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At least 1,081 records · Page 60Linked to original sources
USGS2022

The effects of cheatgrass invasion on US Great Basin carbon storage depend on interactions between plant community composition, precipitation seasonality, and soil climate regime

Annual-grass invasions are transforming desert ecosystems in ways that affect ecosystem carbon (C) balance, but previous studies do not agree on the pattern, magnitude and direction of changes. A recent meta-analysis of 41 articles and 386 sites concludes that invasion by annual grasses such as cheatgrass ( Bromus tectorum L) reduces C in biomass across the Great Basin (Nagy et al., 2021). Reanalysis reveals that whether cheatgrass affects biomass C stocks is not generalizable, but rather depends on the considerable variation in climate across the subject sites. Our analysis suggests that accurate Great Basin-scale estimates of cheatgrass effects on C balance are not yet possible. Addition of climate variables to the meta-analysis reveals that cheatgrass invasion (a) reduced C in above-ground biomass in relatively summer-wet sites but not in summer-dry sites, (b) increased surface soil C in sites with intermediate resistance and resilience classifications (R&R) but not in low R&R sites—that is, mesic/aridic soil climates and (c) did not affect deep soil C. Considering that cheatgrass has expanded most in relatively summer-dry sites and mesic/aridic sites, omission of climate factors leads to model overestimates of cheatgrass effects on C when extrapolating to larger areas. Estimates of cheatgrass effects on C would also be improved if the analysis considered that (a) perennial grasslands are a common community state in the Great Basin that have intermediary C relative to annual grasslands and sagebrush stands, that is the omission of perennial grasslands from analysis inflates the baseline C storage of uninvaded Great Basin ecosystems, and( b) cheatgrass does not often exist in stable monocultures and soil carbon can reflect current or recent presence of other species. Synthesis and applications . Invasions often reveal heterogeneity in ecosystem structure and function that is not otherwise evident, and the heterogeneity can influence estimation of the net impacts of the invaders. For cheatgrass and other invaders, we propose that formally accounting for the spatial variability of invasion on ecosystem functions will improve the estimation of their net effect on ecosystem C, and thus improve prospects for adjusting management practices to optimize C sequestration.

western United States↗
USGS2005

A space‐for‐time substitution reveals the long‐term decline in genotypic diversity of a widespread salt marsh plant, Spartina alterniflora, over a span of 1500 years

Clonal populations face a trade‐off between sexual recruitment and vegetative growth and, once established, may undergo continuous declines in genotypic diversity if their sexual recruits make poor competitors. The geological history of delta formation in the Lower Mississippi River Valley was used to age eight S. alterniflora marshes for use in a space‐for‐time substitution ranging over 1500 years, in order to determine the long‐term effects of clonal growth on genotypic diversity in natural populations. 2 We also predicted that highly heterozygous clones are competitively superior, leading to an increase in the overall level of genetic diversity as a marsh ages and/or to an increasingly positive relationship between clone size and individual heterozygosity, and that the clumping of ramets within clones will occur over increasingly large distances as populations age, while the clumping of genetically related clones will become less pronounced as intraclonal competition begins to obscure the initial effects of localized seedling recruitment. Using molecular markers to differentiate clones, we documented a decline in clonal richness at the rate of approximately 1% 100 years −1 that was accompanied for the first 300–500 years by an increase in the distance over which clumping of ramets within genets occurred. Older populations, in the 500–1500‐year range, showed evidence of clone fragmentation. The spatial clustering of kin was observed for only two marshes, and exhibited no clear relationship with marsh age. Whereas the overall level of genetic diversity was consistent among marshes and showed no clear relationship with marsh age, the relationship between heterozygosity and individual clone size became increasingly pronounced within older marshes. Our results suggest that under natural conditions S. alterniflora marshes will rarely reach ages sufficient for the loss of all clonal diversity, or for the effects of inbreeding and drift to pose a significant threat to population viability.

