Article

Regional vegetation die-off in response to global-change-type drought. Proc Natl Acad Sci USA

School of Natural Resources, Institute for the Study of Planet Earth, and Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721-0043, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 11/2005; 102(42):15144-8. DOI: 10.1073/pnas.0505734102
Source: PubMed

ABSTRACT

Future drought is projected to occur under warmer temperature conditions as climate change progresses, referred to here as global-change-type drought, yet quantitative assessments of the triggers and potential extent of drought-induced vegetation die-off remain pivotal uncertainties in assessing climate-change impacts. Of particular concern is regional-scale mortality of overstory trees, which rapidly alters ecosystem type, associated ecosystem properties, and land surface conditions for decades. Here, we quantify regional-scale vegetation die-off across southwestern North American woodlands in 2002-2003 in response to drought and associated bark beetle infestations. At an intensively studied site within the region, we quantified that after 15 months of depleted soil water content, >90% of the dominant, overstory tree species (Pinus edulis, a piñon) died. The die-off was reflected in changes in a remotely sensed index of vegetation greenness (Normalized Difference Vegetation Index), not only at the intensively studied site but also across the region, extending over 12,000 km2 or more; aerial and field surveys confirmed the general extent of the die-off. Notably, the recent drought was warmer than the previous subcontinental drought of the 1950s. The limited, available observations suggest that die-off from the recent drought was more extensive than that from the previous drought, extending into wetter sites within the tree species' distribution. Our results quantify a trigger leading to rapid, drought-induced die-off of overstory woody plants at subcontinental scale and highlight the potential for such die-off to be more severe and extensive for future global-change-type drought under warmer conditions.

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    • "These woodlands are not expanding everywhere. For example, there has been extensive drought-induced woodland mortality, especially of Pinus edulis trees, in parts of the southwestern US (Breshears et al., 2005;Mueller et al., 2005;Floyd et al., 2009). Where these woodlands have expanded into surrounding sagesteppe and forest ecosystems and are considered to impact species of economic or conservation concern, natural resource managers have reduced pinyon and juniper overstory to limit its spread (Miller and Wigand, 1994;Belsky, 1996;Noson et al., 2006). "
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    ABSTRACT: Pinyon and juniper (Pinus spp., Juniperus spp.) woodlands are expanding into shrublands and grasslands throughout much of western North America. Woodland reduction is frequently used to mitigate the effects of conifer encroachment on game species (e.g. mule deer Odocoileus hemionus) and shrub and grassland-obligate species (e.g. sage grouse Centrocercus spp.). Although these practices are widespread, previous studies on the effects of woodland reduction on animal communities have not yet been synthesized, making it difficult to set priorities for future research and practice. To address this gap, we first summarize the history of pinyon and juniper reduction in western North America and characterize known wildlife habitat associations in pinyon and juniper ecosystems. We then review and synthesize evidence from the scientific literature on wildlife responses to pinyon and juniper woodland reduction. We tallied the outcomes of these studies to determine the relative proportions of positive, negative, and non-significant responses by different taxonomic groups and functional groups. The majority (69%) of animal species responses to woodland reduction treatments were non-significant. However, particular groups of species (taxonomic and/or functional) were more likely to respond positively or negatively, depending on the woodland reduction treatment method. Unexpectedly, investigators often found non-significant or negative responses by ungulates to woodland reduction, and non-significant responses by sagebrush obligate species. However, few studies measured effects on sagebrush obligate species, which limits inference for this group. Indeed, our review demonstrates that the effects of woodland reduction are well-understood for only a subset of taxonomic groups (e.g. birds and small mammals); whereas other groups (e.g. reptiles and terrestrial invertebrates) are consistently under-studied. Further, a shortage of large-scale and long-term research limits our ability to fully understand spatial and temporal wildlife responses to woodland reduction. We encourage practitioners to design and implement pinyon and juniper reduction projects to experimentally assess the effects of these practices on both target and non-target species. Adopting consistent monitoring protocols across projects would also facilitate greater understanding of how factors such as treatment type, size, location and duration result in positive or negative impacts to diverse wildlife of conservation concern.
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    • "Semiarid forests including those of lodgepole and ponderosa pine may be highly impacted by climate change (Diffenbaugh et al., 2008; Allen et al., 2010; Herrero et al., 2013), which is predicted to produce warmer and drier conditions across much of western North America in the coming century (Seager et al., 2007; Gutzler and Robbins, 2011; IPCC, 2013). There is concern that ecological disturbances will be intensified by climate change, resulting in largescale degradation of and vegetation compositional changes in these forests in the coming decades (Breshears et al., 2005; Aitken et al., 2008; Allen and Breshears, 1998; Allen et al., 2010; Feddema et al., 2013; Hanberry, 2014), altering and possibly reducing the persistence of these forests and the services they provide. "
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    • "We observed just such a decline in our study plots: there was a significant decrease in green canopy cover for all treatments between 2007 and 2012, as well as high tree mortality (Fig. 4;Gaylord et al. 2013). Drought, fires and bark beetle outbreaks have reduced the coverage of pi~ non-juniper woodlands over the last few decades driving a shift to juniper and/or grassland dominated systems (Breshears et al. 2005;Williams et al. 2013). The loss of tree cover and decrease in canopy quality in this system may have severe consequences for A. exsanguis and other ectotherms even beyond the loss of protective microhabitat. "

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