Plant Dynamics

Research in the plant dynamics group at the KBS LTER focuses on two major questions: (1) How do differences in cropping systems management impact the diversity, composition and abundance of weeds in row-crops and how does this impact crop yield? and (2)What are the long-term impacts of chronic nitrogen inputs on successional plant communities? Our research utilizes experimental treatments on the LTER main site row-cropping treatments (T1-T4), successional treatments (T7, T8), the Biodiversity Experimental Plots (BExP) and old fields in the surrounding landscape (SF1-3). Changes in plant diversity and abundance is characterized in all of these communities from annual biomass harvests and sampling soil seed banks on a 3-6 year schedule.

Impacts of Cropping System Diversity on Weeds

We have examined a variety of responses to and effects of variation on cropping system management on weeds including: impacts of weed community composition, seed predation, and crop yields (Smith and Gross 2007). The BExP was established in Spring 2000 to examine how manipulations of crop diversity alone can impact weed communities and crop yield. A recent paper (Smith et al 2008) from the BExP study documented that corn grain yields (but not soybean or wheat) increased in response to increasing crop diversity and was strongly correlated with spring soil nitrogen levels, which was associated with the number of legume species in the rotation. Ongoing studies on these plots are focusing on how changes in cropping system impact soil microbial communities and associated biogeochemical processes.

Seed Bank Dynamics

Soil seed banks provide a reservoir of diversity and legacy of past community composition in both successional and row crop communities. We have sampled seed banks on the LTER every 3-6 years since 1999.  We have shown that weed seed banks change in response to cropping treatments and that these shifts can occur rapidly (Smith and Gross 2006). However, the correlation and ability to predict weed communities remain tenuous (Davis et al. 2005, Smith and Gross 2006). Protocols, methods and data from seed bank surveys are available from the LTER Data Catalog.

Plant Community Responses to Chronic Nitrogen Fertilization

Long-term fertilization experiments were established in the early successional treatment (T7) in 1989 and has  provided for testing hypotheses relating resource alterations on local  successional dynamics to collaborate with other LTER sites in cross-site synthesis on the impacts of nitrogen addition on productivity, diversity and species loss in grasslands (Suding et al. 2005). Data from used for these analyses are published in Ecological Archives (Cleland et al. 2008). The cross-site work continues as the Productivity-Diversity-Traits Network (PDTNet) a collaboration involving researchers from 10 LTER sites and long-term experiments on grasslands at Jasper Ridge, CA. The most recent paper from this collaboration showed that species with a tall-runner growth form respond strongly to N-fertilization and in many grasslands determined the magnitude of species loss (Gough et al. 2012)

Selected Publications

  • Cleland, E.E., C.M. Clark, S.L. Collins, J.E. Fargione, L. Gough, K.L. Gross, D.G. Milchunas, S.C. Pennings, W.D. Bowman, I.C. Burke, W.K. Lauenroth, G.P. Robertson, J.C. Simpson, D. Tilman, and K.N. Suding. 2008. Species responses to nitrogen fertilization in herbaceous plant communities, and associated species traits. Ecology 89:1175.
  • Davis, A., K.A. Renner, K.L. Gross. 2005. Weed seedbank and community shifts in a long-term cropping system experiment. Weed Science 53: 296-306.
  • Gough, L., K.L. Gross, E. Cleland, C. Clark, S. Collins, J. Fargione, S. Pennings, and K. Suding. 2012. Incorporating clonal growth form clarifies the role of plant height in response to nitrogen addition. Oecologia (in press).
  • 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, R.G., K.L. Gross, G.P. Robertson. 2008. Effects of crop diversity on agroecosystem function: Crop yield response. Ecosystems 11: 355-366.
  • Smith, R.G. and K.L. Gross. 2007. Assembly of weed communities along a crop diversity gradient. J. Appl. Ecology 44:1046-1056.
  • Smith, R.G., K.L. Gross, G.P. Robertson. 2008. Effects of crop diversity on agroecosystem function: Crop yield response. Ecosystems 11: 355-366.
  • Suding, K.N., SL Collins, L. Gough, C.M. Clark, E.E. Cleland, K.L. Gross, D. Milchunas, and S. Penning. 2005. Functional- and abundance-based mechanisms explain diversity loss due to N fertilization. PNAS 102: 4387-4392.