THE FATE OF HERBACEOUS SEEDS DURING TOPSOIL STOCKPILING: GERMINATION RATE AND VIABILITY Rivera, D. *1, Jáuregui, BM. 2, de La Rosa, G. & Peco, B.

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1 THE FATE OF HERBACEOUS SEEDS DURING TOPSOIL STOCKPILING: GERMINATION RATE AND VIABILITY Rivera, D. *1, Jáuregui, BM. 2, de La Rosa, G. & Peco, B. *1 1 Departamento Ecología, Universidad Autónoma de Madrid. C/ Darwin, 2 C.P Madrid. * desi.rivera@gmail.com 2 Servicio I+D+I. OHL, Obrascón Huarte Lain. S.A., Paseo de la Castellana 259D, Torre Espacio, planta 9 norte. C.P Madrid Abstract: Topsoil removed during linear infrastructure construction is one of the most valuable resources for the ecological restoration of roadslopes, as it contains the highest concentration of micro-organisms, nutrients and seeds in the soil. During construction work, topsoil is stockpiled in a way that can harm seed germination and survival capacity. In order to assess the effects of topsoil storage time and seed burial depth on germination and viability, an experiment with three replicates was conducted using two factors: time (1 to 6 months) and burial depth (0, 5, 30 and 50 cm). At each depth of the stockpile we burried 25 seeds from 10 natural grassland species -belonged to 5 families- in permeable nylon sachets. Germination rate and viability were analyzed using binomial GLM, with family as random factor and four covariates time, depth of burial, seed weight and light response. Germination rate increased with time (X 2 =552.87; p<0.001), seed weight (X 2 =14.80; p<0.001), but decreased with burial depth (X 2 = ; p<0.001) and light response (X 2 =15.3; p<0.001). Family was also significant (X 2 =477.94; p<0.001), higher in Poaceae and lower in Caryophyllaceae. Viability decreased significantly with storage time (X 2 =888.88; p<0.001) and depth (X 2 =70.56; p<0.001), while it increased significantly with seed weight (X 2 =49.25; p<0.001). Family also had a significant influence (X 2 =239.85; p<0.001) on viability. It was lower in Compositae than in the rest of the families. The results show that there may be a loss of viable seeds in topsoil stockpiles, particularly in the case of large seeds. Keywords: restoration, roadslope, topsoil, seedbank, germination rate and viability. Introduction During construction of lineal infrastructures like highways and railways, major earthworks are needed to adapt topography to the infrastructure, generating roadslopes: either roadcuts or roadfills. The environmental law says that this work on landscape must have a minimal impact, so restorations plans are implemented. Many of this restoration projects involve removal of topsoil, the first 30 cm, which afterwards will be spread out (Balaguer, 2002; Rokich et al., 2000). Since topsoil contains the highest concentration of seeds, nutrients and microorganisms in the soil, it is one of the most valuable resources for the ecological restoration of roadslopes. Therefore, handled correctly, the topsoil seedbank can be used to successfully revegetate disturbed areas (Rokich et al., 2000). But, before topsoil is spread out, it is stockpiled in ellipsoidal mounds in a way that can harm the success of seed germination and survival capacity. So, as topsoil s seedbank will be the first source of colonization, it is very important to consider what processes affect seeds during the stockpiling period. The main objective of this study was to assess the effects of topsoil storage time and seed burial depth on germination rates and viability. Materials and methods The study area was in central Spain, in an abandoned pasture area near Universidad Autónoma de Madrid (40º N; 3º O). The experimental design consisted of three factors: storage time, from 1 to 6 months; burial depth (0; 5; 30 and 50 cm) and family; with three replicates (n=720). First of all, seeds were collected in July 2009 in Mediterranean pastures of the north of Madrid. After cleaning, seeds were put into permeable nylon sachets (25 seeds per sachet). The first 30 cm of topsoil were removed and stockpiled with a digger in September. In October, sachets were buried at each depth level along the stockpile, two temperature and moisture sensors were placed at each depth as well. 1

2 Ecological Restoration and Sustainable Development - Establishing Links Across Frontiers Three sachets from each depth were exhumed monthly. Seeds were removed from the sachets and classified into germinated or not germinated seeds and viability of the non-germinated seeds was assessed with a pressure test. Thus, non-germinated seeds were classified into hard or soft seeds. The ten species used in this experiment were: Tolpis barbata, Crepis capillaris (Compositae), Bromus hordaceous, Vulpia muralis-myurus (Poaceae), Trifolium dubium, Trifolium campestre (Fabaceae), Petrorhagia nanteuilii, Spergularia purpurea (Caryophyllaceae), Plantago coronopus, Plantago lagopus (Plantaginaceae). Figure 1. a) Study area, b) seed collection, c) seed sachets, d, e, f) stockpile construction, g) seed burial, h) sachets at surface, and i) seed sachets after exhumation. Three response variables were used in the analysis. The first one, restoration potential, calculated as the number of hard seeds divided by total seeds, is an index of seed survival. The restoration capacity decreases with this index, mainly caused by seed germination or death. Germination was calculated as number of germinated seeds divided by total seeds, and mortality was the number of soft seeds divided by non germinated seeds. In a previous germination test, an index of light response was generated for each species. It was calculated as seed germination percentage in light divided by seed germination percentage in light plus seed germination percentage in darkness. Seed weight was also used as indicator, obtained as the mean weight of ten samples of 50 seeds. We used ANOVA and Bonferroni s post-hoc tests to analyze significant differences for environmental parameters. Binomial Generalized Linear Model with logit link function was used to analyze the response variables. Family was introduced as the random factor while the rest of explanatory variables were introduced as covariates: stockpiling time, burial depth, light response index and seed weight. AIC criterion was used to choose the best model. Results and discussion The best fitting model for restoration potential included all the explanatory variables except seed weight (Table 1). Stockpiling time had a negative effect on restoration potential. Seeds with high light response index showed higher survival capacity in the stockpile. For the germination model, light response index was the only non significant variable. Stockpiling time and seed weight were positively related with seed germination, suggesting that germination of large seeds is faster, probably, to avoid predation (Peco et al., 2003). And seed germination, decrease with burial depth (Chen & Maun, 1999; Ren et al., 2002). The best model for mortality included all explanatory variables. Mortality increased significantly with stockpiling time and burial depth. Loss of seeds due to mortality was lower for species with large seeds, probably, because they have more resources and better protection 2

