Aspen Regeneration along a Burn Intensity Gradient in Relation to Pre-fire Aspen Abundance in Mixed Conifer Forests

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1 Aspen Regeneration along a Burn Intensity Gradient in Relation to Pre-fire Aspen Abundance in Mixed Conifer Forests By A. Alisa Royem December 4, 2006 Abstract Aspen (Populus tremuloides) is the most widely distributed tree species in North America having tremendous ecological breadth. The most important disturbance agent in aspen stands of the southern Rocky Mountains is fire. Aspen is a fast growing clonal tree that commonly regenerates via root suckering after disturbances such as fire remove or kill the aboveground portion of the tree. In summer 2002, the Missionary Ridge fire burned 28,525 hectares near Durango, Colorado. In this study, I looked at the effects of fire severity and pre-existing aboveground aspen abundance on aspen regeneration in correlation with elk browsing. Results were variable with problems occurring with sampling protocol and plot locations. Introduction Aspen (Populus tremuloides) is the most widely distributed tree species in North America having tremendous ecological breadth (Campbell and Bartos, 2001). Aspen is the only deciduous tree found in the inter-rocky Mountains northward of southern Arizona and New Mexico (Peet, 2000). Aspen is found below tree-line between 2370 and 3330 m in the San Juan Mountains, providing important habitat for wildlife, biodiversity, and aesthetic value (Elliott and Baker, 2004). Aspen groves tend to harbor a rich diversity of understory plants, insects, and birds due to its deciduous nature and commonly being found in moist areas (Hessl, 2002). Aspen also provide forage and cover for native and domestic ungulates. The most important disturbance agent in aspen stands of the southern Rocky Mountains is fire (Romme, 1995). Intentional fire suppression since the early 1900 s, along with the removal of fuels by cattle and sheep grazing, are thought to have unnaturally decreased fire frequency and therefore aspen regeneration below 2,743 meters where aspen co-exists with pure ponderosa pine (Pinus ponderosa) and warm, dry mixed conifer stands (Romme, 2001). Above this elevation, where fire return intervals range from years, aspen regeneration may be threatened due to a natural absence of fire (Brown and DeByle, 1987). Human land use practices over the past century have altered fire regimes and elk (Cervus elephas) populations in lower elevation forests, but ecologists do not agree on the extent to which elk, fire, and climate have been affecting aspen in the Rocky Mountain West (Romme et al., 1995; Hessl, 2002).

2 Aspen is a fast growing clonal tree that commonly regenerates via root suckering after disturbances such as fire remove or kill the aboveground portion of the tree (Frey et al., 2003). This regeneration takes place almost entirely by vegetative reproduction, as aspen rarely propagates from seeds. Clones grow from the roots of a parent tree, and these stay connected underground even after the shoots have matured into trees. The clonal habit of aspen adds to its uniqueness among tree species (Campbell and Bartos, 2001). It is possible for a clone with as many as 50,000 stems to occupy more than 80.9 hectares and trace to a single common ancestor (Barnes, 1975). Aspen clones also exhibit high genetic diversity (Campbell and Bartos, 2001). Although individual stems may only survive for a maximum of 200 years, the clone itself lives for much longer as new stems grow to replace those which die, and viable clones existing without aboveground stems may be present for thousands of years (Featherstone, 2003). Higher soil temperatures following disturbances are considered to be the most important environmental factor controlling sucker initiation (Frey et al., 2003). High root temperatures have been thought to facilitate auxin degradation and promote root growth and cytokinin synthesis (Frey, 2003). Research exists for post-fire vegetation recovery in forests, such as aspen, but only when the species was a part of the aboveground vegetation before the fire occurred (Wang, 2003). Little information exists on post-fire vegetation establishment under various burn severities where no aboveground aspen was present prior to the fire (Korb et al. 2003). In summer 2002, the Missionary Ridge fire burned 28,525 hectares near Durango, Colorado. The fire burned a diversity of forest types from Gamble oak (Quercus gambelii) shrublands to spruce-fir (Picea englemanni/abies lasiocarpa) forest under various severities. Understanding where and under what environmental conditions aspen will regenerate is important because there currently is a debate whether aspen stands are stable or decreasing in extent within Colorado (Crawford et al. 1998; Kulakowski et al. 2004; Smith and Smith 2005). In this study, I looked at the effects of fire severity and pre-existing aboveground aspen abundance on aspen regeneration in correlation with elk browsing. It has been observed in northern Arizona and Utah that elk browsing inhibits aspen regeneration, while other research suggests that elk browsing is not a problem in the Rocky Mountain west (Romme, 2003). I hypothesized there would be a difference in post-fire vegetation recovery in relation to burn

