tests indicated that no particular salt ion significantly affected growth differently than the others (P . 0.1). Pairwise comparisons of the means revealed that moderately saline soils (4–8 dS m21 ) would decrease the relative height growth by 30–40 %. Leaf tissue analysis showed that all treatment groups accumulated salt ions. This finding suggests reduced growth and leaf burn even at the lowest ion concentrations if salts are not periodically leached from the soil. Regardless of the specific ions in the irrigation water, the results suggest that growth and appearance of coast redwoods will be negatively impacted when recycled water electrical conductivity exceeds .1.0 dS m21 . This information will prove valuable to many metropolitan areas faced with conserving water while at the same time maintaining healthy verdant landscapes that include coast redwoods and other long-lived conifers. Keywords California; drought; Mediterranean climate; reclaimed water; urban forestry; urban horticulture. <p> <br></p></div>

Thesis Overview

<p> <b></b></p> <b><i></i>1. INTRODUCTION</b>&nbsp;<div>Water used to irrigate important verdant, social landscapes (e.g. arboreta, public parks and golf courses) faces competition with other uses of fresh water including increasing agricultural and urban demands (Hamilton et al. 2005). Recycled wastewater has been highlighted as one of the most affordable alternative resources for agricultural, industrial and urban non-potable purposes in arid and semi-arid regions like California, where current fresh water reserves are at a critical limit (Lazarova et al. 2001). In California natural prolonged periods of summer drought have been exacerbated in recent years by low winter rainfall. California’s 2014 Water Year, which ended&nbsp; 30 September 2014 was the third driest in 199 years of record; and was the warmest year on record (USGS 2015). In addition, California’s population is estimated to increase by 15.4 million residents (a 39 % increase) over the next 50 years (Palmer and Schooling 2013). Both the rise in population and the uneven distribution of these new inhabitants will cause an increase in water demand (Hanak and Davis 2006).&nbsp;</div><div><br></div><div>To mitigate the effects of increased competition for limited potable water, horticulturalists and municipalities in California and in arid and semi-arid climates around the world are developing sources of recycled wastewater (Hamilton et al. 2005; Miller 2006; Toze 2006). Types of waste waters used for recycling include treated and untreated sewage effluent, storm water runoff, domestic grey water and industrial wastewater (Toze 2006). Although recycled water meets many social and environmental objectives by reducing competition for fresh water, there are some drawbacks that make it less suitable than potable water for horticultural applications. Primarily, recycled water often has a greater salt concentration than potable water.&nbsp;</div><div><br></div><div>Although the salinity of recycled water is not usually high enough to make it unsuitable for irrigation (Vartanian 2008), it can contain 10 times more salt (e.g. 1.0–2.0 dS m21 ) than potable water (0.1 dS m21 ). Thus, recycled water can be harmful to salt-intolerant plants (Maas 1986). Salinity in low concentrations (,2.0 dS m21 ) has been shown to have adverse effects on growth and physiology of many plants (Kozlowski 1997; Chaves et al. 2009). Salinity impacts plant growth by decreasing the osmotic potential of the soil and imposing physiological drought, or through toxic effects from high concentrations of particular ions, such as sodium or chloride that can injure the plant (Chaves et al. 2009). Although there is an extensive literature on the negative effect of salt on plant growth for many agricultural crops (Sohan et al. 1999; Sultana et al. 1999; Katerji et al. 2003; Zheng et al. 2008), there is a limited amount of information quantifying growth responses to salt for important horticultural species. In particular, there is only one report published about the salt tolerance of the coast redwood tree (Sequoia sempervirens) (Wu and Guo 2006), which is surprising given this species’ important ecological and horticultural value. The coast redwood is emblematic of western US conifers known for its towering height (.100 m) and longevity (.1500 years). This charismatic tree species’ native range extends along the fog-belt of the Pacific coast from southern Oregon to central California. The coast redwood is an important timber species, prized in building for its burnt-sienna coloured wood that is naturally decay resistant. Coast redwoods are also used extensively in Pacific horticulture (CA, OR, WA), in public parks, golf&nbsp; courses, highways and private landscapes; and are popular horticultural specimens used throughout the USA and in temperate climates around the world. Although the coast redwood is indigenous within a Mediterranean climate, which is typified by long periods of summer drought, coast redwoods thrive in areas with significant summertime moisture, typically derived from abundant marine fog. Moisture input from fog drip in the summer can constitute 30 % or more of the total water input each year (Dawson 1998).