3 against pathogen attacks. On the other hand, seeds that have a greater response to light have lower mortality,since the response to light is a mechanism which allows seed to identify emerging gaps. The family factor was significant in all the three models (Figure 2). In our study, the family with the highest restoration potential was Caryophyllaceae while Poaceae had the lowest one. In the case of germination, Poaceae had the highest value and Cariophyllace the lowest, which also agreed with restoration potential results. Fabaceae showed the lowest value of seed mortality while Compositae was the family with the highest mortality. Along time, the intense loss of more than 40% of the restoration potential with the stockpiling time, seemed to be caused by the increase of germination and seed mortality (Figure 3). Germination increased until the fourth month, when it remained constant. However, in this study, germination was low, probably because the optimum period for germination of these seeds is autumn and during the study there was an extremely dry autumn. The first rains were in early winter and germination was limited by cold temperatures (Figure 5 and 6). Mortality increased with time, possibly due to pathogen attacks. Regarding depth, the restoration potential was lower at the stockpile surface (Figure 4) because germination mainly occurs at the surface, induced by daily oscillation temperature (Figure 7), light and oxygen conditions (Fenner & Thompson, 2005). Seeds that germinate between 0 and 5 cm can be lost because of suicide germination (germinated seeds can t reach the surface) or because the emerged plants die before seed production (Chen & Maun, 1999; Rokich et al., 2000). Lack of light, oxygen or moisture could explain the decrease in germination at higher depths since these factors can completely suppress germination (Fenner & Thompson, 2005). Regarding mortality, it was low and remained constant with depth. Table 1. Binomial GLM results for restoration potential, germination and mortality. Response variable Explanatory variables X 2 de Wald P-value β Restoration potential Stockpiling time < (n=720; AIC= ; d.f.=7; p=0.000) Burial depth < Light response index 3.85 < Family < Germination (n=720; AIC= ; d.f.=7; p=0.000) Mortality (n=720; AIC= ; d.f.=8; p=0.000) Stockpiling time < Burial depth < Seed weight < Family < Stockpiling time < Burial depth 4.69 < Light response index 4.71 < Seed weight < Family < Figure 2. Plant families. Figure 3. Stockpiling time + SD Figure 4. Burial depth + SD. 3

4 Ecological Restoration and Sustainable Development - Establishing Links Across Frontiers During the experiment, mean daily temperature in time didn t show any significant difference between the depth levels. For mean daily oscillation temperature, ANOVA s post hoc test showed that at 5 cm depth, mean daily oscillation was bigger than at the two other burial depths. And finally, mean daily soil moisture decreased with depth and was higher at 5 cm than at the two other depths. Figure 5. Mean daily temperature (⁰C) + SD. Figure 6. Mean daily soil moisture (%) + SD. Figure 7. Mean daily temperature oscillation (⁰C) + SD. Conclusions The restoration potential gradually decreased with stockpiling time. Since this loss is mainly produced by seed germination and mortality increase. Germination basically occurs at surface while mortality is low and independent of depth. Seed loss due to germination in the stockpile was mainly drawn by the germination of species with larger seeds. Seed loss due to mortality was higher in species with small seeds and in species without physiological mechanisms of light detection related to germination. Restoration potential is conditioned by taxonomic groups. Recommendations From the results of this study, we recommend to minimize stockpiling time to avoid seed germination and, consequently to maintain the restoration potential of seeds. Furthermore, in case stockpiles remain on field for long periods, before being spread out, it is recommended to wait until the production of new seeds. Thus, an increase of seeds in the stockpile is possible. Acknowledgements This study is part of research by OHL company in collaboration with the Ecology Department of the Autonoma s University of Madrid, part of a bigger industrial research project called CENIT-OASIS and funded by Spain's Research and Science Ministry. This project was also cofunded by Comunidad de Madrid, REMEDINAL 2 S-2009/AMB/783. References Balaguer, L. (2002). Las limitaciones de la restauración de la cubierta vegetal. Ecosistemas, Año XI, Nº1. Chen, H. & Maun, M.A Effects of sand burial depth on seed germination and seedling emergence of Cirsium pitcheri. Plant Ecology 140: Fenner & Thompson, The Ecology of Seeds. Cambridge University Press. Peco, B., Traba, J., Levassor, C., Sanchez, A.M. & Azcárate, F.M Seed size, shape and persistence in dry Mediterranean grass and scrublands. Seed Science Research, 13:

5 Ren, J., Tao, L., & Liuz, X Effect of sand burial depth on seed germination and seedling emergence of Calligonum L. species. Journal of Arid Environments, 51: Rokich, D., Dixon, K., Sivasithamparam, K. & Meney, K Topsoil Handling and Storage Effects on Woodland Restoration in Western Australia. Restoration Ecology. 8 (2):