3 intensity and preexisting aspen abundance in the aboveground vegetation. Specifically, areas of high burn severity and high pre-fire aspen abundance are predicted to have the highest aspen regeneration due to stimulation of cloning sucker roots. The objectives of this study are to: 1) determine what pre-fire aspen abundance, if any, is needed to have successful aspen regeneration post-fire; 2) determine if burn severity influences post-fire aspen regeneration abundance; 3) quantify ungulate elk browsing on post-fire aspen regeneration under different burn severities and pre-fire aspen abundance; and, 4) determine if there is a difference between one and four years post-fire aspen regeneration under different burn severities and prefire aspen abundance. Methods Study Area Missionary Ridge is located 8 kilometers north, northeast of Durango, Colorado in the arid Southwest. The Missionary Ridge Fire began on June 9th, 2002 after a record drought year resulting from a low winter snowpack (only 1.3 inches of precipitation occurred from January to June 2002; (Baer, 2002). The normal accumulative precipitation by June is 5.18 in, while the average annual rainfall within the region is in (Western Regional Climate Center, 2006). Durango s average maximum summer temperature, from May to August, is 79.8 degrees F [Western Regional Climate Center, (1990 to 1991)]. The Missionary Ridge fire burned approximately 28,525 hectacres of mostly National Forest lands, with Bureau of Land Management, Bureau of Reclamation, Colorado State, and private lands also burned to a lesser extent. Extremely dry fuels resulting from long-term drought conditions, dense forests with high ladder fuels, and high winds allowed the fire to quickly turn into a plume dominated blaze (Baer Plan, 2002). The fire spread through areas of Gamble oak, ponderosa pine, mixed conifer, aspen, and spruce-fir burning at different intensities and causing a wide range of ecological effects on the landscape. The fire was contained on July 17, 2002, only after winds settled down and cooler, more humid weather moved into the area. Experimental Design A GIS database constructed by the USFS, Columbine District vegetation manager, was originally used to stratify the Missionary Ridge burn area by seven variables: vegetation type, burn severity, pre-fire

4 aspen abundance (no aspen, 1-15%, 16-30%, >30%), slope (<=35%, >35%), salvage logging areas, and historic activities (thinning, prescribed burns) (Korb et. al., 2003). A total of 847 permanent sampling plots were established in different combinations of these stratified units and were surveyed by the USFS Columbine District in the summers of 2003 and The plots that were resurveyed include those stratified by vegetation type (warm-dry mixed conifer), burn severity (low or high), and pre-fire aspen abundance (0 or 16-30%) USFS (Korb et al, 2003). Burn severity was determined from visual analysis of aerial photographs after the fire and pre-fire abundance from preexisting Common Stand Exam (CSE) surveys prior to the fire by the USFS. Four permanent plots were be randomly chosen within each of these four different stratified units to resurvey for a total of 16 plots: (1) warm-dry mixed conifer, low severity burn, 0% pre-fire aspen abundance; (2) warm-dry mixed conifer, low severity burn, 16-30% pre-fire aspen abundance; (3) warm-dry mixed conifer, high severity burn, 0% pre-fire aspen abundance and, (4) warmdry mixed conifer, high severity burn, 16-30% pre-fire aspen abundance. Within each stratified unit, changes in aspen density and elk damage to aspen between 2003 and 2006 were quantified. Only data from 2003 were analyzed because I am only interested in the greatest change over time. Field Methods The sampling protocol in 2006 followed the same methodology used for the pre- and post-fire vegetation sampling by the USFS. A Garmin GPS unit was used to navigate to permanent plots. CSE is the standardized method for vegetation data collection used by the USFS. All data will be collected using the Quick Plot Tree Form following CSE standards. The Tree Form method collects data on fixed radius plots of 5.09 meter radius. For this study, subsets of variables generally were recorded on the form: all live and dead aspen (density), and signs of elk damage. Elk browsing will be quantified by recording nipped tops and scarred markings on trees. One digital photograph was taken to assist in visual comparisons of aspen between the 2003 and 2006 data. Statistics I used the Mann-Whitney statistical test to quantify changes between post-fire aspen regeneration one year after the burn and four years after the burn. Kruskal-Wallis statistical test was used to quantify changes in

