Copper River Basin Research Articles

Long-term change and geospatial patterns of river ice cover and navigability in Southcentral Alaska detected with remote sensing

ABSTRACT People who travel on ice-covered rivers to access traditional lands and resources can be profoundly impacted by effects of climate change on river ice seasonality. We used remote sensing, bolstered by citizen science, to assess trends and geospatial patterns of the ice cover in the Copper River Basin of Southcentral Alaska. Our analysis of Landsat imagery from water years (WYs) 1973 to 2021 suggests a severely diminishing season of river ice travel (delayed or incomplete freezeup, early breakup) due to increasing air temperatures. The weekly probability of an adequate ice cover for river crossings declined by an average of 53 percentage points. Ice extent was closely related to accumulated freezing degree days (AFDD). AFDDOct-Apr decreased by 15% since WY 1943, a significant warming trend. We mapped the spatiotemporal variation of ice and open water extent with multispectral and synthetic aperture radar (SAR) imagery (Sentinel-2, Sentinel-1). We identified reaches with more reliable opportunities for winter access and others susceptible to extensive open water, differences related to flow energy and channel form. The results of this study can support local decision making and adaptation in response to rapidly changing river ice conditions, and our approach can be applied elsewhere to document change and improve travel safety.

Creating a Software Methodology to Analyze and Preserve Archaeological Legacy Data

AbstractSoftware now allows archaeologists to document excavations in more detail than ever before through rich, born-digital datasets. In comparison, paper documentation of past excavations (a valuable corpus of legacy data) is prohibitively difficult to work with. This pilot study explores creating custom software to digitize paper field notes from the 1970s excavations of the Gulkana site into machine-readable text and maps to be compatible with born-digital data from subsequent excavations in the 1990s. This site, located in Alaska's Copper River Basin, is important to archaeological understanding of metalworking innovation by precontact Northern Dene people, but is underrepresented in the literature because no comprehensive map of the site exists. The process and results of digitizing this corpus are presented in hopes of aiding similar efforts by other researchers.

Terminal Pleistocene human occupation of the upper Copper River basin, southern Alaska: Results of test excavations at Nataeł Na’

After decades of debate, the homeland of the First Americans is now generally understood to be northeast Asia; however, the process of Late Pleistocene peopling remains unresolved. As more archaeological sites south of the continental ice sheets are discovered that predate the opening of the interior “ice-free” corridor, interest in a coastal Pacific dispersal route has grown, and previously overlooked regions proximal to the Pacific coast have become a central focus of exploration efforts. The Copper River basin of southern Alaska is one such region. Here we present the results of 2019 archaeological excavations at Nataeł Na’, a buried and stratified archaeological site situated along the upper Copper River. The site contains a robust occupation dating to the late Younger Dryas climate reversal as well as an earlier occupation dating to the late Allerød interstadial. This discovery demonstrates that Pleistocene hunter-gatherers inhabited the Pacific basin of southern Alaska during the same time Clovis peoples inhabited temperate North America. The occupations at Nataeł Na’ join a growing body of evidence suggesting that the early inhabitants of eastern Beringia were geographically more widely dispersed than previously documented.

Long-term recovery of soil carbon stocks and permafrost depth lags recovery of organic layer thickness following fire in black spruce forests of the Copper River Basin, Alaska

This study investigated differences in forest structure, organic layer thickness, soil organic carbon, and permafrost depth between late-successional (LS) and postfire (PF; 90–120 years since burn) plots under black spruce (Picea mariana (Mill.) B.S.P.) growing on fine-textured, poorly drained lacustrine sediments in the Copper River Basin, Alaska. Although live stem and seedling density and organic layer thickness were not significantly different between PF and LS plots (28 ± 7 cm and 31 ± 10 cm, respectively), we did find a significant difference in soil organic carbon (SOC) stocks (30 ± 10 kg·m−2 and 46 ± 12 kg·m−2, respectively) permafrost depth (90 ± 28 cm and 56 ± 12 cm, respectively). OLT was linearly related to 1 m SOC stocks for LS plots but not for PF plots, and LS plots had a greater proportion of highly decomposed (humic) material in the organic layer. The soil properties of PF plots appear to be on a trajectory of recovery toward those of LS plots with respect to SOC stocks, permafrost depth, and organic layer composition. However, PF plots remain different despite nearly 100 years since fire disturbance and thus they are potentially more sensitive to changes in future fire frequency or climate.

