Variation in wood density across South American tropical forests.

Wood density is a critical control on tree biomass, so poor understanding of its spatial variation can lead to large and systematic errors in forest biomass estimates and carbon maps. The need to understand how and why wood density varies is especially critical in tropical America where forests have exceptional species diversity and spatial turnover in composition. As tree identity and forest composition are challenging to estimate remotely, ground surveys are essential to know the wood density of trees, whether measured directly or inferred from their identity. Here, we assemble an extensive dataset of variation in wood density across the most forested and tree-diverse continent, examine how it relates to spatial and environmental variables, and use these relationships to predict spatial variation in wood density over tropical and sub-tropical South America. Our analysis refines previously identified east-west Amazon gradients in wood density, improves them by revealing fine-scale variation, and extends predictions into Andean, dry, and Atlantic forests. The results halve biomass prediction errors compared to a naïve scenario with no knowledge of spatial variation in wood density. Our findings will help improve remote sensing-based estimates of aboveground biomass carbon stocks across tropical South America

Extraction of nitrate reductase from members of the South African Proteaceae

The inhibition of in vitro nitrate reductase activity in extracts of Protea spp. is shown to be due to polyphenolic constituents of the roots and shoots of the plants (which can be adsorbed by insoluble polyvinylpyrrolidone) rather than to the activity of endogenous proteases. The in vitro nitrate reductase activity in shoots of Protea repens and Protea cynaroides fed 2mmoldm−3 NO3− for 24h prior to nitrate reductase extraction show a nitrate reductase activity of 2–4μmol NO2− h−1 while the roots of P. repens show an in vitro nitrate reductase activity of 0,2μmol NO2− h−1 (g fresh mass)−1. The low nitrate reductase activity of these plants possibly reflects their adaptation to growth under the low nutrient condition of the soils of the South Western Cape, South Africa

The Influence of nutrient nitrogen source on the growth and productivity of sunflower (Helianthus annuus var. Dwarf Sungold)

Helianthus annuus L. plants were germinated and grown under environmentally controlled conditions in Long Ashton nutrient medium using nitrogen supplied in the form of 2mM nitrate, 2mM ammonium or 1mM nitrate+1mM ammonium and harvested after either 4 or 10 weeks of growth. Ammonium oxidation was prevented by the inclusion of nitrapyrin to the nutrient solutions. After 4 weeks of growth the mixed feed plants had 18% greater leaf area than the nitrate-only and 23% greater leaf area than the ammonium-only fed plants. Both the 4- and 10-week-old mixed-feed plants had greater fresh and dry mass, lower transpiration rates and greater water use efficiency than plants of either of the other two nutrient regimes. The fruit mass for the mixed feed plants was 25% greater than the nitrate-only and more than double that of the ammonium-only fed plants. The bound nitrogen content of both the nitrate-only and mixed-feed plants was over 30% greater than that of the ammonium-only fed plants, although individual seed nitrogen concentration was the same. Combined ammonium+nitrate is therefore considered to be a more efficient nitrogen source for sunflower production than either nitrate or ammonium fed singly

Atmospheric input of nitrogen to a coastal fynbos ecosystem of the south-western Cape Province, South Africa

Results are presented of nitrogen input via bulk precipitation over a two-year period, to coastal fynbos vegetation occurring on soils of low nutrient status. The annual precipitation recorded during the study periods of 1980 – 81 and 1981 – 82 was 381mm and 466mm respectively which was below the 40-year mean of 577mm recorded for the area. Total nitrogen input during 1980 – 81 was 1,99kg N ha−1 y−1. Of this 1,12kg N ha−1 y−1 was inorganic nitrogen in the form of NO3-N and NH4-N. Inorganic N input occurred predominantly in the winter rainfall months of June to August. Inorganic N input during 1981 – 82 was 1,79kg N ha−1 y−1 with an NH4-N to NO3-N quotient of 1. Logarithmic relationships exist between NO3-N and NH4-N concentrations in bulk precipitation and the volume of precipitation collected at weekly intervals. Bulk precipitation collectors at 1,5m and 4m were not suitable to determine the infuence of local recycling of dust aerosols. It is probable that although atmospheric N deposition may contribute a portion of the extra-system N, it alone is not sufficient to replenish N lost by coastal fynbos ecosystems as a result of recurrent short interval fires. Rather the low input of nitrogen to the Pella site reported gives an indication of the levels of N deposition to be expected in the fynbos region where N is derived mainly from oceanic sources and few anthropogenic activities influence precipitation composition

