Soil Ecology Restoration Group

last updated July 17, 2000

FORT IRWIN - Plant Community Restoration

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Results

Native Plant Windbreak

In June of 1998, overall survivorship was 87 percent, 443 of the 508 plants installed at the Fifth Street windbreak remained alive. Both Prosopis glandulosa and P. pubescens (honey and screwbean mesquite) had the highest survivorship at 98 and 97 percent respectively. At 23 percent, Acacia gregii had the lowest survivorship of all the species planted in the windbreak. The number of plants installed and survivorship eight months after planting are summarized in Table 3.

 

     Table 3: Survivorship of 5th street windbreak eight months after planting

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Scientific Name	Number Planted	Number Alive 6/26/98	Percent Survival
Prosopis glandulosa	137	135	99
Cercidium floridum	128	96	75
Chilopsis linearis	85	81	95
Prosopis pubescens	78	76	97
Isomeris arborea	67	52	78
Acacia gregii	13	3	23
Total	508	443	87

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In October of 1998, one year after planting, overall survivorship dropped slightly to 84 percent, 427 of the original 508 plants at the Fifth Street Windbreak remained alive. Prosopis glandulosa at 97 percent and P. pubescens at 96 percent again had the highest survivorship while Acacia gregii had the lowest. No significant amount of training manuever damage had occurred since the plants were installed. Survivorship at the one year mark is summarized in Table 4 and photographs are shown in Figure 14.

Table 4: Survivorship of 5th street windbreak Twelve months after planting
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Scientific Name	Number Planted	Number Alive 10/27/98	Percent Survival
Prosopis glandulosa	137	133	97
Cercidium floridum	128	87	68
Chilopsis linearis	85	81	95
Prosopis pubescens	78	75	96
Isomeris arborea	67	49	73
Acacia gregii	13	2	15
Total	508	427	84

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Erosion Control Site

On the erosion control site wooden stakes marked at one cm increments were used to determine the amount of soil accumulation or loss from the surface treatment plots and behind check dams. The soil level on the wooden stakes was read in July 1998, nine months after installation, and in May 1999, nineteen months after installation. A subjective determination of check dam condition was also made at the same time.
After seventeen months, erosion gullies did not form on any of the surface treatment plots or control. On the surface treatment plots, an average of 0.19 cm of soil was accumulated on the coir fence treatment, 0.30 cm of soil on the punched straw plot and 0.41 cm of soil on the control. A non-parametric ANOVA, Kruskal-Wallace test was used to determine if the amount of soil accumulated from the surface treatment plots was statistically significant. No statistical significance was found.

Chi square contingency tables were used to determine if the amount of soil accumulation and checkdam condition were independent of the various construction techniques used. Final analysis revealed that the amount of soil accumulation was independent of the check dam construction techniques used, however, with a p-value of 0.06, two trends were apparent. First, straw bale dams tended to trap more soil, and second, coir fences tended to trap very little soil. Chi square analysis revealed that check dam condition was dependent on the construction technique used (p = 0.005). Check dams that were built out of straw bales and straw flakes were ranked more often in excellent condition. Check dams built out of coir, straw and rock and straw and soil were ranked more often as fair. These data and the average amount of soil accumulated behind the check dams and on the surface treatment plots are summarized in Table 5 for the nine month measurements and Table 6 for the nineteen month measurements. Photographs at nineteen months are shown in Figure 15.

Table 5: Summary of soil accumulation behind the erosion control check dams and on the surface treatment plot nine months after installation
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Check Dam Type	Number of Observations	Average Soil Accumulation (cm)	Std Err	Dam Condition	Excellent	Fair	Poor
Coir	8	0	0.39		1	7	0
Straw Flake	7	0.29	0.18		6	1	0
Straw and Rock	5	0.6	0.25		1	4	0
Straw and Soil	6	0.83	0.17		0	6	0
Straw Bale	7	3.14	1.75		5	1	1
Surface Treatment Plots
Punched Straw	4	0	0
Control	4	0.13	0.13
Coir Fences	4	0.25	0.25