5 differing treatment types within the same year. Data and Results Figure 2a. Aspen regeneration (dbh < 3in) in 2003 across treatment types within the Missionary Ridge burn area (N=4, ±SEM). The Kruskal-Wallis statistical test indicated no significant difference among treatment types within the same year (p= 0.509). b Figure 2b. Aspen regeneration (dbh <3in) in 2006 across treatment types within the Missionary Ridge burn area (N=4; ±SEM) The Kruskal-Wallis statistical test indicated significant differences among treatment types within the same year (p= 0.084). Post-hoc analysis indicates a significant p-value (p= 0.058) between the high burn, 0% prefire aspen composition and the rest of the treatment types in 2006.

6 Figure 1a. Change in aspen regeneration (dbh < 3in) in the Missionary Ridge burn area in low burn intensity, 0% pre fire aspen composition between permanent plots in 2003 and 2006 (N=4, ±SEM). Values indicated by a different letter are significantly different at the p < 0.10 level between different years. The Mann-Whitney U statistical analysis indicates a p-value of.245 representing that there is no significant change in aspen regeneration from 2003 to 2006 in the same treatment area. Figure 1b. Change in aspen regeneration (dbh< 3in) in the Missionary Ridge burn area in low burn intensity, 16-30% pre fire aspen composition against permanent plots in 2003 and 2006 (N=4, ±SEM). Values indicated by a different letter are significantly different at the p < 0.10 level between different years. The Mann-Whitney U statistical analysis indicates a p-value of 1.0 representing that there is no significant change in aspen regeneration from 2003 to 2006 in the same treatment area.

7 Figure 1c. Change in aspen regeneration (dbh< 3in) in the Missionary Ridge burn area in high burn intensity, 0% pre fire aspen composition against permanent plots in 2003 to 2006 (N=4, ±SEM). Values represent four sampled plots taken at the same location in 2003 and repeated in 2006 then converted to trees per acre. Values indicated by a different letter are significantly different at the p < 0.10 level between different years. The Mann-Whitney U statistical analysis indicates a p-value of.013 representing a highly significant change in aspen regeneration from 2003 to 2006 in the same treatment area. Figure 1d. Change in aspen regeneration (dbh<3 in) in the Missionary Ridge burn area in high burn intensity, 16-30% pre fire aspen composition against permanent plots in 2003 and 2006 (N=4, ±SEM). Values indicated by a different letter are significantly different at the p < 0.10 level between different years. The Mann- Whitney U statistical analysis indicates a p-value of 1.0 representing no significant change in aspen regeneration from 2003 to 2006 in the same treatment area. (2

8 a) b) c) d) Figure 3a-d. Permanent photographs of low burn intensity, 0% pre-fire aspen composition for 2003 and 2006; a) plot 306 b) plot 307, c) plot 583, d) 303. No photograph was taken for plot 583 in 2006.

9 e) f) g) h) Figure 3e-h. Permanent photographs of low burn intensity, 16-30% pre-fire aspen composition for 2003 and 2006; e) plot 300 f) plot 257, g) plot 265, h) plot 270. No photograph was taken for plot 257 in 2006.

10 i) j) k) l) Figure 3i-l. Permanent photographs of high burn intensity, 0% pre-fire aspen composition for 2003 and 2006; a) plot 504 b) plot 505, c) plot 517, d) plot 518.

11 m) n) o) p) Figures 3m-p. Permanent photographs of high burn intensity,16-30% pre-fire aspen composition for 2003 and 2006; a) plot 430 b) plot 431, c) plot 450, d) plot 438. Results Using the Kruskal-Wallis statistical test there was no difference (X 2 = 6.638, p= 0.509) among the four treatment types within 2006 when comparing aspen regeneration (dbh <3in) (Figure 2a) in the

