Geology topics

Nicholas J. Ampersee

Publications and source records attributed to Nicholas J. Ampersee.

3 recordsLinked to original sources

USGS2020

Tree mortality in blue oak woodland during extreme drought in Sequoia National Park, California

Blue oak woodlands in California have been a focus of conservation concern for many years. Numerous studies have found that existing seedling and sapling numbers are inadequate to sustain current populations, and recent work has suggested that blue oak woodlands might be particularly vulnerable to a warming climate. California has recently experienced a drought of historically unprecedented severity, resulting in the mortality of tens of millions of trees, including an apparent spike in mortality in oak communities. Here we present the results of a survey of tree mortality and composition in blue oak woodlands in Sequoia National Park. We found that 18% (95% CI = 14–24,) of all standing trees and 23% (95% CI = 17–30) of standing Quercus douglasii Hook. & Arn. (blue oak) were dead, substantially higher than proportions of dead trees recorded in pre-drought datasets, which showed 4% (95% CI = 2–9) standing dead for all trees and 5% (95% CI = 4–7) dead or 8% (95% CI = 4–16) standing dead for blue oak. Furthermore, much of this mortality appeared to be recent. Based on foliage or fine twig retention, 19% (95% CI = 14–26) of blue oak and 23% (95% CI = 16–31) of Quercus wislizeni A. DC. (interior live oak) appear to have died recently. In contrast, only 5% (95% CI = 3–8) of Aesculus californica (Spach) Nutt. (California buckeye) and 5% (95% CI = 2–11) of Fraxinus dipetala Hook. & Arn. (California ash) appear to have died recently. Even after such high mortality, with blue oak basal area dropping by 26% (from 9.5 m2/ha [95% CI = 7.4–11.6] to 7.0 m2/ha [95% CI = 5.3–8.7]), blue oak remains the dominant species in these ecosystems. However, given the lack of recruitment and the apparent vulnerability to extreme drought, blue oak populations may be at risk for severe decline if such mortality events become more frequent.

California↗

USGS2019

Which trees die during drought? The key role of insect host-tree selection

1. During drought, the tree subpopulations (such as size or vigor classes) that suffer disproportionate mortality can be conceptually arrayed along a continuum defined by the actions of biotic agents, particularly insects. At one extreme, stress dominates: insects are absent or simply kill the most physiologically stressed trees. At the opposite extreme, host selection dominates: outbreaking insects kill trees independently of their stress, instead selecting trees based on size or other traits. Intermediate responses are also possible. Yet for mixed-species forests, we lack a broad understanding of the relative importance of insects in determining exactly which subpopulations of trees suffer disproportionate mortality during drought, and whether these subpopulations differ among co-occurring tree species. 2. During an extreme drought, we documented the roles of native bark beetles in the mortality of five tree species in California’s Sierra Nevada. We analyzed patterns and agents of tree mortality in 12 permanent plots, and patterns of mortality in 89 temporary plots. 3. Most tree mortality was associated with bark beetles. But the growth rates (an indicator of chronic stress) and sizes of trees that suffered greatest bark-beetle-related mortality differed sharply among tree taxa, variously conforming with domination by stress (Abies concolor), domination by host selection (Pinus lambertiana and P. ponderosa), or a mix of the two (Calocedrus decurrens). Quercus kelloggii mortality remained relatively low. Thus, even during extreme drought substantial proportions of stressed trees survived because they were of sizes that mostly avoided fatal insect attack. Conversely, substantial proportions of comparatively unstressed trees died because they were of sizes that were selectively killed by outbreaking insects. 4. Synthesis. Native bark beetles were primarily responsible for determining which subpopulations of trees suffered greatest mortality during drought. However, idiosyncratic host-tree selection by the different bark beetle taxa meant that the tree subpopulations suffering greatest mortality differed strikingly among tree taxa – for example, high mortality of small trees of one species, but of large trees of another. If idiosyncratic host-tree selection by biotic mortality agents proves to be a generally common phenomenon, it could help explain weak broad-scale correlations between tree traits and tree mortality during drought.

Journal of Ecology↗

USGS2018

Patterns and correlates of giant sequoia foliage dieback during California’s 2012–2016 hotter drought

Hotter droughts – droughts in which unusually high temperatures exacerbate the effects of low precipitation – are expected to increase in frequency and severity in coming decades, challenging scientists and managers to identify which parts of forested landscapes may be most vulnerable. In 2014, in the middle of California’s historically unprecedented 2012–2016 hotter drought, we noticed apparently drought-induced foliage dieback in giant sequoias ( Sequoiadendron giganteum Lindl. [Buchholz]) in Sequoia and Kings Canyon national parks, California. Characteristics of the dieback were consistent with a controlled process of drought-induced senescence: younger (distal) shoots remained green while older (proximal) shoots were preferentially shed. As part of an ongoing interdisciplinary effort to understand and map sequoia vulnerability to hotter droughts, we reviewed historical records for evidence of previous foliage dieback events, surveyed dieback along trail corridors in eight sequoia groves, and analyzed tree-ring data from a high- and a low-foliage-dieback area. In sharp contrast to the greatly elevated mortality of other coniferous species found at low and middle elevations, we estimate that <1% of sequoias died during the drought. Foliage dieback was notably elevated in 2014 – the most severe single drought year in our 122-year record – but much lower in subsequent years. We found no historical records of similar foliage dieback during previous droughts. Dieback in 2014 was highly variable both within and among groves, ranging from virtually no dieback in some areas to nearly 50% in others. Dieback was highest (1) at low elevations, probably due to higher temperatures, reduced snowpack, and earlier snowmelt; (2) in areas of low adult sequoia densities, which likely reflect intrinsically more stressful sites; and (3) on steep slopes, probably reflecting reduced water availability. Average sequoia ring widths were narrower at the high-dieback than the low-dieback tree-ring site, but for reasons that remain unclear the sites did not differ in their proportional ring-width responses to past droughts. Collectively, our results suggest that giant sequoia vulnerability to hotter droughts may be spatially quite variable, and that at least some of that variability can be explained by metrics related to site water balance. Future research will focus on integrating our results with physiological and remote-sensing data, including tracking sequoias as they recover from the drought.

California↗