Leaf—area index of a forest can be measured by determining the ratio of light at 800 μm to that at 675 μm on the forest floor. It is based on the principle that leaves absorb relatively more red than infrared light, and therefore, the more leaves that are present in the canopy, the greater will be the ratio.

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 I 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/m 2 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 (~2m tall) present on the site after 5 yr. More than halfofthese were primary forest species. Primary forest species were most common in the understory. These trees averaged >I 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!m 2 at the end of the I st yr, but ""I 000 g!m 2 were added in each subsequent year. The standing crop of biomass (shoot and root) at 5 yr was 4840 g!m 2 , 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!m 2 (year one) to 825 g!m 2 (year five) and tended to be inversely related to rainfall. Total aboveground production was highest for the 5th yr (1940 g!m 2 ) 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% forK, 48% for Ca, and 45% for Mg.A large increase in the leachate concentrations of K, Mg, and N03 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 site's 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.

No abstract

Surface root mats on oxisols and spodosols near San Carlos de Rio Negro, Venezuela, were sprayed with (45)Ca and (32)P to simulate the addition of nutrients to the soil surface through leaching of decomposing litter, and through precipitation and throughfall. Collections from lysimeters placed below the root mat and humus layer showed that in all but 1 case, <0.1% of the radiotracers leached past the root–organic mat, and leaching stopped completely after 1 to 2 months. Through analysis of root mat samples, the radioisotopes were found to have been taken up and translocated by living roots. This is the first evidence supporting the ability of the root map on the surface of poor, depleted tropical soils to efficiently take up dissolved nutrients before they percolate down to mineral soil. Other studies have shown up that one of the uptake mechanisms is mycorrhizal fungi.

The Everglades in southern Florida, U.S.A., is a major focus of conservation activities. The freshwater wetlands of the Everglades do not have high species richness, and no species of threatened aquatic animals or plants live there. We have, however, identified a distinctive ecological feature of the Everglades that is threatened by canal construction, draining, and nutrient enrichment from agricultural runoff. Compared to values reported from other freshwater systems, standing stocks of periphyton in relatively undisturbed areas of the Everglades were unusually high, and standing stocks of invertebrates and fish were unusually low. Averaging data gathered from nine sites and five sampling periods spanning 1 year, we found that periphyton standing crop was 88.2 g/m2 (ash‐free dry mass), invertebrate standing stock was 0.64 g/m2 (dry mass), and fish standing stock was 1.2 g/m2 (dry mass of large and small species combined). We found that fish standing stocks were much higher in phosphorus‐enriched sites than in nearby reference sites but that invertebrate standing stocks were similar in enriched and reference sites. Our results support the notion that oligotrophy is at least partially responsible for the low standing stocks of fish, but they also suggest that species interactions and a paucity of deep‐water refugia are important. Anthropogenic eutrophication in Everglades marshes will lead to the loss of distinctive ecosystem features. A focus on species richness and “hot spots” of threatened species provides no basis for conservation of ecosystems like the Everglades. If oligotrophic ecosystems often have low species richness, they will be underrepresented in preservation networks based on some common criteria for establishing conservation priorities.