Summary• Dark respiration and photosynthesis were measured in leaves of poplar Populus deltoides × nigra ('Veronese') saplings to investigate the extent of respiratory and photosynthetic acclimation in pre-existing and newly emerged leaves to abrupt changes in air temperature.• The saplings were grown at three temperature regimes and at high and low nitrogen availabilities. Rates of photosynthesis and dark respiration (R d ) were measured at the initial temperature and the saplings were then transferred to a different temperature regime, where the plants remained for a second and third round of measurements on pre-existing and newly emerged leaves.• Acclimation of photosynthesis was limited following transfer to warmer or cooler growing conditions. There was strong evidence of cold and warm acclimation of R d to growth temperature, but this was limited in pre-existing leaves. Full acclimation of R d was restricted to newly emerged leaves grown at the new growth temperature.• These findings indicate that the extent of thermal acclimation differs significantly between photosynthesis and respiration. Importantly, pre-existing leaves in poplar were capable of some respiratory acclimation, but full acclimation was observed only in newly emerged leaves. The R d /A max ratio declined at higher growth temperatures, and nitrogen status of leaves had little impact on the degree of acclimation.

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We investigated seasonal variation in dark respiration and photosynthesis by measuring gas exchange characteristics on Pinus radiata and Populus deltoides under field conditions each month for 1 year. The field site in the South Island of New Zealand is characterized by large day-to-day and seasonal changes in air temperature. The rate of foliar respiration at a base temperature of 10 1C (R 10 ) in both pine and poplar was found to be greater during autumn and winter and displayed a strong downward adjustment in warmer months. The sensitivity of instantaneous leaf respiration to a 10 1C increase in temperature (Q 10 ) was also greater during the winter period. The net effect of this strong acclimation was that the long-term temperature response of respiration was essentially flat over a wide range of ambient temperatures. Seasonal changes in photosynthesis were sensitive to temperature but largely independent of leaf nitrogen concentration or stomatal conductance. Over the range of day time growth temperatures (5-32 1C), we did not observe strong evidence of photosynthetic acclimation to temperature, and the long-term responses of photosynthetic parameters to ambient temperature were similar to previously published instantaneous responses. The ratio of foliar respiration to photosynthetic capacity (R d /A sat ) was significantly greater in winter than in spring/ summer. This indicates that there is little likelihood that respiration would be stimulated significantly in either of these species with moderate increases in temperature -in fact net carbon uptake was favoured at moderately higher temperatures. Model calculations demonstrate that failing to account for strong thermal acclimation of leaf respiration influences determinations of leaf carbon exchange significantly, especially for the evergreen conifer.

Pinus radiata L. were grown in climate-controlled cabinets under three night/day temperature treatments, and transferred between treatments to mimic changes in growth temperature. The objective was to determine the extent to which dark respiration and photosynthesis in pre-existing and new needles acclimate to changes in growth temperatures. We also assessed whether needle nitrogen influenced the potential for photosynthetic and respiratory acclimation, and further assessed if short-term (instantaneous, measured over a few hours) respiratory responses are accurate predictors of long-term (acclimated, achieved in days--weeks) responses of respiration to changing temperature. Results show that respiration displayed considerable potential for acclimation. Cold and warm transfers resulted in some acclimation of respiration in preexisting needles, but full acclimation was displayed only in new needles formed at the new growth temperature. Short-term respiratory responses were poor predictors of the long-term response of respiration due to acclimation. There was no evidence that photosynthesis in pre-existing or new needles acclimated to changes in growth temperature. N status of leaves had little impact on the extent of acclimation. Collectively, our results indicate that there is little likelihood that respiration would be significantly stimulated in this species as night temperatures increase over the range of 10--20 C, but that inclusion of temperature acclimation of respiration would in fact lead to a shift in the balance between photosynthesis and respiration in favour of carbon uptake.

Research relating to the use of organic amendments on soils has focused largely on agricultural soils, and there is a lack of information worldwide on their efficacy as amendments for urban soil management, especially in tropical urban environments. A pot experiment was conducted to assess the influence of biochar and organic compost on urban soil properties and on tree growth performance in Singapore. Biochar and compost were mixed with topsoil in different proportions, and two urban tree species commonly grown in Singapore (Samanea saman and Suregada multiflora) were used. There were significant additional height increments for both the tree species following application of biochar. S. saman exhibited greater stem elongation compared with S. multiflora in response to organic amendments. A significantly higher foliar N content was found in both tree species in biocharamended treatments along with significant increases in P and K. Increases in soil nutrient concentrations were also observed in combined biochar-compost treatments for both species. Combined compost and biochar had the strongest effects on soils and growth of the two urban tree species examined and applications containing biochar resulted in the most significant soil improvements.