Toward a Coordinated Understanding of Hydro-Biogeochemical Root Functions in Tropical Forests for Application in Vegetation Models

Tropical forest root characteristics and resource acquisition strategies are underrepresented in vegetation and global models, hampering the prediction of forest–climate feedbacks for these carbon-rich ecosystems. Lowland tropical forests often have globally unique combinations of high taxonomic and functional biodiversity, rainfall seasonality, and strongly weathered infertile soils, giving rise to distinct patterns in root traits and functions compared with higher latitude ecosystems. We provide a roadmap for integrating recent advances in our understanding of tropical forest belowground function into vegetation models, focusing on water and nutrient acquisition. We offer comparisons of recent advances in empirical and model understanding of root characteristics that represent important functional processes in tropical forests. We focus on: (1) fine-root strategies for soil resource exploration, (2) coupling and trade-offs in fine-root water vs nutrient acquisition, and (3) aboveground–belowground linkages in plant resource acquisition and use. We suggest avenues for representing these extremely diverse plant communities in computationally manageable and ecologically meaningful groups in models for linked aboveground–belowground hydro-nutrient functions. Tropical forests are undergoing warming, shifting rainfall regimes, and exacerbation of soil nutrient scarcity caused by elevated atmospheric CO2. The accurate model representation of tropical forest functions is crucial for understanding the interactions of this biome with the climate

Long-term thermal sensitivity of Earth’s tropical forests

The sensitivity of tropical forest carbon to climate is a key uncertainty in predicting global climate change. Although short-term drying and warming are known to affect forests, it is unknown if such effects translate into long-term responses. Here, we analyze 590 permanent plots measured across the tropics to derive the equilibrium climate controls on forest carbon. Maximum temperature is the most important predictor of aboveground biomass (−9.1 megagrams of carbon per hectare per degree Celsius), primarily by reducing woody productivity, and has a greater impact per °C in the hottest forests (>32.2°C). Our results nevertheless reveal greater thermal resilience than observations of short-term variation imply. To realize the long-term climate adaptation potential of tropical forests requires both protecting them and stabilizing Earth’s climate

Data from: Rapid nitrogen fixation by canopy microbiome in tropical forest determined by both phosphorus and molybdenum

Biological nitrogen fixation is critical for the nitrogen cycle of tropical forests, yet we know little about the factors that control the microbial nitrogen-fixers that colonize the microbiome of leaves and branches that make up a forest canopy. Forest canopies are especially prone to nutrient limitation because they are (1) disconnected from soil nutrient pools, and (2) often subject to leaching. Earlier studies have suggested a role of phosphorus and molybdenum in controlling biological N-fixation rates, but experimental confirmation has hitherto been unavailable. We here present the results of a manipulation of canopy nutrient availability . Our findings demonstrate a primary role of phosphorus in constraining overall N-fixation by canopy cyanobacteria, but also a secondary role of molybdenum in determining per-cell fixation rates. A conservative evaluation suggests that canopy fixation can contribute to significant N fluxes at the ecosystem level, especially as bursts following atmospheric inputs of nutrient-rich dust

Data from: Nitrogen-fixing tree abundance in higher-latitude North America is not constrained by diversity

The rarity of nitrogen (N)-fixing trees in frequently N-limited higher-latitude (here, > 35°) forests is a central biogeochemical paradox. One hypothesis for their rarity is that evolutionary constraints limit N-fixing tree diversity, preventing N-fixing species from filling available niches in higher-latitude forests. Here, we test this hypothesis using data from the USA and Mexico. N-fixing trees comprise only a slightly smaller fraction of taxa at higher vs. lower latitudes (8% vs. 11% of genera), despite 11-fold lower abundance (1.2% vs. 12.7% of basal area). Furthermore, N-fixing trees are abundant but belong to few species on tropical islands, suggesting that low absolute diversity does not limit their abundance. Rhizobial taxa dominate N-fixing tree richness at lower latitudes, whereas actinorhizal species do at higher latitudes. Our results suggest that low diversity does not explain N-fixing trees' rarity in higher-latitude forests. Therefore, N limitation in higher-latitude forests likely results from ecological constraints on N fixation

