Woody plant proliferation in desert grasslands: perspectives from roots and ranchers

Abstract

The widespread proliferation (or 'encroachment') of trees and shrubs in grasslands over the past 150 years is embedded in both natural and human systems. This dissertation addressed the following ecological and ethnoecological questions. Can seedling traits help us understand why so few woody species have encroached markedly into North American desert grasslands, and the conditions likely to promote their proliferation? What is the role of informal knowledge of the environment in efforts to manage woody plant abundance? Woody seedling survival often depends on rapid taproot elongation. In glasshouse experiments, initial water supply markedly affected taproot elongation in young seedlings. Response patterns may help explain recruitment patterns in Larrea tridentata, the principal evergreen woody encroacher in Sonoran and Chihuahuan Desert grasslands, and in Prosopis velutina and Prosopis glandulosa, the principal deciduous woody encroachers in Sonoran and Chihuahuan Desert grasslands, respectively. P. velutina and P. glandulosa showed greater sensitivity to water supply levels at the seedling stage than did the similar, related non-encroachers, Acacia greggii, Parkinsonia florida and Parkinsonia aculeata . This enabled the Prosopis species to overcome lower seed and seedling biomass to achieve similar taproot length to A. greggii and the Parkinsonia species. Consequently, population level advantages of lower seed mass, such as high seed numbers, may enhance encroachment potential in the Prosopis species without being negated by corresponding seedling survivorship disadvantages. I used semi-structured interviews to document informal rangeland monitoring by ranchers in southeast Arizona. Ranchers used qualitative methods to assess forage availability, rangeland trends and responses to woody plant suppression measures. Informal rangeland assessments informed ranchers' management decisions on sub-yearly, yearly and multi-year timescales. Informal monitoring appeared largely compatible with formal monitoring and natural science, and most ranchers integrated the two systems. Informal rangeland assessments can be valuable in planning woody plant suppression measures, particularly in light of the small number of formal long-term studies of brush suppression. Ecological studies may help predict places and periods of relatively rapid encroachment, perhaps enabling early or pre-emptive brush suppression measures. Thus, both seedling ecology and informal environmental knowledge are likely to be useful in managing woody plant populations in desert grasslands.