Papers by Jonathan Bakker
Background/Question/Methods Globally, grasslands are threatened by the invasion of woody plants. ... more Background/Question/Methods Globally, grasslands are threatened by the invasion of woody plants. In many grasslands, vigorous growth of mature trees suggests that invasion is regulated not by constraints on adult growth, but by factors that limit establishment. Predicting and managing invasion thus requires an understanding of the factors that influence tree seedling establishment and growth. While top-down effects such as fire, climate, and grazing are often used to explain the dynamics of woody plant invasions, biotic interactions can also be important. Yet, few studies have considered the potential for complex interactions among multiple factors, thus limiting our ability to predict the conditions under which invasions will occur. We examined the role of established vegetation in regulating the invasion of the conifer Pseudotsuga menziesii in western Washington prairies. Using a fully factorial randomized block design we tested the influence of aboveground competition (yes/no, i....
Journal of Applied Ecology, 2008
New Forests, 2002
In temperate climates, conifer seedlings are often held in frozen storage (22 8C) for extended pe... more In temperate climates, conifer seedlings are often held in frozen storage (22 8C) for extended periods and then placed in cool storage (12 8C) so the root plug can thaw prior to outplanting. Two plug temperature treatments were used to test the hypothesis that thawing seedlings prior to outplanting may be unnecessary: seedlings were planted with frozen root plugs ('frozen seedlings') and with thawed root plugs ('thawed seedlings'). The experiment was conducted under two watering regimes (irregular, regular) and with three conifer species (lodgepole pine [Pinus contorta var. latifolia], western larch [Larix occidentalis], interior spruce [Picea glauca 3 engelmannii]) to increase the generality of the results. The warming of root plugs after planting was examined. Thawed root plugs warmed to soil temperature rapidly (about 30 min) while frozen root plugs took longer (to 2 h) because ice in the plug had to melt before temperatures rose. Larger root plugs took longer to warm to soil temperature. Several aspects of seedling field performance were also assessed. For all species, variable fluorescence did not differ between frozen and thawed seedlings. Bud break was faster for thawed than frozen western larch seedlings but did not differ between frozen and thawed seedlings for either lodgepole pine or interior spruce. Height increment differed significantly between frozen and thawed seedlings that received the irregular watering regime; this effect was likely a response to the positioning of irrigation nozzles, which resulted in sporadic and non-uniform irrigation patterns. Height increment did not differ between frozen and thawed seedlings that received the regular watering regime. Root collar diameter and volume increments were not significantly affected by plug temperature treatment under either watering regime. Planting seedlings with frozen root plugs did not hinder field performance over one growing season under these watering regimes.
Nature, Jan 13, 2016
How ecosystem productivity and species richness are interrelated is one of the most debated subje... more How ecosystem productivity and species richness are interrelated is one of the most debated subjects in the history of ecology. Decades of intensive study have yet to discern the actual mechanisms behind observed global patterns. Here, by integrating the predictions from multiple theories into a single model and using data from 1,126 grassland plots spanning five continents, we detect the clear signals of numerous underlying mechanisms linking productivity and richness. We find that an integrative model has substantially higher explanatory power than traditional bivariate analyses. In addition, the specific results unveil several surprising findings that conflict with classical models. These include the isolation of a strong and consistent enhancement of productivity by richness, an effect in striking contrast with superficial data patterns. Also revealed is a consistent importance of competition across the full range of productivity values, in direct conflict with some (but not all...
ABSTRACT Background/Question/Methods The establishment of parasitic plants may be limited both by... more ABSTRACT Background/Question/Methods The establishment of parasitic plants may be limited both by their requirements and by their host plants. These issues are magnified when the parasitic species is rare. Castilleja levisecta (golden paintbrush) is a federally threatened hemiparasite associated with one of the most endangered ecosystems in the United States, the prairies of western Washington. Recovery of this species requires understanding how sensitive it is to presence and identity of host plants, and whether this sensitivity depends on plant size. We tested the effects of host plant presence and identity (none, Festuca roemeri, or Eriophyllum lanatum) and initial C. levisecta size (grown in 49 or 393 ml pots) on the performance of 560 C. levisecta ouplanted in a tilled field. Performance was characterized using four metrics: survival, vigor, number of flowering stems, and seed production one growing season after outplanting. Results/Conclusions C. levisecta survival was high (81%) when planted with no host and enhanced by the presence of a host plant. Host plant presence also enhanced the other C. levisecta performance metrics (all p < 0.05). Host plant identity did not affect vigor or flowering stem production, but strongly affected seed production: each surviving C. levisecta plant produced ~ 500 seeds if grown with no host compared to ~1,400 if grown with F. roemeri and ~3,000 if grown with E. lanatum. Host plant identity also appeared to affect the flowering phenology of C. levisecta. C. levisecta plants grown in smaller pots had higher survival but poorer vigor than those grown in large pots. Significant host plant × pot size interactions were detected for some metrics, but were generally much smaller than main effects. These results improve our understanding of interactions between parasitic plants and their hosts. Further research should test whether C. levisecta can use other host plants and the mechanisms by which host plant identity might affect seed production, including differences in rooting morphology, haustorial connections, and ability to attract pollinators that also visit C. levisecta. Recovery of C. levisecta would be enhanced by planting E. lanatum as its host.
