Abstract
Low mortality rates and slow growth differentiate shade-tolerant from shade-intolerant species and define the survival strategy of juvenile trees growing in deep shade. While radial stem growth has been widely used to explain mortality in juvenile trees, the leaf area ratio (LAR), known to be a key component of shade tolerance, has been neglected so far. We assessed the effects of LAR, radial stem growth and tree height on survival time and the age-specific mortality rate of juvenile Fagus sylvatica L. (European beech), Acer pseudoplatanus L. (sycamore maple) and Acer platanoides L. (Norway maple) in a primeval beech forest (Ukraine). Aboveground and belowground biomass and radial stem growth were analysed for 289 living and 179 dead seedlings and saplings. Compared with the other species, F. sylvatica featured higher LAR, slower growth and a lower mortality rate. The average survival time of F. sylvatica juveniles (72 years) allows it to reach the canopy more often than its competitors in forests with low canopy turnover rate. In contrast, a combination of lower LAR, higher growth rate and higher age-specific mortality rate of the two Acer species resulted in their shorter survival times and thus render their presence in the canopy a rare event. Overall, this study suggests that shade tolerance, commonly defined as a relationship between sapling mortality and growth, can alternatively be formulated as a relationship between survival time and the interplay of growth and LAR.
Highlights
The success of forest regeneration in reaching the canopy is related to species-specific patterns of radial and height growth, with shade-tolerant species growing better in the shade than shade-intolerant ones and vice versa (Givnish 1988; Popma and Bongers 1988)
We focused on the Uholka part of the reserve because it has a larger share of Acer species than the Shyrokyi Luh part (Commarmot et al 2013)
The mean age of dead trees was substantially higher for F. sylvatica in the height classes from 36 to 500 cm compared with Acer spp., i.e. F. sylvatica survived for a longer time (Fig. 1)
Summary
The success of forest regeneration in reaching the canopy is related to species-specific patterns of radial and height growth, with shade-tolerant species growing better in the shade than shade-intolerant ones and vice versa (Givnish 1988; Popma and Bongers 1988). The better performance of shade-tolerant species under low-light conditions has been attributed to a more efficient use of light due to a larger leaf area per plant mass (leaf area ratio = LAR), a larger and deeper crown, and faster growth (Bazzaz 1979; Givnish 1988). It has been argued that survival is more important than growth in determining regeneration success at low light levels (Canham et al 1999; Kitajima 1994; Kobe et al 1995; Veneklaas and Poorter 1998). The high survivorship of shade-tolerant species under low-light conditions has been attributed to efficient carbon allocation and storage, acting as a buffer against stress (Canham et al 1999; Kobe 1997; Reich et al 1998). The survival of regeneration may play a more important role than growth in determining plant success at low-light levels, and the trade-off between growth at high-light levels and survival (rather than growth) at low-light levels may be the key explanation for the shade
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