of perennial shrubs like Larrea divaricata, and reproduces only by seeds .T. pasacana blooms during spring, from October through December. Fruits are fleshy ...
Tropical Ecology 40(1):79-84, 1999
ISSN 0564-3295
© International Society for Tropical Ecology
Seed production and seed bank of Trichocereus pasacana (Cactaceae) in northwestern Argentina MARTA L. DE VIANA
Universidad Nacional de Salta, Facultad de Ciencias Naturales, Catedra de Ecología Buenos Aires 177, 4400, Salta, Argentina Abstract: Fruit and seed production and seed bank of Trichocereus pasacana were studied for two years at two different sites (Cardonal and Duendeyacu) in the National Park Los Cardones (N. W. Argentina). The sites differed in T. pasacana density, plant species diversity and altitude. Wet and dry fruit weight, pulp content per fruit and mean seed weight of the T.pasacana population at the Cardonal site exceeded that of Duendeyacu, but mean seed number per fruit was similar at both sites. The number of fruits produced by the adult population was variable in space and time. Mean seed rain over the two year period was 150±194 and 149±206 m¯² for Cardonal and Duendeyacu, respectively. Seed density and viability were low at both sites and in both years’ samples. No seed were found in samples taken of bare soil. The number of germinable seeds (estimated at 0.2 m¯²) was similar at both sites and in both years. Resumen: En dos sitios del parque Nacional Los Cardones (noroeste de Argentina), se estudió la producción de frutos y semillas y el banco de semillas de T.pasacana. Los sitios difieren en densidad y composición por clases de altura de T.pasacana, diversidad de especies vegetales y altitud. El peso fresco y seco, el contenido de pulpa por fruto y el peso promedio de las semillas fue mayor en la población del Cardonal que en la de Duendeyacu, aunque el número promedio de semillas por fruto fue similar en ambos sitios. El número de frutos producidos por la población de adultos vario´ en espacio y tiempo. La lluvia promedio de semillas en los dos años de estudio fue 150 ± 194 y 149 ± 206/m² en ambos sitios, respectivamente. La densidad y viabilidad de las semillas fue baja en ambos sitios y años muestreados. No se detectaron semillas en las muestras de duelo descubierto. La cantidad de semillas germinables (estimada en 0.2/m²) fue similar en ambo sitios y años. Resumo: A produção de semente e o banco seminal de Trichocereus pasacana foram estudados durante dois anos em duas estações diferentes (Cardonal e Aguada Duendeyacu) no Parque Nacional de Los Cardones (N.W. Argentina). As estações apresentavam diferenças no que concerne à densidade de T. pasacana, na diversidade específica e na altitude. O peso húmido e seco, o conteúdo em polpa por fruto e o peso médio da semente da população de T. pasacana, na estação de Cardonal, foi superior a encontrada em Duendeyacu, se bem que o numero médio de sementes por fruto fosse similar nas duas estações. O numero de frutos produzidos pela população adulta foi variável no espaço e no tempo. O numero médio de sementes para o período dos dois anos foi de 150±194 e 149±206 m¯² para Cardonal e Duendeyacu respectivamente. A densidade das sementes e a sua viabilidade eram baixas nos dois locais e nos dois anos amostrados. Não foram encontradas sementes nas amostras tiradas do solo nu. O número de sementes germináveis (estimadas em 0.2 m¯²) foi similar, nos dois anos do ensaio, em ambas as estações.
Key words: Columnar cacti, fruit and seed production, germination, northwestern Argentina, seed bank, seed rain
T. PASACANA SEED PRODUCTION AND BANK production, the storage of seeds in the seed bank, and germination were studied in two environments of the National Park Los Cardones, with high and low abundance of recruits.