Menge_etal_ELE_00062_2017_Figs

This is the R script that opens the data file and makes the figures

Climate windows of opportunity for plant expansion during the Phanerozoic

International audienceEarth's long-term climate may have profoundly influenced plant evolution. Local climatic factors, including water availability, light, and temperature, play a key role in plant physiology and growth, and have fluctuated substantially over geological time. However, the impact of these key climate variables on global plant biomass across the Phanerozoic has not yet been established. Linking climate and dynamic vegetation modelling, we identify two key `windows of opportunity' during the Ordovician and Jurassic-Paleogene capable of supporting dramatic expansions of potential plant biomass. These conditions are driven by continental dispersion, paleolatitude of continental area and a lack of glaciation, allowing for an intense hydrological cycle and greater water availability. These windows coincide with the initial expansion of land plants and the later angiosperm radiation. Our findings suggest that the timing and expansion of habitable space for plants played an important role in plant evolution and diversification

Data used in Lai et al. (2018). Nitrogen fixer abundance has no effect on biomass recovery during tropical secondary forest succession. Journal of Ecology.

<div><b>Data from: </b>Lai, H.R., J.S. Hall, S.A. Batterman, B.L. Turner & M. van Breugel (2018). Nitrogen fixer abundance has no effect on biomass recovery during tropical secondary forest succession. Journal of Ecology</div><div><br></div><div><b>Methods and materials;</b></div><div>See 'Methods & Materials' section in Lai et al. 2018 for details </div><div><br></div><div><div><b>Summary</b>:</div><div>1) Nitrogen-fixing trees (N2 fixers) provide new nitrogen critical for rapid biomass accumulation of tropical forests during early secondary succession, but it remains unclear how the abundance of N2 fixers in the forest community affects the growth of non-fixers or the primary productivity of the whole forest. </div><div>2) On the one hand, N2 fixers may enhance forest productivity by providing a facilitative effect through the provision of plant-available nitrogen to non-fixing trees. On the other hand, N2 fixers may suppress the growth of non-fixers by growing faster and competing more vigorously for light and other resources. A third alternative is that the growth of N2 fixers themselves accumulate biomass rapidly, while having a neutral effect on non-fixers, leading to an overall increase in forest biomass.</div><div>3) We examine these alternative hypotheses using five-year tree census data from 88 plots in 44 seasonal tropical moist secondary forests (3–32 years old) across a human-modified landscape in central Panama. We examined whether N2 fixers accumulated biomass more rapidly than non-fixers, and how relative biomass of N2 fixers as a functional group and as individual species influenced the growth of non-fixer and whole stand primary productivity.</div><div>4) Surprisingly, we found no evidence for either a net competitive or a facilitative effect of N2 fixers as a functional group or individual species on the biomass recovery in these young forests. N2 fixers did not grow faster than non-fixers. Individual mortality rates were lower among N2 fixers, but biomass losses due to mortality were similar between the two groups. Overall, we found no relationship between the relative abundance of N2 fixers and stand primary productivity during succession. </div><div>5) Synthesis: N2-fixing trees may be critical for reducing nitrogen limitation and accelerating biomass growth during tropical secondary forest succession, thereby impacting the global carbon cycle. However, our findings indicate that, in early successional seasonal tropical moist forests, N2 fixers provide neither a net competitive nor a facilitative effect on non-fixing trees or the whole forest stand, likely because tropical N2 fixers utilize facultative fixation and hence abundance poorly approximates the ecosystem function of fixation. Our results indicate that models should not simply scale symbiotic fixation and its effects from N2-fixing tree abundance.</div></div><div><br></div