Meeting Report

Root Dynamics and Global Change: An Ecosystem Perspective
Martyn Caldwell, Utah State University, Logan, Utah, USA

This meeting was the fifth annual symposium sponsored by the New Phytologist journal and the first to be held in North America. It was superbly conceived and organised by Richard Norby (with Robert Jackson and Alastair Fitter as co-organisers) and was held in conjunction with the GCTE (Global Change and Terrestrial Ecosystems) Core Project of the IGBP (International Geosphere-Biosphere Programme). It took place in Townsend, Tennessee, 19-22 October, 1999.

In field CO₂-fumigation experiments, more carbon clearly appears to flow belowground in many ecosystems, but the fate of this carbon remains one of the most important and perplexing questions in assessing ecosystem effects of global change and ultimately carbon sequestration in soils. Similarly, other facets of global change such as warming, increased nitrogen deposition and changing soil moisture also affect root and soil system processes. Ecosystem-level observations of root and soil processes as influenced by global change are beginning to emerge. The goal of this symposium was to review highlights of such ecosystem studies and to identify important directions for future research in this area. Correspondingly, the GCTE identified these issues in its new implementation plan for upcoming research initiatives in this important component of global change research.

Of course, addressing questions of root and rhizosphere function in the field is difficult and vexed with problems and uncertainties. However, the participants (some 70 investigators representing 15 countries) have passed beyond the hand-wringing stage and have undertaken experiments to answer these questions, despite the obvious problems. Some of the same questions remain, e.g., how long do fine roots live and what characteristics of these roots relate to longevity? Yet, some surprising advances have also emerged, some of which have been afforded by new technologies. A series of interesting oral presentations covered general themes while posters portrayed many specific experiments, but also revealed some of the most interesting advances. Much of this will appear in the July 2000 issue of New Phytologist.

David Eissenstat (with C.E. Wells and R.D. Yanai) reviewed comprehensive studies indicating fine root longevity to be related to a number of root characteristics such as diameter, tissue density, mycorrhizal infection, nitrogen concentration, basal respiration rates and accumulation of condensed tannins and lignin. While the very small diameter roots are generally thought to turnover most rapidly and many observations support this view, very fine roots (especially of an earlier order in the branching sequence) may live a remarkably long time. At the end of the observation period of 800 days, some of the same very small roots of trees were still present and apparently alive in minirhizotron observations. An innovative technique portrayed in a poster by Julia Gaudinski (with Trumbore, Cook and Richter) utilized ¹⁴C released during the early '60's from thermonuclear testing to date the carbon in fine roots collected from a forests. Though very fine (less than 1 mm in diameter), these roots were surprisingly long-lived, ranging from 2 to 16 years!. Though an expensive approach that will mostly likely not be partical in routine experiments, it causes much rethinking about the nature of fine "ephemeral" roots. While many of the small root traits that relate to turnover and longevity are not easily assessed, if there are analogs with leaf traits aboveground, much could be learned. This is the finding in studies shown in a poster by Craine, Wedin, Reich, Chapin and Tilman for prairie species in Minnesota. More widespread attempts to link leaf and root characteristics in other systems seems warranted. If fruitful, this short cut could save much painstaking work belowground.

Just as investigators of root system dynamics lament the neglect of belowground activity by many plant biologists and ecologists, the "root community" of investigators must not dismiss the close coupling of root activity with the status of shoot phenology, photosynthetic activity and source/sink balances. This point was highlighted in laboratory physiological studies presented by John Farrar (& D. Jones) showing the importance of both root and shoot control of root growth, supporting their hypothesis of "distributed control". Similarly, much empirical evidence in field studies indicates the importance of shoot phenological status and photosynthetic control of root growth in agronomic annual plants (Rogers, Pritchard, Prior and van Santen)and root respiration (Atkin, Edwards and Byrne). By the same token, perhaps it is not so surprising that root turnover in field studies in upland grasslands in the United Kingdom was better related to daily solar radiation rather than soil temperature (in results discussed by Fitter). And, root system response to nitrogen deposition can only be understood in the context of accompanying shoot responses to excess nitrogen as described by Knute Nadelhoffer.

Although work with belowground systems remains quite limited by available experimental technology, there have been advances both in the experimental tools and in modelling that are allowing notable major steps forward, especially if combined. Robert Jackson reported on use of molecular techniques to identify individual trees aboveground whose roots had penetrated to caves below --- yielding an assessment of substantial rooting depths of different species in a Texas savannah. Jackson emphasised the importance of considering deep roots in ecosystem and global function. A clever inverse modeling approach taken by Kleidon and Haiman (portrayed by both Jackson and Ian Woodward in their lectures) estimated maximum rooting depth needed to match water uptake on a global scale. These estimates converge to some degree with estimates of Jackson and colleagues based on extrapolations of rooting depth profiles. In a similar apparent convergence some careful small-scale mechanistic studies may be related to large-scale modeling results. Some "bottom-up" approaches to estimating the contribution of different root and rhizosphere processes to soil CO₂ efflux from forests exposed to elevated CO₂ were portrayed in the lecture by Zak, Pregitzer, King and Holmes (also posters by Weizin Cheng and by Bovard and Curtis). Their results can be compared with results of an innovative "top-down" deconvolution modeling approach described in a poster of Yiqi Luo. He is using the time course of soil CO₂ efflux following a "perturbation", namely the imposition of elevated CO₂ at the Duke FACE (free air CO₂ enrichment) site. Molecular techniques are also allowing a more exacting assessment of mycorrhizal responses to global change including the measurement of mycorrhizal hyphae in soil and the discrimination of different types of mycorrhizae in field soils in work described by Michael Allen and coworkers.

Root physiological properties, such as uptake kinetics (BassiriRad, Gutschick and Zerihun), respiration (Pregitzer and Atkin et al.) and mycorrhizal associations (Allen and Fitter, Heinemeyer and Staddon) were described as they relate to warming, elevated CO₂ and nitrogen deposition. Yet, some seemingly tractable issues remain such as the degree to which roots can acclimate to temperature under field conditions as other factors are changing, e.g., water stress. Mycorrhizal root associations respond to global change conditions, but it is not so clear how much of this is a response of the fungus or the host plant.

Overall, the goals of this meeting were admirably met. Of course, there was much emphasis on field research in North America and Europe where societal affluence provides the expertise and wherewithal to accomplish this expensive research. There was much emphasis on forests because of their obvious importance in global fluxes. However, those working with 'charismatic megaphyta' may tend to overlook the importance of the grasslands, deserts, rangelands and agricultural fields that constitute most of the globe's terrestrial ecosystems. Jackson emphasised the importance of assembling gridded global data sets on belowground turnover, rooting depth, etc. Though many open grid squares result from such gridded data assemblages, they indicate the challenge of what is needed for truly global assessments. Also, beyond global climate change, land-use changes probably represent the greatest perturbations of belowground processes and carbon storage. Hugo Rogers represented the agricultural community well in this meeting and highlighted many questions seemingly related primarily to agriculture. However, if we can answer these, we will understand less perturbed lands much better. For example, if we can better understand the optimal timing of nutrient and water applications in agronomic fields, we may better assess when to sample and measure belowground activity in nonagricultural systems. We still grapple with issues of what we should measure, at what scale, how often and how best to synthesise rapidly accumulating data sets at different scales.

(The report first appeared in the Bulletin of the Ecological Society of America, vol. 81, no. 1, January 2000.)