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

Interspecific variation in vitamin E levels and the extent of lipid peroxidation in pioneer and non-pioneer species used in tropical forest restoration

Reforestation projects have gained interest over recent years due to the loss of biodiversity in tropical regions as a result of large deforestation by anthropogenic actions. However, better knowledge on the tolerance of plant species to environmental stresses is needed for reforestation success. Here, we evaluated the photoprotective and antioxidant capacity, in terms of vitamin E accumulation, of five pioneer (Platypodium elegans Vogel, Schinus terebinthifolius Raddi, Lafoensia pacari A. St.-Hil, Cecropia pachystachya Trécul. and Aegiphila sellowiana Cham.) and five non-pioneer (Myracrodruon urundeuva Allemão, Cedrela fissilis Vell., Genipa americana L., Copaifera langsdorffii Desf. and Hymenaea courbaril L.) species, in relation to the extent of lipid peroxidation in leaves. Furthermore, we examined differences between sun and shade leaves on vitamin E accumulation and the extent of lipid peroxidation. Pioneer plants showed on average 33% higher malondialdehyde levels, an indicator of lipid peroxidation, than non-pioneer species, but no significant differences in vitamin E contents. In contrast, a marked interspecific variation was observed in the levels of α-tocopherol and its precursor, γ-tocopherol. Natural variation revealed interesting relationships between vitamin E levels and the extent of lipid peroxidation in leaves. The pioneer species, P. elegans, did not accumulate α-tocopherol and displayed the highest levels of malondialdehyde. Sun and shade leaves accumulated vitamin E levels to a similar extent, except for the pioneer L. pacari and the non-pioneer C. langsdorffii, the former accumulating more α-tocopherol in sun leaves and the latter in shade leaves. We conclude that interspecific variation is higher than both leaf type and successional-group variation in terms of vitamin E accumulation and the extent of lipid peroxidation, and that vitamin E levels, particularly those of α-tocopherol, negatively correlate with the extent of lipid peroxidation, thus supporting a photoprotective and antioxidant function for vitamin E in plants growing in tropical environments.

Network Connectance Analysis as a Tool to Understand Homeostasis of Plants under Environmental Changes

The homeostasis of plants under environmental constraints may be maintained by alterations in the organization of their physiological networks. The ability to control a network depends on the strength of the connections between network elements, which is called network connectance. Herein, we intend to provide more evidence on the existence of a modulation pattern of photosynthetic networks, in response to adverse environmental conditions. Two species (Glycine max-C3 metabolism, and Brachiaria brizantha-C4 metabolism) were submitted to two environmental constraints (water availability, and high and low temperatures), and from the physiological parameters measured, the global connectance (Cg_total) and the modules connectance (gas exchange-Cg_ge and photochemical-Cg_pho) were analyzed. Both types of environmental constraints impaired the photosynthetic capacity and the growth of the plants, indicating loss of their homeostasis, but in different ways. The results showed that in general the Cg_total of both species increased with temperature increment and water deficit, indicating a higher modulation of photosynthetic networks. However, the Cg variation in both species did not influence the total dry biomass that was reduced by environmental adversities. This outcome is probably associated with a loss of system homeostasis. The connectance network analyses indicated a possible lack of correspondence between the photosynthetic networks modulation patterns and the homeostasis loss. However, this kind of analysis can be a powerful tool to access the degree of stability of a biological system, as well as to allow greater understanding of the dynamics underlying the photosynthetic processes that maintain the identity of the systems under environmental adversities.

Exergetic Model of Secondary Successions for Plant Communities in Arid Chaco (Argentina)

Ecosystems are open systems where energy fluxes produce modifications over plant communities. According to the state and transition model, plant formations are defined by changes in natural conditions and disturbs. Based on these changes, it is possible to define vectors that show the tendencies of the communities towards other states. Within the subregion of Arid Chaco, mature communities of Aspidosperma quebracho blanco represent the quasistable equilibrium communities or “climax,” similar to that observed in the Chancaní Natural Reserve (Córdoba, Argentina). Biodiversity values and Lyapunov coefficients were calculated based on plant abundance and cover data. Lyapunov coefficients were calculated as the Euclidean distance of each site with respect to reference condition (community of Aspidosperma quebracho blanco), representing for each state the necessary exergy to reach the reference condition. When Lyapunov coefficients decrease in time, it is expected for the system to drive towards a quasistationary state; otherwise, the equilibrium is unstable and becomes less resilient. The diversity of species has a significant effect over the resistance to perturbations but equivocal for the recovery rate. Lyapunov coefficients may be more precise succession indicators than biodiversity indexes, representing the amount of exergy needed for a vegetation state to reach the reference condition.

Functional regeneration and spectral reflectance of trees during succession in a highly diverse tropical dry forest ecosystem

PREMISE OF THE STUDY

The function of most ecosystems has been altered by human activities. To asses the recovery of plant communities, we must evaluate the recovery of plant functional traits. The seasonally dry tropical forest (SDTF), a highly threatened ecosystem, is assumed to recover relatively quickly from disturbance, but an integrated evaluation of recovery in floristic, structural, and functional terms has not been performed. In this study we aimed to (a) compare SDTF plant functional, floristic, and structural change along succession; (b) identify tree functional groups; and (c) explore the spectral properties of different successional stages.

