Shifts in biomass and nitrogen allocation to tree seedlings in response to root-zone temperature

Root-zone warming of trees can result in an increase in biomass production but the mechanisms for this increase may differ between evergreen and deciduous species. The leaf gas exchange, carbohydrate and nitrogen partitioning of two Australian evergreens, Acacia saligna and Eucalyptus cladocalyx, were compared to the deciduous Populus deltoides and Acer negundo after exposure to cool or warm soil during spring. The warm treatment stimulated aboveground biomass production in all four species, however the form of this increase was species dependent. Compared with the evergreens, soluble sugars were mobilised from the above- and belowground components to a greater extent in the deciduous species, especially during root-zone warming. Photosynthesis, stomatal conductance and transpiration were increased in the warm soil treatment for the two evergreens and P. deltoides only. In P. deltoides and A. saligna the new fine roots contained greater starch concentrations when grown in warm soil but only in A. negundo was new root growth greater. Compared with the other three species, the leguminous A. saligna contained the highest N and most of this was concentrated in the phyllodes of warmed plants with no apparent mobilisation from the existing biomass. In the other evergreen, E. cladocalyx, the existing leaves and stems were a N source for new growth, while in the two deciduous species N was derived from the woody components and structural roots. These data show that the carbohydrate movement and nitrogen partitioning patterns in response to soil warming differ between perennial and deciduous plants and are likely responsible for the different forms of biomass accumulation in each of these species.