Growth response of spruce saplings in relation to climatic conditions along a gradient of gap size

To evaluate the relative importance of climatic factors and the level of natural canopy disturbance on sapling growth rates, terminal shoot increment of Norway spruce (Picea abies (L.) Karst.) saplings was analyzed in old-growth Sphagnum-Myrtillus forests of the European southern boreal zone (Tver Region, Russia). For a 5-year period, terminal increments were retrospectively measured in 225 saplings in a range of naturally created canopy gaps. Climatic variability was estimated by Seljaninov hydrothermal coefficient. Variation in the growth rate was partitioned as (i) annual variation observed within a particular sapling over a 5-year period ("within-stem variation," WSV) and (ii) variation of 5-year cumulative height increments within a particular location ("within-location variation," WLV). Sapling growth was positively related to gap size and, except when under canopy location, with the height of the saplings. For the growth, differences in sapling location along a gradient of gap sizes were more important than annual dynamics of water availability. Impact of the annual climatic variability was less pronounced in large gap, compared with other locations. Absolute values of WSV and WLV were similar under an intact canopy. WSV had a tendency to decrease in greater gaps, whereas WLV variation increased.