The Role of Plant Competition During Early, Secondary Succession: An Experimental Approach

Photos from the Starrbright study site where we are conducting long-term species removal experiments to examine the role of interspecific competition during the early stages of succession in the Pacific Northwest.

Broadcast burning of the study site, September 11, 1991

One day after broadcast burning. Rebar posts mark the locations of experimental plots.

Ground surface conditions in a 1 x 1 m experimental plot shortly after broadcast burning.
A "community removal" plot 3 yr after burning. Note the numerous
dead stems of Senecio sylvaticus (center) which dominated
the previous year (see Abstract below).

An experimental block, 3 yr after burning. Dominant residual
(resprouting) species is Rhododendron macrophyllum; dominant early seral species is the short-lived perennial, Gnaphalium microcephalum.
Sampling plant species density and cover, 7 yr after broadcast burning. Flowering stems are Crepis capillaris.

From:   Halpern, C. B., J. A. Antos, M. A. Geyer, and A. M. Olson.  1997.  Species replacement during early secondary succession:  the abrupt decline of winter annual.  Ecology 78:621-631. (.pdf file)

The factors that contribute to species establishment and decline determine the rate and pattern of successional change.  We tested a commonly-held assumption that competitive displacement is responsible for the loss of species during succession.  Manipulative field experiments were used to examine the effects of interspecific competition on the population dynamics of Senecio sylvaticus, a winter annual that briefly dominates post-harvest sites in the western Cascade Range of Oregon.  Senecio increased in density 400-fold from the first to the second growing season after disturbance, but decreased precipitously in year 3 to 10% of the density and 0.5% of the biomass/plot of the previous year.  Although interspecific competition reduced the cover and biomass of Senecio during its peak year, it had little or no effect on either the population increase or decline; the pattern of change was similar among all treatments.  These counterintuitive results underscore the importance of testing, not simply assuming, that interspecific competition is responsible for the replacement of a species during succession.

Temporal changes in the performance of Senecio sylvaticus in species removal and control treatments for three growing seasons after broadcast burning (downward arrow). Population density, cover, and biomass peak in year 2, but r
emoving some or all competitors does not affect the population decline in year 3.

From:   Antos, J. A., and C. B. Halpern.  1997.  Root system differences among species:  implications for early successional changes in forests of western Oregon.  American Midland Naturalist 138:97-108.

Differences in root systems among species may contribute to compositional changes during succession.  We excavated all belowground parts of species in an early successional community 2-3 yr after logging and burning of an old Pseudotsuga menziesii forest.  Annual species had a similar overall root system morphology, but varied in characteristics such as rooting depth and root/shoot ratio.  Seedlings of perennial species generally had higher root/shoot ratios than did annuals.  Mature perennials had extensive root systems with high root/shoot ratios and most species were clonal.  Species typical of later successional stages had more extensive and deeper root systems than did species of earlier stages, a factor that may influence compositional change.  Some species that persist throughout succession may do so because their root systems are flexible, changing in origin and distribution.  Differences in root systems among species are consistent with their successional roles and habitat affinities, and thus, are important in understanding species replacement during secondary succession.


Root system of a 2-yr-old individual of Gnaphalium microcephalum,
a short-lived, monocarpic perennial herb

Root system of a 2-yr-old individual of Epilobium angustifolium,
a long-lived perennial herb

From:   Antos, J. A., C. B. Halpern, R. E. Miller, K. Cromack, Jr., and M. G. Halaj.  2003.  Temporal and spatial changes in soil carbon and nitrogen after clearcutting and burning of an old-growth Douglas-fir forest.  USDA Forest Service Research Paper PNW-RP-552. (.pdf file)

We assessed the immediate and longer term effects of clearcut logging and broadcast burning on the concentration and spatial variability of carbon (C) and nitrogen (N) in the mineral soil (0 to 10 cm) of a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forest in western Oregon. The 4-ha study site, the Starrbright timber sale, is located in the Cascade Range about 25 km south of the H.J. Andrews Experimental Forest. Our specific objectives were to quantify and interpret (1) initial effects of logging and burning on the concentrations of total C, total N, and extractable mineral N (NH4+-N + NO3--N); (2) correlations between the magnitude of initial effects on extractable mineral N and predisturbance concentrations of N or burn severity; (3) longer term (3-year) changes in N and how these changes correlate with increases in plant biomass during succession; and (4) variability in C and N as a function of spatial scale, and whether this variability is affected by logging and broadcast burning. The study site was clearcut logged in late May and early June 1991. Logs were removed with a tracked skidder that moved between experimental blocks; yarding was completed in mid July. The site was broadcast burned on 11 September 1991 with a uniform, moderate-intensity fire. We used 135 permanent plots (4 m2) within 15 blocks (121 m2) and installed before logging and burning to quantify changes in concentration and spatial variation of carbon and nitrogen.

