Regionale Verbreitungsmuster in tropischen Wäldern: Die direkte und indirekte Bedeutung von Trockenperioden
Final Report Abstract
Tropical forests are among the most diverse plant communities on earth. Up to 473 tree and liana species occur in a single hectare. A central question in ecology concerns the controls of diversity in such species-rich forests. Indeed, "What determines species diversity?" was recently selected by the prestigious Science magazine as one ofthe 25 most important research questions for contemporary science. The most prominent pattern of diversity in tropical forests is an increase of species numbers with increasing rainfall and decreasing dry season length. While the patterns of distribution and diversity are welldocumented, the causes underlying these patterns remain highly disputed. In this project I examined the two main hypotheses for why forest diversity increases with rainfall: one suggests lhat drought directly affects diversity: species vary in their physiological tolerance to drought, so lhat drought sensitive tree species are excluded from drier forests, with leaves them with a lower diversity. The alternative hypothesis suggests, that not drought itself is driving the patterns, but that instead indirect processes are at work: Pests, such as insects and diseases, are more abundant in wetter forests, and keep any one tree species from 'taking over the forest". This mechanism would allow more tree species to coexist in those forests that are wetter. Understanding which one of the mechanisms is underlying the changes in tropical forest diversity along with rainfall is extremely important for making sound predictions about the fate of these forest in the face of global climate change: pronounced alterations of rainfall patterns are projected for the tropics. The forests along the rainfall gradient across the Isthmus of Panama were used as a model system: rainfall almost doubles from the Pacific to the Caribbean side at a distance of only 65km. A wealth of existing data from a long and active history of forest research at the Smithsonian Tropical Research Insiitute in Panama allowed to address the complex question. I could show, together with colleagues, that differential drought sensitivity of the species shapes plant distributions in tropical forests at both regional and local scales, suggesting that niche differentiation with respect to soil water availability is a direct determinant of the distribution of tropical trees. These analyses were based on experimental field assessments of drought sensitivity of 48 species of trees and shrubs, on their local and regional distributions within a network of 122 inventory sites spanning a rainfall gradient across the Isthmus ofPanama, and extensive data on environmental conditions. A transplant experiment of 36 species to six forest sites across the rainfall gradient gave further insight about the mechanisms that are driving seedling performance and distribution patterns, and the results underline the pervasive direct role of drought. Annual seedling survival significantly increased with soil moisture across the rainfall gradient. Seedlings died mostly in the dry season, and the drier the site was, the more seedlings died. At the same time, those species 1 had previously identified as most drought sensitive were the ones that had the highest mortality. Fertilization did not increase seedling survival or growth, and I also found no evidence that pest pressure may be limiting seedlings in wetter forests or in the wet season. Surprisingly, the patterns of seedling survival did not translate into pallerns of species distributions: Even seedlings of the species that naturally occur in dry forests did better in wet forests, suggesting that mechanisms at other life stages, for example the seed, are limiting their distribution. Despite the overwhelming evidence from this project for a strong and direct role of drought in driving species distribution and diversity patterns, I also found compelling support for an additional role of plant diseases. When following naturally growing seedlings, I found that the probability for a seedling to survive decreased when more seedlings of its own species were around (i.e. negative density dependent survival), which is likely to be a consequence of specialized plant diseases. This effect, which can allow many species to coexist, got stronger, the wetter the forest was. The results from this project therefore suggest that both direct and indirect effects of drought play an important role for species distribution and forest diversity in tropical forests: drought directly eliminates drought sensitive species from drier forests and reduces species numbers there, whereas high moisture conditions support high abundances of pathogens, which in turn allows many plant species to coexist in wet forests. At the same time the results emphasize the sensitivity of tropical forests to water availability and suggest that any future changes in soil moisture availabilily caused by global climate change, forest fragmentation or changes of land-use will alter tropical species distributions, community composition and diversity and ecosystem function.
Publications
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(2006) Species drought sensitivity shapes loca! and regional distribution patterns in tropical forests. Connecting microbes, plants, animals, and human impact. Annual Meeting German Association for Tropical Ecology, Kaiserslautern
Engelbrecht BMJ
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(2007) Drought sensitivity shapes species distribution patterns in tropical forests. Nature 447:80-82
Engelbrecht BMJ, Comita LS, Condit R, Kursar TA, Tyree MT, Turner BL, Hubbell SP
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(2007) Studying drought in the rain. STRI News. May 18, 2007
Engelbrecht BMJ
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(2008) Drought shapes species distribution in tropical forests - consequences of climate change for biodiversity. Climate Change and Biodiversity in the Americas. Smithsonian Tropical Research Institute, Panama
Engelbrecht BMJ, Comita LS, Condit R, Kursar TA, Tyree MT, Turner BL, Hubbell SP
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(2008) Drought shapes tree distribution in tropical forests - consequences of climate change for biodiversity? University of California Berkeley, Berkeley, U.S.A.
Engelbrecht BMJ
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(2008) The chemistry and dynamics of soil organic phosphorus in tropical rain forests. American Society of Agronomy. Joint Annual Meeting. Houston. Texas, U.S.A
Turner BL, Engelbrecht BMJ, Romero TE, Tanner EVD, Vincent AG, Wright SJ
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(2008) Tolerance to low leaf water status of tropical tree seedlings is related to drought performance and distribution. Functional Ecology 23: 93 - 102
Kursar TA, Engelbrecht BMJ, Burke A, Tyree MT, El Omari B, Giraldo JP
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(2009) Drought in tropical forests: Linking mechanisms of drought sensitivity with plant dynamics and distributions. Dimensions of ecology: from global change to molecular ecology. GfÖ - Meeting in Bayreuth
Engelbrecht MBJ, Comita LS, Kursar TA, Tyree M, Turner B, Hubbell S, Condit R
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(2009) Regional variation of dry season intensity affects seedling survival and tree distribution in tropical forests. Impacts of Global Change on Tropical Ecosystems - cross-culling the Abiotic, Biotic and Human Spheres. Joint meeting of the GTÖ and ATBC in Marburg Presented within ihe Symposium entitled: Climate effecis on tropical seedling regeneration — implications for vegetation under climate change?, organized by Liza Comita and Bettina Engelbrecht
Engelbrecht BMJ
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(2009) Seasonal and spatial variation in water availability drive habitat associations in a tropical forest. Ecology 90 (10): 2755-2765
Comita LS, Engelbrecht BMJ