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Table 6: Summary of soil accumulation behind the erosion control check dams and on the surface treatment plot nineteen months after installation
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Check Dam Type	Number of Observations	Average Soil Accumulation (cm)	Std Err	Dam Condition	Excellent	Fair	Poor
Coir	8	0.19	0.27		1	7	0
Straw Flake	7	0.60	0.31		5	2	0
Straw and Rock	6	0.69	0.21		0	6	0
Straw and Soil	5	0.77	0.17		1	4	0
Straw Bale	7	3.09	1.61		4	2	1
Surface Treatment Plots
Punched Straw	4	0.31	0.20
Control	4	0.41	0.23
Coir Fences	4	0.19	0.12

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Direct Seeding Site

One-way analysis of variance (ANOVA) and multiple comparisons tests were computed and used to detect significant improvements in cover and density on the experimental surface treatments (bark, xanthan gum and Soil-Sement). After damages caused by vehicular traffic during training manuevers, all the peat moss plots were lost while only one replicate plot per bark, xanthan gum and Soil-Sement treatment remained undisturbed. However, the damage occurred before the seedlings sprouted so data could still be taken in June 1998. At that time, six months after installation, a significant increase in density (p ² 0.10) was detected for the Soil-Sement treatment. Percent cover averaged the highest on the untreated sampling plot (7.40 %) and lowest on the xanthan gum plot (5.40%). Average density was highest on the plot treated with Soil-Sement (13.10 seedlings/1.2 m2) and lowest on the plot covered with fir bark (3.70 seedlings/1.2 m2).

ANOVA was also used to determine if seedling density and percent cover were influenced by trail usage. A significant difference in total percent cover (p ² 0.05 significance level) was found between the high use vehicle trail and the undisturbed pitted and seeded areas. Average percent cover was lower on the high use trails (1.25%) versus the unused area (6.40%). Surprisingly, density was significantly (p ² 0.08) higher on the low use trail than the unused areas (8.30 seedlings/1.2m2 versus 5.30 seedling/1.2m2).

ANOVA was used to determine the effect seeding rate (high and low seeding rate areas) on percent cover and density. No significant difference was found between the high and low density seeded areas. These data along with seedling density and percent cover for the surface treatment plots and the trail crossings are summarized in Table 7.

Table 7: Summary of percent vegetation cover and seedling density six months after installation.
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	Mean % Cover		Std Error			Mean Density		Std Error
Trail Crossings
Heavy Use	1.25	±	0.19	S1		5.00	±	1.25
Moderate Use	3.95	±	1.11			3.50	±	0.82
Low Use	3.45	±	0.85			8.30	±	1.68
No Use	6.40	±	1.69			5.30	±	1.29
P-Value	0.02					0.08
Surface Treatments
Xanthan Gum	5.40	±	1.79			7.50	±	2.22
Fir Bark	5.70	±	1.89			3.70	±	1.34
Soil Sement	7.00	±	2.06			13.10	±	4.44	S2
No Treatment	7.40	±	1.15			6.60	±	0.73
P-Value	0.82					0.10
Varied Seeding Rate
High	6.30	±	1.57			8.90	±	2.84
Low	7.40	±	1.15			6.60	±	0.73
P-Value	0.58					0.44
S1 at .05 level S2 at .10 level

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TV Hill Revegetation Sites

A four month survivorship was taken in June 1998 at the south site. An ANOVA was computed to determine if catchment size had a significant effect on survivorship. No significant response was found. It is interesting to note that Salazaria and Achnatherum had the poorest survival rate in spite of the high winter and spring rainfall. Survivorship at the south revegetation site is summarized in Table 8. It appeared that survivorship was higher for most species inside the catchments than outside. To investigate this hunch, Chi square contingency tables were calculated at both the north and south sites to determine if survival was independent of installation inside or outside catchments. Results showed survival at the south site was independent of installation.

