Effect of redistributing windrowed topsoil on growth and development of ponderosa pine plantations
Introduction
Windrowing is a site preparation operation, usually performed by machine, which piles debris and shrub slash in linear rows immediately prior to planting (Helms, 1998). The primary objectives are to increase survival and growth of planted trees by improving the seedling microsite and controlling competing vegetation. Increased access, lower fire risk, and reduced pests are secondary objectives (Atzet et al., 1989). This practice, however, directly impacts surface soil, or topsoil, where organic matter and labile nutrients are disproportionally concentrated in the soil profile (Powers, 1990, Powers et al., 1990). Tew et al. (1986) estimated that displacement of surface materials into windrows removed two to three times more N and P than does whole-tree harvesting. Because temperate and boreal forests can store as much as three times more nutrients in the forest floor than in the standing forest (McColl and Powers, 1984), windrowing has a significant impact on nutrient displacement and subsequently site productivity (Morris et al., 1983, Dyck and Beets, 1987, Powers et al., 1988, Fox et al., 1989).
During the 1950s and 1960s, many plantations were established using windrowing methods in the United States and across the world (Fig. 1). The direct impact of such practices on early plantation productivity often was confounded by reduced weed competition (Powers et al., 1990). For example, volume growth for a Pinus taeda plantation grown on windrowed sites as compared to those with intact topsoil significantly increased at age 3 in Alabama, which was mainly due to plant competition reduction (Tuttle et al., 1985). However, volume was similar at age 12 in Louisiana (Haywood and Burton, 1989). Because topsoil displacement also reduced weed competition, fertility losses were confounded with reduced competition.
In New Zealand’s pumice region, displacing logging debris and a thin layer of topsoil into windrows during site preparation produced nutrient deficiency and led to a 30% loss in volume growth in a 17-year-old Pinus radiata plantation (Dyck and Beets, 1987). In the North Carolina Piedmont, windrowing on a Typic Hapludult soil led to a 23% volume growth reduction at 25 years (Fox et al., 1989). Powers et al. (1988) compared nutritional characteristics of 22 established plantations of ponderosa pine (Pinus ponderosa) in California and Oregon that had or had not been windrowed during site preparation. Although results were confounded somewhat by differing soil types, windrowed plantations averaged one-third less mineralizable soil N, one-tenth less foliar N, and one-third lower site indices than non-windrowed plantations. Nitrogen fertilization produced four times the relative volume growth response in windrowed plantations versus non-windrowed ones.
Mitigating the impacts of windrowing on these maturing plantations can be difficult. As far as we are aware, no studies have been reported showing before and after results from redistributing topsoil. Moreover, few studies have aimed to test if windrowing affects long-term stand productivity and overall ecosystem health. Here, we report results from a well-designed experiment initiated by the late Dr. Robert Powers in 1989 to determine if and how topsoil redistribution and shrub control affects soil productivity and tree growth in an established plantation.
Section snippets
Study site
The study is located in Northeastern California on the Doublehead Ranger District, Modoc National Forest (Lat. 41.33N; Long. 121.27W). Elevation is about 1650 m. Site index is 22 m at 50 years. Slopes average about 15% gradient with an easterly aspect. Soils are of volcanic origin where pumice, ash and cinders were deposited on the lower side slopes of Medicine Lake volcano. The USDA soil series is Tionesta, classified as a pumiceous or ashy-pumiceous over medial-skeletal, mixed, frigid, Typic
Tree growth
Because SHRUB and SOIL effects were significant for height, QMD, BA, and volume measured in 1989, we analyzed treatment effect on stand growth using 1989 data as a covariate. Neither SHRUB or SOIL effects, nor their interactions, were found to be significant for height in 1994 (P > 0.17). However, the effect of soil manipulation was significant (P < 0.02) in 2005 and 2010. In 2010, over 20 years after the treatments, shrub removal effect was only significant at P = 0.07. Multiple comparisons showed
Discussion
Results from this study provide some quantitative measures of amounts of soil and soil nitrogen and carbon that were displaced when windrowing site preparation was used to establish tree plantations, a widely used practice in the West in the 1950s and 1960s. The windrows in this study were substantially larger due to wider inter-windrow widths (approx. 28–35 m) and a more dense brush community than what was reported in the South (Morris et al., 1983, Pye and Vitousek, 1985, Tew et al., 1986).
Acknowledgements
We thank many former and current personnel from Pacific Southwest Research Station and Region 5 for installing, maintaining, and measuring this study, especially Robert F. Powers, Cristina Siegel-Issem, Rose Leonard, Terrie Alves, Bert Spear, Bob Carlson, and John Anstead. A great collaboration between generations of Forest Service researchers and Modoc National Forest personnel made this study possible. We also thank our station statistician James Baldwin for his assistances in the data
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