Search USGS⌕ Search

Geology topics

Craig D. Allen

Publications and source records attributed to Craig D. Allen.

At least 73 records · Page 4Linked to original sources
USGS2013

Quantifying tree mortality in a mixed species woodland using multitemporal high spatial resolution satellite imagery

Widespread tree mortality events have recently been observed in several biomes. To effectively quantify the severity and extent of these events, tools that allow for rapid assessment at the landscape scale are required. Past studies using high spatial resolution satellite imagery have primarily focused on detecting green, red, and gray tree canopies during and shortly after tree damage or mortality has occurred. However, detecting trees in various stages of death is not always possible due to limited availability of archived satellite imagery. Here we assess the capability of high spatial resolution satellite imagery for tree mortality detection in a southwestern U.S. mixed species woodland using archived satellite images acquired prior to mortality and well after dead trees had dropped their leaves. We developed a multistep classification approach that uses: supervised masking of non-tree image elements; bi-temporal (pre- and post-mortality) differencing of normalized difference vegetation index (NDVI) and red:green ratio (RGI); and unsupervised multivariate clustering of pixels into live and dead tree classes using a Gaussian mixture model. Classification accuracies were improved in a final step by tuning the rules of pixel classification using the posterior probabilities of class membership obtained from the Gaussian mixture model. Classifications were produced for two images acquired post-mortality with overall accuracies of 97.9% and 98.5%, respectively. Classified images were combined with land cover data to characterize the spatiotemporal characteristics of tree mortality across areas with differences in tree species composition. We found that 38% of tree crown area was lost during the drought period between 2002 and 2006. The majority of tree mortality during this period was concentrated in piñon-juniper ( Pinus edulis-Juniperus monosperma ) woodlands. An additional 20% of the tree canopy died or was removed between 2006 and 2011, primarily in areas experiencing wildfire and management activity. -Our results demonstrate that unsupervised clustering of bi-temporal NDVI and RGI differences can be used to detect tree mortality resulting from numerous causes and in several forest cover types.

Remote Sensing of Environment↗
USGS2013

The Malthusian-Darwinian dynamic and the trajectory of civilization

Two interacting forces influence all populations: the Malthusian dynamic of exponential growth until resource limits are reached, and the Darwinian dynamic of innovation and adaptation to circumvent these limits through biological and/or cultural evolution. The specific manifestations of these forces in modern human society provide an important context for determining how humans can establish a sustainable relationship with the finite Earth.

Trends in Ecology and Evolution↗
USGS2012

Temperature as a potent driver of regional forest drought stress and tree mortality

s the climate changes, drought may reduce tree productivity and survival across many forest ecosystems; however, the relative influence of specific climate parameters on forest decline is poorly understood. We derive a forest drought-stress index (FDSI) for the southwestern United States using a comprehensive tree-ring data set representing AD 1000-2007. The FDSI is approximately equally influenced by the warm-season vapour-pressure deficit (largely controlled by temperature) and cold-season precipitation, together explaining 82% of the FDSI variability. Correspondence between the FDSI and measures of forest productivity, mortality, bark-beetle outbreak and wildfire validate the FDSI as a holistic forest-vigour indicator. If the vapour-pressure deficit continues increasing as projected by climate models, the mean forest drought-stress by the 2050s will exceed that of the most severe droughts in the past 1,000 years. Collectively, the results foreshadow twenty-first-century changes in forest structures and compositions, with transition of forests in the southwestern United States, and perhaps water-limited forests globally, towards distributions unfamiliar to modern civilization.

Nature Climate Change↗
USGS2012

The macroecology of sustainability

The discipline of sustainability science has emerged in response to concerns of natural and social scientists, policymakers, and lay people about whether the Earth can continue to support human population growth and economic prosperity. Yet, sustainability science has developed largely independently from and with little reference to key ecological principles that govern life on Earth. A macroecological perspective highlights three principles that should be integral to sustainability science: 1) physical conservation laws govern the flows of energy and materials between human systems and the environment, 2) smaller systems are connected by these flows to larger systems in which they are embedded, and 3) global constraints ultimately limit flows at smaller scales. Over the past few decades, decreasing per capita rates of consumption of petroleum, phosphate, agricultural land, fresh water, fish, and wood indicate that the growing human population has surpassed the capacity of the Earth to supply enough of these essential resources to sustain even the current population and level of socioeconomic development.

