▪ Abstract The use of stable isotope techniques in plant ecological research has grown steadily during the past two decades. This trend will continue as investigators realize that stable isotopes can serve as valuable nonradioactive tracers and nondestructive integrators of how plants today and in the past have interacted with and responded to their abiotic and biotic environments. At the center of nearly all plant ecological research which has made use of stable isotope methods are the notions of interactions and the resources that mediate or influence them. Our review, therefore, highlights recent advances in plant ecology that have embraced these notions, particularly at different spatial and temporal scales. Specifically, we review how isotope measurements associated with the critical plant resources carbon, water, and nitrogen have helped deepen our understanding of plant-resource acquisition, plant interactions with other organisms, and the role of plants in ecosystem studies. Where possible we also...

Stable Isotopes in Plant Ecology

With the development of rapid measurement techniques, stable oxygen isotope analysis of plant tissue is poised to become an important tool in plant physiological, ecological, paleoclimatic and forensic studies. Recent advances in mechanistic understanding have led to the improvement of process-based models that accurately predict variability in the oxygen isotope composition of plant organic material (δ18Op). δ18Op has been shown to reflect the isotope composition of soil water, evaporative enrichment in transpiring leaves, and isotopic exchange between oxygen atoms in organic molecules and local water in the cells in which organic molecules are formed. This review presents current theoretical models describing the influences on δ18Op, using recently published experimental work to outline strengths and weaknesses in the models. The potential and realised applications of the technique are described.

Stable oxygen isotope composition of plant tissue: a review

he use uf slable isolOpes of carbon, njlTogen. oxygen. and hydrogen to sludy J1h y~ iu l og i cil l rroces~es has increa sed exponemiaJly in the past three decades. Whe n Harm llO Cra ig (195 3. 1954) . a geochemist and early pioneer of natural abundance ,t a ble i,otopes. fi r, t mea ~ured iso topic values of plant materials. he f()und that plams tended to have a fairly na rrow ODC range of -25 tu 35'J!.o . In these inilial su rveys. he was unable ro find large taxonomic or en vironme ntal ('Hens on these values. Since that rime ecologists have ide nti ­ Ilt'd clea r isotopic signatures bJsed not only on diifc.:rem photosynthetic palhways, b UI also on ecophysiological differences. such as photosynthetic wa t(' r u~(' efficiell cy (WUE) and sources 01 wa te r and nilTogen used. As large l'n tp irica l dal abase, have accumulaled and our Iheoretical understanding uf i~otupic cOlTIrosilion bas impruved, sciemists have continued to discove r miS l1la lCll e, uelwet:' n Iheo retical and observed values. as well as confounding l'lfecls from ,ources and factors n ot previously considered . In the besl tradi­ lion ll f scie nce. these discoveries have led ro important new inSights into ph y~iologi cal or ecological processes. as well as new uses of stable isotopes in plant ccopbysiology. This chapter revie ws the most common applications of ~ l a ble isolope anal YSis in plant ecophysiology.

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Sources of Variation in the Stable Isotopic Composition of Plants

The mechanistic understanding of isotope fractionation processes is increasing but we still lack detailed knowledge of the processes that determine the isotopic composition of the tree-ring archive over the long term. Especially with regard to the path from leaf photosynthate production to wood formation, post-assimilation fractionations/processes might cause at least a partial decoupling between the leaf isotope signals that record processes such as stomatal conductance, transpiration and photosynthesis, and the wood or cellulose signals that are stored in the paleophysiological record. In this review, we start from the rather well understood processes at the leaf level such as photosynthetic carbon isotope fractionation, leaf water evaporative isotope enrichment and the issue of the isotopic composition of inorganic sources (CO2 and H2O), though we focus on the less explored 'downstream' processes related to metabolism and transport. We further summarize the roles of cellulose and lignin as important chemical constituents of wood, and the processes that determine the transfer of photosynthate (sucrose) and associated isotopic signals to wood production. We cover the broad topics of post-carboxylation carbon isotope fractionation and of the exchange of organic oxygen with water within the tree. In two case studies, we assess the transfer of carbon and oxygen isotopic signals from leaves to tree rings. Finally we address the issue of different temporal scales and link isotope fractionation at the shorter time scale for processes in the leaf to the isotopic ratio as recorded across longer time scales of the tree-ring archive.

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Stable isotopes in tree rings: towards a mechanistic understanding of isotope fractionation and mixing processes from the leaves to the wood.

A small proportion of water molecules contain the heavier isotopes of hydrogen and oxygen. There is a tendency for these heavier molecules of water to accumulate in leaves during transpiration. This has several interesting repercussions, including effects on the isotopic composition of organic

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Heavy water fractionation during transpiration

Based on measurements of δ18O and δ13C in organic matter of C3-plants, we have developed a conceptual model that gives insight into the relationship between stomatal conductance (g l) and photosynthetic capacity (A max) resulting from differing environmental constraints and plant-internal factors. This is a semi-quantitative approach to describing the long-term effects of environmental factors on CO2 and H2O gas exchange, whereby we estimate the intercellular CO2 concentration (c i) from δ13C and the air humidity from δ18O. Assuming that air humidity is an important factor influencing g l, the model allows us to distinguish whether differences in c i are caused by a response of g l or of A max. As an application of the model we evaluated the isotope data from three species in plots differing in intensity of land use (hay meadows and abandoned areas) at three sites along a south north transect in the Eastern Alps. We found three different δ18O-δ13C response patterns in native and planted grassland species (cultivated in the greenhouse). After preliminary confirmation by gas-exchange measurements we conclude that the proposed model is a promising tool for deriving carbon water relations in different functional groups from δ18O and δ13C isotope data.

Linking stable oxygen and carbon isotopes with stomatal conductance and photosynthetic capacity: a conceptual model.

We determined vertical oxygen isotope gradients of leaf organic matter for a grassland in Switzerland and a mountain beech forest (Fagus sylvatica) in Northern Italy A distinctly positive (18)O/(16)O gradient with height above ground was found for the grassland (79/1000 m(-1), p < 0001), whereas the gradient was negative for the forest (-0077/1000 m(-1), p < 0001) The results are consistent with microclimatic measurements, although large isotope variations between the species have to be taken into account for the grassland A conceptual scheme is shown which relates the isotope enrichment to the canopy density, considering the effects of transpiration and canopy structure We conclude that the analysis of the within canopy variation in delta(18)O of organic matter can be used to provide long-term estimates of leaf water isotope composition, thus improving existing isotope methods to determine the gas-exchange between vegetation and atmosphere

Y. Scheidegger

Papers

Linking stable oxygen and carbon isotopes with stomatal conductance and photosynthetic capacity: a conceptual model.

Canopy gradients in delta(18)O of organic matter as ecophysiological tool.