Disentangling the role of climate and soil on tree growth and its interaction with seed origin.

When considering options for adapting forests under climate change, climate is treated as the dominant driver of forest growth, while soil properties are often ignored mainly due to shortage of accurate data. The effects of climate and soil on forest growth may vary due to local adaptation to both climate and soil, and these local adaptations might need to be considered when transferring seed provenances under climate change. Data from 29 provenance trials of Norway spruce (Picea abies (L.) Karst.) across a wide gradient of planting conditions in Austria was used to develop Structural Equation Models (SEMs) to quantified the role of climatic and soil drivers and their interactions on juvenile growth performance and to test if provenance origin affects the relative importance of these drivers. Climate and soil of the planting site location were found to have similar direct effects on juvenile tree growth, however, climate was found to be more important because of additional indirect effects via interactions with soil parameters. Notably, the relative effects of climate and soil vary among different provenance groups. Climate constraints are dominant for seed sources originating from colder and/or high altitude locations, while test site climate and soil are equally important contributors of growth for provenances originating from warmer origin and lower elevation sites. Together with the better growth performance of the latter provenance group their plasticity allows them to utilize a wide range of soil conditions.