Tansley Review No. 104 - Calcium physiology and terrestrial ecosystem processes

Calcium occupies a unique position among plant nutrients both chemically an d functionally. Its chemical properties allow it to exist in a wide range o f binding states and to serve in both structural and messenger roles. Despi te its importance in many plant processes, Ca mobility is low, making Ca up take 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 am ong these are: water and solute movement, influenced through effects on mem brane structure and stomatal function; cell division and cell wall synthesi s; direct or signaling roles in systems involved in plant defense and repai r 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 ex amined for evidence of limitations imposed by the timing and level of Ca su pply. Examination of Ca physiology and biogeochemical cycling for forested systems reveals many indications that Ca supply places important limitation s on forest structure and function. These limitations are likely to be most significant with older trees, later successional stages, high levels of so il acidity and/or high canopy Ca leaching losses, or under conditions vc he re plant competition is high or transpiration is limited by high humidity o r low soil moisture. Evidence of structural and physiological adaptations o f forests to limited Ca supply; indicators of system dysfunction at many le vels under reduced Ca supply; and the positive responses of diverse indicat ors of forest vitality in liming experiments indicate that Ca is more impor tant to forest function and structure than has generally been recognized. L ack of recognition of Ca limitations is due in part to that fact some impor tant plant functions are controlled by changes in very small physiologicall y active pools within the cytoplasm, and whole-leaf Ca levels may not refle ct these limitations. An additional aspect is the fact that Ca availability has declined significantly for man!, forests in just the past few decades. Additional research on the role of Ca supply in resistance of forests to d isease, changes in structural integrity of woody tissues, restrictions on r ooting patterns and function, and uptake of other nutrients, notably N, is needed. Increased understanding of the physiological ecology of Ca supply c an be anticipated to provide important insights that will aid in future pro tection and management of both natural and commercial forest systems.