Robert L. Sanford

Litter Decomposition on the Mauna Loa Environmental Matrix, Hawai'i: Patterns, Mechanisms, and Models

We determined controls on litter decomposition and nutrient release for the widespread native tree Metrosideros polymorpha in 11 sites arrayed on gradients of elevation, precipitation, and substrate age on Hawaiian lava flows. The effects of site characteristics were evaluated using three common substrates (Metrosideros leaf litter from one of the sites, wood dowels, and filter paper) decomposed in each of the sites, and the inherent decomposability of tissue (substrate quality) was evaluated using Metrosideros leaf litter from each of the sites decomposed in a common site. Site characteristics were responsible for most of the variation in rates of decomposition in the range of sites and substrates examined. Common substrates decomposed much more rapidly in warm, low elevation sites; apparent Q10 values, calculated on the basis of variation in mean annual temperature with elevation on individual lava flows, ranged from 4 to 11. Litter decomposed slowly in the dry sites, but leaf litter produced in the dry sites decomposed more than twice as rapidly as litter from wet sites when both were measured in the same site. The higher substrate quality of litter from dry sites could be due to trade—offs among nutrient—use efficiency, water—use efficiency, and carbon gain by water—limited Metrosideros. We used these results to test a revision of the CENTURY soil organic matter model that had been designed to simulate the decomposition of surface litter. Simulations accurately matched the pattern but underestimated the magnitude of among—site differences in the decomposition of common substrates in a range of sites. Analyses of both field and simulation results suggested that the decomposition of Metrosideros leaf litter could be limited by nitrogen availability.

Amazon Rain-Forest Fires

Charcoal is common in the soils of mature rain forests within 75 kilometers of San Carlos de Rio Negro in the north central Amazon Basin. Carbon-14 dates of soil charcoal from this region indicate that numerous fires have occurred since the mid-Holocene epoch. Charcoal is most common in tierra firme forest Oxisols and Ultisols and less common in caatinga and igapo forest soils. Climatic changes or human activities, or both, have caused rain-forest fires.

The effects of slash burning on ecosystem nutrients during the land preparation phase of shifting cultivation

The most commonly observed change in soil following slash-and-burn clearing of tropical forest is a short-term increase in nutrient availability. Studies of shifting cultivation commonly cite the incorporation of nutrient-rich ash from consumed aboveground biomass into soil as the reason for this change. The effects of soil heating on nutrient availability have been examined only rarely in field studies of slash-and-burn, and soil heating as a mechanism of nutrient release is most often assumed to be of minor importance in the field. Few budgets for above and belowground nutrient flux have been developed in the tropics, and a survey of results from field and laboratory studies indicates that soils are sufficiently heated during most slash-and-burn events, particularly in dry and monsoonal climates, to cause significant, even substantial release of nutrients from non-plant-available into plant-available forms in soil. Conversely, large aboveground losses of nutrients during and after burning often result in low quantities of nutrients that are released to soil. Assessing the biophysical sustainability of an agricultural practice requires detailed information about nutrient flux and loss incurred during management. To this end, current conceptual models of shifting cultivation should be revised to more accurately describe these fluxes and losses.

Hurricane effects on soil organic matter dynamics and forest production in the Luquillo experimental forest, Puerto Rico : results of simulation modeling

The forests and soils at Luquillo Experimental Forest (LEF), Puerto Rico, are frequently disturbed by hurricanes occurring at various frequencies and intensities. We have derived a forest version of the Century soil organic matter model to examine the impact of hurricanes on soil nutrient availability and pool sizes, and forest productivity in the tabonuco forest at Luquillo. The model adequately predicted aboveground plant production, soil carbon, and soil nitrogen levels for forest conditions existing before Hurricane Hugo. Simulations of Hurricane Hugo and of an historical sequence of hurricanes indicated a complex pattern of recovery, especially for the first 10 yr after the hurricanes. After repeated hurricanes, forest biomass was reduced, while forest productivity was enhanced. Soil organic matter, and phosphorus and nitrogen mineralization stabilized at higher levels for the LEF than for hurricane-free tabonuco forest, and organic soil phosphorus was substantially increased by hurricanes. Results from these simulations should be regarded as hypotheses. At present there is insufficient data to validate the results of hurricane model simulations.

Holocene fires in Costa Rica

Charcoal fragments in soils and sediments document Holocene fires in the rain forests of the La Selva Biological Station in the northern Caribbean lowlands of Costa Rica, and in the paramo surrounding Cerro Chirrip6 in the Cordillera de Talamanca. Radiocarbon determinations on soil charcoal from La Selva and charcoal-rich lake sediment from Chirrip6 cluster at 2430 yr B.P. and at 1110-1180 yr B.P.; dates in each cluster are coeval, suggesting that the rain forest and paramo fires occurred at similar times. Fires at La Selva were likely set by human activity but may have spread into intact rain forest during exceptionally dry periods; fires at Chirrip6 were set by people or lightning during what may have been lower lake stands. The drought periods suggested by our charcoal samples may have been associated with short-term atmospheric anomalies such as El Niiio, or with longer-term shifts in climate.