ABSTRACT Background/Question/Methods Many ecological phenomena are scale-dependent, and these rel... more ABSTRACT Background/Question/Methods Many ecological phenomena are scale-dependent, and these relationships can change among habitats and with disturbance. We studied the importance of spatiotemporal scale in understanding the effects of afforestation and livestock grazing in Campos grasslands of southern South America. Afforestation is a common land-use change across this region; in Uruguay alone, ~19% of the land base has been or will be impacted by afforestation. Livestock grazing was widespread before afforestation and continues in and around plantations. Our study had two objectives: 1) compare species richness for four treatment types (grazed grassland, ungrazed grassland, grazed pine forest, and ungrazed pine forest) at various spatial scales, and 2) investigate time since exclosure effects between grazed and ungrazed habitats. We calculated species richness in modified-Whittaker nested plots of varying sizes (1, 4, 9, 36, and 144 m2) for each treatment type at 5 sites. Data were collected in spring and fall for two years after exclosures were established. Each season was analyzed separately, testing the effects of afforestation status (forest or grassland), management (grazed or ungrazed), time since exclosure, and their interactions on species richness for each plot size using generalized linear mixed models. Results/Conclusions Afforestation and grazing had interactive effects on species richness: richness was greater in ungrazed than grazed pine forests but lower in ungrazed than grazed grasslands, especially at large scales. At small scales, afforestation status was most important: richness was greater in grasslands than forests, and the effects of grazing were only significant for grasslands (richness was greater in grazed than ungrazed). Species richness was greater in spring than fall, but effects of grazing on richness were greater in fall (grazed compared to ungrazed habitats at time exclosure was built and 1 yr later). Our results illustrate the importance of spatial and temporal scale when examining habitats and disturbance regimes. Similar to previous research conducted in other regions, we found scale-dependent relationships, and that to fully understand the effects of complex land-use change, multiple spatiotemporal scales must be examined.
Forest Science, 2014
Forecasting diameter growth accurately is essential for long-term forest management. The large-tr... more Forecasting diameter growth accurately is essential for long-term forest management. The large-tree diameter growth submodel for ponderosa pine in the Southwest Ponderosa Pine model type of the Central Rockies variant of the Forest Vegetation Simulator (FVS) was evaluated using a long-term data set. Small trees were predicted least accurately, with a large proportion being overpredicted in long projections. Sensitivity analysis methods were used to assess how the model could be improved by ranking input variables in terms of their importance to model predictions. Based on these results, a new 10-year periodic diameter model was developed using regression techniques. Compared with the FVS model, the new model reduced both the short-term (10-year) prediction error by 0.18 in. and the overprediction bias and error propagation in the long-term (40-year) projection. The improvement of the new model is attributed to the addition of top height and quadratic mean diameter. These variables describe the vertical and horizontal distribution of the canopy and account for the effect of intertree competition for light and nutrients. Describing the stand structure comprehensively helped to more accurately assess the resource environment that affects tree productivity. Comprehensive measures of intertree competition were found to be more important in long-term projections.
