Modeling high-severity fire, drought and climate change impacts on ponderosa pine regeneration

We describe a model to investigate the effects of high-severity fire and drought on ponderosa pine regeneration using a water balance approach. Based on literature reviews and an analysis of annual and monthly correlations against tree regeneration, climate envelopes are constructed to simulate conditions during the flowering, seed production, germination phase, seedling growth in the season following germination, and seedling growth in the 2 years following germination. The model was tested against observed regeneration at five sites in the Southwest that experienced high-severity fires during a drought from ca. 1945 to 1956. For validation purposes single fire events, as occurred at each field site, were simulated by altering maximum and minimum temperatures and runoff conditions in declining stages of severity for 7 years. To evaluate long term fire and climate impacts four sensitivity tests were conducted on climate records from 1914 through 2009: (1) a climate control with no modifications to temperature and precipitation inputs; (2) a permanently burned condition simulation; (3) a simulation where climate conditions were altered based on IPCC future climate change projections and; (4) a simulation with both climate change and fire modifications. Results show that annual data correlate poorly with ponderosa pine regeneration compared to monthly correlations. On average the model correctly predicts regeneration outcomes 62% of the time at the field sites. The sensitivity tests suggest that regeneration rates are reduced by 43% in post high-severity fire environments because of changed conditions during germination and post-germination establishment phases. Future climate projections result in mixed outcomes across the 5 sites and an average reduction of regeneration rates by 8%. Ponderosa pine forests on the dry end of the climate envelope are predicted to experience severe reduction in regeneration.