The density of aspen regeneration following prescribed fire in LVNP was highly variable within and among sites, although the overall density range among individual transects (0 stems ha−1 to 36 667 stems ha−1) was similar to that reported three years after prescribed fire in the Sierra Nevada (Krasnow et al. 2012) and four years after mechanical removal of conifers on the Lassen National Forest adjacent to LVNP (Jones et al. 2005). These aspen densities are slightly lower than the range of 8000 stems ha−1 to 40 000 stems ha−1 reported five years following mechanical thinning in the Sierra Nevada (Krasnow et al. 2012). Interestingly, aspen regeneration densities from the few studies in the southern Cascades and the Sierra Nevada (including our study) are at the low end of the range of regeneration densities reported following fire or clear-cutting in the Rocky Mountains (e.g., 35 800 stems ha−1 four years following high severity fire in spruce-fir in New Mexico, 30 000 stems ha−1 to 150 000 stems ha−1 following fire in western Wyoming, and 76 600 stems ha−1 following a clear cut in Colorado; Jones and Debyle 1985, Schier et al. 1985). It is possible that aspen regeneration densities at LVNP will continue to increase in the burned transects in the future, but total aspen regeneration tends to stabilize or decline in the first two years following disturbance (Shepperd 1993).
Post-fire conifer basal area was negatively associated with post-fire aspen regeneration density among transects (Figure 2). This is consistent with many studies throughout the western USA, indicating that increased conifer competition is one cause of deteriorating aspen stands and reduced regeneration (St. Clair et al. 2013). Although the highest post-fire aspen regeneration densities at LVNP occurred in transects with no conifers post-fire, other transects with no post-fire conifers had low regeneration (e.g., SL and BUT; Figure 2). This suggests that lack of conifer shading and competition is important, but not sufficient by itself to result in high post-fire aspen regeneration density. Pre-treatment aspen density (of all size classes) has been shown to be positively associated with post-treatment regeneration response in the adjacent Lassen National Forest, California (Jones et al. 2005). Low pre-fire total aspen density could possibly explain some of the low regeneration response at LVNP where there was seemingly a suitable post-fire regeneration environment (i.e., low post-fire conifer density), but these data were not available for our sites.
Although aspen regeneration density is an important component of aspen stand dynamics, this metric alone can potentially be misleading if regeneration is in poor condition. Height growth and vigor of aspen regeneration will ultimately determine the potential to replace a declining aspen overstory, particularly when there is heavy browse pressure (Jones et al. 2005, Shepperd et al. 2006). In aspen stands with heavy browsing pressure, a height of greater than 150 cm is considered a conservative estimate for the tops of the trees to escape browsing by deer (Odocoileus spp.), and are therefore deemed “successful” in terms of having a high probability of recruitment into the overstory (assuming subsequent growth is not inhibited by conifer competition) (Mueggler and Bartos 1977). The small percentage (5 % at all sites and 7 % at 11-year post-fire sites) and low density (282 stems ha−1 to 333 stems ha−1) of aspen regeneration greater than 150 cm tall in burned sites at LVNP suggests that browsing is greatly reducing the number of stems that reach sufficient height to replace the declining aspen overstory (Table 3). It is possible that regeneration at recently burned sites (BUT, 1-year post-fire, and GR2, 3-years post-fire) may not have had sufficient time to reach 150 cm height (e.g., Jones et al. 2005). However, asexual aspen regeneration are commonly observed growing greater than 150 cm within one or two years after disturbance because of large clonal root system reserves (Jones and Schier 1985), which would be expected at mesic sites associated with springs and streams in our study areas. Moreover, low numbers of aspen regeneration greater than 150 cm at the two sites (SL and BLK) that were 11 years post fire suggest that browsing is preventing the post-fire regeneration from recruiting into the overstory (Figure 3, Table 3). These results are consistent with Jones et al. (2009), who found that simulated browsing and the presence of overstory conifers reduced aspen regeneration height growth in three aspen stands on the adjacent Lassen National Forest. This is also consistent with experimental observations in the Rocky Mountains (Calder et al. 2011). Thus, in the absence of elk, which can significantly affect aspen regeneration in the Rocky Mountains (Seager et al. 2013), mule deer browsing within small groves at LVNP is likely effectively reducing aspen regeneration height and the potential for recruitment into the overstory.
