Drought-induced tree death in savanna (original) (raw)

Abstract

Increasing densities of woody plants in savannas has been attributed to both elevated atmospheric CO2 and reduced burning with grazing management, such that the biome could represent a substantial carbon sink. However, we show that extreme droughts (less than two-thirds expected rainfall over 3 years) occur in the drier half of the savanna biome and can cause substantial tree death. An Australian case study reveals that a net increase in tree cover over five decades of above-average rainfall was offset by sudden tree death during drought. The relationship between woody cover change and rainfall is moderated by competition with growth being facilitated by low woody cover and drought-induced death more likely as the woody component of savanna increases. The results are not supportive of a sustained increase in the woody component of xeric savannas resulting from CO2 fertilization or land management. Extensive tree death in savanna regions will become a stark consequence of climate change if predictions of increasing severity and frequency of drought are realized.

Key takeaways

sparkles

AI

  1. Extreme droughts can lead to significant tree death in savanna ecosystems, impacting carbon dynamics.
  2. A net increase in tree cover over decades can be negated by sudden drought-induced mortality events.
  3. Drought index values below -1, indicating severe rainfall deficits, correlate with increased tree mortality.
  4. Studies indicate no sustained increase in woody biomass due to CO2 fertilization or land management practices.
  5. Drought frequency in savannas is predicted to double, exacerbating the effects of climate change on tree survival.

Loading...

Loading Preview

Sorry, preview is currently unavailable. You can download the paper by clicking the button above.

References (40)

