Fire feedbacks, alternative stable states and the role of livestock on post-fire subalpine N. pumilio forests

PARITSIS, JUAN; HOLZ, ANDRES; BLACKHALL, MELISA; KITZBERGER, THOMAS; RAFFAELE, ESTELA; TIRIBELLI, FLORENCIA; MORALES, JUAN MANUEL; GOWDA, JUAN H.; VEBLEN, THOMAS T.

Chico Hot Springs, Montana

Workshop; WildFIRE PIRE Workshop: Understanding climate-human-fire interactions at different temporal and spatial scales; 2017

Montana State University

Fire feedback processesthrough which pyrophobicvegetation communities are replaced by alternative stablestates dominated by pyrophytic vegetation have been increasingly documented in multiplecommunities and have profound implications in ecosystem functioning. A thorough understanding of the mechanismsresponsible for these feedbacks demands the evaluation of multiple processes, whichmay synergistically  or antagonisticallyaffect the feedbacks. Inthe easternslopes of the Patagonian Andes two woody communities grow in juxtaposition: pyrophobic forests dominated by the non­ resprouter tree Nothofagus pumilioand pyrophytic mixed shrublands. Our overall objective is to quantify fuel properties (amountand vertical distribution), vegetation structure and microclimate in unburned forests and in post-fire plant communities as mechanisms that might explain flammability differences and fire vegetation feedbacks involving these communities. Additionally, we experimentally evaluatedthe role of livestock on the transition and maintenance of the phyrophytic shrubland alternative stable state that results from the initial severeburning of pyrphobic tall N. pumilioforest. Fuel amountandarrangement in unburned N. pumilio forestsare unfavorable for fire activitycompared with post-fire N. pumilio forests and shrublands. Microclimatic conditions following burning of N. pumilioforests and shrublands were significantly warmer and drier than in the unburned forest, and more frequently exceeded thresholds associated with fire activity in this region. These results suggestthat once N. pumilio forests are transformed to the alternative state of shrublands, return to a forest coveris unlikely due to increased probability of burning in shrublands. Livestock presence generated antagonistic effects on flammability. Herbivores decreased tissue ignitability, total fine fuel and litter depth, and disrupted the vertical and horizontal fine fuel continuity, therefore reducing the probability of fire propagation. However, livestock also increased ratiosof dead to live fine fuels, reduced soil moisture, and inhibited tree height growth to canopysize, consequently impeding the development of a closedpyrophobic forest canopy,which may promotethedevelopment of novelpost-fire transitional states.