Sam Johnson Pine and Oak Response to Fire in the Sierra Nevada Foothill Savanna May 8, 2006p. 1Pine and Oak Response to Fire in the Sierra Nevada Foothill SavannaSam JohnsonAbstract The pine-oak savanna of California's Sierra Nevada foothills has been greatly alteredby the introduction of cattle grazing, exotic annual grasses and the suppression of fire. Fire is animportant element of the disturbance regime of an ecosystem and is a common management toolfor wildlands. Therefore it is important to understand the effects of fire on the growth rate of treespecies native to this ecosystem. I used woody debris line-intercept sampling and Brown fueltransects to quantify fuels consumed by a controlled burn treatment at the University ofCalifornia's Sierra Foothill Research and Extension Center (SFREC) in the northern SierraNevada mountains. I used the difference in ground fuels before and after the burn as a proxy forthe fire intensity and compared the difference against tree diameter growth rates before and aftertreatment. Quercus douglasii and Pinus sabiniana both showed a significant negative differencein normalized rates of radial growth in the first year following the prescribed burn. I did not finda significant correlation between radial growth and fuels consumed on a sub-watershed scale. Ideveloped a model to explain the relationship between woody debris and Brown fuel transects.This pilot study suggests that a more in-depth study is needed to understand the relationshipbetween fire and tree growth here. Longer term trends can only be determined futuremeasurements of tree growth. Further work needs to be conducted on the specific parameterswhich best suit the implementation of woody debris and/or Brown fuel transects in the savannaecosystem.Sam Johnson Pine and Oak Response to Fire in the Sierra Nevada Foothill Savanna May 8, 2006p. 2IntroductionCalifornia's Sierra Nevada foothills contain extensive blue oak savanna ecosystems, whichhave attracted attention as part of ongoing debates about the ecological role of fire andappropriate use of fire as a management tool. However, very few studies have been conducted onthe blue oak savanna, and those studies which have examined these ecosystems have treatedthem as co-existing grassland and a woodland/forest ecosystems, not as a form in its own right.Approaching a savanna as a distinct ecosystem has the potential to more precisely describe andunderstand the unique dynamics of the system. Some past research has been conducted onsimilar pine-wiregrass systems in the southeastern US (Mitchell 1999), but conundrums outlinedby House et al. (2003) limit and restrict the results of savanna study. They noticed that in order tobetter understand savanna dynamics, research must acknowledge three conundrums – whichmost studies of savanna systems fail to thoroughly address. House et al. pose the conundrums asthree questions which most studies of savanna systems fail to thoroughly address. How is thewoody and herbaceous plants ratio controlled? How do they interact? How does the net primaryproductivity (NPP) change with changes in the woody-herbaceous plant ratio? To address thesequestions, the savanna ecosystem must be viewed as a continuous ecosystem – not a mixture ofgrasslands and woodlands. Previous studies on blue oak savanna (e.g. Callaway 1991, Dahlgern1997, Kay 1987) examined tree growth at the University of California's Sierra Field Researchand Extension Center (SFREC), but none of these studies took a continuous ecosystem approach.Ecosystem scientists recognize a model developed by Hans Jenny as one of the seminalworks in the field. This model consists of five state factors which act upon ecosystem processesand affect what organisms exists for a particular set of state factors as shown in Fig. 1. Jenny(1941) originally developed the idea of five independent state factors to explain soil processes,but ecologists later used this model to explain ecosystem processes in general. Chapin et al.(1996) revised Jenny's model to include the concept of four interactive controls which, unlikestate factors, both affect and are affected by ecosystem processes (Fig. 1). Many ecosystemshave evolved with particular disturbance regimes - an interactive control of the state factorsclimate and time - of which fire is an essential component (Chapin et al. 1996). Savanna systemsare one such ecosystem. John Battles (2005) has proposed that three major changes have takenplace in California savanna systems. First, native grasses have been replaced with non nativegrasses. Second, fire has been removed from the ecosystem. Third, cattle grazing has beenSam Johnson Pine and Oak Response to Fire in the Sierra Nevada Foothill Savanna May 8, 2006p. 3introduced to the ecosystem. Interestingly enough, these ecosystems seem to be fairly stable.Battles (2005) has further suggested that grazing has taken the place of fire to keep r-selectedspecies (herbaceous plants) in check while giving k-selected species (woody plants) a chance tothrive, but the effects of fire on growth have not been studied on a blue oak savanna as acontinuum of woody and herbaceous plants.My study examines how tree diameter growth correlates with fire intensity in a savannadominated by blue oak (Quercus douglasii) and ghost pine (Pinus sabiniana) trees on plot andwatershed scales. It also quantifies fire intensity by the difference in the amount of ground fuelbefore and after the fire using downed woody debris and Brown fuel transects (1974). In thisstudy, I test the hypothesis that in areas of my study site with significantly less ground fuels aftera prescribed burn than before the burn correlates with greater increase in tree diameter thanwould otherwise occur. I am also testing the hypothesis that trees in the watershed subjected to aprescribed burn will exhibit a more positive response in radial growth than occurs in the controlwatershed. In essence I am asking if fire is beneficial to the trees in a the blue oak savanna of theSierra Nevada Foothills.MethodsMy study is a subset of a larger project conducted in the UC Berkeley department ofEnvironmental Science Policy and Management which attempts to answer the NPP question inthe blue oak systems at my site. Forest ecologist John Battles, and rangeland ecologists BarbaraAllen-Diaz and James Bartolome focus on using a continuous ecosystem approach, which Houseet al. (2003) recommends for addressing the three conundrums of savanna ecosystems.
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