Application of randomization methods to study the association ofTrichocereus pasacana (Cactaceae) with potential nurse plants

摘要

Spatial distribution of plants and seeds of Trichocereus pasacana was studied in relation to bush species abundance, and of cacti in relation to the available number of seeds under bush species, in Los Cardones National Park, Argentina. Randomization tests, carried out to find the significance level of Chi squared test statistics used to test for non-random association hypothesis, proved that T. pasacana plants, seeds, and seedling establishment is related to the different bush species that co-occur with it. Positive interactions were found mostly with L. divaricata, but also with P. ferox and A. spartioides, suggesting that they act as nurse plants. Negative interactions were found with B. boliviensis, A. spartioides, S. crassiramea, and Verbena sp, in spite of the high seed storage beneath their canopies. Cacti are more frequent than expected in relation to seed relative abundance under L. divaricata and P. ferox and less frequent under P. rougesii, B. boilviensis, Verbena sp, G. glutinosa, S. crassiramea, and A. spartioides, due to differential seed or seedling mortality when emergence occurs.