Differential Frost Tolerance of Ponderosa and Lodgepole Pine Megasporangiate Strobili
Note by Frank C. Sorensen and RichardS. Miles
Abstract. Lodgepole pine megasporangiate stro­
bili were much less damaged by a late spring
frost than were ponderosa strobili at the same
stage of development in three mixed ponderosa­
lodgepole pine stands in central Oregon. Forest
Sci. 20:377-378.
Additional key words. Pinus ponderosa, Pinus
contorta, frost hardiness.
COCHRAN AND BERNTSEN (1973) studied
causes for prevalence of lodgepole pine
(Pinus contorta Dougl.) in frost pocket areas
m central Oregon to the exclusion of ponder­
osa pine (P. ponderosa Laws.). They re­
ported that lodgepole pine is more tolerant
to low temperatures during the seedling
emergence period than is ponderosa pine.
The purpose of this note is to report that
this differential frost sensitivity is also a fea­
ture of the floral structure of the two species.
Methods and Materials. The observations
were made incidental to controlled pollina­
tiOns in three mixed lodgepole-ponderosa
stands about 20 km southwest of Bend,
Oregon, at elevations of 1,350, 1,500, and
1,550 m. The trees at 1,350 m were situated
m a shallow drainage, those at 1,500 m on
a broad saddle, and those at 1,550 m on a
flat. Later, when frost damage was tallied,
lodgepole and ponderosa individuals were
paired based on proximity and topographic
location. Eight, three, and seven pairs were
m the three locations.
Developing conelets were enclosed in kraft
paper isolation bags (sturdy kraft bags used
m maize breeding) on May 22 and May 23.
Pollination was between June 7 and 12. Pol­
len was applied when megasporangiate strobili
were judged to be at maximum receptivity.
Dates of pollen application were recorded
and averaged June 9.8 for ponderosa pine and
June 9.2 for lodgepole pine.
The authors are, respectively, Principal Plant
Geneticist and Forest Research Technician, Pa­
Cific Northwest Forest and Range Exp. Stn.,
Forestry Sciences Laboratory, USDA Forest
Service, Corvallis, Oreg. 97331. Manuscript re­
ceived Feb. 8, 1974.
The damaging frost occurred the night of
June 10--11, or just slightly after the average
date of pollination. A minimum of - 2.2°C
was recorded at the Bend Weather Station
(1,100 m). Minima of - 3.9°, -3.1 °, and
- 2.5°C were recorded the same night at 1 4
m above ground surface in three different
frost pocket areas at about 1,300 m approxi­
mately 30 km SSW of these study areas 1
The previous 6 days had been warm, with
Bend temperature maximums ranging from
21.1° to 31.6°C. Frosts of - 3.8° and
- 2.2°C were also recorded on the nights of
June 1-2 and 2-3, respectively, and may have
contributed to the damage. No record was
made of the floral stage at which damage
occurred, but we believe almost all damage
was caused by the June 10-11 frost.
Final pollinations were made on June 11
and 12, and some early signs of floral damage
were observed at that time. Isolation bags
were removed July 19-20 (about 6 weeks
after the frost); damage was found to be
severe and was tallied.
Percentage abortion in the bags was deter­
mined from a count of aborted and healthy
conelets and outside from counts of aborted
and healthy conelets on 5-l 0 branch tips m
the vicinity of the bagged twigs.
Results. For the bagged twigs, 86 percent of
the ponderosa pine megasporangiate strobili
were killed, but only 22 percent of the lodge­
pole strobili. On the unbagged twigs, the
respective figures were 88 and 26 percent
All conelets were killed on nine of the pon­
derosa but on none of the lodgepole pines.
Frost damage to ponderosa pollen catkins
was also observed on June 11 and 12, but
this could not be tallied at the later date.
Discussion. Cochran and Berntsen (1973)
suggested that elimination of ponderosa pine,
but not lodgepole pine, in some areas may be
due to differential frost tolerance of the two
species at the time of seedling emergence
Natural reproduction, of course, depends on
seed production, seed germination, and seed­
ling survival. Floral structures must also be
' Patrick H. Cochran, Bend, Oregon, personal
communication.
volume 20, number 4, 1974 I 377
tolerant to environmental extremes. The pres­
ent observations indicate differential frost
tolerance of the two species at still another
phase in the regeneration cycle.
The - 2.2°C on June 10-11 greatly differ­
entiated the two species. The probability of
such a frost at the Bend Weather Station on
or after this date is 0.32 (Climatological
Handbook 1968). For a true expectation of
floral damage, probability of a frost during
sensitive stages in strobilus development and
possible differences in specific flowering times
from year to year would have to be taken
mto account. Little information of this type
ts available. However, where temperature
seems to be such an important factor, it
would be interesting to know whether these
probabilities are related to stand composition.
Finally, although various types of isolation
bags have been observed to influence the en­
VIronment around the twig (Boyer and Woods
1973, Rohmeder and Eisenhut 1959) and to
affect cone (Plym Forshell 1953) and seed
stze (Sorensen 1973), there was no indication
m this test that they affected the amount of
frost damage.
Literature Cited
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of pollen shedding by longleaf pine advanced
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Sci 19:315-318.
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CocHRAN, P. H., and C. M. BERNTSEN. 1973
Tolerance of lodgepole and ponderosa pine
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Sci 19:272-280.
PLYM FoRSHELL, C. 1953. The development
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Fornes annosus
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Printed by the Southeastern Forest Experiment Station, USDA Forest Service,
Asheville, N.C. 1974. 289 p.
Scientists from 14 countries met to discuss this world-wide root disease. Their
deliberations are presented in 30 papers plus summary discussions of 7 main topics.
378 I Forest Science