Succession
Invasion
Predation
We are interested in the environmental constraints, organismal tradeoffs, and feedbacks that control the rate, pattern, and direction of succession (e.g., Huston and Smith 1987; Theme 2) because these give insights into the mechanisms controlling diversity, community assembly and ecosystem functioning (Theme 3). Moreover, successional patterns demonstrate the impacts and recovery from agricultural disturbance (Theme 1). The assembly of communities and the maintenance of diversity within them may relate to several potential mechanisms:
A key part of our study of disturbance, succession, and community assembly comes from long-term observations in 22 fields of different successional ages (E014), a periodically resampled chronosequence. In 1983 we established 100 permanent 0.5 m2 plots along four parallel 80m long transects in each of 22 fields ranging in age from 1 to 56 years (Pierce 1954, Inouye et al, 1987c). All 2200 plots have been sampled every 5 or 6 years (1983, 1989,1994, and 1997). All plants in a plot are identified to species and their cover estimated for all plots (Inouye et al. 1987a). Soil cores from each plot are collected, analyzed for total N and C, and archived for future analysis. Each field also is sampled annually for abundances of grasshoppers, the major herbivore (Huntly & Inouye 1988), small mammals (Huntly & Inouye 1987), and pocket gophers (Geomys bursarius, Inouye et al. 1987c). These abundance data are critical in assessing the role of trophic interactions in regulating species composition and diversity (see Section 2.B.D). A 0.1 x 3 m strip from a 5 x 5 m plot at the start of each transect in each field is annually clipped for aboveground biomass and sorted to species (E054). This provides annual information on long-term productivity patterns.
These results suggest that succession emerges from a complex interaction of plant colonization of abandoned fields, of local competition for N, and of herbivory that limits the abundance and impacts of N-fixers (see Section 2.B.D). Because it is well documented and data rich, this chronosequence has been used for short term “snapshots” of successional patterns of abundances of mycorrhizal fungal species (Johnson et al. 1991), microbial biomass and organic matter dynamics (Zak et al. 1990), litter mass (Inouye et al. 1987c), plant allocation to roots, leaves, stems, and reproduction (Gleeson & Tilman 1990; Craine et al. 1999a), plant tissue C and N, and arthropod diversity and abundances (Siemann et al. 1996, 1999a). These snapshots have been instrumental in showing that succession is driven more by colonization limitation than by the resource ratio hypothesis (Tilman 1985) that initially motivated this work. They continue to provide insights into mechanisms maintaining diversity and regulating C and N budgets following agricultural abandonment.
We will resample soils and vegetation of the full chronosequence, E014, in 2002, giving us five sampling dates over a 19 year period. In order to examine the temporal patterns of C and N accumulation, especially at greater soil depths, we will pull deeper cores (to 60 cm depth), which has not been done since our original sampling in 1983. We will continue sampling plant biomass (sorted to species), insect abundances, and small mammal abundances annually.
At Cedar Creek, we are uniquely poised to explore how patterns in insect and plant diversity are influenced by habitat fragmentation and disturbance across 4 orders of magnitude in spatial scale [plot (1 m), transect (80 m), field (100-500 m, and all of Cedar Creek (12 km)], and 2.5 orders of magnitude in temporal scale (1-240 months). Plant diversity patterns will be discerned from the cover plots sampled in each field. To explore this question for insects, we will undertake an extensive insect fauna survey of 8 major orders of insects in each field in 2002, using monthly sweep and pitfall trap samples within the area proscribed by the transects in each field in E014. This is possible because of the expertise of our on-site entomologist, John Haarstad. Using the annual grasshopper data, we will also explore patterns of grasshopper diversity and community patterns across different temporal scales. This survey will measure size-diversity relationships at 4 different spatial scales, following Siemann et al. (1996). These data should provide unique insights into the causes of variation in insect and plant diversity.
To further explore species’ effects on ecosystem function and succession, J. Knops and D. Wedin propose to examine the mechanism by which species impact ecosystem N cycling. This will use the 16 most abundant plant species of the chronosequence. They will perform a container experiment to determine if plant nutrient use (e.g. allocation patterns, photosynthetic N use efficiency, tissue longevity and stoichiometry) leads to positive feedbacks resulting in diverging productivity and N and C pools on initially identical soils. They will also determine species impacts on N input and losses and the consequence of such differential rates on primary productivity and ecosystem N and C pools. This will provide insights into the aspects of a species’ biology that determine the long-term consequences of successional replacements on ecosystem C and N dynamics. This also will test if short term studies of species impacts on N and C can predict long-term successional patterns.
