in situ seed banks and via newly arrived disseminules. Considering the
climate of this region, crop yields are low: 0.7-1.5 tons grain ha -1 yr -1 [Del
Pozo, et al. 1989b]. With improved varieties and fertilization, however,
significantly higher wheat yields (2.5 tons) can be achieved [Del Canto 1983].
In a very few ranches, e.g. La Estrella, large, individual espinos are pruned
and cared-for on a permanent basis. Direct benefit to crops however, is only
rarely perceived as justification for conservation of these trees. Rather trees
are left in fields as 'pajareros', or roosting sites for birds. Thus they may have
more sentimental than economic value in the view of the farmers.