Irrigation and Surface Mulch to Improve Transplant Survival
David Bainbridge, John Tiszler, Robert MacAller, and Mike Allen
Supplemental water is usually need for successful shrub and treeestablishment in arid lands, but limited water supplies often restrict wateravailability and increase water cost. This make plant establishment costlyand challenging unless water requirements for establishment can beminimized. With the United Nations Environment Program estimating that 3.3billion hectares are in need of repair and revegetation improving plant survivalat reduced cost is critical (UNEP, 1991). Various irrigationtechniques have been devised over the centuries to increase the effectiveness ofwater application. Deep-pipe and buried clay pot irrigation are bothsimple, low cost techniques for improving watering effectiveness (Bainbridge andVirginia, 1990). Deep-pipe irrigation supplies water directly to a plant's lowerroot zone through an open vertical tube (Mathew, 1976). Buried unglazedclay pots also introduce water directly into subsurface soils, but not asdeeply, and the water seeps through the clay walls and is made available tonearby plants over a period of many hours to several days (Sheng Han, 1974;Bainbridge et al. 1998).
Surface mulches have been used to improve soil waterretention, reduce soil temperature, and reduce wind velocity at the soil surfaceand arid lands (Kay, 1978; Jalota and Prihar, 1998). Surface mulches can alsoimprove water penetration by impeding runoff and protecting the soil fromraindrop splash and reducing soil crusting (Munshower, 1994). Verticalmulches pushed or crimped into the ground can funnel precipitation to substratesoils (Bainbridge, 1996). An early demonstration plot at the mesquitemounds site showed much improved survival with early bark mulch, and similarresults have been found in other arid sites (Zink and Allen, 1995).
We compared the effectiveness of deep-pipe clay pot handwatered irrigation compared to conventional surface watering on honey mesquite (Prosopisglandulosa var. torreyana) on seedling establishment beginning inNovember 1995, figure 3.3-1. We combined this study of irrigation methods with atest of horizontally spread and vertically placed soil surface matches, figure3.3-2.
Materials and Methods
The study as conducted in an abandoned highway department borrow pit at thesouthwest corner of the intersection of Highway 86 and 78m near the lower SanFelipe Creek in the Sonoran Desert east of San Diego, California. Thisarea of the Imperial Valley receives about 65 mm of rainfall annually (data forBrawley), with nearly equal maxima occurring in January, from winter frontalstorms from the Pacific, and in August, from summer storms moving north from theGulf of California. Evaporation potential from a class A evaporation pan is over3,000 mm per year (Hughes, 1963). Rainfall averages less than 1 mm in Mayand in most years there is no rainfall in June. This is an exposed sitewith very dry and often high wind speeds, particularly in spring. Temperatures may exceed 40°C any month of the year.
Irrigation and mulching treatments applied in thisexperiment are as follows:
1. Deep pipe irrigation: 5 cm (2") diameter 100 cm long PVC piped wereinserted vertically into the ground with the open base of each pipe at 50 cmdepth.
2. Buried clay pot irrigation: Unglazed, low fired 20 cm (8")diameter clay pots (red clay garden pots) with sealed drainage holes were buriedwith the lip of each pot flush to the ground surface. The pots werecovered with a weighted aluminum cover with holes punched to allow rainwaterentry.
3. Surface watering: Water was poured from a container onto the soil aroundthe plant.
A. Spread Mulch: Medium-sized pine bark chips were spread over plots tocreate a continuous 2 cm deep mulch layer.
B. Vertical Mulch: Dried broom corn sprigs (Sorghum vulgare) were inserted onto the soil at 20 cm intervals, with 25 sprigs per square meter. The broom corn was buried to a depth of 10-15 cm depth with the exposed portion standing 45 cm tall.
C. Open ground (control).
Treatments were applied in a full 3x3 factorial design, creating nineirrigation-mulch (I-M) treatment combinations. Replicate treatments wereorganized into eight 5m x5 blocks with one of each I-M treatment combinationrepresented in each block. Within block, treatments were randomly assigned to 1m˛ plots spaced 1 m apart. Each I-M treatment combination was replicatedeight times and each individual treatment 25 times On November 16, 1995each plot was planted with two 20 cm tall mesquite seedlings grown in 20 cu inchplant bands (Bainbridge, 1994b; Bainbridge et al., 1995). Seedlings wereplaced near the center of each plot, within 10 cm and on opposite sides of eachdeep pipe of clay pot. In plots receiving surface watering mesquite werespaced from the plots midpoint. A total of 144 mesquite were planted andenclosed in SupertubeŠ tree shelters which have been found to improve plantsurvival in arid environments (Bainbridge, 1994a; Grantz et al ., 1998).