Introduction Until recently, compared to other topics ecology, seed populations have received little attention. However, the study of seed populations is essential for understanding plant responses to environmental heterogeneity, when recruitment occurs only by seeds and reproduction is unpredictable in space and time as, for example, in stress tolerant species sensu (Cavers 1983; Fenner 1985; Grime 1979; Harper 1977; Hickman 1979; Putwain & Gillham 1990). The number and quality of the seeds produced may be limited by external or internal plant factors which may vary over space and time. Plants may adjust resource allocation to seed production by altering the number of flowers, the number of ovules per flower, the number of aborted fruits and seeds, and the mass of mature seeds (Lee 1988; Primack 1978; Stephenson 1984, 1992). Trichocereus pasacana (Web.) Britton & Rose (Cactaceae), grows in the northwestern arid regions of Argentina, and characterizes the prepuna vegetation (Cabrera 1976). It is commonly found under the canopy of perennial shrubs like Larrea divaricata, and reproduces only by seeds .T. pasacana blooms during spring, from October through December. Fruits are fleshy, very attractive to birds, mammals (domestic and wild), and ants. Maturation starts in March (end of the rainy season), continuing up to the end of May. However, some fruits persist on the plant until July (de Viana 1995). T. pasacana is a long lived columnar cacti that may reach 8 m in height. The adults of this species are exploited as a source of wood for buildings and handicrafts. The continued selective exploitation of T.pasacana cacti of certain size classes by humans has produced a biased distribution of height in the population and has led to local extinctions (Acreche et al .1989; de Viana 1993).Within our research area in the Tin Tin Valley (covering some 9000 ha), recruits (< 10 cm height) have a patchy distribution. Inspite of the fact that juveniles and adults are distributed over the entire area, the density of recruits varies widely, from zero to high values (Acreche et al. 1989; de Viana et al. 1990). In this context, the absence of recruits may be attributed to failures in fruit and/or seed production, lack of viable seed, or to external factors that control seed survival, germination and establishment (Lee 1988; Primack1978; Stephenson1984, 1992). Seed
Methods Background to study area As part of a larger study of overall ecology of T.pasacana, seed production, seed storage in the seed bank, and germination were studied at two different sites located in the National Park Los Cardones (65º03’66º05’W,25º03’-25º23’S),Salta province, northwestern Argentina. The two sites (Cardonal and Aguada Duendeyacu) are both located within the Tin Tin Valley within the confines of the Park, but are situated 7 km apart. The sites differ in T.pasacana abundance and height class distribution, shrub species diversity and altitude (2800 and 2600 masl, respectively)(de Viana 1993)(Table 1).Since 1987,all individuals of T.pasacana present in a 3 ha plot at Cardonal, and a 1 ha plot at Duendeyacu, have been marked and studied for growth patterns (Acreche et al. 1989). At both sites, all adult individuals (150 cm
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DE VIANA 27.4° C and – 7.1°C, respectively. Throughout the year, there is a high diurnal thermal amplitude (over 20°C). The vegetation of the Tin Tin Valley is dominated by perennial shrubs including Larrea divaricata, L. nitida, Gochnatia glutinosa, Plectrocarpa tetracantha, Baccharis boliviensis, Flourensia fiebrigii, Bulnesia schickendantzii, Aphyllocladus spartioides, and Senna crassiramea. The only tree species in the area is Prosopis ferox and total plant cover is about 25% at both sites (de Viana et al 1989).
cm deep layer of soil in plastic trays in a germination chamber (20-24°C). Samples were watered every two days and seedling emergence was used to determine the number of viable seeds. The trays were searched for new seedlings over a period of a month. There was a high variability in soil volume per sample (due to differences in soil development, i.e., different rock content), but there was no correlation between number of seeds and sample volume (Spearman’s rank correlation coefficient 0.04; P>0.5).Because the seed data are highly skewed, distribution-free statistics were used in the analysis.
Fruit and seed production In 1992, at each site, a pooled sample of ripe fruits of T.pasacana (Cardonal N=23: Duendeyacu N=16) was weighed, fresh and dry. Later, seeds were separated from the pulp and weighed. In order to estimate the number of seeds per fruit, all seeds per fruit and there samples of 50 seeds per fruit, were weighed. Fruit production was assessed in two consecutive years (1991 and 1992) by counting the number of fruits produced by the adult population (marked individuals >150 cm in height) at both study sites. In 1991, the sample of adult cacti was slightly smaller than in 1992 (Cardonal=42 in 1991 and N= 54 in 1992; Duendeyacu: N=16 in 1991 and 24 in 1992). This was partly due to problems encountered in relocating some of the cacti previously marked in 1987 and it took time to reidentify the individuals.
Results Fruit traits varied from site to site. Fresh and dry fruit weights, and pulp content per fruit, were higher at Cardonal. Mean seed weight was higher at Cardonal than at Duendeyacu, but mean seed number per fruit was similar at both sites (Table 2). The number of fruits produced by the adult population was variable in space and time. In 1991 the fruit crop at Cardonal was lower than that at Duendeyacu in the same year, and lower than the fruit crop in 1992 at both sites. However, there were no significant differences in fruit crop between sites or within sites over the two year period. Taking into account fruit crop and mean seed number per fruit in the adult population, the potential mean seed rain/m² was estimated. Although there were annual differences between and among sites, mean seed rain, over the two years period was similar at both sites (Table 3). In both years sampled, total seed number was low (compared to other studies with perennial pastures in Patagonia, Bertiller & Aloia 1995) as was germinability (less than 27%), in the two sites studied. No seeds were found in bare soil samples. At both sites, higher seed numbers (total and germinable) were found near T. Tabla 2. . T.pasacana mean weight (± standard error in brackets) of fresh and dry fruits, pulp and seeds (**P150 cm height) at the study sites. (Student-t, *difference in fruit crop between years, Cardonal, P