&nbsp;</div><div>The coast redwood is characterized as having low to moderate drought tolerance (Sunset Books 2000) and requires supplemental irrigation where fog or summer precipitation events are lacking. Without natural precipitation (rain or fog) or supplemental irrigation, dry summer conditions may inhibit the performance of mature individuals of coast redwood in urban settings where signs of water stress often include leaf senescence and stem die back (Litvak et al. 2011) (Fig. 1). The work presented herein was initiated to fill a knowledge gap by determining the level of tolerance of coast redwood to sodium and chloride. The research was designed in response to reports from water districts in the San Francisco Bay Area, which claimed that coast redwoods within public parks had shown signs of decline or death after irrigation with recycled water. To determine the effects of sodium and chloride ions on the growth and health of redwoods, Sequoia sempervirens ‘Aptos Blue’ specimens were placed in a greenhouse and irrigated daily with one of 17 treatments represented by a non-saline nutrient solution that was used as the control treatment plus four different salt solutions at four different concentrations. We hypothesized that redwoods&nbsp; would be classifiable as a ‘salt-sensitive’ species, demonstrated by declines in growth at soil salinity concentrations ,3.0 dS m21 . Further, we hypothesized that different salt solutions would be more toxic than others, represented by statistically different growth responses.</div><div><br></div><div> <b>2.</b> <b>DESIGN METHOD</b></div><div>The experiment was conducted in a glasshouse at the UC Davis Environmental Horticulture Complex (Davis, CA, USA). Greenhouse daytime low and high temperatures were maintained between 21 and 24 8C, and night-time low and high temperatures were maintained between 13 and 17 8C. No artificial lighting was supplied to the plants. The glasshouse was divided into two blocks to control for natural gradients of sunlight, temperature and humidity. Pots were placed 1 m apart throughout the two blocks. One hundred and two Sequoia sempervirens ‘Aptos Blue’ saplings in 8 L pots (21 cm tall, with a 21 cm diameter tapering to 18.5 cm) were obtained from Generation Growers, Modesto, CA, USA. Potting media contained a mix of humus and sand in a 4 : 1 volumetric ratio, 6.0 kg m23 dolomite, 0.6 kg m23 calcium nitrate, 1.2 kg m23 ferrous sulfate heptahydrate, 3.0 kg m23 nitroform, 2.4 kg m23 double super phosphate and 1.2 kg m23 oyster shell lime. The salinity treatments consisted of a control, as well as four different salts: sodium chloride (NaCl), calcium chloride (CaCl2), sodium chloride and calcium chloride (NaCl + CaCl2) and sodium sulfate (Na2SO4).&nbsp;</div><div><br></div><div>Each salt was applied at four different concentrations represented by electrical conductivity (EC) of 1.0, 3.0, 4.5 and 6.0 dS m21 . NaCl was selected because it is the most common salt in recycled water. Na2SO4 was used to isolate Na symptoms, whereas CaCl2 served to isolate Cl symptoms. The combination of NaCl and CaCl2 provided a treatment simulating environmental conditions, where combinations of monovalent and multivalent cations would be present in the irrigation water and/or soil. Each salt type was added to a onequarter strength Hoagland’s fertilizer ‘Solution 2’ which had an EC of 0.5 dS m21 (Epstein and Bloom 2005). The control treatment received only the modified Hoagland’s, without additional salt. Six trees were replicated in each of 17 treatments. Treatments were initialized on 15 October 2005. Dosatronw DI-16 injectors (Dosatron USA, Clearwater, FL, USA) were used to mix the salinity treatments into the irrigation water. Three Netafimw Woodpecker pressure-compensating emitters (Netafim Irrigation, Fresno, CA, USA, rated 4 L h21 ) at each pot produced an average total flow rate of 12.8 L h21 (SE ¼ 0.08, n ¼ 9). Multiple emitters at each pot allowed for uniform saturation of the container medium. Daily irrigations were&nbsp;</div><div><br></div><div> Table 1. Mean (+SE) cumulative treatment and leachate EC values from the testing period 12 July 2005 to 1 September 2007. A leaching fraction of 0.4–0.5 was applied to all treatments independently. Irrigation treatment salinity concentrations were evaluated weekly by collecting solute from an emitter tube at each tree prior to the day’s irrigation cycle.