Potential distribution of the invasive tree Triadica sebifera (Euphorbiaceae) in the United States:: evaluating CLIMEX predictions with field trials

Models that project introduced species distributions based on the climates in native and potential introduced ranges can provide valuable insights on the extent of a species' future spread. Yet, the lack of direct field evaluation of these range projections remains a major limitation. We evaluated results from the CLIMEX model in conjunction with results from seed and plant field trials in assessing environmental constraints to spread of the invasive tree Triadica sebifera (Chinese tallow tree) in the southeastern USA. CLIMEX incorporates key climatic parameters to generate large-scale projections of potential distributions based on the climate across the species' current distribution. By employing field trials within microhabitats within and beyond the tree's current range, we were able to determine seed and young plants' response to the heterogeneity of the environment at regional scales. Based on projections of the CLIMEX model, T. sebifera has the potential to spread 500 km northward beyond its current distribution in the southeastern USA; minimum temperature and limited precipitation are the key climatic constraints in the eastern and western USA, respectively. CLIMEX results correlate strongly with seed germination across sites in the southeastern USA. These results do not however correlate with plant growth rates, which were often higher in sites with low projected climatic suitability. Competition and herbivory were not constraints on the growth of T. sebifera in our field trials and were therefore not responsible for the lack of correlation between model results and plant growth rates. If the minimum and maximum temperatures were to rise by 2 degrees C, the range of T. sebifera could extend northward 700 km beyond its current distribution. While both CLIMEX and the field trials indicate that T. sebifera is capable of extensive northward spread in the eastern USA, results of field trials indicate that the patterns of invasion within the region are likely to vary substantially with local site conditions.