Arch. Biol. Sci., Belgrade, 64 (4), 1543-1554, 2012 DOI:10.2298/ABS1204543C EFFECTS OF BURIAL DEPTH ON SEED GERMINATION AND SEEDLING EMERGENCE OF MEXICAN OAKS: A GLASSHOUSE EXPERIMENT J. FLORES-CANO1, E. I. BadaNO2 and J. FLORES2* 1 Facultad de Agronomía, Universidad Autónoma de San Luis Potosí, C.P. 78321. San Luis Potosí, México 2 Instituto Potosino de Investigación Científica y Tecnológica, A.C. (IPICYT), División de Ciencias Ambientales, 78216, San Luis Potosí, México Abstract - Despite the interest in restoring oak forests in Mexico, very little is known about their regeneration ecology. We assessed the influence of acorn burial depth on seed germination and seedling emergence for eight Mexican oak species (Q. affinis, Q. castanea, Q. coccolobifolia, Q. laeta, Q. mexicana, Q. polymorpha, Q. tinkhamii and Q. viminea). We performed a glasshouse experiment in which acorns were buried at five soil depths (0, 2, 4, 6 and 8 cm). After four months, acorn germination and seedling emergence were recorded. Buried acorns showed higher germination and seedling emergence than unburied ones, but burial depth also influenced these responses. The optimum burial depth for seedling emergence of each species was 2, 4, 6 and 8 cm depth for four species (Q. castanea, Q. mexicana, Q. tinkhamii, and Q. viminea); 2 and 4 cm for Q. laeta; as well as 2, 4 and 8 cm for Q. coccolobifolia. Key words: Acorns, Erythrobalanus, Lepidobalanus, North American forests, Quercus, regeneration ecology INTRODUCTION to earlier successional species (Livingston and Alle- sio, 1968). The first stages of plant life cycles strongly influence recruitment patterns in natural plant populations Moreover, as most seeds on the soil surface need (Grubb, 1977). One of the most obvious factors in- light to germinate, burial is an essential prelude to fluencing germination and seedling emergence is the dormancy in several species (Thompson et al., 1993; seed burial depth in the soil (Fuchs et al., 2000; Traba Grime, 2001). From this point of view, burial may et al., 2004). Seed burial may be promoted by abiot- provide safe sites for seeds until germination occurs, ic-driven soil disturbances, such as those caused by although other processes such as seed predation, wind and water, as well as by biotic factors, including fungal attack, etc. can also be operating (Borchert earthworm activity and the foraging behavior of seed et al., 1989; Vander-Wall, 1993; Seiwa et al., 2002). dispersers (Vander-Wall 1990, 1993; Chambers and Nevertheless, while absolute darkness is required for MacMahon, 1994; Renard et al., 2010). Seed burial triggering the germination of some seeds (negative helps seeds to escape from post-dispersal predation photoblastic species), other seeds require full expo- and prevents the death of the embryo by desiccation sure to light in order to germinate (positive photo- or extreme temperatures during the unfavorable sea- blastic species) (Baskin and Baskin, 1998; Flores et son (Borchert et al., 1989; Vander-Wall, 1990; Seiwa al., 2011). Furthermore, between these two extreme et al., 2002; Cheng et al., 2007). On the other hand, light responses, there is a wide range of germination the presence of buried viable seeds has been related response to different light intensities (Pons, 2000). 1543 1544 J. FLORES-CANO ET AL. For example, light availability had a negative effect on cultural values that they have for Mexico (Luna-José the germination of Quercus suber and Q. canariensis, et al., 2003). but a small positive effect onQ. pyrenaica (Urbieta et al., 2008). Therefore, since the influence of sunlight Despite the current interest of governmental and radiation decreases with increasing soil depth, ger- non-governmental agencies in restoring Mexican oak mination responses are expected to change depend- forests, little is known about the factors that affect ing on the seed burial depth and the specific light their regeneration (López-Barrera and González-Es- requirements of each species (Fenner and Thomp- pinosa, 2001; Zavala-Chávez, 2004; Martínez-Pérez son, 2005). Findings reported for oaks from Asia, et al., 2006). It is therefore important to understand Europe and the USA show lower germination when the requirements for oak germination and establish- acorns are exposed to direct sunlight at the soil sur- ment; studies about the oak requirements to germi- face (Shaw, 1968; Borchert et al., 1989; Kollmann and nate and establish are critical. This study focuses on Schill, 1996; Nilsson et al., 1996; Khan and Shankar, determining the impact of burial depth on acorn 2001; García et al., 2002; Li and Ma, 2003; Xiao et al., germination and seedling emergence for eight repre- 2004; Gómez et al., 2004; Guo et al., 2009). sentative Mexican oak species. Oaks (genus Quercus L., Fagaceae) are perhaps MATERIALS AND METHODS the most important group of trees in northern hemi- sphere forests. This is because of their high commer- Species cial value for timber, as well as their significance in supporting