The effects of apical meristem damage on growth and defences of two Acacia species in the Negev Desert

Question: Does removal of the apical meristem result in compensatory responses or overcompensation? Hypotheses: There will be increased investment in axillary meristems and lateral branches and reduced investment in defences. There will be trade-offs between investment in different types of defence and trade-offs between defence and growth. Organisms: Young Acacia gerrardii and A. raddiana trees. Field site: Greenhouse, Blaustein Desert Research Institutes, Negev Desert, Israel. Methods: The treatments were: (I) a control (no removal of either the apical meristem or biomass); (2) removal of the apical meristem only; (3) removal of 75% of the biomass of the plant including removal of the apical meristem; and (4) removal of 75% of the biomass of the plant excluding removal of the apical meristem. Conclusions: There was no sign of overcompensation or compensatory responses in the experimentally manipulated plants. Plants with the apical meristem removed did not produce more lateral branches. Protecting the apical meristem appeared to be important, although this was only apparent when 75% of the biomass was also removed. There were trade-offs between chemical defences (condensed tannins) and mechanical defences (mean number of thorns) in A. gerrardii but not in A. raddiana. There were also trade-offs between defence and growth in A. gerrardii. I detected several positive correlations between mechanical defences and growth parameters in both species, indicating that mechanical defences may be part of a suite of growth-related traits.