L. Valbuena

Seed banks of Erica australis and Calluna vulgaris in a heathland subjected to experimental fire

Soil samples were collected before and after an experimental fire on a heathland in the province of Le6n (Spain). The seed banks were assessed by counting the num- bers of seedlings appearing of Erica australis and Calluna vulgaris. A total of 2285 germinable seeds/m2 before burning and 1177 germinable seeds/m2 after the fire were estimated for Erica australis, and 90 and 690 germinable seeds/m2, respec- tively, for Calluna vulgaris. After the fire the number of germinable seeds/m2 of Erica australis had decreased, whilst there was a considerable increase in the number of germinable seeds/m2 of Calluna vulgaris. Neither seedlings nor sprouts of Calluna vulgaris had appeared in the field plots 10 months after the fire. Erica australis did recover mainly by sprouting after fire.

Eleven years of recovery dynamic after experimental burning and cutting in two Cistus communities

Recovery after cutting and burning in two series of experimental shrubland plots dominated by Cistus laurifolius and Cistus ladanifer, respectively, was studied over 11 years. The two communities tend to recover by an autosuccession process after disturbances, with Cistus species being dominant from the first or second year. The greatest diversity is recorded in the first few years after the disturbances, with higher species richness, basically herbaceous ones, and lower dominance effect. Cover by woody species tends to increase but from the fourth year the differences are not significant. The highest herbaceous cover is after one year and the peak is more marked in the cut plots. Herbaceous species cover decreases afterwards and there are small changes in the different years but without clear trends. When all the samplings are compared by a TWINSPAN, the first division separates both communities and the second division the initial and the final phases from each community. The separation between cut and burnt plots is only observed in the final phases.

Effect of high temperatures on seed germination and seedling survival in three pine species (Pinus pinaster, P. sylvestris and P. nigra)

In the present study, we analyse the germination and seedling growth of three Pinus species (P. pinaster, P. sylvestris and P. nigra) under laboratory conditions after thermal shocks simulating the temperature effects of fire. Temperatures up to 150°C and 5 min exposure show negative effects on the percentage germination of P. pinaster seeds. P. sylvestris and P. nigra cannot stand temperatures greater than 110°C and 5 min exposure. In all three species, thermal shocks delay the start of germination. In addition, seeds from P. pinaster germinate over a longer time range. Seedling aerial growth after germination is influenced by high temperatures in P. sylvestris and P. nigra, but not in P. pinaster. Finally, thermal shocks produce a reduction in root growth in comparison with aerial growth in P. sylvestris.

Influence of tree age on seed germination response to environmental factors and inhibitory substances in Pinus pinaster

The present study analyses the reproductive behaviour of Pinus pinaster at different ages of the parent trees after subjecting the seeds to variations in environmental factors and inhibitory substances. An experiment was designed in which seeds from mature (43 years old) and young (13 years old) trees were subjected to different environmental factors: darkness, thermal shocks and the presence of inhibitory substances from the needles and the undergrowth humus of P. pinaster. A control treatment was also carried out for the seeds of each age. The results obtained indicate that the age of the parent trees has a significant effect on mean germination time after experimental treatments: shock temperatures, darkness, and leaf and humus exudates. Seeds from young populations have shorter mean germination times than those from adult populations. Similarly, mature P. pinaster populations show more diversity in the length of innate dormancy of their seeds than younger populations. However, age has no significant effect on total percentage germination. Significant reductions in germination were observed when seeds from mature and young trees were exposed to temperatures equal to or above 150°C for 5 min. Increased temperature also produces a delay in the onset of germination in both populations.

Comparison between the soil seed banks of a burnt and an unburnt Quercus pyrenaica Willd. forest

Soil samples from burnt and unburnt sites dominated by Quercus pyrenaica Willd. forest in León province (NW Spain) were collected, separated into an upper layer (0–2 cm depth) from a lower layer (2–5 cm), put in a greenhouse, and seeds allowed to germinate. A total of 670 identified seedlings comprising 56 species and belonging to 24 families were recorded. Most seedling were herbaceous perennials (hemicryptophytes: 59%), followed by annuals (therophytes: 23%). Many species that germinated from buried seeds were not observed as plants at any site, and came from exogenous communities. The principal means of seed dissemination were anemochory (45%) and autochory (23%). The number of species as well as seedlings was not significantly different between soils from the burnt and unburnt sites, but seedlings were more numerous in samples from the unburnt sites, as the number of seedlings was always highest in the upper soil layer. Germination behaviour of four particular species was characterized. Seedlings from the seed bank temporarily were found in the early stages of recovery of the burnt Quercus pyrenaica forests.

Influencia del calor y del aclarado sobre la germinación de Cistus laurifolius y Cistus ladanifer

El objetivo de este estudio es determinar si el aumento de temperatura que se produce en el suelo durante los incendios favorece la germinacion de las semillas de Cistus laurliolius y Cistus ladanifer. Se realizan ensayos de germinacion en el laboratorio, calentando las semillas a diferentes temperaturas durante distintos tiempos. Paralelamente, se estudia la regeneracion de ambas especies en condiciones de campo, en parcelas experimentales sometidas a quema controlada y corta de la biomasa aerea. Cistus ladanifer se regenera mejor que Cistus laurifolius, en terminos de numero de plantulas, y mejor en las parcelas quemadas que en las cortadas. La altura en cada caso sigue patrones diferentes. Los resultados de campo concuerdan con los de laboratorio.

Ten years of recovery of Cistus ladanifer after experimental disturbances.

The aim of this study is to compare the recovery response of Cistus ladanifer L. tothree experimental treatments in terms of seedling growth and population dynamics.A uniform area was chosen in the province of Le& oacute; n (NW Spain), in which C. ladanifer was clearly dominant with a mean cover of 70%. Three plots were established and subjected to experimental burning, cutting, and plowing in July1989. The changes in the C. ladanifer population were surveyed over 10 years (from1990 to 1999). The number of Cistus seedlings, the height of each, and the total Cistus cover percentage were recorded for every sampling period. There were no Cistus shoots from vegetative sprouting. The lowest density was recorded in the plowed plot and the highest in the burned one, as was expected from a species whose germination is stimulated by heat; ten years after treatments, density in the burned plot is still significantly higher. A tendency towards stabilization in cover was observed in the three plots, with the highest ...

Models of Structural Variation of the Herbaceous Community with Tree Influence in “Dehesa” Ecosystems of Quercus Pyrenaica

The interactions between the trees and the herbaceous community near them are multiple and complex (1, 2, 4). The objective of this work was to determine the manner in which these interactions affect the structure of the herbaceous community and whether this effect can be described as a function of distance from the trees and of the orientation.