Carl F. Jordan

SUCCESSION AND NUTRIENT DYNAMICS FOLLOWING FOREST CUTTING AND BURNING IN AMAZONIA

Plant establishment, plant mortality, vegetation productivity, nutrient accumulation, and nutrient leaching were studied during the first 5 yr of succession following the cutting and burning of a forest plot in the upper Rio Negro region of the Amazon Basin. The speed and vigor of forest regeneration following disturbance of this type provide a measure of the recuperative potential of this ecosystem. The study plot (0.09 ha) was dominated by forbs during the 1 st yr and then by the pioneer tree species Cecropiaficifolia Snethlage during year two. Tree mortality exceeded establishment during the 3rd yr because the C. ficifolia trees died nearly in unison. During the 4th yr, tree density increased sharply as successional and forest tree species grew in the space vacated by C. ficifolia. The canopy was dominated at that time by the pioneer species Vismia japurensis Reich. and Vismia lauriformis (Lam.) Choisy. By the 5th yr, all establishment space had apparently been preempted: there were only 0.07 new establishments/M2 during year five, while 95% of the trees present at the end of year four survived through the 5th yr. There were 56 tree species (? 2 m tall) present on the site after 5 yr. More than half of these were primary forest species. Primary forest species were most common in the understory. These trees averaged > 1 m in growth height each year and should eventually grow into the canopy. Hence, it appears that most individuals that will participate in the aggrading phase of forest development were already present on the site within 5 yr of burning. Biomass was only 66 g/m2 at the end of the 1 st yr, but g1000 g/m2 were added in each subsequent year. The standing crop of biomass (shoot and root) at 5 yr was 4840 g/m2, a value 16% that of the mature forest which occupied the site prior to cutting. Approximately 27% of the dead trunk and branch biomass from the pre-existing forest remained on the site after 5 yr. Litter production increased each year from 39 g/m2 (year one) to 825 g/m2 (year five) and tended to be inversely related to rainfall. Total aboveground production was highest for the 5th yr (1940 g/m2) and was probably close to a maximum for the site. Total live-plant nutrient stocks after 5 yr, as a percentage of the precut forest stocks, were 15% for N, 23% for P, 39% for K, 48% for Ca, and 45% for Mg. A large increase in the leachate concentrations of K, Mg, and NO3 occurred during the first 2 yr following burning. By the 5th yr, concentrations were not significantly different from those in the undisturbed forest. After 5 yr soil nutrient levels were also similar to those of the preburn forest. Overall, nutrient losses apparently were not great enough to destroy the sites ability to return eventually to a state similar to that which existed prior to disturbance. This does not mean that a more severe treatment, such as conversion of forest to pasture, would not affect the regrowth ability of a site.

Ecosystem Recovery in Amazon Caatinga Forest after Cutting, Cutting and Burning, and Bulldozer Clearing Treatments

A study of ecosystem recovery following forest cutting, forest cutting and burning, and forest clearing by bulldozing was conducted in Amazon caatinga forest in the upper Rio Negro Region of southern Venezuela. Ecosystem recovery was evaluated by measuring vegetation composition, biomass, nutrient accumulation, soil characteristics, and nutrient leaching. As disturbance increased in intensity, the early successional vegetation changed from primary forest trees (cut treatment) to successional woody species (cut and burned treatment) to forbs and grasses (bulldozed treatment). Soil nutrient levels were greater in both the cut and the cut and burned treatment plots than in the control forest three years after the disturbances presumably because of steady transfers of nutrients from the forest slash to the soil. Soil nutrient levels in the bulldozed plot were much lower than the control forest because of top soil removal. Aboveground biomass at three years was 1291 g m2 in the cut site, 870 g m2 in the cut and burned site, and 77 g m2 in the bulldozed site. Given these rates of biomass accumulation, approximately 100 yr will be required for both the cut and the cut and burned sites to reach biomass levels characteristic of mature caatinga forest, while more than 1000 yr will be necessary in the case of the bulldozed site.

Nitrogen conservation in a tropical rain forest

SummaryA series of plots in an Amazonian Rain Forest were trenched and treated with calcium to determine the effects of perturbation on numbers and activity of nitrifying bacteria. Although treatment resulted in between 2 and 26% of the nitrogen being lost from the humus layer, virtually all of it was in the NH4+form. Numbers of nitrifying bacteria in the plots were relatively low. The low numbers and low activity of nitrifying bacteria is attributed to the low pH and high concentration of tannins in the root mat. It is hypothesized that the suppression of nitrifying bacteria results in nitrogen conservation in the rain forest.

Soil phosphorus fractions, aluminum, and water retention as affected by microbial activity in an Ultisol

Increased organic matter input into weathered and infertile soils through agricultural techniques such as minimum tillage or agroforestry can improve P availability to crops. Organic matter is an energy source for microbes, and their activity may be responsible in part for increased levels of labile P. The objective of the work reported here was to examine, in a highly weathered Ultisol, the influence of microbial activity in mobilizing P, maintaining it in a plant-available state, and preventing its fixation, and the effect of N and biocides on these processes. Exchangeable aluminum and soil moisture were also determined, since they interact with microbes and soil P.Results showed that increased microbial activity reduced sorption of dissolved and organic P by soil, maintained inorganic P in soluble and labile pools, increased microbial P, decreased mineral P, increased exchangeable Al, and increased water retention. Additions of N and biocides had variable effects, probably due to complex interactions between N, degrading biocides, and microbial populations.