DOC export is exceeded by C fixation in May Creek: A late-successional watershed of the Copper River Basin, Alaska.

Understanding the entirety of basin-scale C cycling (DOC fluxes and CO2 exchanges) are central to a holistic perspective of boreal forest biogeochemistry today. Shifts in the timing and magnitude of dissolved organic carbon (DOC) delivery in streams and eventually into oceans can be expected, while simultaneously CO2 emission may exceed CO2 fixation, leading to forests becoming stronger CO2 sources than sinks amplifying rising trace gases in the atmosphere. At May Creek, a representative late-successional boreal forest watershed at the headwaters of the Copper River Basin, Alaska, we quantified the seasonality of DOC flux and landscape-scale CO2 exchange (eddy covariance) over two seasonal cycles. We deployed in situ fDOM and conductivity sensors, performed campaign sampling for water quality (DOC and water isotopes), and used fluorescence spectroscopy to ascertain DOC character. Simultaneously, we quantified net CO2 exchange using a 100 ft eddy covariance tower. Results indicate DOC exports were pulse-driven and mediated by precipitation events. Both frequency and magnitude of pulse-driven DOC events diminished as the seasonal thaw depth deepened, with inputs from terrestrial sources becoming major contributors to the DOC pool with decreasing snowmelt contribution to the hydrograph. A three-component parallel factorial analysis (PARAFAC) model indicated DOC liberated in late-season may be bioavailable (tyrosine-like). Combining Net Ecosystem Exchange (NEE) measurements indicate that the May Creek watershed fixes 142-220 g C m-2 yr-1 and only 0.40-0.57 g C m-2 yr-1 is leached out as DOC. Thus, the May Creek watershed and similar mature spruce forest dominated watersheds in the Copper River Basin are currently large ecosystem C sinks and exceeding C conservative. An understanding of DOC fluxes from Gulf of Alaska watersheds is important for characterizing future climate change-induced seasonal shifts.

Cryoturbation and Carbon Stocks in Gelisols under Late‐Successional Black Spruce Forests of the Copper River Basin, Alaska

In the Alaskan discontinuous permafrost zone, soils developed under black spruce [Picea mariana (Mill.) Britton, Sterns & Poggenb.] forests are affected by fire over return intervals ranging from decades to centuries and cycle between recently burned and late‐successional stages. Soils under late‐successional forests therefore provide foundational information regarding the potential recovery of soil and permafrost characteristics following disturbance by fire. Twenty‐two permafrost‐affected soils (predominantly Histoturbels with minor components of Sapristels and Aquiturbels) were investigated under late‐successional black spruce in the Copper River Basin, which contains the largest expanse of glaciolacustrine sediments in Alaska. Although these soils are well insulated (organic layer thicknesses of 31 ± 10 cm) and fine‐textured with a relatively shallow permafrost table (69 ± 19 cm), 87% of the 129 described mineral soil horizons across these sites showed morphological evidence of cryoturbation. Depth trends in water content and bulk density were consistent with an ice‐rich upper permafrost, with suspended cryostructures most common in the upper permafrost. Soil organic carbon (SOC) stocks to 1 m averaged 46 ± 12 kg m−2, with 44% of the SOC stocks contained in cryoturbated mineral horizons, many of which were identified well below the current permafrost table. The most likely mechanism of cryoturbation in these soils is post‐fire active layer deepening followed by solutioning and diapirism, generating SOC stocks in cryoturbated mineral materials of the upper permafrost. Future changes in climate or fire frequency that affect active layer depth may therefore have the potential to affect cryoturbation processes and carbon stocks in these soils.

