Geology topics

Thomas M. Powell

Publications and source records attributed to Thomas M. Powell.

4 recordsLinked to original sources

USGS2018

Drivers and mechanisms of tree mortality in moist tropical forests

Tree mortality rates appear to be increasing in moist tropical forests (MTFs) with significant carbon cycle consequences. Here, we review the state of knowledge regarding MTF tree mortality, create a conceptual framework with testable hypotheses regarding the drivers, mechanisms and interactions that may underlie increasing MTF mortality rates, and identify the next steps for improved understanding and reduced prediction. Increasing mortality rates are associated with rising temperature and vapor pressure deficit, liana abundance, drought, wind events, fire and, possibly, CO 2 fertilization‐induced increases in stand thinning or acceleration of trees reaching larger, more vulnerable heights. The majority of these mortality drivers may kill trees in part through carbon starvation and hydraulic failure. The relative importance of each driver is unknown. High species diversity may buffer MTFs against large‐scale mortality events, but recent and expected trends in mortality drivers give reason for concern regarding increasing mortality within MTFs. Models of tropical tree mortality are advancing the representation of hydraulics, carbon and demography, but require more empirical knowledge regarding the most common drivers and their subsequent mechanisms. We outline critical datasets and model developments required to test hypotheses regarding the underlying causes of increasing MTF mortality rates, and improve prediction of future mortality under climate change.

New Phytologist↗

USGS2010

Biological communities in San Francisco Bay track large‐scale climate forcing over the North Pacific

Long‐term observations show that fish and plankton populations in the ocean fluctuate in synchrony with large‐scale climate patterns, but similar evidence is lacking for estuaries because of shorter observational records. Marine fish and invertebrates have been sampled in San Francisco Bay since 1980 and exhibit large, unexplained population changes including record‐high abundances of common species after 1999. Our analysis shows that populations of demersal fish, crabs and shrimp covary with the Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO), both of which reversed signs in 1999. A time series model forced by the atmospheric driver of NPGO accounts for two‐thirds of the variability in the first principal component of species abundances, and generalized linear models forced by PDO and NPGO account for most of the annual variability of individual species. We infer that synchronous shifts in climate patterns and community variability in San Francisco Bay are related to changes in oceanic wind forcing that modify coastal currents, upwelling intensity, surface temperature, and their influence on recruitment of marine species that utilize estuaries as nursery habitat. Ecological forecasts of estuarine responses to climate change must therefore consider how altered patterns of atmospheric forcing across ocean basins influence coastal oceanography as well as watershed hydrology.

California↗

USGS1986

Phytoplankton spatial distribution in south San Francisco Bay: mesoscale and small-scale variability

Horizontal transects of surface salinity and in-vivo fluorescence indicate the existence of three distinct spatial regimes in South San Francisco Bay. A mid-Bay region of low phytoplankton biomass with little small-scale variance is bounded to the north and south by water masses having higher in-vivo fluorescence and enhanced small-scale variability. Autocorrelation analyses demonstrate that the length scale of phytoplankton patchiness is longest in the mid-Bay region. The persistent discontinuities of in-vivo fluorescence and salinity are associated with topographic features—a large shoal to the north and a constriction to the south. The three spatial regimes are consistent with measured zooplankton distributions, existing current meter data, estimated longitudinal transports, and numerical simulations of residual circulations that show one (and perhaps two) large-scale gyre(s) bounded by the northern shoal and southern constriction. Topographic features are the most important physical factors controlling mesoscale (~ 10 km) variability of phytoplankton in South San Francisco Bay. We speculate that vertical current shear and salinity stratification (and their effects upon turbulence and diffusion) control small-scale patchiness, but quantitative estimates are needed to determine the influence of large-scale (and local) phytoplankton growth and loss processes.

Book chapter↗

USGS1976

Numerical models of wind-driven circulation in lakes

The state-of-the-art of numerical modelling of large-scale wind-driven circulation in lakes is presented. The governing equations which describe this motion are discussed along with the appropriate numerical techniques necessary to solve them in lakes. The numerical models are categorized into three large primary groups: the layered models, the Ekman-type models, and the other three-dimensional models. Discussions and comparison of models are given and future research directions are suggested.

Applied Mathematical Modelling↗