Trees are not always carbon limited: Evidence for sink regulation of photosynthesis

Background/Question/Methods

Carbon limitation of tree growth has been a fundamental assumption in understanding the carbon physiology of trees.  Most models estimate carbon cycling in ecosystems implicitly assuming carbon limitation, and thus predict increased tree growth due to rising concentration of atmospheric CO₂.  However, growth responses under elevated CO₂ depend on water and nutrient status, and some trees even fail to respond at all.  These observations diverge from the model predictions, and suggest that other factors need to be considered.  One potential factor is the sink regulation of photosynthesis because it allows for an adjustment of photosynthetic carbon gain according to sink demand.  However, very little evidence exists to show that trees do posses the ability for sink regulation of photosynthesis.  Here, we examine for this ability in trees by manipulating its source-sink relationship.  In uniform age and single species stands at canopy closure, we girdled branches to reduce sink demand, and surrounded an untreated branch with girdled ones to increase sink demand.  We then tracked the changes in leaf gas exchange measures.

Results/Conclusions

The girdled branches decreased photosynthesis by an average of 60%, and each untreated branch surrounded by girdled branches increased photosynthesis by an average of 12%. The treatments also changed the intercellular CO₂ response curve of photosynthesis and stomatal conductance to water vapor.  However, leaf dark respiration did not change under the treatments, suggesting that the carbon balance at leaf level was controlled mainly through the feedback regulation.  These results show trees have the ability for sink regulation of photosynthesis.  They do so through changes in photosynthetic capacity and stomatal behavior.  Sink regulation of photosynthesis is therefore a possible factor that could help improve the predictions of carbon cycle models.