METHODS

Across a SDTF successional gradient, we evaluated the change of species composition, vegetation structure, and leaf spectral reflectance and functional traits (related to water use, light acquisition, nutrient conservation, and CO(2) acquisition) of 25 abundant tree species.

KEY RESULTS

A complete recovery of SDTF takes longer than the time period inferred from floristic or structural data. Plant functional traits changed along succession from those that maximize photoprotection and heat dissipation in early succession, where temperature is an environmental constraint, to those that enhance light acquisition in late succession, where light may be limiting. A spectral indicator of plant photosynthetic performance (photochemical reflectance index) discriminated between early and late succession. This constitutes a foundation for further exploration of remote sensing technologies for studying tropical succession.

CONCLUSIONS

A functional approach should be incorporated as a regular descriptor of forest succession because it provides a richer understanding of vegetation dynamics than is offered by either the floristic or structural approach alone.

Diurnal and seasonal carbon balance of four tropical tree species differing in successional status

This study addressed some questions about how a suitable leaf carbon balance can be attained for different functional groups of tropical tree species under contrasting forest light environments. The study was carried out in a fragment of semi-deciduous seasonal forest in Narandiba county, São Paulo Estate, Brazil. 10-month-old seedlings of four tropical tree species, Bauhinia forficata Link (Caesalpinioideae) and Guazuma ulmifolia Lam. (Sterculiaceae) as light-demanding pioneer species, and Hymenaea courbaril L. (Caesalpinioideae) and Esenbeckia leiocarpa Engl. (Rutaceae) as late successional species, were grown under gap and understorey conditions. Diurnal courses of net photosynthesis (Pn) and transpiration were recorded with an open system portable infrared gas analyzer in two different seasons. Dark respiration and photorespiration were also evaluated in the same leaves used for Pn measurements after dark adaptation. Our results showed that diurnal-integrated dark respiration (Rdi) of late successional species were similar to pioneer species. On the other hand, photorespiration rates were often higher in pioneer than in late successional species in the gap. However, the relative contribution of these parameters to leaf carbon balance was similar in all species in both environmental conditions. Considering diurnal-integrated values, gross photosynthesis (Pgi) was dramatically higher in gap than in understorey, regardless of species. In both evaluated months, there were no differences among species of different functional groups under shade conditions. The same was observed in May (dry season) under gap conditions. In such light environment, pioneers were distinguished from late successional species in November (wet season), showing that ecophysiological performance can have a straightforward relation to seasonality.

The complexity-stability hypothesis in plant gas exchange under water deficit

We hypothesized that more complex, i.e. irregular, temporal dynamics and a more interconnected overall network supports greater stability to gas exchange parameters (herein, CO2 net assimilation and transpiration) in plants under water deficit. To test this hypothesis two genotypes of Phaseolus vulgaris were subjected to a period of absence of irrigation, and subsequent rewatering to achieve recovery. Gas exchanges parameters were measured each 10 s during 6 h to obtain time series to evaluate complexity by Approximate Entropy (ApEn) calculations, and network connectance in each water regime. Notably, the Jalo Precoce genotype showed significantly more stability than the Guarumbé genotype under system perturbation, coincident with greater irregularity in each gas exchange parameter and greater overall connectance for Jalo Precoce. This conclusion is consistent with other observations of greater homeostasis in more complex networks, seen in broad contexts such as cardiac rhythms and respiratory dynamics

Changes in network connectance and temporal dynamics of gas exchange in Citrus sinensis under different evaporative demands

Stomatal aperture is an essential factor both in regulation of transpiration and net photosynthesis. This regulation is especially important in the response of plants to drought or to an increase in leaf-to-air vapor pressure difference (VPD); however, such a regulation is part of a complex dynamical environment, associated with multiple regulatory pathways. Accordingly, we studied the effects of VPD on gas exchange of Citrus sinensis via the evaluation of two complementary analytic approaches, to approach an understanding of the full scope of the system interactions. First, we used classical statistical methodologies, e.g., means, coefficient of variation, and linear correlation. Second, we used measures developed for more model-independent applications, Approximate Entropy (ApEn) to evaluate the irregularity or complexity of gas exchange time-series, and network connectance to evaluate changes in the extent of linkage among specified gas exchange parameters. The analyses of experiments carried out under constant environmental conditions in each VPD treatment (1.0, 2.0 and 3.0 kPa) showed a number of relatively subtle results of physiological consequence, such as differences in network connectance during the period of measurements at the same condition showing different patterns of gas exchange regulation. Additionally, VPD changes affect the dynamics of gas exchange by alterations in the irregularity of the time-series. These experiments highlight the endogenous and self-organized mechanisms that underlie the gas exchange process with further theoretical findings and possible practical applications.