12 Missionary Ridge burn area. There were significant differences (X 2 = 2.320, p= 0.084) among the four treatment types with 2203 when comparing aspen regeneration (dbh <3) (figure 2b) in the Missionary Ridge burn area. Post-hoc analysis indicates a significant p-value (p= 0.058) between the high burn, 0% pre-fire aspen composition and the rest of the treatment types in Change in aspen regeneration (dbh <3in) since the 2002 Missionary Ridge Fire has not significantly changed (Z= ; p=.245) between 2003 (4375 trees/ acre) and 2006 (1925 trees/ acre) in the low burn, 0% pre-fire aspen composition treatment type (Figure 1a) although there was a decline in aspen regeneration between the years. Aspen regeneration (dbh <3in) in the low burn, 16-30% pre fire aspen composition treatment area has not significantly changed (Z= 0.0; p= 1.0) between 2003 (1675 trees/ acre) and 2006 (1837 trees/ acre) (Figure 1b), although there was an increase in aspen regeneration between the years. Aspen regeneration (dbh <3in) in high burn, 0% pre-fire aspen treatment area has significantly changed (Z= ; p= 0.013) between 2003 (1262 trees/ acre) and 2006 (0 trees/ acre) (Figure 1c). A decline in aspen regeneration between the two years was observed. Change in aspen regeneration (dbh <3in) since the 2003 Missionary Ridge fire has not significantly changed (Z= 0.0; p= 1.0) between 2003 (1255 trees/ acre) and 2006 (6587 tress/ acre) in the high burn, 16-30% pre fire aspen composition treatment area, (Figure 1d) although an increase in aspen regeneration has occurred between the years. Repeat photo analysis was used to qualify aspen regeneration over time at the permanent sampled plots. When comparing plot 583 ( Figure 3c) in the low burn intensity, 0% pre-fire aspen composition treatment area repeat photo analysis indicates no change in the above ground aspen regeneration. Plot 300, the low burn intensity, 16-30% pre-fire aspen composition treatment area, (Figure 3e) indicates only slight regeneration since the 2002 burn indicated by small aspen samplings in the repeat photo analysis. Repeat photo analysis of plot 518 (Figure 3l) in the high burn, 0% pre-fire aspen composition indicates aboveground regeneration of grasses, yet no aspen regeneration. Plot 431 (Figure 3n) in the high burn intensity, 16-30% pre-fire aspen composition indicates no aspen regeneration with when comparing against repeat photo analysis taken in 2003 and again in To quantify elk browsing within the Missionary Ridge burn area evidence of bedding, droppings, and

13 nipped tops were looked for within each sampled plot. No evidence of elk browsing was observed within the sampled plots and therefore elk browsing cannot be quantified. Evidence of bedding and droppings were present outside permanent plots. Discussion Fire s effect on aspen regeneration has been studied throughout the inter Rocky Mountain West (Murry et al. 1998, Romme et al. 2001, Campbell & Bartos 2001, Kaye et al. 2003). Fire has historically influenced the composition and structure of forest ecosystems and this is directly linked to dominant historical fire intensities (Romme et al. 2001). Fire regime indicates which tree species were most abundant and whether overstory trees typically survived fires because fires produce variable mortality (Arno & Allison-Bunnell, 2002). The 2002 Missionary Ridge fire burned at varying intensities across a varying landscape resulting in differential patterns of regeneration in relation to pre-fire vegetation composition. Short term fire intervals are likely to maintain aspen dominated communities looking at fire affects in interior Alaska (Epting & Verbyla 2005). Variability in fire weather and fuel conditions create variation in burn intensities and burn severities, leading to heterogeneity in factors that can effect post fire establishment (Epting & Verbyla 2005). A study examining aspen response to a mixed-severity wildfire in the Black Hills, South Dakota by Keyser and colleges (2005) found that regardless of severity, fire did not cause an increase in the area occupied by individual aspen clones. Clones affected by high severity fire had the greatest suckering response producing an average of 31,930 sprouts/ha. Sprout growth in high severity clones was 135% and 60% greater than sprout growth in unburned and low severity clones. Since the 2002 fire, in my study, regeneration has proven to be diverse across the landscape due to the high heat intensity of the fire and the historical fire regimes native to the area. Aspen regeneration in the Missionary Ridge burn area exhibited a variety of successional patterns following the 2002 fire disturbance in the permanent plots of varying burn severity and pre-fire aspen composition that were surveyed for this study. No significant regeneration was observed in the low burn, 0% pre-fire aspen treatment area, or the low burn, 16-30% pre-fire aspen composition treatment area, when compared from 2003 to 2006 which was congruent with my hypothesis. The low burn areas within the mixed conifer forest type were variable in regeneration and damage to the ecosystem. Historically, warm dry, mixed conifer forest types experienced

