Fire Severity and Western Juniper
Fire severity varied by time of burning and site (Appendices 1 through 3). At all sites, fire severity was rated light in JAN and MAR treatments as only 1-hr fuels were consumed and shrub mortality was less than 10 %. At the Bluebunch and NGBER sites, severity was rated high in the SEP and the APR treatments as 1-hr, 10-hr, and 100-hr western juniper fuels were consumed, 1000-hr fuels were partly consumed and charred, and shrub mortality was 80 % (APR) to 100 % (SEP) (Appendices 1 and 2). All junipers remaining after prep-cutting were killed by fire. In the CUT, JAN, and MAR treatments, all trees >1.5 m height were killed by cutting; however, 50 % to 100 % of small juniper (<1.5 m) survived (Figure 2A and 2B). Juniper cover was eliminated on the SEP treatment and in the other treatments (CUT, JAN, MAR, APR), cover was below 0.1 %.
At the Fescue site, fire was of moderate severity in the SEP treatment as juniper 1-hr and 10-hr fuels were fully consumed, 100-hr fuels were partially consumed, and shrub mortality was less than 75 % (Appendix 3). Fire severity was moderately high in the APR treatment as 1-hr, 10-hr, and 100-hr fuels were consumed; 1000-hr fuels were partly consumed and charred; and shrub mortality averaged 35 %. Most small juniper (<1.5 m height) survived the CUT, JAN, and MAR treatments (Figure 2C), and juniper cover was reduced to less than 0.5 % on all treatments.
Ground Cover, All Sites
Prior to juniper treatment, ground cover response variables did not differ among treatments and the control; after treatments, there were strong year-by-treatment interactions. Beginning in the second year after treatment, total herbaceous cover increased (P < 0.001), and by the fifth year after treatment applications (2011), herbaceous cover was 2.5 times greater (34.9 ± 4.2 %) than the control (Figures 3 and 4A; P = 0.004). Herbaceous cover among the five juniper treatments, however, was not different (P = 0.415). In the first year (2007) after treatment, litter cover increased in CUT (+50%), Jan (+33%), MAR (+38%), and APR (+ 27%) treatments, and decreased in the SEP (−40%) treatment (Figure 4B, P < 0.001). By 2010, differences in litter cover among most treatments (APR, JAN, MAR, SEP) and the control were equalizing and did not differ from pre-treatment values. Litter cover was greater in the CUT than in other treatments for most of the study, and in 2011 was 21 % to 59 % greater than the other treatments and control (P < 0.006). Bare ground increased in 2007 in SEP (+20 %) and APR (+8 %) treatments, and declined 14 % to 19 % in the other treatments (Figure 4C; P = 0.006). Bare ground eventually declined in all treatments to below pre-treatment levels (P < 0.001); however, it remained 9 % to 20 % greater in the SEP treatment than in the CUT, JAN, and MAR treatments (P = 0.046). Biocrust cover declined from 3 % to below 1 % in all treatments and were less than the control (Figure 4D, P < 0.007).
Life Form Canopy Cover
Poa secunda. Treatment response of P. secunda was site dependent. On the Bluebunch site, cover declined among treatments from 2.2% in 2006 (pre-treatment) to less than 0.5% in 2011, significantly less than in the control (P = 0.021). On the Fescue site, P. secunda cover fluctuated in response to year (P < 0.001); however, treatment differences were not significant (P = 0.283), with cover averaging 3.5 ± 0.2%. On the NGBER site, cover of P. secunda treatments did not differ (P = 0.719); however, cover increased across years from 1.8% to 2.5 % (P = 0.002).
Perennial bunchgrasses. Cover of perennial bunchgrasses increased the third year (2009) after treatment at all sites (P < 0.001), and by 2011 were 2.5 to 3 times greater on the treatments than the control on the Bluebunch site (Figure 5A, P < 0.001); 1.5 to 2 times greater than pre-treatment (2006) values on the NGBER site (Figure 5B, P < 0.001); and 2 to 2.8 times greater on the treatments than the control on the Fescue site (Figure 5C, P < 0.001). On the Bluebunch site, bunchgrass cover was greater in the JAN treatment than in the SEP and APR treatments (P < 0.045). At the NGBER site, perennial bunchgrass cover was 3 % to 5 % greater in the JAN, MAR, and CUT treatments compared to the SEP treatment from 2009 to 2011 (P < 0.001). On the Fescue site, bunchgrass cover was greater by 3 % to 7 % in the JAN and MAR treatments compared to the CUT, SEP, and APR treatments (P = 0.011).
Perennial forbs. On the Bluebunch site, perennial forbs did not respond to treatment as dramatically as other life forms (P = 0.007; Figure 6A). In three of the post-treatment years, (2007 to 2009) perennial forb cover in all treatments (except APR) was greater than in the control; however, forb cover never exceeded 2 %, nor did it differ from the controls during the final two years of measurement. On the NGBER site, perennial forb cover increased in all treatments except the SEP treatment compared to pre-treatment values (P = 0.002). Perennial forb cover in the SEP treatment was consistently 2 % to 3 % lower than the other treatments (Figure 6B; P < 0.001). On the Fescue site, perennial forb cover increased in all treatments and in the control (Figure 6C, P = 0.019); however, cover was about 50 % greater in the treatments than in the control (P = 0.001).
