3.1 Tree level synchrony and growth
Bivariate relationships demonstrated the strength of the correlation
between growth synchrony and BAIs. Growth synchrony generally increased
with altitude and BAI declined with altitude. Fagus growth
synchrony and BAI also increased with species diversity. By contrast,Picea growth synchrony declined with species diversity and BAI of
understory Picea trees was prominently negatively correlated with
species diversity. See figure 3 for correlation estimates.
We integrated these results into
regression models of growth synchrony and BAIs (all effect sizes
reported in table 1). For both species, diameter was a prominent
determinant of both ring-width synchrony and BAI, though stronger for
BAI. In general, effect sizes of biotic and abiotic factors were
stronger for synchrony than BAI. Understory synchrony was lower for both
species. Elevation has a positive effect on growth synchrony for both
species and all competition classes, though this effect was strongest
for understory Fagus. BAI declined significantly with elevation except
for understory Picea , which was non-significant. The positive
effect of elevation on growth synchrony was apparent for beech and
similar for both canopy classes. For spruce, elevation effects were
larger for overstory trees than for understory individuals. Understory
beech appear to increase greatly in ring-width synchrony as species
diversity in the canopy increases. Species diversity had a positive
effect on Fagus synchrony and a negative effect on Piceasynchrony. Understory Picea had the strongest BAI response to
species diversity, which was negative.
3.2 Spatial variation in \(\overset{\overline{}}{r}\) and
mean BAI
The mean \(\overset{\overline{}}{r}\) among overstory trees within
inventory plots was 0.309, with a standard deviation of 0.089. Multiple
regression indicated that geographic factors (elevation and latitude),
structural composition (stem density and basal area) and species
composition collectively explain an appreciably high degree of the
spatial variation in \(\overset{\overline{}}{r}\) (r2= 0.59), and therefore indicated where forest productivity is most
tightly coupled to climate. Climate sensitivity increased prominently
with elevation (Standardized effect size = 0.42) and latitude (SEF =
0.26). We also detected a positive interaction between elevation and
latitude (0.150), suggesting that elevation effects increase with
latitude. The effect of species diversity was similar in magnitude to
elevation, but opposite in sign (-0.43) implying that ring-width
synchrony declines with species diversity. Stem densities have a
negative effect on \(\overset{\overline{}}{r}\) while the effect of
basal area was positive.
The same ecological factors, with the addition of\(\overset{\overline{}}{r}\) were fit to the variation in mean BAI c.
1980-2010 (r2 = 0.364). Elevation and its interaction
with latitude was the primary constraining factor, followed by the
negative effects of stem densities (0.-29) and\(\overset{\overline{}}{r}\) (-0.27). Plot-level basal area was
positively correlated with mean BAI (0.202). Species diversity did not
have a significant effect on mean BAI (Table 2).