Geology topics

Kevin R. Hultine

Publications and source records attributed to Kevin R. Hultine.

12 recordsLinked to original sources

USGS2022

Dryland mechanisms could widely control ecosystem functioning in a drier and warmer world

Responses of terrestrial ecosystems to climate change have been explored in many regions worldwide. While continued drying and warming may alter process rates and deteriorate the state and performance of ecosystems, it could also lead to more fundamental changes in the mechanisms governing ecosystem functioning. Here we argue that climate change will induce unprecedented shifts in these mechanisms in historically wetter climatic zones, towards mechanisms currently prevalent in dry regions, which we refer to as ‘dryland mechanisms’. We discuss 12 dryland mechanisms affecting multiple processes of ecosystem functioning, including vegetation development, water flow, energy budget, carbon and nutrient cycling, plant production and organic matter decomposition. We then examine mostly rare examples of the operation of these mechanisms in non-dryland regions where they have been considered irrelevant at present. Current and future climate trends could force microclimatic conditions across thresholds and lead to the emergence of dryland mechanisms and their increasing control over ecosystem functioning in many biomes on Earth.

Nature Ecology & Evolution↗

USGS2020

Regional coordination between riparian dependence and atmospheric demand in willows (Salix L.) of western North America

Aim Plants vary in their hydrological and climatic niches. How these niche dimensions covary among closely related species can help identify co‐adaptations to hydrological and climatic factors, as well as predict biodiversity responses to environmental change. Location Western United States. Methods Relationships between riparian dependence and climate niches of willows ( Salix L.) were assessed, incorporating phylogenetics and functional traits to understand the adaptive nature of those relationships. The riparian dependence niche was estimated as the mean distance between georeferenced occurrence records and the nearest stream based on the National Hydrography Database. Results were compared to oaks ( Quercus L.), a less riparian‐dependent clade, with the expectation of different niche relationships. Results Willows generally occurred closer to streams than expected by chance, but riparian dependence varied substantially among species. Riparian dependence was positively correlated with mean annual temperature and diurnal temperature range niche, both indicators of atmospheric demand on evapotranspiration. Phylogenetic independent contrast correlations for these relationships were significant as well, and the high degree of niche convergence among species indicated evolutionarily labile co‐adaptations to riparian dependence and atmospheric demand. Plant height increased with mean annual temperature niche, and specific leaf area increased with residual variation in height, indicating underlying morphological correlates of niche variation. Oaks, on the other hand, exhibited no relationship between atmospheric demand and riparian dependence, and weaker niche relationships with riparian dependence overall. Main conclusions These results support the assertion that hydric‐adapted, woody riparian plants compensate for increased atmospheric demand on transpiration with a reliable supply of water provided by riparian habitats and that this trade‐off may be unique from mesic–xeric woody plants. Conservation of warm‐adapted riparian trees and shrubs under increasing temperatures and atmospheric demand may necessitate reversal of groundwater depletion. Cool‐adapted species may be best conserved through maintenance or expansion of riparian buffers as they become more riparian obligate with warming.

Diversity and Distributions↗

USGS2020

An integrative ecological drought framework to span plant stress to ecosystem transformation

Droughts have increased globally in the twenty-first century and are expected to become more extreme and widespread in the future. Assessments of how drought affects plants and ecosystems lack consistency in scope and methodology, confounding efforts to mechanistically interpret structural and functional impacts and predict future transformations under climate change. To promote integration among studies, we identify water deficit conditions that are ecologically meaningful, clarify the stages in which ecological drought progresses, and consider approaches to synthesize drought effects across multiple species and ecosystems. This improved ecological drought framework reveals advantages of using different ecological drought metrics and strengthens approaches to distinguish ecosystem stress from crossing an irreversible threshold. We employ several well-studied examples from water-limited ecosystems, which contain plants that are often at their physiological limits and highly responsive to climate variability. We suggest that emerging research on early warning signs, drought recovery, and the effects of land management interventions be incorporated into the ecological drought framework. An integrative approach to understand ecological drought can accelerate scientific advancement and create opportunity to adapt and prepare for crossing irreversible ecosystem thresholds.

