Tansley Review No. 104

Summary Calcium occupies a unique position among plant nutrients both chemically and functionally. Its chemical properties allow it to exist in a wide range of binding states and to serve in both structural and messenger roles. Despite its importance in many plant processes, Ca mobility is low, making Ca uptake and distribution rate a limiting process for many key plant functions. Ca plays an essential role in regulating many physiological processes that influence both growth and responses to environmental stresses. Included among these are: water and solute movement, influenced through effects on membrane structure and stomatal function; cell division and cell wall synthesis; direct or signaling roles in systems involved in plant defense and repair of damage from biotic and abiotic stress; rates of respiratory metabolism and translocation; and structural chemistry and function of woody support tissues. Forest trees, because of their size and age capacity, have been examined for evidence of limitations imposed by the timing and level of Ca supply. Examination of Ca physiology and biogeochemical cycling for forested systems reveals many indications that Ca supply places important limitations on forest structure and function. These limitations are likely to be most significant with older trees, later successional stages, high levels of soil acidity and/or high canopy Ca leaching losses, or under conditions where plant competition is high or transpiration is limited by high humidity or low soil moisture. Evidence of structural and physiological adaptations of forests to limited Ca supply; indicators of system dysfunction at many levels under reduced Ca supply; and the positive responses of diverse indicators of forest vitality in liming experiments indicate that Ca is more important to forest function and structure than has generally been recognized. Lack of recognition of Ca limitations is due in part to that fact some important plant functions are controlled by changes in very small physiologically active pools within the cytoplasm, and whole‐leaf Ca levels may not reflect these limitations. An additional aspect is the fact that Ca availability has declined significantly for many forests in just the past few decades. Additional research on the role of Ca supply in resistance of forests to disease, changes in structural integrity of woody tissues, restrictions on rooting patterns and function, and uptake of other nutrients, notably N, is needed. Increased understanding of the physiological ecology of Ca supply can be anticipated to provide important insights that will aid in future protection and management of both natural and commercial forest systems. contents Summary I. introduction – a hypothesis 374 II. effects of calcium on physiological processes 376 III. calcium uptake and distribution at the whole ‐ plant level 382 IV. ecosystem processes and calcium supply 387 V. plant and ecosystem responses to human alterations in calcium supply 391 VI. conclusion 407 VII. evaluation of the hypothesis 410 Acknowledgements 411 References 411