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Acacia tree density strongly affects N and P fluxes in savanna

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Abstract

Studies of nutrient cycling in savanna ecosystems rarely consider how fluxes are affected by local variations in tree density and nutrient redistribution by herbivores. We studied how the density of Acacia zanzibarica trees in a humid savanna ecosystem affected the input of nitrogen (N) through N2-fixation and N and phosphorus (P) outputs through fire and also internal pathways of N and P return through herbivores. We found that N inputs and P outputs both increased with increasing density of N2-fixing trees, the N effect being due to tree density rather than to differences in the rate of N2-fixation. However, total N outputs due to fire did not vary with tree density because losses from the herb layer decreased as losses from the tree layer increased. In contrast, total P outputs did increase with tree density because P losses from the tree layer exceeded those from the herb layer. We suggest that variation in the density of N2-fixing trees coupled with the effects of fire can cause substantial differences in the local N and P balances in savanna vegetation. To some extent, these differences may be evened out by the tendency for browsing herbivores to transfer nutrients from Acacia- to grass-dominated areas. We conclude that encroachment by N2-fixing trees and shrubs has important consequences for ecosystem properties such as N and P dynamics.

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References

  • Abbadie L (2006) Nitrogen inputs to and outputs from the soil-plant system. In: Abbadie L, Gignoux J, LeRoux X, Lepage M (eds) Lamto: structure, functioning, and dynamics of a savanna ecosystem. Springer, New York, pp 255–275

    Chapter  Google Scholar 

  • Abbas F, Merlet J, Morellet N et al (2012) Roe deer markedly alter forest nitrogen and phosphorus budgets across Europe. Oikos 121:1271–1278

    Article  Google Scholar 

  • Aranibar JN, Otter L, Macko SA et al (2004) Nitrogen cycling in the soil-plant system along a precipitation gradient in the Kalahari sands. Glob Change Biol 10:359–373

  • Augustine DJ, McNaughton SJ, Frank DA (2003) Feedbacks between soil nutrients and large herbivores in a managed savanna ecosystem. Ecol Appl 13:1325–1337

    Article  Google Scholar 

  • Bakker ES, Olff H, Boekhoff M, Gleichman JM, Berendse F (2004) Impact of herbivores on nitrogen cycling: contrasting effects of small and large species. Oecologia 138:91–101

    Article  Google Scholar 

  • Blaser WJ, Shanungu GK, Edwards PJ, Olde Venterink H (2014) Woody encroachment reduces nutrient limitation and promotes soil carbon sequestration. Ecol Evol. doi:10.1002/ece3.1024

    Google Scholar 

  • Bond WJ (2008) What limits trees in C4 grasslands and savannas? Annu Rev Ecol Evol Syst 39:641–659 (Palo Alto)

    Article  Google Scholar 

  • Bond WJ, Midgley GF, Woodward FI (2003) The importance of low atmospheric CO2 and fire in promoting the spread of grasslands and savannas. Glob Change Biol 9:973–982

    Article  Google Scholar 

  • Bond WJ, Woodward FI, Midgley GF (2005) The global distribution of ecosystems in a world without fire. New Phytol 165:525–537

    Article  Google Scholar 

  • Burnham KP, Anderson DR (2002) Model selection and multimodel inference: a practical information-theoretic approach. Springer, New York

    Google Scholar 

  • Cech PG (2008) Impact of fire, large herbivores and N2-fixation on nutrient cycling in humid savanna, Tanzania. Dissertation, Swiss Federal Institute of Technology (ETH), Zurich

  • Cech PG, Kuster T, Edwards PJ, Olde Venterink H (2008) Effects of herbivory, fire and N2-fixation on nutrient limitation in a humid African savanna. Ecosystems 11:991-1004

  • Cech PG, Olde Venterink H, Edwards PJ (2010a) N and P cycling in Tanzanian humid savanna: influence of herbivores, fire, and N2-fixation. Ecosystems 13:1079–1096

    Article  Google Scholar 

  • Cech PG, Edwards PJ, Olde Venterink H (2010b) Why is abundance of herbaceous legumes low in African savanna? A test with two model species. Biotropica 42:580–589

  • Chen Y, Randerson JT, van der Werf GR, Morton DC, Mu M, Kasibhatla PS (2010) Nitrogen deposition in tropical forests from savanna and deforestation fires. Glob Chang Biol 16:2024–2038

    Article  Google Scholar 

  • Cleveland CC, Townsend AR, Schimel DS et al (1999) Global patterns of terrestrial biological nitrogen (N2) fixation in natural ecosystems. Glob Biogeochem Cycles 13:623–645

