My research interests are mainly in plant ecology, and specifically in the ecology of plant roots.  The guiding question of my research is how structure and function of plants belowground relate to structure and function aboveground.  In my work I address this question at different levels of organization, ranging from the individual plant to the ecosystem and global level, using approaches that include field and laboratory studies, ecological models, as well as analyses of large, newly compiled databases.  
 

Predicted probability of deep rooting (>2 m).  From Schenk & Jackson 2005.

Global ecology and biogeography of roots. The major goal of this research is to quantify global patterns of rooting depths and to determine their relationship to climate, soil, and aboveground vegetation structure (Schenk & Jackson 2002a, 2002b, 2005).  This project was initiated to add a belowground perspective to plant biogeography, which in the past has been largely restricted to vegetation structure aboveground.  A better knowledge about global patterns of rooting depths will contribute to a further understanding of the role of plants in the global cycles of carbon, water, and plant nutrients ( Jackson, Schenk, Jobbágy, et al. 2000 ).  For this reason, I developed a global database of vertical root profiles that is available online at ORNL DAAC web site.  These data were used to develop global maps of rooting depths for the NASA International Satellite Land Surface Climatology Project ( ISLSCP II ).  The maps can be used to parameterize rooting depths in global hydrological and biogeochemical models.  In addition, I have developed a mechanistic model to predict vertical root distributions from soil characteristics and hydrological variables, which is currently being tested against field data.  The ultimate goal of this work is to move global root ecology from a purely empirical science to a process-based one that can be used to predict effects of global change on the belowground structure and function of ecosystems.

Roots of the desert semi-shrub Ambrosia dumosa.

In addition to these studies at the ecosystem- or biosphere-level, I am also interested in the ecological and evolutionary determinants of root system sizes of individual plants, especially in water-limited ecosystems, where the sizes of root systems may be crucial for survival (Schenk & Jackson 2002a ).  In a project done in collaboration with Karl Niklas at Cornell University, using large, newly compiled databases of plant sizes and biomass, we found a linear relationship between the volumes of plant canopies and the volumes occupied by their root systems.  This linear relationship spanned ten orders of magnitude of plant sizes from small annuals to large trees and included phyletically and ecologically diverse species (Niklas, Schenk & Jackson, in revision).

Relationships between structure and function in roots and shoots of desert shrubs Soil water and nutrients are the growth-limiting factors in deserts, which means that efficient root systems are of critical importance for desert plants.  In field studies in the Mojave Desert, I found that many desert shrub species develop split root- and shoot-axes or develop other forms of hydraulic axis segmentation (e.g., in the genera Ambrosia and Acamptopappus in the Asteraceae).  A review of the literature showed that this is a general, but little-studied, phenomenon found in desert plants worldwide ( Schenk 1999 ).  This suggests that it may be advantageous for desert plants to minimize lateral water flow among individual roots or branches, possibly to maximize vertical transport from roots to shoots.  In collaboration with Cynthia Jones at the University of Connecticut I am currently working on a project to study this phenomenon by comparing desert shrubs with hydraulic segmentation to species that are not segmented (e.g., the genus Encelia in the Asteraceae).  The guiding questions of this project are: (1) How does axis splitting relate to wood anatomy and xylem function in roots and shoots? (2) How does it affect drought tolerance and longevity? (3) How is it related to water uptake and transport by roots? (4) Is the functional morphology of these desert shrubs related to their biogeographical distributions?

Axis splitting in Ambrosia dumosa (Asteraceae).

Role of root interactions in determining the spatial structure of plant communities Interactions between roots often strongly influence the structure of plant communities aboveground.  While the aboveground structure of a plant community is relatively easy to study, researchers of belowground vegetation structure face many obstacles.  Various approaches are available for determining the belowground size of the volume occuppied by the roots of an individual plant, and these are discussed in an upcoming paper by Casper, Schenk, & Jackson (in press). Because soil resources tend to be growth-limiting in deserts, root interactions often play an important role in structuring desert communities, with some desert species having evolved mechanisms that may create root territories by minimizing root system overlap among neighbors ( Schenk et al. 1999 ).  In field studies conducted at a site in the western Mojave Desert, I found that negative and positive root interactions strongly affected both the spatial distribution of individual shrubs and their sizes (Schenk & Mahall 2002, Schenk et al. 2003).  The next step of this research projec will be to determine whether the self/non-self recognition system observed in the laboratory among roots of one of the species studied in the Mojave Desert, Ambrosia dumosa, contributes to spatial segregation of root systems in the field, and whether root territories are maintained only among genetically identical clones or also among different individuals.  These observations could throw light on the question of how root territoriality may have evolved in this species (Schenk et al. 1999).

Map of desert shrubs in the Mojave Desert.

The mechanisms underlying the distributions of plants were also the topic of an earlier study on the biogeography of tree species.  Here, my research has focused on the question how tree species respond to climatic factors, such as temperature, how such responses affect geographical range boundaries, and how responses can be modeled in individual-based tree population models to predict effects of global change (Schenk 1996) .

Another interest of mine is to integrate concepts from behavioral ecology into plant community ecology.  Plants interact with other organisms (including plants) in many ways; for example, they exchange signals, exude defense compounds, and show plastic responses to stimuli, all of which processes are not fundamentally different from those observed in animals.  In this area, I have been especially interested in signal and energy exchanges between plants and in the phenomenon of spatial root segregation or root territoriality .
 

Suaeda linifolia in Nevada, a C 3 species, newly introduced to North America from Asia.  Photograph by H. J. Schenk.

Taxonomy and phylogeny of the genus Suaeda In addition to my ecological research, I am also interested in the systematics and phylogeny of plants.  My main research subject in these fields has been the halophytic genus Suaeda in the Chenopodiaceae.  With Wayne Ferren (University of California Santa Barbara) I have co-authored a treatment of this genus for the Flora of North America project (Ferren & Schenk 2004) , a nomenclatural study (Schenk & Ferren 2001) , and a study of the global distribution of C3 and C4 photosynthesis in the genus (Fisher et al. 1997). Currently I am collaborating on a global phylogenetic treatment of the genus, based on DNA, anatomical, and morphological analyses.  Suaeda includes many widespread, polymorphic taxa, and one of my research interests is to study ecotypic and genetic differentiation within these taxa, and relate the distributions of microspecies and ecotypes to environmental variables. Links to more detailed images of Suaeda species: Suaeda linifolia:  Pic 1,  Pic 2,  Pic 3, Pic 4 Suaeda calceoliformis:  Pic 1, Pic 2 Suaeda occidentalis:  Pic 1, Pic 1 Suaeda taxifolia:  Pic 1

Suaeda calceoliformis at Snow Water Lake in Nevada,

Author:  Jochen Schenk (jschenk@fullerton.edu ) Last Updated:  16 February, 2006
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