The increasing size and severity of wildfires in the western United States has created large, contiguous areas of very high tree mortality and correspondingly low recruitment of conifers that depend on seed rain from surviving or unburned trees. Planting trees can alleviate seed availability constraints, but reforestation often involves intensive (and expensive) management of densely planted stands, such as through the removal of competing vegetation. While intensive management is common, there has been little exploration of the tradeoffs involved in using this strategy in ecosystems where the desired objective is a relatively low-density stand – as is the case for fire-resilient pine-dominated and mixed conifer forests in the Sierra Nevada, CA.
We investigated this question by simulating the long-term outcome of planted stands with a tree growth-and-yield model (the Forest Vegetation Simulator). We parameterized the model using field-collected data from 132 paired plots planted after 8 wildfires; half had competing vegetation removed (“treated”) and half did not (“control”). We asked whether treated stands had a greater likelihood of reaching the historical reference density (~25-80 trees per hectare), whether they had a shorter time to reach this density, and whether simulated wildfires in the near- or mid- term influenced these findings.
Results/Conclusions
We found that the likelihood of reaching reference density levels within the 100-year simulation period was strongly influenced by simulated fire characteristics: under high-intensity wildfire, only 10-20 % of plots reached reference density levels, whereas 75% percent reached target density levels under more moderate fire conditions. We did not find that treatment status significantly influenced the likelihood of reaching target densities, but heat load index (a proxy for productivity) and distance to seed sources were significant in most fire scenarios. Our findings are consistent with prior work showing that the influence of early competing vegetation removal on tree growth and forest structure declines over time. We anticipate that these results will be useful for planning reforestation strategies--such as whether to prioritize intensive or extensive management--in circumstances where the management objective is the restoration of fire-resilient forest structure.
