9 - Fluvial Geomorphic Disturbances and Life History Traits of Riparian Tree Species

The eastern, equatorial and interior basins from Northeastern Brazil contain a diverse sedimentary record representative of the events that culminated in the rupture of the Gondwana megacontinent and the genesis of the South Atlantic Ocean. Those events that occurred between the Neoaptian and Neoalbian (Mid cretaceous) were associated with deep climatic changes that, besides involving particular depositional systems from continental to lacustrine-evaporitic-open marine, allowed the development of a wide and diversified vegetation adapted to the new ecological niches then created. Paleobotanical data were analyzed from five intervals defined according to the Brazilian palynological zoning. The middle section of the Upper Aptian (P-260) suggests the existence of highlands with cold to hot but dry climate in the interior basins, with high humidity in the interior plains and stressful conditions along the coast. Warmer, but not arid climate prevailed on the east margin, being even hotter and arid on the equatorial margin. The Upper Aptian (P-270) is suggestive of highlands in the interior basins and at the eastern and equatorial margins, with hot and humid climate in the interior basins and the eastern margin. The climate would be significantly more arid in the equatorial margin, where coastal evaporitic systems would have been developed. From the Neoaptian (P-280) the records for the interior basins are not significant. For the eastern and equatorial margin basins would persist the presence of highlands, with a mild climate, with frequent humid, locally hot and arid environments, but not stressful. During early Albian (P-320), the phytogeographic distribution reflects a significant climate change in the east and equatorial margins, with a marked reduction of humidity and reduction or absence of highlands. The climate would have been warm, with increasing aridity and presence of more stressful environments. During Mid Albian (P-330), the highlands would no longer exist on the eastern and equatorial margins, the climate being predominantly hot and dry, but milder on the eastern margin, while on the equatorial margin there would be an increase in aridity from east to west. During the late Albian (P-355 / P-360), in addition to the absence of highlands, the phytogeographic distribution suggests the maintenance of a hot and arid climate, probably representing the hottest climate of the entire studied section

From a life-cycle perspective seed dispersal is a key factor for population dynamics. However, little is known on seed dispersal mechanisms in riparian hydrochorous and anemochorous canopy tree species. We studied Ulmus laevis Pallas reproductive biology in a human transformed population with lack of recruitment, during three consecutive years, in order to evaluate the species’ recovery capabilities. We investigated fruit release timing and rates, and how these are affected by meteorological variables with generalized linear models. Seed production, dispersal distances, dispersal patterns and predation rates were evaluated with inverse modeling. Results show that the stand did not suffer from source limitation, in the fecundity sense. Despite samaras being winged nuts, wind did not disperse these long distances, as 95% of full samaras land at less than 30 m from the mother tree. We did not observe secondary dispersal in this stand. Therefore, we hypothesize that for this species water may be responsible for long distance dispersal, and wind would be effective in short distance dispersal. This coupled system would increase efficiency in seed arrival at appropriate habitats for colonization, at both short and long distance scale. Empty samara production and the proportion of these in relation to full samaras are important for diminishing pre- and post-dispersal predation rates. Our results also shed light on how human-induced changes in water-table and river regulation may affect seed dispersal and recruitment in riparian forests. Finally, U. laevis short-term conservation measures are discussed, while long term conservation requires natural hydrological regimes restoration so as to facilitate seed transport and deposition in adequate microsites.

We examined the relationships between floodplain forest structure and disturbance frequencies in bar-braided and incised-meandering channel sections of the Rekifune River, northern Japan. This was undertaken with special reference to the life history traits at seedling and reproductively mature stages of eight dominant species (Chosenia arbutifolia, Populus maximowiczii, Toisusu urbaniana, Salix sachalinensis, Salix pet-susu, Alnus hirsuta, Ulmus japonica, and Fraxinus mandshurica var. japonica). These species were generally arranged along the flood frequency and intensity gradient represented by five geomorphic surface types: gravel bar, lower and upper floodplain, secondary channel, and terrace. However, habitat separation between seedlings and conspecific mature stands occurred with geomorphic surface type in four of the eight species, suggesting that these species require more than one geomorphic surface type to complete their life cycles. Comparisons of stand-replacement disturbances between the bar-braided and incised-meandering rivers suggested that the shifting mosaic pattern in association with the evolution of land surfaces plays a vital role in maintaining variety in regeneration habitats and development of riparian forests in a half-century time scale. We conclude that the key factors maintaining the diversity of floodplain tree species in the studied northern temperate forests are: (1) the development of diverse geomorphic surfaces providing regeneration and habitat niches, and (2) the timing of maturation and lifespan of early and mid-successional species consistent with the disturbance frequency.