Ecosystems↗

USGS2019

Early-warning signals of individual tree mortality based on annual radial growth

Tree mortality is a key driver of forest dynamics and its occurrence is projected to increase in the future due to climate change. Despite recent advances in our understanding of the physiological mechanisms leading to death, we still lack robust indicators of mortality risk that could be applied at the individual tree scale. Here, we build on a previous contribution exploring the differences in growth level between trees that died and survived a given mortality event to assess whether changes in temporal autocorrelation, variance, and synchrony in time-series of annual radial growth data can be used as early warning signals of mortality risk. Taking advantage of a unique global ring-width database of 3065 dead trees and 4389 living trees growing together at 198 sites (belonging to 36 gymnosperm and angiosperm species), we analyzed temporal changes in autocorrelation, variance, and synchrony before tree death (diachronic analysis), and also compared these metrics between trees that died and trees that survived a given mortality event (synchronic analysis). Changes in autocorrelation were a poor indicator of mortality risk. However, we found a gradual increase in inter-annual growth variability and a decrease in growth synchrony in the last ∼20 years before mortality of gymnosperms, irrespective of the cause of mortality. These changes could be associated with drought-induced alterations in carbon economy and allocation patterns. In angiosperms, we did not find any consistent changes in any metric. Such lack of any signal might be explained by the relatively high capacity of angiosperms to recover after a stress-induced growth decline. Our analysis provides a robust method for estimating early-warning signals of tree mortality based on annual growth data. In addition to the frequently reported decrease in growth rates, an increase in inter-annual growth variability and a decrease in growth synchrony may be powerful predictors of gymnosperm mortality risk, but not necessarily so for angiosperms.

Frontiers in Plant Science↗

USGS2016

A synthesis of radial growth patterns preceding tree mortality

Tree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still limited. We compiled a new pan-continental tree-ring width database from sites where both dead and living trees were sampled (2970 dead and 4224 living trees from 190 sites, including 36 species), and compared early and recent growth rates between trees that died and those that survived a given mortality event. We observed a decrease in radial growth before death in ca. 84% of the mortality events. The extent and duration of these reductions were highly variable (1–100 years in 96% of events) due to the complex interactions among study species and the source(s) of mortality. Strong and long-lasting declines were found for gymnosperms, shade- and drought-tolerant species, and trees that died from competition. Angiosperms and trees that died due to biotic attacks (especially bark-beetles) typically showed relatively small and short-term growth reductions. Our analysis did not highlight any universal trade-off between early growth and tree longevity within a species, although this result may also reflect high variability in sampling design among sites. The intersite and interspecific variability in growth patterns before mortality provides valuable information on the nature of the mortality process, which is consistent with our understanding of the physiological mechanisms leading to mortality. Abrupt changes in growth immediately before death can be associated with generalized hydraulic failure and/or bark-beetle attack, while long-term decrease in growth may be associated with a gradual decline in hydraulic performance coupled with depletion in carbon reserves. Our results imply that growth-based mortality algorithms may be a powerful tool for predicting gymnosperm mortality induced by chronic stress, but not necessarily so for angiosperms and in case of intense drought or bark-beetle outbreaks.

Global Change Biology↗

USGS2016

Climate change perils for dioecious plant species

Climate change, particularly increased aridity, poses a significant threat to plants and the biotic communities they support. Dioecious species may be especially vulnerable to climate change given that they often exhibit spatial segregation of the sexes, reinforced by physiological and morphological specialization of each sex to different microhabitats. In dimorphic species, the overexpression of a trait by one gender versus the other may become suppressed in future climates. Data suggest that males will generally be less sensitive to increased aridity than co-occurring females and, consequently, extreme male-biased sex ratios are possible in a significant number of populations. The effects of male-biased sex ratios are likely to cascade to dependent community members, especially those that are specialized on one sex.

Nature Plants↗

USGS2014

Rapid dispersal of saltcedar (Tamarix spp.) biocontrol beetles (Diorhabda carinulata) on a desert river detected by phenocams, MODIS imagery and ground observations

