A comprehensive review of herbaceous layer ecology (2005)

Gilliam, F.S. & Roberts, M.R. (eds) ( 2003 ) The herbaceous layer in forests of eastern North America . Oxford University Press, Oxford, UK . xvi +408 pp . , figs, tables, line diagrams, halftones, index . Hardback : price £26.99 , ISBN 0-19-514088-5 . The herbaceous layer represents a characteristic and a vital component of eastern North American forests. This stratum typically comprises the largest proportion of forest diversity and tends to be strongly responsive to environmental gradients (e.g. topography, soil moisture and fertility, light availability), allowing the use of herbaceous species as indicators of altered edaphic and environmental conditions, particularly relative to anthropogenic disturbance. Additional importance of the herbaceous layer can be found in its influence on nutrient cycling and effects on woody species success (through competitive interactions) in reaching the forest canopy. Despite the ecological significance of herbaceous layer vegetation in maintaining the ‘functional integrity’ of forest ecosystems, studies of vegetation dynamics in eastern forests have historically ignored this layer of ‘step-overs’ in favour of larger and more economically important overstorey species. The Herbaceous Layer in Forests of Eastern North America explores the ecology of the herbaceous layer as a vital component of eastern forests, seeking to synthesize isolated studies of herbaceous layer structure and dynamics and set priorities for future research. This multi-authored volume, comprising 14 chapters, follows a well-received 1998 Ecological Society of America symposium on the ecology of herbaceous-layer communities in eastern forests. In the preface and the introductory chapter, the editors discuss the scope and objectives of the book, providing useful information on its timeliness and a historical perspective. As a conceptual framework for studies of the herbaceous layer, the editors suggest division of this stratum into two functional groups: resident species (those confined to the herbaceous layer — annuals, herbaceous perennials and low-growing shrubs) and transient species (those capable of emerging to higher forest strata — larger shrubs and juvenile trees). They support their argument by contrasting resource use, competitive interactions, reproductive modes and dispersal abilities within these functional groups. Surprisingly, the editors place less emphasis on what seems to be a more common division of understorey vegetation — woody vs. herbaceous species. Marked differences in life-history strategies, patterns of resource use and seasonal variations, environmental responses, diversity and conservation status seem to support additional discussion of woody and herbaceous components, an approach taken by many authors in this volume. Intermediate chapters are divided into four sections. The first section, ‘The Environment of the Herbaceous Layer’, includes an excellent review of the unique attributes of deciduous forest herbs, their contribution to ecosystem-level nutrient dynamics and a synthesis of more than 25 years of research on the ‘vernal dam’ hypothesis. Additional chapters provide comprehensive discussions of morphological and physiological adaptations of forest herbs to variable light (particularly light-limited) environments and physiological responses of herbs to natural and anthropogenically induced nutrient variations, including influences on herb–mycorrhiza and herb–herbivore interactions. Section two, ‘Population Dynamics of the Herbaceous Layer’, reviews the population biology of a large number of threatened and endangered forest herbs and the primary anthropogenic threats to these species. Community ecology receives the greatest attention in the book. Section three, ‘Community Dynamics of the Herbaceous Layer Across Spatial and Temporal Scales’, discusses a range of topics including: the history and ecology of eastern old-growth forests and the structure, diversity and invasibility of their herbaceous communities; variations in mound–pit microtopography relative to stand history and site factors and their influence on understorey composition and diversity; interactions between overstorey and understorey vegetation in disturbed and mature forest stands and a mechanistic explanation for this relationship via parallel environmental responses; changing herb layer–environmental relationships through secondary succession and a review of classic successional theories relative to the herbaceous layer; and controls on understorey composition and dynamics in boreal forests. This last chapter deviates from the traditional focus on deciduous forests by discussing boreal forest disturbance regimes and vegetation dynamics. The fourth section, ‘Community Dynamics of the Herbaceous Layer and the Role of Disturbance’, includes discussions of the influence of understorey vegetation on spatial patterning and species-specific regeneration of canopy trees; the distribution and impacts of non-indigenous species and implications for forest management; and the effects common disturbances (natural and anthropogenic) on forest overstorey and understorey layers. Throughout the book, each contributing author takes a relatively broad-scale approach to reviewing his or her own work, synthesizing historical and contemporary literature and reflecting on advances in herbaceous-layer ecology. Topics covered represent multiple levels of ecological organization, from physiological and morphological adaptations at the subcellular and the organismal levels to population- and community-level dynamics. The integration of basic and applied concepts by nearly all authors is particularly noteworthy, incorporating discussions of sampling and analytical methods, anthropogenic influences (e.g. invasive species, climate change, forest management, altered disturbance regimes), and suggestions for conservation and wise management of eastern forests. The final chapter by the editors provides an excellent synthesis of the book, highlighting the major points in each section and providing an impressive list of future research needs based on suggestions by chapter authors. I believe that this book and its extensive collection of references on the herbaceous layer will be of value and interest to researchers, graduate students and others in community and forest ecology and conservation biology.