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  • Germination, Emergence, and Survival of Buddleja cordata in an Urban Forest
  • Pedro E. Mendoza-Hernández, Alma Orozco-Segovia, and Irene Pisanty

Mexico City, one of the world’s largest cities, has grown at the expense of different ecosystems. The Parque Ecológico de la Ciudad de México (PECM) is located on Mt. Ajusco south of Mexico City (19°14′N and 99°15′W, elev. 2,600–2,900 m asl), one of the surrounding mountains that are important for conserving biodiversity and recharging aquifers. This park includes 730 ha, 200 of which were deeply disturbed in the early 1980s by an urban shantytown settlement (Cano-Santana et al. 2006).

Vegetation includes oak, pine, and pine-oak forests as well as xerophylous shrubland growing on basaltic substrate. Soils are well developed in the forests but scarce in the shrublands. The basaltic substrate has low water [End Page 263] retention and wide temperature variation (0°C–50°C) (Olvera-Carrillo et al. 2009). Average annual temperature is 12°C–14°C, and annual precipitation, concentrated in the summer rainy season, is 700–1,000 mm. In the most disturbed area, volcanic rock was fragmented and removed. The most common pioneer trees in this area are known locally as tepozán (Buddleja cordata) and chapulixtle (Dodonaea viscosa), while mala mujer (Wigandia urens) is the most common shrub. However, the harsh environmental conditions make plant recruitment difficult, and intervention to restore this area is necessary.

Tepozán is common in disturbed habitats from Mexico and Guatemala (Rzedowski and Rzedowski 2001). It flowers between July and December, and seed shedding occurs from October to March. The tiny seeds (0.0278 ± 0.0005 mg) are wind dispersed, need light to germinate, have high viability, and are partially dormant after collection (González-Zertuche et al. 2002). Tepozán has high litter production and growth rates, which make it a suitable species to restore ecosystem structure and functionality in the area (Vázquez-Yanes and Bátis 1996, Mendoza-Hernández 2003)

To make better use of tepozán in the restoration of disturbed shrubland, we tested the effects of seed age and collection time on the germination of this species and measured the effects of substrate and watering on the emergence and survival of seedlings. We seek a better understanding of these early life stages that are crucial to establishment of this important pioneer species.

We collected seeds from ten trees in October 1992 (autumn), November 1993 (autumn), January and February 1994 (winter), and March 1995 (winter). Seeds were cleaned, air dried, and stored at room temperature (23°C–25°C; 20%–50% RH), until use in June 1995. To compare germination among seeds produced during the same season, we used seeds collected in November 1993 and January and February 1994. Seeds from each collection date were divided into three replicates of 50 seeds each and germinated in Petri dishes on a 1% agar plate in a growth chamber (Model 125L, Conviron, Winnipeg, Canada) at 25°C and 12 h of light. We recorded the number of germinated seeds every three days. Final germination percentages were arcsine transformed and compared using an ANOVA test (Statistica, vers. 5, Statsoft, Tulsa OK). Under constant temperature, seeds germinated simultaneously, beginning on the fifth day and ending on the tenth day. Seeds collected in the rainy autumn had higher germination percentages (77.8 ± 2.4) than seeds collected in the dry winter (35.6 ± 1.1 and 28.5 ± 2.5, for the January and February seed lots, respectively) (F2,8 = 22.937, p = 0.0001), suggesting that environmental conditions had a significant effect on seed dormancy and vigor.

Next, we determined the effect of soil type, watering regime, and seed age on seedling emergence. We filled 24 pots with forest soil; half were sown with seeds collected in 1992 and the other 12 with seeds collected in 1995, using 50 seeds per pot. Pots were placed in a shadehouse in the PECM. To emulate precipitation variability, six pots from each cohort were watered every three days (frequent watering), and the other six only once a week (scarce watering). The same procedure was carried out with...

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