Vegetation structure: Fine scale relationships with soil in a cerrado site

Abstract

There is a lack of understanding on factors influencing the occurrence of high species heterogeneity at fine scale in the Brazilian cerrado. Soil is a major determinant of vegetation in the Brazilian cerrado and an important candidate to influence species distribution at fine scales, since soil features vary at very small distances, whereas many environmental variables are relatively homogeneous at such scale. We tested plant–soil relationships at fine scales in a cerrado site. We placed 100 contiguous 25 m² plots, where we identified all woody individuals and measured several soil features. We did partial redundancy analysis, controlling for spatial autocorrelation, to test for relationships between soil features and floristic composition. We also did multiple regressions or spatial autoregressive models to test for relationships between soil features and: (1) the abundance of the five most common species, (2) total abundance, (3) richness, (4) evenness, and (5) diversity. We found weak relationships between soil and floristic composition, richness, and total abundance, which, coupled with also weak relationship found in another study with plant available water, indicate there is no major environmental variable influencing vegetation at fine scales, but several of them interacting. Organic matter was positively related with the abundance of Myrsine umbellata and was negatively related to evenness. Although a causal relationship cannot be inferred with certainty, the dominance of Myrsine umbellata seems to be related to a positive feedback with soil.

Introduction

The Brazilian cerrado is the richest savanna in the world, with about 7000 plant species, of which about 1500 are shrubs or trees (Castro et al., 1999). Due to its high species richness, high degree of endemism, and present conservation status, the cerrado is one of the biodiversity hotspots in the world (Myers et al., 2000). High species turnover can be found at very small distances in cerrado, and there is a great number of species even in small areas (Carvalho and Martins, 2009, Silva and Batalha, 2009). However, there is a lack of studies about the determinants of this heterogeneity at fine scales (Ferreira et al., 2009).

Together with fire and climatic seasonality, soil is a main determinant of changes in plant species and vegetation structure in the Brazilian cerrado, whose physiognomic variation is considered by some authors (for example, Goodland and Pollard, 1973) to be a fertility gradient. The cerrado tends to occur on well-drained, acid, and nutrient-poor soils, with high levels of exchangeable aluminum, and, at increased water availability or soil fertility, it tends to be replaced by forest (Goodland and Ferri, 1979, Oliveira-Filho and Ratter, 2002). The cerrado productivity gradient is related to higher availability of bases in the soil (Goodland and Ferri, 1979), whereas the sclerophyllous features of the cerrado vegetation are attributed to direct and indirect effects of high aluminum contents and low nutrient availability (Arens, 1958, Arens, 1963, Goodland and Ferri, 1979, Sarmiento, 1984). Aluminum is also related to changes in species richness between cerrado physiognomies (Carvalho and Martins, 2009).

Although many studies found relationships between soil conditions and plant species composition when moving from cerrado to other vegetation types (Amorim and Batalha, 2007, Ruggiero et al., 2002), the relationship between soil conditions and the occurrence of different cerrado physiognomies is controversial, since some studies did not find it (Amorim and Batalha, 2007, Marimon and Haridasan, 2005, Ruggiero et al., 2002), whereas others did (Carvalho and Martins, 2009, Goodland and Pollard, 1973). However, there is a lack of studies in cerrado about the determinants of vegetation at fine scales (Ferreira et al., 2009). Since in the cerrado many woody species can be found in relatively small areas (Oliveira-Filho and Ratter, 2002), fine scale studies may allow to understand what drives cerrado plant responses to the environment. As long as changes in soil features can be found at distances as small as 1 m (Souza and Martins, 2004), soil is an important candidate to exert fine scale effects upon the cerrado vegetation, because, at such a fine scale, other factors like climate and fire frequency, are more homogeneous.

We assumed soil to be an important environmental filter at fine scale in cerrado plant communities. Thus, we expected floristic composition to change with fertility, reflecting, to some extent, success in exploiting resources and competing with other species (Magurran, 2004). Therefore we looked for relationships between soil and vegetation at fine scale in a cerrado site, trying to answer the following questions: (1) Is floristic composition related to soil features? (2) Are total abundance, richness, evenness, and diversity related to soil features? Moreover, species not only respond to soil, but also influence it as well (Ruggiero et al., 2002, Silva et al., 2008, Sparovek and Camargo, 1997). Additionally, we expected that, on nutrient-rich soils, where competition may be lower (Bertness and Callaway, 1994, MacArthur, 1972), total abundance, richness, evenness, and diversity would be higher.