Journal of Ecology↗
USGS2009

The changing global carbon cycle: Linking plant-soil carbon dynamics to global consequences

Most current climate-carbon cycle models that include the terrestrial carbon (C) cycle are based on a model developed 40 years ago by Woodwell & Whittaker (1968) and omit advances in biogeochemical understanding since that time. Their model treats net C emissions from ecosystems as the balance between net primary production (NPP) and heterotrophic respiration (HR, i.e. primarily decomposition). Under conditions near steady state, geographic patterns of decomposition closely match those of NPP, and net C emissions are adequately described as a simple balance of NPP and HR (the Woodwell-Whittaker model). This close coupling between NPP and HR occurs largely because of tight coupling between C and N (nitrogen) cycles and because NPP constrains the food available to heterotrophs. Processes in addition to NPP and HR become important to understanding net C emissions from ecosystems under conditions of rapid changes in climate, hydrology, atmospheric CO 2 , land cover, species composition and/or N deposition. Inclusion of these processes in climate-C cycle models would improve their capacity to simulate recent and future climatic change. Processes that appear critical to soil C dynamics but warrant further research before incorporation into ecosystem models include below-ground C flux and its partitioning among roots, mycorrhizas and exudates; microbial community effects on C sequestration; and the effects of temperature and labile C on decomposition. The controls over and consequences of these processes are still unclear at the ecosystem scale. Carbon fluxes in addition to NPP and HR exert strong influences over the climate system under conditions of rapid change. These fluxes include methane release, wildfire, and lateral transfers of food and fibre among ecosystems. Water and energy exchanges are important complements to C cycle feedbacks to the climate system, particularly under non-steady-state conditions. An integrated understanding of multiple ecosystem-climate feedbacks provides a strong foundation for policies to mitigate climate change. Synthesis . Current climate systems models that include only NPP and HR are inadequate under conditions of rapid change. Many of the recent advances in biogeochemical understanding are sufficiently mature to substantially improve representation of ecosystem C dynamics in these models.

Journal of Ecology↗
USGS2012

Changes in avian and plant communities of aspen woodlands over 12 years after livestock removal in the northwestern Great Basin

Riparian and quaking aspen (Populus tremuloides) woodlands are centers of avian abundance and diversity in the western United States, but they have been affected adversely by land use practices, particularly livestock grazing. In 1990, cattle were removed from a 112,500-ha national wildlife refuge in southeastern Oregon. Thereafter, we monitored changes in vegetation and bird abundance in years 1–3 (phase 1) and 10–12 (phase 2) in 17 riparian and 9 snow-pocket aspen plots. On each 1.5-ha plot, we sampled vegetation in 6 transects. Three times during each breeding season, observers recorded all birds 50 m to each side of the plot's 150-m centerline for 25 minutes. We analyzed data with multivariate analysis of variance and paired t tests with p values adjusted for multiple comparisons. In both periods, riparian and snow-pocket aspen produced extensive regeneration of new shoots (x̄ = 2646 stems/ha and 7079 stems/ha, respectively). By phase 2, a 64% increase in medium-diameter trees in riparian stands indicated successful recruitment into the overstory, but this pattern was not seen in snow-pocket stands, where the density of trees was over 2 times greater. By phase 2 in riparian and snow-pocket stands, native forb cover had increased by 68% and 57%, respectively, mesic shrub cover had increased by 29% and 58%, and sagebrush cover had decreased by 24% and 31%. Total avian abundance increased by 33% and 39% in riparian and snow-pocket aspen, respectively, ground or understory nesters increased by 133% and 67% and overstory nesters increased by 34% and 33%. Similarly, ground or understory foragers increased by 25% and 32%, aerial foragers by 55% and 57%, and overstory foragers by 66% and 43%. We interpreted the substantial regeneration of aspen shoots, increased densities of riparian forbs and shrubs, and increased avian abundances as a multitrophic-level response to the total removal of livestock and as substantial movement toward recovery of biological integrity.