We measured the rate of dispersal of saltcedar leaf beetles ( Diorhabda carinulata ), a defoliating insect released on western rivers to control saltcedar shrubs ( Tamarix spp.), on a 63 km reach of the Virgin River, U.S. Dispersal was measured by satellite imagery, ground surveys and phenocams. Pixels from the Moderate Resolution Imaging Spectrometer (MODIS) sensors on the Terra satellite showed a sharp drop in NDVI in midsummer followed by recovery, correlated with defoliation events as revealed in networked digital camera images and ground surveys. Ground surveys and MODIS imagery showed that beetle damage progressed downstream at a rate of about 25 km yr −1 in 2010 and 2011, producing a 50% reduction in saltcedar leaf area index and evapotranspiration by 2012, as estimated by algorithms based on MODIS Enhanced Vegetation Index values and local meteorological data for Mesquite, Nevada. This reduction is the equivalent of 10.4% of mean annual river flows on this river reach. Our results confirm other observations that saltcedar beetles are dispersing much faster than originally predicted in pre-release biological assessments, presenting new challenges and opportunities for land, water and wildlife managers on western rivers. Despite relatively coarse resolution (250 m) and gridding artifacts, single MODIS pixels can be useful in tracking the effects of defoliating insects in riparian corridors.

Arizona, Nevada↗

USGS2014

Riparian restoration framework for the Upper Gila River, Arizona

This technical report summarizes the methods and results of a comprehensive riparian restoration planning effort for the Gila Valley Restoration Planning Area, an approximately 53-mile portion of the upper Gila River in Arizona (Figure 1-1). This planning effort has developed a Restoration Framework intended to deliver science-based guidance on suitable riparian restoration actions within the ecologically sensitive river corridor. The framework development was conducted by a restoration science team, led by Stillwater Sciences with contributions from researchers at the Desert Botanical Garden (DBG), Northern Arizona University (NAU), University of California at Santa Barbara (UCSB), and U.S. Geological Survey (USGS). All work was coordinated by the Gila Watershed Partnership of Arizona (GWP), whose broader Upper Gila River Project Area is depicted in Figure 1-1, with funding from the Walton Family Foundation’s Freshwater Initiative Program.

Arizona↗

USGS2013

Detection of tamarisk defoliation by the northern tamarisk beetle based on multitemporal Landsat 5 thematic mapper imagery.

The spread of tamarisk ( Tamarix spp., also known as saltcedar) is a significant ecological disturbance in western North America and has long been targeted for control, leading to the importation of the northern tamarisk beetle ( Diorhabda carinulata ) as a biological control agent. Following its initial release along the Colorado River near Moab, Utah in 2004, the beetle has successfully established and defoliated tamarisk across much of the upper Colorado River Basin. However, the spatial distribution and seasonal timing of defoliation are complex and difficult to quantify over large areas. To address this challenge, we tested and compared two remote sensing approaches to mapping tamarisk defoliation: Disturbance Index (DI) and a decision tree method called Random Forest (RF). Based on multitemporal Landsat 5 TM imagery for 2006-2010, changes in DI and defoliation probability from RF were calculated to detect tamarisk defoliation along the banks of Green, Colorado, Dolores and San Juan rivers within the Colorado Plateau area. Defoliation mapping accuracy was assessed based on field surveys partitioned into 10 km sections of river and on regions of interest created for continuous riparian vegetation. The DI method detected 3711 ha of defoliated area in 2007, 7350 ha in 2008, 10,457 ha in 2009 and 5898 ha in 2010. The RF method detected much smaller areas of defoliation but proved to have higher accuracy, as demonstrated by accuracy assessment and sensitivity analysis, with 784 ha in 2007, 960 ha in 2008, 934 ha in 2009, and 1008 ha in 2010. Results indicate that remote sensing approaches are likely to be useful for studying spatiotemporal patterns of tamarisk defoliation as the tamarisk leaf beetle spreads throughout the western United States.

Arizona, Colorado, New Mexico, Utah, Wyoming↗

USGS2010

A river system to watch: documenting the effects of saltcedar ( Tamarix spp.) biocontrol in the Virgin River valley

