In situ and ex situ soil respiration in natural, Acacia-invaded and cleared riparian ecotones in the Fynbos Biome

[摘要] ENGLISH ABSTRACT: Soil respiration (Rs) is a major component of CO2 emissions and the global carbon balance. In the context of global change it of interest to understand seasonal patterns of RS in fynbos riparian ecosystems, particularly in invaded-riparian ecotones of these Mediterranean type ecosystems (MTE's) in the Western Cape, South Africa. Riparian ecotones are three dimensional transitional zones that provide multiple ecosystem services and functions and they act as the linkage between terrestrial and aquatic ecosystems where key ecological and geomorphological processes occur. Riparian ecotones are highly prone to disturbance, and because of this reason are also vulnerable to invasion by invasive alien plants (IAPs), notably Acacia species.Invasion by IAPs is considered one of the major threats to global change and biodiversity causing extensive ecological, economical, and social impacts. In south-western Cape, more than two thirds of the riparian environment is invaded to some extent, IAPs replacing the well adapted native species along river systems. In particular, impact of IAPs on soil respiration (Rs) may be relevant, with consequences for ecosystem function and services. Clearing of invaded riparian zones initiated by the Working for Water program has been a successful in eradicating alien plants within riparian areas even though recovery after alien clearing is lagging at many sites, and knowledge on repair of ecosystem function is lacking. Various studies have generated knowledge on carbon cycling and Rs in forests, savanna, grasslands, tundra and Mediterranean shrublands, but little is known about Rs in riparian zones, and even less about soil CO2 efflux in invaded riparian fynbos riparian ecotones.The objective of this study was to contribute to a better understanding and quantifying the effect and impact of IAPs on carbon cycling between and across riparian ecotones with different invasion status: natural, invaded, and cleared. The study areas were located in the south-western Cape and measurements of Rs, soil temperature, soil moisture, root mass, litter mass, and soil properties were carried out in riparian soils of the mountain and transitional stream longitudinal river sections, and uplands fynbos areas of six different perennial river systems. In each site, four to five transects were laid out with one sampling site of each landscape position (wet bank, dry bank, and terrestrial areas) giving a total of 12 to 15 samples per site. Soil respiration measurements were taken over a period of two years, and were done seasonally.Results from this study showed that Rs was different among seasons with highest soil respiration rates in summer. Soil CO2 efflux increased in response to warm and dry conditions during summer, while seasonal soil CO2 efflux declined in autumn and winter in response to wet and cold soil conditions. The large increase in soil CO2 efflux response to warm and dry periods when temperature was 25 to 30 °C over all riparian sites and was highest in invaded sites compared to the natural and cleared sites. A significant difference was found between sites with different statuses with invaded sites leading seasonal Rs rates. Natural and cleared sites did not differ significantly in their CO2 efflux rates, suggesting that clearing of IAPs may put invaded ecosystems on a trajectory of restoration. There were also differences in terms of landscape positions; dry banks zones of the invaded sites had higher rates compared to wet banks and the uplands areas.Our results further suggest that roots are the most important component of overall Rs rates, rather than microbial respiration. When we incubated soils minus roots, little difference was evident, either when viewing the results by invasion status or by landscape position, which suggest that inherent soil differences in terms of microbial respiration were not different. We also use a trenching approach to further investigate this, and though we found Rs to decline significantly, trends later suggest that decomposition of fine and course roots likely obscured the decline in overall Rs due to root respiration.Overall, our results showed that clearing of invaded riparian zones will likely lead to successful restoration of soil functioning in terms of C cycling. Clearing of Acacia-invaded riparian ecotones will likely lead to a decline in root density, and which removes a major component of overall Rs. These results make the investigation of the C balance of invaded riparian ecotones and terrestrial areas critical in order to assess their contribution to regional C cycles.

[发布机构] Stellenbosch University