THE ROLE OF PARTIAL LIMB BREAKS IN THE GROWTH AND PERSISTENCE OF ARROYO WILLOW (SALIX LASIOLEPIS)

This paper examines the occurrence of limb breaks in Salix lasiolepis Benth. (Arroyo Willow) in a natural riparian area, and it explores the morphological and ecological consequences of those breaks. Six S. lasiolepis stands of different ages (0.5–40 yr old) containing individuals of different average sizes (0.3–122 m2 crown areas) were studied in the Tijuana River Valley, California. Results showed that limb breaks were common, and their numbers increased significantly with the size of the individual. Most limb breaks were partial, with sufficient vascular connection remaining for the broken limb to survive its break and fall. Each break was examined for the presence of included bark, i.e., a wedge of bark in the limb junction usually considered to be a defect because it weakens junctions and causes junctions to break easily. Included bark was observed in 76% of the limb-junction breaks, and its presence was associated with significantly lower xylem damage and higher survivorship of broken limbs. The ecological consequences of partial limb breaks were considerable. Most fallen limbs remained alive, touched the ground and produced vigorous new growth of epicormic shoots and adventitious roots. The fallen limbs allowed individuals to spread laterally, usurp space, and outcompete neighbors. Over time, repeated limb breaks changed the growth form of S. lasiolepis individuals from upright trees or shrubs to decumbent, clonal sprawlers. This study concludes that, in S. lasiolepis, limb breaks are an integral part of growth and maturation, included bark facilitates survivable limb breaks, and limb breaks are the unusual mechanism by which individuals spread laterally and achieve long-term persistence.