Ciclo de vida de populações de bambu (Guadua spp.), no tempo e no espaço, no sudoeste da Amazônia

Abstract

Open forests dominated by semi-scandent woody bamboos of the genus Guadua Kunth. are found in the southwest Amazon as a mosaic of patches, each being 102 to 104 km2 in size. Each patch is a single population of semelparous plants with a synchronized life cycle. The individuals flower and fruit only once in their lifetime then die, opening canopy space for the new cohort of juveniles that will gradually regain dominance of the forest canopy. At the adult stage, areas dominated by Guadua can be detected on images captured by orbital sensors equipped with bands in the optical portion of the spectrum. Adult bamboo patches exhibit a spectral pattern similar to that of secondary forests, distinct from the darker areas covered by forest without bamboo or with juvenile bamboo still confined to the forest understory. The short post-reproductive stage is also spectrally distinct due to the leafless sunlit stems of dead bamboo. This study examined temporal and spatial patterns in the spectral behavior of Guadua populations over their life cycle in the southwest Amazon, using images acquired by optical orbital sensors between 1975 and 2008. The sensors were LANDSAT Multispectral Scanner (MSS), LANDSAT Thematic Mapper (TM), Enhanced Thematic Mapper (ETM+) and the Moderate Resolution Imaging Spectrometer (MODIS). To describe the spectral pattern of bamboo at different life stages, reflectance values for populations of different ages were obtained from the MODIS sensor. Product 43B4 was chosen as it provides atmosphere-free reflectance adjusted to emulate nadir view and solar angles, thus removing most confounding artifacts. Temporal mosaics of MODIS 43B4 for the prior 16 days are produced every 8 days. For each year, one mosaic was chosen, being that with the least cloud and haze over the SW Amazon. Three images were used, one from each year 2001, 2002 and 2003. Cloud contamination prevented collecting reflectances from mosaics for the years 2004 to 2008. Changes with the age of a bamboo population are reported for all seven optical MODIS bands and for the vegetation indices NDVI and EVI. Ages were determined by finding mortality patches in over 200 LANDSAT and MODIS images from 1975 to 2002. As expected, the two MODIS bands located on the near-infrared plateau of a typical green leaf spectrum, showed the greatest absolute differences in reflectance between the different life stages. EVI showed greater sensitivity than did NDVI for distinguishing different ages of bamboo. All life stages had a spectral pattern typical of healthy green leaves, indicating that even during a mortality event the bamboo-dominated forest is actually dominated by trees with green leaves. Despite filtering bad pixels, consistent biases (possible processing errors in the product) were found between the three best images of MODIS product 43B4. Length of the bamboo life cycle was estimated by two methods. First, repeated mortality events in the same geographic patch were observed directly, by examining over 200 LANDSAT images of the SW Amazon from 1975 to 2008 and annual MODIS 43B4 images from 2001 to 2008. Life cycles of 27 and 28 years were found. Second, the life cycle was inferred for all populations within a 34,000 km2 area that had almost no man-caused deforestation. Patches with bamboo dominant in the forest canopy were mapped within this area for 21 different dates spanning 1975–2008. The pair of maps with highest spatial congruence was taken to be one life cycle apart in time. This method gave a life cycle of 28 years. The final question examined was whether separate bamboo populations time their flowering events independently from their neighbors. Because these populations do not share genes, no relationship is expected to be found between proximity in time and proximity in space, for the flowering of different populations. Flowering itself is not visible, so all post-reproductive dieback events were mapped using the annual wall-to-wall MODIS coverage of the SW Amazon for 2001-2008. Reproductive events of each patch (each population) occurred in a highly contagious fashion, forming super-patches of temporally clustered events. A possible explanation for the shared genetic knowledge between several contiguous populations may be a succession of derivations from a single ancestral population, a process similar to allochronic speciation. One flowering wave was also observed, i.e., contiguous patches flowering like falling dominoes. But most temporally and spatially proximal flowering events did not occur in this fashion.