After long-term decline, are aspen recovering in northern Yellowstone?
Introduction
In northern Yellowstone National Park (YNP), stands of quaking aspen (Populus tremuloides) declined during the 20th century as mature trees died but were not replaced by new trees (Romme et al., 1995). This lack of new aspen trees was primarily due to intensive herbivory by elk (Cervus elaphus) in winter, which suppressed the growth of young aspen (Kay, 2001, NRC (National Research Council), 2002, Barmore, 2003, Larsen and Ripple, 2003, Kauffman et al., 2010). The decline of aspen recruitment (i.e., growth of sprouts into saplings and trees) on the northern Yellowstone elk winter range (“northern range”) roughly coincided with the extirpation of wolves (Canis lupus). Some researchers (Ripple and Larsen, 2000, Ripple et al., 2001) hypothesized that the removal of these large predators contributed to aspen decline through a trophic cascade (Schmitz et al., 2000, Terborgh and Estes, 2010) when elk were released from predation pressure. Other factors that may have suppressed aspen recruitment in addition to herbivory included suppression of fire, and a period of drought in the 1930s (Houston, 1982, Romme et al., 1995, YNP (Yellowstone National Park), 1997, Eisenberg et al., 2013).
Reintroduction of wolves to YNP in 1995–1996, and a concurrent increase in grizzly bears (Ursus arctos) (Schwartz et al., 2006, Barber-Meyer et al., 2008), provided an opportunity to observe the effects of large carnivore restoration on elk and possible effects on plants, with potential for increased survival and height of young aspen. After the return of wolves, Ripple and Beschta, 2007, Ripple and Beschta, 2012b found a decrease in browsing associated with “the first significant growth of young aspen in the northern range for over half a century,” and hypothesized that this was the result of a trophic cascade resulting from wolf reintroduction. Kauffman et al. (2010), using different methods, did not find evidence of reduced browsing or aspen recovery and concluded that no trophic cascade benefiting aspen had yet begun. These disparate findings and the ensuing debate demonstrated a need for further investigation of the extent and timing of a possible aspen recovery (Beschta and Ripple, 2013, Kauffman et al., 2013).
Trophic cascades in Canadian parks involving wolves, elk, and aspen have been attributed to a combination of low elk densities and predation-risk avoidance behavior (White et al., 1998, White et al., 2003, Hebblewhite et al., 2005, Beschta and Ripple, 2007, Hebblewhite and Smith, 2010). As in Yellowstone, bears (Ursus spp.) were present in these areas but it was wolves that were associated with lower elk densities and greater aspen recruitment. Evidence for trophic cascades involving wolves and cervids has also been found in the Great Lakes region (Callan et al., 2013) and in a national park in Poland (Kuijper et al., 2013).
Since the return of wolves to YNP, elk numbers have declined substantially on the northern range (White et al., 2012), so it may be reasonable to expect some response in plants browsed by elk. Conversely, relatively low elk numbers in the 1950s and 1960s did not result in new aspen recruitment (YNP (Yellowstone National Park), 1997, Barmore, 2003, Wagner, 2006), so aspen recovery with reduced elk numbers is not a foregone conclusion. If aspen recovery has now begun with similar elk numbers to those of 1950–1970, this would suggest a role for behavioral or trait-mediated responses to predation (Schmitz et al., 2004) in addition to simple reduction of elk numbers. Other factors besides predation also have affected elk population dynamics, including hunting outside the park, a severe winter in 1997, and perhaps a period of drought in the early 2000s. Some researchers have argued that these factors were more important than predation as causes of elk decline prior to 2006 (Vucetich et al., 2005, Eberhardt et al., 2007). However, winters after 1999 were mild, hunting was greatly reduced after 2005, and the drought ended by 2007, with little change in trends of declining elk density and shifting distribution (Hamlin and Cunningham, 2009, White et al., 2012, White and Garrott, 2013). In the same period of time, wolves became the primary cause of elk mortality in the northern Yellowstone herd (White and Garrott, 2005a, Hamlin et al., 2009, White et al., 2010), while bears became the leading cause of elk calf mortality (Barber-Meyer et al., 2008). Densities of these predators has been greatest in the park, while the winter elk hunt north of the park has been eliminated (White et al., 2012), allowing elk to reduce predation risk by wintering outside the park.
