Juan J. Martínez-Sánchez

Influence of heat on seed germination of seven Mediterranean Leguminosae species

The influence of high temperatures (dry heat and hot water) on germination of seven Mediterranean Leguminosae species typical of fire-prone ecosystems in southern Spain is analyzed, in order to know the response of seeds to wildfires and the possible implications in their regeneration after this disturbance. Seeds were heated to a range of temperatures (50 °–150 °C) and exposure times (1–60 min) similar to those registered in the upper soil layers during wildfires. Germination tests were carried out in plastic Petri dishes over 60 days. In general, the degree of seed germination promotion by dry heat treatments showed a wide interspecific variation, although the final germination level was increased in all the studied species except for Scorpiurus muricatus. The thermal pretreatment of 50 °C, however, was not effective for germination in any species, and rising the temperature to 70 °C only slightly enhanced the germination in Cytisus patens. The preheatings of 90 °C (5 and 10 min), 120 °C (5 and 10 min), and 150 °C (1 min) were the most effective in promoting seed germination. Hot water (100 °C) scarification also increased the final germination level in all cases, with the exception of C. patens. The germination rates after preheating were much lower than in mechanically scarified seeds and closely resembled those of the untreated seeds, except for C. reverchonii, whose seed germination rate decreased with heat. The response of species to heat shock had no clear relationship with life trait or with the specific post-fire regeneration strategy (obligate seeder or facultative resprouter). Those species coexisting in the same habitats had different heat optimal requirements for seed germination, an strategy suggested by some authors as minimizing interspecific competition in the secondary succession started after fire.

Effects of high temperatures on germination ofPinus halepensis Mill. andP. pinaster Aiton subsp.pinaster seeds in southeast Spain

Seeds ofPinus halepensis andPinus pinaster subsp.pinaster were subjected to four heat treatments (90, 110, 150 and 200 °C) for 1 and 5 min to investigate their tolerances to high temperatures resulting from fires. Another group of seeds of each species were not heated and considered as a control. Information on responses of seeds to high temperatures would help to determine whether or not the species are pyrophytes. Heating for 1 min at all temperatures did not affect the average germination ofPinus halepensis seeds whereas that ofP. pinaster subsp.pinaster seeds was decreased significantly when they were heated to 200 ° C. HoweverP. pinaster seeds were more resistant than that ofP. halepensis when heat treatment was 150 °C for 5 min. Data obtained also indicated that, in general,P. halepensis andP. pinaster subsp.pinaster seed germination is not favoured by a temperature increase than can be reached in soils during a fire. Thus, the species are not pyrophytes but colonizers of burnt sites.

Postfire regeneration of Pinus halepensis Miller in a semi-arid area in Albacete province (southeastern Spain)

Abstract:Natural regeneration of Pinus halepensis Mill. was studied for the first three years after fire in a semi-arid area of southeastern Spain. In order to establish the main periods of emergence and seedling mortality, 465 seedlings were labelled in ten permanent 50 m2 plots. The results showed that pine recolonization was accomplished during the first 21 months after fire. No new seedling was detected after that time. P. halepensis seedling mortality after the first post-germination summer ranged between 14.94% and 47.35%. After the second post-germination year, the increase of seedling mortality rate was not always lower than that recorded after the first year. Seedling density decreased from 0.66 seedling/m2, 9 months after fire, to 0.24 seedling/m2, 39 months after fire. This density should be high enough to guarantee the persistence of the pine forest in the burnt area.

A Conceptual Model of Salt Marsh Plant Distribution in Coastal Dunes of Southeastern Spain

The main objectives of this study were to identify a small of edaphic factors that could be related to vegetation distribution in a coastal dune salt marsh system in the Southeast of Spain and to establish a simple conceptual model to describe the relationships between these soil factors and the main plant communities. Soil and vegetation data were obtained from 87 sampling plots. The plant communities studied were dominated by Crucianella maritima, Teucrium dunense, Ammophila arenaria, Lygeum spartum, Schoenus nigricans, Juncus maritimus, Limonium cossonianum, Sarcocornia fruticosa, Arthrocnemum macrostachyum, and co-dominance of Sarcocornia fruticosa and Arthrocnemum macrostachyum. The first four communities occupied summit positions and the rest of communities interdune depressions. In addition, we sampled plots in bare soil at interdune depressions. The soil parameters studied were soil salinity, soil moisture, the ground-water level, the depth to gleyed matrix, and the distance to the shoreline. Soils at interdune depressions were consistently more saline, wetter, and with a shallower water table and gleyed matrix than soils at summit positions. Soil moisture, salinity, and the distance to the shoreline were parameters related to plant distribution at summit positions. However, at interdune depressions species distribution was mainly related to salinity, moisture, the depth of the ground water, and the depth to gleyed matrix. In the conceptual model proposed, bare soils are characterized by their extreme salinity in the growing season (spring) and a shallower ground-water level, which leads to a shallower gleyed matrix.