PLoS Biology↗
USGS2012

Modeling aeolian transport in response to succession, disturbance and future climate: Dynamic long-term risk assessment for contaminant redistribution

Aeolian sediment transport is a fundamental process redistributing sediment, nutrients, and contaminants in dryland ecosystems. Over time frames of centuries or longer, horizontal sediment fluxes and associated rates of contaminant transport are likely to be influenced by succession, disturbances, and changes in climate, yet models of horizontal sediment transport that account for these fundamental factors are lacking, precluding in large part accurate assessment of human health risks associated with persistent soil-bound contaminants. We present a simple model based on empirical measurements of horizontal sediment transport (predominantly saltation) to predict potential contaminant transport rates for recently disturbed sites such as a landfill cover. Omnidirectional transport is estimated within vegetation that changes using a simple Markov model that simulates successional trajectory and considers three types of short-term disturbances (surface fire, crown fire, and drought-induced plant mortality) under current and projected climates. The model results highlight that movement of contaminated soil is sensitive to vegetation dynamics and increases substantially (e.g., > fivefold) when disturbance and/or future climate are considered. The time-dependent responses in horizontal sediment fluxes and associated contaminant fluxes were sensitive to variability in the timing of disturbance, with longer intervals between disturbance allowing woody plants to become dominant and crown fire and drought abruptly reducing woody plant cover. Our results, which have direct implications for contaminant transport and landfill management in the specific context of our assessment, also have general relevance because they highlight the need to more fully account for vegetation dynamics, disturbance, and changing climate in aeolian process studies.

Aeolian Research↗
USGS2012

Soil C and N patterns in a semiarid piñon-juniper woodland: Topography of slope and ephemeral channels add to canopy-intercanopy heterogeneity

Carbon and nitrogen are crucial to semiarid woodlands, determining decomposition, production and redistribution of water and nutrients. Carbon and nitrogen are often greater beneath canopies than intercanopies. Upslope vs. downslope position and ephemeral channels might also cause variation in C and N. Yet, few studies have simultaneously evaluated spatial variation associated with canopy–intercanopy patches and topography. We estimated C and N upslope and downslope in an eroding piñon–juniper woodland for canopies beneath piñons ( Pinus edulis ) and junipers, ( Juniperus monosperma ), intercanopies, and ephemeral channels. Soil C and N in the surface and profile beneath canopies exceeded that of intercanopies and channels. Relative to intercanopies, channels had more profile C upslope but less downslope (profile N was not significant). Relative to upslope, profile C downslope for intercanopies was greater and for channels was less (profile N was not significant). Relative to profile, surface soil C and N exhibited less heterogeneity. Although some topographic heterogeneity was detected, results did not collectively support our redistribution hypotheses, and we are unable to distinguish if this heterogeneity is due to in situ or redistribution effects. Nonetheless, results highlight finer topographical spatial variation in addition to predominant canopy and intercanopy variation that is applicable for semiarid woodland management.

New Mexico↗
USGS2011

Effects of biotic disturbances on forest carbon cycling in the United States and Canada

Forest insects and pathogens are major disturbance agents that have affected millions of hectares in North America in recent decades, implying significant impacts to the carbon (C) cycle. Here, we review and synthesize published studies of the effects of biotic disturbances on forest C cycling in the United States and Canada. Primary productivity in stands was reduced, sometimes considerably, immediately following insect or pathogen attack. After repeated growth reductions caused by some insects or pathogens or a single infestation by some bark beetle species, tree mortality occurred, altering productivity and decomposition. In the years following disturbance, primary productivity in some cases increased rapidly as a result of enhanced growth by surviving vegetation, and in other cases increased slowly because of lower forest regrowth. In the decades following tree mortality, decomposition increased as a result of the large amount of dead organic matter. Net ecosystem productivity decreased immediately following attack, with some studies reporting a switch to a C source to the atmosphere, and increased afterward as the forest regrew and dead organic matter decomposed. Large variability in C cycle responses arose from several factors, including type of insect or pathogen, time since disturbance, number of trees affected, and capacity of remaining vegetation to increase growth rates following outbreak. We identified significant knowledge gaps, including limited understanding of carbon cycle impacts among different biotic disturbance types (particularly pathogens), their impacts at landscape and regional scales, and limited capacity to predict disturbance events and their consequences for carbon cycling. We conclude that biotic disturbances can have major impacts on forest C stocks and fluxes and can be large enough to affect regional C cycling. However, additional research is needed to reduce the uncertainties associated with quantifying biotic disturbance effects on the North American C budget.

