|
Mixed
effects on meadow species. Meadow
species cover, but not richness, declined with tree age. However,
the rate and magnitude of decline did not differ under lodgepole
pine and grand fir (figure, right; Table, lower right).
Facilitation
of forest species.
Cover and richness of forest understory species increased steeply
with tree age under grand fir, but not under pine (figure, right; Table, lower right).
 |
| Effects
of tree age and species on meadow and forest species.
Negative
values indicate lower cover (or richness) under tree canopies
than in open meadow. Regression lines are shown for significant
slopes. Where slopes differ between tree species, separate
regressions are shown (forest species cover and richness). |
|
Conclusions
Grand
fir and lodgepole pine clearly differ in the degree to which they
facilitate forest understory species. Differences in canopy architecture
and foliar density lead to greater shading below grand fir.
Our
methods and results suggest an approach to prioritizing tree removal
as part of meadow restoration in this system: Removing grand fir
of any age is a higher priority than removing pine.
More generally, our approach can be employed in other invaded systems
to detect the tree species with the strongest effects, and the age
at which these effects become apparent. |
| Tree effects on vegetation |
 |
| Sharp
transition from meadow grasses to forest herbs under a
mature grand
fir. |
|
|
| Levels
of significance (p values) for predictors of the cover
and richness of meadow and forest species. |
| |
Meadow
species |
|
Forest
species |
| |
Cover |
Richness |
|
Cover |
Richness |
|
Adjusted R2: |
0.54
|
0.18 |
|
0.50
|
0.49 |
|
Full model (p): |
<0.001 |
0.005 |
|
<0.001 |
<0.001 |
| Predictors |
|
|
|
|
|
|
Age |
<0.001
|
0.15 |
|
<0.001 |
<0.001 |
|
Species |
0.40 |
0.31 |
|
0.006
|
0.005 |
|
Age x species |
0.19
|
0.93
|
|
<0.001 |
<0.001 |
| |
|
|
|
|
 |
|
|
|
|
|
Introduction
& methods