Establishment of Pinus halepensis Mill, saplings following fire: effects of competition with shrub species

An early study analysing the effects of competition from Cistus monspeliensis-dominated shrub canopy on Pinus halepensis saplings, both colonising a recently burnt area, has been extended in order to test initial predictions. Inter-specific competition effects on P. halepensis were experimentally analysed by a shrub thinning-out treatment carried out 1 year after fire. The extension of the recorded period confirmed (i) a significant increase in height, and (ii) the lack of variation in density of P. halepensis saplings when the shrub layer was removed. In contrast, the increase in relative growth rate in height (RGRh) and the decrease in mortality recorded during early post-fire stages for treated units did not persist in subsequent years. These two treatment-induced effects disappeared 1 year after the shrub clearing (29 months after fire). It is hypothesised that this time should represent a culminating point in the inter-specific competition established between Cistus and Pinus saplings simultaneously colonising recently disturbed areas and be a critical period for pine sapling survival. After this time, a reduction in relatively short-lived Cistus populations and an increase in P. halepensis abundance should be expected in the community. It is concluded that a shrub-clearing treatment could be recommendable if the initial post-fire pine sapling density is not high enough to successfully face the early critical competitive period. Consequences of early shrub competition on forest productivity during mature phases are also discussed.

Influence of a low-intensity fire on a Pinus halepensis Mill. forest seed bank and its consequences on the early stages of plant succession

The effect of an experimental low-intensity fire on the soil seed bank and its consequences on the early stages of post-disturbance succession were analyzed. The hypothesis that low-intensity fire may produce a population decline of hard-seeded species by the elimination of established plants and the lack of a seed germination promotion effect was investigated. Consecutive pre- and post-burning soil samplings revealed extremely low impact of fire on seed banks. In contrast, some specific seed banks increased with fire, probably due to a seed rain from fire-opening of capsule fruits. Fire neither promoted germination of hard-coated seeds nor affected their viability. Both the length of the seed-unproductive juvenile phase and post-fire habitat conditions were revealed as critical in determining the recovery of hard-seeded species after a low-intensity fire. Species with a short life cycle produced seeds in the year subsequent to the disturbance, ensuring a progressive moderate recovery from the few individ...

Comparative root system structure of post-fire Pinus halepensis Mill, and Cistus monspeliensis L saplings.

To describe root system topology of Pinus halepensis and Cistus monspeliensis saplings co-inhabiting natural post-fire sites, 55 P. halepensis and 26 C. monspeliensis saplings were extracted by the total excavation method from a burnt pine stand. Seedlings were individually labelled when emerging after fire and extracted three years later, at the sapling phase. In order to evaluate the effect of inter-specific competition of C. monspeliensis on P. halepensis root system, a stratified sampling was carried out according to density and height of the saplings. Topological parameters considered in the analysis were magnitude, total external pathlength, and altitude of the root systems. Weight and length of roots were also measured in order to estimate the specific root length, an index commonly used in morphological studies. Results clearly evidenced greater variability in root system topology of P. halepensis than C. monspeliensis saplings. Herringbone architecture (i.e., the most ordered pattern possible, with branching confined to the main axis) characterised small pine saplings, regardless of competition from C. monspeliensis, which changed to random branching in large saplings. In medium sized saplings, the root system was affected by inter-specific competition, which delayed changes in root branching. In contrast, C. monspeliensis invariably adopted randomly branched architecture, regardless of intra-specific competition. It is concluded that such different topological patterns make C. monspeliensis more competitive during the early stages of post-fire succession, because its root system is much more transport-efficient in the nutrient-rich environment. Those pines which finally branch roots by random pattern will reach higher stem height and magnitude, a factor which allows them to successfully compete with C. monspeliensis for soil nutrients and water. The morphological analysis showed a significant increase in the specific root length with competition, both in P. halepensis and C. monspeliensis saplings, which could be interpreted as a consequence of the reduction of root diameter in response to nutrient depletion.

