Book review: Tackling thorny issues in seasonally dry tropical forests

Abstract

news and update ISSN 1948‐6596 book review Tackling thorny issues in seasonally dry tropical forests The ecology and conservation of seasonally dry forests in Asia, by William J. McShea, Stuart J. Davies and Naris Bhumpakphan (editors) 2011, Smithsonian Institution Scholarly Press, 418 pp. ISBN: 978‐1‐935623‐02‐1 Price: $69.95 / £44.95 (Hardback); http://www.scholarlypress.si.edu/ Seasonally dry tropical forests: ecology and conservation, by Rodolfo Dirzo, Hillary S. Young, Harold A. Mooney and Gerardo Ceballos (editors) 2011, Island Press, 391 pp.ISBN: 978‐1‐59726‐703‐8/978‐1‐59726‐704‐5 Price: $95.00 (Hardback)/$50.00 (Paperback); http://islandpress.org/ The first difficulty in reviewing (and I presume writing) any book on seasonally dry tropical for‐ ests (SDTF) is to come to some understanding as to what SDTF actually are. I hazard a guess that what is a seasonally dry tropical forest to one per‐ son is a seasonal rain forest to another. I myself have recently treated ‘seasonally dry tropical for‐ est’ formations as various types of ‘seasonal rain forest’ (Ghazoul & Sheil, 2010). What is clear is that aseasonal tropical wet forests grade almost imperceptibly into seasonally dry evergreen for‐ est, variably deciduous forest and, ultimately, xe‐ rophytic woodland at the other extreme. Imposing clear‐cut system definitions is always going to be problematic, but it is the nature (and necessity) of human enquiry to impose some sort of classifica‐ tion system on hopelessly complex realities. In this vein two recent edited books on Neotropical (Dirzo et al.) and Asian (McShea et al.) forests de‐ fine SDTF as tropical forests with predictable, regular dry seasons lasting 4‐6 (Neotropics) or 2‐6 months (Asia), while also recognising variations on this theme. It is not so much the duration or inten‐ sity of the dry season that matters, but rather the seasonal water stress that plants are subjected to, and this is as much a function of local topography, soil and disturbance history as it is of climate. Thus in Asia SDTF can occur alongside wet ever‐ green formations in a mosaic of different vegeta‐ tion types, adding to the complexities of distin‐ guishing and mapping SDTF. Both these books begin similarly, with a couple of chapters on floristic composition and biogeography of seasonally dry tropical forests in each respective region. In the Neotropics Linares‐ Palomino et al. identify close floristic affinities among isolated SDTF regions that are distributed across a formerly hypothesised ‘Pleistocene Arc’ of SDTF that extends from Argentina in the south through Paraguay and across Brazil in the north. The Andean dry forests might represent an exten‐ sion of this arc, although the data on this point are ambiguous. Yet these authors argue that there is little evidence for a wide‐ranging Pleistocene SDTF formation, citing long‐distance dispersal events to explain the distribution of widespread species. The floristic ordination of continental Southeast Asian SDTF conducted by Bunyavejchewin et al. separates formations by elevation, topography, soil chemistry and soil physical features, all of which are related to soil water‐holding capacity, as well as the length of the dry season. A striking feature of SDTF and other tropical forests in Asia is their abundance of trees belonging to the Dip‐ terocarpaceae, yet some formations (e.g. mixed deciduous forests and lower montane forests) are notable by the near absence of dipterocarps. A biogeographic approach might provide some in‐ sights into such patterns because the Asian mixed deciduous and lower montane forests are domi‐ nated largely by families that are thought to have warm‐temperate Eurasian origins (e.g. Fagaceae, Lythraceae), while the dipterocarps originated in the southern hemisphere (Gondwana) and colo‐ nised the Asian tropics via India’s collision with Asia around 50 Mya. A fascinating chapter on the evolution of dry forest gingers by John Kress is unfortunately not placed in the context of the dis‐ tribution of forest formations outlined in the ear‐ lier chapters. The central defining feature – a predictable dry season – shapes phenological, behavioural, © 2012 the authors; journal compilation © 2012 The International Biogeography Society — frontiers of biogeography 3.4, 2012