Patterns of mast production in pinyon and juniper woodlands along a precipitation gradient in central New Mexico (Sevilleta National Wildlife Refuge)

This paper reports on an 8-year continuing project with the Sevilleta Long-Term Ecological Research program (LTER) in central New Mexico aimed at the study of dynamics of woodland ecosystems. The study of mast production was conducted at six experimental woodland sites situated across an elevational gradient of 1500–2000 m in proximity to permanent meteorological stations. The dominant woody species were Juniper monosperma, Pinus edulis, and Quercus turbinella. Functioning of arid ecosystems of the Southwest undergoes seasonal, annual and multidecadal fluctuations which link to corresponding dynamics of available moisture. Results of the 1997–2004 study show a gradual decrease in mast production of all three species at all six sites, due to a continuing drought in this period. Mast production in a given year is determined to a considerable extent by available moisture during the entire growth year, from the previous spring to the current summer. The areas of oak–pinyon–juniper woodlands with higher elevation and highest precipitation generally have the highest yield of juniper berries, pinyon nuts and acorns. The severe drought of 2001–2003 has been accompanied by a mass dying of mature pinyon pines and loss of foliage in junipers. According to our observations, there is no correlation between mast production and amount of precipitation in the current year's monsoon period (June–September). A high correlation between acorn production and non-monsoonal precipitation (October–May) was established, but no such correlation was found for juniper berries and pinyon nuts. Weather conditions (wind, temperature, and moisture) of late May are a critical factor determining the success of fertilization and subsequent mast production in all three species.