If aspen have begun to recover due to a reduction in elk herbivory, we would expect to find reduced rates of browsing associated with greater recruitment of tall aspen saplings above the browse level of elk, >200 cm in height (Kay, 1990, White et al., 1998). Reduced browsing intensity would also be likely to result in greater variation in the height of young aspen, due to differences in the amount of time since release from browsing or stand productivity affecting height after browsing is reduced. To test these hypotheses, we evaluated aspen stand conditions on the YNP northern range in the summer of 2012 and compared our results to similar data collected in 1997–1998, 14 years earlier (Larsen and Ripple, 2005), when wolves returned and the elk population began to decline. We used more extensive random sampling of aspen stands than in other recent studies of northern range aspen (Kauffman et al., 2010, Ripple and Beschta, 2012b) and sampled not only the population of young aspen within each stand, but also the tallest five as an indication of recent recruitment. We considered the possible effects of site productivity, climate, and annual snow accumulation on browsing intensity and aspen height, and analyzed the age distribution and recruitment history of trees in aspen stands.
Section snippets
Study area
Valleys of the upper Yellowstone River and its tributaries comprise YNP’s northern range, the wintering grounds for elk, bison (Bison bison), deer (Odocoileus spp.), and small numbers of pronghorn (Antilocapra americana) and moose (Alces alces). In these valleys, dry grasslands and sagebrush (Artemisia spp.) steppe are interspersed with groves of aspen. The upper slopes are forested with Douglas fir (Pseudotsuga menziesii), lodgepole pine (Pinus contorta) and Engelmann spruce (Picea engelmannii
Climate data
Long-term climate data (for years 1895–2012) in the form of the Palmer Z Index were obtained for the Yellowstone Drainage Climate Division from two data sources, the National Climatic Data Center (NCDC, 2013) and the Western Regional Climate Center (WRCC, 2013). This Climate Division includes most of YNP (excluding the southwest portion). We averaged monthly values of the Palmer Z Index for each water year (October–September) to derive annual values, and compared results from the two models.
Climate
The two long-term climate data sources (NCDC and WRCC) gave somewhat different results (Fig. 4a and b), with only moderate correlation between the two datasets (R2 = 0.46). Both datasets showed a recent drought, but data from NCDC, a widely used source (e.g., McMenamin et al., 2008), showed the drought as extreme and unprecedented with a strong trend of increasing drought over the century. These indications may be misleading given the likelihood that WRCC data based on PRISM are more
Discussion
Almost all overstory aspen trees in our sampling plots were established before 1930 (Fig. 4c), consistent with previous research (Kay, 1990, Romme et al., 1995, Ripple and Larsen, 2000, Halofsky and Ripple, 2008, Beschta and Ripple, 2013, Kauffman, 2013). However, by 2012 the multi-decade hiatus in aspen recruitment appeared to be ending. In the last decade some saplings survived to grow above the reach of elk, in contrast with the absence of tall saplings in sampling plots in 1997–1998 (Fig. 5
Conclusions
Widespread but patchy recruitment of saplings above the browse height of elk demonstrates that an important shift in aspen dynamics has occurred on Yellowstone’s northern range, a change that did not occur in the period 1930–1998, even when overall elk numbers were low in the 1950s and 1960s. Many aspen stands are in the early stages of recovery as indicated by decreased browsing and increased height of young aspen. It is unlikely that climate caused these recent changes, which happened despite
Acknowledgements
We received financial support from the University of Wyoming – National Park Service Research Station. We thank field technicians Jeff Stephens and Jonathan Batchelor. Ariel Muldoon assisted with the statistical analysis. Doug Smith provided helpful comments on an early draft. The manuscript was greatly improved by comments from two anonymous reviewers. Our thanks also to Henry Finkbeiner and Doug McLaughlin of Silver Gate Lodging for their hospitality and support.
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2020, Food WebsCitation Excerpt :With the return of wolves in 1995–96, completing the park's large predator guild, browsing pressure on young aspen sprouts was reduced in many northern range aspen stands. This ongoing recovery started slowly but has nevertheless increased in strength over time (Painter et al., 2014; Beschta et al., 2018). For example, Painter et al. (2014) found that the 5-tallest young aspen heights, in 87 randomly selected aspen stands from across the northern range, increased from an average height of 35 cm in 2003 to nearly 170 cm in 2012.
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2018, Forest Ecology and ManagementCitation Excerpt :We used the five-tallest because they (1) could be consistently identified in an aspen stand, given the history of long-term height suppression, (2) likely denoted the first young aspen in a given stand to experience a reduction in browsing pressure, which we could identify over the life of each plant via measurements of plant architecture, and (3) represented a “leading edge” indication of a broader shift in plant community dynamics for northern range aspen stands. For example, the heights of the five-tallest have been found to be positively correlated with the heights of all young aspen in northern range aspen stands (r2 = 0.59, p < 0.001) (Painter et al., 2014). To be included in this study, an aspen stand had to have one or more live overstory trees that were >30 m from those in any adjacent stand.