[R] difference between linear model & scatterplot matrix

[Francesco Nutini]

Dear R-users,
I'm studing a DB, structured like this (just a little part of my dataset): 
_____________________________________________________________________________________________________________









  Site
  Latitude
  Longitude
  Year
  Tot-Prod
  Total_Density
  dmp



  Dendoudi-1
  15.441964
  -13.540179
  2005
  3271.16
  1007
  16993.25


  Dendoudi-2
  15.397321
  -13.611607
  2005
  1616.84
  250
  25376.67


  …
  …
  …
  …
  …
  …
  …

_____________________________________________________________________________________________________________

If I made a scatterplotmatrix with the command show below I obtain a matrix (visible in the image) that show which variables is more correlated with dmp data (violet color).
But, if I made a linear model between the dependent variable (dmp) and  many independent variables
I get different information about the significativity of the variable. 
I mean, variables that appear correlated with dependent variable in the matrix result not correlated in the summary of linear model, and vice versa. Have I made a mistake in the interpretation of the result, or not?

Thank you in advance,
Francesco



#command for matrix-plot


>dta <-
senegal5[c(  2,4,5,6,7,8,9,13,15,17,21,
39,44,45)]

>dta.r <-
abs(cor(dta))

>dta.col
<- dmat.color(dta.r)

>dta.o <-
order.single(dta.r) 

>cpairs(dta,
dta.o, panel.colors=dta.col, gap=.5,

>main="Variables Ordered and Colored by
Correlation")
#command for linear model and summary()


>a<- lm ( dmp ~ Latitude
+ Longitude +  Year +  Tot.Prod +    Herbaceous.Prod.kg.ha. +  Leaf.Prod +  Tree.bio  + Total_Density  + X1st.SpecieDensity.trunk.ha.+
X2nd.SpecieDensity.trunk.ha.+ Herb_Specie_Index1 +  iNDVI.JASO. 
+ 
RFE.Cum.JASO., data=senegal5 )




>summary(a)



Call:

lm(formula = dmp ~
Latitude + Longitude + Year + Tot.Prod + Herbaceous.Prod.kg.ha. + 

    Leaf.Prod + Tree.bio + Total_Density +
X1st.SpecieDensity.trunk.ha. + 

    X2nd.SpecieDensity.trunk.ha. +
Herb_Specie_Index1 + iNDVI.JASO. + 

    RFE.Cum.JASO.,
data = senegal5)

Residuals:

    Min     
1Q  Median      3Q    
Max 

-676.49 -195.77  -33.06 
113.34  816.17 



Coefficients:

                               Estimate Std. Error
t value Pr(>|t|)    

(Intercept)                  -3.283e+05  4.505e+04 
-7.288 4.41e-11 ***

Latitude                     -6.100e+01  1.990e+02 
-0.307   0.7598    

Longitude                    -3.617e+02  8.639e+01 
-4.187 5.60e-05 ***

Year                          1.604e+02  2.300e+01  
6.973 2.15e-10 ***

Tot.Prod                     -4.893e+00  1.565e+02 
-0.031   0.9751    

Herbaceous.Prod.kg.ha.        4.905e+00  1.565e+02  
0.031   0.9751    

Leaf.Prod  
                  4.842e+00  1.565e+02  
0.031   0.9754    

Tree.bio                     -4.241e+01  2.771e+02 
-0.153   0.8786    

Total_Density                -1.930e+00  8.933e-01 
-2.160   0.0329 *  

X1st.SpecieDensity.trunk.ha.  1.992e+00 
9.246e-01   2.154  
0.0333 *  

X2nd.SpecieDensity.trunk.ha.  3.416e+00 
1.642e+00   2.080   0.0398 * 


Herb_Specie_Index1           -1.091e+00  1.844e+00 
-0.592   0.5552    

iNDVI.JASO.                   8.914e+02  6.076e+01 
14.670  < 2e-16 ***

RFE.Cum.JASO.                 2.525e+00  4.529e-01  
5.575 1.68e-07 ***

---

Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’
1 



Residual standard
error: 295.3 on 114 degrees of freedom

Multiple R-squared:
0.9206,     Adjusted R-squared: 0.9116 

F-statistic: 101.7 on
13 and 114 DF,  p-value: < 2.2e-16