Throughout riparian areas of the southwestern United States, non-native saltcedar (also known as tamarisk; Tamarix spp.) can form dense, monotypic stands and is often reported to have detrimental effects on native plants and habitat quality (Everitt 1980; Shafroth et al. 2005). Natural resource managers of these riparian areas spend considerable time and resources controlling saltcedar using a variety of techniques, including chemical (Duncan and McDaniel 1998), mechanical, and burning methods (Shafroth et al. 2005). Approximately one billion dollars are spent each year on river restoration projects nationally (Bernhardt et al. 2005), and a majority of these projects focus on invasive species control in the Southwest (Follstad Shah et al. 2007). A technique that has drawn much attention is the use of the saltcedar leaf beetle ( Diorhabda spp.), a specialist herbivore, as biological control of saltcedar (Lewis et al. 2003). Research testing was conducted with beetles housed in secure enclosures in six states in 1998 and 1999 (Dudley et al. 2001), followed by open release at some of those sites starting in 2001 (DeLoach et al. 2004). By 2005, full-scale saltcedar biocontrol was implemented in 13 states, led by the USDA Animal and Plant Health Inspection Service (APHIS), the agency that oversees biological control programs, and with the participation and support of the U.S. Fish and Wildlife Service (USFWS). Despite the widespread application of Diorhabda , however, only limited research has quantified the consequences (benefits and costs) on biotic communities and ecosystem services. Alterations to riparian areas caused by various non-native species control activities have the potential to affect a variety of habitat types used by wildlife (Bateman et al. 2008a); processes like water availability, fluvial deposition, and erosion; and the establishment of other non-native species (Carruthers and D'Antonio 2005, Shafroth et al. 2005, DeLoach et al. 2006). Similarly, biocontrol is expected to modify riparian ecosystems, and it is imperative to document and evaluate both the environmental benefits and the potential costs of this tamarisk management method.

Virgin River↗

USGS2008

Wide-area estimates of saltcedar (Tamarix spp.) evapotranspiration on the lower Colorado River measured by heat balance and remote sensing methods

In many places along the lower Colorado River, saltcedar ( Tamarix spp) has replaced the native shrubs and trees, including arrowweed, mesquite, cottonwood and willows. Some have advocated that by removing saltcedar, we could save water and create environments more favourable to these native species. To test these assumptions we compared sap flux measurements of water used by native species in contrast to saltcedar, and compared soil salinity, ground water depth and soil moisture across a gradient of 200–1500 m from the river's edge on a floodplain terrace at Cibola National Wildlife Refuge (CNWR). We found that the fraction of land covered ( f c ) with vegetation in 2005–2007 was similar to that occupied by native vegetation in 1938 using satellite-derived estimates and reprocessed aerial photographs scaled to comparable spatial resolutions (3–4 m). We converted f c to estimates of leaf area index (LAI) through point sampling and destructive analyses ( r 2 = 0·82). Saltcedar LAI averaged 2·54 with an f c of 0·80, and reached a maximum of 3·7 with an f c of 0·95. The ranges in f c and LAI are similar to those reported for native vegetation elsewhere and from the 1938 photographs over the study site. On-site measurements of water use and soil and aquifer properties confirmed that although saltcedar grows in areas where salinity has increased much better than native shrubs and trees, rates of transpiration are similar. Annual water use over CNWR was about 1·15 m year −1 .

Arizona, California↗

USGS2008

Scaling sap flux measurements of grazed and ungrazed shrub communities with fine and coarse-resolution remote sensing

We measured transpiration by black greasewood ( Sarcobatus vermiculatus ) (SAVE) and fourwing saltbush ( Atriplex canescens ) (ATCA) over a nitrate-contaminated aquifer in Monument Valley, Arizona, on the Colorado Plateau. Heat balance sap flow sensors were used to measure transpiration by shrubs in 2006 and 2007 and results were scaled to larger landscape units and longer time scales using leaf area index (LAI), fractional vegetation cover, meteorological data, and the enhanced vegetation index (EVI) from MODIS sensors on the Terra satellite. Transpiration was high depending on leaf area (2·95–6·72 kg m −2 d −1 ) and was controlled by vapour pressure deficit ( D ) in the atmosphere. SAVE tended to have higher transpiration rates than ATCA and had a steeper response to D , but both exhibited midday depression of leaf conductance. Over most of the site, fractional vegetation cover ( f c ) and area-wide LAI were low (0·10 and 0·37, respectively) due to heavy grazing by cattle and sheep. However, a portion of the plume that had been protected from grazing for 10 years had f c = 0·75, LAI = 2·88. Transpiration rates on a ground-area basis varied with LAI, with midsummer daily values ranging from 1·44 mm d −1 (LAI = 0·36) to 13·1 mm d −1 (LAI = 2·88 mm) over the site, corresponding to projected annual values of 159–1447 mm year −1 . Controlling grazing could, theoretically, slow or halt the movement of the contamination plume by allowing the shrub community to extract more water than is recharged in the aquifer.

Utah↗