Differences in response between nitrate- and ammonium-fed maize to salinity stress and its amelioration by potassium

The effects of salinity (80mM NaCl) on the growth in hydroponics of ammonium- and nitrate-fed maize and the ameliorative role of potassium on salinity toxicity in these plants was assessed from measurements of growth rates and gas exchange characteristics. Ammonium-fed plants were more sensitive to salinity stress than their nitrate-fed counterparts. The photosynthetic and transpiration rates of ammonium-fed plants were significantly reduced by salinity whereas those of nitrate-fed plants were not significantly affected. Salinity caused 59 and 60% reductions in the shoot and root growth of ammonium-fed plants, respectively, whereas shoot and root growth of nitrate-fed plants was reduced by only 27 and 34%, respectively. In both nitrate- and ammonium-fed plants, raising the potassium concentration from 0.2 to 5mM in the presence of salinity produced a significant increase in maize growth.Die uitwerking van saliniteit (80mM NaCl) op die groei in waterkultuur van mielies wat met ammonium en nitraat gevoed is en die verbeterende rol van kalium op toksiese saliniteit by hierdie plante is met behulp van metings van groeitempo’s en gaswisselingskenmerke bepaal. Plante wat met ammonium gevoed was, was meer gevoelig vir saliniteitstremming as die wat met nitraat gevoed was. Die fotosintese- en transpirasie-tempo’s van plante wat met ammonium gevoed was, is betekenisvol deur saliniteit verminder, terwyl die van plante wat met nitraat gevoed was, nie noemenswaardig beïnvloed is nie. Saliniteit het verminderings van onderskeidelik 59 en 60% veroorsaak in die groei van stingels en wortels van plante wat met ammonium gevoed was, terwyl stingel- en wortelgroei van plante wat met nitraat gevoed was, slegs met onderskeidelik 27 en 34% gereduseer is. By plante wat met nitraat gevoed was sowel as by dié wat met ammonium gevoed was, het ’n verhoging van die kaliumkonsentrasie vanaf 0.2 na 5mM in die teenwoordigheid van saliniteit tot ’n betekenisvolle toename in die groei van mielies gelei

Growth of calcicole and calcifuge Agulhas Plain Proteaceae on contrasting soil types, under glasshouse conditions

Seven species of Proteaceae — Protea compacta, P. obtusifolia, Leucadendron xanthoconus, Ld. meridianum, Leucospermum cordifolium, Ls. truncatum and two populations of Aulax umbellata — from the Agulhas Plain, south-western Cape, were grown in acid sand and neutral limestone sand in pots in a greenhouse. Heights were measured monthly, and after 400 days the plants were harvested and the dry weights of shoots and roots determined. Species normally growing on acid sands showed signs of chlorosis and necrosis when grown on limestone soils, and weights at harvesting were significantly less than for the same species grown on acid sand. Species normally growing on limestone soils were not significantly different in height or weight when grown on either soil type. Mean species root weight in all but one case was more for plants grown on acid sands than on limestone soils. It is suggested that soil type plays the major role in the failure of calcifuge species to establish themselves on limestone soils, but that competition is probably more important in the reverse case. Some suggestions for future research into the physiological tolerances of conspecifics under controlled conditions and field conditions are made