Canopy gap dynamics and tree understory release in a virgin beech forest, Slovakian Carpathians

Canopy gaps play a crucial role for forest dynamics processes, as they largely determine light transmission to lower canopy strata, thereby controlling the turnover of tree individuals in the stand. Even though their functional importance is undisputed, quantitative data on the rate of gap creation and gap closure, and the temporal change in gap size distribution patterns in temperate virgin forests are scarce. We used a repeated inventory (line-intercept sampling) of gap size frequency and fraction in a virgin beech (Fagus sylvatica) forest in the Slovakian Carpathians over a 10-year interval (2003-2013) to test the hypotheses that (i) disturbance intensity and thus gap creation and gap closure rate change only little over time, (ii) gaps persist or even expand, until they are filled primarily by vertical ingrowth of trees from lower strata, and (iii) gap creation promotes the height growth of released saplings and sub-canopy trees. In the 2003 and 2013 inventories, 37 and 30 gaps >20 m2 size were mapped along a total of 3217 m transect line investigated. The large majority of gaps was <100 m2 in size; large gaps >500 m2 were very rare. Gap fraction decreased significantly from 13.6% in 2003 to 8.2% in 2013 (associated with a reduction in mean gap size from 261 to 96 m2), indicating considerable variation in disturbance intensity in the past decades. Before 2003, both large gaps (probably caused by wind throw) and small gaps (from dying trees) have been formed, while only small gaps developed in the period 2003-2013. Small gaps were closed within a few years through rapid horizontal canopy expansion of neighboring beech trees, while vertical gap filling through ingrowth of lower canopy layers and regeneration was the dominant process in larger gaps. Saplings and trees in lower canopy layers formed a heterogeneous understory in large parts of recently formed gaps and responded to this process with increased height growth. We conclude that, despite considerable variation in disturbance intensity over time, this beech virgin forest responds to gap formation with high resilience through rapid lateral canopy expansion in small gaps and ingrowth of saplings and sub-dominant tree layers in larger gaps.