Interferometric synthetic aperture radar study of recent eruptive activity at Shrub mud volcano, Alaska

Shrub mud volcano is one of three large mud volcanoes that comprise the Klawasi Group in the Copper River Basin of southcentral Alaska. Except for minor discharges in the mid-1950s when the group was first described, Shrub was dormant prior to its reactivation in summer 1996. From 1997 to 1999, Shrub vigorously erupted more than 5 × 105 m3 of saline mud and CO2-rich gas at temperatures as high as 54 °C. Thereafter, activity waned but continued at least through 2015. We analyzed 192 interferograms derived from 106 synthetic aperture radar (SAR) images acquired by the JERS-1 (L-band), ERS-1/2 (C-band), RADARSAT-1 (C-band), and ALOS PALSAR (L-band) satellites to characterize ground deformation at Shrub before, during, and after its reactivation. Collectively, the interferograms span 1992–2000 and 2006–2011. We fit the observations with two deformation sources: a deflating, steeply-dipping, pipe-like body under the summit area and an inflating, shallow-dipping, sill-like body under the southwest flank. Both sources are shallow, with centroids less than 1 km beneath the summit. Prior to reactivation, the flank source inflated ~0.35 × 105 m3/yr from July 1992 to May 1996. During eruptive activity, the summit source deflated at higher rates that peaked at ~8.71 × 105 m3/yr during May–November 1997 and continued at ~0.95 × 105 m3/yr during the 2006–2011 observation window. Cumulative source-volume loss is comparable to the volume of mud erupted. We interpret the summit source as the volcano's feeder conduit that pressurized prior to the first SAR observation in 1992. Also before 1992, the conduit ruptured to feed a lateral intrusion of mud under the southwest flank, perhaps along a bedding plane in underlying glaciolacustrine deposits. The growing sill caused the southwest flank to inflate while it accommodated the mud supply from depth, which explains why we observed pre-eruptive inflation of the flank but not the summit. The summit began deflating when the conduit ruptured to the surface at the onset of eruptive activity. The flank source did not deflate concurrently because the weight of the thin overburden was insufficient to collapse the sill. There is a suggestion in the modern topography that lateral intrusions under Shrub's southwest flank are a common feature of activity there.

A Physically Based Daily Simulation of the Glacier‐Dominated Hydrology of the Copper River Basin, Alaska

AbstractThe large, highly glacierized Copper River basin is an important water resource for the south‐central region of Alaska. Thus, information is needed on the reaction of its hydrologic timing and streamflow volumes to historical changes in climate, in order to assess the possible impact of future changes. However, the basin is remote, and therefore, it has proved difficult to collect field data in a frequent temporal and spatial manner. An extension of the distributed‐parameter, physical‐process code Precipitation Runoff Modeling System, PRMSglacier, has been specifically developed to simulate daily hydrology without requiring extensive input data. In this study, PRMSglacier was used to characterize the hydrology of the Copper River basin from 1959 to 2015. The basin was split into subbasins for specific regional climatic calibrations and finer resolution characterization. The model was calibrated and performed well against data of glacier mass balance, glacier area change, snow cover, gaged streamflow, evapotranspiration, and solar radiation. Ice melt contributed 26% of the total basin streamflow, with differences temporally from climate oscillations. Furthermore, differences were seen geographically in subbasins depending on the state of the glaciers in each subbasin. Decreasing trends in ice melt volume were mostly seen on smaller steeper glaciers responding to a critical level of glacier recession, while increasing trends in ice melt volume were mostly seen on larger valley glaciers responding to increasing temperature. The areas with substantially decreasing ice melt had decreasing streamflow, possibly indicating health concerns for the ecosystems therein.

