Although trees have responded to global warming in the past - to temperatures higher than they are now - the rate of change predicted in the 21st century is likely to be unprecedented. Greenhouse gas emissions could cause a 3-6°C increase in mean land surface temperature at high and temperate latitudes. Despite this, few experiments have isolated the effects of temperature for this scenario on trees and forests. This review focuses on tree and forest responses at boreal and temperate latitudes, ranging from the cellular to the ecosystem level. Adaptation to varying temperatures revolves around the trade-off between utilizing the full growing season and minimizing frost damage through proper timing of hardening in autumn and dehardening in spring. But the evolutionary change in these traits must be sufficiently rapid to compensate for the temperature changes. Many species have a positive response to increased temperature - but how close are we to the optima? Management is critical for a positive response of forest growth to a warmer climate, and selection of the best species for the new conditions will be of vital importance. Contents Summary 369 I. Introduction 370 II. Photosynthesis and respiration 370 III. Soil organic matter decomposition and mineralization 373 IV. Phenology and frost hardiness 376 V. Whole tree experimental responses to warming 380 VI. Changes in species distribution at warmer temperatures 381 VII. Adaptation and evolution 383 VIII. Ecosystem level responses to warming 387 Acknowledgements 390 References 390 Appendix I. Temperature response functions 399.
Keywords: global warming; phenology; physiology; tree and forest functioning.