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Gregory P Asner

Publications and source records attributed to Gregory P Asner.

10 recordsLinked to original sources
PubMed2006

Genetic variation in leaf pigment, optical and photosynthetic function among diverse phenotypes of Metrosideros polymorpha grown in a common garden.

Coordinated variation has been reported for leaf structure, composition and function, across and within species, and theoretically should occur across populations of a species that span an extensive environmental range. We focused on Hawaiian keystone tree species Metrosideros polymorpha, specifically, 13-year old trees grown (2-4 m tall) in a common garden (approximately 1 ha field with 2-3 m between trees) from seeds collected from 14 populations along an altitude-soil age gradient. We determined the genetic component of relationships among specific leaf area (SLA), the concentrations of nitrogen (N) and pigments (chlorophylls, carotenoids, and anthocyanins), and photosynthetic light-use efficiency. These traits showed strong ecotypic variation; SLA declined 35% with increasing source elevation, and area-based concentrations of N, Chl a + b and Car increased by 50, 109 and 96%, respectively. Concentrations expressed on a mass basis were not well related to source elevation. Pigment ratios expressed covariation that suggested an increased capacity for light harvesting at higher source elevation; Chl/N and Car/Chl increased with source elevation, whereas Chl a/b declined; Chl a/b was higher for populations on younger soil, suggesting optimization for low N supply. Parallel trends were found for the photosynthetic reactions; light-saturated quantum yield of photosystem II (Phi (PSII)) and electron transport rate (ETR) increased with source elevation. Correlations of the concentrations of photosynthetic pigments, pigment ratios, and photosynthetic function across the ecotypes indicated a stoichiometric coordination of the components of the light-harvesting antennae and reaction centers. The constellation of coordinated morphological, biochemical and physiological properties was expressed in the leaf reflectance and transmittance properties in the visible and near-infrared wavelength region (400-950 nm), providing an integrated metric of leaf status among and between plant phenotypes.

Altitude↗
PubMed2006

Condition and fate of logged forests in the Brazilian Amazon.

The long-term viability of a forest industry in the Amazon region of Brazil depends on the maintenance of adequate timber volume and growth in healthy forests. Using extensive high-resolution satellite analyses, we studied the forest damage caused by recent logging operations and the likelihood that logged forests would be cleared within 4 years after timber harvest. Across 2,030,637 km2 of the Brazilian Amazon from 1999 to 2004, at least 76% of all harvest practices resulted in high levels of canopy damage sufficient to leave forests susceptible to drought and fire. We found that 16+/-1% of selectively logged areas were deforested within 1 year of logging, with a subsequent annual deforestation rate of 5.4% for 4 years after timber harvests. Nearly all logging occurred within 25 km of main roads, and within that area, the probability of deforestation for a logged forest was up to four times greater than for unlogged forests. In combination, our results show that logging in the Brazilian Amazon is dominated by highly damaging operations, often followed rapidly by deforestation decades before forests can recover sufficiently to produce timber for a second harvest. Under the management regimes in effect at the time of our study in the Brazilian Amazon, selective logging would not be sustained.

Biomass↗
PubMed2006

Recovery of forest structure and spectral properties after selective logging in lowland Bolivia.

Effective monitoring of selective logging from remotely sensed data requires an understanding of the spatial and temporal thresholds that constrain the utility of those data, as well as the structural and ecological characteristics of forest disturbances that are responsible for those constraints. Here we assess those thresholds and characteristics within the context of selective logging in the Bolivian Amazon. Our study combined field measurements of the spatial and temporal dynamics of felling gaps and skid trails ranging from <1 to 19 months following reduced-impact logging in a forest in lowland Bolivia with remote-sensing measurements from simultaneous monthly ASTER satellite overpasses. A probabilistic spectral mixture model (AutoMCU) was used to derive per-pixel fractional cover estimates of photosynthetic vegetation (PV), non-photosynthetic vegetation (NPV), and soil. Results were compared with the normalized difference in vegetation index (NDVI). The forest studied had considerably lower basal area and harvest volumes than logged sites in the Brazilian Amazon where similar remote-sensing analyses have been performed. Nonetheless, individual felling-gap area was positively correlated with canopy openness, percentage liana coverage, rates of vegetation regrowth, and height of remnant NPV. Both liana growth and NPV occurred primarily in the crown zone of the felling gap, whereas exposed soil was limited to the trunk zone of the gap. In felling gaps >400 m2, NDVI, and the PV and NPV fractions, were distinguishable from unlogged forest values for up to six months after logging; felling gaps <400 m2 were distinguishable for up to three months after harvest, but we were entirely unable to distinguish skid trails from our analysis of the spectral data.

Bolivia↗
PubMed2005

Selective logging in the Brazilian Amazon.

