(Photograph = Neobuxbaumia tetetzo, Amante Darmanin)
Introduction
Columnar cacti sometimes need nurse shrubs to help them become established because seedling cacti survive better under the canopy of some shrubs than they do in open areas. Neobuxbaumia tetetzo uses the nurse shrub, Mimosa luisana in the Tehuacan Valley of Mexico.
The Details
In a series of studies, cactus seed germination was lowest in open areas and highest under M. luisana plants. This effect was seen no matter if predators (small rodents, birds) were excluded from the seedlings or not. After two years no seedlings at all were observed except under nurse plants.
Enhanced germination and survival of seedlings could be due to various factors including:
- Seeds may accumulate more densely under nurse plants due to animals or other factors
- Greater predation of seedlings may occur in open areas where the seedlings are not hidden from view
- Nurse plants may protect seedlings from freezing temperatures
- Soil under nurse plants may be richer than in open areas
- Greater shade under nurse plants may create conditions favorable for seedlings.
One large study showed that shade is a critical factor. Shade under nurse plants (reduced solar radiation) cools the soil and water is more available in such situations.
One interesting note is that N. tetetzo eventually outgrows the nurse plant and displaces it, perhaps by being more successful at acquiring water and nutrients.
Additional Reading: Neobuxbaumia, Mimosa, and Shade
