Bruce Walker Nelson

Improved allometric models to estimate the aboveground biomass of tropical trees

Terrestrial carbon stock mapping is important for the successful implementation of climate change mitigation policies. Its accuracy depends on the availability of reliable allometric models to infer oven-dry aboveground biomass of trees from census data. The degree of uncertainty associated with previously published pantropical aboveground biomass allometries is large. We analyzed a global database of directly harvested trees at 58 sites, spanning a wide range of climatic conditions and vegetation types (4004 trees ≥ 5 cm trunk diameter). When trunk diameter, total tree height, and wood specific gravity were included in the aboveground biomass model as covariates, a single model was found to hold across tropical vegetation types, with no detectable effect of region or environmental factors. The mean percent bias and variance of this model was only slightly higher than that of locally fitted models. Wood specific gravity was an important predictor of aboveground biomass, especially when including a much broader range of vegetation types than previous studies. The generic tree diameter-height relationship depended linearly on a bioclimatic stress variable E, which compounds indices of temperature variability, precipitation variability, and drought intensity. For cases in which total tree height is unavailable for aboveground biomass estimation, a pantropical model incorporating wood density, trunk diameter, and the variable E outperformed previously published models without height. However, to minimize bias, the development of locally derived diameter-height relationships is advised whenever possible. Both new allometric models should contribute to improve the accuracy of biomass assessment protocols in tropical vegetation types, and to advancing our understanding of architectural and evolutionary constraints on woody plant development.

Natural forest disturbance and change in the Brazilian Amazon

Abstract Landsat Thematic Mapper images covering the entire 3.9 million km2 of forested Brazilian Amazon reveal natural change and disturbance occurring on a scale of decades to centuries. These include: 92,000 km2 of bamboo forests undergoing synchronous mortality and regrowth; 1500 km2 of recently active dune fields; 900 km2 of recent downburst blowdowns; > 500 km of recent forest fire scars; and an unknown area of forest mortality from flooding of high ground and alluvial forests. Fire subclimax fern savannas created by Yanoama Indians cover an additional 600 km2. Natural and indigenous disturbances and synchronized phenologies are therefore responsible for a dynamic spectral bahavior in large portions of the Amazon Basin. Disturbances and disturbance indicators not easily detected include blowdown sites > 30 yrs old, blowdowns < 30 hectares in size, liana forests and babassu palm forests.

Leaf development and demography explain photosynthetic seasonality in Amazon evergreen forests.

Leaf seasonality in Amazon forests Models assume that lower precipitation in tropical forests means less plant-available water and less photosynthesis. Direct measurements in the Amazon, however, show that production remains constant or increases in the dry season. To investigate this mismatch, Wu et al. use tower-based cameras to detect the phenology (i.e., the seasonal patterns) of leaf dynamics in tropical tree crowns in Amazonia, Brazil, and relate this to patterns of CO2 flux. Accounting for age-dependent variation among individual leaves and crowns is necessary for understanding the seasonal dynamics of photosynthesis in the entire ecosystem. Leaf phenology regulates seasonality of the carbon flux in tropical forests across a gradient of climate zones. Science, this issue p. 972 Camera recordings of the age distribution of leaves coupled with carbon dioxide flux data show the phenological basis of photosynthesis. In evergreen tropical forests, the extent, magnitude, and controls on photosynthetic seasonality are poorly resolved and inadequately represented in Earth system models. Combining camera observations with ecosystem carbon dioxide fluxes at forests across rainfall gradients in Amazônia, we show that aggregate canopy phenology, not seasonality of climate drivers, is the primary cause of photosynthetic seasonality in these forests. Specifically, synchronization of new leaf growth with dry season litterfall shifts canopy composition toward younger, more light-use efficient leaves, explaining large seasonal increases (~27%) in ecosystem photosynthesis. Coordinated leaf development and demography thus reconcile seemingly disparate observations at different scales and indicate that accounting for leaf-level phenology is critical for accurately simulating ecosystem-scale responses to climate change.

