[VIPS] Crown ideotype for hybrid poplar

'Toby' H D Bradshaw toby at u.washington.edu
Wed Oct 4 18:46:14 EST 1995

There was considerable discussion at the recent International 
Poplar Symposium (IPS; point your Web browser at 
http://poplar1.cfr.washington.edu for more information) on 
ideotype breeding of hybrid poplar to maximize 
productivity in short (4-9 years) and medium (10-20 years) 
rotation plantations under intensive culture.  To continue the 
discussion beyond the end of the meeting, and to include as 
many new participants as possible, I would like to have a 
Virtual International Poplar Symposium (VIPS).  Hence, 
the [VIPS] in the header.

Of many possible elements to be included in an ideotype 
for a domesticated tree growing in a monoclonal stand, 
crown architecture is one element with the potential for 
strong input from various disciplines, including anatomy, 
physiology, evolutionary biology, ecology, and genetics.  
The two Populus species most often involved in commercial 
hybrid poplar plantations in Pacific Northwestern North 
America are P. trichocarpa (black cottonwood; PNW native) 
and P. deltoides (eastern cottonwood; from eastern and central 
NA).  These species have very different crowns:

		P. trichocarpa		P. deltoides
leaf angle
  hang		large			small
  tilt		small			large
sylleptic	many			few
proleptic	small			large
branch		steep			flat
light		high			lower
apical		high			lower

F1 hybrids between P. trichocarpa and P. deltoides are 
intermediate for many of these crown characters, and 
superior to either parental species in growth (at least in the 
Pacific Northwest) and light interception.  However, there is 
nothing to indicate that the F1 hybrids have optimized 
crowns for productivity, and so there is a strong interest in 
understanding the critical components of a crown ideotype.

Ideotypes can be formulated on purely theoretical grounds, 
then tested empirically by breeding and selection.  The 
ability to clonally propagate ideotypes (and their sub-
optimum relatives) gives these experiments considerable 
statistical power.

Several key questions should be answered in order to 
formulate a crown ideotype:

1) Which crown characters (e.g., leaf orientation, branch 
number) are most influential in light interception?

2) Which Populus species can contribute to the net 
improvement of each important crown architecture trait?

3) How do crown ideotypes vary across latitude (i.e., 
changes in sun angle), or gradients of temperature, or 
changes in light intensity, or other physical features?

4) What is the evolutionary/ecological reason for the 
difference in crown structure between P. trichocarpa and P. 

5) How do crown ideotypes differ between short rotations 
for pulpwood and medium-length rotations for veneer or 

6) How much of the variation in crown traits is genetic, and 
how much is due to phenotypic plasticity?

Toby Bradshaw                       | (206)616-1796 (voice)
Center for Urban Horticulture       | (206)616-1826 (FAX)
Box 354115                          | toby at u.washington.edu
University of Washington            | 47.39.496N 122.17.404W
Seattle WA 98195                    | Will make linkage maps for food.

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