Expansion of gallery forests into central Brazilian savannas

Abstract

Upland tropical forests have expanded and contracted in response to past climates, but it is not clear whether similar dynamics were exhibited by gallery (riparian) forests within savanna biomes. Because such forests generally have access to ample water, their extent may be buffered against changing climates. We tested the long-term stability of gallery forest boundaries by characterizing the border between gallery forests and savannas and tracing the presence of gallery forest through isotopic analysis of organic carbon in the soil profile. We measured leaf area index, grass vs. shrub or tree coverage, the organic carbon, phosphorus, nitrogen and calcium concentrations in soils and the carbon isotope ratios of soil organic matter in two transitions spanning gallery forests and savanna in a Cerrado ecosystem. Gallery forests without grasses typically show a greater leaf area index in contrast to savannas, which show dense grass coverage. Soils of gallery forests have significantly greater concentrations of organic carbon, phosphorus, nitrogen and calcium than those of savannas. Soil organic carbon of savannas is significantly more enriched in 13 C compared with that of gallery forests. This difference in enrichment is in part caused by the presence of C 4 grasses in savanna ecosystem and its absence in gallery forests. Using the 13 C abundance as a signature for savanna and gallery forest ecosystems in 1 m soil cores, we show that the borders of gallery forests have expanded into the savanna and that this process initiated at least 3000-4000 BP based on 14 C analysis. Gallery forests, however, may be still expanding as we found more recent transitions according to 14 C activity measurements. We discuss the possible mechanisms of gallery forest expansion and the means by which nutrients required for the expansion of gallery forest might accumulate.

The importance of evolutionary history in studies of plant physiological ecology: examples from cerrados and forests of central Brazil

“…Boundaries between cerrado and forest are dynamic systems, having undergone past shifts (Silva et al, 2008) and will probably respond to future climate change (Salazar, 2007). Humans contribute to these dynamics through deforestation and increased fire frequency in some cases (Cochrane et al, 1999) and by suppressing fire in others (Durigan and Ratter, 2006).…”

“…Light tends to be more abundant in savanna, though seedlings in savanna may experience very low light levels (Nardoto et al, 1998, Kanegae et al, 2000 and forest canopy trees experience very high light intensities. Relative to savanna, forest soils tend to have higher nutrient availability (Furley, 1992;Ruggiero et al, 2002, Silva et al, 2008, though this is not always the case for cerradão (Haridasan, 1992). Similarly, forests are associated with greater water availability in the case of riparian forests and, at regional scales, in the Amazon and Atlantic forest regions that border the Cerrado.…”

Plant- and stand-level variation in biophysical and physiological traits along tree density gradients in the Cerrado

“…Gallery forest soils have relatively high nutrient content compared to soils in savanna physiognomies (e.g. 4.8 (pH); 11.2 (%OM); 0.3 (%N); 4 mg dm -3 (P), 0.2 cmol dm -3 (Ca) to upper layer (0-100cm)) (Silva Jr., 2005;Silva et al, 2008) Long term fertilization with N, P and N+P affected chemical properties of soils in the cerrado sensu stricto plots used in this study (Kozovits et al, 2007). Plots fertilized with N and N+P had lower values of pH.…”

“…The reasons why no forest with a continuous canopy develops in the Brazilian savanna region are still debated (Hoffmann et al, 2005b;Silva et al, 2008). The possible explanations include climatic, edaphic, and anthropogenic factors (e.g.…”

Spatial and temporal variability of canopy cover and understory light in a Cerrado of Southern Brazil

“…In a recent study, Bucci et al (2008) showed that transpiration increases with LAI in all Cerrado studied physiognomies in both the dry and wet seasons. Also recently, Silva et al (2008) observed that the differential LAI values of wood species and grass in a transect between a gallery forest and Cerrado were highly correlated with δ 13 C values of the soil organic carbon. They showed an increase of δ 13 C with the increase of grass LAI values and a decrease with the increase of LAI from woody species.…”