A comunidade herbáceo-arbustiva de cerrado sensu stricto frente às alterações de luminosidade e de serapilheira em condição de adensamento: dinâmica de espécies, germinação e respostas funcionais
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Data
2020-12-15
Autores
Pinheiro, Luiz Felipe Souza [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
O adensamento vegetacional consiste no aumento da densidade e da cobertura de espécies lenhosas em vegetações com um expressivo estrato graminoso. Este processo vem acontecendo em ritmo acelerado nas savanas da América do Sul. Particularmente no Brasil, muitas áreas de cerrado stricto sensu (CSS) vêm apresentando perda de espécies herbáceo-arbustivas típicas de áreas abertas em áreas adensadas. Assim, as mudanças microambientais causadas pelo adensamento podem estar filtrando essas espécies. Tais mudanças envolvem, por exemplo, a diminuição da luminosidade incidente no sub-bosque e o aumento na espessura da camada de serapilheira; juntos esses dois fatores podem causar redução na flutuação de temperatura no solo superficial e diminuição na razão de vermelho e vermelho-extremo (V:VE). Por meio de experimentação em campo, este estudo objetivou investigar o papel da redução da luminosidade e do aumento da camada de serapilheira causados pelo adensamento sobre a diversidade, estrutura e fisiologia da comunidade herbáceo-arbustiva de CSS. Além disso, foram estudados aspectos da regeneração por sementes de espécies de CSS relacionados às alterações no regime de temperatura, na razão de V:VE e espessura da camada de serapilheira. Os efeitos da serapilheira sobre a estrutura e a diversidade florística e funcional da comunidade herbáceo-arbustiva foram discretos, assim como para os parâmetros foliares funcionais. Entretanto, a presença de uma camada de serapilheira diminuiu drasticamente a emergência de plântulas de espécies herbáceo-arbustivas. Por outro lado, o sombreamento foi um fator de grande importância na estrutura e florística da comunidade do estrato rasteiro, causando redução nas riquezas florística e funcional desse estrato, assim como na densidade, cobertura e biomassa de partes aéreas ao longo do tempo. Dentre as formas de crescimento, as plantas herbáceas foram as mais forte e rapidamente impactadas pelo sombreamento. Ainda, com o sombreamento imposto, as espécies herbáceo-arbustivas foram capazes de apresentar alguns ajustes relacionados à melhora da captura luminosa, tais como o aumento na área foliar específica e nos teores de clorofilas e carotenoides. Estes ajustes, no entanto, não foram suficientes para a manutenção do desempenho fotoquímico e da assimilação de carbono destas espécies. Com relação aos aspectos da regeneração por sementes, além dos amplos efeitos da serapilheira mencionados anteriormente, a diminuição da razão de V:VE e do regime de amplitude térmica também afetaram negativamente a germinação de algumas espécies. Os resultados indicam que as alterações ambientais causadas pelo adensamento representam fortes filtros ambientais, moldando a montagem da comunidade herbáceo-arbustiva. Essas alterações atuam limitando tanto a sobrevivência como a regeneração de espécies herbáceo-arbustivas típicas de CSS em condições adensadas.
Woody plant encroachment refers to the increase in density and cover of woody species in vegetation with an expressive herbaceous-grassy layer. This process is going rapidly in South America savannas, where many Brazilian savanna areas have undergone losses of herbaceous-shrubby species in encroached sites. Thus, environmental changes caused by woody encroachment may be filtering these species. Such changes comprise understory light reduction and higher litter deposition in encroached sites; these two factors combined can induce narrowing of the soil surface temperature alternation and red and far-red ratio (R:FR) decrease. By means of field manipulation, this study investigated the role of light reduction and litter layer increase caused by woody encroachment in diversity, structure and physiology of savanna ground layer community. Furthermore, seed regeneration aspects of savanna ground layer species were evaluated according to alterations in temperature alternation, R:FR ratio and litter layer thickness. We found that the litter had only small effects in floristic and functional diversity of savanna ground layer community as well as structural and functional parameters. However, litter layer presence drastically reduced seedling emergence of savanna ground layer species. In the other hand, shading was an important factor for savanna ground layer structure and floristic. Shading caused significant reductions in floristic and functional richness as well as in density, cover and aboveground biomass over time. Among growth forms, herbaceous plants were most strongly and rapidly affected by shading. Moreover, savanna ground layer species were capable of some adjusts related to light capture improvement in shaded conditions, such as higher leaf specific area values and chlorophyll and carotenoid content. However, these adjustments were not enough to allow the photochemical and carbon assimilation performance maintenance. Regarding seed regeneration aspects, besides the large litter effects mentioned before, reductions in R:FR and temperature alternation regime also negatively affected some species. In conclusion, results indicate that environmental changes caused by woody encroachment represent strong environmental filters shaping ground layer community assembly. These changes act limiting both survival and regeneration of savanna ground layer species in encroached situations.
Woody plant encroachment refers to the increase in density and cover of woody species in vegetation with an expressive herbaceous-grassy layer. This process is going rapidly in South America savannas, where many Brazilian savanna areas have undergone losses of herbaceous-shrubby species in encroached sites. Thus, environmental changes caused by woody encroachment may be filtering these species. Such changes comprise understory light reduction and higher litter deposition in encroached sites; these two factors combined can induce narrowing of the soil surface temperature alternation and red and far-red ratio (R:FR) decrease. By means of field manipulation, this study investigated the role of light reduction and litter layer increase caused by woody encroachment in diversity, structure and physiology of savanna ground layer community. Furthermore, seed regeneration aspects of savanna ground layer species were evaluated according to alterations in temperature alternation, R:FR ratio and litter layer thickness. We found that the litter had only small effects in floristic and functional diversity of savanna ground layer community as well as structural and functional parameters. However, litter layer presence drastically reduced seedling emergence of savanna ground layer species. In the other hand, shading was an important factor for savanna ground layer structure and floristic. Shading caused significant reductions in floristic and functional richness as well as in density, cover and aboveground biomass over time. Among growth forms, herbaceous plants were most strongly and rapidly affected by shading. Moreover, savanna ground layer species were capable of some adjusts related to light capture improvement in shaded conditions, such as higher leaf specific area values and chlorophyll and carotenoid content. However, these adjustments were not enough to allow the photochemical and carbon assimilation performance maintenance. Regarding seed regeneration aspects, besides the large litter effects mentioned before, reductions in R:FR and temperature alternation regime also negatively affected some species. In conclusion, results indicate that environmental changes caused by woody encroachment represent strong environmental filters shaping ground layer community assembly. These changes act limiting both survival and regeneration of savanna ground layer species in encroached situations.
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Atributos funcionais, Filtros ambientais, Fotossíntese, Germinação, Luz, Savana, Environmental filters, Functional traits, Germination, Photosynthesis, Light, Savanna