Table 8: Survivorship at TV Hill south revegetation site after four months
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	Outside Catchments				Within Catchments
Species	Planted	Alive	Percent		Planted	Alive	Percent
Ambrosia dumosa	167	147	88		83	76	91
Atriplex spp.	52	41	79		90	83	92
Encelia farinosa	82	77	94		56	53	96
Ephedra nevadensis	100	97	97		53	53	100
Hymenoclea salsola	33	26	79		45	35	77
Isomeris arborea	31	27	87		43	40	93
Larrea tridentata	35	32	91		93	91	98
Prosopis glandulosa	21	17	81		45	42	93
Salazaria mexicana	23	5	22
Sphaeralcea ambigua	29	27	93
Achnatherum hymenoides	73	35	48
Total planted per treatment	646	531	82		508	473	93
Total planted on site	1154
Overall survival	87%

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The ANOVA and Chi square tests described for the four-month data were repeated for data taken after eight months. Survivorship data taken in November 1998 are summarized in Table 9. ANOVA results for catchment size were the same with no significant difference noted, but the Chi square test this time showed a significant relationship between survival and installation within a catchment (planting within catchments improved survivorship).
One year survivorship data taken in February 1999 at TV Hill south is summarized in Table 10. No significant results for survivorship of plants in the varying catchment sizes were shown. The highest survival (87%) was found in the 4 m2 size catchments, and next highest in the 25 m2 size catchments (86%) with survivorship lowest (80%) on the 9 m2 and 16 m2 catchments. As with the eight month data, there is a significant difference of survivorship between the shrubs planted in the catchments and the shrubs planted outside the catchments (p-value ² 0.0001). Overall survival inside the catchments was 84%, nearly 20% higher than the 65% recorded outside the catchments. Out of the eight comparable species planted, three species (Larrea tridentata, Encelia farinosa and Isomeris arborea) had only slight increases inside the catchments. The other five species (Ephedra nevadensis, Ambrosia dumosa, Hymenoclea salsola, Atriplex spp. and Prosopis glandulosa) had significantly higher survival inside the catchments (p ² 0.1)

Table 9: Survivorship at TV Hill south revegetation site after eight months
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	Outside Catchments				Within Catchments
Species	Planted	Alive	Percent		Planted	Alive	Percent
Ambrosia dumosa	167	137	82		83	72	87
Atriplex spp.	52	23	44		90	61	68
Encelia farinosa	82	62	76		56	47	84
Ephedra nevadensis	100	90	90		53	52	98
Hymenoclea salsola	33	22	67		45	35	78
Isomeris arborea	31	17	55		43	35	81
Larrea tridentata	35	32	91		93	86	93
Prosopis glandulosa	21	12	57		45	33	73
Salazaria mexicana	23	3	13
Sphaeralcea ambigua	29	24	83
Achnatherum hymenoides	73	16	22
Total planted per treatment	646	438	68		508	421	83
Total planted on site	1154
Overall survival	74%

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Table 10: Survivorship at TV Hill south revegetation site after twelve months
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	Outside Catchments				Within Catchments
Species	Planted	Alive	Percent		Planted	Alive	Percent
Ambrosia dumosa	167	137	82		83	75	90
Atriplex spp.	52	23	44		90	59	66
Encelia farinosa	82	64	78		56	48	85
Ephedra nevadensis	100	82	82		53	51	96
Hymenoclea salsola	33	21	63		45	37	83
Isomeris arborea	31	23	74		43	35	81
Larrea tridentata	35	31	88		93	86	92
Prosopis glandulosa	21	7	32		45	37	83
Salazaria mexicana	23	4	17
Sphaeralcea ambigua	29	16	55
Achnatherum hymenoides	73	10	14
Total planted per treatment	646	418	65		508	428	84
Total planted on site	1154
Overall survival	73%

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The results for the Encelia frutescens mycorrhizal inoculation experiment proved to be insignificant. Using a one-way ANOVA test, no significant effects of adding either Glomus intradices or native mycorrhizae to the containers of the outplanted shrubs were noted. The survival rate was 55% for native mycorrhizae treated plants, 65% for Glomus intradices treated plants and 70% for the non-treated plants. There was also no significant difference in height between the treated and non-treated plants. The surviving non-treated plants had an average height of 40 cm, the native mycorrhizae treated plants 41 cm, and the Glomus intradices treated plants 47 cm. The results of the height measurements are somewhat skewed because a number of the tubex had been blown off of the plants, causing them to grow in their more natural rounded low profile shape and/or to be herbivorized.
When survival was collected at the north site in July 1998, three months after installation, survival was found to be dependent on installation inside or outside the catchments (p = 0.0001). Survivorship inside the catchments was 85% compared to 70% for those planted outside the catchments in clusters. Survivorship was slightly lower on the experimental irrigation lines at 67%. A Chi square contingency table was calculated to determine the affect of straw and bark mulch surface treatment on survivorship within the catchments at the north revegetation site. No significant difference among treatments was found. Survivorship at the TV hill north revegetation site after three months is summarized in Table 11.