Recent paleoseismicity record in Prince William Sound, Alaska, USA

Sedimentological and geochemical investigation of sediment cores collected in the deep (>400 m) central basin of Prince William Sound, along with geochemical fingerprinting of sediment source areas, are used to identify earthquake-generated sediment gravity flows. Prince William Sound receives sediment from two distinct sources: from offshore (primarily Copper River) through Hinchinbrook Inlet, and from sources within the Sound (primarily Columbia Glacier). These sources are found to have diagnostic elemental ratios indicative of provenance; Copper River Basin sediments were significantly higher in Sr/Pb and Cu/Pb, whereas Prince William Sound sediments were significantly higher in K/Ca and Rb/Sr. Within the past century, sediment gravity flows deposited within the deep central channel of Prince William Sound have robust geochemical (provenance) signatures that can be correlated with known moderate to large earthquakes in the region. Given the thick Holocene sequence in the Sound (~200 m) and correspondingly high sedimentation rates (>1 cm year−1), this relationship suggests that sediments within the central basin of Prince William Sound may contain an extraordinary high-resolution record of paleoseismicity in the region.

Late Quaternary megafloods from Glacial Lake Atna, Southcentral Alaska, U.S.A.

Geomorphic, stratigraphic, geotechnical, and biogeographic evidence indicate that failure of a Pleistocene ice dam between 15.5 and 26 ka generated a megaflood from Glacial Lake Atna down the Matanuska Valley. While it has long been recognized that Lake Atna occupied ≥ 9000 km 2 of south-central Alaska's Copper River Basin, little attention has focused on the lake's discharge locations and behaviors. Digital elevation model and geomorphic analyses suggest that progressive lowering of the lake level by decanting over spillways exposed during glacial retreat led to sequential discharges down the Matanuska, Susitna, Tok, and Copper river valleys. Lake Atna's size, ∼ 50 ka duration, and sequential connection to four major drainages likely made it a regionally important late Pleistocene freshwater refugium. We estimate a catastrophic Matanuska megaflood would have released 500–1400 km 3 at a maximum rate of ≥ 3 × 10 6 m 3 s − 1 . Volumes for the other outlets ranged from 200 to 2600 km 3 and estimated maximum discharges ranged from 0.8 to 11.3 × 10 6 m 3 s − 1 , making Lake Atna a serial generator of some of the largest known freshwater megafloods.

Stand and landscape level effects of a major outbreak of spruce beetles on forest vegetation in the Copper River Basin, Alaska

From 1989 to 2003, a widespread outbreak of spruce beetles ( Dendroctonus rufipennis) in the Copper River Basin, Alaska, infested over 275,000 ha of forests in the region. During 1997 and 1998, we measured forest vegetation structure and composition on one hundred and thirty-six 20-m × 20-m plots to assess both the immediate stand and landscape level effects of the spruce beetle infestation. A photo-interpreted vegetation and infestation map was produced using color-infrared aerial photography at a scale of 1:40,000. We used linear regression to quantify the effects of the outbreak on forest structure and composition. White spruce ( Picea glauca) canopy cover and basal area of medium-to-large trees [≥15 cm diameter-at-breast height (1.3 m, dbh)] were reduced linearly as the number of trees attacked by spruce beetles increased. Black spruce ( Picea mariana) and small diameter white spruce (<15 cm dbh) were infrequently attacked and killed by spruce beetles. This selective attack of mature white spruce reduced structural complexity of stands to earlier stages of succession and caused mixed tree species stands to lose their white spruce and become more homogeneous in overstory composition. Using the resulting regressions, we developed a transition matrix to describe changes in vegetation types under varying levels of spruce beetle infestations, and applied the model to the vegetation map. Prior to the outbreak, our study area was composed primarily of stands of mixed white and black spruce (29% of area) and pure white spruce (25%). However, the selective attack on white spruce caused many of these stands to transition to black spruce dominated stands (73% increase in area) or shrublands (26% increase in area). The post-infestation landscape was thereby composed of more even distributions of shrubland and white, black, and mixed spruce communities (17–22% of study area). Changes in the cover and composition of understory vegetation were less evident in this study. However, stands with the highest mortality due to spruce beetles had the lowest densities of white spruce seedlings suggesting a longer forest regeneration time without an increase in seedling germination, growth, or survival.