Background/Question/Methods Today, native grasslands are one of the most threatened ecosystems in... more Background/Question/Methods Today, native grasslands are one of the most threatened ecosystems in the United States. Native prairie habitats have been nearly extirpated from the Pacific Northwest and are the most endangered ecosystem in Washington State. Past efforts to restore these landscapes have focused primarily on outplanting containerized seedlings of native plant species important for rare butterflies, an extremely labor- and resource-intensive approach. Large-scale seeding of these species may be a viable option to increase efficiency, although it is unknown which seeding techniques are most effective for germination and establishment. Because seed availability is often a limiting factor in large-scale restoration, it is also important to evaluate seed application rate to determine the lowest rate that provides the desired density of adult plants. We assessed three seeding techniques (drill, broadcast, hydroseed) and five seed application rates (0, 350, 700, 1050, 1400 seed...
Background/Question/Methods Experiments under carefully controlled conditions have been criticize... more Background/Question/Methods Experiments under carefully controlled conditions have been criticized as having limited applicability because they do not adequately reflect the heterogeneity of natural environments. This heterogeneity, arising from multiple sources, can affect the magnitude and direction of ecological interactions. We evaluated the importance and magnitude of spatiotemporal variability relating to plant establishment in a field restoration experiment in western Washington. Our experiment was conducted at four sites (two each in North and South Puget Sound) and in three seeding years (2009-2011), for a total of ten site-year combinations. Sites were abandoned agricultural fields initially dominated by non-native grasses and agricultural weeds. We killed the extant vegetation using two site preparation treatments (glyphosate-burn-glyphosate (burned) or glyphosate-mow-till-solarize (solarized)). All plots were broadcast seeded with a forb-rich mixture in autumn, keeping t...
Journal of Applied Ecology, 2008
1. The identification of species associated with or indicative of groups of samples is a common a... more 1. The identification of species associated with or indicative of groups of samples is a common aspect of ecological research, including studies of environmental management. Indicator Species Analysis (ISA) permits statistically rigorous assessments of these indicator species, but its usage is currently restricted to simple designs. 2. I describe three improvements that greatly increase the utility of ISA. First, exact permutation tests require that the correct exchangeable units be permuted; these exchangeable units may vary among factors. Secondly, the consistency of indicators can be assessed using meta-analytic techniques to combine the results from multiple sites. Thirdly, while ISAs are classically conducted using abundance data, a simplified ISA can be conducted using binary (presence/absence) data. 3. These improvements are illustrated by identifying indicators of grazing treatment (inside or outside exclosure) at three sites in a southwestern ponderosa pine Pinus ponderosa forest. Most indicators were consistent among sites, and the number of significant indicators was reduced 37-40% by combining results from multiple sites. Species that occurred at multiple sites were more likely to be indicators than those present at a single site. 4. Simplified ISAs produced very similar results to classical ISAs: both methods identified the same group as having the maximum Indicator Value in > 93% of tests. Compared to abundance data, however, the presence/absence data used in a simplified ISA are easily collected and efficiently published in data tables or appendices. 5. Synthesis and applications . Meta-analytic techniques and simplified Indicator Species Analyses can increase the ability to analyse new or previously published data, and permit rigorous assessments of the consistency of indicator status spatially or temporally. Other recommendations to improve the utility of ISA include identifying organisms to as fine a taxonomic resolution as possible, providing detailed descriptions of groups and typologies, ensuring adequate sample sizes within groups, reporting the sample size and frequency of all species in all groups, and publishing data for all species -whether or not they are significant indicators -to prevent publication bias in future meta-analyses.
Nature Plants, 2015
Terrestrial ecosystem productivity is widely accepted to be nutrient limited(1). Although nitroge... more Terrestrial ecosystem productivity is widely accepted to be nutrient limited(1). Although nitrogen (N) is deemed a key determinant of aboveground net primary production (ANPP)(2,3), the prevalence of co-limitation by N and phosphorus (P) is increasingly recognized(4-8). However, the extent to which terrestrial productivity is co-limited by nutrients other than N and P has remained unclear. Here, we report results from a standardized factorial nutrient addition experiment, in which we added N, P and potassium (K) combined with a selection of micronutrients (K+μ), alone or in concert, to 42 grassland sites spanning five continents, and monitored ANPP. Nutrient availability limited productivity at 31 of the 42 grassland sites. And pairwise combinations of N, P, and K+μ co-limited ANPP at 29 of the sites. Nitrogen limitation peaked in cool, high latitude sites. Our findings highlight the importance of less studied nutrients, such as K and micronutrients, for grassland productivity, and point to significant variations in the type and degree of nutrient limitation. We suggest that multiple-nutrient constraints must be considered when assessing the ecosystem-scale consequences of nutrient enrichment.
