Mariano Oyarzabal

Trait differences between grass species along a climatic gradient in South and North America

Abstract Question: Are trait differences between grasses along a gradient related to climatic variables and/or photosynthetic pathway? Location: Temperate grassland areas of South and North America. Methods: In a common garden experiment, we cultivated C3 and C4 grasses from grasslands under different climatic conditions, and we measured a set of 12 plant traits related to size and resource capture and utilization. We described (1) interspecific plant trait differences along a climatic gradient defined by the precipitation and temperature at the location where each species is dominant and (2) the association between those plant trait differences and the photosynthetic pathway of the species. Results: Trait differences between grasses were related to the precipitation at the area where each species is dominant, and to the photosynthetic pathway of the species. Leaf length, leaf width, plant height, leaf area per tiller, specific leaf area, leaf δ13C ratio, and nitrogen resorption efficiency increased while leaf dry matter content and nitrogen concentration in senesced leaves decreased as precipitation increased. A proportion of these changes along the gradient was related to the photosynthetic pathway because dominant grass species in cold areas with low precipitation are mainly C3 and those from warm and wet areas are C4. Conclusions: A previous worldwide analysis showed that traits of graminoid species measured in situ changed slightly along climatic gradients (< 10% variance explained). In contrast, under a common environment we observed that (1) grass traits changed strongly along a climatic gradient (30–85% variance explained) and, (2) a proportion of those changes were related to the association between photosynthetic pathway of the species and precipitation. Nomenclature: Cabrera & Zardini (1978); Correa (1971–1988); Anon. (2005).

Phosphorus reserves increase grass regrowth after defoliation

Accumulation of P above levels that promote growth, a common plant response called “luxury consumption”, can be considered as a form of reserve to support future growth when the nutrient can subsequently be mobilized. However, the effect of P reserves on regrowth following defoliation has not been demonstrated. We tested the hypothesis that P luxury consumption increases plant tolerance to defoliation. We performed two experiments with four grass species from a continuously grazed temperate grassland in the Flooding Pampa (Argentina). The first experiment, aimed at generating P luxury consumption by fertilization, resulted in one species (Sporobolus indicus) showing luxury consumption. In this way, we were able to obtain plants of S. indicus with similar biomass but contrasting amounts of P reserves. The second experiment evaluated the subsequent regrowth following defoliation on a P-free medium of these plants differing in P reserves. Regrowth was larger for plants that had shown P luxury consumption during a previous period than for plants with lower levels of P reserves. During regrowth these plants showed a clear pattern of P remobilization from the stubble, crown, and root compartments to the regrowing tissue, in addition to a likely reutilization of P present in leaf-growth zones. This work is the first showing that high levels of P reserves can confer tolerance to defoliation by promoting compensatory growth under P deficiency.

Land cover and precipitation controls over long‐term trends in carbon gains in the grassland biome of South America

Carbon gains are a key aspect of ecosystem functioning since they represent the energy available for upper trophic levels. Carbon gains (or primary production) are strongly correlated with other ecosystem attributes such as secondary production and they are also the support for the provision of many ecosystem services. Given the documented dependency of primary production on precipitation, we expect that altered precipitation regimes, such as those projected by climate models, will have a significant impact on carbon gains. Land use and land cover changes are also expected to have a significant impact on the dynamics of carbon gains. We generated a spectral database of the fraction of photosynthetically active radiation intercepted by vegetation (fPAR), in order to study long-term trends (i.e., decades) in carbon gains and its spatial and temporal relationships with precipitation and land cover patterns in Uruguay, which is part of the Rio de la Plata Grasslands, one of the largest temperate grasslands biome of the world. We found that carbon gains of native forests and grassland afforestation exhibited the strongest positive spatial response to precipitation, whereas crops and rangelands the weakest. In addition, we found that the temporal response of carbon gains to precipitation was strong and positive for all land uses. Although there were not clear trends in precipitation, we found strong negative trends in carbon gains through time, particularly in rangelands of the “Northern Campos” of Uruguay, where these trends represent a decrease between 10% and 25% of the annual aboveground net primary production. On the other hand, positive trends in carbon gains through time were associated to grassland afforestation and native forests. Therefore, during the period analyzed, land cover had a stronger influence on the observed trends in carbon gains than precipitation. These patterns emerged as a consequence of the interaction among precipitation, temperature, edaphic factors and management. Present trends in the controlling factors of C gains would exacerbate the observed patterns with serious consequences for the provision of ecosystems services.

Climatic and land‐use drivers along a latitudinal gradient: species diversity in temperate grasslands on agricultural soils

Questions Do remaining mesophytic grasslands on soils with agricultural potential respond to a latitudinal gradient? Are climatic or land use factors the principal drivers of regional and local diversity of these grasslands? Location The mesophytic grasslands of the Argentine Pampas, between 32°S and 38°S, along 600km within the Mesopotamic, Rolling and Flooding Pampas of the Rio de la Plata grasslands. Methods The species presence and cover was recorded in each of 96 remnant grasslands on soils suitable for cropping, grouped in five sampling locations. In each region, confidence intervals for gamma diversity and for the slope of species/area curves were estimated. Unconstrained ordination was applied to detect the principal gradients in floristic composition and correlation analysis to identify their main drivers. Mantel test was used to evaluate the correlation between floristic similarity and geographical distance, and with Euclidean distance in fragmentation and climatic variables. To rank the climatic and land use factors that account for local species richness and percent number of exotics, we applied correlation analysis and regression models. Results Community composition (73% native species) was mainly related to climatic variables determined by latitude, with a slight influence of fragmentation variables. Regional species richness responded to latitude: gamma diversity decreased linearly with increasing distance from the Equator, and so did the rate of species accumulation with expanding area. Alpha diversity did not vary in a systematic way with latitude, being associated with landscape fragmentation and mean annual precipitation. Relative importance of C4 grasses and mean percent exotics showed opposite latitudinal tendencies, the former decreasing and the latter increasing towards the South. Conclusions Latitude was a strong determinant of regional diversity and community composition, but a partial driver for local species richness mainly influenced by landscape fragmentation. While climatic and geographical drivers determined gradual latitudinal turnover in regional species pools, within regions land-use history, stochastic processes and biotic interactions were also important. All these patterns need to be considered together when delimiting grassland nature reserves for conservation of the most diverse ecosystems of this region, threatened by agricultural expansion and intensification. This article is protected by copyright. All rights reserved.