Size and frequency of natural forest disturbances and the Amazon forest carbon balance

Forest inventory studies in the Amazon indicate a large terrestrial carbon sink. However, field plots may fail to represent forest mortality processes at landscape-scales of tropical forests. Here we characterize the frequency distribution of disturbance events in natural forests from 0.01 ha to 2,651 ha size throughout Amazonia using a novel combination of forest inventory, airborne lidar and satellite remote sensing data. We find that small-scale mortality events are responsible for aboveground biomass losses of ~1.7 Pg C y−1 over the entire Amazon region. We also find that intermediate-scale disturbances account for losses of ~0.2 Pg C y−1, and that the largest-scale disturbances as a result of blow-downs only account for losses of ~0.004 Pg C y−1. Simulation of growth and mortality indicates that even when all carbon losses from intermediate and large-scale disturbances are considered, these are outweighed by the net biomass accumulation by tree growth, supporting the inference of an Amazon carbon sink.

Sediments of the Northern Arabian Sea.

Abstract Topographically the northern Arabian Sea can be divided into two basins separated by the northeast-southwest trending Murray Ridge. Northwest of the ridge is the Oman Basin which exhibits the more irregular topography, particularly along the continental margin. The southeast or Arabian Sea Basin is smoother because it is completely dominated by the large sediment cone of the Indus River. The nature of the sediments in the two basins differs in several respects including clay mineralogy. Terrigenous sedimentation is the dominant process throughout the entire area due to the close proximity of land on three sides. Besides fluvial material, aeolian derived sediment is of considerable importance and is relatively most important in the Oman Basin.

Bamboo-dominated forests of the southwest Amazon: detection, spatial extent, life cycle length and flowering waves.

We map the extent, infer the life-cycle length and describe spatial and temporal patterns of flowering of sarmentose bamboos (Guadua spp) in upland forests of the southwest Amazon. We first examine the spectra and the spectral separation of forests with different bamboo life stages. False-color composites from orbital sensors going back to 1975 are capable of distinguishing life stages. These woody bamboos flower produce massive quantities of seeds and then die. Life stage is synchronized, forming a single cohort within each population. Bamboo dominates at least 161,500 km2 of forest, coincident with an area of recent or ongoing tectonic uplift, rapid mechanical erosion and poorly drained soils rich in exchangeable cations. Each bamboo population is confined to a single spatially continuous patch or to a core patch with small outliers. Using spatial congruence between pairs of mature-stage maps from different years, we estimate an average life cycle of 27–28 y. It is now possible to predict exactly where and approximately when new bamboo mortality events will occur. We also map 74 bamboo populations that flowered between 2001 and 2008 over the entire domain of bamboo-dominated forest. Population size averaged 330 km2. Flowering events of these populations are temporally and/or spatially separated, restricting or preventing gene exchange. Nonetheless, adjacent populations flower closer in time than expected by chance, forming flowering waves. This may be a consequence of allochronic divergence from fewer ancestral populations and suggests a long history of widespread bamboo in the southwest Amazon.

Sedimentary Phosphate Method for Estimating Paleosalinities

The widespread occurrence of sedimentary phosphate in argillaceous sediments provides the basis for a new method of paleosalinity estimation. Sedimentary phosphate contains iron- and calcium-phosphate fractions, the relative proportions of which are sensitive to salinity of the water at sites of deposition. The sedimentary phosphate method provides direct estimates of paleosalinity throughout the freshwater to marine range.