Effects of sample mass and macrofossil type on radiocarbon dating of arctic and boreal lake sediments

Dating lake sediments by accelerator mass spectrometry (AMS) 14C analysis of terrestrial plant macrofossils overcomes one of the main problems associated with dating bulk sediment samples, i.e., the presence of old organic matter. Even so, many AMS dates from arctic and boreal sites appear to misrepresent the age of the sediment. To understand the nature of these apparent dating anomalies better, we conducted a series of 14C dating experiments using samples from Alaskan and Siberian lake-sediment cores. First, to test whether our analytical procedures introduced a sample-mass bias, we obtained 14C dates for different-sized pieces of single woody macrofossils. In these sample-mass experiments, statistically equivalent ages were found for samples as small as 0.05 mg C. Secondly, to assess whether macrofossil type influenced dating results, we conducted sample-type experiments in which 14C dates were obtained for different macrofossil types sieved from the same depth in the sediment. We dated materials from multiple levels in sediment cores from Upper Capsule Lake (North Slope, northern Alaska) and Grizzly Lake (Copper River Basin, southern Alaska) and from single depths in other records from northern Alaska. In several of the experiments there were significant discrepancies between dates for different plant tissues, and in most cases wood and charcoal were older than other macrofossil types, usually by several hundred years. This pattern suggests that 14C dates for woody macrofossils may misrepresent the age of the sediment by centuries, perhaps because of their longer terrestrial residence time and the potential in-built age of longlived plants. This study identifies why some 14C dates appear to be inconsistent with the overall age-depth trend of a lake-sediment record, and it may guide the selection of 14C samples in future studies.

SHORT-TERM DEMOGRAPHIC RESPONSE OF THE RED-BACKED VOLE TO SPRUCE BEETLE INFESTATIONS IN ALASKA

Abstract How small mammals are affected by habitat changes caused by forest insect epidemics is largely unknown. Our objective was to determine the influence of spruce beetle (Dendroctonus rufipennis) epidemics on the dynamics of northern red-backed vole (Clethrionomys rutilus) populations approximately 10 years post-infestation. We conducted a mark–recapture study on northern red-backed voles for 2 field seasons in the Copper River Basin, Alaska, USA, where recent beetle infestations were widespread. Using the robust sampling design, we produced estimates of vole abundance, survival, and recruitment in 3 locations that varied in their degree of beetle-induced spruce mortality. Vole abundance inversely related to the level of spruce mortality. Vole recruitment showed a larger contribution from both immigration and in situ reproduction in the low infestation site than in the medium and heavy infestation sites. No differences in vole survival were detectable across the 3 locations with varied beetle-induced...

The impact of tree and stand characteristics on spruce beetle (Coleoptera: Scolytidae) induced mortality of white spruce in the Copper River Basin, Alaska

I examined the relationships between individual and stand-level characteristics of white spruce, Picea glauca (Moench) Voss, and the incidence of spruce beetle, Dendroctonus rufipennis Kirby, induced mortality. The study region, in the Kennicott Valley of the Copper River Basin, Alaska, has contained an active spruce beetle epidemic since 1989. I investigated the relationship among the individual traits of host age, size (diameter at breast height, DBH), and growth rate (basal area increment, BAI) and mortality from the spruce beetle. I also examined the effects of stand density, mean DBH, and mean BAI on percent mortality within plots. Survival was higher for younger, smaller, and faster-growing trees. However, the effect of age is not significant when included in a logistic regression model examining the effect of individual host traits on host survival. Mortality increased with increasing DBH and decreasing BAI, and there was a significant interaction between DBH and BAI. While the proportion of individuals killed by the spruce beetle significantly differed between stands, I found no significant relationships between stand-level characteristics and mortality rate. This research suggests that the individual traits of host size and growth rate, as well as their interaction are the best predictors of susceptibility to spruce beetle-induced mortality in this system.

Active mud volcanism observed with Landsat 7 ETM+

Mud volcanoes are relatively small spatter cones that erupt water-laden mud and gases, and occur throughout the world. For many mud volcanoes, the eruption of warm mud (10–40°C) can be detected with high-resolution thermal satellite imagery. We demonstrate the utility of Landsat 7 Enhanced Thematic Mapper Plus (ETM+) imagery for thermal monitoring of active mud volcanism. We constrain the temperature and area of active mud discharge and estimate surface heat flux for two isolated mud volcanoes in the Copper River Basin, Alaska using Band 6 (10.4–12.5 μm). The heat flux results span a wide range due to uncertainties in the environmental conditions at the time of image acquisition, but can be constrained to be less than 0.24 MW for each of the two mud volcanoes considering previously published field measurements. With this higher-resolution Band 6 on the ETM+ sensor, as well as the high-resolution thermal bands on the ASTER sensor, reliable monitoring of mud volcanism on this scale is possible for the first time.