Before harvest, surface soils averaged total C (7.2 percent), total N (0.19 percent), extractable NH4+-N (5.2 ug/g), extractable NO3--N (0.19 ug/g), and pH (5.3). Samples collected 9 months after burning showed a 26 percent decline in concentration of total C. As organic matter burns, C is volatilized, lost as particulates or converted to inorganic carbon. However, addition of C to the surface soil also is possible if charcoal particles from burned duff or logging slash are leached into the soil. Because we used the dry-combustion analytical method that does not distinguish between organic and inorganic forms of C, the decline we observed is a minimum estimate of loss of organic C, which was likely greater, but partly offset by addition of charcoal.

Concentrations of total N changed little after logging and burning, but extractable mineral N showed a fivefold increase 9 months after burning followed by an equally dramatic decline to preburn values in year 2. The coefficient of variation in extractable mineral N increased more than twofold after burning, which, together with the fivefold increase in the mean, yielded extremely high variation in available N after fire. In contrast to extractable mineral N, soil pH remained elevated through final sampling in 1994. Changes in concentration of extractable mineral N were weakly, but significantly, correlated with preharvest concentrations, but were unrelated to burn severity. Increases were observed in nearly all plots, and relatively large increases of extractable N were found in plots that had low, as well as high, initial concentrations. Nine months after burning (1992), concentration of extractable mineral N showed no relation to total plant biomass, which was invariably low during the first postburn growing season. Two and 3 years after burning, however, extractable mineral N showed a significant and increasingly strong negative relation with plant biomass (which had increased substantially), suggesting that N availability was measurably reduced by plant uptake.

Most variation in soil C and N before harvest occurred at small spatial scales (within and among the 2 by 2 m plots); logging and broadcast burning had little effect on this pattern. The large, but transient increase, and the high spatial variation in available N after burning may contribute to the rapid changes in abundance and the patchy spatial distributions of early successional plants that benefit from catastrophic disturbance.

From:   Rozzell, L. R. 2003.  Species pairwise associations over nine years of secondary succession: assessing alternative explanations and successional mechanisms. M.S. Thesis. Utah State University, Logan, Utah. 56 p. (.pdf file)

The importance and mechanisms of species interactions are undetermined in most successional systems. I used correlations and null modeling to detect pairwise species associations between 33 plant species in the first nine years of secondary succession after logging and burning in a western Oregon Cascade forest. I tested for correlations between each species and soil nutrients, nonvegetative ground cover, and surrounding vegetation. More positive than negative associations were found at all sampling times. The proportion of positive associations decreased and negative associations increased through time. Up to 42% of associations at a sampling time were explicable by shared positive correlations with surrounding vegetation. One dominant shrub species, Berberis nervosa, may be primarily responsible for the decline of four early seral species. The associations indicated diffuse facilitation is of primary importance in the stressful early successional environment, and microsite availability and interspecific competition become more important later.

From:   Compagnoni, A., and C. B. Halpern. 2009.  Properties of native plant communities do not determine exotic success during early forest succession. Ecography 32:449-458. (.pdf file)

Considerable research has been devoted to understanding how plant invasions are influenced by properties of the native community and to the traits of exotic species that contribute to successful invasion. Studies of invasibility are common in successionally stable grasslands, but rare in recently disturbed or seral forests. We used 16 yr of species richness and abundance data from 1-m2 plots in a clearcut and burned forest in the Cascade Range of western Oregon to address the following questions: (1) Is invasion success correlated with properties of the native community? Are correlations stronger among pools of functionally similar taxa (i.e., exotic and native annuals)? Do these relationships change over successional time? (2) Does exotic abundance increase with removal of potentially dominant native species? (3) Do the population dynamics of exotic and native species differ, suggesting that exotics are more successful colonists? Exotics were primarily annual and biennial species. Regardless of the measure of success (richness, cover, biomass, or density) or successional stage, most correlations between exotics and natives were non-significant. Exotic and native annuals showed positive correlations during mid-succession, but these were attributed to shared associations with bare ground rather than to direct biotic interactions. At peak abundance, neither cover nor density of exotics differed between controls and plots from which native, mid-successional dominants were removed. Tests comparing nine measures of population performance (representing the pace, magnitude, and duration of population growth) revealed no significant differences between native and exotic species. In this early successional system, local richness and abundance of exotics are not explained by properties of the native community, by the presence of dominant native species, or by superior colonizing ability among exotics species. Instead natives and exotics exhibit individualistic patterns of increase and decline suggesting similar sets of life-history traits leading to similar successional roles.