14 fires once every thirty to fifty years in the San Juan Mountains, resulting in evolutionary advantages favoring species that have resistance to low intensity fire regimes, while aspen regeneration has remained variable in response to fire patterns and herbivory (Romme et al., 2001). High levels of aboveground herbaceous vegetation were present even when aspen regeneration wasn t observed (personal observation). High levels of aboveground biomass regeneration since the fire in the low burn areas were observed, not necessarily high levels of aspen regeneration were observed at each site location. Aspen regeneration within the high burn intensity treatment areas was variable. The high burn, 0% pre-fire aspen treatment area significantly decreased from 2003 to 2006 in aspen regeneration (dbh< 3in). A decline in aspen regeneration may be attributed to the sampling technique and above ground thinning of saplings. No aspen where found within any sampled permanent plots in the high severity treatment area, yet this is not necessarily representative of the ecosystem on the whole. Aspen sprouts decrease exponentially from the time of the disturbance that stimulated the sprouting; Johnston (2001) found a decrease from 32,000 aspen stems per acre to 9,000 trees per acre in a four year interval since the disturbance in mixed conifer forests on the Uncompahgre Plateau, West-Central Colorado. Significant change in regeneration in the high burn, 16-30% pre-fire aspen treatment area did not occur since A study by Wang (2003) of early regeneration and growth of aspen suckers in relation to fire severity indicated that fire severity significantly affected post-fire aspen sucker density, with significantly lower aspen density found on areas of high burn severity. Over a three year time span, Wang (2003) discovered significant changes only on scorched and lightly burned plots; sucker mortality was positively related to the initial sucker density, with more than 80% of the total variance being explained. In the first year fire severity significantly affected the growth of the dominant aspen suckers, but this was not the case in the second and third post-fire years (Wang, 2003). This gives support to my findings of no significant change in the high burn intensity areas over a three year period. No elk browsing was observed within any permanent sampled plots, but evidence of elk within the landscape was evident by means of droppings and bedding areas. After stands of aspen are cut or burned, browsing by an elk can eliminate a sprout crop completely or damage all sprouts so that all tress in a clone

15 will have poor growth for a long time (Johnston, 2001). Large burns over 1,000 acres are needed to perpetrate aspen because high populations of elk can consume and kill most of the seedlings or sprouts in small burned areas (Arno & Allison-Bunnel, 2002). No decline in aspen regeneration due to elk browsing was observed within any sampled plots or the observed landscape. Future studies in the Missionary Ridge burn area could be greatly improved by changing sampling technique and plot analysis. The Common Stand Exam (CSE) used by the USFS in 2003 and 2004 was not large enough to account for ecosystem diversity and changes in the landscape. Instead of a 5.09m radius plot, I would recommend a 25m transect to be used to capture more diversity across the landscape. In my study plot locations could be void of aspen saplings within the plot, but densely covered with aspen just outside of the radius boundary. This was a common occurrence, especially in the high burn, 0% pre-fire aspen composition treatment area, where no aspen was observed in All 841 originally sampled plots would need to be resurveyed in order to make a full assessment of aspen regeneration since the 2002 fire. Ease of permanent plot re-sampling would be greatly facilitated by larger stakes used in determining plot centers. Since the 2002 fire disturbance, a plethora of the aboveground biomass has regenerated, completely covering the 10cm high stakes and making them close to impossible to locate in highly vegetated areas. A greater number of sampled plots would be necessary for more accurate data concerning change over time within the Missionary Ridge burn area, especially when using the CSE method of sampling. More time, energy, and money would be needed to hire field assistants and sample at least 10 plots within each treatment type. Future research that would complement this study would be to spread the study to different burns (fires) with aspen to quantify aspen regeneration not just within the Missionary Ridge burn area. Acknowledgments A special thanks to Patrick Wright for providing data collection assistance, as well as Dr. Julie Korb, whose assistance throughout the entirety of my thesis project made it possible. Funding for this project was provided by the Mountain Studies Institute and the Fort Lewis Biology Department. Works Cited

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17 Epting, J., Verbyla, D Landscape-level interaction of prefire vegetation, burn severity, and postfire vegetation over a 16 year period in interior Alaska. Canadian Journal of Forest Restoration.35: Keyser, T. L., Smith, F. W., Shepperd, W. D Trembling aspen response to a mixed-severity wildfire in the Black Hills, South Dakota, USA. Canadian Journal of Forest Research. Vol. 35: 11; Johnston, B. C Multiple Factory Affect Aspen Regeneration on the Uncompahgre Plateau, West-Central Colorado. USDA Forest Service Proceedings. Unpublished USDA document. Wang, G.G Early regeneration and growth dynamics of Populus tremuloides suckers in relation to fire severity Canadian Journal of Forestry Restoration 33(10):