Vegetation processes are fundamentally limited by nutrient and water availability, the uptake of which is mediated by plant roots in terrestrial ecosystems. While tropical forests play a central role in global water, carbon, and nutrient cycling, we know very little about tradeoffs and synergies in root traits that respond to resource scarcity. Tropical trees face a unique set of resource limitations, with rock-derived nutrients and moisture seasonality governing many ecosystem functions, and nutrient versus water availability often separated spatially and temporally. Root traits that characterize biomass, depth distributions, production and phenology, morphology, physiology, chemistry, and symbiotic relationships can be predictive of plants’ capacities to access and acquire nutrients and water, with links to aboveground processes like transpiration, wood productivity, and leaf phenology. In this review, we identify an emerging trend in the literature that tropical fine root biomass and production in surface soils are greatest in infertile or sufficiently moist soils. We also identify interesting paradoxes in tropical forest root responses to changing resources that merit further exploration. For example, specific root length, which typically increases under resource scarcity to expand the volume of soil explored, instead can increase with greater base cation availability, both across natural tropical forest gradients and in fertilization experiments. Also, nutrient additions, rather than reducing mycorrhizal colonization of fine roots as might be expected, increased colonization rates under scenarios of water scarcity in some forests. Efforts to include fine root traits and functions in vegetation models have grown more sophisticated over time, yet there is a disconnect between the emphasis in models characterizing nutrient and water uptake rates and carbon costs versus the emphasis in field experiments on measuring root biomass, production, and morphology in response to changes in resource availability. Closer integration of field and modeling efforts could connect mechanistic investigation of fine-root dynamics to ecosystem-scale understanding of nutrient and water cycling, allowing us to better predict tropical forest-climate feedbacks.
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The adaptive challenge of extreme conditions shapes evolutionary diversity of plant assemblages at continental scales
The tropical conservatism hypothesis (TCH) posits that the latitudinal
gradient in biological diversity arises because most extant clades
of animals and plants originated when tropical environments were
more widespread and because the colonization of colder and more
seasonal temperate environments is limited by the phylogenetically
conserved environmental tolerances of these tropical clades. Recent
studies have claimed support of the TCH, indicating that temperate
plant diversity stems from a fewmore recently derived lineages that
are nested within tropical clades, with the colonization of the
temperate zone being associated with key adaptations to survive
colder temperatures and regular freezing. Drought, however, is an
additional physiological stress that could shape diversity gradients.
Here, we evaluate patterns of evolutionary diversity in plant
assemblages spanning the full extent of climatic gradients in North
and South America. We find that in both hemispheres, extratropical
dry biomes house the lowest evolutionary diversity, while tropical
moist forests and many temperatemixed forests harbor the highest.
Together, our results support a more nuanced view of the TCH, with
environments that are radically different from the ancestral niche of
angiosperms having limited, phylogenetically clustered diversity
relative to environments that show lower levels of deviation from
this niche. Thus, we argue that ongoing expansion of arid environments
is likely to entail higher loss of evolutionary diversity not just
in the wet tropics but in many extratropical moist regions as well.
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- Award ID(s):
- 1934790
- NSF-PAR ID:
- 10294223
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences of the United States of America
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/doi.org/10.1073/pnas.2021132118
