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Predicting forest recovery following high-severity fire - Final Report to the Joint Fire Science Program

Author(s): Carmen L. Tubbesing, Scott L. Stephens
Year Published: 2020

Novel combinations of fire regime and forest type are emerging in areas affected by climate change, fire exclusion, and other stressors. Species interactions following wildfire in these areas are not well understood. In Sierra Nevada mixed conifer forests, large patches of stand-replacing fire were once rare but are becoming increasingly prevalent and are quickly revegetated by native shrubs. There is uncertainty as to which tree species are best adapted to recover in the resulting post-fire environments. We introduce a conceptual framework for understanding how the altered fire regime in the Sierra Nevada may affect species composition. We investigate an understudied link in this framework: how juvenile ponderosa pine (Pinus ponderosa) and white fir (Abies lowiana) growth and mortality rates are affected by shrub competition following stand-replacing fire. We measured juvenile conifer growth in relation to shrub competition in five fire footprints ranging from 8 to 35 years old and > 400 ha in size. To test whether reductions in conifer growth may lead to increased mortality, we also evaluated how recent tree growth predicts mortality of similarly aged juvenile trees in nearby managed stands. We then developed a data-driven simulation model that we use to predict conifer emergence above the shrub canopy. We found that juvenile ponderosa pine growth was negatively associated with nearby shrub competition, but white fir growth was not. Both species grew slower preceding mortality, with a steeper relationship found in ponderosa pine. However, our model results showed that ponderosa pine (Pinus ponderosa) emerged at a faster rate than white fir (Abies lowiana) under whitethorn ceanothus (Ceanothus cordulatus) and deerbrush (Ceanothus integerrimus) and at a similar rate under greenleaf manzanita (Arctostaphylos patula). Across all shrub species, ponderosa pine had a relative advantage over fir in the period between conifer establishment and peak shrub competition, requiring a mean of 18 ± 2 years for 50% of individuals to emerge compared to 21 ± 1 years for white fir. Fir emergence rates then surpassed those of pine, leading to similar overall emergence by the end of the simulation: 82% ± 6% for pine and 83% ± 5% for fir. These results show that, on net, shrub neighborhood dynamics do not produce an ecological filter favoring firs, but that emergence patterns are sensitive to shrub species. Further modeling based on this data-driven simulation framework could improve understanding of other important components of post-fire succession, including the understudied process of conifer seedling establishment under shrubs.

Citation: Tubbesing, Carmen; Stephens, Scott. 2020. Predicting forest recovery following high-severity fire - Final Report to the Joint Fire Science Program for contract 17-2-01-23, Berkeley, CA: University of California-Berkeley. 39 p.
Topic(s): Fire Behavior, Simulation Modeling, Weather, Fire Effects, Recovery after fire, Restoration
Ecosystem(s): None
Document Type: Technical Report or White Paper
NRFSN number: 22241
FRAMES RCS number: 61818
Record updated: Dec 7, 2020