Environmental influences on density and height growth of natural ponderosa pine regeneration following wildfires

Authors: HAMMOND, DARCY - University Of Idaho; STRAND, EVA - University Of Idaho; MORGAN, PENELOPE - University Of Idaho; HUDAK, ANDREW - Us Forest Service (FS); Newingham, Beth

Submitted to: Fire
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/18/2021
Publication Date: 10/21/2021
Citation: Hammond, D.H., Strand, E.K., Morgan, P., Hudak, A.T., Newingham, B.A. 2021. Environmental influences on density and height growth of natural ponderosa pine regeneration following wildfires. Fire. 4(4). Article 80. https://doi.org/10.3390/fire4040080.
DOI: https://doi.org/10.3390/fire4040080

Interpretive Summary: Over the past century the size and severity of wildfires, as well as post-fire recovery processes (e.g., seedling establishment), have been altered from historical levels due to management policies and changing climate. Tree seedling establishment and growth drive future overstory tree dynamics after wildfire. Post-fire tree regeneration can be highly variable depending on burn severity, pre-fire forest condition, tree regeneration strategies, and climate; however, few studies have examined how different abiotic and biotic factors impact seedling density and growth and the interactions among those factors. We measured seedling density and height growth in the period 2015–2016 on three wildfires that burned in ponderosa pine (Pinus ponderosa) forests in the period 2000–2007 across broad environmental and burn severity gradients. Using a non-parametric multiplicative regression model, we found that downed woody fuel load, duff depth, and fall precipitation best explained variation in seedling density, while the distance to nearest seed tree, a soil productivity index, duff depth, and spring precipitation as snow best explained seedling height growth. Overall, results highlight the importance of burn severity and post-fire climate in tree regeneration, although the primary factors influencing seedling density and height growth vary. Drier conditions and changes to precipitation seasonality have the potential to influence tree establishment, survival, and growth in post-fire environments, which could lead to significant impacts for long-term forest recovery.

Technical Abstract: Over the past century the size and severity of wildfires, as well as post-fire recovery processes (e.g., seedling establishment), have been altered from historical levels due to management policies and changing climate. Tree seedling establishment and growth drive future overstory tree dynamics after wildfire. Post-fire tree regeneration can be highly variable depending on burn severity, pre-fire forest condition, tree regeneration strategies, and climate conditions; however, few studies have examined how different abiotic and biotic factors impact seedling density and growth and the interactions among those factors. We measured seedling density and height growth in 2015-2016 on three wildfires that burned in ponderosa pine (Pinus ponderosa) forests during the time period 2000-2007 across broad environmental and burn severity gradients. Using a non-parametric multiplicative regression model, we found that downed woody fuel load, duff depth, and fall precipitation best explained variation in seedling density, while the distance to nearest seed tree, a soil productivity index, duff depth, and spring precipitation as snow best explained seedling height growth. Overall, burn severity and climate were found important in tree regeneration although the primary factors influencing density and growth vary. Drier conditions and changes to precipitation seasonality have the potential to influence tree establishment, survival, and growth in post-fire environments, which could lead to significant impacts for long-term forest recovery.