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"Wheeler, George"
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Functional traits’ annual variation exceeds nitrogen-driven variation in grassland plant species
Effective application of functional trait approaches to ecological questions requires understanding the patterns of trait variation within species as well as between them. However, few studies address the potential for intraspecific variation to occur on a temporal basis and, thus, for trait-based findings to be contingent upon sampling year. To quantify annual variation in the functional traits of grassland plant species, we measured specific leaf area, leaf dry matter content, plant height, and chlorophyll content in 12 shortgrass prairie plant species. We repeated these measurements across 4 years, both in long-term nitrogen addition plots and in corresponding control plots. Three of the four traits showed significant year-to-year variation in a linear mixed model analysis, generally following a pattern of more acquisitive leaf economics spectrum traits in higher rainfall years. Furthermore, two of the measured traits responded interactively to nitrogen addition and sampling year, although only one, leaf dry matter content, showed the expected pattern of stronger nitrogen responses in high rainfall years. For leaf dry matter content and specific leaf area, trait responses to sampling year were larger than responses to the nitrogen addition treatment. These findings illustrate that species’ functional traits can respond strongly to environmental changes across years, and thus that trait variation in a species or community is likely to extend beyond the values and patterns observed in any single year.
Journal Article
The synergistic response of primary production in grasslands to combined nitrogen and phosphorus addition is caused by increased nutrient uptake and retention
Background and aimsA synergistic response of aboveground plant biomass production to combined nitrogen (N) and phosphorus (P) addition has been observed in many ecosystems, but the underlying mechanisms and their relative importance are not well known. We aimed at evaluating several mechanisms that could potentially cause the synergistic growth response, such as changes in plant biomass allocation, increased N and P uptake by plants, and enhanced ecosystem nutrient retention.MethodsWe studied five grasslands located in Europe and the USA that are subjected to an element addition experiment composed of four treatments: control (no element addition), N addition, P addition, combined NP addition.ResultsCombined NP addition increased the total plant N stocks by 1.47 times compared to the N treatment, while total plant P stocks were 1.62 times higher in NP than in single P addition. Further, higher N uptake by plants in response to combined NP addition was associated with reduced N losses from the soil (evaluated based on soil δ15N) compared to N addition alone, indicating a higher ecosystem N retention. In contrast, the synergistic growth response was not associated with significant changes in plant resource allocation.ConclusionsOur results demonstrate that the commonly observed synergistic effect of NP addition on aboveground biomass production in grasslands is caused by enhanced N uptake compared to single N addition, and increased P uptake compared to single P addition, which is associated with a higher N and P retention in the ecosystem.
Journal Article
Frequent failure of nutrients to increase plant biomass supports the need for precision fertilization in agriculture
Implementing precision fertilization to maximize crop yield while minimizing economic and environmental impacts has become critical for agriculture. Variability in biomass response to fertilization within fields, among regions, and over time creates simultaneous risks of under-yielding and overfertilization. We quantify factors determining fertilization responsiveness (i.e., biomass increases with fertilization) up to 15 years in 61 unfertilized rangelands on six continents. We demonstrate widespread multi-year variability in responsiveness, with fertilization increasing average yield by 43% but failing to improve biomass 26% of the time. All sites were responsive at least once, but only four of 61 responded in all plots and years. Modelled management scenarios highlighted that fertilizer cessation is likely to generate sizable economic savings but always reduces yield because of the difficulty in predicting when and where biomass will be unresponsive. This work reveals substantial scale-dependent variability in fertilization responsiveness globally, while clarifying the prospects and pitfalls of managing more spatially and temporally precise nutrient application.
Journal Article
Local nutrient addition drives plant diversity losses but not biotic homogenization in global grasslands
Nutrient enrichment typically causes local plant diversity declines. A common but untested expectation is that nutrient enrichment also reduces variation in nutrient conditions among localities and selects for a smaller pool of species, causing greater diversity declines at larger than local scales and thus biotic homogenization. Here we apply a framework that links changes in species richness across scales to changes in the numbers of spatially restricted and widespread species for a standardized nutrient addition experiment across 72 grasslands on six continents. Overall, we find proportionally similar species loss at local and larger scales, suggesting similar declines of spatially restricted and widespread species, and no biotic homogenization after 4 years and up to 14 years of treatment. These patterns of diversity changes are generally consistent across species groups. Thus, nutrient enrichment poses threats to plant diversity, including for widespread species that are often critical for ecosystem functions.
