Growth and physiological response of tropical lianas and trees to elevated CO2 and soil nutrient availability

Background/Question/Methods

The recent increase in the abundance and size of native lianas (woody climbing vines) in tropical forests may lead to changes in species community composition and decreased carbon storage capacity. Lianas are associated with an increased risk of tree mortality and decreased tree growth due to intense above and belowground competition with trees for light, water, and soil nutrients. Increasing atmospheric CO₂ and nitrogen deposition are potential drivers of the liana increase. Phosphorus availability, often assumed to be of key importance in constraining the productivity of lowland tropical forests, may decline as a consequence of increased nitrogen deposition. Our goal was to determine whether there is any difference in the growth and physiological response of tropical lianas and trees under elevated CO₂, and whether any response differs as soil nitrogen and phosphorus availability change. We investigated locally abundant tropical liana and tree species grown in open-top chambers in Panama, half of which were maintained at twice-ambient levels of CO₂. In two separate studies, seedlings were grown in pots with either reduced soil nitrogen or phosphorus. Half the pots in each experiment received weekly nutrient additions to return soil nutrients to current levels found in neotropical forests. 

Results/Conclusions

The relative effect of CO₂ on the response of lianas tended to be larger under low phosphorus conditions, although not always statistically significant. Nitrogen fertilization interacted with elevated CO₂ to produce significant differences in liana vs. tree height and leaf area. Variation within species and across treatments was very high, and lianas gained relatively more stem height and leaf area than trees under elevated CO₂ alone. These results suggest lianas will continue to increase as elevated CO₂ rises and phosphorus limitation is strengthened by increasing nitrogen deposition.