Close association of RGR, leaf and root morphology, seed mass and shade tolerance in seedlings of nine boreal tree species grown in high and low light

Abstract

1. To test hypotheses concerning adaptation and acclimation of tree species to shaded habitats we determined the growth, biomass partitioning and morphology of seedlings of nine near‐boreal tree species in high‐ and low‐light greenhouse environment (25 and 5% of full sunlight, respectively), comparable to sunlit gap and shaded microsites in boreal forests. The species differ widely in shade tolerance, seed size and leaf life span. 2. In low light, all species allocated proportionally more biomass to stems and less to roots, but the same to foliage, compared with the high‐light environment. At a common size, all species had finer leaf morphology (higher specific leaf area, SLA) but coarser root morphology (lower specific root length, SRL) in low than high light. From a whole plant perspective, all species enhanced leaf area per unit plant mass (leaf area ratio, LAR) in low light and root length per unit plant mass (root length ratio, RLR) in high light. 3. Shade‐intolerant deciduous species had higher RGR, SLA and SRL than larger seeded evergreens: ranking from Populus, Betula and Larix spp., then to five evergreen Pinus, Picea and Thuja spp., which were generally comparable in these traits. There were no changes in growth rankings of species between high‐ and low‐light environments, nor consistent differences among species in biomass partitioning. Hence, species differences in leaf and root morphology (SLA, SRL) drove whole plant patterns, such as Populus, Betula and Larix had greater total leaf area and root length per unit plant mass (LAR and RLR, respectively) than the evergreens. Interspecific variation in RGR in both high and low light was positively correlated (r≈ 0·9) with SLA, SRL, LAR and RLR, and negatively correlated (r≈–0·9) to seed mass and leaf life span. 4. These data suggest that SLA, SRL, NAR and RGR are closely associated with variation in life‐history traits and that variation in leaf and root structure more strongly influences patterns of RGR among species and light environments than does biomass partitioning.

Seedling growth of fifteen Brazilian tropical tree species differing in successional status

“…Other responses presented by the plants under both shade conditions included lower R:S and higher LAR, LMR and SLA, with no distinct treatments differences between AS and NS. This growth pattern has been observed by others (Popma & Bongers 1988, 1991, Walters et al 1993, Osunkoya et al 1994, Reich et al 1998 and is considered an adaptation to shade. It is assumed that these responses taken together can increase the ratio of photosynthesis to respiration at the whole-plant level and contribute to the maintenance of a positive carbon balance and maximization of growth in shade (Kitajima 1994).…”

“…Moreover, in contrast to other studies where lower SLA, LAR and LMR and higher R:S were observed for shadetolerant in comparison with shade-intolerant species (Rao & Singh 1989, Kitajima 1994, no consistent trend regarding successional positions was found. In a study with nine boreal tree species, Reich et al (1998) also failed to find any correlation between successional status and biomass allocation or LMR, yet they still observed significant correlations with respect to SLA and LAR. DeLucia et al (1998) also did not find any association of SLA and LAR with shade tolerance status.…”

“…In the present study, there was a tendency for early-successional species to present higher growth rates than late-successional ones, regardless of light environment, and, in fact, significant correlations were found between growth in FS and shade treatments. Other studies comparing species from different successional positions also confirm higher growth for early shade-intolerant than for late shadetolerance species (Poorter 1999, Walters et al 1993, Osunkoya et al 1994, Reich et al 1998. It seems, therefore, that the characteristic RGR from each species under a certain condition reflects its successional position, although the utilization of this or any other growth parameter, when isolated, requires caution.…”

“…Contrasting results in relation to the determinants of RGR have been found in other studies. Osunkoya et al (1994) and Reich et al (1998) found that RGR was correlated not only with NAR but also with LAR. Poorter (1999), based on results obtained with 15 rain-forest tree species, suggested that the different components of RGR become important at different irradiances.…”

“…However, although it is know that the spectral composition of radiation changes as it is filtered by green canopies, most studies on shade effects have been restricted to investigations of the effects of decreasing the photon flux density without ascribing for changes in spectral composition (Osunkoya & Ash 1991, Thompson et al 1992, Walters et al 1993, Osunkoya et al 1994, Bartlett & Remphrey 1998, Reich et al 1998. Exceptions are the studies of Kwesiga & Grace (1986), Kitajima (1994), Tinoco-Ojanguren & Pearcy (1995) and Lee et al (1996), where natural shade was simulated under experimental conditions by reducing both photon flux density and red/far-red ratio.…”