-(Growth response of three woody species seedlings from the Tropical Atlantic rain forest to changing light conditions). The growth response of seedlings of three woody species from the Tropical Atlantic rain forest to changing light conditions was studied. The species were the early successional Cecropia glazioui Sneth., the intermediary in succession Cedrela fissilis Vell. and the late successional Bathysa australis (A. St.-Hil.) Hook. ex Sch. These species showed, within the range of light gradient, plasticity to increase light interception at lower light levels (through increasing leaf area ratio -LAR, and decreasing root/shoot ratio -R/SH); and plasticity to increase carbon gain and decreasing transpiration at high light levels (through increasing stomatal density and R/SH, and decreasing LAR). Species responses to irradiance varied with position along the light gradient. Plants at the lower end of the light gradient were more responsive than plants at the higher end of the light gradient. C. glazioui seemed to be more plastic than C. fissilis to alter several features. The behavior showed by the three species in response to changing light was consistent with their environmental occurrence.Key words -Atlantic rainforest, growth, light, seedlings, tropical trees RESUMO -(Crescimento inicial de três espécies arbóreas da Floresta Atlântica em resposta à variação na quantidade de luz). Verificou-se a resposta de crescimento à variação na intensidade de luz de plântulas de três espécies arbóreas da Floresta Tropical Atlântica, Cecropia glazioui Sneth., Cedrela fissilis Vell. e Bathysa australis (A. St.-Hil.) Hook. ex Sch., respectivamente de estádios inicial, intermediário e final de sucessão. As três espécies mostraram, dentro de um determinado gradiente de luz, plasticidade para aumentar a captação de luz quando em baixa irradiância (através de aumento da razão de área foliar -RAF e diminuição da razão entre raiz e parte aérea -R/PA) e plasticidade para aumentar o ganho de carbono e diminuir a transpiração quando em alta irradiância (através dos aumentos da razão R/PA e densidade estomática, e da diminuição da RAF). A plasticidade das espécies em variar determinado parâmetro em função da intensidade de luz foi dependente do gradiente de intensidade de luz aplicado. A plasticidade foi maior nas intensidades mais baixas de luz tanto para C. glazioui quanto para C. fissilis. Para a maior parte dos parâmetros analisados, C. glazioui mostrou maior parte plasticidade para aclimatar-se à maior irradiância, que C. fissilis. As variações apresentadas pelas espécies na morfologia e fisiologia em relação à variação na intensidade de luz são consistentes com o local de ocorrência de cada espécie.Palavras-chave -arbóreas tropicais, crescimento, Floresta Atlântica, luz, plântulas IntroduçãoA luz é um dos fatores físicos mais importantes no controle do desenvolvimento de plântulas de espécies arbóreas em florestas tropicais úmidas (Lee et al. 1997). As condições de luz para as plântulas, no nível do chão da floresta...
This study analysed the behaviour of Cecropia glazioui Sneth. and Cedrela fissilis Vell. seedlings across a 2-50% full sun light gradient. The morphological adjustments of the seedlings to the different light regimes were interpreted as means of light interception in low light and avoidance of water stress in high light conditions. Cecropia seedlings showed greater trait plasticity at the 2-15% light interval, whereas Cedrela seedlings showed a slight tendency to present greater trait plasticity at the 15-30% light interval than the 2-15% interval. The seedlings of both species did not fully acclimate to decreasing light, showing lower values for the mean relative growth rate (RGR) components, leaf area ratio (LAR) and net assimilation rate (NAR) than seedlings under constant low light, but acclimated to increasing light, showing higher values for RGR components than seedlings under constant high light. The general responses to light gradient in these two species indicate their relative tolerance and ability to grow at low and high light and provide traits used in coping with gap dynamics. The results also indicated that the two species differed in their shade tolerance.
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