Benton N. Taylor

Changes in root architecture under elevated concentrations of CO2 and nitrogen reflect alternate soil exploration strategies

Predicting the response of fine roots to increased atmospheric CO₂ concentration has important implications for carbon (C) and nutrient cycling in forest ecosystems. Root architecture is known to play an important role in how trees acquire soil resources in changing environments. However, the effects of elevated CO₂ on the fine-root architecture of trees remain unclear. We investigated the architectural response of fine roots exposed to 14 yr of CO₂ enrichment and 6 yr of nitrogen (N) fertilization in a Pinus taeda (loblolly pine) forest. Root traits reflecting geometry, topology and uptake function were measured on intact fine-root branches removed from soil monoliths and the litter layer. CO₂ enrichment resulted in the development of a fine-root pool that was less dichotomous and more exploratory under N-limited conditions. The per cent mycorrhizal colonization did not differ among treatments, suggesting that root growth and acclimation to elevated CO₂ were quantitatively more important than increased mycorrhizal associations. Our findings emphasize the importance of architectural plasticity in response to environmental change and suggest that changes in root architecture may allow trees to effectively exploit larger volumes of soil, thereby pre-empting progressive nutrient limitations.

Nitrogen-fixing trees inhibit growth of regenerating Costa Rican rainforests

Significance Regrowing tropical forests are critical for global biodiversity conservation and carbon capture. Nitrogen availability often controls how fast these forests can regrow. Because nitrogen-fixing plants are the primary source of new nitrogen into these forests, one might expect that more nitrogen fixers lead to faster forest regrowth. However, here we show that nitrogen fixers actually slow forest regrowth. Their competitive influence on neighboring trees outweighs any growth enhancement from their nitrogen inputs at this site. Our results call for a more critical evaluation of how nitrogen fixers influence the surrounding forest, especially given the large uncertainty in global climate projections that hinges on the role of nitrogen fixers during tropical forest regeneration. More than half of the world’s tropical forests are currently recovering from human land use, and this regenerating biomass now represents the largest carbon (C)-capturing potential on Earth. How quickly these forests regenerate is now a central concern for both conservation and global climate-modeling efforts. Symbiotic nitrogen-fixing trees are thought to provide much of the nitrogen (N) required to fuel tropical secondary regrowth and therefore to drive the rate of forest regeneration, yet we have a poor understanding of how these N fixers influence the trees around them. Do they promote forest growth, as expected if the new N they fix facilitates neighboring trees? Or do they suppress growth, as expected if competitive inhibition of their neighbors is strong? Using 17 consecutive years of data from tropical rainforest plots in Costa Rica that range from 10 y since abandonment to old-growth forest, we assessed how N fixers influenced the growth of forest stands and the demographic rates of neighboring trees. Surprisingly, we found no evidence that N fixers facilitate biomass regeneration in these forests. At the hectare scale, plots with more N-fixing trees grew slower. At the individual scale, N fixers inhibited their neighbors even more strongly than did nonfixing trees. These results provide strong evidence that N-fixing trees do not always serve the facilitative role to neighboring trees during tropical forest regeneration that is expected given their N inputs into these systems.

Why are nitrogen-fixing trees rare at higher compared to lower latitudes?

Symbiotic nitrogen (N) fixation provides a dominant source of new N to the terrestrial biosphere, yet in many cases the abundance of N-fixing trees appears paradoxical. N-fixing trees, which should be favored when N is limiting, are rare in higher latitude forests where N limitation is common, but are abundant in many lower latitude forests where N limitation is rare. Here, we develop a graphical and mathematical model to resolve the paradox. We use the model to demonstrate that N fixation is not necessarily cost effective under all degrees of N limitation, as intuition suggests. Rather, N fixation is only cost effective when N limitation is sufficiently severe. This general finding, specific versions of which have also emerged from other models, would explain sustained moderate N limitation because N-fixing trees would either turn N fixation off or be outcompeted under moderate N limitation. From this finding, four general hypothesis classes emerge to resolve the apparent paradox of N limitation and N-fixing tree abundance across latitude. The first hypothesis is that N limitation is less common at higher latitudes. This hypothesis contradicts prevailing evidence, so is unlikely, but the following three hypotheses all seem likely. The second hypothesis, which is new, is that even if N limitation is more common at higher latitudes, more severe N limitation might be more common at lower latitudes because of the capacity for higher N demand. Third, N fixation might be cost effective under milder N limitation at lower latitudes but only under more severe N limitation at higher latitudes. This third hypothesis class generalizes previous hypotheses and suggests new specific hypotheses. For example, greater trade-offs between N fixation and N use efficiency, soil N uptake, or plant turnover at higher compared to lower latitudes would make N fixation cost effective only under more severe N limitation at higher latitudes. Fourth, N-fixing trees might adjust N fixation more at lower than at higher latitudes. This framework provides new hypotheses to explain the latitudinal abundance distribution of N-fixing trees, and also provides a new way to visualize them. Therefore, it can help explain the seemingly paradoxical persistence of N limitation in many higher latitude forests.

