Explaining interspecific differences in sapling growth and shade tolerance in temperate forests

1. Species differences in growth and shade tolerance might contribute to coexistence of tree species. To explore how such differences depend on underlying plant traits, 14 tree species were investigated in temperate forests on sand and loess soils in the Netherlands. Plant traits were measured for 0.5-1 m tall saplings: 10 saplings growing at low light conditions and 10 at high light conditions. Growth was determined from annual lengths of leader shoots, and shade tolerance from an independently tested scale. 2. Shade tolerance and extension growth of the leader shoot were negatively correlated. Plant traits that related positively with shade tolerance were negatively related to extension growth, and vice versa. Photosynthetic capacity, leaf dark respiration and total leaf mass related weakly to shade tolerance and extension growth. Specific leaf area (SLA, leaf area/leaf mass) and wood density were the traits most strongly correlated to shade tolerance and extension growth. 3. Shade-tolerant species had denser wood and slower extension growth. They also had larger crowns, suggesting that dense stems provide strength for supporting larger crowns. 4. More shade-tolerant species had a higher SLA, which is opposite to tree communities with larger leaf life span variation. In winter deciduous tree communities, more shade-tolerant species benefit from investing leaf area at low costs (high SLA) rather than from investing in durable leaves (low SLA). 5. Species on sand had higher growth rates and higher light requirements than species on loess. In line with the resource-ratio hypothesis, the dominance of the more shade-tolerant species on nutrient rich loess soils may be attributed to the denser vegetation supported and the resultant lower light availability for saplings compared with saplings on infertile and drought-prone sand soils. 6. Synthesis: These results provide plant trait-based predictions for the regeneration success and composition of species of temperate forests with management based on natural regeneration. These forests are expected to become dominated by shade-tolerant species with high wood density and high SLA on the more productive soils, and by light demanding species with low wood density and low SLA on poorer soils.