Dynamics of energy and nutrient concentration and construction cost in a native and two alien C4 grasses from two neotropical savannas

Abstract

In Venezuela, the alien grasses Melinis minutiflora Beauv. and Hyparrhenia rufa (Nees.) Stapf tend to displace the native savanna plant community dominated by Trachypogon plumosus (Humb. and Bonpl.) Nees. This occurs in either relatively wetter and fertile highland savannas or in drier and less fertile lowland savannas. Although the native and aliens are perennial C4 grasses, higher net assimilation leaf biomass per plant and germination rate of the latter are some causes for their higher growth rates and for their competitive success. The objective of this study is to compare seasonal tissue energy, N, P and K concentrations and the calculated construction costs (CC) between the native grass and either one of the alien grasses from lowland and highland savannas. We predict that, in order to out-compete native plants, alien grasses should be more effcient in resource use as evidenced by lower tissue energy and nutrient concentrations and CC.Tissue energy and nutrient concentration were measured throughout the year and compared between M. minutiflora and the co-occurring local population of T. plumosus in a highland savanna and between H. rufa and its neighbor local population of T. plumosus in a lowland savanna. CC was calculated from energy, N and ash concentrations considering ammonium as the sole N source. Differences between co-occurring species, T. plumosus populations, seasons, and organs were analyzed with ANOVA.Highland and lowland grasses differed in concentration and allocation of energy and nutrients whereas the differences between alien and native grasses were specific for each pair considered. Highland grasses had higher energy, N, P and CC than lowland grasses. These variables were always lowest in the culms. In the more stressed lowland site, tissue energy and nutrient concentrations decreased significantly during the dry season except in the roots of both grasses which had the highest energy and nutrients concentrations during the drought. This seasonal response was more marked in the local lowland population of T. plumosus in which maximum CC alternated seasonally between leaves and roots. Energy and nutrient concentrations and CC were the lowest in H. rufa. In the lowland savannas, the higher efficiency of resource use in the invader grass contributes to its higher competitive success through increased growth rate. In the highlands, overall tissue energy concentration and CC, but not N nor P concentration, were lower in the fast growing M. minutiflora but seasonal differences were lacking. The higher leaf CC in T. plumosus can be attributed to the higher proportion of sclerenchyma tissue which is more expensive to construct. Considering CC, both fast growing alien grasses are more efficient in resource use than the co-occurring native grass. However, the role of CC explaining the competitive success of the former, through higher growth rates, is more evident in the more stressful environment of the lowland savanna.

Poaceae communities in the savannas of the Amazon Estuary in Brazil

“…Four species correlated with the different environments: Trachypogon spicatus was correlated with soils with more organic matter; Axonopus polydactylus was correlated with soils with a greater sand content; and Paspalum pulchellum and Mesosetum loliiforme were correlated with soils with greater silt content. Although there are many factors that limit the occurrence of the Poaceae in the savannas of the Americas (Allem & Valls 1987), low fertility was the only one of those factors observed in the savannas of the Amazon Estuary in the present study, corroborating the results obtained by Baruch et al (1985), Baruch & Gómez (1996), and Baruch (2005).…”

Effects of fire and defoliation on the life history of native and invader C 4 grasses in a Neotropical savanna

“…Several causes for the displacement of Trachypogon by Hyparrhenia have been postulated. The latter had higher laboratory photosynthetic rates (Baruch et al 1985), lower tissue construction costs (Baruch and Go mez 1996), coped better with seasonal drought (Baruch and Ferna ndez 1993), was more tolerant to defoliation under semi-controlled conditions (Simoes and Baruch 1991), and had higher germination potential than native grasses (Z. Baruch, unpublished results).…”

Seasonal Dynamics in Carbon Dioxide Flux Es of the Herbaceous Layer of a Moist Kenyan Savannah

“…The shoot and root N levels were tightly inter-linked in this ecosystem. Shoot N was high and root N low during the periods of peak active growth and biomass accumulation (May) but shoot N declined as root N increased by 50% at the end of the growing season pointing to possible relocation and accumulation of N in the roots (Baruch & Gómez, 1996;Wedin, 1996). Furthermore the plant roots had higher N levels than the shoots during drought and shoot C:N ratio was high during drought but low during wet periods with active growth implying transportation of N out of the shoots possibly to the roots.…”