Amazon deforestation has been measured by remote sensing for three decades. In comparison, selective logging has been mostly invisible to satellites. We developed a large-scale, high-resolution, automated remote-sensing analysis of selective logging in the top five timber-producing states of the Brazilian Amazon. Logged areas ranged from 12,075 to 19,823 square kilometers per year (+/-14%) between 1999 and 2002, equivalent to 60 to 123% of previously reported deforestation area. Up to 1200 square kilometers per year of logging were observed on conservation lands. Each year, 27 million to 50 million cubic meters of wood were extracted, and a gross flux of approximately 0.1 billion metric tons of carbon was destined for release to the atmosphere by logging.

Brazil↗
PubMed2005

Ground-based and remotely sensed nutrient availability across a tropical landscape.

Tropical soils often are assumed to be highly weathered and thus nutrient-depleted, but this prediction applies primarily to geomorphically stable surfaces. Topography complicates the assumption of nutrient depletion, because erosion can enhance the supply of nutrients to tropical ecosystems. Consequently, understanding nutrient availability across landscapes requires a spatially explicit assessment of the relative strength of depletion and enhancement. We document the relationship between foliar nutrients and topographic position across a 20-km(2), 4- to 5-million-year-old eroded landscape in Kaua'i, Hawai'i, and use this relationship to build a bottom-up map of predicted nutrient availability across this landscape. Only approximately 17% of the landscape is nutrient-poor, mostly on stable uplands; nutrient availability on slopes and valley bottoms is much higher, in some cases similar to the most fertile montane forests in the Hawaiian Islands. This pattern was corroborated by top-down remote sensing of area-integrated canopy phosphorus concentrations.

Ecosystem↗
PubMed2005

Global consequences of land use.

Land use has generally been considered a local environmental issue, but it is becoming a force of global importance. Worldwide changes to forests, farmlands, waterways, and air are being driven by the need to provide food, fiber, water, and shelter to more than six billion people. Global croplands, pastures, plantations, and urban areas have expanded in recent decades, accompanied by large increases in energy, water, and fertilizer consumption, along with considerable losses of biodiversity. Such changes in land use have enabled humans to appropriate an increasing share of the planet's resources, but they also potentially undermine the capacity of ecosystems to sustain food production, maintain freshwater and forest resources, regulate climate and air quality, and ameliorate infectious diseases. We face the challenge of managing trade-offs between immediate human needs and maintaining the capacity of the biosphere to provide goods and services in the long term.

Agriculture↗
PubMed2005

Remote analysis of biological invasion and biogeochemical change.

We used airborne imaging spectroscopy and photon transport modeling to determine how biological invasion altered the chemistry of forest canopies across a Hawaiian montane rain forest landscape. The nitrogen-fixing tree Myrica faya doubled canopy nitrogen concentrations and water content as it replaced native forest, whereas the understory herb Hedychium gardnerianum reduced nitrogen concentrations in the forest overstory and substantially increased aboveground water content. This remote sensing approach indicates the geographic extent, intensity, and biogeochemical impacts of two distinct invaders; its wider application could enhance the role of remote sensing in ecosystem analysis and management.

Biodiversity↗
PubMed2004

Drought stress and carbon uptake in an Amazon forest measured with spaceborne imaging spectroscopy.

Amazônia contains vast stores of carbon in high-diversity ecosystems, yet this region undergoes major changes in precipitation affecting land use, carbon dynamics, and climate. The extent and structural complexity of Amazon forests impedes ground studies of ecosystem functions such as net primary production (NPP), water cycling, and carbon sequestration. Traditional modeling and remote-sensing approaches are not well suited to tropical forest studies, because (i) biophysical mechanisms determining drought effects on canopy water and carbon dynamics are poorly known, and (ii) remote-sensing metrics of canopy greenness may be insensitive to small changes in leaf area accompanying drought. New spaceborne imaging spectroscopy may detect drought stress in tropical forests, helping to monitor forest physiology and constrain carbon models. We combined a forest drought experiment in Amazônia with spaceborne imaging spectrometer measurements of this area. With field data on rainfall, soil water, and leaf and canopy responses, we tested whether spaceborne hyperspectral observations quantify differences in canopy water and NPP resulting from drought stress. We found that hyperspectral metrics of canopy water content and light-use efficiency are highly sensitive to drought. Using these observations, forest NPP was estimated with greater sensitivity to drought conditions than with traditional combinations of modeling, remote-sensing, and field measurements. Spaceborne imaging spectroscopy will increase the accuracy of ecological studies in humid tropical forests.

Brazil↗
PubMed2003

Plants reverse warming effect on ecosystem water balance.

Models predict that global warming may increase aridity in water-limited ecosystems by accelerating evapotranspiration. We show that interactions between warming and the dominant biota in a grassland ecosystem produced the reverse effect. In a 2-year field experiment, simulated warming increased spring soil moisture by 5-10% under both ambient and elevated CO2. Warming also accelerated the decline of canopy greenness (normalized difference vegetation index) each spring by 11-17% by inducing earlier plant senescence. Lower transpirational water losses resulting from this earlier senescence provide a mechanism for the unexpected rise in soil moisture. Our findings illustrate the potential for organism-environment interactions to modify the direction as well as the magnitude of global change effects on ecosystem functioning.

Carbon Dioxide↗