Gap dynamics in a near-natural spruce forest at Mt. Brocken, Germany

Abstract

We analyzed indices of gap dynamics, i.e. gap formation and gap closure rates as well as turnover rates, in a montane conifer forest in the temperate zone and asked: (1) Are the characteristics of gap dynamics comparable with those of other temperate forests? (2) Do gap dynamics exhibit temporal trends, and (3) if so, are they related to recent climate change? We also addressed methodological issues concerning the differences observed at differing temporal and spatial scales of the used datasets. Thus, we asked further how the indices of gap dynamics are influenced (4) by differences in the length of observation periods and (5) by differences in gap size categories included in the study?The study area was located in the near-natural Picea abies forests in the core zone of the Harz National Park on the north-eastern slopes of Mt. Brocken, Germany. From this area, aerial photographs from the last 60yrs (1945, 1991, 2000, 2003) were analyzed by GIS techniques. Total gap area, number of gaps, gap size and area-based turnover rate, rotation time, gap formation and gap closure rate were calculated and correlated with climate variables.The total gap area varied between 10.9ha and 19.0ha for the study area of 225.2ha between the different years. Similarly, the median of the gap size was 155.6m2 and 87.0m2 in 1945 and 2003, respectively. A rotation time of 228yrs was calculated for the study area. Area-based turnover rates and gap formation rates increased with time, while mean gap size decreased. The most frequently recorded gaps were those in the category <100m2. In particular, small gaps at the level of single trees (≤40m2) became more frequent in the last decade and showed a higher gap formation rate than larger gaps. Temperature was significantly related to both turnover and gap closure rates.In conclusion, for the near-natural spruce stands at Mt. Brocken indices of gap dynamics were of a magnitude comparable to those described from other forest types in the temperate zone. However, the indices of gap dynamics increased with time, which coincided with increasing temperatures over the last few decades.