﻿insects, 
  diseases, 
  fires, 
  and 
  storms 
  range 
  from 
  

   small 
  percentages 
  of 
  potential 
  production 
  to 
  

   total 
  wipsout 
  of 
  individual 
  stands. 
  Certain 
  

   species 
  are 
  more 
  susceptible 
  to 
  fire 
  and 
  storm 
  

   damage, 
  and 
  to 
  attack 
  by 
  insects 
  or 
  disease. 
  

   Some 
  respond 
  more 
  dramatically 
  than 
  others 
  

   to 
  such 
  injuries. 
  The 
  threat 
  of 
  the 
  unknown, 
  

   especially 
  in 
  view 
  of 
  past 
  insect 
  and 
  disease 
  

   problems, 
  injects 
  into 
  the 
  problem 
  of 
  species 
  

   selection 
  more 
  speculation 
  than 
  is 
  comfortable 
  

   for 
  the 
  land 
  manager. 
  However, 
  some 
  esti- 
  

   mate 
  of 
  risk 
  is 
  a 
  necessary 
  part 
  of 
  the 
  species 
  

   evaluation, 
  even 
  if 
  it 
  must 
  be 
  based 
  on 
  judg- 
  

   ment 
  rather 
  than 
  measurement. 
  

  

  For 
  individual 
  stands, 
  catastrophic 
  losses 
  

   from 
  fires 
  and 
  storms 
  cannot 
  be 
  predicted. 
  

   Insect 
  and 
  disease 
  attacks 
  cannot 
  ordinarily 
  

   be 
  predicted 
  with 
  the 
  surety 
  desired. 
  For 
  large 
  

   areas, 
  however, 
  risk 
  is 
  usually 
  assigned 
  a 
  value 
  

   in 
  terms 
  of 
  a 
  discount 
  applied 
  to 
  the 
  gross 
  

   volume 
  yield 
  normally 
  expected 
  from 
  stands 
  

   of 
  certain 
  species. 
  The 
  greater 
  the 
  discount 
  

   applied 
  to 
  gross 
  yield, 
  the 
  less 
  attractive 
  the 
  

   species 
  or 
  species 
  mixture 
  may 
  be 
  for 
  a 
  timber 
  

   production 
  program. 
  However, 
  the 
  compara- 
  

   tive 
  importance 
  of 
  the 
  risk 
  factor 
  is 
  apparent 
  

   only 
  in 
  the 
  net 
  volume 
  yield 
  estimate. 
  A 
  

   stand 
  of 
  one 
  species 
  discounted 
  15 
  percent 
  

   may 
  produce 
  more 
  net 
  volume 
  than 
  another 
  

   species 
  discounted 
  only 
  10 
  percent. 
  

  

  It 
  is 
  obvious 
  that 
  the 
  present 
  risk 
  in 
  grow- 
  

   ing 
  white 
  pine 
  is 
  too 
  high. 
  The 
  volume 
  yields 
  

   expected 
  from 
  stands 
  established 
  during 
  the 
  

   past 
  25 
  years 
  will 
  probably 
  have 
  to 
  be 
  dis- 
  

   counted 
  75 
  percent 
  or 
  more 
  because 
  of 
  mor- 
  

   tality 
  and 
  growth 
  losses 
  from 
  blister 
  rust 
  and 
  

   bark 
  beetles. 
  Other 
  species 
  could 
  be 
  similarly 
  

   devastated. 
  Blister 
  rust 
  and 
  bark 
  beetles 
  are 
  

   the 
  current 
  sources 
  of 
  extreme 
  risk, 
  but 
  other 
  

   serious 
  pest 
  problems 
  are 
  evident. 
  