Assessment of a phosphorus fractionation method for soils: problems for further investigation

Abstract A sequential extraction method developed by Canadian researchers is described which allows the fractionation of biologically meaningful soil phosphorus (P) pools. The method was originally developed for use on neutral to mildly alkaline soils. Determination of microbial P as part of the fractionation scheme by the flush of NaHCO3-extractable P following CHCl3 fumigation is erratic in acid, highly weathered soils. This is explainable based on the large capacities of these soils to sorb P. Sequential extraction of both fumigated and unfumigated samples to determine the fate of P of microbial origin is an alternative. Sample processing can affect results. Incubation of previously air-dried soil reduces bioavailable (resin) P below that present in the air-dried soil. Grinding soil releases some microbial P. Although this is a valuable method, more research is needed on its application to a variety of soils.

Natural vs. plantation forests: A case study of land reclamation strategies for the humid tropics

Biomass and productivity were compared in two plantations and in one stand of natural regeneration on similar sites in a premontane moist forest region of Puerto Rico. While initial growth rates of plantation species were higher, after four decades productivity of the natural regeneration plots was equal to or greater than productivity of the plantations. For the first 44 years, aboveground biomass of natural regeneration increased at an average annual rate of 3.8t·ha−1·yr−1, but the last year of the study it was 14.7t·ha−1. Biomass increment of a pine plantation averaged between 8 and 10.5t·ha−1·yr−1 except for one year when the rate was much lower, possibly because of hurricane damage. A tropical hardwood plantation averaged close to 4t·ha−1·yr−1 for 41 years. It is suggested that in countries where funds for land reclamation are limited, intensive plantations may not always be the best strategy. Natural regeneration or shelterbelt plantations may be suitable alternatives.

The nitrogen cycle in a ‘Terra Firme’ rainforest on oxisol in the Amazon territory of Venezuela@@@Ciclo de nitrógeno de un bosque pluvial de Tierra Firme sobre oxisol en el Territorio Amazonas de Venezuela

Standing stocks and fluxes of nitrogen, including nitrogen fixation and denitrification, were measured in a tropical rainforest on Oxisol in the Amazon Territory of Venezuela. The standing stock of nitrogen was comparable to that of temperate forests, but was higher than that in an adjacent forest on Spodosol. Fluxes were higher than in forests in the temperate zone, but lower than in another tropical forest on more fertile soil. Even though nitrogen was abundant, this does not mean that nitrogen could not be limiting to agriculture if the forest is cleared and the land cultivated. The nitrogen fixing and nitrogen conserving mechanisms are dependent upon the structure of the undisturbed forest, and destruction of the forest would eventually decrease the input of nitrogen to the soil.ResumenLos contenidos y flujos de nitrógeno, incluyendo fijación de nitrógeno y denitrificación, se midieron en un bosque lluvioso tropical que crece sobre oxisoles en el Territorio Amazonas de Venezuela. El contenido de nitrógeno era comparable con el de bosques templados, pero era más elevado que en un bosque adyacente sobre spodosoles. Los flujos eran más elevados que en bosques de zonas templadas, pero más bajos que en otro bosque tropical de suelos más fértiles. A pesar de que el nitrógeno era abundante, esto no significa que este elemento no podría ser un factor limitante para la agricultura si el bosque se corta, y se cultiva la tierra. Los mecanismos de fijación y conservación de nitrógeno dependen de la estructura del bosque no perturbado, y la destrucción del bosque eventualmente disminuiría el suministro de nitrógeno al suelo.

Genetic Engineering, the Farm Crisis, and World Hunger

The US farm belt has been mired in recession. Between 1960 and 2000, the food price index for major commodities dropped almost in half, and as a result, farm income has declined (Halweil 2002). Overall conditions in the farm economy in early 2000 were largely a replay of the previous year. Markets for major commodities, particularly field crops, were very weak as supplies remained large (ERS 2000). Crop prices projected for the 2001–2005 period are at, or near, lows experienced over the previous 20 years (Womack et al. 2001). The cause is overproduction (Halweil 2002).

The nitrogen cycle in a ‘Terra Firme’ rainforest on oxisol in the Amazon territory of Venezuela

Standing stocks and fluxes of nitrogen, including nitrogen fixation and denitrification, were measured in a tropical rainforest on Oxisol in the Amazon Territory of Venezuela. The standing stock of nitrogen was comparable to that of temperate forests, but was higher than that in an adjacent forest on Spodosol. Fluxes were higher than in forests in the temperate zone, but lower than in another tropical forest on more fertile soil. Even though nitrogen was abundant, this does not mean that nitrogen could not be limiting to agriculture if the forest is cleared and the land cultivated. The nitrogen fixing and nitrogen conserving mechanisms are dependent upon the structure of the undisturbed forest, and destruction of the forest would eventually decrease the input of nitrogen to the soil.

Nitrogen Dynamics during Conversion of Primary Amazonian Rain Forest to Slash and Burn Agriculture

The standing stocks and fluxes of nitrogen were measured for four years in an experimental slash and burn agricultural site and control forest in the Amazon Basin of Venezuela. During the study, there was a decrease of about 15% in nitrogen in the system as a whole due to leaching, harvesting, and denitrification. However, nitrogen levels in the soil did not change, due to replenishment by nitrogen entering the soil from decomposing slash. Since nitrogen levels in the soil remained high, scarcity of nitrogen is probably not responsible for the observed decline in crop yields, and lack of nitrogen is not expected to affect secondary succession in the abandoned agricultural plot.