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Seedling size influences relationships of shade tolerance with carbohydrate-storage patterns in a temperate rainforest

Authors


†Author to whom correspondence should be addressed. E-mail: clusk@bio.mq.edu.au

Summary

  • 1Carbohydrate storage has been attributed an important role in the ability to tolerate shade, yet empirical support for this idea has been patchy. We asked if carbohydrate-storage patterns of seedling evergreens in low light are correlated with variation in shade tolerance, and how these patterns change with seedling size.
  • 2We measured biomass distribution and total non-structural carbohydrate (NSC) concentrations of leaves, stems and roots of two seedling size classes of six evergreens growing in a temperate rainforest understorey. Light requirements of the six species were quantified by calculating the 10th percentile of the distribution of established seedlings in relation to canopy openness.
  • 3NSC averaged 14% of the dry mass of small seedlings (40–60 mm tall), and 22% of that of large seedlings (400–600 mm tall). This difference was entirely due to variation in starch reserves, which on average accounted for 60% of NSC in small seedlings and 84% in large seedlings.
  • 4NSC concentrations of leaves and roots (but not stems) of large seedlings were negatively correlated with species’ shade tolerance, but no such pattern was found in small seedlings. Leaf NSC on an area basis was not related to species’ shade tolerance in either size class.
  • 5Partitioning of the NSC pool between leaves, stems and roots of small seedlings was closely related to variation in shade tolerance. Small seedlings of shade-tolerant species had a relatively low proportion of their NSC pool in leaves and a high proportion in roots. This is likely to ensure the retention of the greater part of the NSC pool even in the event of extensive defoliation, and the availability of reserves to replace lost leaves. In contrast, the large leaf-mass fraction of large seedlings of shade-tolerant species (46–47% of biomass) meant that these plants had a large proportion of their NSC pool in foliage.
  • 6Results suggest that, in Chilean rainforest evergreens, any adaptive relationship of carbohydrate storage with shade tolerance may be confined to young seedlings, involving interspecific variation in the partitioning of reserves between leaves and other organs, rather than especially high NSC concentrations in shade-tolerant species.

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