Light regulates tropical symbiotic nitrogen fixation more strongly than soil nitrogen

Nitrogen limits primary production in almost every biome on Earth1,2. Symbiotic nitrogen fixation, conducted by certain angiosperms and their endosymbiotic bacteria, is the largest potential natural source of new nitrogen into the biosphere3, influencing global primary production, carbon sequestration and element cycling. Because symbiotic nitrogen fixation represents an alternative to soil nitrogen uptake, much of the work on symbiotic nitrogen fixation regulation has focused on soil nitrogen availability4–8. However, because symbiotic nitrogen fixation is an energetically expensive process9, light availability to the plant may also regulate symbiotic nitrogen fixation rates10,11. Despite the importance of symbiotic nitrogen fixation to biosphere functioning, the environmental factors that most strongly regulate this process remain unresolved. Here we show that light regulates symbiotic nitrogen fixation more strongly than does soil nitrogen and that light mediates the response of symbiotic nitrogen fixation to soil nitrogen availability. In a shadehouse experiment, low light levels (comparable with forest understories) completely shut down symbiotic nitrogen fixation, whereas soil nitrogen levels that far exceeded plant demand did not fully downregulate symbiotic nitrogen fixation at high light. For in situ forest seedlings, light was a notable predictor of symbiotic nitrogen fixation activity, but soil-extractable nitrogen was not. Light as a primary regulator of symbiotic nitrogen fixation is a departure from decades of focus on soil nitrogen availability. This shift in our understanding of symbiotic nitrogen fixation regulation can resolve a long-standing biogeochemical paradox12, and it will improve our ability to predict how symbiotic nitrogen fixation will fuel the global forest carbon sink and respond to human alteration of the global nitrogen cycle.Symbiotic nitrogen fixation is one of the most important sources of nitrogen in terrestrial ecosystems. In this study, the researchers demonstrate that light availability is an outstanding driver of symbiotic nitrogen fixation in tropical leguminous trees.

Logarithmic scales in ecological data presentation may cause misinterpretation

Scientific communication relies on clear presentation of data. Logarithmic scales are used frequently for data presentation in many scientific disciplines, including ecology, but the degree to which they are correctly interpreted by readers is unclear. Analysing the extent of log scales in the literature, we show that 22% of papers published in the journal Ecology in 2015 included at least one log-scaled axis, of which 21% were log–log displays. We conducted a survey that asked members of the Ecological Society of America (988 responses, and 623 completed surveys) to interpret graphs that were randomly displayed with linear–linear or log–log axes. Many more respondents interpreted graphs correctly when the graphs had linear–linear axes than when they had log–log axes: 93% versus 56% for our all-around metric, although some of the individual item comparisons were even more skewed (for example, 86% versus 9% and 88% versus 12%). These results suggest that misconceptions about log-scaled data are rampant. We recommend that ecology curricula include explicit instruction on how to interpret log-scaled axes and equations, and we also recommend that authors take the potential for misconceptions into account when deciding how to visualize data.Logarithmic scales are frequently used in ecological data display, but the degree to which they are understood is not clear. Here, the authors survey members of the Ecological Society of America and find that only 56% of respondents correctly interpreted data presented on log–log axes.

Seed Removal by an Introduced Scatter-Hoarder on a Caribbean Island

Abstract The introduction of non-native seed dispersers has the potential to significantly alter distributions and relative abundances of native plants. Although effects of introduced seed predators have been documented, little is known about how introduced dispersers influence seed movement patterns. We investigated seed removal of seven rainforest species on the island of Dominica in the Lesser Antilles by the entire seed-remover community and specifically by the Red-rumped Agouti, Dasyprocta leporina, a scatter-hoarding rodent introduced to the island approximately 2500 years ago. We recorded removal rates in three regions of Dominica from 168 experimentally placed seed groups containing a total of 1356 seeds. Seed groups were either accessible to the entire seed-remover community or placed within exclosures designed to exclude agoutis. Within 13 days, 47 percent and 28 percent of seeds had been removed from control groups and agouti exclosure groups, respectively, leading to 19 percent of seed removal being attributed to agoutis. Species with smaller seeds were preferentially taken by seed removers other than agoutis, whereas agoutis were responsible for the majority of the removal of larger-seeded species. Seed removal was greater in areas with higher regional conspecific adult densities regardless of treatment, but agoutis had a greater impact relative to other seed removers on the seed removal of the studys rarest species. The results of this study highlight the potential impacts that introduced dispersers may have on native plant communities and call for further study of disperser introductions worldwide.