Response to fire of a predominantly transient seed bank in a Mediterranean weedy pasture (eastern-central Spain)

Abstract The impact of fire on the viability of the entire soil seed bank in a Mediterranean pasture dominated by weeds originating from arable fields, and its recovery in the first post-fire year, were investigated. Soil samples were excavated separately in 0-2 cm and 2-5 cm deep layers, and their seed content was assessed using the seedling emergence method. Emergent seedling counts were carried out in the field during the spring following the fire. Analysis of samples from an unburnt stand revealed that annuals (83% species, 86% seeds), and weeds (50% species, 30% seeds) were a major component in the seed bank. Seed banks of most species (85%) were accumulated in the upper soil layer, denoting their transient nature. Fire severely reduced the density and species richness of the seed bank, as expected for a mainly transient soil seed bank in a community not exposed to a recurrent wildfire regime throughout its evolutionary history. One year after the fire, the seed bank increased in density and species richness, particularly in the upper soil layer. The recovery occurred in two ways: i) in-site seed production from seeds which survived the fire and germinated, and ii) wind-borne seed invasion from adjacent unburnt vegetation. The first type of recovery was mainly the result of relatively large-seeded species with short-distance seed dispersal mechanisms, whereas the second was observed for anemochorous and/or tiny-seeded species. We conclude that the ability of weedy communities to respond to fire is based on the colonizing and invasive nature of the species. Consequences for the use of fire in the management of such communities are discussed.

Morphology, anatomy and germination response of heteromorphic achenes of Anthemis chrysantha J. Gay (Asteraceae), a critically endangered species.

This study demonstrates that Anthemis chrysantha , a ‘Critically Endangered’ annual plant, produces two morphs of achenes: white and dark achenes, which differ in size, mass, anatomy and germination behaviour. Fresh white achenes germinated at all temperatures assayed from 10 to 25°C in both continuous darkness and 12-h photoperiod, ranging between 24% at 25°C in darkness and 89% at 12/20°C in light, whereas fresh dark achenes did not germinate under any temperature or light conditions. To identify differences in dormancy type between the two morphs, germination of dry-stored achenes, and achenes stratified at 5 or 25°C for 2 months were tested in both darkness and light at 5, 15 and 12/20°C for dry-stored and warm-stratified (25°C) achenes; and at 15, 25 and 12/20°C for cold-stratified (5°C) achenes. Of the white achenes, 90% germinated during the cold stratification period. In general, dry storage and warm stratification did not increase germination compared to fresh achenes. However, dark achenes did not germinate under any conditions. Dark achene dormancy was only broken by mechanical scarification or by excising the embryo (germination reached 71%). An anatomical study showed that the mesocarp of dark achenes had no intercellular spaces and was much thicker and stronger than that of white achenes, making the entry of water difficult, and also preventing germination by mechanical restriction. This study demonstrated that dormancy in the dark achenes is likely caused by the thickness of their pericarp, physically impeding germination and hampering imbibition of water.

STAGES OF PLANT SUCCESSION IN FAGUS SYLVATICA L. AND PINUS SYLVESTRIS L. FORESTS OF TEJERA NEGRA NATURAL PARK (CENTRAL SPAIN), THREE YEARS AFTER FIRE

In August 1991 a fire in Tejera Negra Natural Park (Guadalajara Province, Central Spain) burned 300 ha. A study of the stages of succession in Fagus sylvatica L. and Pinns sylvestris L. forests was carried out during the first 3 years after the fire (1992–1994) in order to evaluate the dynamics of floristic richness, vegetation cover, diversity, plant density, and fugacity of the colonizing species by sampling of 10 permanent plots. Two years after the fire, vegetation cover in the burnt plots was similar to that of unburnt plots. However, floristic richness, diversity, and fugacity values were significantly higher in the burnt plots. At the end of the study, Fagus sylvatica was replaced by Genista florida shrubs and Pinus sylvestris was replaced by Cytisus scoparius.