New Forests, 2002
In temperate climates, conifer seedlings are often held in frozen storage (-2 °C) for extended pe... more In temperate climates, conifer seedlings are often held in frozen storage (-2 °C) for extended periods and then placed in cool storage (+2 °C) so the root plug can thaw prior to outplanting. Two plug temperature treatments were used to test the hypothesis that thawing seedlings prior to outplanting may be unnecessary: seedlings were planted with frozen root plugs (‘frozen
Nature communications, 2015
Exotic species dominate many communities; however the functional significance of species' bio... more Exotic species dominate many communities; however the functional significance of species' biogeographic origin remains highly contentious. This debate is fuelled in part by the lack of globally replicated, systematic data assessing the relationship between species provenance, function and response to perturbations. We examined the abundance of native and exotic plant species at 64 grasslands in 13 countries, and at a subset of the sites we experimentally tested native and exotic species responses to two fundamental drivers of invasion, mineral nutrient supplies and vertebrate herbivory. Exotic species are six times more likely to dominate communities than native species. Furthermore, while experimental nutrient addition increases the cover and richness of exotic species, nutrients decrease native diversity and cover. Native and exotic species also differ in their response to vertebrate consumer exclusion. These results suggest that species origin has functional significance, and...
We describe an individual-tree model that uses contemporary measurements to "grow trees back... more We describe an individual-tree model that uses contemporary measurements to "grow trees backward" and reconstruct past tree diameters and stand structure in ponderosa pine dominated stands of the Southwest. Model inputs are contemporary structural measurements of all snags, logs, stumps, and living trees, and radial growth measurements, if available. Key steps include the application of inverse decay functions to estimate snag and log death dates, and the estimation of tree size in the reconstruction year via radial growth data or accrued basal area increment. The model is provided as a function for R, and can be modified for other species and regions.
The Fort Valley Experimental Forest has contributed many long-term studies and ecological lessons... more The Fort Valley Experimental Forest has contributed many long-term studies and ecological lessons to forest and range research since its inception in 1908. In 1909, T. S. Woolsey (Regional Forester) and G. A. Pearson (Director, Fort Valley) established a network of permanent sample plots in the ponderosa pine, mixed conifer, and spruce forest types throughout Arizona and New Mexico. We revisited many of the ponderosa pine-dominated plots and used ledger data, contemporary data, and dendrochronological techniques to quantify changes in species composition, tree density, and tree size over the past century. We used historical stem-maps to examine variations in tree spatial patterns over time as well. The Coulter Ranch site (24 plots) was part of the nation-wide Methods of Harvest study established in 1913. We examined the short- and long-term consequences of historical harvest method on contemporary pine forest structure and recruitment patterns at this northern Arizona site. In addit...
Much has been written about the many agents that have caused structural changes in ponderosa pine... more Much has been written about the many agents that have caused structural changes in ponderosa pine (Pinus ponderosa var. scopulorum) forests of the Southwest since Euro-American settlement. Few studies, however, mention the importance of spatial pattern, and even fewer provide quantitative data describing how spatial patterns have changed. The resulting descriptions of stand dynamics and reference conditions are not very satisfying. We describe the spatial patterns of two large (S1A: 2.59 ha; S4A: 3.24 ha) historically stem-mapped ponderosa pine sites at four points in time: contemporary (2002)(2003)(2004), before and after selective harvests (ca. 1894-1909), and before Euro-American settlement (~1876). For each site and date, we used first-and second-order nearest neighbor analyses (nearest neighbor indices, Ripley's univariate and bivariate statistic) and geostatistical analyses (autocorrelograms, semivariograms, block kriging) to quantify the spatial pattern of stems ≥ 9.14 cm diameter at breast height (1.37 m). Trees were spatially aggregated at presettlement and before selective harvests. Harvesting exaggerated the patchiness of these stands by selectively removing groups of trees. At present, one site is one large patch while the other has a spatial pattern similar to the presettlement pattern; these differences are due in part to differences in soil parent material: tertiary basalt flows at S1A and cinders at S4A. Tree recruitment patterns are not spatially random, especially at small scales, and differ based on soil parent material. Spatial pattern is an important component of ponderosa pine forests, affecting wildlife habitat, understory production, and other factors, and therefore should be explicitly quantified and incorporated into management activities.
