Interactions of multiple disturbances in shaping boreal forest dynamics: a spatially explicit analysis using multi‐temporal lidar data and high‐resolution imagery

Abstract

Summary1. Mixed-wood boreal forests are often considered to undergo directional succession from shadeintolerant to shade-tolerant species. It is thus expected that overstorey gaps should lead to the recruitment of shade-tolerant conifers into the canopy in all stand development stages and that the recruitment of shade-intolerant hardwoods would be minimal except in the largest gaps. 2. We analysed short-term gap dynamics over a large 6-km 2 spatial area of mixed-wood boreal forest across a gradient of stands in different developmental stages with different times of origin since fire (expressed as stand 'age') that were affected differentially by the last spruce budworm (SBW) outbreak. Structural measurements of the canopy from lidar data were combined with spectral classification of broad species groups to characterize the gap disturbance regime and to evaluate the effect of gap openings on forest dynamics. 3. Estimated annual gap opening rates increased from 0.16% for 84-year-old stands to 0.88% for 248-year-old stands. Trees on gap peripheries in all stands were more vulnerable to mortality than interior canopy trees. 4. Due to recovery from the last SBW outbreak 16 years previously, gap closure rates were higher than opening rates, ranging from 0.44% to 2.05% annually, but did not show any relationship with stand age. There was, however, a continuing legacy of the last SBW outbreak in old-conifer stands in terms of a continued high mortality of conifers. In all stands, the majority of the openings were filled from below, although a smaller but significant proportion filled from lateral growth of gap edge trees. 5. Synthesis. The forest response to moderate-to small-scale disturbances in old-growth boreal forest counters the earlier assumption that the transition from one forest state to the next is slow and directional with time since the last fire. Overall, a small 6% increase in hardwoods was observed over 5 years, largely due to regeneration in-filling of hardwoods in gaps instead of successional transition to more shade-tolerant conifers. Gaps are vital for hardwood maintenance while transition to softwoods can occur without perceived gap-formation as overstorey trees die, releasing understorey trees.

Using Unmanned Aerial Vehicles (UAV) to Quantify Spatial Gap Patterns in Forests

“…While we have not analyzed aerial images in consecutive years, it is quite likely that the very small gaps detected here are part of repeated gap formation induced by small-scale disturbance. For example, trees of gap peripheries are more vulnerable to mortality and injury than interior canopy trees, which could lead to small openings [37]. Overall, it has been shown that repeated disturbances are important for the regeneration of species that can tolerate intermediate light levels and that are able to survive several periods of suppression from neighboring trees before growing into the canopy [38].…”

Gap pattern of the largest primeval beech forest of Europe revealed by remote sensing

“…Provided that an accurate digital terrain model (DTM) exists, canopy height models, which have become popular for disturbance analysis by means of canopy gaps or canopy height profiles (Fujita et al 2003, Henbo et al 2004, Vepakomma et al 2010, could be calculated by subtracting the terrain model from the surface model , Vepakomma et al 2008. However, for our study area in the Ukraine, as well as for many other countries, accurate DTMs are lacking.…”

Automated Detection of Forest Gaps in Spruce Dominated Stands Using Canopy Height Models Derived from Stereo Aerial Imagery

“…Gap size and distribution pattern were mapped from satellite imagery by Garbarino et al [23] and Hobi et al [24]. Vepakomma [25,26] used Light Detection and Ranging (LiDAR) to assess spatial contiguity and continuity of canopy gaps over time in mixed wood boreal forests. Rugani et al [27] based a similar research question using visual stereo interpretation of scanned color-infrared (CIR) aerial photographs.…”