Summary1. The responses of night-time dark respiration ( R d ) to temperature and leaf characteristics were measured through the canopies of tree species from two distinct forests -an oak-dominated deciduous forest in north-eastern USA, and a conifer-dominated temperate rainforest in New Zealand. These were chosen to examine the extent to which canopy level changes in dark respiration can be applied across forest biomes, and the appropriateness of scaling rules to calculations of whole-canopy carbon efflux. 2. The response of respiration to temperature differed significantly between species and with height in the canopy. This involved changes in both R d at a reference temperature, and the extent to which R d increased with temperature (described by the energy of activation, E o , or the change in R d over a 10 ° C range, Q 10 ). E o ranged from 25 (lowercanopy leaves) to 53·8 kJ mol − 1 K − 1 (upper-canopy leaves) in the deciduous forest, and from 24-37 kJ mol − 1 K − 1 in the temperate rainforest site. 3.Relationships between respiratory and leaf characteristics indicated that the instantaneous rate of respiration covaries with soluble sugar concentration and leaf nitrogen, but the temperature response of respiration ( E o or Q 10 ) appears to be driven by leaf N. 4. Scaling leaf respiratory carbon loss to the whole-canopy level indicated that simplifying assumptions regarding the variation in respiration and its temperature response with canopy height tend to underestimate carbon loss if the assumptions are based on lower-canopy leaf physiology, but overestimate carbon loss if the assumptions are based on upper-canopy physiology. Thus, canopy-level differences in leaf respiratory characteristics should be considered in modelling efforts attempting to estimate wholecanopy respiration.

We measured responses of leaf respiration to temperature and leaf characteristics in three deciduous tree species (Quercus rubra L., Quercus prinus L. and Acer rubrum L.) at two sites differing in water availability within a single catchment in the Black Rock Forest, New York. The response of respiration to temperature differed significantly among the species. Acer rubrum displayed the smallest increase in respiration with increasing temperature. Corresponding Q(10) values ranged from 1.5 in A. rubrum to 2.1 in Q. prinus. Dark respiration at ambient air temperatures, expressed on a leaf area basis (Rarea), did not differ significantly between species, but it was significantly lower (P < 0.01) in trees at the wetter (lower) site than at the drier (upper) site (Q. rubra: 0.8 versus 1.1 micromol m(-2) s(-1); Q. prinus: 0.95 versus 1.2 micromol m(-2) s(-1)). In contrast, when expressed on a leaf mass basis (R(mass)), respiration rates were significantly higher (P < 0.01) in A. rubrum (12.5-14.6 micromol CO(2) kg(-1) s(-1)) than in Q. rubra (8.6-9.9 micromol CO(2) kg(-1) s(-1)) and Q. prinus (9.2-10.6 micromol CO(2) kg(-1) s(-1)) at both the lower and upper sites. Respiration on a nitrogen basis (R(N)) displayed a similar response to R(mass). The consistency in R(mass) and R(N) between sites indicates a strong coupling between factors influencing respiration and those affecting leaf characteristics. Finally, the relationships between dark respiration and A(max) differed between sites. Trees at the upper site had higher rates of leaf respiration and lower A(max) than trees at the lower site. This shift in the balance of carbon gain and loss clearly limits carbon acquisition by trees at sites of low water availability, particularly in the case of A. rubrum.

Measurements of photosynthesis and respiration were made on leaves in summer in a Quercus rubra L. canopy at approximately hourly intervals throughout 5 days and nights. Leaves were selected in the upper canopy in fully sunlit conditions (upper) and in the lower canopy (lower). In addition, leaves in the upper canopy were shaded (upper shaded) to decrease photosynthesis rates. The data were used to test the hypothesis that total night-time respiration is dependent on total photosynthesis during the previous day and that the response is mediated through changes in storage in carbohydrate pools. Measurements were made on clear sunny days with similar solar irradiance and air temperature, except for the last day when temperature, especially at night, was lower than that for the previous days. Maximum rates of photosynthesis in the upper leaves (18.7 lmol m À2 s À1 ) were approximately four times higher than those in the lower leaves (4.3 lmol m À2 s À1 ) and maximum photosynthesis rates in the upper shaded leaves (8.0 lmol m À2 s À1 ) were about half those in the upper leaves. There was a strong linear relationship between total night-time respiration and total photosynthesis during the previous day when rates of respiration were normalized to a fixed temperature of 20 1C, removing the effects of temperature from this relationship. Measurements of specific leaf area, nitrogen and chlorophyll concentration and calculations of the maximum rate of carboxylation activity, V cmax , were not significantly different between upper and upper shaded leaves 5 days after the shading treatment was started. There were small, but significant decreases in the rate of apparent maximum electron transport at saturating irradiance, J max (P40.05), and light use efficiency, « (Po0.05), for upper shaded leaves compared with those for upper leaves. This suggests that the duration of shading in the experiment was sufficient to initiate changes in the electron transport, but not the carboxylation processes of photosynthesis. Support for the hypothesis was provided from analysis of soluble sugar and starch concentrations in leaves. Respiration rates in the upper shaded leaves were lower than those expected from a relationship between respiration and soluble sugar concentration for fully exposed upper and lower leaves. However, there was no similar difference in starch concentrations. This suggests that shading for the duration of several days did not affect sugar concentrations but reduced starch concentrations in leaves, leading to lower rates of respiration at night. A model was used to quantify the significance of the findings on estimated canopy CO 2 exchange for the full growing season. Introducing respiration as a function of total photosynthesis on the previous day resulted in a decrease in growing season night-time respiration by 23% compared with the value when respiration was held constant. This highlights the need for a process-based approach linking respiration to photosynthesis when modelling long-term carbon exchange in forest ...