wildlife (McWilliams et al., 2002). Acorn The studied species wereQ. affinisScheidw., Q. cas- germination is one of the main determinants of the tanea Née, Q. coccolobifolia Trel., Q. laeta Liebm., natural regeneration of oak forests (Day, 2002), but Q. mexicana Bonpl., Q. polymorpha Schltdl. et little is known about the influence of different burial Cham., Q. tinkhamii C.H. Mull., and Q. viminea depths on the germination of American oak seeds. Trel. (Nomenclature of species and authorities fol- As far as we know, the few studies performed on this lowing the TROPICOS® database of the Missouri issue were conducted in China, showing that acorn Botanical Garden; available at http://www.tropicos. germination decreases as burial depth increased org). We chose these eight species for a number of (Guo et al., 2001; 2009). Studies on oak and acorn reasons. First, they are widely distributed through- ecology are particularly important for Mexico, be- out the Mexican forests (Llorente-Bousquets and cause this country is the center of diversification of Ocegueda, 2008). Second, more than half are en- the genus Quercus in the Western hemisphere (Nix- demic (Llorente-Bousquets and Ocegueda, 2008). on, 1993). Mexico contains the largest oak diversity Third, they include representatives of the two main (161 of the estimated 400 species), and the highest Quercus subgenera of Mexico, Lepidobalanus (white number of endemic species (109 species) in the world oaks) and Erythrobalanus (red oaks), which are (Valencia-A., 2004). Oak forests and mixed pine-oak thought to differ in several seed traits, such as the forests cover 15-18% of the Mexican land surface and time required for fruit maturation, chemical com- strongly contribute to the high diversity of this coun- position of their stored food reserves, and degree of try (Mittermeier and Goettsch-Mittermeier, 1997). dormancy (Zavala-Chávez and García, 1996; Bon- However, almost all of them show clear signs of en- ner, 2008). Moreover, while white oaks seem to be vironmental degradation because they are located at more abundant than red oaks in northern Mexico, sites that are particularly well suited for human set- the converse situation occurs in southern Mexico tlement, farming and ranching (Bonfil and Soberón, (Zavala-Chavez, 1998). Finally, a recent study indi- 1999). Therefore, these forests require to be urgently cated that these oak species recruit at different forest restored because of the high economic, social and successional stages (Castillo-Lara et al., 2008). De- tails on infrageneric taxonomy, distribution range EFFECTS OF BURIAL DEPTH ON SEED GERMINATION AND SEEDLING EMERGENCE OF MEXICAN OAKS 1545 Table 1. Oak species that were considered in this study. The table indicates the subgenus of each species, as well as the distribution range and successional stages to which each oak species belongs. The last column indicates the time that seeds were stored at 5°C after harvesting Seed storing Species Subgenus Distribution range Successional stage time (months) Quercus affinis Erythrobalanus Endemic for Mexico Late 1 Quercus castanea Erythrobalanus Mexico and USA Intermediate 0 Quercus coccolobifolia Erythrobalanus Endemic for Mexico Pioneer 2 Quercus laeta Lepidobalanus Endemic for Mexico Pioneer 1 Quercus mexicana Erythrobalanus Endemic for Mexico Intermediate 0 Quercus polymorpha Lepidobalanus Mexico and USA Intermediate 1 Quercus tinkhamii Lepidobalanus Endemic for Mexico Pioneer 1 Quercus viminea Erythrobalanus Mexico and USA Intermediate 2 Table 2. Acorn quality characteristics from eight Quercus species from San Luis Potosí, México. Species Seed viability (%) Seed mass (mg) Q. affinis 98.8 1784.797 Q. castanea 100 743.009 Q. coccolobifolia 97.6 230.727 Q. laeta 100 1015.990 Q. mexicana 99 1032.990 Q. polymorpha 100 1381.189 Q. tinkhamii 90 603.062 Q. viminea 97.5 174.986 and successional stages to which each oak species tion and exceeds 800 mm above 2200 m.a.s.l. (Ar- belongs are given in Table 1. riaga et al., 2000). Collecting site In late spring 2009, 20-30 adult oaks of each spe- cies were selected across this site when mature acorns We collected acorns of all these species at Sierra were directly collected from them. Acorns from trees de Álvarez (21°42’32’’–22°23’ 21’’ N; 100°05’24’’– belonging to the same species were mixed and stored 100°49’30’’ W), a priority conservation area for in polyethylene bags to avoid desiccation. Approxi- Mexico located in the State of San Luis Potosí (Ar- mately 800-1000 acorns were collected for each oak riaga et al., 2000). This mountain range covers 2265 species. Bags with seeds were stored at 5ºC to inhibit km2. Climate and vegetation both vary strongly germination (Zavala-Chávez, 2004). Acorns of some with elevation. The prevailing climate below 2000 species were ready for harvesting in July, but others m.a.s.l. is semi-temperate and semi-arid, and the were not ready until October; thus, the storing time vegetation is mainly composed of xerophytic plants.