&nbsp;</div><div><br></div><div>Treatment      Cumulative mean treatment EC     Cumulative mean leachate EC</div><div><u>               (dS m21)+1 SE          (dS m21)+1 SE&nbsp;    </u></div><div><br></div><div>Control 0.5 dS m21        0.57+0.01             0.66+0.01&nbsp;</div><div>NaCl 1.0 dS m21         1.05+0.01             1.67+0.05&nbsp;</div><div>NaCl 3.0 dS m21         3.12+0.03             4.52+0.11&nbsp;</div><div>NaCl 4.5 dS m21         4.32+0.05             5.71+0.11&nbsp;</div><div>NaCl 6.0 dS m21         5.72+0.08             7.08+0.12&nbsp;</div><div>CaCl2 1.0 dS m21         1.06+0.01             1.54+0.02&nbsp;</div><div>CaCl2 3.0 dS m21        &nbsp; &nbsp;2.95+0.02             5.08+0.13&nbsp;</div><div>CaCl2 4.5 dS m21        &nbsp; &nbsp;4.52+0.04             7.10+0.16&nbsp;</div><div>CaCl2 6.0 dS m21         6.12+0.04             8.83+0.17</div><div>&nbsp;NaCl + CaCl2 1.0 dS m21     &nbsp; 1.09+0.01             1.61+0.03&nbsp;</div><div>NaCl + CaCl2 3.0 dS m21      2.94+0.03             4.60+0.11&nbsp;</div><div>NaCl + CaCl2 4.5 dS m21      4.59+0.03             6.83+0.16&nbsp;</div><div>NaCl + CaCl2 6.0 dS m21      6.10+0.04             8.40+0.15&nbsp;</div><div>Na2SO4 1.0 dS m21        1.09+0.01             &nbsp; 1.73+0.05&nbsp;</div><div>Na2SO4 3.0 dS m21        3.10+0.04              4.68+0.11&nbsp;</div><div>Na2SO4 4.5 dS m21        4.71+0.01              6.08+0.09&nbsp;</div><div>Na2SO4 6.0 dS m21        6.10+0.02              7.37+0.11&nbsp;</div><div><br></div><div> scheduled with a Hunterw ICC irrigation timer (Hunter Industries Inc., San Marcos, CA, USA). A leaching fraction of 0.4–0.5 was applied to all treatments independently. The leaching fraction is defined as the ratio of the quantity of water draining past the root zone to that infiltrated into the soil’s surface. This fraction was used to isolate symptoms related to the salt treatments by eliminating stress due to both insufficient water and increasing container EC due to evapo-transpiration. Further, this leaching fraction was designed to provide sufficient irrigation treatment volume to allow for uniform saturation of the container medium. Irrigation treatment salinity concentrations were evaluated weekly by collecting solute from the emitter tube at each tree during the day’s irrigation cycle. After the irrigation cycle, a portable meter was used to test the EC and pH of each sample leachate (Table 1). &nbsp; &nbsp;&nbsp;</div><div><br></div><div> <b><i>Data collection&nbsp;</i></b></div><div>Stem diameter and stem length (i.e. tree height) were measured every second week starting on 25 September 2005 and ending 3 January 2007. A set of digital calipers (Fisher Scientific, Pittsburgh, PA, USA) was placed around the trunk at a height of 3 cm above the potting medium in a constant orientation for each tree. The trunk was marked to indicate the points of contact for the calipers and the diameter was measured across these points each time. Tree height was evaluated every third week starting 15 September 2005 and ending 8 January 2007. Height was measured with a tape from an indicated point on the pot rim to the apex of the central leader of the tree.&nbsp;</div><div><br></div><div> The concentration of salt ions accumulated in the leaves was determined from analyses of leaves sampled from the previous flush of growth. These leaves were identified as originating from lignifying stem segments occurring directly behind the youngest, light green leaves on solid green stems. Consistency of tissue maturity has been shown to be an important characteristic for obtaining comparable results (Mills and Jones 1996). Leaf tissue-sampling events occurred on 17 October 2005, 9 January 2006, 18 May 2006, 22 September 2006 and 15 January 2007. The experiment was terminated shortly after the fifth sampling. Both proximal (P) and distal (D) leaf blade sections were collected on each date. The distal portions of leaves were removed first; the halfway cut point was determined visually. Then the basal sections of the cut leaves were removed by cutting them as closely to the stem as possible. A minimum of 1.5 g dry weight (3.8 g fresh weight, 39 % dry: fresh weight ratio) was collected for each sample. The dried samples were analysed for % Ca+, % Cl2 and % Na+ by using the ‘Nitric/Perchloric Wet Ashing Open Vessel’ (P – 3.10) technique, and Cl was analysed using the ‘2 % Acetic Acid Extraction’ (P – 4.20) technique by Dellavallew Laboratory, Inc. (Fresno, CA, USA). Ion accumulation rates were evaluated for the different concentrations within each salinity treatment type (e.g. 1.0 dS m21 NaCl vs. 6.0 dS m21 NaCl), as well as within treatment concentration level between the various salinity treatment types (e.g. 1.0 dS m21 NaCl vs. 1.0 dS m21 CaCl2). &nbsp; <br>&nbsp; <p> <br></p></div>