Forest Nutrient Availability Research Articles

Increased Nitrogen Availability in the Soil Under Mature Picea abies Trees Exposed to Elevated CO2 Concentrations

The response of trees to increasing atmospheric CO2 concentrations is often mediated by the availability of nutrients. However, little is known about the influence of CO2 enrichment on nutrient availability in forests with mature trees. We studied processes in the soil under five 35-m-tall Norway spruce trees (Picea abies) in NW Switzerland that were exposed to a mean CO2 concentration of 550 ppm for 5 growing seasons using free air CO2 enrichment (FACE). We compared them with values from the soil under five control trees. Ceramic suction cups were installed in the soil under each tree to collect soil solution, and ion-exchange resin bags were buried in the soil to absorb ammonium and nitrate. Soil cores taken at the end of the experiment were used to measure the gross production of ammonium and nitrate by the 15N dilution technique. Although temporally and spatially variable, the nitrate concentration was higher in the soil solution under CO2-enriched trees. This effect was reflected in the resin bag extracts, which additionally indicated a trend of increased ammonium availability. Dissolved reduced N concentration (mainly dissolved organic N), however, was lower in the soil solution under CO2-enriched trees. K and Mg, and to a lesser extent Na and sulfate concentrations increased in the soil solution. P concentrations mostly remained below the detection limit. In spite of the higher concentrations of nitrate in soil extracts, gross N mineralization and nitrification rates were not affected by FACE. Needles from CO2-enriched trees contained slightly more N. No difference was observed for other nutrients. Overall, these results support the hypothesis of a priming effect, i. e. that FACE led to the production of more root exudates, which in turn stimulated soil biological activity, including mineralisation, over a time-span of at least several years. However, these tall trees showed no growth response to elevated CO2; hence, they gained no advantage from increased nitrate in the soil solution, presumably owing to other growth constraints including P and Mg availability.

Disentangling Effects of Time Since Fire, Overstory Composition and Organic Layer Thickness on Nutrient Availability in Canadian Boreal Forest

Wildfire is the primary abiotic disturbance in the boreal forest, and its long-term absence can lead to large changes in ecosystem properties, including the availability and cycling of nutrients. These effects are, however, often confounded with the effects of successional changes in vegetation toward nutrient-conservative species. We studied a system of boreal forested lake islands in eastern Canada, where time since last fire ranged from 50 to 1500 years, and where the relative abundance of the most nutrient-conservative tree species, black spruce, was largely independent of time since last fire. This allowed us to disentangle the effects of time since fire and the dominant vegetation on ecosystem properties, including nutrient stocks and concentrations. Effects of time since fire independent of vegetation composition mostly involved an increase in the thickness of the organic layer and in nitrogen concentration in both soil and leaves. Domination by black spruce had strong negative effects on nutrient concentrations and was associated with a shift toward more fungi and Gram-positive bacteria in the soil microbial community. Path modeling showed that phosphorus concentration was inversely related to organic layer thickness, which was in turn related to both time since fire and black spruce abundance, while nitrogen was more directly related to time since fire and the composition of the overstory. We conclude that discriminating between the effects of vegetation and time since fire is necessary for better understanding and predicting the long-term changes that occur in forest nutrient availability and ecosystem properties.

Composition and carbon dynamics of forests in northeastern North America in a future, warmer worldThis article is one of a selection of papers from NE Forests 2100: A Synthesis of Climate Change Impacts on Forests of the Northeastern US and Eastern Canada.

Increasing temperatures, precipitation extremes, and other anthropogenic influences (pollutant deposition, increasing carbon dioxide) will influence future forest composition and productivity in the northeastern United States and eastern Canada. This synthesis of empirical and modeling studies includes tree DNA evidence suggesting tree migrations since the last glaciation were much slower, at least under postglacial conditions, than is needed to keep up with current and future climate warming. Exceedances of US and Canadian ozone air quality standards are apparent and offset CO2-induced gains in biomass and predispose trees to other stresses. The deposition of nitrogen and sulfate in the northeastern United States changes forest nutrient availability and retention, reduces reproductive success and frost hardiness, causes physical damage to leaf surfaces, and alters performance of forest pests and diseases. These interacting stresses may increase future tree declines and ecosystem disturbances during transition to a warmer climate. Recent modeling work predicts warmer climates will increase suitable habitat (not necessarily actual distribution) for most tree species in the northeastern United States. Species whose habitat is declining in the northeastern United States currently occur in Canadian forests and may expand northward with warming. Paleoecological studies suggest local factors may interact with, even overwhelm, climatic effects, causing lags and thresholds leading to sudden large shifts in vegetation.

A plot study of forest floor litter frogs, Central Amazon, Brazil

ABSTRACTAbundance and distribution of frogs inhabiting the litter layer of an area of primary lowland rain forest in Central Amazonia were studied over a period of 15 months by sampling 498 plots each 5m × 5m. The litter frog fauna of the area consists of 23 species, but only 12 of these were encountered in the plots, and 84% of the frogs encountered belonged to only six species. Total abundance and diversity within the plot data are strongly seasonal and peak in the late wet season. Both are positively correlated with litter volume and moisture. Most of this variation is due to seasonality of reproduction, as indicated by patterns of occurrence of juveniles of the most abundant species.These results indicate that the plot sampling method docs not sample the entire fauna adequately. Since this technique has been used to study other tropical forest litter herpetofaunas, however, comparison with other studies may be useful. Species diversity of litter frogs appears to be approximately the same in lowland primary forest sites studied, averaging around 20 ‘regular’ species. Abundances, however, vary widely. Central American communities contain 14–15 frogs (100m)-2, African 9–10, South American 4–6, and South-east Asian 1–2. These differences may be due to differential nutrient availability in forests of different ages and/or on different soils.