Germanium/silicon ratios in the Copper River Basin, Alaska: Weathering and partitioning in periglacial versus glacial environments

Glacial to interglacial variation in the germanium to silicon ratio (Ge/Si) of biogenic opal in oceanic sediment cores has been proposed to record terrestrial silicate weathering intensity. However, data connecting Ge/Si ratios to weathering are sparse, especially for cold regions. To characterize Ge/Si of the dissolved loads of rivers draining modern glacial and periglacial regions, we measured Ge and major element chemistry of more than 30 streams in the Copper River Basin of south Alaska. We measured significantly higher Ge/Si in rivers with greater than 15% glacial cover in their basins than in rivers draining areas with less glacial cover, which we attribute to preferencial subglacial weathering of biotite. We also ran laboratory experiments to investigate the role of differential sorption of Ge and Si on hydrous iron oxides in moderating the Ge/Si ratio of rivers in recently deglaciated basins. These experiments indicate that Ge is preferentially adsorbed over Si by hydrous iron oxides over a wide range of pH values and support differential sorption as a potentially significant fractionation process. Our measured high Ge/Si in glaciated basins is in striking contrast to the low Ge/Si in the oceanic record during glacial periods. The glacial to interglacial changes in marine Ge/Si cannot simply reflect changes in glacier cover on land, as the mineralogy of silicates weathered and processes partitioning Ge in soils must also strongly influence the fluvial Ge and Si inputs to the ocean.

Facies architecture within a regional glaciolacustrine basin: Copper River, Alaska

This paper defines the principal architectural elements present within the Pleistocene, glaciolacustrine basin-fill of the Copper River Basin in Alaska. The Copper River drains an intermontane basin via a single deeply incised trench through the Chugach Mountains to the Gulf of Alaska. This trench was blocked by ice during the last glacial cycle and a large ice-dammed lake, referred to as Lake Atna, filled much of the Copper Basin. Facies analysis within the basin floor allows a series of associations to be defined consistent with the basinward transport of sediment deposited along calving ice margins and at the basin edge. Basinward transport involves a continuum of gravity driven processes, including slumping, cohesive debris flow, hyperconcentrated/concentrated density flows, and turbidity currents. This basinward transport results in the deposition of a series of subaqueous fans, of which two main types are recognised. (1) Large, stratified, basin floor fans, which extend over at least 5 km and are exposed in the basin centre. These fans are composed of multiple lobes, incised by large mega-channels, giving fan architectures that are dominated by horizontal strata and large, cross-cutting channel-fills. Individual lobes and channel-fills consist of combinations of: diamict derived from iceberg rainout and the ice-marginal release of subglacial sediment; multiple units of fining upward gravels which grade vertically into parallel laminated and rippled fine sands and silts, deposited by a range of density flows and currents derived from the subaqueous discharge of meltwater; and rhythmites grading vertically into diamicts deposited from a range of sediment-density flows re-mobilising sediment deposited by either iceberg rainout or the ice-marginal release of sediment. (2) Small, complex, proximal fans, which extend over less than 2 km, and are exposed in the southern part of the basin. These fans are composed of coalescing and prograding lobes of diamict and gravel deposited both directly by subaqueous meltwater and from sediment-density flows. These lobes are cross-cut by a range of sand and gravel-filled troughs and channels cut by subaqueous outwash, and either overlie or are overlain by horizontal sheets of gravel and diamict deposited from a range of sediment-density flows. The fans are, therefore, characterised by a complex, and laterally variable facies, architecture. Water depth, proglacial topography, stability of meltwater portals and sediment supply may all be important in determining the type of subaqueous fan present at any one location. We suggest that the Copper River basin-fill is dominated by packages of sediment containing multiple subaqueous fans with individual fans separated by units of diamict. Each sediment package is in turn separated from the next by a palaeo-landsurface shaped by interstadial/interglacial fluvial processes and by volcanic debris flows.

