Environmental triggers of germination and phenological events in an arid savannah region of northern Kenya

Germination, seedling emergence, growth and phenological patterns of two plant species found in an arid savannah region of Kenya were studied in relation to watering, rainfall and soil moisture under laboratory and field conditions. An initial 5 mm watering event triggered seedling emergence ofLeptothrium senegalense(Kunth.) under laboratory conditions. Under field conditions, 24•3 mm and 9•6 mm of rainfall evoked emergence of the grass seedlings in rain-fed and irrigated plots, respectively, while 26•9 mm was necessary to evoke emergence ofIndigofera spinosa(Forsk.). Watering alone did not evoke seedling emergence under field conditions. Seedling emergence was also associated with high relative humidity and low evaporative demand. Results confirmed a staggered seedling emergence pattern in both species. Seedling density was higher forL. senegalensethanI. spinosa, indicating their r/k strategies. Grass seedlings were able to achieve reproductive maturity even under conditions of severe moisture stress (within 34 days) after 31•9 mm of rain.Indigofera spinosaseedlings, however, never attained the reproductive stage. A 2-month dry season spell resulted in 80–100% seedling mortality. Green-up ofI. spinosaoccurred with 5 mm of rainfall in 4–5 days. Podding of rain-fed and irrigated non-clippedI. spinosawas achieved in 29 and 14 days, respectively, with 31•9 mm of rain.