Does the timing of litter inputs determine natural abundance of (13)C in soil organic matter? Insights from an African tiger bush ecosystem.

We investigated total primary production and natural abundance of (13)C in soil and plants in the landscape of tiger bush, Niger. Tiger bush is viewed as a natural cyclic succession of several types of vegetation (grasses, living trees and senescent vegetation) occurring over very small areas, on soils with similar chemical and physical characteristics. Under the pioneer front, production was 130 g m(-2) year(-1) of which 23% came from C4 plants; under the thicket of mature trees, grass production was 190 g m(-2) year(-1) (all C3 grasses) and under senescent vegetation, 40 g m(-2) year(-1) of which 1.5% came from C4 plants. Total above- and belowground primary production was estimated to be 890-4880 g m(-2) year(-1) of which 0.4-0.5% was contributed by C4 plants. From 29 to 45% of the soil organic carbon originated from C4 plants even though the contribution of C4 grasses to total primary production did not exceed 0.5%. We suggest that the order in which the different sources of organic matter entered the soil could lead to the overlabelling of soil organic matter with a C4 print. Because all C4 plants are grasses located in the pioneer front of tiger bush bands, their C4 organic matter enters the soil first and fixes onto clays. The C3 organic matter enters the soil several years later and is also fixed by the clays but in a lower proportion. Therefore it is less protected from microbial activity and quickly decomposes. We postulate that the repetition of this pattern over many decades (incorporation of a pure C4 material to soil, followed by the incorporation of a C3-dominated material), leads to the overaccumulation of C4 compounds on the most protective sites.

Which functional processes control the short-term effect of grazing on net primary production in grasslands?

Grazing has traditionally been viewed as detrimental to plant growth, but it has been proposed that under certain conditions, grazing may lead to compensatory or overcompensatory growth. However, comprehensive information on the relative role of the main functional processes controlling the response of net primary production (NPP) to grazing is still lacking. In this study, a modelling approach was used to quantify the relative importance of key functional processes in the response of annual canopy NPP to grazing for a West African humid grassland. The PEPSEE-grass model, which represents radiation absorption, NPP, water balance and carbon allocation, was used to compute total and aboveground NPP in response to grazing pressure. Representations of grazing and mineral nitrogen input to the canopy were simplified to focus on the vegetation processes implemented and their relative importance. Simulations were performed using a constant or resource-driven root/shoot allocation coefficient, and dependence or independence of conversion efficiency of absorbed light into dry matter on nitrogen availability. There were three main results. Firstly, the response of NPP to grazing intensity emerged as a complex result of both positive and negative, and direct and indirect effects of biomass removal on light absorption efficiency, soil water availability, grass nitrogen status and productivity, and root/shoot allocation pattern. Secondly, overcompensation was observed for aboveground NPP when assuming a nitrogen-dependent conversion efficiency and a resource-driven root/shoot allocation. Thirdly, the response of NPP to grazing was mainly controlled by the effect of plant nitrogen status on conversion efficiency and by the root/shoot allocation pattern, while the effects of improved water status and reduced light absorption were secondary.

Comparing the performances of Diggle's tests of spatial randomness for small samples with and without edge-effect correction: application to ecological data.

Diggle's tests of spatial randomness based on empirical distributions of interpoint distances can be performed with and without edge-effect correction. We present here numerical results illustrating that tests without the edge-effect correction proposed by Diggle (1979, Biometrics 35, 87-101) have a higher power for small sample sizes than those with correction. Ignoring the correction enables detection of departure from spatial randomness with smaller samples (down to 10 points vs. 30 points for the tests with correction). These results are confirmed by an example with ecological data consisting of maps of two species of trees in a West African savanna. Tree numbers per species per map were often less than 20. For one of the species, for which maps strongly suggest an aggregated pattern, tests without edge-effect correction enabled rejection of the null hypothesis on three plots out of five vs. on only one for the tests with correction.