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Question

What is the relative importance of solar radiation intensity and soil origin for cactus establishment at the spatial scales of microhabitat and plant association?

Location

Tehuacan-Cuicatlan region, central Mexico.

Methods

The seedling survivorship of Myrtillocactus geometrizans, Neobuxbaumia tetetzo and Opuntia pilifera was assessed. A garden experiment was conducted involving two light treatments (direct solar radiation and artificial shade) for seedlings growing in soil from different microhabitats (under the canopy of a Parkinsonia praecox nurse plant and in an open area) from three plant associations (tetechera, sotolinera and mezquitera) in a semi-arid inter-tropical ecosystem. It was predicted that seedling survival would be higher: (1) under the artificial shade treatment; (2) in soil from the microhabitat under the canopy of the nurse tree; (3) in soil from the plant association with the highest concentrations of carbon (C), nitrogen (N) and phosphorus (P); and (iv) in the species with the largest seed mass.

Results

Seedlings of the three species survived better in the shade. Despite the fact that soil taken under nurse plants has a higher concentration of nutrients than that from open areas, seedling survivorship did not differ between these microhabitats. Opuntia pilifera, the species with higher seed mass, survived better than M.geometrizans and N.tetetzo, whose seedling survival did not differ. Seedling survivorship differed between soils taken from the different plant associations. Higher survival rates were observed in soil from plant associations with the lowest concentration of total and available P (i.e. sotolinera and tetechera) but with a high concentration of C, total N, nitrate and ammonium.

Conclusions

The relative importance of shade and soil origin on seedling survivorship depends on the spatial scale. At the microhabitat level, shade is crucial for seedling establishment, whereas soil type is not. However, soil seems important at the plant association scale. An adaptation to the most common habitat of the Zapotitlan Valley, where soil has high quantities of Ca (as carbonate), little available P but relatively high available N, could explain the better early cactus seedling survivorship in the tetechera and sotolinera.