Heavy browsing pressure at all four sites may also be influencing aspen growth form. We found the ubiquitous presence of multi-stemmed aspen regeneration clusters (e.g., Figure 3) to be in contrast to the structure of the overstory aspen, which were single-stemmed, tall “trees.” In the Intermountain West, aspen has been observed to regenerate in clusters from a single point on a root, but most clusters self-thin to a single dominant stem by 5 to 10 years after the regeneration event (Schier et al. 1985). We found no relationship between the number of stems per cluster and years-since-fire, which suggests that self-thinning is not occurring at LVNP. At the two sites that burned over 11 years ago (SL and BLK), the mean number of stems per cluster is still high (1.5 to 3). We hypothesized that another potential cause of multi-stemmed regeneration could be persistent browsing, such that as the apical meristem is repeatedly removed by browsing additional stems are produced. The positive correlation between browsing and the number of aspen stems per cluster at all but one transect supports this hypothesis. Multi-stemmed aspen “bushes” associated with repeated mid- to late-summer mule deer browse has been observed in the nearby northern Sierra Nevada (Shepperd et al. 2006). The possible link between browsing and a trend toward change in aspen vegetative structure should be a topic of further research.
Prescribed Fire
Although the four aspen groves in this study all burned, fire intensity and severity varied at fine-scales between (and even within) individual transects, which has important implications for these very small groves. These three prescribed burns were not designed specifically for restoration of aspen groves with high intensity fire, as is typically recommended in the Rocky Mountains or Intermountain West (e.g., Keyser et al. 2005). Rather, they were initial (first-entry) burns during cool conditions aimed at restoring low severity fire regimes that historically burned in surrounding coniferous forests (Taylor 2000), and limiting old-growth conifer mortality. We did not have access to data on pre-fire overstory density or transect-scale fire severity. However, we observed that overall fire intensity in many of the aspen stands and transects was high enough to girdle and kill fire-sensitive overstory aspen stems, but too low to kill most conifers with thicker bark. Thus, after the fire, the few regenerating shade-intolerant aspen stems were still in an unfavorable environment, shaded by conifers and heavy coarse woody debris (e.g., Figure 4, top panels). This is consistent with our observed general trend of lower aspen regeneration density with increasing post-fire conifer basal area (i.e., lower fire severity; Figure 2). The main mechanism to stimulate aspen regeneration is an event that alters the auxin-cytokinin balance and induces sprouting from the roots (e.g., high severity crown fire that top-kills aspen stems) (Schier et al. 1985). It has been shown that increased soil temperature (e.g., from increased solar radiation following reduced canopy cover) can also increase cytokinin levels in roots, which signals regeneration (Schier et al. 1985). Thus, low intensity fire that killed dominant aspen, but did not sufficiently reduce shading from live conifers or forest floor cover, would likely not be beneficial for aspen regeneration and could possibly explains the poor post-fire response in some transects (e.g., SL and GR2 transects with high conifer basal area and low aspen regeneration density; Figure 2). This pattern would be consistent with Keyser et al. (2005), who found higher levels of suckering in high severity burns compared to unburned or low severity burns. After our study was completed, a large wildfire (2012 Reading Fire) burned the GR2 transects and additional aspen groves in LVNP with varying severity, so continued monitoring and research of those sites will help quantify effects of fire severity on aspen regeneration in the region.
Although fire of higher intensity and severity may be important for successful aspen regeneration, localized high heat loads associated with long flame residence times appear to have occurred at some sites (e.g., BUT) where large logs were present and probably contributed to lower post-fire aspen regeneration. We observed that, where fire consumed the duff layer, aspen roots were exposed, burned, and killed, and thus were unable to regenerate post-fire; whereas, areas within the same transect (<10 m apart) with intact duff had successful aspen regeneration. This could not be observed or quantified for the older burns, so we could not analyze this systematically. However, on the adjacent Lassen National Forest, Jones et al. (2005) observed a similar absence of post-fire aspen regeneration in areas where slash piles were burned, possibly because the fire intensity and residence time killed the aspen roots. In declining, small aspen groves with heavy fuel loading, mechanical treatment (including removal of activity fuels) may be beneficial prior to burning, and regeneration may need initial protection from browse until stems reach heights of greater than 150 cm.
The prescribed burns we studied may have benefited quaking aspen in some individual transects by increasing mean regeneration density or heights relative to untreated or pre-fire numbers (although only one site had statistically significant changes). However, heavy post-fire mule deer browsing is reducing the number of aspen regenerations that reach sufficient height (>150 cm) to replace the declining aspen overstory, and may even be producing multi-stemmed trees or aspen “shrubs.” Low intensity burning followed by heavy browsing could ultimately lead to reduced aspen regeneration density, as was observed at two sites. Fires likely need to be intense enough to kill conifers that are out-competing the shade-intolerant aspen regenerations, or aspen stands may decline further. In areas where aspen restoration is the primary goal, multiple risk factors should be considered (e.g., numbers of live aspen regeneration, conifer cover, and browse pressure) for prioritizing and designing restoration treatments (Bartos and Campbell 1998). Additional research is needed to improve our understanding of the interactions of fire severity, competing vegetation and browsing on aspen in this understudied region. This is especially the case in small, declining groves where low intensity fires are being implemented that are consistent with historical fire intensity, but may be ineffective at reducing conifer density or regenerating aspen under current conditions.