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

Spatial distribution of plants and seeds of Trichocereus pasacana was studi ed in relation to bush species abundance, and of cacti in relation to the a vailable 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 hyp othesis, proved that T. pasacana plants, seeds, and seedling establishment is related to the different bush species that co-occur with it. Positive in teractions were found mostly with L. divaricata, but also with P. ferox and A. spartioides, suggesting that they act as nurse plants. Negative interac tions were found with B. boliviensis, A. spartioides, S. crassiramea, and V erbena sp, in spite of the high seed storage beneath their canopies. Cacti are more frequent than expected in relation to seed relative abundance unde r L. divaricata and P. ferox and less frequent under P. rougesii, B. boilvi ensis, Verbena sp, G. glutinosa, S. crassiramea, and A. spartioides, due to differential seed or seedling mortality when emergence occurs.