Table 11: Survivorship at TV Hill north revegetation site after three months
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	Outside Catchments				Within Catchments				On Irrigation Lines
Species	Planted	Alive	Percent		Planted	Alive	Percent		Planted	Alive	Percent
Achnatherum hymenoides	4	3	75		23	7	30		78	31	40
Ambrosia dumosa	96	76	79		36	22	61		32	18	56
Atriplex polycarpa/canescens	1	1	100
Encelia farinosa	2	2	100						7	7	100
Ephedra nevadensis					32	28	88		34	32	94
Eriogonium fasciculatum	46	44	96
Hymenoclea salsola	57	50	88		12	10	83		2	2	100
Isomeris arborea					4	2	50		4	0	0
Larrea tridentata	7	7	100		31	29	94		42	38	91
Lepidium fremonii	42	39	93		17	11	65		69	53	77
Lycium andersonii	7	6	86		1	1	100		8	6	75
Prosopis glandulosa					8	7	88
Opuntia acanthocarpa	12	12	100
Opuntia basilaris	2	1	50
Senna armata	14	6	43		6	3	50		23	15	65
Sphaeralcea ambigua	15	14	93
Yucca shidigera	32	28	88
Per treatment	337	289	86		170	120	71		300	203	68
Total planted on site	807
Overall survival	76%

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Survivorship at the TV Hill north site after six months was taken in November 1998 and is summarized in Table 12. Chi square tests were again performed, as described for the three-month data, with the following results; significant differences were found in survivorship between plants inside catchments and those outside (p ² 0.05), and between plants inside catchments and those on irrigation lines (p ² 0.05). No significant differences in survivorship were found between plants installed outside of catchments and those on irrigation lines, and no significant difference existed among the plants installed within catchments to which different surface amendments were applied.

Table 12: Survivorship at TV Hill north revegetation site after six months
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	Outside Catchments				Within Catchments				On Irrigation Lines
Species	Planted	Alive	Percent		Planted	Alive	Percent		Planted	Alive	Percent
Achnatherum hymenoides	4	2	50		23	18	78		78	32	41
Ambrosia dumosa	96	72	75		36	19	53		32	16	50
Atriplex polycarpa/canescens	1	1	100
Encelia farinosa	2	2	100						7	7	100
Ephedra nevadensis					32	25	78		34	29	85
Eriogonium fasciculatum	46	43	94
Hymenoclea salsola	57	49	86		12	10	83		2	2	100
Isomeris arborea					4	2	50		4	0	0
Larrea tridentata	7	7	100		31	29	94		42	36	86
Lepidium fremonii	42	37	88		17	12	71		69	54	78
Lycium andersonii	7	4	57		1	1	100		8	5	63
Prosopis glandulosa					8	7	88
Opuntia acanthocarpa	12	10	83
Opuntia basilaris	2	1	50
Senna armata	14	7	50		6	1	17		23	14	61
Sphaeralcea ambigua	15	14	93						1	1	100
Yucca shidigera	32	25	78
Per treatment	337	274	81		170	124	73		300	196	65
Total planted on site	807
Overall survival	74%

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One year results were taken in May 1999. There were still significant differences in survivorship between plants inside catchments and those outside catchments without irrigation lines (p = 0.0001) and those outside catchments with irrigation lines (p = 0.0001). Unlike the previous results, however, a significant difference existed among the plants installed within catchments as to which surface amendments were applied (p = 0.008). Compared to the control catchment survivorship of 73%, those shrubs planted in catchments amended with straw had an 88% survival and those planted in catchments with bark had an 84% survival. Photographs of TV Hill are shown in Figure 16 and survivorship data after one year is shown in Table 13.


