Photosynthesis is a vital process for trees, one in which CO₂ plays a major role. At the same time, the tree’s own metabolism produces CO₂. For this PhD thesis, I investigated the fate of respiratory CO₂ – is it all released back into the atmosphere or is it re-used in photosynthesis? The new LeafWeb-model uses gas exchange measurements of leaves to estimate the percentage of respiratory CO₂ that is being refixed in photosynthesis (P_r). This thesis examined the effectson P_r of physiological, anatomical, and morphological traits; those of light availability and temperature; and P_r variations between species, functional groups or biomes. In addition, an experimental study explored the effect of drought on carbon allocation, which implied potential advantages of high P_r.

I found that P_r is increased with high mesophyll resistance to CO₂ diffusion (r_m), high maximum carboxylation rate of Rubisco (V_cmax), and low stomatal conductance to CO₂ diffusion (g_s). This suggests that these physiological states slow CO₂ diffusion out of the leaf (high r_m and low g_s) while increasing the draw on CO₂ at the photosynthetic sites (high V_cmax). Thus, all three increase refixation probability. High leaf mass per area (LMA) and thick cell walls, traits known to correlate positively with r_m, also increased refixation. The fact that both morphological and anatomical traits that are known to correlate with high r_m also correlated with high P_r in my findings supports the assumption of the model regarding the relationship between P_r and r_m. Furthermore, P_r increasing with r_m is likely the reason P_r was highest in evergreen needle species and in the boreal biomes where this trait is prevalent. Species with high P_r might be less dependent on uptake of atmospheric CO₂ and can close more of their stomata to conserve water. Models calculating terrestrial CO₂-uptake should therefore consider including P_r, and assume that plants with high r_m and high V_cmax refixate most of their (photo)respiratory-derived CO₂.

The thesis also includes a study that found that drought during early development of P.mariana shoots affected carbon partitioning and shoot morphology. The shoots allocated carbon away from structural components andtowards respiratory or osmoregulation processes. This might result in mature shoots with lower r_m, which could reduce their P_r permanently. High P_r couldbe an advantage in conditions where stomata are closed; efficiently reusing(photo)respiratory CO₂ during winter or mild drought could make it possible to maintain some photosynthesis even with very low g_s. If this is true, new shoots with less efficient refixation might make the tree as a whole less resilient during future droughts. A possible implication is that while the higher P_r of boreal biomes and evergreen conifers may make them better able to tolerate future dry-periods, such periods may weaken this effect if they happen during shoot development.