Vertical Vegetation Structure Below Ground: Scaling from Root

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364 Ecology7 Conclusions and Future ChallengesRoots experience a resource environment that has a strong vertical dimension.Just as leaves respond to the vertical distribution of light in a canopy,roots will respond to vertical gradients of nutrients,water,and other soilproperties. Just as the top of the canopy is generally the optimal locationfor leaves,so the upper soil layers are the optimal location for roots. Someplants do not compete for the best spot in the canopy; they are shadeadaptedand specialized to live at the lower edge of the canopy. Similarly,some plants will be adapted to be deeply rooted and will not compete withother species for the abundant resources in upper soil layers. However,unlike the top of the canopy which may be out of reach for any but the tallestspecies,the upper soil layers are reachable by all plants,and thereforevertical stratification of the soil profile by different species is less likely tobe as pronounced as vertical stratification of the canopy.Roots live in a heterogeneous and complex environment,but,as thischapter shows,their average responses to soil conditions are predictable toa large degree. Moreover,roots modify the soil environment,and thepositive feedback effects of root-mediated soil modifications make rootdistributions more stable and predictable than they would be without sucheffects.Root research faces many methodological challenges,but it now hasaccess to a large variety of tools and methods (Smit et al. 2000). Over recentdecades,it has moved from description to explanation and,by takingadvantage of descriptive work accumulated over many decades,is now wellon its way towards prediction. Because of the large number of variablesaffecting root growth,our predictions about root behavior will never bevery precise,but the increasing number of ecological models that successfullypredict general patterns of root distributions demonstrates that ourunderstanding of root ecology has greatly progressed in recent years.Obviously,being able to predict root distributions is still a long way fromunderstanding the spatial distribution of root function. We are only beginningto understand the functional diversity of roots within root systems(Pregitzer 2002). There is mounting evidence that water and nutrient uptakefrom a soil volume are not always closely related to root length densityor root surface area within that volume (Adiku et al. 2000; Robinson 2001),proving that roots,just like leaves,are not all alike and are capable ofadjusting their physiologies in response to external or internal signals(Forde and Lorenzo 2001; Pregitzer 2002). Root dormancy is another neglectedissue that will have large effects on the distribution of functionswithin root systems (North and Nobel 1998). Studies of root functionrequire a different set of tools from root distribution studies,such as tracers
Vertical Vegetation Structure Below Ground: Scaling from Root to Globe 365of nutrient and water uptake (Casper et al. 2003) and inverse modelingapproaches that calculate root function from changes in the distribution ofsoil resources (Adiku et al. 2000; Hupet et al. 2003). The greatest challengein root research remain mycorrhizae and their effects on root distributionand function. The vertical distribution of mycorrhizal hyphae below theupper soil layers is an open research question for most terrestrial ecosystems(Rosling et al. 2003).Ecologists cannot be accused to ever ‘scoff at what their eyes cannot see’– to paraphrase the German poet Matthias Claudius – but too often in thepast have they ignored what their eyes did not see. Too often the soilremains the black box of studies in terrestrial ecology (Hammer 1998). Thevisible and hidden halves of plants,of vegetation,and of ecosystems deserveand need equal attention.Acknowledgements. The author wishes to thank Susan Schwinning and David Robinson forhelpful comments on the manuscript and Laura Middlebrooke for help with data processing.This research was made possible in part by a postdoctoral fellowship from the NationalCenter for Ecological Analysis and Synthesis [a Center funded by NSF (DEB-94-21535),theUniversity of California at Santa Barbara,and the State of California] and by a grant fromthe Andrew W. Mellon Foundation.ReferencesAdiku SGK,Braddock RD,Rose CW (1996) Modelling the effect of varying soil water on rootgrowth dynamics of annual crops. Plant Soil 185:125–135Adiku SGK, Rose CW, Braddock RD, Ozier-Lafontaine H (2000) On the simulation of rootwater extraction: examination of a minimum energy hypothesis. Soil Sci 165:226–236Anten NPR,Hirose T (2001) Limitations on photosynthesis of competing individuals instands and the consequences for canopy structure. Oecologia 129:186–196Bais HP,Vepachedu R,Gilroy S,Callaway RM,Vivanco JM (2003) Allelopathy and exoticplant invasion: from molecules and genes to species interactions. Science 301:1377–1380Baitulin IO (1979) Kornevaja sistema rastenij aridnoj zony Kazakhstana (in Russian). (Rootsystems of plants of the arid zone of Kazakhstan) Nauka, Alma-AtaBaitulin IO (ed) (1993) Fitoékologicheskie issledovanija v juzhnoj Gobi (in Russian withEnglish summary). (Phytoecological investigations in southern Gobi) Gylim,Alma-AtaBaligar VC,Fageria NK,Elrashidi MA (1998) Toxicity and nutrient constraints on rootgrowth. HortScience 33:960–965Ball-Coelho BR, Roy RC, Swanton CJ (1998) Tillage alters corn root distribution in coarsetexturedsoil. Soil Till Res 45:237–249Benasher J,Silberbush M (1992) Root distribution under trickle irrigation – factors affectingdistribution and comparison among methods of determination. J Plant Nutr 15:783–794Bengough AG (2003) Root growth and function in relation to soil structure,composition,and strength. In: de Kroon H,Visser EJW (eds) Root ecology,vol 168. Springer,BerlinHeidelberg New York,pp 151–171Bingham IJ,Bengough AG (2003) Morphological plasticity of wheat and barley roots inresponse to spatial variation in soil strength. Plant Soil 250:273–282Biondini M (2001) A three-dimensional spatial model for plant competition in an heterogeneoussoil environment. Ecol Model 142:189–225
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Vertical Vegetation Structure Below Ground: Scaling from Root

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