|
| The processes causing the latitudinal gradient in species richness
remain elusive. Ecological theories for the origin of biodiversity
gradients, such as competitive exclusion, neutral dynamics, and
environmental filtering, make predictions for how functional diversity
should vary at the alpha (within local assemblages), beta
(among assemblages), and gamma (regional pool) scales. We test
these predictions by quantifying hypervolumes constructed from
functional traits representing major axes of plant strategy variation
(specific leaf area, plant height, and seed mass) in tree assemblages
spanning the temperate and tropical New World. Alpha-scale trait
volume decreases with absolute latitude and is often lower than
sampling expectation, consistent with environmental filtering
theory. Beta-scale overlap decays with geographic distance fastest
in the temperate zone, again consistent with environmental
filtering theory. In contrast, gamma-scale trait space shows a
hump-shaped relationship with absolute latitude, consistent with
no theory. Furthermore, the overall temperate trait hypervolume
was larger than the overall tropical hypervolume, indicating that
the temperate zone permits a wider range of trait combinations
or that niche packing is stronger in the tropical zone. Although
there are limitations in the data, our analyses suggest that
multiple processes have shaped trait diversity in trees, reflecting
no consistent support for any one theory. | |
|