Vegetation dynamics following compound disturbance in a dry pine forest: fuel treatment then bark beetle outbreak

Abstract

In the western United States, restoration of forests with historically frequent, low‐severity fire regimes often includes fuel reduction that reestablish open, early‐seral conditions while reducing fuel continuity and loading. Between 2001 and 2016, fuel reduction (e.g., thinning, prescribed burning, etc.) was implemented on over 26 million hectares of federal lands alone in the United States, reflecting the urgency to mitigate risk from high‐severity wildfire. However, between 2001 and 2012, nearly 20 million hectares in the United States were impacted by mountain pine beetle (MPB; Dendroctonus ponderosae), compounding restoration effects in wildfire‐hazard‐treated stands. Knowledge of the effects of treatments followed by natural disturbance on long‐term forest structure and communities is needed, especially considering that fuel treatments are increasingly being implemented and warming climate is predicted to exacerbate disturbance frequency and severity. We tested the interacting effects of treatments designed to reduce high‐severity wildfire hazard in stands subsequently challenged by MPB outbreak on vegetation dynamics using a factorial experimental design (control, thin only, burn only, thin + burn) in a ponderosa pine (Pinus ponderosa)‐dominated forest. Stands were treated by 2002, then impacted by MPB outbreak from 2005 to 2012. We assessed change in overstory and understory forest community structure, composition, and diversity over time. There were distinct thinning, burning, and year effects. Thinning immediately reduced overstory density; pine density then declined 4.5 times more in unthinned than thinned treatments due to MPB. Burning immediately reduced graminoid, shrub, and total understory cover by as much as 52%, resulting in greater species evenness than unburned treatments, but differences disappeared by 2016 due to growth and MPB outbreak. Similarly, multivariate analyses indicated forest communities were starkly different after treatment but became more similar over time, though key understory and overstory attributes still distinguish control and thin + burn. This study shows the value of long‐term silvicultural experiments to evaluate treatment longevity and the compounded effects of treatment and natural disturbance. We demonstrate the homogenizing effects of treatment‐induced growth coupled with MPB‐caused tree mortality on management strategies that just treat the overstory (thinning) or understory (burning), showing that only combined treatments can provide the unique structural and compositional outcomes expected of restoration.

Restoration Treatments Improve Overstory Tree Resistance Attributes and Growth in a Ponderosa Pine/Douglas-Fir Forest

“…The FFS treatments at LEF had substantial impacts on forest structure and composition (Figure 1; Table 1), as characterized by past studies [21,24,25,35,48]. Broadly summarized, the burn-only treatment had little evident effect on overstory trees and composition, as evidenced by the high residual densities of small diameter trees.…”

“…An important consideration is that the gap between surface and overstory fuel strata may be short-lived due to combined slowed recession from above, as this study demonstrates in the thinned treatments, and growth from the understory below. Although trees in both the thin-only and the thin + burn treatments had slowed crown recession, the gap between overstory crown bases (i.e., canopy base height) and understory fuels was more than twice as large in thin + burn because prescribed fire killed understory shrubs, advanced regeneration, and lifted crown base heights [21,48].…”

“…Prescribed fire treatments were low-severity spring burns, with windspeeds less than 13 km h −1 and flame lengths typically less than 0.6 m, but ranging up to 2.7 m in slash piles. Stand conditions have been documented to assess treatment effects on fire hazard and overall vegetation dynamics from 4 to 14 years [21,24,35].…”

“…Growth efficiency, here defined as wood growth per tree leaf area, quantifies tree vigor [20] and theoretically reflects tree capacity to withstand bark-beetle attack. However, it is unclear how restoration treatment, which immediately alters forest structure and subsequent vegetation dynamics [21], impacts these attributes in the years following.…”

Variation in fine root traits with thinning intensity in a Chinese fir plantation insights from branching order and functional groups

“…Thinning is a widely used practice in forest management to develop complex stand structures and compositions for fire and insect resistance 1 and to improve tree growth and stand regeneration 2 – 5 . The effects of thinning on the aboveground structure and processes have been widely investigated 2 , 6 , 7 .…”