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).