Abstract
Anticipating how boreal forest landscapes will change in response to changing fire regime requires disentangling the effects of various spatial controls on the recovery process of tree saplings. Spatially explicit monitoring of post-fire vegetation recovery through moderate resolution Landsat imagery is a popular technique but is filled with ambiguous information due to mixed pixel effects. On the other hand, very-high resolution (VHR) satellite imagery accurately measures crown size of tree saplings but has gained little attention and its utility for estimating leaf area index (LAI, m2/m2) and tree sapling abundance (TSA, seedlings/ha) in post-fire landscape remains untested. We compared the explanatory power of 30 m Landsat satellite imagery with 0.5-m WorldView-2 VHR imagery for LAI and TSA based on field sampling data, and subsequently mapped the distribution of LAI and TSA based on the most predictive relationships. A random forest (RF) model was applied to assess the relative importance and causal mechanisms of spatial controls on tree sapling recovery. The results showed that pixel percentage of canopy trees (PPCT) derived from VHR imagery outperform all Landsat-derived spectral indices for explaining variance of LAI (R2VHR = 0.676 vs. R2Landsat = 0.427) and TSA (R2VHR = 0.508 vs. R2Landsat = 0.499). The RF model explained an average of 55.5% (SD = 3.0%, MSE = 0.382, N = 50) of the variation of estimated LAI. Understory vegetation coverage (competition) and post-fire surviving mature trees (seed sources) were the most important spatial controls for LAI recovery, followed by burn severity (legacy effect), topographic factors (environmental filter) and nearest distance to unburned area (edge effect). These analyses allow us to conclude that in our study area, mitigating wildfire severity and size may increase forest resilience to wildfire damage. Given the easily-damaged seed banks and relatively short seed dispersal distance of coniferous trees, reasonable human help to natural recovery of coniferous forests is necessary for severe burns with a large patch size, particularly in certain areas. Our research shows the VHR WorldView-2 imagery better resolves key characteristics of forest landscapes like LAI and TSA than Landsat imagery, providing a valuable tool for land managers and researchers alike.
Highlights
Wildfire has been well recognized as a crucial process governing the dynamics of forest structure, composition and function in boreal forests [1,2]
Our study is one of very few to investigate the utility of very-high resolution (VHR) satellite imagery for monitoring post-fire forest structural attributes in terms of Leaf area index (LAI) and Tree sapling abundance (TSA)
We found that WorldView-2 VHR imagery outperforms Landsat imagery but still has some limitations
Summary
Wildfire has been well recognized as a crucial process governing the dynamics of forest structure, composition and function in boreal forests [1,2]. The resilience of boreal forests is decreasing due to a misalignment between changing fire regimes and vulnerable ecosystem states [9,10,11]. Biome transitions in boreal forests, including shifts from forests to treeless steppes and an increase in deciduous trees, are thought to be exacerbated by changing fire regimes [12,13,14,15]. Both fire frequency and severity in boreal ecosystems is predicted to increase in the future [16,17,18,19]. Research on the spatial controls of forest restoration post-fire is essential for understanding how boreal ecosystems respond to changing fire regimes and advancing ecologically sound policies in boreal fire management and restoration
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