Ecology of Knobcone Pine in the Santa Ana Mountains, California

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Abstract
Knobcone pine (Pinus attenuata) is restricted in the Santa Ana Mountains to hydrothermally modified serpentinite which supports only limited shrub growth as opposed to the surrounding dense chaparral on nonspecialized substrates. Ceanothus papillosus var. rowaenus and Ribes malvaceum var. viridifolium are also restricted locally. The pines survive on the serpentinite by tolerating the existing edaphic conditions, including nitrogen and phosphate deficiencies and low pH, and because the otherwise dense competitive growth has been minimized. The water—retaining capacity of the serpentinite is nearly double that of the chaparral soils. This soil characteristic, the frequent fogs, the location of the pines in fog gaps, the scattered growth, multiple—trunked trees, spreading crowns, and medium—length needles all contribute to the persistence of the pines by enhancing their ability to intercept marine air and produce considerable fog drip which is readily held by the soils. The controlling influences of the serpentinite diminish as soil genesis advances. This is countered by the dynamic expansion and uplifting of the serpentine body and by continuous erosion which is linked to the friable geology, typically steep slopes, thin soils, sparse cover, and repeated disturbance by fire. The life cycle of these knobcone pines is related to fire, and its periodic occurrence is a necessity for survival. The pines possess a strict closed—cone habit, with firmly attached cones accumulating throughout the life of each tree. The seed remains until heat generated by a fire opens the cones. Cones are seldom burned, and the seed is not shed until well after the fire, remaining viable for at least 3 years in opened cones. Fire creates pioneer conditions necessary for seedling establishment. Trees have a short lifespan and occur in even—aged groves that date back to known fires.

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