Document: SAM-3-33-27

Mechanisms and implications of herbivory-induced decrease in litter C/N ratio in pinyon pine.

CHAPMAN, S.K.*, G.W.KOCH and N.S.COBB

Northern Arizona University, Flagstaff, AZ 86011 USA ¹

Abstract:
Herbivory effects on nutrient cycling have been demonstrated in a number of different ecosystems, but have not been examined in the arid coniferous systems found throughout the intermountain and southwestern U.S. In the pinyon juniper woodland near Sunset Crater National Monument, Arizona, a large proportion of pinyon pines (Pinus edulis ) are chronically infested with a scale insect, Matsucoccus acalyptus, while others are resistant to this herbivore. Previous research has demonstrated that scale infestation induces premature needle abscission, reducing mean needle lifetime from six to two years. We have found that this herbivore-induced response reduces nitrogen (N) retranslocation from senescing needles, decreasing the C/N ratio of needle litter by nearly 33%. We measured mass-specific rates of needle ethylene production as an indicator of tissue senescence and found an average of 0.24 nmol g^-1 h^-1 in scale-infested needles compared to 0.07 nmol g^-1 h^-1 in the senescent age class of scale-resistant trees and 0.05 nmol g^-1 h^-1 for scale-free, non-senescent needles on resistant trees. We hypothesize that scale infestation induces ethylene production and needle senescence; thus, increasing availability of mobile forms of N to the mesophyll feeding herbivore. At the tree level, herbivory causes a large reduction in nutrient resorption efficiency, while at the system level the reduced C/N ratio may stimulate litter decomposition and increase soil N availability.

Keywords: nutrient resorption efficiency, senescence, C/N ratio

This abstract is being presented at: 9:45 AM in session:
Oral Session #44: Terrestrial Invertebrates: Foodwebs and Plant Responses.