Table 13: Survivorship at TV Hill north revegetation site after twelve months.
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	Outside Catchments				Within Catchments				On Irrigation Lines
Species	Planted	Alive	Percent		Planted	Alive	Percent		Planted	Alive	Percent
Achnatherum hymenoides	4	1	25		23	19	83		78	30	38
Ambrosia dumosa	96	74	77		36	21	58		32	15	47
Atriplex polycarpa/canescens	1	1	100
Encelia farinosa	2	2	100						7	7	100
Ephedra nevadensis					32	11	34		34	26	76
Eriogonium fasciculatum	46	39	85
Hymenoclea salsola	57	51	89		12	10	83		2	2	100
Isomeris arborea	4				4	1	25		4	0	0
Larrea tridentata	7	6	86		31	28	90		42	39	93
Lepidium fremonii	42	34	81		17	13	76		69	59	86
Lycium andersonii	7	4	57		1	1	100		8	6	75
Prosopis glandulosa					8	4	50
Opuntia acanthocarpa	12	11	92
Opuntia basilaris	2	1	50
Senna armata	14	8	57		6	1	17		23	14	61
Sphaeralcea ambigua	15	9	60						1	1	100
Yucca shidigera	32	20	63
Per treatment	337	261	77		170	90	53		300	199	66
Total planted on site	807
Overall survival	68%

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Discussion and Recommendations

Native Plant Windbreak

One year after planting survivorship remained very high at 84%. After planting in October 1997, plants installed at the Fifth Street windbreak had approximately one month of relatively mild conditions to overcome transplant shock and acclimatize to the site before the onset of cooler winter temperatures. The unusually wet El Niño winter of 1997-98 provided above average winter precipitation that encouraged strong root development. Together, the fall planting and above average precipitation are responsible for the excellent survival. There do not appear to be any large gaps in the windbreak resulting from mortality such as occurred on the Goldstone windbreak and replanting is not anticipated. Survival was poorest for Acacia. This species was outplanted from deep pipe containers which were difficult to transplant from and root damage probably occurred. Poor Acacia survival probably was the result of transplant shock and a general lack of vigor resulting from the age of the plants at the time of planting. Furthermore, they may have been more susceptible to the added physiological stress caused by dust from vehicle traffic. It may be worthwhile trying this species in future windbreak plantings. The use of smaller container sizes in conjunction with an early fall or spring planting when temperatures are warmer should improve success.

The loose alluvial soils and windbreak positioning relative to drainage patterns has been problematic. Flooding and erosion of the berm, with burial of some seedlings in the eastern section of the windbreak, has resulted from winter rainfall. These problems were corrected in January and February 1998 by SERG personnel with the ITAM tractor used to make repairs to the damaged sections. Sand bags were also used to reinforce vulnerable points. In March 1998 ITAM arranged for a motor grader to place a second berm alongside the eastern most section of the existing windbreak to protect the seedlings from further damage. This effectively protected the seedlings through the vulnerable first year seedling stage.

Wheeled vehicle traffic cutting across the berm was also a problem. During installation, one section of perforated pipe was crushed by traffic driving over the freshly buried pipe and had to be repaired. Off limits markers placed by ITAM along with the highly visible protective treeshelters placed around the seedlings appear to be partially effective. Once seedlings have outgrown the treeshelters, we recommend that the shelters be removed and placed alongside the plant as a visual deterrent to traffic. Watering into the perforated pipe began in May 1998 and continued monthly throughout the summer. Individual hand watering of the seedlings as was done on the Goldstone tank trail windbreak will not be necessary because rooting to the depth of the perforated pipe appears to have occurred. Future maintenance of this windbreak should include grading every spring to restore the berm where flooded out or damaged by vehicle traffic.

Erosion Control Site

The amount of soil translocated on the surface treatment plots was monitored in July 1998 and May 1999. This marked the end of one wet winter season for the site in 1997-98 and one dry winter season in 1998-99. Erosion gullies did not form on any of the surface treatment plots or control. At the same time, soil accumulation behind the check dams was recorded. No check dam failures were discovered, no repairs were necessary nor is any further maintenance or monitoring planned for the site.

