To better understand the effects vegetation has on forest soils, we established a number of sampling transects running from old-growth (OG) forests into stands with different vegetation or transects within different vegetation types without an OG component. Each transect was made up of 75 meter segments in both the OG and “treatment” stands. Soil samples and field observations were made at 5 meter-intervals along these segments. Where indicated, the OG portion of the transect acted as a pseudocontrol. The types of vegetation assemblages studied were: (1) a 26 year-old young stand (YS), (2) 6 sites showing normal to fast recovery (FAST) ranging in age from 29 to 36 years, (3) 5 sites showing slow recovery (SLOW) after clear-cutting ranging in age from 27 to 36 years, (4) 4 degraded (DEGRAD) sites ranging in age from 26 to 35 years, (5) 2 grass sites (GRASS), 26 years and undisturbed, and (6) a bracken fern site (FERN) aged at 26 years.
Of these, the DEGRAD, GRASS and FERN sites showed much higher levels of denitrification potential than the other sites suggesting that mineralized fixed nitrogen was being lost from these sites at higher rates than the other vegetation types. Ectomycorrhizal mats were also essentially absent from sites as well. The concentration of living roots was highest in the YS and GRASS sites. The lowest concentrations of labile or biologically active organic carbon as measured by laboratory respiration rates, was found in the DEGRAD sites. The lowest levels of mineralizable (labile) organic nitrogen were found in the FERN site. Litter depth was lowest in the YS and GRASS sites and highest in the FERN site. There were a number of differences found between FAST and SLOW sites that reflected the different NNP activities in these stands. The concentration of ectomycorrhizal mats was greater in the FAST stands. Additionally, litter depth, field respiration rates were all greatest in the FAST stands, all of these patterns would be expected from stands with greater NPP. The concentration of mineralizable nitrogen, extractable ammonium and denitrification potentials were all lowest in the FAST stands suggesting that more organic nitrogen is being cycled and utilized by the faster growing trees. The higher concentration of mycorrhizal mats in these sites could provide the mechanism for cycling organic nitrogen a more rapid rate that in the SLOW sites where they are not as numerous.
Alan K. Swanson, Robert P. Griffiths
Vegetation can profoundly influence both the chemistry and biology of soils; altering soils in a way that enhances plant community resiliency to perturbation (Perry et al.1989). To a large degree, this finding explains why forests that have been disturbed by fire, disease, wind-throw, harvesting or other factors typically return to the same vegetative assemblage that was present before the disturbance. For instance comparative studies between grasslands and forests have shown large differences in soil chemistry (Göceoðlu, 1988; Hart et al., 1992; Popenoe et al., 1992; Ross et al., 1996; Yakimenko, 1997); litter decomposition rates (Hunt et al., 1988; Köchy and Wilson, 1997) and food web compositions (Hunt et al., 1987; Ingham at al., 1989).
The main objective of this study was to run a survey on the effects of different types of vegetation and rates of early succession on forest soil properties. In past studies, we had observed significant differences in soil properties in a chronosequence of post-clear-cut stands ranging in age from 5 to 40 years (seed data in study code SP07: “Disturbance effects on soil processes (stand age study))”. We found significant differences in soil properties between 5 year-year old stands and old-growth stands. The differences were essentially nonexistent after 40 years. We wanted to determine if we could detect differences in soils associated with different vegetation types (i.e. GRASS and FERN) as well as soils associated with stands with different rates of recovery after clear-cutting. The extreme on this continuum is the DEGRAD sites where essentially no conifers have become reestablished after harvest. The SLOW sites were those that did not have canopy closure after approximately 30 years and the FAST sites were ones that did.
The main focus of Synthesis Area “B” of LTER4 is early plant succession. Our study was designed to provide preliminary information about how the rate of early succession might impact soil processes. The current study was an important first step before conducting a much more comprehensive study of early succession sites conducted during 1998 and 1999. The results of this more recent study are reported in the data file entitled “Influence of coniferous tree invasion on forest meadow soil properties” (study code SP012). The current study also laid the groundwork for a data set entitled “Effects of bracken fern invasions on soil processes in clear-cut marginal sites” completed in 1997. In addition, it laid the groundwork for a 1998 study of forest meadow invasion by conifers in a data set entitled “Influence of coniferous tree invasion on forest meadow soil” (See SP016).