DOI: 10.5061/dryad.n2z34tmsw
title: Small seed bank in grasslands and tree plantations in former grassland sites in the South Brazilian highlands

description: The soil seed bank can be an important source for vegetation regeneration,
 and data on the similarity between aboveground vegetation and the seed
 bank can provide information about successional pathways after
 disturbances or land-use change. We conducted this study in natural
 grasslands in the subtropical highland region in southern Brazil. We
 evaluated the effect of silviculture on richness, density, and composition
 of the seed bank at former grassland sites converted to pine plantations
 25 years ago. We worked at six grassland sites and three pine plantation
 sites and used the seedling emergence method. Seed bank density and
 richness in grasslands was lower than those reported in similar
 environments in other regions. Species richness and density varied
 considerably within each vegetation type therefore, richness and density
 were not statistically significant, while composition varied among
 vegetation types. In terms of species, the pine plantation seed bank was a
 small subset of the grassland seed bank. Seeds of typical grassland
 species were missing in the pine plantation, but also had only low
 abundances in the grassland, and similarity of seed bank and vegetation
 was low (less than 20%). The low seed density found in this study,
 including in grasslands areas, indicates that regeneration of species from
 the soil seed bank likely is of a limited role for the maintenance of
 plant populations after disturbances in this system. Our data further
 suggest that natural regeneration after tree planting in grasslands is
 reduced due to seed limitation.  
description: STUDY AREA—Our study sites are located in the highland grassland
 region in the southern part of Brazil’s Atlantic Forest domain (29°04’12’’
 S, 50°00’49’’ W). Regional climate is Cfb according to Köppen climate classification,
 and altitude approximately 1000 m. Mean annual temperature is 15°C and
 mean annual precipitation is 1881 mm (climate-data.org). The region is a
 plateau formed by basalt, rhyolite and rhyodacit rocks of Serra Geral
 formation. Soils are classified as Cambisoils according to FAO, 1997
 (Cambissolos in the Brazilian classification; Embrapa 2013). Natural
 vegetation in the region is composed of mosaics of
 <i>Araucaria</i> forest, cloud forest and grasslands (Leite
 & Klein 1990). These highland grasslands have been used for
 livestock grazing since European colonization. However, the presence of
 large herbivores – today extinct – even before the arrival of native
 American people is confirmed by the fossil record in the region (Scherer
 et al. 2007). Based on charcoal records from peat bogs, we know that fire
 has been rare during the Glacial maximum but became more frequent at the
 beginning of the Holocene (Behling & Pillar 2006). Today, fire,
 usually every other year, is used as a management tool to remove
 accumulated biomass to stimulate young leaf regrowth after winter. In
 terms of their floristic composition, the highland grasslands are
 dominated by C4 tussock grasses such as <i>Andropogon lateralis
 </i>Nees, <i>Sorgastrum scaberrimum </i>(Nees) Herter,
 <i>Axonopus pellitus</i> (Nees ex Trin.) Hitchc. &
 Chase and a high representation of Fabaceae family (Andrade et al.
 2019)<i>. </i>The region encompasses two important national
 parks, Aparados da Serra and Serra Geral, and other state and private
 protected areas. In the region, we find vast areas of pine plantations,
 with single planting cycles of 30 years on average, causing loss and
 fragmentation of natural areas (Hermann et al. 2016).
 For this study, we chose six well conserved grasslands, four
 located in Serra Geral National Park and two in Aparados da Serra National
 Park (Fig.1), and three pine plantations established in former grasslands
 areas. Two of them were in the buffer zone of the National parks, and one
 of them at the edge of the park. Pine plantations were initiated about 25
 years ago. Sites were situated in three blocks, each with one pine
 plantation and two natural grassland areas, with the same history and
 similar floristic composition of grasslands. Distance of blocks varied
 from 2 to 20 km, and areas within each block had distances of 500 to 2000
 m (see Fig. 1 for scheme of study design). VEGETATION
 SAMPLING—Quantitative vegetation sampling at the grassland sites was
 conducted in December 2014, in 10 plots of 1 m², randomly allocated, per
 grassland area. Distance between plots was approx. 50 m. Cover of all
 vascular species was recorded using the Londo decimal scale (Londo, 1976).
 In the pine plantation areas, no vegetation survey was conducted, as
 ground layer vegetation was completely absent. SEED
 BANK SAMPLING AND ASSESSMENT—The seed bank study was carried out using the
 seedling emergence method, which evaluates only the viable seeds in the
 soil (Thompson & Grime 1979). Soil samples for the seed bank study
 were collected in
 grasslands and current pine plantations. Samples were
 collected in two seasons (spring and autumn) with the
 intention of accessing both the transient seed bank and persistent seed
 bank (Thompson & Grime 1979). We used five sampling points in each
 study area, totaling 30 samples from grassland and 15 from pine
 plantations (five per area). Distances between sampling points were
 approx. 50 m. Soil samples were collected with an auger (diameter: 5 cm;
 depth: 10 cm). At each sample point we collected four sub-samples which
 were mixed, resulting in one composite sample per point. All sample points
 were randomly selected. For seedling emergence,
 we used 50% of the soil collected in the field. Soil was mixed with
 vermiculite (50:50), to maintain humidity, and spread in trays (soil
 depth: 2-3 cm). Samples were kept in a greenhouse with irrigation for one
 year and were monitored weekly. Trays with sterilized soil were
 distributed among the soil samples from the grasslands to control possible
 contaminations by plants dispersed close to the experimental facilities.
 Emerging seedlings were identified, counted, and removed as soon as
 possible. For species that could not be identified right away, at least
 one specimen was transplanted into a larger container for development of
 the reproductive phase, for later identification. Most taxa (83%) were
 identified to the species level and 92% to the genera level. Some
 individuals died in the trays or transplanted pots before identification
 was possible, or there was little development of individuals impeding
 identification. DATA
 ANALYSIS—Data of seeds per sampling point unit were converted to density
 (seeds per square meter) with the aim of facilitating comparison with
 other studies. We averaged seed density data from the two seasons together
 for each sampling point. For statistical analysis, mean values of each
 studied area were considered, resulting in six average values for the
 grassland areas and three average values for the Pinus areas. For all
 analyses, we used randomization tests, with 10.000 iterations. This method
 (also referred to as permutation test), based on resampling, is also
 adequate for multivariate data sets, such as compositional data, and has
 been proposed specifically for vegetation data (details in Pillar
 & Orlóci 1996). Another advantage is that it does not require
 normal distribution of data, while preventing robust test results (Pillar
 & Orlóci 1996); this also makes the method especially appropriate
 for our data set. For analysis of richness and density data, we used
 Euclidean distance as dissimilarity measure and for analysis of the seed
 bank composition chord distance as dissimilarity measure. We analyzed
 composition similarities among pine plantations soil seed bank, grassland
 seed bank and aboveground vegetation on grassland areas with Sørensen’s
 Index (2a/2a + b + c), where a = number of species common to both seed
 banks, b = number of species unique to the first seed bank, and c = number
 of species unique to the second seed bank, considering all the data set of
 the seed bank (two seasons). Principal Coordinate Analysis was conducted
 to visualize difference in seed bank composition between the grasslands
 and pine plantations, using chord distance as the similarity measure. For
 all analyses, we used the software MULTIV (Pillar, 2006). We used alpha =
 0.05 as significance level.