NEWS & COMMENT Korea and China, vaccines based on inac- tivated virus are already widely used, and also the development of a recombinant vaccine is in progress (Connie Schmaljohn, USAMRIID, Ft Detrick, MD, USA). Heikki Henttonen Finnish Forest Research Institute, POB 18, FIN-01301 Vantaa, Finland Olli Vapalahti Antti Vaheri Dept of Virology, University of Helsinki, POB 21, FIN-00014 Helsinki, Finland References 1 Schrag, S.J. and Wiener, P. (1995) Emerging infectious disease: what are the relative roles of ecology and evolution? Trends Ecol. Euol. 10,319-324 Wilson, D.E. and Reeder, D.M., eds (1993) Mammal species of the world: a taxonomic and geographic reference, Smithsonian Institution Press Brummer-Korvenkontio, M., Henttonen, H. and Vaheri, A. (1982) Hemorrhagic fever with renal syndrome in Finland: ecology and virology of Nephropathia epidemica, &and. J. Infect. Dis. Suppl. 36,88-91 Niklasson, B., HGrnfeldt, B., Lundkvist, A., BjGrsten, S. and LeDuc, J. (1995) Temporal dynamics of Puumala virus antibody prevalence in voles and of Nephropathia epidemica incidence in humans, Am. J. Trop. Med. Hyg. 53,134-140 Clement, J. et al. (1994) Hantavirus epidemic in Europe, 1993, Lancet 343, 114 Pucek, Z., Jedrzejewski, W., Jedrzejewska, B. and Pucek, M. (1993) Rodent population dynamics in a primeval deciduous forest (BiaIowieza National Park) in relation to weather, seed crop, and predation, Actu Theriol. 38, 199-232 Spatial heterogeneity in tropical forest structure: canopy palms as landscape mosaics B y definition, a forest is simply an area covered with trees. The structure and species composition of a forest-the physi- cal placement of individuals and species - are what distinguish one type of forest from another. How we characterize forest structure and how we measure the basic ecosystem or community properties of a forest depend upon the spatial scale con- sidered and the spatial resolution of the methodology used. In a new paper, Clark et al. l have shown elegantly that, at least for a guild of canopy palm species, old- growth lowland rain forest is a mosaic of community structure. This landscape- level study of seven canopy palm species showed that total palm density, species richness and species abundance patterns varied significantly with edaphic factors and past human activity. Their findings confirm the widely held view that the green cloak of species-rich rain forests is not made of a homogeneous fabric, but rather is a richly textured tapestry of species distributions. Botanists and phytogeographers have long recognized the mosaic nature of tropical forest structure2J. Spatial hetero- geneity in forest structure and the high species richness of tropical wet forests have often been assessed in terms of gap dynamics and resulting successional mosaics of regeneration4J. Other studies 8 0 1996, Elsevier Science Ltd emphasize that tree species distributions in tropical forests can be linked to small- scale topographic variation6 or edaphic factors7J. Ultimately, forest regeneration cycles interact with species differences in response to soil, drainage and slope to cre- ate patchy community structure within tropical forests3. Clark et al.1 go beyond these earlier approaches, setting a new methodologi- cal standard for landscape-level studies of rain forest structure. By restricting their focus to seven species of sub-canopy and canopy palm species, this team spread their sampling effort across 568 ha of contiguous lowland rain forest. Their for- est laboratory was well prepared for this study, following the establishment of a 50 x 100 m grid system throughout the en- tire forest arid the installation of a state-of- the-art Geographical Information System (GIS). La Selva Biological Station is hardly an unstudied area, but it took a study at this large spatial scale to reveal some pre- viously unknown details about the seem- ingly well-known palms. A five-minute walk in the La Selva for- est reveals the striking prevalence of palms, which constitute 26% of all stems >lO cm dbh, but only 2.6% of the number of specie@. Spatial variation in palm dis- tributions and abundance exerts a strong influence on local populations of plants 7 8 9 10 11 Parmenter, R.R., Brunt, J.W., Moore, D.I. and Ernest, S. (1993) The Hantavirus epidemic in the southwest: rodent population dynamics and the implications for transmission of hantavi-iated adult respiratory dii syndrome (HARDS) in the Four Corners region, Seoi/letu LTER fubl. No. 41, University of New Mexico Bignall, J. (1995) Hantaviruses: the rodents take revenge, Lancet 345, 1564 Vapalahti, O., Plyusnin, A., Vaheri, A. and Henttonen, H. (1995) Hantavirus antibodies in European mammalogists, Lancet 345,1569 Yanagihara, R., Gajdusek, D.C. and Gibbs, C.J. (1984) Prospect Hill virus: serologic evidence for infection in mammaiogists, New Engl. J Med. 310, 1325-1326 Mills, J.N. et al. (1995) Guidelines for working with rodents potentially infected with hantavirus, J. Mammal. 76,716-722 and animals and on the ecosystemg. From any vantage point within La Selva, palms are ubiquitous elements of the landscape. But the view from the valley floor of El Salto stream (highly acidic, poorly drained soils) looks different from a scene near the Experimental Trail Swamp (more fertile, recent alluvium). For reasons that remain obscure, local species richness of canopy palms is highest on the valley bottom soils, lowest on the more fertile alluvial soils, and intermediate on soils of intermediate fertility’. Moreover, tall palms are more abundant along slope crests rather than on flat terrain. Density of palms greater than 10 m tall decreases from slope crests, to slopes of decreasing steepness, to slope bases and, finally, to flat terrain’. Because of the large number of sam- pling points, effects of topography and soil type could be determined independently for individual species. Eutetpe macraspadix, for example, occurred in 91% of the sam- ple points in weathered, residual soils, but in only 10% of sample points in recent allu- vium. Within the residual soils, however, Euterpe was present in only half of the flat sites, but occurred in over 90% of the points on steeper topography. Other spe- cies, such as Socrates exorrhiza and Welfia georgii, occurred in all sites, but varied widely in abundance across the different soil types. By including a large number of localized samples, Clark et al. 1 were able to resolve small-scale spatial variation across a large landscape unit. Other palm studies throughout the tropics have also revealed significant vari- ation in density and species composition according to topography and edaphic con- ditions. Along a soil moisture gradient in TREE vol. II, RO. 1 January 1996