Trait–climate relationships in 454 eucalypt taxa: common garden vs field-derived trends

Understanding the role of site climate in driving geographic trait variation and revealing the relative contributions of adaptation and plasticity are key goals in plant sciences. We tested mechanistic hypotheses for trait–climate relationships for mature eucalypt trees grown in common garden and in situ field conditions, quantifying joint and individual effects of mean annual precipitation (MAP) and temperature (MAT) on eight functionally important traits. Trait–MAP relationships were particularly strong, with all but one trait consistently related to precipitation in both growth conditions. Trait–MAT relationships were notably weaker, but where relationships existed, most traits responded to low temperature in the same direction as to low rainfall, as predicted. Comparing cross-species trait–climate relationships in situ with trait–climate-of-origin relationships in the common garden indicated substantial contributions from both adaptation and plasticity, with plasticity contributing more to variation in photosynthetic traits than in leaf structural or wood traits. Two key advances were: teasing apart the roles of site temperature and rainfall on trait variation, which are often confounded; and inferring the contributions of adaptation and plasticity to observed trait patterns. The relative importance of these processes may determine the timescales over which plant traits shift with climate change.