Functional groups of forest succession as dissipative structures: an applied study

Abstract

This study tested the hypothesis that dissipative efficiency of tropical tree species could be an ecological advantage in the forest succession process. Daily leaf gas exchanges of a pioneer species (Guazuma ulmifolia) and a late successional species (Cariniana legalis) were evaluated under well-irrigated conditions and by withholding irrigation. Analyses of network connectance (Cg) and plant autonomy (At) were carried out in order to assess metabolic network changes in response to environmental perturbation. As a global estimation of latent heat dissipation, the capacity to both maintain and cool leaf temperature in response to air temperature changes (∆T = T o C air -T o C leaf ) was evaluated. The changes observed in both the systemic parameters (Cg and At) and the physiological ones brought about by water deficit, associated with discrepant growth rates between both species, suggested that the initial formation of gap canopies composed by pioneer species could simply be a result of the higher photosynthetic rates of these species, and not necessarily because late successional species cannot cope with such a heterogeneous environment as that of a gap. Our results indicate that, in the absence of water constraints, the highest CO 2 assimilation rates of pioneer species are supported by the efficiency of the whole dissipative structure, involving both degradation and dissipative processes. As a practical result, our study suggests the ∆T analysis in order to evaluate the efficiency of dissipative structures and as a aid in characterizing functional groups.Key words: dissipative structures, tropical ecophysiology, functional groups, gas exchanges. RESUMO Grupos funcionais da sucessão florestal como estruturas dissipativas: um estudo aplicadoEste estudo testou a hipótese de que maior eficiência dissipativa de espécies arbóreas tropicais poderia apresentar vantagem ecológica no processo de sucessão florestal. Medidas diárias de trocas gasosas de uma espécie pioneira (Guazuma ulmifolia) e de uma espécie secundária tardia (Cariniana legalis) foram realizadas em plantas irrigadas e com deficiência hídrica. Análises da conectância da rede de trocas gasosas (Cg) e da autonomia (At) da planta em relação ao ambiente foram realizadas com o objetivo de verificar mudanças metabólicas na rede em resposta à deficiência hídrica. Como estimativa global da dissipação de calor latente, foi avaliada a capacidade de manutenção e resfriamento da temperatura foliar diante das mudanças na temperatura do ar (∆T = T o C ar -T o C folha ). As mudanças observadas nos parâmetros sistêmicos (Cg e At) e fisiológicos em resposta à perturbação ambiental, associadas à discrepante taxa de crescimento entre as espécies, sugeriram que a formação inicial do dossel de uma clareira por espécies pioneiras poderia ser resultado das maiores taxas de fotossíntese dessas espécies, e não necessariamente decorrentes do fato de as espécies secundárias tardias não suportarem o ambiente heterogêneo da clareira. Nossos resultados indicam que os maiores ...

Network connectance and autonomy analyses of the photosynthetic apparatus in tropical tree species from different successional groups under contrasting irradiance conditions

“…However, there are few comparative studies to approach the question if early and late successional plants differ in relation to the dissipative efficiency. Our results based on DT values (figure 2) and transpiration rates (table 1) indicated that species of the early successional stages have more efficient dissipative capacity, which is in agreement with Souza et al (2004) and supports the statement above.…”

“…Therefore, approaches that access and quantify system interactions, considering the networks of relationships among system elements, are required in order to improve the classical physiological methods. Prado et al (2004) and Souza et al (2004) have suggested that the effects of environmental disturbances on the photosynthetic network may be evaluated through the strength of the network connectance, according to the approach suggested by Amzallag (2001). This author suggests the network connectance evaluation, a statistical method based on path analysis (Li 1975), through the analysis of the normalized correlation coefficient between network parameters.…”

“…Accordingly, changes in the system connectance, mostly connectance increases, could be considered as acclimatory responses to environmental disturbances. Souza et al (2004) have suggested a new approach in order to analyze the ecophysiological responses of tropical tree species to environmental changes. In that report the authors used the difference between air and leaf temperature (∆T) to access the global dissipative capacity of plants under water deficit.…”

“…In that report the authors used the difference between air and leaf temperature (∆T) to access the global dissipative capacity of plants under water deficit. In addition, Souza et al (2004) analyzed systemic changes in the network connectance of gas exchange, as well as the relationship of each gas exchange parameter with the environment to estimate plant autonomy in relation to the external conditions.…”

“…In addition, the photochemical activity involves an intricate network of regulation processes that can be influenced by external factors such as temperature, water deficit and irradiance (Yordanov et al 1997, Critchley 1998, Krause et al 2001. Thus, the photosynthetic responses to increasing irradiance could have their understanding complemented by the approach proposed by Amzallag (2001) and Souza et al (2004), since such responses result from the interactions between a very complex photosynthetic network and the environment.…”