Global Change Biology↗
USGS2011

The effects of α-cellulose extraction and blue-stain fungus on retrospective studies of carbon and oxygen isotope variation in live and dead trees†

Tree-ring carbon and oxygen isotope ratios from live and recently dead trees may reveal important mechanisms of tree mortality. However, wood decay in dead trees may alter the δ 13 C and δ 18 O values of whole wood obscuring the isotopic signal associated with factors leading up to and including physiological death. We examined whole sapwood and α-cellulose from live and dead specimens of ponderosa pine ( Pinus ponderosa ), one-seed juniper ( Juniperous monosperma ), piñon pine ( Pinus edulis ) and white fir ( Abies concolor ), including those with fungal growth and beetle frass in the wood, to determine if α-cellulose extraction is necessary for the accurate interpretation of isotopic compositions in the dead trees. We found that the offset between the δ 13 C or δ 18 O values of α-cellulose and whole wood was the same for both live and dead trees across a large range of inter-annual and regional climate differences. The method of α-cellulose extraction, whether Leavitt-Danzer or Standard Brendel modified for small samples, imparts significant differences in the δ 13 C (up to 0.4‰) and δ 18 O (up to 1.2‰) of α-cellulose, as reported by other studies. There was no effect of beetle frass or blue-stain fungus ( Ophiostoma ) on the δ 13 C and δ 18 O of whole wood or α-cellulose. The relationships between whole wood and α-cellulose δ 13 C for ponderosa, piñon and juniper yielded slopes of ~1, while the relationship between δ 18 O of whole wood and α-cellulose was less clear. We conclude that there are few analytical or sampling obstacles to retrospective studies of isotopic patterns of tree mortality in forests of the western United States.

Rapid Communications in Mass Spectrometry↗
USGS2011

Salvage logging versus the use of burnt wood as a nurse object to promote post-fire tree seedling establishment

Intense debate surrounds the effects of post-fire salvage logging (SL) versus nonintervention policies on forest regeneration, but scant support is available from experimental studies. We analyze the effect of three post-fire management treatments on the recruitment of a serotinous pine ( Pinus pinaster ) at a Mediterranean mountain. Treatments were applied 7 months after the fire and differ in the degree of intervention, ranging from “no intervention” (NI, all trees left standing) to “partial cut plus lopping” (PCL, felling most of the trees, cutting the main branches, and leaving all the biomass in situ without mastication), and “SL” (felling and piling the logs, and masticating the woody debris). Seedling survival after 3 years was the highest in PCL (47.3% versus 38.7% in SL). This was associated with the amelioration of microclimatic conditions under the scattered branches, which reduced radiation and soil temperature while increasing soil moisture. Seedling density after 2 years was approximately 5.5 times higher in PCL than in SL, as in SL a large fraction of seedlings was lost as a consequence of mechanized mastication. The NI treatment showed the lowest seedling survival (17.3%). Nevertheless, seedling density was similar to SL. Seedling growth scarcely differed among treatments. Our results show that branches left onsite acted as nurse objects that improved key microclimatic conditions for seedling recruitment. This creates a facilitative interaction ideal for seedling establishment in moisture-deficient ecosystems, as it provides the benefit of a shading overstory but without underground competition.

Restoration Ecology↗
USGS2011

Climate-induced tree mortality: Earth system consequences

One of the greatest uncertainties in global environmental change is predicting changes in feedbacks between the biosphere and the Earth system. Terrestrial ecosystems and, in particular, forests exert strong controls on the global carbon cycle and influence regional hydrology and climatology directly through water and surface energy budgets [ Bonan , 2008; Chapin et al. , 2008]. According to new research, tree mortality associated with elevated temperatures and drought has the potential to rapidly alter forest ecosystems, potentially affecting feedbacks to the Earth system [ Allen et al. , 2010]. Several lines of recent research demonstrate how tree mortality rates in forests may be sensitive to climate change—particularly warming and drying. This emerging consequence of global change has important effects on Earth system processes (Figure 1).