Aspen Woodlands;Great Basin↗
USGS2019

Fire severity and changing composition of forest understory plant communities

Questions Gradients of fire severity in dry conifer forests can be associated with variation in understory floristic composition. Recent work in dry conifer forests in California, USA, has suggested that more severely burned stands contain more thermophilic taxa (those associated with warmer and drier conditions), and that forest disturbance may therefore accelerate floristic shifts already underway due to climate change. However, it remains unknown how rapidly thermophilic taxa shifts occur following disturbance, how long such shifts are likely to persist, and how different thermophilic post‐disturbance communities are from pre‐disturbance communities. Location Colorado Front Range, USA. Methods We investigated these questions using a unique 15‐year vegetation plot dataset that captures pre‐ and post‐fire understory community composition across a gradient of fire severity in dry conifer forests, classifying taxa using the biogeographic affinity concept. Results Thermophilization (defined here as a decrease in the ratio of cool‐mesic taxa to warm‐xeric taxa, based on biogeographic affinity of paleobotanical lineages) was observed as early as one year post‐fire for all fire severity classes, but was stronger at sites that burned at higher severity. The ratio of cool‐mesic to warm‐xeric taxa recovered to pre‐fire levels within 10 years in stands that burned at low severity, but not in stands that burned at moderate or high severity. The process of thermophilization after high‐severity fire appears to be driven primarily by the gain of warm‐xeric taxa that were absent before the fire, but losses of cool‐mesic taxa, which did not return during the duration of the study, also played a role. Conclusions Decreases in canopy cover appear to be a main contributor to understory thermophilization. Fine‐scale heterogeneity in post‐fire forest structure is likely an important driver of floristic diversity, creating the microclimatic variation necessary to maintain floristic refugia for species mal‐adapted to increasingly warm and dry conditions.

Colorado↗
USGS2012

Response to comments on "Productivity is a poor predictor of plant species richness"

Pan et al. claim that our results actually support a strong linear positive relationship between productivity and richness, whereas Fridley et al. contend that the data support a strong humped relationship. These responses illustrate how preoccupation with bivariate patterns distracts from a deeper understanding of the multivariate mechanisms that control these important ecosystem properties.

Science↗
USGS1984

Disruption of the terrestrial plant ecosystem at the Cretaceous-Tertiary boundary, western interior

The palynologically defined Cretaceous-Tertiary boundary in the western interior of North America occurs at the top of an iridium-rich clay layer. The boundary is characterized by the abrupt disappearance of certain pollen species, immediately followed by a pronounced, geologically brief change in the ratio of fern spores to angiosperm pollen. The occurrence of these changes at two widely separated sites implies continentwide disruption of the terrestrial ecosystem, probably caused by a major catastrophic event at the end of the period.

North America western interior↗
USGS2016

Comment on "Worldwide evidence of a unimodal relationship between productivity and plant species richness"

Fraser et al . (Reports, 17 July 2015, p. 302) report a unimodal relationship between productivity and species richness at regional and global scales, which they contrast with the results of Adler et al . (Reports, 23 September 2011, p. 1750). However, both data sets, when analyzed correctly, show clearly and consistently that productivity is a poor predictor of local species richness.

Science↗
USGS1997

Paleobiotic and isotopic analysis of mollusks, fish and plants from core OL-92: Indicators for an open or closed lake system

Intervals of open versus closed lake systems for Pleistocene Owens Lake in California are suggested by a comparison of paleobiotic and isotopic evidence recovered from core samples of OL-92. Mollusks and fish were identified from 67 core samples, and their ecological requirements were noted. Carbon dioxide extractions for stable isotopes of 13 C and 18 O from aragonite of the molluscan shell material were obtained by standard procedures, and isotopic compositions were measured using a mass spectrometer. The values of 13 C and 18 O from sediment samples taken previously (Benson and Bischoff, 1993) were compared with 13 C and 18 O values from mollusk shells recovered from the core. These data indicate at least two times of open lake, very low salinity (or “fresh” water) episodes, which are in agreement with the interpretation of fish and mollusk paleoecology supporting open systems between 207 m and 208 m (ca. 450 ka to ca. 453 ka) and between 309 m and 313 m (ca. 765 ka to ca. 775 ka).

