[VIPS] Crown ideotype for hybrid poplar

['Toby' H D Bradshaw]

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
  branches
proleptic       small                   large
  branches
branch          steep                   flat
  angle
light           high                    lower
  interception
apical          high                    lower
  control

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. 
deltoides?

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

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.