Research Overview

The KBS LTER Site

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Plant Dynamics

Research in the plant dynamics group at the KBS LTER focuses on two major questions: (1) What are the impacts of cropping systems management on the diversity, composition and impact of weeds in row-crop agriculture? and (2) What are the long-term impacts of chronic nitrogen inputs on successional plant communities? Our research activities utilize experimental treatments on the LTER main site row-cropping treatments (T1-T4), successional treatment (T7, 8), a set of abandoned fields and forests in the surrounding KBS landscape, LFL, the Biodiversity Experimental Plots (BExP): The BExP was established in Spring 2000 to examine how manipulations of crop diversity alone can impact the weed communities, crop yield and a  host of ecosystem services through both direct and indirect effects.

Impacts of cropping system diversity on weeds. We have examined a variety of responses to and effects of variation on cropping system diversity, including: impacts of weed community composition, seed predation (Menalled et al 2007), and crop yields. A recent paper from the BExP study documented the impact of increasing crop diversity, with no other changes in management on grain yields in corn (but not other crops in the rotation. The variation in corn yield is strongly correlated with spring soil nitrogen levels, which is associated with the number of legume species in the rotation.

Seed bank dynamics.We have sampled seed banks on the main experimental LTER site as part of our baseline work every 3 years (1990-2002; now every 6 yrs, 2008) and on a more irregular schedule on other experimental plots. Results to date have shown that seed bank species composition is responsive to treatments and that these shifts can occur rapidly. However, the correlation to above-ground weed communities remain tenuous. Protocals, methods and DATA from past seed bank surveys are (or soon will be) available.

Plant community responses to chronic nitrogen fertilization. The successional treatments (T7, T8) were established on the KBS LTER to provide a reference community for comparisons of ecological processes in response to disturbance. They have provided us with opportunities for cross-site analysis and synthesis on the impacts of nitrogen addition on grassland communities, particularly productivity, diversity and species loss. This work, is now done by the Productivity-Diversity-Traits Network (PDTNet) is a collaboration of researchers (PIs, former and current postdocs and graduate students) from 10 LTER sites and long-term experiments on grasslands at Jasper Ridge, CA. A key paper from this synthesis showed that local diversity declined with N-addition, and used a trait-based approach to identify if there were consistent predictors of species loss across sites. A more recent analysis, examined how environmental factors may also drive species loss. Data from used for these analyses are published in Ecology. We are continuing research to examine the long term impacts of chronic fertilization on community assembly and to explore how variation in abundance and composition of species traits across communities may impact the responsiveness to nitrogen addition and predicted variation in precipitation patterns.

Selected Publications

Clark, CM, EE Cleland, SL Collins, JE. Fargione, L Gough, KL Gross , SC. Pennings , KN Suding and JB Grace 2007. Title: Environmental and plant community determinants of species loss following nitrogen enrichment Ecology Letters 10: 596-607.

Cleland, EE, CM. Clark, SL Collins, JE Fargione, L Gough, KL. Gross, DG Milchunas, SC Pennings, WD Bowman, IC Burke, WK Lauenroth, GP Robertson, JC Simpson, D Tilman, and KN Suding. 2008. Species responses to nitrogen fertilization in herbaceous plant communities, and associated species traits. Ecology 89:1175.

Davis, A, KA Renner, KL Gross. 2005. Weed seedbank and community shifts in a long-term cropping system experiment. Weed Science 53: 296-306.

Gross, K.L., M.R. Willig, L. Gough, R. Inouye, and S. B. Cox. 2000. Species density and productivity at different spatial scales in herbaceous plant communities. Oikos 89: 417-427.

Smith, RG, KL Gross, GP Robertson, 2008. Effects of crop diversity on agroecosystem function: Crop yield response Ecosystems 11: 3550366.

Smith, RG and KL Gross. 2007. Assembly of weed communities along a crop diversity gradient. J. Appl Ecology 44:1046-1056.

Suding, KN, SL Collins, L Gough, CM Clark, EE Cleland, KL Gross, D Milchunas, and S Penning. Functional traits predict species loss in response to resource enhancement. PNAS 102: 4387-4392.