On the surface treatment plots, an average of 0.19 cm of soil accumulated on the coir fence treatment, 0.30 cm on the punched straw plot and 0.41 cm on the control. While not statistically significant, these differences are understansable when considering the ripped and loosened condition of the soil surface. On the control and coir plots, the loosened soil shifted during winter rains and accumulated against the coir fences and measuring stakes. On the punched straw plots, the surface was better stabilized. Winter rainfall did not shift the loose soil surface and the straw facilitated water infiltration; the overall effect was no soil loss or accumulation. For Fort Irwin, using punched straw at the conclusion of training activities to protect vulnerable areas appears to be highly practical. Because the major expense involved with using punched straw is labor, and labor is plentiful during cleanup activities following training, work parties can be arranged to install punched straw immediately after damage has occurred and before erosion can aggravate problem areas. Another possible option for slowing down sheet erosion would be the construction of catchments in a fishscale pattern.

While statistical analysis did not reveal a statistically significant link between check dam construction material and soil accumulation in the gullies, two trends were apparent and should be considered when planning future check dam installations. First, straw bale dams tend to trap more soil and, second, coir fences tend to trap very little soil. Straw bales were placed in larger drainages; therefore, higher runnoff and soil accumulation would be expected. Why coir dams did not hold as much soil as the other check dam types is probably due to the coarse sandy soil being able to move through the coir fences. Future installations may want to avoid coir check dams at sites with very sandy soils and use straw check dams instead. Another alternative would be to alternate check dams of various construction techniques since in areas that receive large amounts of runnoff coir check dams are still desirable because of their higher porosity and reduced likelihood of washout.

Direct Seeding Site

At the seeding site, the surface treatment experiment was modified in response to training activities that impacted the area in February 1998. The removal of markers and the loss of plots was the most significant disturbance to the site. Originally there were three blocks of each surface treatment. All of the peatmoss plots were lost, while only one bark, one xanthan gum and one Soil-Sement plot were able to be salvaged and sampled in May 1998. A statistically significant increase in seedling density was found on the plot treated with Soil Sement. This suggests that soil tackifiers may be useful in improving direct seeding success. It is interesting to note that the Xanthan gum plots did not follow a similar trend. This suggests the Xanthan gum may not be as effective as the Soil-Sement in glueing the seed down. The lowest seedling density was observed on the fir bark plot. This suggests gluing seed down may be more effective than covering the seed up with mulch; however, observations by Bainbridge et al. (1995) suggest that a covering of peat moss may facilitate germination on desert sites. Further research is needed to elucidate this problem. Based on the small sample size used in this analysis, conclusions drawn from this study are tentative at best. If large scale seeding is planned for Fort Irwin, a repeat of this investigation with additional surface treatments would be a prudent investment that could improve seeding effectiveness and reduce future costs.

Damage to the seeding area in February 1998 included vehicle tracks over much of the site. This presented an opportunity to investigate such an impact on seedling establishment. Not surprisingly, a significantly lower total percent cover (at 0.05 significance level) was found on the high use vehicle trails . This is probably the result of heavy soil compaction. Surprisingly, density was significantly (at the 0.08 level) higher on the low use vehicle trail. The light vehicle traffic may have facilitated germination by scarifying hard seed coats and improved seed/soil contact by pushed the seed firmly into the ground. This area needs further investigation. Results may give insight into the level of soil preparation necessary prior to direct seeding needed to produce adequate results.

Two different seeding densities, 100 live seeds/m2 and 250 live seeds/m2, were tested at the seeding site. No significant difference in percent cover or density were found between the high and low density seeded areas. There are two possible explanations for this lack of response. First, it was very windy when the high density area was seeded - much of the seed could have been lost due to the wind. Second, and more likely the probable cause, the size of both areas changed after grading.

