Does age matter? Grass density differences under new and old mesquite. McClaran, Mitchel^*,1, Angell, Deborah², ¹ School of Natural Resources, Tucson, Arizona, U.S.A.² Sonoran Desert Network, Tucson, Arizona, U.S.A.

ABSTRACT- Availability of light decreases and soil nutrients increase as the length of mesquite occupation increases. For grass species common under mesquite (facultative skiophytes), the abundant nutrients are thought to compensate for the reduced light; whereas, those species rarely found under mesquite (obligate heliophytes) are intolerant of reduced light. Therefore, we hypothesized that 1) the abundance of facultative skiophytes will be less under newer than older tree cover, and 2) the abundance of obligate heliophytes will not differ between newer and older tree cover. We compared grass density (measured in 1997, 2000, and 2003) among mesquite-free areas, under newer mesquite cover that was <20 years old, and older mesquite cover that was >40 years old on the Santa Rita Experimental Range, Arizona. We accepted our hypotheses for the facultative skiophyte Muhlenbergia porteri and the obligate heliophyte Bouteloua rothrockii. Aristida spp. did not conform to our expectations. Their absence from older mesquite cover and equal abundance under newer mesquite cover and mesquite-free areas suggests a limited tolerance to shade and no compensation from increased nutrients. The density of Eragrostis lehmanniana and Digitaria californica did not differ between new and old mesquite or mesquite-free areas, suggesting that they were indifferent to these levels and ages of mesquite cover. For some grass species, the age of mesquite cover does matter, but other species are less discriminating.

KEY WORDS: heliophytes, Arizona, skiophytes