﻿ANNUAL 
  RINGS 
  OF 
  TREES 
  — 
  DOUGLASS. 
  233 
  

  

  VI. 
  CORRELATIONS. 
  

  

  It 
  is 
  no 
  surprise 
  that 
  variations 
  in 
  climate 
  can 
  be 
  read 
  in 
  the 
  

   growth 
  rings 
  of 
  trees, 
  for 
  the 
  tree 
  ring 
  itself 
  is 
  a 
  climatic 
  product. 
  

   It 
  is 
  an 
  effect 
  of 
  seasons. 
  The 
  geologists 
  use 
  the 
  absence 
  of 
  rings 
  

   in 
  certain 
  primitive 
  trees 
  as 
  an 
  indication 
  that 
  no 
  seasons 
  existed 
  

   in 
  certain 
  early 
  times. 
  Whatever 
  may 
  have 
  been 
  the 
  cause 
  of 
  that 
  

   absence, 
  we 
  recognize 
  that 
  the 
  ring 
  is 
  caused 
  primarily 
  by 
  changes 
  

   in 
  temperature 
  and 
  moisture. 
  Now 
  if 
  successive 
  years 
  were 
  exactly 
  

   alike, 
  the 
  rings 
  would 
  be 
  all 
  of 
  the 
  same 
  size 
  with 
  some 
  alteration 
  

   with 
  age 
  and 
  injury. 
  But 
  successive 
  years 
  are 
  not 
  alike 
  and 
  in 
  

   that 
  difference 
  there 
  may 
  be 
  some 
  factor 
  which 
  appeals 
  strongly 
  

   to 
  the 
  tree. 
  In 
  northern 
  Arizona, 
  with 
  its 
  limited 
  moisture 
  and 
  

   great 
  freedom 
  from 
  pests 
  and 
  with 
  no 
  dense 
  vegetable 
  population, 
  

   this 
  controlling 
  factor 
  may 
  reasonably 
  be 
  identified 
  as 
  the 
  rainfall. 
  

   If 
  the 
  trees 
  have 
  all 
  the 
  moisture 
  they 
  can 
  use, 
  as 
  in 
  north 
  Europe 
  

   about 
  the 
  Baltic 
  Sea 
  and 
  other 
  wet 
  climates, 
  we 
  look 
  for 
  it 
  in 
  some- 
  

   thing 
  else. 
  It 
  could 
  be 
  — 
  I 
  do 
  not 
  say 
  that 
  it 
  is 
  — 
  some 
  direct 
  form 
  

   of 
  solar 
  radiation. 
  It 
  could 
  be 
  some 
  special 
  combination 
  of 
  the 
  

   ordinary 
  weather 
  elements 
  with 
  which 
  we 
  are 
  familar. 
  Shreve 
  

   has 
  studied 
  this 
  phase 
  in 
  the 
  Catalinas. 
  If 
  the 
  abundance 
  of 
  mois- 
  

   ture 
  is 
  so 
  great 
  as 
  actually 
  to 
  drown 
  the 
  tree, 
  then 
  decrease 
  in 
  rain- 
  

   fall 
  which 
  lowers 
  the 
  water 
  table 
  below 
  ground 
  will 
  be 
  favorable. 
  

   A 
  fact 
  often 
  forgotten 
  is 
  that 
  more 
  than 
  one 
  factor 
  may 
  enter 
  into 
  

   the 
  tree 
  rings 
  at 
  the 
  same 
  time, 
  for 
  example, 
  rainfall, 
  temperature, 
  

   and 
  length 
  of 
  growing 
  season. 
  These 
  may 
  be 
  isolated 
  in 
  two 
  ways. 
  

   We 
  may 
  select 
  a 
  special 
  region, 
  as 
  northern 
  Arizona, 
  where 
  nature 
  

   has 
  standardized 
  the 
  conditions, 
  leaving 
  one 
  of 
  them, 
  the 
  rainfall, 
  

   of 
  especial 
  importance. 
  Or 
  we 
  may 
  isolate 
  certain 
  relationships 
  

   as 
  in 
  any 
  other 
  investigation, 
  by 
  using 
  large 
  numbers 
  of 
  observa- 
  

   tions, 
  that 
  is, 
  many 
  trees, 
  and 
  averaging 
  them 
  with 
  respect 
  to 
  one 
  

   or 
  another 
  characteristic. 
  For 
  example, 
  I 
  can 
  determine 
  the 
  mean 
  

   growth 
  curve 
  of 
  the 
  Vermont 
  hemlocks 
  and 
  then 
  compare 
  it 
  sepa- 
  

   rately 
  with 
  rainfall 
  and 
  solar 
  activity, 
  and 
  I 
  may, 
  and 
  do, 
  find 
  a 
  

   response 
  to 
  each. 
  For 
  that 
  reason, 
  I 
  have 
  felt 
  quite 
  justified 
  in 
  

   seeking 
  first 
  the 
  correlation 
  with 
  moisture. 
  A 
  temperature 
  cor- 
  

   relation 
  doubtless 
  exists 
  and 
  in 
  fact 
  has 
  been 
  noted, 
  but 
  its 
  less 
  

   minute 
  observance 
  does 
  not 
  lessen 
  the 
  value 
  of 
  the 
  rainfall 
  rela- 
  

   tionship. 
  

  

  The 
  first 
  real 
  result 
  obtained 
  in 
  this 
  study 
  was 
  in 
  1906, 
  when 
  it 
  

   became 
  apparent 
  that 
  a 
  smoothed 
  curve 
  of 
  tree 
  growth 
  in 
  northern 
  

   Arizona 
  matched 
  a 
  smoothed 
  curve 
  of 
  precipitation 
  in 
  southern 
  

   California 
  since 
  1860. 
  That 
  degree 
  of 
  correlation 
  is 
  now 
  extensively 
  

   used 
  in 
  the 
  Forest 
  Service. 
  This 
  was 
  followed 
  almost 
  at 
  once 
  by 
  

   noting 
  a 
  strikingly 
  close 
  agreement 
  between 
  the 
  size 
  of 
  individual 
  

  

  