Avaliação do lançamento de folhas novas em uma floresta de terra firme na amazônia central

Abstract

The gross primary productivity (GPP) in central Amazonia is highly seasonal. Subsequently, an increase in the greenness of the forest (green-up) was observed during the dry season, coinciding with the increase in the PPB. Recently, other works showed that the seasonality of the PPB can be explained by a seasonality of the photosynthetic capacity (CF) of the forest. The seasonality of FC would be due to the interaction of two phenomena: I) at the leaf level there would be a decay of FC with the aging of the leaves; and II) at the canopy level there would be a change in the age structure of the leaves that compose it, with a more accentuated leaf renewal (turnover) during the dry season. Three hypotheses were proposed in previous works in the literature: H1) Hydric hypothesis, in which leaves released at the beginning of the dry season are more efficient in the use of water; H2) Availability of radiation, in which leaves released at the beginning of the dry season have greater absorption power and use of photosynthetically active radiation (PAR) when they mature, coinciding with the highest PAR values during the year; and H3) Ecological Hypothesis, where young leaves are more susceptible to attack by herbivores, with a reduction in the activity of herbivores and pathogens in the dry season. Thus, the release of new leaves in the dry season would be an escape strategy for herbivores and pathogens. Given this, and also as these hypotheses have not yet been tested, the present study had as a guideline to answer the following question: new leaves are emitted as an attempt to escape from herbivory, or to better take advantage of resources, such as photosynthetically active radiation (PAR) and water, in a terra firme forest in the Central Amazon? The study was conducted at the Amazon Tall Tower Observatory (ATTO) project site. A model to estimate leaf mass and area was developed and validated, which allowed evaluating the herbivory action. Subsequently, the relationships between seasonal leaf development with water availability and PAR were studied. As main results, consistent models were created and validated that allow the determination of mass consumed by herbivory, it was found that the forest does not escape herbivory, since in the period of greater leaf release there is also greater herbivory activities, twice as much. of those verified in the rainy season. No morphoanatomical changes were detected that could support the water or PAR hypotheses. It was not possible to sustain that the massive release of leaves in the dry period is correlated with the hydrous, PAR or ecological hypotheses.