Eos, Transactions, American Geophysical Union↗
USGS2011

Millennial precipitation reconstruction for the Jemez Mountains, New Mexico, reveals changing drought signal

Drought is a recurring phenomenon in the American Southwest. Since the frequency and severity of hydrologic droughts and other hydroclimatic events are of critical importance to the ecology and rapidly growing human population of this region, knowledge of long-term natural hydroclimatic variability is valuable for resource managers and policy-makers. An October–June precipitation reconstruction for the period AD 824–2007 was developed from multi-century tree-ring records of Pseudotsuga menziesii (Douglas-fir), Pinus strobiformis (Southwestern white pine) and Pinus ponderosa (Ponderosa pine) for the Jemez Mountains in Northern New Mexico. Calibration and verification statistics for the period 1896–2007 show a high level of skill, and account for a significant portion of the observed variance (>50%) irrespective of which period is used to develop or verify the regression model. Split-sample validation supports our use of a reconstruction model based on the full period of reliable observational data (1896–2007). A recent segment of the reconstruction (2000–2006) emerges as the driest 7-year period sensed by the trees in the entire record. That this period was only moderately dry in precipitation anomaly likely indicates accentuated stress from other factors, such as warmer temperatures. Correlation field maps of actual and reconstructed October–June total precipitation, sea surface temperatures and 500-mb geopotential heights show characteristics that are similar to those indicative of El Niño–Southern Oscillation patterns, particularly with regard to ocean and atmospheric conditions in the equatorial and north Pacific. Our 1184-year reconstruction of hydroclimatic variability provides long-term perspective on current and 20th century wet and dry events in Northern New Mexico, is useful to guide expectations of future variability, aids sustainable water management, provides scenarios for drought planning and as inputs for hydrologic models under a broader range of conditions than those provided by historical climate records.

New Mexico↗
USGS2011

Extended megadroughts in the southwestern United States during Pleistocene interglacials

The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern. Multi-year droughts during the instrumental period and decadal-length droughts of the past two millennia were shorter and climatically different from the future permanent, ‘dust-bowl-like’ megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C 4 plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ∼ 2 °C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4–6 °C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase.

Nature↗
USGS2011

Seeing the forest and the trees: USGS scientist links local changes to global scale

The recent recipient of two major awards, Craig D. Allen, a research ecologist with the U.S. Geological Survey Fort Collins Science Center, has loved trees since childhood. He is now considered an expert of world renown on the twin phenomena of forest changes and tree mortality resulting from climate warming and drought, and in 2010 was twice recognized for his scientific contributions. In December 2010, Dr. Allen was named a 2010 Fellow of the American Association for the Advancement of Science “for outstanding leadership in the synthesis of global forest responses to climate change, built from worldwide collaboration and a deep understanding of the environmental history of the southwestern United States.” In March 2010, he was honored with the Meritorious Service Award from the U.S. Department of the Interior (DOI) in recognition of his outstanding vision, initiative, and scientific contributions to the USGS, DOI, and U.S. Department of Agriculture in establishing a model science program to support adaptive land management at the new Valles Caldera National Preserve in north-central New Mexico. Dr. Allen has authored more than 85 publications on landscape ecology and landscape change, from fire history and ecology to ecosystem responses to climate change. He has appeared on NOVA discussing fire ecology and on The Discovery Channel and Discovery Canada explaining the links between drought-induced tree mortality and climate warming, in addition to being interviewed and quoted in innumerable newspaper articles on both topics. But how did this unassuming scientist grow from nurturing maple saplings on 40 acres in Wisconsin to understanding forest system stress worldwide?