Each plant was initially supplied with on liter of water biweekly, applied directly inside the tree shelter. After eight weeks, the irrigation treatments were begun, with each plot receiving 2.4 liters of water every other week. Beginning in Mach, watering intervals were lengthened, and irrigation continued for 4 months. Plants were watered once in 1997 and once in 1998. There was no mesquite mortality during the first winter and plant growth was rapid. Monitoring has continued and the results after almost three and a half years are discussed in this paper.
The experimental site experienced a not uncommon drought during the three anda half years of the study, with only 1.77 inches of rain in the 1998-99season. Severe herbivory by jack rabbits (Lepus californicus) limitedgrowth to the top of the tree shelters in most cases, although some treeseventually escaped, figure 3.3-3.
Mulch treatments were still intact and functional after three and a halfyears. As irrigation was discontinued the survival gradually declined, butnow appears to have stabilized, with overall survival at 44%. The survivalby irrigation treatment, mean height, and health ranking on 6/21-99 (1=barelyalive, 4=excellent) are shown in Table 3.3-1
Table 3.3-1 Irrigation effects
|Irrigation Method||Survival (percent)||Mean height(cm)||Rating|
Significant difference in height and ranking is indicated by letter, with asignificant level of 0.05 for Fisher's Protected LSD using SuperAnova computersoftware. There were no significant differences or trends in survival orheight based on mulch treatment. Analysis of mulch by irrigation alsoshowed no statistically significant differences. There was a very weaksuggestion that the broom corn may have had a slightly negative effect and thebark may have provided a weak benefit.
The benefits of deep pipe irrigation were clearly confirmed. This method of irrigating should be more widely used for planting in arid environments. The deep pipe system was most effective in enabling plants to escape the severe herbivory, suggesting that a more robust plant with extensive root system had developed. Nine of the ten biggest plants, ranging from 87-210 cm tall, were on deep pipe systems. This clearly confirms the benefits first demonstrated by Mathew (1987). The benefits of clay pot irrigation were less than expected, probably the result of miscommunication during setup, with the seedlings planted too far from the pots. In other cases, clay pot irrigation has proved to be very effective in very arid environments (Shiek'h and Shah, 1983; Bainbridge and Virginia, 1990).
The use of mulch to conserve moisture in very arid environments was notsupported by this study, although it has been effective in other cases. Itappears that the use of mulch, like may other arid zone techniques, is agamble. We have seen benefits from mulch when sufficient rain fell tocause surface ponding and sheet flow. It is likely this condition occurredrarely, if at all, during the course of this study. Jalota and Prihar(1998) reviewed the effects of mulch on soil moisture content and found thatmulch may be of little value or detrimental in low rainfall periods. Themulch can intercept moisture from light rains before it reaches the soil. Mulch can also wick moisture from the soil into the air, increasingevaporation. In their own experiments Jalota and Prihar determined thatwith evaporation rates of 15.4 mm day (common at this site) unmulched soilconserved more moisture than mulched soil (Jalota and Prihar, 1990).
The general recommendation from this study would b to use deep pipe irrigation to establish shrubs and trees in very arid environments. The use of organic mulch is problematic because it can limit movement of moisture from light rains to the soil and increase evaporation from the soil surface. Burying the mulch 5 cm deep appears to provide increased benefits (Minhas and Gill, 1985), while the benefit of surface mulch may be realized only with heavy rains or regular irrigation.
Despite severe herbivory and limited rainfall more than 70% of the mesquiteseedlings started on deep pipe irrigation were still alive after three and ahalf years. Without intervention it may take centuries for degraded sitesto recover in these arid environments (Webb et al., 1983), yet this studysuggests that with minimal water and sufficient plant protection even arid sitescan be revegetated. It is not easy or inexpensive, but it can be done,figure 3.3-4.
Figure 3.3-4. Growth after three years
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