    Article  Google Scholar 

  • Cochard R, Edwards PJ (2011a) Structure and biomass along an Acacia zanzibarica woodland-savanna gradient in a former ranching area in coastal Tanzania. J Veg Sci 22:475–489

    Article  Google Scholar 

  • Cochard R, Edwards PJ (2011b) Tree dieback and regeneration in secondary Acacia zanzibarica woodlands on an abandoned cattle ranch in coastal Tanzania. J Veg Sci 22:490–502

    Article  Google Scholar 

  • Cramer MD, Chimphango SBM, Van Cauter A, Waldram MS, Bond WJ (2007) Grass competition induces N2 fixation in some species of African Acacia. J Ecol 95:1123–1133

    Article  Google Scholar 

  • Delon C, Galy-Lacaux C, Adon M et al (2012) Nitrogen compounds emission and deposition in West African ecosystems: comparison between wet and dry savanna. Biogeosciences 9:385–402

    Article  Google Scholar 

  • Edwards PJ, Hollis S (1982) The distribution of excreta on New Forest grassland used by cattle, ponies and deer. J Appl Ecol 19:953–964

    Article  Google Scholar 

  • Eriksen C (2007) Why do they burn the ‘bush’? Fire, rural livelihoods, and conservation in Zambia. Geogr J 173:242–256

    Article  Google Scholar 

  • Fornara DA, Du Toit JT (2008) Browsing-induced effects on leaf litter quality and decomposition in a southern African savanna. Ecosystems 11:238–249

    Article  Google Scholar 

  • Frank DA, Inouye RS, Huntly N, Minshall GW, Anderson JE (1994) The biogeochemistry of a north-temperate grassland with native ungulates: nitrogen dynamics in Yellowstone National Park. Biogeochemistry 26:163–188

    Article  Google Scholar 

  • Galang MA, Markewitz D, Morris LA (2010) Soil phosphorus transformations under forest burning and laboratory heat treatments. Geoderma 155:401–408

    Article  Google Scholar 

  • Geesing D, Felker P, Bingham RL (2000) Influence of mesquite (Prosopis glandulosa) on soil nitrogen and carbon development: implications for global carbon sequestration. J Arid Environ 46:157–180

    Article  Google Scholar 

  • Jeltsch F, Weber GE, Grimm V (2000) Ecological buffering mechanisms in savannas: a unifying theory of long-term tree-grass coexistence. Plant Ecol 150:161–171

    Article  Google Scholar 

  • Jewell PL, Käuferle D, Güsewell S, Berry NR, Kreuzer M, Edwards PJ (2007) Redistribution of phosphorus by cattle on a traditional mountain pasture in the Alps. Agric Ecosyst Environ 122:377–386

    Article  Google Scholar 

  • Kauffman JB, Cummings DL, Ward DE (1994) Relationships of fire, biomass and nutrient dynamics along a vegetation gradient in the Brazilian Cerrado. J Ecol 82:519–531

    Article  Google Scholar 

  • Laclau JP, Sama-Poumba W, Nzila JD, Bouillet JP, Ranger J (2002) Biomass and nutrient dynamics in a littoral savanna subjected to annual fires in Congo. Acta Oecol 23:41–50

    Article  Google Scholar 

  • Langkamp PJ, Swinden LB, Dalling MJ (1979) Nitrogen fixation (acetylene reduction) by Acacia pellita on areas restored after mining at Groote Eylandt, Northern Territory. Aust J Bot 27:353–361

    Google Scholar 

  • Liousse C, Guillaume B, Grégoire JM et al (2010) Updated African biomass burning emission inventories in the framework of the AMMA-IDAF program, with an evaluation of combustion aerosols. Atmos Chem Phys 10:9631–9646

    Article  Google Scholar 

  • López-Hernández D, Santaella S, Chacón P (2006) Contribution of nitrogen-fixing organisms to the N budget in Trachypogon savannas. Eur J Soil Biol 42:43–50

    Article  Google Scholar 

  • Pearson HL, Vitousek PM (2001) Stand dynamics, nitrogen accumulation, and symbiotic nitrogen fixation in regenerating stands of Acacia koa. Ecol Appl 11:1381–1394

    Article  Google Scholar 

  • R Development Core Team (2011) R: a language and environment for statistical computing. R Foundation for Statistical Computing, Vienna

    Google Scholar 

  • Riginos C, Grace JB (2008) Savanna tree density, herbivores, and the herbaceous community: bottom-up vs. top-down effects. Ecology 89:2228–2238

    Article  Google Scholar 

  • Robertson GP, Rosswall T (1986) Nitrogen in West Africa: the regional cycle. Ecol Monogr 56:43–72