Report↗
USGS2010

Forest responses to increasing aridity and warmth in the southwestern United States

In recent decades, intense droughts, insect outbreaks, and wildfires have led to decreasing tree growth and increasing mortality in many temperate forests. We compared annual tree-ring width data from 1,097 populations in the coterminous United States to climate data and evaluated site-specific tree responses to climate variations throughout the 20th century. For each population, we developed a climate-driven growth equation by using climate records to predict annual ring widths. Forests within the southwestern United States appear particularly sensitive to drought and warmth. We input 21st century climate projections to the equations to predict growth responses. Our results suggest that if temperature and aridity rise as they are projected to, southwestern trees will experience substantially reduced growth during this century. As tree growth declines, mortality rates may increase at many sites. Increases in wildfires and bark-beetle outbreaks in the most recent decade are likely related to extreme drought and high temperatures during this period. Using satellite imagery and aerial survey data, we conservatively calculate that ≈2.7% of southwestern forest and woodland area experienced substantial mortality due to wildfires from 1984 to 2006, and ≈7.6% experienced mortality associated with bark beetles from 1997 to 2008. We estimate that up to ≈18% of southwestern forest area (excluding woodlands) experienced mortality due to bark beetles or wildfire during this period. Expected climatic changes will alter future forest productivity, disturbance regimes, and species ranges throughout the Southwest. Emerging knowledge of these impending transitions informs efforts to adaptively manage southwestern forests.

PNAS↗
USGS2010

A 15 000-year record of climate change in northern New Mexico, USA, inferred from isotopic and elemental contents of bog sediments

Elemental (C, N, Pb) and isotopic ( δ 13 C, δ 15 N) measurements of cored sediment from a small bog in northern New Mexico reveal changes in climate during the Late Pleistocene and Holocene. Abrupt increases in Pb concentration and δ 13 C values ca. 14 420 cal. YBP indicate significant runoff to the shallow lake that existed at that time. Weathering and transport of local volcanic rocks resulted in the delivery of Pb-bearing minerals to the basin, while a 13 C-enriched terrestrial vegetation source increased the δ 13 C values of the sedimentary material. Wet conditions developed over a 300 a period and lasted for a few hundred years. The Younger Dryas period (ca. 12 700–11 500 cal. YBP) caused a reduction in terrestrial productivity reflected in decreasing C/N values, δ 15 N values consistently greater than 0‰ and low organic content. By contrast, aquatic productivity increased during the second half of this period, evidenced by increasing δ 13 C values at the time of highest abundance of algae. Dry conditions ca. 8 000–6 000 cal. YBP were characterised by low organic carbon content and high Pb concentrations, the latter suggesting enhanced erosion and aeolian transport of volcanic rock. The range in δ 13 C, δ 15 N and C/N values in the sedimentary record fall within the range of modern plants, except during the periods of runoff and drought. The sedimentary record provides evidence of natural climate variability in northern New Mexico, including short- (multi-centennial) and long-(millennial) term episodes during the Late Pleistocene and Holocene.

Journal of Quaternary Science↗
USGS2010

A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests

Greenhouse gas emissions have significantly altered global climate, and will continue to do so in the future. Increases in the frequency, duration, and/or severity of drought and heat stress associated with climate change could fundamentally alter the composition, structure, and biogeography of forests in many regions. Of particular concern are potential increases in tree mortality associated with climate-induced physiological stress and interactions with other climate-mediated processes such as insect outbreaks and wildfire. Despite this risk, existing projections of tree mortality are based on models that lack functionally realistic mortality mechanisms, and there has been no attempt to track observations of climate-driven tree mortality globally. Here we present the first global assessment of recent tree mortality attributed to drought and heat stress. Although episodic mortality occurs in the absence of climate change, studies compiled here suggest that at least some of the world's forested ecosystems already may be responding to climate change and raise concern that forests may become increasingly vulnerable to higher background tree mortality rates and die-off in response to future warming and drought, even in environments that are not normally considered water-limited. This further suggests risks to ecosystem services, including the loss of sequestered forest carbon and associated atmospheric feedbacks. Our review also identifies key information gaps and scientific uncertainties that currently hinder our ability to predict tree mortality in response to climate change and emphasizes the need for a globally coordinated observation system. Overall, our review reveals the potential for amplified tree mortality due to drought and heat in forests worldwide.

Forest Ecology and Management↗