TV Hill Revegetation Sites

At both the south and north revegetation sites, survival inside and outside the catchments was compared. Initially no significant difference was found at the south site. This was probably caused by the site being planted during the winter and receiving above average precipitation which equalized soil moisture inside and outside of the catchments. However, at the south site after eight months and at the north site for the four, eight and twelve month survivorship measurements, survival was significantly higher within the catchments. It appears that the use of catchments for the planting of desert perennial shrubs at Fort Irwin is effective in increasing survivability and should therefore be integrated into future restoration projects as a common planting procedure.

All species benefited from being planted in the catchments, however, caveats do exist in their use. After an intense rainstorm, such as those that occurred in February 1998, catchments filled with water and remained flooded for several days. During this time, the roots and lower stem suffer from lack of aeration and may have been vulnerable to fungal disease. For this study, shrubs were planted one meter apart in the deepest corner of the catchment and, on several occasions, water was observed pooling in the catchments. Though the survival rate difference was not significant, the Larrea at this site may have suffered from this. On several occasions Larrea transplants inside the catchments did not green up as rapidly as those planted outside. Knowing the infiltration rate of the soil should help determine the best planting spot within the catchment. Fast draining soils would be better candidates for planting at the lowest point in the basin while catchments constructed in slow draining soils should be planted up on the basin border to take advantage of the soil moisture above the water line caused by capillary action.

The results of this study are inconclusive as to which catchment size was the most effective. Catchment size did not significantly effect survival at any of the samplings. One year after planting, the highest survival (87%) was found in the 4 m2 size catchments, with next highest in the 25 m2 size catchments (86%). Survivorship was lowest (80%) on the 9 m2 and 16 m2 catchments. With time, catchment size and the water harvesting characteristics of larger catchments may become more important and lead to higher long term plant survival. However, this experiment suggests that, at least on this resoration site, building catchments larger than four square meters for native species may not be worth the added construction expense. It is also worth considering that several smaller individual catchments have higher relative water yield per unit surface area than one larger catchment.

The transplanted shrubs within the catchments benefited from the addition of bark and straw as soil amendments. Although the results of increased survivorship did not become apparent until the final monitoring event, the control group survivorship of 73% was lower than the catchments amended with straw (88% survival) or bark (84% survival). The benefits of the mulches may not have become apparent during the earlier samplings for two reasons. First, since there was relatively very little rainfall during the winter 1998-99 season, supplemental watering did not occur after fall 1998. The bark and straw may have helped retain moisture in the soil for a longer period of time than in the catchments without amendments. Second, since bark is highly recalcitrant, it may have taken more than six months for mineralization to commence and begin releasing nutrients into the soil. As a result, more water and nutrients during the six to twelve month interval were available to shrubs growing in amended catchments than the shrubs growing in non-amended catchments. This condition is most likely the reason for the increased survivability.

As a result of the February 1998 storms, nine out of the 128 catchments on the south site washed out. These catchments had been placed in drainages that received large amounts of surface runoff. Five of the catchments were in the same drainage fed by a bare, heavily compacted slope on which catchments could not be constructed. It appears that the first catchment failed after receiving too much water, causing a cascade effect that washed out the next four catchments. The fifth catchment in the series had been reinforced with rocks as it was being built and did not fail. Based on these observations, we suggest that if catchments are to be placed in gullies or drainages that will receive large amounts of runoff, they should be reinforced in some way. It is probably not worth the extra cost to reinforce all the catchments on a site if they are fed only by sheet runoff.

No significant effects of increased survivorship or height of plants occurred after adding mycorrhizal inoculum, either native or commercial stock, to the Encelia frutescens shrubs planted at the south site. The non-treated plants had a higher survival rate than either of the treated plants. This suggests that, at least for the Encelia frutescens shrubs, there was enough native mycorrhizae already present in the soil to benefit the plants or the plants do not necessarily need an abundance of mycorrhizal infected roots to survive. Perhaps other species of plants would benefit more from mycorrhizal inoculation treatments.

Based on the results so far, where topography and soil conditions permit, catchments appear not only to be effective in improving survivorship but also are likely to reduce surface runoff and erosion. Care must be taken to place catchments where they will not receive excessive runoff or reinforcement using rocks may be necessary to prevent washouts. At Fort Irwin, catchments offer a practical revegetation technique that can passively provide additional soil moisture and improve revegetation success.

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