Enhancement of photosynthesis and growth of an aridland perennial in response to soil nitrogen pulses generated by mule deer

Abstract

The altitudinal range of the aridland herbaceous perennial Cryptantha flava overlaps with the wintering range of mule deer ( Odocoileous hemionus) in the Colorado Plateau region of the western United States. Deer do not feed on C. flava, but do influence its microhabitat via excretion of nitrogenous compounds. We were interested in determining the importance of Mule deer in redistributing N across the landscape as well as determining if C. flava was capable of exploiting the ephemeral pulse of this redistributed N. Based on pellet group counts, we found that mule deer deposited on average 16 g N m −2 per year to our study site in northeastern Utah, but these deposits were highly variable spatially. The distribution of pellet groups varied from 3 to 49 per 100 m 2, resulting in 6% of individual plants in the population occurring within 20 cm of a pellet group. We applied a one-time treatment of fresh fecal pellets (0.53 g N per pellet group) and experimental urine (6.4 g urea N) to small pre-reproductive plants to examine plant response to N pulses. Within 1 week of application, leaf N concentrations, instantaneous photosynthetic rates, carboxylation efficiencies, and CO 2 saturated photosynthetic rates were significantly higher for plants receiving simulated urine. Increased photosynthetic rates were maintained for at least 3 weeks post treatment. Over the next 2 years, urine-treated plants had higher growth rates, greater overall size, higher flowering rates, and produced more seed per plant than those not receiving artificial urine. We conclude that C. flava is capable of rapid root uptake and incorporation of nitrogen into photosynthetic structures in response to nitrogen pulses. Mule deer activity has direct population level consequences on this species by increasing spatial nutrient heterogeneity, and the growth and reproductive rate of individuals.