Fire favours expansion of bamboo-dominated forests in the south-west Amazon

Forests dominated by semi-scandent woody bamboos of the genus Guadua cover about 165 000 km 2 of the south-west Amazon. Because many woody bamboo species are favoured by disturbance some authors have inferred this landscape to be a consequence of indigenous or natural disturbance. As seen in satellite images, the rounded edges of some bamboo-dominated forests indicate expansion into surrounding forest. These edges are unrelated to topography and resemble the borders of ground fires in unlogged Amazon forests, suggesting that bamboo may have been favoured by past fires. We studied the recovery of Guadua sarcocarpa and its competitors in the face of simulated fire by cutting all plant stems at ground level in ten 100-m 2 plots, compared with ten control plots, and by burning a 2500-m 2 plot. In the clear-cuts, bamboos recovered more successfully than did palms and dicots, by two measures: biomass accumulated and per cent recovery of pre-disturbance biomass. Resprouted bamboo attained higher stem densities than in control sites at 11 mo. In the burn plot, bamboo basal area recovered to pre-burn levels after 2 y and approached that of an undisturbed control area after 3 y. Though other natural disturbances are relevant, we conclude that forest fires should favour the spread and dominance of Guadua species in the south-west Amazon.

Fire disturbance in Amazonian blackwater floodplain forests

Background: Large fire scars were detected in floodplain forests of the middle Rio Negro in dry years of the 1990s, using satellite data. Aim: To relate fire years and river level anomalies to the Tropical Pacific Southern Oscillation Index (SOI); to measure fire damage and post-fire succession rate. Methods: We analysed the relationship between the SOI and water levels for 1968–2010. In Landsat images of the 1990s we determined fire scar ages. Using QuickBird images, we measured forest cover loss in 36 of these scars, covering 873 ha. This was validated by field measurements of tree mortality in 15 scars. As a metric of post-fire succession, we compared change in the Enhanced Vegetation Index (EVI) in 10 large floodplain fire scars and 12 terra firme slash-and-burn fallows. Results: SOI explained 32% of the variation in annual low water level. Forest cover loss in the 36 burn scars was 88% ± 8% (mean ± SD), range 67–98%. Post-fire tree mortality was 91%, ±8%, range 75–100%. Correlation between cover loss and mortality was 73% (P < 0.002). Forest recovery was very slow. EVI values typical of bare soil were still present 13 years after the fires, indicating a successional stage similar to the first year in terra firme sites. Conclusions: Results suggest a very low resistance and resilience of blackwater floodplain forests to fire disturbance associated with drought during El Niño events.

Repeated fires trap Amazonian blackwater floodplains in an open vegetation state

Climate change may increase the occurrence of droughts and fires in the Amazon. Most of our understanding on how fire affects tropical ecosystems is based on studies of non-flooded forest–savanna ecotones. Nonetheless, tropical floodplain forests in the Amazon can burn severely during extreme droughts. The mechanisms slowing down forest regeneration in these ecosystems remain poorly understood and have never been assessed in the field. We studied the recovery of Amazonian blackwater floodplain forests after one and two fire events. We used Landsat images to map fire history and conducted field surveys to measure forest structure, tree species richness, tree seed bank and post-fire invasion of herbaceous plants. Sites burnt once had on average 0% trees, 6% tree seed abundance, 23% tree seed species richness and 8% root mat thickness compared to unburnt forests. In contrast, herbaceous cover increased from 0 to 72%. Nonetheless, forest structure and diversity recovered slowly towards pre-burn levels, except for tree seed banks that remained depleted even 15 years after fire. Sites burnt twice had on average 0% trees, 1% tree seed abundance, 3% tree seed species richness and 1% root mat thickness compared to unburnt forests. Herbaceous cover increased to 100%. Mean recovery of tree basal area was 50% slower and of root mat thickness 93% slower compared to recovery in sites burnt once. Tree seed banks did not recover at all, and herbaceous cover persisted close to 100% for more than 20 years after the second fire. Synthesis and applications. Our results indicate that after a second fire event, Amazonian blackwater floodplain forests lose their recovery capacity, and persist in a non-forested state dominated by herbaceous vegetation. Such fragility implies that preventing human ignited fires during drought episodes is a particularly important conservation strategy for these ecosystems.