  1. Ainsworth EA, Long SP (2005) What have we learned from 15 years of free-air CO 2 enrichment (FACE)? A meta-analytic re- view of the responses of photosynthesis, canopy properties and plant production to rising CO 2 . New Phytologist, 165, 351-372.
  2. Archer S (1995) Tree-grass dynamics in a Prosopis-thornscrub savanna parkland: reconstructing the past and predicting the future. Ecoscience, 2, 83-99.
  3. Archer S, Scifres C, Bassham CR, Maggio R (1988) Autogenic succession in subtropical savanna: conversion of grassland to thorn woodland. Ecological Monographs, 58, 111-127.
  4. Asner GP, Archer S, Hughes RF, Ansley RJ, Wessman CA (2003) Net changes in regional woody vegetation cover and carbon storage in Texas drylands, 1937-1999. Global Change Biology, 9, 316-335.
  5. Bennett MM (1928) Christison of Lammermoor. Alston Rivers, London.
  6. Berry SL, Roderick ML (2004) Gross primary productivity and transpiration flux of the Australian vegetation from 1788 to 1988 AD: effects of CO 2 and land use change. Global Change Biology, 10, 1884-1898.
  7. Berry SL, Roderick ML (2006) Changing Australian vegetation from 1788 to 1988: effects of CO 2 and land-use change. Australian Journal of Botany, 54, 325-338.
  8. Bond WJ, Midgley GF (2000) A proposed CO 2 controlled me- chanism of woody plant invasion in grasslands and savannas. Global Change Biology, 6, 865-869.
  9. Bowman DMJS, Prior LD (2005) Why do evergreen trees dominate the Australian seasonal tropics? Australian Journal of Botany, 53, 379-399.
  10. Breshears DD, Cobb NS, Rich PM et al. (2005) Regional vegeta- tion die-off in response to global-change-type drought. Pro- ceedings of the National Academy of Sciences of the United States of America, 102, 15144-15148.
  11. Burke EJ, Brown SJ, Christidis N (2006) Modeling the recent evolution of global drought and projections for the twenty-first century with the Hadley Centre Climate Model. Journal of Hydrometeorology, 7, 1113-1125.
  12. Burrows WH, Henry BK, Back PV et al. (2002) Growth and carbon stock change in eucalypt woodlands in northeast Australia: ecological and greenhouse sink implications. Global Change Biology, 8, 769-784.
  13. FAO (2001) Global ecological zoning for the Global Forest Resources Assessment 2000 final report. Food and Agricultur- al Organization of the United Nations, Rome.
  14. Fensham RJ (2008) Leichhardt's maps: one hundred years of change in vegetation structure in inland Queensland. Journal of Biogeography, 35, 141-156.
  15. Fensham RJ, Fairfax RJ (2007a) Drought-related tree death of savanna eucalypts: species susceptibility, soil conditions and root architecture. Journal of Vegetation Science, 18, 71-80.
  16. Fensham RJ, Fairfax RJ (2007b) Effect of photo scale, interpreter bias and land type on woody crown cover estimates from aerial photography. Australian Journal of Botany, 55, 457-463.
  17. Fensham RJ, Fairfax RJ, Archer S (2005) Rainfall, land-use and woody vegetation cover change in semi-arid Australian savanna. Journal of Ecology, 93, 596-606.
  18. Fensham RJ, Holman JE (1999) Temporal and spatial patterns in drought-related tree dieback in Australian savanna. Journal of Applied Ecology, 36, 1035-1050.
  19. Fernandez-Illescas CP, Rodriguez-Iturbe I (2003) Hydrologically driven hierarchical competition-colonization models: the im- pact of interannual climate fluctuations. Ecological Monographs, 73, 207-222.
  20. Foley JC (1957) Droughts in Australia. Review of records from earliest years of settlement to 1955. Bulletin No. 47, Bureau of Meteor- ology, Commonwealth of Australia, Melbourne.
  21. Gitlin AR, Sthultz CM, Bowker MA et al. (2006) Mortality gradients within and among dominant plant populations as barometers of ecosystem change during extreme drought. Biology, 5, 1477-1486.
  22. Gonzalez P (2001) Desertification and shift of forest species in the West African Sahel. Climate Research, 17, 217-228.
  23. Hoffmann WA, Bazzaz FA, Chatterton NJ et al. (2000) Elevated CO 2 enhances resprouting of a tropical savanna tree. Oecologia, 123, 312-317.
  24. Houghton RA, Hackler JL, Lawrence KT (1999) The U.S. carbon budget: contributions from land-use change. Science, 285, 574-578.
  25. Keyantash J, Dracup JA (2002) The quantification of drought: an evaluation of drought indices. Bulletin of the American Meteor- ological Society, 83, 1167-1180.
  26. Khan JA, Rodgers WA, Johnsingh AJT, Mathur PK (1994) Tree and shrub mortality and debarking by sambar Cervus unicolor (Kerr) in Gir after a drought in Gujarat, India. Biological Conservation, 68, 149-154.
  27. Mitchell TD, Jones PD (2005) An improved method of construct- ing a database of monthly climate observations and associated high-resolution grids. International Journal of Climatology, 25, 693-712.
  28. Moleele NM, Ringrose S, Matheson W, Vanderpost C (2002) More woody plants? The status of bush encroachment in Botswana's grazing areas. Journal of Environmental Management, 64, 3-11.
  29. New M, Hulme M, Jones PD (1999) Representing twentieth- century space-time climate variability. Part II. Development of 1961-90 mean monthly terrestrial climatology. Journal of Climatology, 13, 2217-2238.
  30. Polley HW, Mayeux HS, Johnson HB, Tischler CR (1997) View- point: atmospheric CO 2 , soil water, and shrub/grass ratios on rangelands. Journal of Rangeland Management, 50, 278-284.
  31. Poupon H, Bille JC (1974) Recherches ecologiques sur une savane sahelienne du Ferlo septentrional, Senegal: influence de la secheresse de l'annee 1972-1973 sur la strate ligneuse. La Terre et la Vie, 28, 49-75.
  32. Rice KJ, Matzner SL, Byer W, Brown JR (2004) Patterns of tree dieback in Queensland, Australia: the importance of drought stress and the role of resistance to cavitation. Oecologia, 139, 190-198.
  33. Sankaran M, Hanan NP, Scholes RJ et al. (2005) Determinants of woody cover in African savannas. Nature, 438, 846-849.
  34. Scheffe ´H (1953) A method for judging all contrasts in the analysis of variance. Biometrika, 40, 87-104.
  35. Schlesinger WH, Gramenopoulos N (1996) Archival photo- graphs show no climate-induced changes in woody vegetation in the Sudan, 1943-1994. Global Change Biology, 2, 137-141.
  36. Scholes RJ, Archer SR (1997) Tree-grass interactions in savannas. Annual Review of Ecology and Systematics, 28, 517-544.
  37. Silva JF, Zambrano A, Farinas MR (2001) Increase in the woody component of seasonal savannas under different fire regimes in Calabozo, Venezuela. Journal of Biogeography, 28, 977-983.
  38. Viljoen AJ (1995) The influence of the 1991/92 drought on the woody vegetation on the Kruger National Park. Koedoe, 38, 85-97.
  39. Walker BH, Noy-Meir I (1982) Aspects of the stability and resilience of savanna ecosystems. In: Ecology of Tropical Savan- nas. Ecological Studies 42 (eds Huntley BJ, Walker BH), pp. 556- 590. Springer, Berlin.
  40. Western AW, Grayson RB, Blo ¨schl G (2002) Scaling of soil moisture: a hydrologic perspective. Annual Review of Earth and Planetary Sciences, 30, 149-180.

FAQs

sparkles

AI

What relationship exists between multi-year drought and tree death in savannas?add

The study finds that tree death is significantly correlated with multi-year droughts, as shown by the drought index falling below -1, associated with severe declines in tree stock in regions like Senegal and Australia.

What impacts does increasing atmospheric CO2 have on woody plant biomass?add

The research reveals that atmospheric CO2 fertilization could double woody plant biomass over 200 years in Australian savannas, enhancing primary productivity through increased water use efficiency.

How does competition affect woody cover change in savanna ecosystems?add

Findings indicate that competitive effects significantly influence tree dynamics, with lower initial cover promoting higher rates of increase in woody cover, particularly when relative rainfall is favorable.

What are the predicted future trends for drought severity in savannas?add

The study predicts a doubling in the frequency of severe droughts in savanna regions, which may exacerbate tree mortality due to elevated temperatures and stressed soil moisture conditions.

How has land use impacted woody plant dynamics historically in Australian savannas?add

Historical analysis suggests that rangeland pastoralism has not led to sustained increases in woody cover, as evidenced by longstanding drought influences on vegetation structure since the mid-19th century.