Atmospheric nitrogen deposition in world biodiversity hotspots: the need for a greater global perspective in assessing N deposition impacts
Phoenix, G.K. ; Hicks, W.K. ; Cinderby, S. ; Kuylenstierna, J.C.I. ; Stock, W.D. ; Dentener, F.J. ; Giller, K.E. ; Austin, A.T. ; Lefroy, R.D.B. ; Gimeno, B.S. ; Ashmore, M.R. ; Ineson, P. - \ 2006
Global Change Biology 12 (2006)3. - ISSN 1354-1013 - p. 470 - 476.
terrestrial ecosystems - species richness - chalk-grassland - consequences - ecoregions - pollutants - population - vegetation - scenarios - ammonia
Increased atmospheric nitrogen (N) deposition is known to reduce plant diversity in natural and semi-natural ecosystems, yet our understanding of these impacts comes almost entirely from studies in northern Europe and North America. Currently, we lack an understanding of the threat of N deposition to biodiversity at the global scale. In particular, rates of N deposition within the newly defined 34 world biodiversity hotspots, to which 50% of the world's floristic diversity is restricted, has not been quantified previously. Using output from global chemistry transport models, here we provide the first estimates of recent (mid-1990s) and future (2050) rates and distributions of N deposition within biodiversity hotspots. Our analysis shows that the average deposition rate across these areas was 50% greater than the global terrestrial average in the mid-1990s and could more than double by 2050, with 33 of 34 hotspots receiving greater N deposition in 2050 compared with 1990. By this time, 17 hotspots could have between 10% and 100% of their area receiving greater than 15 kg N ha1 yr1, a rate exceeding critical loads set for many sensitive European ecosystems. Average deposition in four hotspots is predicted to be greater than 20 kg N ha1 yr1. This elevated N deposition within areas of high plant diversity and endemism may exacerbate significantly the global threat of N deposition to world floristic diversity. Overall, we highlight the need for a greater global approach to assessing the impacts of N deposition
Long-term effects of elevated atmospheric CO2 on species composition and productivity of a southern African C4 dominated grassland in the vicinity of a CO2 exhalation.
Stock, W.D. ; Ludwig, F. ; Morrow, C. ; Midgley, G.F. ; Wand, S.J.E. ; Allsopp, N. ; Bell, T.L. - \ 2005
Plant Ecology 178 (2005)2. - ISSN 1385-0237 - p. 211 - 224.
tallgrass prairie ecosystem - soil carbon - biomass production - plant-communities - water relations - gas-exchange - growth - enrichment - responses - dynamics
We describe the long-term effects of a CO2 exhalation, created more than 70 years ago, on a natural C4 dominated sub-tropical grassland in terms of ecosystem structure and functioning. We tested whether long-term CO2 enrichment changes the competitive balance between plants with C3 and C4 photosynthetic pathways and how CO2 enrichment has affected species composition, plant growth responses, leaf properties and soil nutrient, carbon and water dynamics. Long-term effects of elevated CO2 on plant community composition and system processes in this sub-tropical grassland indicate very subtle changes in ecosystem functioning and no changes in species composition and dominance which could be ascribed to elevated CO2 alone. Species compositional data and soil ¿13C isotopic evidence suggest no detectable effect of CO2 enrichment on C3:C4 plant mixtures and individual species dominance. Contrary to many general predictions C3 grasses did not become more abundant and C3 shrubs and trees did not invade the site. No season length stimulation of plant growth was found even after 5 years of exposure to CO2 concentrations averaging 610 ¿mol mol-1. Leaf properties such as total N decreased in the C 3 but not C4 grass under elevated CO2 while total non-structural carbohydrate accumulation was not affected. Elevated CO2 possibly lead to increased end-of-season soil water contents and this result agrees with earlier studies despite the topographic water gradient being a confounding problem at our research site. Long-term CO2 enrichment also had little effect on soil carbon storage with no detectable changes in soil organic matter found. There were indications that potential soil respiration and N mineralization rates could be higher in soils close to the CO2 source. The conservative response of this grassland suggests that many of the reported effects of elevated CO2 on similar ecosystems could be short duration experimental artefacts that disappear under long-term elevated CO2 conditions