Graduate Thesis Or Dissertation

Vegetation and soil response to tree removal methods in invasive western juniper woodlands

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  • On piñon-juniper encroached sites that lack the understory fuels to carry a prescribed fire, treatment options are limited to mechanical methods. Cutting with chainsaws and leaving the trees on site has been the primary treatment method for such sites, however this method creates a potential fire hazard, particularly in the first 2-3 years when needles remain suspended on downed trees. Follow-up treatments to remove juniper fuels, such as broadcast burning downed trees or moving slash into piles with machinery followed by burning are becoming more common practices on private and public lands. There is limited information on the impacts of mechanical and fire treatments on herbaceous recovery and impacts to soil nutrients and characteristics. It is important to evaluate these treatments to provide resource managers with ecological information to assist in developing appropriate fuel reduction measures. This study sampled herbaceous vegetation and soil attributes of three common mechanical treatments used to treat invasive western juniper (Juniperus occidentalis var. occidentalis Vasek) woodlands: Cut & Leave, Cut & Burn, and Pile & Burn. Sampling consisted of cover and density of herbaceous species and life forms (groups of species that function similarly), as well as total ground cover, and soil attributes including: carbon (C), nitrogen (N), carbon:nitrogen ratio (C:N), phosphorus (P), potassium (K), sulfate-sulfur (SO4-), calcium(Ca), magnesium (Mg), organic matter (OM~loss on ignition), cation exchange capacity (CEC), and power of hydrogen (pH). A randomized complete block experimental design (RCBD) was used with five 1-hectare blocks and three treatment plots per block (with piling plots being ½ hectare in size, and the other two treatment plots each ¼ hectare in size). Within each treatment plot, herbaceous vegetation sampling was stratified between three microsites: the slash microsite (beneath the three slash treatments; cut trees, burned trees, and burned piles), in the litter deposition microsite (litter zone around the stump) and the interspace microsite (between trees). Soil attributes were only sampled in the slash and interspace microsites, at two depths (0-4cm and 0-25cm). Each microsite within each plot was sampled by 40, 0.2m2 herbaceous frames and three composited soil samples from each depth. Mechanical treatments were completed in the fall of 2005 and prescribed fire treatments were completed in the fall of 2006. Treatment analysis compared pre and post treatment herbaceous data from 2005 and 2007, and soil attribute data from 2006 and 2007. Bare ground increased and litter and cover and density of most herbaceous species/life form groups declined beneath Cut & Burn and Pile & Burn treatments compared to the Cut & Leave treatment. The largest declines in cover and density of herbaceous species/life forms were recorded beneath burned piles. The largest changes measured adjacent to slash treatment locations (in the litter deposition and interspace microsites) were in the Cut & Burn plots due to fire spreading out burned trees. In the litter deposition microsite of the Cut & Burn treatment in 2007 total herbaceous cover was 1/3 and 1/2 that measured in the Cut & Leave and Pile & Burn plots, respectively. Contrarily the interspace microsite of the Cut & Burn treatment in 2007 indicated a slight (although insignificant) increase in herbaceous cover, with annual forb cover twice that measured in either of the other treatments. Concentrations of C, N, OM, and CEC were largely unchanged from pre-treatment levels in the compared treatments, however SO4, Mg, Ca, K, and pH all increased in response to burning compared to the Cut & Leave treatment. Implications of this research are limited to the first year post burn-treatment. However, these findings can be used as a general guide to choosing slash treatments to meet specific objectives. Determination of which of these treatments to use is largely dependent upon management objectives and site conditions. The high degree of vegetation disturbance and nutrient release measured beneath the Pile & Burn slash treatment could pose potential problems with invasive species establishment. However, the Cut & Burn treatment had similar, although less dramatic, vegetation disturbances and nutrient releases beneath burned slash, which covered a larger proportion of area than the Pile & Burn treatment. If a similar degree of invasive species establishment occurs in both of these slash burning treatments, the Pile & Burn treatment may be a better option due to the smaller area impacted. Measurements indicated little impact to vegetation in the Cut & Leave treatment, however wildfire risk could diminish the short term benefits of this treatment. Long term monitoring of herbaceous vegetation and soil attribute response among these three slash treatments is needed to make solid inferences of the site recovery following treatments to assist in land management decisions.
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