The fate of seeds in the soil: a review of the influence of overland flow on seed removal and its consequences for the vegetation of arid and semiarid patchy ecosystems

Abstract

Abstract. Since seeds are the principle means by which plants move across the landscapes, the final fate of seeds plays a fundamental role in the assemblage, functioning and dynamics of plant communities. Once seeds land on the soil surface after being dispersed from the parent plant, they can be moved horizontally by surface runoff. In arid and semiarid patchy ecosystems, where seeds are scattered into a very heterogeneous environment and intense rainfalls occur, the transport of seeds by runoff to new sites may be an opportunity for seeds to reach more favourable sites for seed germination and seedling survival. Although seed transport by runoff may be of vital importance for the recruitment of plants in these ecosystems, it has received little attention in the scientific literature, especially among soil scientists. The main goals are (1) to offer an updated conceptual model of seed fate with a special attention to seed destiny in and on the soil, (2) to review studies on seed fate in overland flow and the ecological implications seed transport by runoff has for the origin, spatial patterning and maintenance of patches and for plant community composition in arid and semiarid patchy ecosystems, and finally (3) to point out directions for future research. Our review shows that seed fate in overland flow may result either in the export of seeds from the system (seed loss) or in the spatial redistribution of seeds within the system through short-distance seed movements (seed displacement). Seed transport by runoff depends on rainfall, slope and soil characteristics. Seed susceptibility to be removed varies highly between species and is mainly related to seed traits, as seed size, seed shape, presence of appendages, and seed ability to secrete mucilage. Although initially considered as a risk of seed loss, seed removal by runoff has recently been described as an ecological driver that shapes plant composition from the first phases of the plant life, by favouring species with seeds able to resist erosion and by selecting for plant traits that prevent seed loss. Moreover, the interaction of seed transport by overland flow with the high seed trapping capacity of vegetated patches results in a "patch-to-patch" transport of seeds that plays a relevant role in vegetation establishment and patterning in arid and semiarid patchy ecosystems. Overall, this review shows how the knowledge about seed fate in overland flow can be used to explain a number of important characteristics of whole plant communities. It also underlines important gaps of knowledge that should be filled in. Future lines of research are proposed in order to broaden our understanding of the origin, maintenance and dynamics of patchiness in arid and semiarid ecosystems and to improve restoration success of intensively eroded ecosystems.

Seed‐bank structure and plant‐recruitment conditions regulate the dynamics of a grassland‐shrubland Chihuahuan ecotone

“…Early seedling dynamics can have a great influence on the spatial pattern and rates of vegetation change in drylands (Espigares and Peco 1993, Kigel 1995, Peters 2002b, Bowers et al 2004, Bochet 2015, Collins and Xia 2015. Our L. tridentata seedling simulations along 1948-1959 suggest that the 1950s dry period provided an ideal window for the expansion of L. tridentata over the SNWR Five Points area (Fig.…”

“…Soil seed-density records in our Chihuahuan study are considerably smaller than typical viable seed amounts in other semiarid environments, for example the shortgrass steppe and mediterranean grasslands, which have average densities generally above 1,000 seeds/m 2 Lauenroth 1989, Espigares andPeco 1993), although they are within the range of viable soil seed densities reported to occur in desert sites (Dwyer andAguirre 1978, Koontz andSimpson 2010). Concentration of the soil seed bank in densely vegetated microsites is characteristic of many arid and semiarid landscapes, where vegetation patches generally act as a sink for water, nutrients, and sediments, leading to improved soil properties (Bochet 2015). Spatial heterogeneity of soil seed density in these systems is usually explained by the patchy distribution of parental plants (i.e., seed pools generally decrease rapidly away from the parental plants for most gravity dispersed seeds) and patch-to-patch transmission of seeds by wind and/or runoff (Guo et al 1998, Koontz and Simpson 2010, Thompson et al 2014.…”

“…An important step toward understanding patterns in plant-species dominance and vegetation change at desert grassland-shrubland ecotones is determining plant-recruitment limitations imposed by the shrubencroachment process. Seed availability and the dynamics of seed germination and early seedling establishment are critical plant recruitment processes that can condition species dominance, plant-regeneration patterns, and the rates of vegetation change in drylands (Coffin and Lauenroth 1989, Espigares and Peco 1993, Kigel 1995, Bowers et al 2004, Koontz and Simpson 2010, Peters et al 2014, Bochet 2015. Previous studies of L. tridentata plant-age structure in a variety of North American desert sites suggests that recruitment of seedlings occurs primarily as rare episodic events that may influence shrubexpansion rates and the frequency of active encroachment pulses for the region (Chew and Chew 1965, Barbour 1969, Allen et al 2008.…”

Diversity without complementarity threatens vegetation patterns in arid lands

“…Different species adapt to harsh environments by changing the soil crust and the soil structure (Seghieri et al, 1997;Valentin et al, 1999) and allocating resources either below (Couteron et al, 2014) or above ground, in order to have access to scarce resources. Species adaptation also influences seed dispersal and seed fate, according to the local erosion rate, water and solid transport (Bochet, 2015;Bochet et al, 2009). Therefore, periodic species zonation may be expected along a tiger bush transect.…”

Changes in Nutrient Contents of Soil across Different Land Uses in a Forest Reserve

“…This process, in turn, leads to a decline in soil fertility and loss of biological activity and diversity . Vegetation cover is, therefore, a key indicator of soil degradation as plants play a role in the control of soil erosion (Cedar, 1998;Keesstra et al, 2009;Kropfl et al, 2013;Tejada and Benitez, 2014;Bochet, 2015).…”