    Article  Google Scholar 

  • Sanhueza E, Crutzen PJ (1998) Budgets of fixed nitrogen in the Orinoco savannah region: role of pyrodenitrification. Glob Biogeochem Cycles 12:653–666

    Article  Google Scholar 

  • Sankaran M, Hanan NP, Scholes RJ et al (2005) Determinants of woody cover in African savannas. Nature 438:846–849

    Article  Google Scholar 

  • Scholes RJ, Archer SR (1997) Tree-grass interactions in savannas. Annu Rev Ecol Syst 28:517–544

    Article  Google Scholar 

  • Schulze ED, Gebauer G, Ziegler H, Lange OL (1991) Estimates of nitrogen fixation by trees on an aridity gradient in Namibia. Oecologia 88:451–455

    Article  Google Scholar 

  • Sitters J, Edwards PJ, Olde Venterink H (2013) Increases of soil C, N and P pools along Acacia tree density gradients and their feedbacks on trees and grasses. Ecosystems 16:347–357

    Article  Google Scholar 

  • Sitters J, Maechler M, Edwards PJ, Suter W, Olde Venterink H (2014) Interactions between C:N:P stoichiometry and soil macrofauna control dung decomposition of savanna herbivores. Funct Ecol 28:776–786

    Article  Google Scholar 

  • Skarpe C (1992) Dynamics of savanna ecosystems. J Veg Sci 3:293–300

    Article  Google Scholar 

  • Smith TM, Goodman PS (1986) The effect of competition on the structure and dynamics of Acacia savannas in Southern Africa. J Ecol 74:1031–1044

    Article  Google Scholar 

  • Tobler MW, Cochard R, Edwards PJ (2003) The impact of cattle ranching on large-scale vegetation patterns in a coastal savanna in Tanzania. J Appl Ecol 40:430–444

    Article  Google Scholar 

  • Treydte AC, Edwards PJ, Suter W (2005) Shifts in native ungulate communities on a former cattle ranch in Tanzania. Afr J Ecol 43:302–311

    Article  Google Scholar 

  • Van Auken OW (2009) Causes and consequences of woody plant encroachment into western North American grasslands. J Environ Manag 90:2931–2942

    Article  Google Scholar 

  • van der Werf GR, Randerson JT, Giglio L et al (2006) Interannual variability in global biomass burning emissions from 1997 to 2004. Atmos Chem Phys 6:3423–3441

    Article  Google Scholar 

  • van der Werf GR, Randerson JT, Giglio L, Gobron N, Dolman AJ (2008) Climate controls on the variability of fires in the tropics and subtropics. Glob Biogeochem Cycles 22:1–13

    Google Scholar 

  • Vitousek PM, Cassman K, Cleveland C et al (2002) Towards an ecological understanding of biological nitrogen fixation. Biogeochemistry 57:1–45

    Article  Google Scholar 

  • Wiegand K, Saltz D, Ward D (2006) A patch-dynamics approach to savanna dynamics and woody plant encroachment—insights from an arid savanna. Perspect Plant Ecol 7:229–242

    Article  Google Scholar 

  • Wolf A, Doughty CE, Malhi Y (2013) Lateral diffusion of nutrients by mammalian herbivores in terrestrial ecosystems. PLoS One 8:e71352

    Article  Google Scholar 

Download references

Acknowledgments

We acknowledge Stefanie Karrer, Marc-Jacques Mächler, Anneke Valk, Amrish Baidjoe, Wilma Blaser, Anka Bedetti and Ruth de Groot for their help in the field and Britta Jahn-Humphrey, Marilyn Gaschen and Adolphe Munyangabe for their help in the lab. We thank Annette Stähli and the authorities of Saadani National Park for their support in Tanzania, and John Williams (†), Hamis Williams and Benjamin Donald for their assistance in the field. Sabine Güsewell is acknowledged for her statistical support. Helpful comments of two anonymous reviewers significantly improved the manuscript. Research was conducted with permission and support given by the Tanzanian Wildlife Research Institute (TAWIRI), Commission of Science and Technology (COSTECH) and Tanzania National Parks (TANAPA). This study was financed by the Swiss National Science Foundation Grant no. 2-77321-08.

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  1. Judith Sitters & Harry Olde Venterink

    Present address: Plant Biology and Nature Management, Vrije Universiteit Brussel, 1050, Brussels, Belgium

Authors and Affiliations

  1. Institute of Integrative Biology, ETH Zurich, Universitätsstrasse 16, 8092, Zurich, Switzerland

    Judith Sitters, Peter J. Edwards & Harry Olde Venterink

  2. Swiss Federal Research Institute WSL, Zürcherstrasse 111, 8903, Birmensdorf, Switzerland

    Werner Suter

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  1. Judith Sitters
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Correspondence to Judith Sitters.