Densities of breeding birds and changes in vegetation in an Alaskan boreal forest following a massive disturbance by spruce beetles

We examined bird and plant communities among forest stands with different levels of spruce mortality following a large outbreak of spruce beetles (Dendroctonus rufipennis (Kirby)) in the Copper River Basin, Alaska. Spruce beetles avoided stands with black spruce (Picea mariana) and selectively killed larger diameter white spruce (Picea glauca), thereby altering forest structure and increasing the dominance of black spruce in the region. Alders (Alnus sp.) and crowberry (Empetrum nigrum) were more abundant in areas with heavy spruce mortality, possibly a response to the death of overstory spruce. Grasses and herbaceous plants did not proliferate as has been recorded following outbreaks in more coastal Alaskan forests. Two species closely tied to coniferous habitats, the tree-nesting Ruby-crowned Kinglet (Regulus calendula) and the red squirrel (Tamiasciurus hudsonicus), a major nest predator, were less abundant in forest stands with high spruce mortality than in low-mortality stands. Understory-nesting birds as a group were more abundant in forest stands with high levels of spruce mortality, although the response of individual bird species to tree mortality was variable. Birds breeding in stands with high spruce mortality likely benefited reproductively from lower squirrel densities and a greater abundance of shrubs to conceal nests from predators.

Densities of breeding birds and changes in vegetation in an Alaskan boreal forest following a massive disturbance by spruce beetles

We examined bird and plant communities among forest stands with different levels of spruce mortality following a large outbreak of spruce beetles (Dendroctonus rufipennis (Kirby)) in the Copper River Basin, Alaska. Spruce beetles avoided stands with black spruce (Picea mariana) and selectively killed larger diameter white spruce (Picea glauca), thereby altering forest structure and increasing the dominance of black spruce in the region. Alders (Alnus sp.) and crowberry (Empetrum nigrum) were more abundant in areas with heavy spruce mortality, possibly a response to the death of overstory spruce. Grasses and herbaceous plants did not proliferate as has been recorded following outbreaks in more coastal Alaskan forests. Two species closely tied to coniferous habitats, the tree-nesting Ruby-crowned Kinglet (Regulus calendula) and the red squirrel (Tamiasciurus hudsonicus), a major nest predator, were less abundant in forest stands with high spruce mortality than in low-mortality stands. Understory-nesting bir...

Reproductive Success of Bald Eagles in Interior Alaska

We compared productivity and nesting success of 2 adjacent populations of bald eagles (Haliaeetus leztcocephalus) near the northern limits of their range in interior Alaska during 1989-94. Productivity (f 2 SE young fledgedloccupied territory) and nesting success differed between populations; pairs in the Gulkana River basin had higher productivity (0.86 2 0.05, n = 274) and nesting success (59%) than those in the Copper River basin (0.71 2 0.04, 48%, n = 471; P < 0.02). Productivity varied both annually and spatially within each basin (P < 0.001). However. brood sizes of successful nests were identical for both basins (1.48 f 0.03). suggesting that variability in productivity resulted largely from differences in nesting success. Patterns of vari- ability in reproductive success within a territory also were similar for both populations. Pairs that were suc- cessful one year fledged more offspring, were more likely to be successful, were more likely to reoccupy the same territory, and were less likely to change nest locations the following year compared to pairs that were unsuccessful the previous year (P < 0.025 for all comparisons). Most nesting failure (92%) occurred during incubation when weather conmtions tend to be most severe. However, reproductive success was not negatively correlated with severity of spring weather (temp or rainfall) or strongly correlated with prey abundance during brood rearing. We hypothesize that annual and spatial variability in reproductive success of these northern bald eagle populations may be associated with variation in prey availability, especially before and during incubation. J. WILDL. MANAGE. 61(4):1313-1321