The influence of high temperatures on germination of four
Cistus and five Halimium taxa is
analysed. Seeds were heated to a range of temperatures (from 50ºC to
150ºC) and a range of exposure times (from 1 to 60 min), simulating those
heat conditions registered on the soil surface during wildfires. After the
thermal pretreatments, seeds were sown in plastic Petri dishes and monitored
for germination over 60 days. For all the species, germination was increased
significantly over the control set (untreated seeds) by at least one of the
thermal pretreatments. Temperatures of 120 and 150ºC were the most
efficient temperatures promoting seed germination, although germination was
interrupted at 150ºC if exposure times were equal to or longer than 5
minutes for the majority of species. The preheating at 50ºC was effective
on;y for C. populifolius, but only at long exposure
times (60 min). H. atriplicifolium had the highest heat
requirements for stimulating germination, requiring at least 120ºC for 5
min. H. halimifolium subsp.
halimifolium seeds showed the highest heat resistance:
the final germination level reached at 150ºC for 7.5 minutes ranged
between 52.5 and 55.5%. The germination rates after preheating were
much lower than in mechanically scarified seeds, and closely resembled those
of the untreated seeds. In general, species sharing the same habitat showed
different heat requirements in promoting germination. For
C. crispus and H. halimifolium
subsp. halimifolium, the experiment was carried out on
seeds collected from two different localities. For both species the
germination patterns were similar between populations, although some high
temperature pretreatments showed different germination percentages.
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