Canopy gap creation and closure in temperate mountain forests identified from multi-temporal lidar data

Abstract

Canopy gaps are the fingerprint of forest disturbances, with forest disturbances being the driving force of forest dynamics. Forest gaps can substantially vary in size, from small groups of trees to several hundreds to thousands of hectares being disturbed by windthrow, outbreaks of tree-killing insects or fire. Gap creation is a complex process, because individual disturbance agents can interact, eventually forming larger gaps from linked disturbance events. We still know little about the emergence of gaps: I.e., do small gaps beget large gaps by continuously growing larger, or are small gaps staying small, and large gaps are created large? Furthermore, in order to understand the effect of gaps on forest dynamics, it is equally important to consider gap closure, as the interplay between gap creation and closure determines how persistent gaps are. To address these issues, we here investigated natural patterns of canopy gap creation and closure in an unmanaged temperate mountain forest ecosystem in the Berchtesgaden National Park. Three repeated lidar acquisitions from 2009, 2017 and 2021, covering 3543 ha of closed forest enabled the analysis of gap creation, pervasiveness and closure rates. We delineated gaps from lidar derived Canopy Height Models by thresholding vegetation < 5m and applying a minimum gap size of 400 m2 (i.e., approximately the size of one large canopy tree). The identification of gap creation, expansion and closure was done by subtracting each year’s individual gap layers. New and expanding gaps were classified by the presence of gaps in the same geographical location in the previous time step. Crown plasticity leading to lateral closure of gaps vs. ingrowth of regenerating trees are differentiated by the distance to the forest edge and a maximum vertical vegetation height gain during the respective observation period. First results indicate a higher annual gap closure rate (0.66% per year) than annual gap creation rate (0.38% per year), reducing the total gap area across the study area from 22.8% in 2009 to 19.1% in 2021. With increasing elevation, both gap creation and closure increase. Gap expansion is the dominant process of gap creation, underlining the spatially contagious nature of the dominant disturbance agents in the area, such as bark beetles and windthrow. Wind creates edges, which are again more susceptible to wind and provide abundant breeding material for bark beetles that can infest neighboring stands, again creating edges susceptible to wind. Regeneration is the dominating process of gap closure, while lateral crown expansion plays a crucial role mainly in broadleaved forests dominated by European beech. Smaller gaps close disproportionally faster than larger gaps and mixed forests close gaps faster than pure forests, likely due to niche complementarity. Overall, our study presents the first landscape-scale assessment of canopy gap creation and closure in temperate mountain forests and adds to our understanding of forest dynamics.