Annual grass. On the Bluebunch site, invasive annual grass (cheatgrass [Bromus tectorum L.] and field brome [Bromus arvensis L.]) began increasing during the third year (2009) following treatment, after which annual grass cover was significantly greater than in the control (Figure 7A; P < 0.001). In the final two measurement years, annual grass cover was nearly 50 % greater in the SEP treatment than in the JAN treatment. On the NGBER site, cheatgrass increased in the SEP and APR treatments and was greater than in the JAN, MAR, and CUT treatments (Figure 7B; P = 0.003). In 2011, cheatgrass cover represented about 12 % and 20 % of total herbaceous cover in SEP and APR treatments, respectively. On the Fescue site, B. tectorum cover was greater in the SEP treatment than in other treatments in 2011 (Figure 7C; P = 0.017); however, cover of B. tectorum in the SEP treatment (1.0 ± 0.3 %) represented only a small part (2.5 %) of total herbaceous cover.
Annual forbs. On the Bluebunch site, annual forb cover increased and was 2 to 6 times greater in all treated areas compared to the control from 2009 to 2011 (Figure 8A; P < 0.001). Annual forb cover was greatest in the SEP and APR treatments, in 2009; however, differences among treatments disappeared by 2011. Annual forbs were almost exclusively comprised of two non-natives (prickly lettuce [Lactuca serriola L.] and desert madwort [Alyssum desertorum Stapf.]) and a native species (western tansy mustard [Descurainia pinnata {Walter} Britton]). On the NGBER site, annual forb cover increased in all treatments (Figure 8B; P < 0.001) and was greatest in the SEP treatment (P < 0.001), although this relationship was not consistent and by 2011 treatment differences had faded. On the Fescue site, annual forb cover increased in all treatments and in the control; however, cover of annual forbs was 2 to 8 times greater in the treatments (Figure 8C, P < 0.001). Among the treatments, annual forb cover was greatest in the SEP treatment in 2009 to 2010. Annual forbs on the Fescue and NGBER sites consisted exclusively of native species.
Shrub cover. On the Bluebunch site, shrubs were largely absent prior to treatment and there was no measurable change or difference in cover six years following treatment applications (P = 0.758). Shrub cover was below 0.5% and species were represented by rubber rabbitbrush (Ericameria nauseosa [Pall. ex Pursh] G.L. Nesom & Baird) and spineless horsebrush (Tetradymia canescens DC). On the Fescue site, Artemisia tridentata ssp. vaseyana cover declined in the SEP treatment to <0.1% and remained about 2% in the other treatments and in the control during the study. On the NGBER site, A.t. ssp. vaseyana cover declined (CUT, JAN, MAR, APR) or was eliminated (SEP) during the year of treatment (P = 0.007). However, by 2009, sagebrush cover did not differ among treatments or from pre-treatment values of about 3.8% to 5.2% (P = 0.097).
Perennial Herbaceous Density
On the Bluebunch site, densities of P. secunda declined in all treatments from pre-treatment values of 6.3 ± 0.3 plants m−2 to 3.8 ± 0.3 plants m−2, and were 75% to 50% less than in the control in 2011 (P = 0.006). Densities of perennial bunchgrasses increased (CUT, JAN, MAR) or recovered to pre-treatment (SEP, APR) values during the third year (2009) after treatment. In 2011, perennial grass densities in the treatments (5.5 plants m−2 to 6.8 plants m−2) were 1.5 to 2 times greater than in the control (Figure 9A; P < 0.007). Perennial forb densities were impacted by year (P < 0.001), increasing from about 1.0 ± 0.4 plants m−2 to peak at 2.8 plants ± 0.4 plants m−2 in the treatments in 2009. Although there were treatment differences for perennial forb densities within individual years, these differences were not sustained at the end of the study (P = 0.266).
On the NGBER site, densities of P. secunda were unaffected by year (P = 0.648) or treatment (P = 0.938l), averaging 14.3 ± 0.9 plants m−2. Densities of perennial bunchgrasses increased in CUT, JAN, and MAR treatments from pre-treatment levels and were greater than the SEP and APR treatments (Figure 9B, P = 0.001). Perennial bunchgrass densities decreased by 20 % and 40 % during the first year after fire in the SEP and APR treatments, respectively. Perennial forb densities were significantly impacted by year (P < 0.004) and, over the course of the study, densities were slightly greater in CUT and JAN treatments (12.3 ± 0.4 plants m−2) than in the SEP treatment (9.6 ± 1.2 plants m−2; P = 0.032).
On the Fescue site, P. secunda density in all treatments and in the control increased 40% from pre-treatment values of 7.8 ± 0.6 plants m−2to 11.7 ± 0.8 plants m−2 by 2011 (P = 0.006). Bunchgrass density increased (CUT, JAN, MAR) or recovered to pre-treatment (SEP, APR) values during the third year (2009) after treatment. By 2011, treatment bunchgrass densities were 1.3 to 1.7 times greater than the control (Figure 9C; P < 0.007). Perennial forb densities increased 2- to 4-fold from a pre-treatment value of 8 ± 0.7 plants m2 in all treatments and in the control (P < 0.001). In the last two measurement years, perennial forb densities were 1.2 to 1.8 times greater in the treatments (25 plants m−2 to 33 plants m−2) than in the control (18.7 ± 0.6 plants m−2; P = 0.027).