Journal Article
Rapid collapse of a population of Dieffenbachia spp., plants used for tadpole-rearing by a poison-dart frog (Oophaga pumilio) in a Costa Rican rain forest
Amphibian populations have been declining worldwide, with multiple potential causes. At La Selva field station in north-eastern Costa Rica, previous work has shown that populations of many amphibians have decreased significantly since the 1970s, especially in primary forest. Starting in 1998, we investigated one of the most common frog species at La Selva, the poison-dart frog Oophaga pumilio (= Dendrobates pumilio). In a survey of 50 plots of 100 m2 in 1998, adult frogs were 4.6 times more abundant in secondary forest than in primary forest. Tadpoles were found only in secondary-forest plots. Almost all (89%) of the tadpoles were found in leaf axils of Dieffenbachia spp., which were much more abundant in secondary-forest than in primary-forest plots. The greater abundance of Dieffenbachia spp. in secondary forest was confirmed in a broad survey of ~11 km of trails within La Selva in 2002. When the same trails were resampled in 2012, Dieffenbachia spp. had been extirpated from 72% of the 50-m segments where plants were present in 2002; abundance was greatly reduced in the few trail segments where any Dieffenbachia spp. remained in 2012. The loss of Dieffenbachia spp., especially in secondary forest, removed the species most often used by O. pumilio for tadpole rearing. Based on counts of calling frogs in 2010, there was no difference in O. pumilio abundance in primary versus secondary forest, in striking contrast to multiple earlier surveys that found much greater frog abundance in secondary forest. We propose that the reason for the rapid decline in Dieffenbachia spp. is herbivory by the collared peccary (Pecari tajacu), which has increased in abundance at La Selva in recent years. A likely consequence is continued reduction in O. pumilio populations.
Journal Article
Interacting Effects of Nutrient Availability and Environmental Change on Grassland Plant Communities
The availability of nutrients plays an important role in shaping the composition, productivity and diversity of plant communities. Such effects are particularly well documented in grassland ecosystems, where both independent studies and collaborations such as the Nutrient Network consistently show increasing productivity and decreasing diversity with the addition of limiting nutrients. The specifics of this pattern, however, vary widely from site to site, and within a single site, they may shift with changes in environmental conditions and disturbance regimes. My research at Cedar Creek Ecosystem Science Reserve (Minnesota, USA), documents such a pattern through resampling of a long-term experiment on pine encroachment into old field grasslands. In burned plots, where pine encroachment is inhibited, nitrogen addition shifts the herbaceous plant community from one dominated by perennial C4 grasses with native forbs and legumes to one where invasive C3 grasses are abundant. In unburned plots subject to pine encroachment, however, invasive C3 grasses are abundant regardless of nitrogen treatment. This community pattern does not, however, extend to the ecosystem’s overall carbon and nitrogen dynamics. While several ecosystem carbon pools respond to fire, they do not respond to nitrogen addition or to fire-nitrogen interactions. Alongside such measures of species composition and resource cycling, plant functional traits may provide valuable insights into community dynamics. Such traits show considerable intraspecific variability, and understanding this variation is critical to their effective use. At Cedar Point Biological Station (Nebraska, USA), grassland plant species’ functional traits show considerable variation with nutrient addition. I found, however, that these traits, and in some cases the strength of their nutrient responses, are also dependent on sampling year. Species display more acquisitive trait values both with nitrogen addition and in high rainfall years, and for leaf dry matter content, these effects interact to produce an elevated nitrogen effect in high rainfall years. Through experimental rainfall manipulation, I found that these patterns can be partially explained by total rainfall but that a substantial component remains to be explained by other components of annual environmental variation.
Dissertation
Conservation Takes a Reflective Turn
Francesca Bewer's new book, published jointly in 2010 by Harvard Art Museum and Yale University Press, takes a step in a new direction for conservation literature. A Laboratory for Art: Harvard's Fogg Museum and the Emergence of Conservation in America, 1900-1950 traces the development of the Conservation Department in the Fogg Museum from its beginnings to just after World War II. That history is told largely by following the personal and professional growth of one man, Edward Waldo Forbes, and those he gathered around him during his tenure as the director of the Fogg Museum from 1909 to 1944. As Bewer (pronounced BAY.ver) points out, Forbes came from good stock - a Boston Brahmin and the grandson of the Transcendentalist poet and philosopher Ralph Waldo Emerson - and his grounding in the American Enlightenment played a large part in shaping both his character and ultimately that of the Fogg's Conservation Department. (Author abstract)
Journal Article