  

  Dwarfmistletoe 
  probably 
  ranks 
  next 
  to 
  blis- 
  

   ter 
  rust 
  as 
  a 
  source 
  of 
  growth 
  loss, 
  chiefly 
  

   affecting 
  lodgepole 
  and 
  ponderosa 
  pine, 
  Doug- 
  

   las-fir, 
  and 
  western 
  larch. 
  Casebearer 
  damage 
  

  

  on 
  western 
  larch 
  has 
  become 
  serious 
  during 
  the 
  

   past 
  few 
  years. 
  Larch 
  stands 
  are 
  expected 
  to 
  

   suffer 
  annual 
  defoliation 
  for 
  at 
  least 
  the 
  next 
  

   decade 
  and 
  perhaps 
  for 
  two 
  decades 
  before 
  

   the 
  casebearer 
  can 
  be 
  effectively 
  controlled 
  

   by 
  natural 
  means. 
  The 
  real 
  effects 
  of 
  such 
  

   defoliation 
  are 
  unknown, 
  of 
  course, 
  but 
  a 
  sub- 
  

   stantial 
  reduction 
  in 
  growth 
  and 
  perhaps 
  some 
  

   mortality 
  are 
  expected. 
  

  

  Currently 
  insects 
  and 
  disease 
  are 
  causing 
  

   loss 
  of 
  timber 
  value 
  in 
  other 
  species 
  — 
  in 
  

   varying 
  degrees 
  depending 
  on 
  locality 
  and 
  

   stand 
  conditions. 
  Any 
  one 
  of 
  these 
  nuisances 
  

   could 
  flare 
  into 
  a 
  serious 
  setback 
  for 
  the 
  

   species. 
  Also, 
  the 
  threat 
  of 
  another 
  imported 
  

   disease 
  reminiscent 
  of 
  blister 
  rust 
  or 
  chestnut 
  

   blight 
  always 
  exists. 
  

  

  Risks 
  are 
  important 
  considerations 
  in 
  tim- 
  

   ber 
  growing 
  decisions, 
  but 
  there 
  is 
  no 
  reason 
  

   to 
  assume 
  timber 
  growing 
  will 
  cease 
  because 
  

   of 
  the 
  risks 
  involved. 
  Land 
  managers 
  must 
  

   increase 
  the 
  biological 
  and 
  economic 
  sound- 
  

   ness 
  of 
  their 
  timber 
  growing 
  projects 
  by 
  care- 
  

   fully 
  considering 
  predictable 
  volume 
  losses 
  and 
  

   by 
  using 
  the 
  best 
  means 
  available 
  for 
  reduc- 
  

   ing 
  the 
  amount 
  of 
  unknown 
  risk 
  that 
  con- 
  

   fronts 
  them 
  in 
  their 
  evaluations. 
  Risk 
  and 
  

   expected 
  losses 
  must 
  be 
  reevaluated 
  as 
  a 
  mat- 
  

   ter 
  of 
  course 
  through 
  the 
  life 
  of 
  the 
  stand, 
  

   because 
  the 
  sources 
  and 
  degree 
  of 
  risk 
  associ- 
  

   ated 
  with 
  an 
  individual 
  stand 
  change 
  with 
  

   time 
  and 
  advances 
  in 
  technology. 
  

  

  Good 
  insurance 
  against 
  the 
  risks 
  associated 
  

   with 
  a 
  single 
  species 
  is 
  to 
  forgo 
  monoculture 
  

   forestry 
  and 
  concentrate 
  on 
  stands 
  of 
  mixed 
  

   compatible 
  species. 
  When 
  choosing 
  the 
  species 
  

   mixture, 
  the 
  manager 
  must 
  consider 
  the 
  de- 
  

   gree 
  of 
  risk 
  (expected 
  loss) 
  associated 
  with 
  

   each 
  alternative 
  species 
  and 
  make 
  some 
  judg- 
  

   ment 
  as 
  to 
  which 
  combination 
  will 
  give 
  the 
  

   best 
  results 
  relative 
  to 
  the 
  expected 
  costs. 
  

   Also 
  he 
  should 
  recognize 
  that 
  mixed 
  stands 
  

   are 
  more 
  difficult 
  to 
  manage 
  than 
  single- 
  

   species 
  stands. 
  

  

  5 
  

  

  