California↗
USGS2006

Recognizing the Albian-Cenomanian (OAE1d) sequence boundary using plant carbon isotopes: Dakota Formation, Western Interior Basin, USA

Analysis of bulk sedimentary organic matter and charcoal from an Albian-Cenomanian fluvial-estuarine succession (Dakota Formation) at Rose Creek Pit (RCP), Nebraska, reveals a negative excursion of ???3???, in late Albian strata. Overlying Cenomanian strata have ??13C values of -24???, to -23???, that are similar to pre-excursion values. The absence of an intervening positive excursion (as exists in marine records of the Albian-Cenomanian boundary) likely results from a depositional hiatus. The corresponding positive ??13C event and proposed depositional hiatus are concordant with a regionally identified sequence boundary in the Dakota Formation (D2), as well as a major regressive phase throughout the globe at the Albian-Cenomanian boundary. Data from RCP confirm suggestions that some positive carbon-isotope excursions in the geologic record are coincident with regressive sea-level phases. We estimate using isotopic correlation that the D2 sequence boundary at RCP was on the order of 0.5 m.y. in duration. Therefore, interpretations of isotopic events and associated environmental phenomena, such as oceanic anoxic events, in the shallow-marine and terrestrial record may be influenced by stratigraphic incompleteness. Further investigation of terrestrial ??13C records may be useful in recognizing and constraining sea-level changes in the geologic record. ?? 2006 Geological Society of America.

Geology↗
USGS2011

Hyperspectral remote sensing tools for quantifying plant litter and invasive species in arid ecosystems

Green vegetation can be monitored and distinguished using visible and infrared multiband and hyperspectral remote sensing methods. The problem has been in identifying and distinguishing the nonphotosynthetically active radiation (PAR) landscape components, such as litters and soils, from green vegetation [35-38]. Additionally, distinguishing different species of green vegetation is challenging using the relatively few bands available on most satellite sensors. This chapter focuses both on previously published work by Nagler et al. [35-38] that identified hyperspectral remote sensing characteristics that distinguish between green vegetation, soil, and litter (or senescent vegetation), and on new research conducted to aid in distinguishing invasive species from the mixed landcover surface.

Book chapter↗
USGS2011

Estimating temporal trend in the presence of spatial complexity: A Bayesian hierarchical model for a wetland plant population undergoing restoration

Monitoring programs that evaluate restoration and inform adaptive management are important for addressing environmental degradation. These efforts may be well served by spatially explicit hierarchical approaches to modeling because of unavoidable spatial structure inherited from past land use patterns and other factors. We developed Bayesian hierarchical models to estimate trends from annual density counts observed in a spatially structured wetland forb (Camassia quamash [camas]) population following the cessation of grazing and mowing on the study area, and in a separate reference population of camas. The restoration site was bisected by roads and drainage ditches, resulting in distinct subpopulations ("zones") with different land use histories. We modeled this spatial structure by fitting zone-specific intercepts and slopes. We allowed spatial covariance parameters in the model to vary by zone, as in stratified kriging, accommodating anisotropy and improving computation and biological interpretation. Trend estimates provided evidence of a positive effect of passive restoration, and the strength of evidence was influenced by the amount of spatial structure in the model. Allowing trends to vary among zones and accounting for topographic heterogeneity increased precision of trend estimates. Accounting for spatial autocorrelation shifted parameter coefficients in ways that varied among zones depending on strength of statistical shrinkage, autocorrelation and topographic heterogeneity-a phenomenon not widely described. Spatially explicit estimates of trend from hierarchical models will generally be more useful to land managers than pooled regional estimates and provide more realistic assessments of uncertainty. The ability to grapple with historical contingency is an appealing benefit of this approach.

PLoS ONE↗