Microsite-specific 25-year mortality of Norway spruce saplings

Abstract

Regeneration of coniferous tree species depends on forest structures related to tree death: canopy gaps, decaying wood, and windthrow mounds and pits. This also applies to Norway spruce (Picea abies), which abundantly regenerates from seeds on dead wood and windthrow mounds, and which benefits from gaps in the forest canopy. Accumulated evidence shows much lower mortality of seedlings on elevated structures than on soil, but for saplings the long-term mortality pattern is poorly known. We studied Norway spruce sapling mortality in a natural old-growth subalpine forest in the Western Carpathians (S Poland), tracking the fate of 1304 saplings growing in different conditions. We found that the 25-year sapling mortality rate significantly differed between three types of microsites, being highest on dead wood (49%), intermediate on undisturbed soil (41%) and lowest on mounds (30%). Sapling mortality decreased with increasing canopy openness. Mortality was highest for medium-sized saplings (2–3 m high), regardless of microsite type and mortality cause. For saplings growing on dead wood, mortality increased with increasing decay class of the wood. For those growing on soil, mortality was higher in patches of Athyrium distentifolium than when surrounded by Vaccinium myrtillus and Dryopteris dilatata. We did not detect differences in mortality between saplings growing next to tree trunks and those growing away from them. The pattern of sapling mortality differed from that of seedlings, but it did not change the regeneration pattern over microsites, which is established at the seedling stage. In particular, we believe that the considerable differences in mortality rates between dead wood and the other microsites at the sapling stage cannot counterbalance increased seedling recruitment and survival on dead wood.