Response of Pinyon-juniper woodlands to fire, chaining, and hand thinning

Pinyon-juniper (Pinus monophylla – Juniperus osteosperma) woodlands have expanded and infilled over the last 150 years to cover more than 40 million ha in the Great Basin. Many land managers seek to remove Pinyon-juniper trees using a variety of treatments. This thesis looks at six different Pinyon-juniper removal projects in Central and Eastern Nevada. We established a total of 73 vegetation and soil monitoring plots (38 treated, 35 adjacent untreated) across six Pinyon-juniper removal projects in Central and Eastern Nevada to look at the effects of fire, hand thinning, and chaining. The four burns examined together in Chapter 1 had similar elevation, precipitation, and pre-treatment vegetation communities in the untreated areas, but the treated areas had significantly different responses to treatment. With nonmetric multidimensional scaling (NMS), we found a useful 3-axis ordination of the plots (stress=7.1, R2=.966). Within ordination space, the treated plots were well grouped by parent material. These results informed a Poisson generalized linear model that found parent material factorized explained 86.5% of the deviance in cheatgrass (Bromus tectorum) cover at the treated plots. The projects on soils derived from welded tuff had very little cheatgrass while soils derived from limestone or mixed limestone/volcanics were dominated by cheatgrass. Parent material should be considered an important factor when planning Pinyon-juniper removal treatments. Chapter 2 examined the effects of a hand thinning. The hand thinning significantly reduced tree cover [F(1,10) = 7.43, p = 0.027] to less than 2%. Perennial grasses on the site are slightly higher in the treated area. There was a significant increase in perennial grass cover from 2013 to 2014 [F(1,10) = 14.5, p = 0.003]. The hand thinning did not have significant effects on shrubs, annual grasses, annual forbs, perennial forbs, ground cover, stability, species richness, diversity, infiltration, or gap structure. Because hand thinning does not remove the shrubs or other perennials, site resistance can be maintained. With sufficient understory vegetation to maintain resistance post treatment (as in phase I or early phase II Pinyon-juniper woodlands), nonnative annual grasses are less likely to dominate after treatment. Chapter 3 examined the effects of a chaining. The effects of the 40-year old chaining are still significant even though Pinyon-juniper trees are reinvading and make up >5% of the cover in the treated area. The treated areas still have a much more productive understory than adjacent untreated areas. Perennial grass cover, frequency, and density was 2-5 times greater in the chained area. The treated area had fewer large gaps (>100 cm). However, interspace infiltration times were slower in the treatment (t(4)=-2.14, p=0.09). Surface and subsurface soil aggregate stability remained significantly lower in the treatment for vegetation-protected and unprotected samples (t(4)=3.53, p=0.024; t(4)=3.10, p=0.036). Chainings have long-term benefits for vegetation, but also long term impacts on soils and hydrologic ecosystem processes. When planning Pinyon-juniper removal treatments, land managers should consider the plant community, temperature and precipitation regime, and soils at the potential treatment location to better achieve desired outcomes