Ingrid Burke Ash vs Non-Ash: Nitrogen & Moisture May 08 2006p. 1Ash and Non-Ash Nitrogen and Moisture Levels in Whitaker’s Forest Canopy GapsIngrid BurkeAbstract Creation of canopy gaps is an innovative forest management technique that providesspace and light for new growth while maintaining the ecology and biodiversity of the forest. Acurrent study on the canopy gap regeneration of giant sequoia (Sequoiadendron giganteum) inWhitaker’s Forest (southern Sierra Nevada Mountains, California) shows a marked difference inthe size and vigor of seedlings growing in ash beds, which resulted from burning logging debris,compared with those growing in bare mineral soil. To investigate this difference, I collected soiland foliar samples, as well as soil moisture measurements, from both ash and non-ash areas offour 1-acre gaps. The soil and foliage samples were analyzed using extractable nitrate andammonium tests. Soil sample nitrogen levels range from 0.1 ppm to 64.5 ppm, foliage samplesrange from 5 ppm to 650 ppm, and soil moisture readings range from –0.044% to 0.36%; none ofthe three shows statistically significant difference between ash and non-ash. Levels of soilnitrogen don’t appear to be related to those of foliar nitrogen, and nitrogen levels do not appearto be related to soil moisture. Neither ash nor non-ash areas in large gaps show the expected highmoisture levels near gap center. This pilot study outlines the high variability in nitrogen andmoisture levels in the system, and also illuminates the spatial scale at which the variabilityoccurs. Future work can use these baselines to continue the examination of potential influencesof ash on conifer regeneration.Ingrid Burke Ash vs Non-Ash: Nitrogen & Moisture May 08 2006p. 2IntroductionIf current even-aged forest management is to be converted into more ecologically beneficialuneven-aged management, we must thoroughly understand the factors influencing seedlinggrowth within canopy gaps (Gagnon et al. 2004). Seedlings within gaps can grow in ash beds andbare mineral soil. To quantify the differences in growing conditions provided by these twosubstrates, I measured their soil nitrogen and moisture content. I also measured the foliarnitrogen content of seedlings grown in ash and soil. The influence of ash on regeneration in gapsis an important consideration because it has the potential to enable increased growth, yet furtherstudy is necessary to determine whether this occurs.Regeneration of the giant sequoia (Sequoiadendron giganteum) within canopy gaps iscurrently being studied at Whitaker’s Forest in the southern Sierra Nevada. Twenty canopy gapswere created in 2003 by small group selection harvests. Differences between gaps andsurrounding forest have been shown to persist for a minimum of two years from gap formation(Ritter et al., 2005a), so these areas can still be expected to have exhibited gap characteristicswhen the data for this study was collected in 2005.Within each canopy gap, logging debris was piled in a north-south strip, called a windrow,and burned. Seedlings and seeds were planted in the resulting ash beds and in the adjacent baremineral soil. Seedlings planted in ash have exhibited higher growth rates when compared tothose in bare mineral soil, but, in the context of York’s study, this difference was less powerfulthan the effect of gap size on growth (R. York 2006, manuscript in progress). However, whenconsidered independently, growth differentials between ash and non-ash hold interestingimplications, and I seek to explore these differences in this study.Seedling growth is regulated by the availability of three basic variables: sunlight, moisture,and nutrients. These three variables are all closely intertwined; for example, rates of soil nutrientcycling are determined by soil temperature and moisture (Prescott 2002) and soil nitrogen levelsare determined partially by soil moisture (Kennedy and Pitman 2004). It is impossible to gain aclear understanding of the seedling responses at Whitaker’s Forest without examining all of thesefactors.Gaps in the forest canopy allow greater amounts of sunlight to reach the ground, which maycause interactions with soil moisture. Despite the increased evaporation implied in gap scenarios,conditions of reduced competition result in higher soil moisture within canopy gaps whenIngrid Burke Ash vs Non-Ash: Nitrogen & Moisture May 08 2006p. 3compared with surrounding forests of both beech and Douglas-fir. In fact, moisture levels areoften at capacity within gaps while being as low as 50% of capacity outside (Gray et al. 2002,Ritter et al. 2005a, Ritter et al. 2005b). Conditions of high water and high sunlight are beneficialto seedling growth, but the color and texture of an ash substrate may react in a different wayfrom soil, and provide very different growing conditions.Nutrient availability is the final essential factor in seedling growth, and nitrogen has beenfound to be the only nutrient that promotes forest growth when administered individually toconifers (Nohrstedt 2001). For centuries, the availability of nutrients such as nitrogen has beenthought to be augmented by the introduction of ash to the soil. This can easily be done inmanaged forests by the on-site burning of logging debris. The comparison of ash and soil isparticularly interesting in my study because the windrow technique of debris burning used hasbeen found to have “dramatic and lasting effects on soil development” in the southeastern UnitedStates (Johnston and Crossley 2002), yet its effects on regeneration have not been investigated.There is some disagreement regarding the effects that wood ash has on seedling growth. Onestudy showed that ash did not affect foliar nutrients, growth, or biomass production in a willowplantation (Park et al. 2005). Another study showed an overall increase in biomass after burningin a pine forest but attributed this partially to an increase in light availability (Skre et al. 1998). Athird study found that ash results in minor conifer growth stimulation in nitrogen rich sites whileit decreases growth in nitrogen poor sites (Nohrstedt 2001). Some canopy gaps, even those assmall as 0.1 hectare, have exhibited increased soil nitrogen availability (Prescott 2002), whichsupports the idea that gaps are
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