Seed production and seedling establishment of a Southwest riparian tree, Arizona walnut ( Juglans major )

-A four-year study offlve populations has revealed influences on seed production and seedling establishment of the Southwest riparian tree Juglans major. Germination is abundant after production of large seed crops (masts), hut masts are produced infrequently. Within years, germination is stimulated by summer rains, enabling seedlings to establish on riparian terraces as well as streambanks. Traits such as capacity for dormancy dUring summer drought allow some seedlings to survive on terraces, but abundant rainfall is essential for high rates ofseedling success. Ranges ofmoisture toleraJJce vary among seeds collected from different populations, suggesting that ecotypes may exist between riparian sites with dissimilar moisture regimes. Population-based differences are associated, in part, with differences in seed size. Arizona walnut Uuglans major [Torr. ] Hel· ler) is a member of the "Interior riparian de· ciduous forest," an assemblage of trees that grow along streams in the Interior South· west (Brown 1982). Walnut sometimes domi· nates this community (Szaro 1989), but more often it occurs at-relatively low densities and frequencies. TIlis distribution pattern, while indicating that establishment occurs infre· quently, does not reveal the stage where re· generation is limited. Sudworth (1934) sug· gested low rates of seed production and high rates of seed predation by tree sqUirrels as possible natural causes of infrequent estab· lishment; seedling mortality from cattle graz· ing also may playa role (Rucks 1984). Lack of suitable germination and establish· ment sites may limit recruitment, but little is known regarding the relationship between these requirements and ]. major. Certain generalizations have been made about the dis· tribution ofwalnut trees, but the habitat char· acteristics of trees may differ from those in which the seedlings established. A study that specifically addressed riparian seedlings re· vealed that seedlings of]. major, a facultative riparian species, establish in many microsites' throughout the riparian zone. This is in con· trast to seedlings of obligate riparian trees such as Arizona alder (Alnus oblongifolia Torr.) that occur only immediately a<;ljacent to the stream (Larkin 1987). Larkin conducted berstudy, however, in a wet. montane area of central Arizona; germination "safe sites" for walnut may be more restricted in drier ripar· ian sites such as along ephemeral streams. For example, seed burial may become a prerequisite ofgennination as soil moisture decreases. as is true for certain oaks (Barrett 1931). Gernlination and establishment of walnut or other riparian species may also vary as a function ofgenetic differences hetween popu· lations. The existence of riparian ecotypes is not a new idea (e.g., Hook and Stubbs 1967), and riparian populations may differ between isolated Southwest watersheds with different hydrologic characteristics. Other studies in this series on walnut reproduction have iden· tmed differences between populations in floral ratios and seed weight (Stromberg 1988), . indicating that]. mnjOf' shows either plastic' responses or genetically based responses to environmental differences within and be· tween riparian sites. The objectives of this study were to: (1) identil)o factors that limit seed production and seedling recruitment of ]. major between sites and between years; (2) determine how gennination requirements vary between microsites; and (3) determine how seedling growth response to soil moisture differs between populations.