Woody-tissue respiration for Simarouba amara and Minquartia guianensis, two tropical wet forest trees with different growth habits

We measured CO2 efflux from stems of two tropical wet forest trees, both found in the canopy, but with very different growth habits. The species were Simarouba amara, a fast-growing species associated with gaps in old-growth forest and abundant in secondary forest, and Minquartia guianensis, a slow-growing species tolerant of low-light conditions in old-growth forest. Per unit of bole surface, CO2 efflux averaged 1.24 μmol m−2 s−1 for Simarouba and 0.83 μmol m−2s−1 for Minquartia. CO2 efflux was highly correlated with annual wood production (r2=0.65), but only weakly correlated with stem diameter (r2=0.22). We also partitioned the CO2 efflux into the functional components of construction and maintenance respiration. Construction respiration was estimated from annual stem dry matter production and maintenance respiration by subtracting construction respiration from the instantaneous CO2 flux. Estimated maintenance respiration was linearly related to sapwood volume (39.6 μmol m−3s−1 at 24.6° C, r2=0.58), with no difference in the rate for the two species. Maintenance respiration per unit of sapwood volume for these tropical wet forest trees was roughly twice that of temperate conifers. A model combining construction and maintenance respiration estimated CO2 very well for these species (r2=0.85). For our sample, maintenance respiration was 54% of the total CO2 efflux for Simarouba and 82% for Minquartia. For our sample, sapwood volume averaged 23% of stem volume when weighted by tree size, or 40% with no size weighting. Using these fractions, and a published estimate of aboveground dry-matter production, we estimate the annual cost of woody tissue respiration for primary forest at La Selva to be 220 or 350 g C m−2 year−1, depending on the assumed sapwood volume. These costs are estimated to be less than 13% of the gross production for the forest.

Effects of harvest intensity, site preparation, and herbicide use on soil nitrogen transformations in a young loblolly pine plantation

Abstract The interactive effects of harvest intensity (whole-tree vs. stem-only), site preparation (chop and burn vs. shear-pile-disk), and silvicultural treatment (herbicide-pesticide vs. none) on N pools and transformations were determined in a young loblolly pine plantation on the North Carolina Piedmont. Harvest intensity had little effect on N transformations in years 3–5 post establishment, but shear-pile-disk site preparation and especially herbicide treatment caused increased net N mineralization and nitrification in the treated plots. Laboratory experiments with 15N label yielded reduced rates of N immobilization in the herbicide treated plots, demonstrating long-term consequences of treatment for soil N transformations.

Canopy openings in a primary neotropical lowland forest

Windthrown trees and large broken branches create a mosaic of light gaps in tropical forests. Viewed from above, these canopy openings are easily recognized and can be surveyed with aerial photos. Within canopy openings, many biological activities take place that occur rarely or not at all elsewhere in the forest near the forest floor. This is mainly a result of the greater amount of sunlight reaching the lowest levels of the canopy strata and frequently but ephemerally, the forest floor. This mosaic of forest structure was first recognized by Aubreville (1938). Recent research has focused on forest regeneration within canopy gaps in primary forest (Brokaw 1982, Hartshorn 1980). Canopy openings are critical for regeneration of many canopy tree species (Denslow 1980, Hartshorn 1978, Lang & Knight 1983, Richards 1952, Whitmore 1978). Nutrient availability and soil nutrient storage may be expected to change in canopy openings (Anderson & Swift 1983, Bazzaz 1983, Orians 1982). Large canopy openings provide one of the few rain forest habitats for many pioneer

Seasonally Dry Tropical Forests: Nutrient cycling in tropical deciduous forests

Introduction Nutrient cycling processes have been well documented for tropical moist forest (Vitousek & Sanford, 1986; Bruijnzeel, 1991) but few comprehensive syntheses exist for tropical and subtropical dry and deciduous forests (Lugo & Murphy, 1986; Singh, 1989). Tropical dry forests are considered among the most threatened tropical ecosystems (Janzen, 1988) because they experience considerable exploitative pressure (Murphy & Lugo, 1986). In India, such pressures have been responsible for the transformation of vast areas of deciduous forest into savanna (Singh, 1989). The current rate of destruction of deciduous forest makes it imperative that we gain a thorough understanding of nutrient cycling in the remaining intact and successional forests. At present one of the principal agricultural practices relies on forest slash burning, resulting not only in recurrent nutrient losses thereby affecting the long-term productivity of the system but also substantially contributing to emissions of C and N to the atmosphere (Kauffman, Sanford & Sampaio, 1990; Maass, Chapter 17). Seasonally dry forest production is controlled by the amount and distribution of annual rainfall (Martinez-Yrizar, Chapter 13), and this may explain why nutrients have not been considered in detail (Murphy & Lugo, 1986; Singh, 1989). For example in a recent review of forest nutrient cycling there was only one reference to tropical dry forest (Vogt, Grier & Vogt, 1986). If we assume that water availability alone limits primary production in tropical deciduous forest, we could conclude that nutrient limitation is not important. However, multiple resource limitation of plant growth is common in natural communities (Chapin et al., 1987).

Effects of slash-and-burn management on soil aggregate organic C and N in a tropical deciduous forest

Our study examined the effect of slash-and-burn management on the distribution of soil organic carbon (SOC) across water-stable aggregate size fractions. Macroaggregates (>250 μm) are an important source of SOC and soil organic nitrogen in forest soil: they account for approximately 80% of the total C and N content. Slashing and burning did not destroy macroaggregates, but the SOC associated with macroaggregates decreased by 32% due to combustion during burning. Fire also disrupted soil aggregate stabilization by changing the chemical nature of SOC. The largest changes were noted after the first growing season following fire: macroaggregates and associated C decreased 50% while microaggregates increased by the same proportion. The changes in organic C observed after the first growing season can be attributed to macroaggregate instability.