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Responsible Editor: Christopher Williams.

Electronic supplementary material

Below is the link to the electronic supplementary material.

10533_2015_69_MOESM1_ESM.pdf

Online Resource 2 Estimates of N and P outputs through fire from herbaceous biomass and Acacia tree biomassSupplementary material 1 (PDF 303 kb)

10533_2015_69_MOESM2_ESM.pdf

Online Resource 1 Estimates of N input through symbiotic N2-fixation by Acacia trees and leguminous herbsSupplementary material 2 (PDF 297 kb)

Online Resource 3 Estimates of herbivore dung and urine deposition ratesSupplementary material 3 (PDF 165 kb)

Online Resource 4 Estimates of Acacia tree litter deposition ratesSupplementary material 4 (PDF 137 kb)

Online Resource 5 Estimates of wet atmospheric deposition ratesSupplementary material 5 (PDF 79 kb)

Online Resource 6 Rough estimates of net N and P budgets Supplementary material 6 (PDF 186 kb)

10533_2015_69_MOESM7_ESM.pdf

Online Resource 7 Estimates of nutrient input rates through grazer dung deposition and output rates through grass consumptionSupplementary material 7 (PDF 83 kb)

Box 1. Hypothesized influence of Acacia tree density on nutrient fluxes

Box 1. Hypothesized influence of Acacia tree density on nutrient fluxes

We have multiple working hypotheses for explaining how an increase in Acacia tree density might influence the main N and P input and output fluxes at the site level. Our predictions are based on results from previous research and are set out below.

N input flux through N2-fixation by trees

The input of N by an individual tree depends on its biomass and N2-fixation rate (see Sitters et al. 2013 and the methods section of this paper). Biomass production of an individual Acacia tree might decrease at higher densities (Smith and Goodman 1986, Pearson and Vitousek 2001, Cochard and Edwards 2011a) likely due to increased intraspecific competition. However, the proportion of N derived from N2-fixation per tree was independent of tree density in our study area (Sitters et al. 2013). We therefore predict the relationship between N input and tree density to be:

  • A1: positive log-linear (a saturated response), if trees do reduce their individual biomass beyond a certain tree density but not so strongly that site level inputs are reduced.

  • A2: quadratic, if trees do reduce their individual biomass beyond a certain tree density so strongly that site level inputs are also reduced.

N input flux through N2-fixation by leguminous herbs

The proportion of N derived from N2-fixation by leguminous herbs decreased with increasing tree density (Sitters et al. 2013), which could additionally limit their productivity. We therefore predict the relationship between N input and tree density to be:

  • B1: negative linear, if trees only reduce N2-fixation rates of leguminous herbs with increasing tree density, but not their biomass.

  • B2: negative exponential (log-linear), if trees reduce both N2-fixation rates and biomass of leguminous herbs with increasing tree density.

N and P output fluxes by fire for the herb layer

Grass biomass at the site level was not related to tree density in our study area (Sitters et al. 2013); hence the amount of herbaceous fuel available is equal in each site. However, trees might act as a buffer against fire at high tree densities and reduce fire probability and intensity (Jeltsch et al. 2000). Additionally, N and P concentrations of the herbaceous biomass might change with increasing tree density. We, for example, observed an increase in herbaceous N concentrations with increasing tree density in our study area (Sitters et al. 2013). We therefore predict the relationship between N and P outputs and tree density to be:

  • C1: negative linear, if a decrease in the proportion of herbaceous biomass burned with increasing tree density is not compensated by an increase in herbaceous N or P concentrations.

  • C2: constant, if an increase in herbaceous N or P concentrations compensates for the decrease in proportion of biomass burned.

N and P output fluxes by fire for the tree layer

As trees might act as a buffer against fire at high densities, the proportion of tree biomass burned at the site level might reduce with increasing tree density. Additionally we did not observe a change in N and P concentrations of tree foliage with increasing tree density (Sitters et al. 2013). We therefore predict the relationship between N and P outputs and tree density to be:

  • D1: positive linear, if the proportion of tree biomass burned is not significantly reduced.

  • D2: positive log-linear (a saturated response), if the proportion of tree biomass burned is reduced but not significantly so to compensate for the increase in site level tree biomass.

  • D3: quadratic, if the proportion of tree biomass burned is significantly reduced to compensate for the increase in site level tree biomass.

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Sitters, J., Edwards, P.J., Suter, W. et al. Acacia tree density strongly affects N and P fluxes in savanna. Biogeochemistry 123, 285–297 (2015). https://doi.org/10.1007/s10533-015-0069-4

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