Ecology, 2015
Humans dominate many important Earth system processes including the nitrogen (N) cycle. Atmospher... more Humans dominate many important Earth system processes including the nitrogen (N) cycle. Atmospheric N deposition affects fundamental processes such as carbon cycling, climate regulation, and biodiversity, and could result in changes to fundamental Earth system processes such as primary production. Both modelling and experimentation have suggested a role for anthropogenically altered N deposition in increasing productivity, nevertheless, current understanding of the relative strength of N deposition with respect to other controls on production such as edaphic conditions and climate is limited. Here we use an international multiscale data set to show that atmospheric N deposition is positively correlated to aboveground net primary production (ANPP) observed at the 1-m 2 level across a wide range of herbaceous ecosystems. N deposition was a better predictor than climatic drivers and local soil conditions, explaining 16% of observed variation in ANPP globally with an increase of 1 kg NÁha À1 Áyr À1 increasing ANPP by 3%. Soil pH explained 8% of observed variation in ANPP while climatic drivers showed no significant relationship. Our results illustrate that the incorporation of global N deposition patterns in Earth system models are likely to substantially improve estimates of primary production in herbaceous systems. In herbaceous systems across the world, humans appear to be partially driving local ANPP through impacts on the N cycle.
Journal of Range Management, 1997
Crested wheatgrass [Agropyron cristatutn (L.) Gaertn.] fields have low plant diversity and a limi... more Crested wheatgrass [Agropyron cristatutn (L.) Gaertn.] fields have low plant diversity and a limited grazing season. We tested whether blue grama [Bouteloua gracilis (HBK.) Lag.] could be established in crested wheatgrass fields to increase plant diversity and grazing season length. Three seeding methods (unseeded, seed broadcast after rototilling, or seed drilled) and 2 herbicide treatments (none or glyphosate [iV (phosphonomethyl)-glycine] sprayed at 1.1 kg a.i. ha") were applied in a complete factorial design to 3 X 10 m plots in 1994 in two 50 year-old crested wheatgrass stands. Blue grama established in seeded plots, but the effectiveness of seeding methods varied between sites. At both sites, herbicide control of crested wheatgrass greatly promoted the establishment of blue grama. Crested wheatgrass biomass and cover were reduced by rototilling and by spraying. The results suggest that the establishment of blue grama in crested wheatgrass stands is possible if seed is added and competition from crested wheatgrass is controlled.
Ecology letters, 2015
Aboveground-belowground interactions exert critical controls on the composition and function of t... more Aboveground-belowground interactions exert critical controls on the composition and function of terrestrial ecosystems, yet the fundamental relationships between plant diversity and soil microbial diversity remain elusive. Theory predicts predominantly positive associations but tests within single sites have shown variable relationships, and associations between plant and microbial diversity across broad spatial scales remain largely unexplored. We compared the diversity of plant, bacterial, archaeal and fungal communities in one hundred and forty-five 1 m(2) plots across 25 temperate grassland sites from four continents. Across sites, the plant alpha diversity patterns were poorly related to those observed for any soil microbial group. However, plant beta diversity (compositional dissimilarity between sites) was significantly correlated with the beta diversity of bacterial and fungal communities, even after controlling for environmental factors. Thus, across a global range of tempe...
Restoration Ecology, 2011
A considerable number of institutions offer courses in restoration ecology and its application, e... more A considerable number of institutions offer courses in restoration ecology and its application, ecological restoration. We quantified the scope and structure of introductory restoration courses at 67 post-secondary institutions by reviewing institutional course catalogs and course syllabi. Most courses were available at Research-level institutions. More than half of the courses were offered within departments that focused on natural resource management, and were required or optional in a degree program. Most courses were taught in classroom settings during the academic year and were geared toward advanced students. Course titles suggested an emphasis on the science of restoration ecology over the practice of ecological restoration, and learning objectives focused primarily on concepts and less on skills and attitudes. Assessment was largely via conventional methods, notably exams. Many courses assigned readings from the primary literature; there was little consensus in terms of text selection. We conclude that restoration is being presented to students as an advanced undertaking and in largely theoretical terms. Although we were unable to consider other important elements such as thematic content, class size, or pedagogical method, our study provides a baseline assessment of introductory restoration courses that can be used to evaluate changes in restoration education or opportunities for restoration education in other countries. These results can inform the development of new introductory restoration courses, and raise important considerations in light of the development of a Practitioners' Certification Program by the Society for Ecological Restoration.
Uploads
Papers by Jonathan Bakker