The effects of harvest-created gaps on plant species diversity, composition, and abundance in a Maine oak–pine forest

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Abstract

In forests where large, stand-replacing disturbances are infrequent, small-scale disturbances associated with the mortality and replacement of individual trees are a primary source of heterogeneity in forest composition and structure. The disturbances considered in this study were canopy gaps created by a partial harvest in the winter of 1987–1988 in an oak–pine (Quercus–Pinus) forest that is part of a long-term ecosystem study at the Holt Research Forest in coastal Maine. This study examined the gap phase of the forest turnover cycle: the brief episodes of rapid change when processes determining the structure and composition of a forest community occur. The objective was to evaluate the effects of harvest-created gaps and soil moisture (as reflected by soil drainage classes) on woody and herbaceous species diversity, composition, and abundance relative to unharvested control areas. Because the vegetation was sampled in both 1993 and 1998, detection of short-term changes was possible.

Harvest gaps had higher total plant species richness in the understory (vegetation <1 m tall) than controls, primarily due to more infrequently-occurring species. Average species richness varied by soil drainage class but not treatment, whereas the Shannon–Wiener diversity index was not affected by treatment, drainage class, or time. Species abundance was more evenly distributed (i.e., higher evenness) in controls than in gaps. Species unique to harvest gaps decreased in number and abundance over time. Detrended correspondence analysis (DCA) revealed that harvest gaps exhibited greater variability in species composition than controls, although many of the gaps and controls were similar in composition. Shifts in individual species abundance were evident with treatment, drainage, and time.

Diversity measures calculated for sapling density showed a significant difference among drainage classes for species richness, but no significant difference between treatments for species richness, the Shannon–Wiener index, or the evenness index. Gap size was positively correlated with the total density of saplings, the density of several species, and species richness. There were significant differences between treatments for the density of all species tested in at least one of the four height classes. DCA of sapling data revealed some separation of harvest gaps and controls but there was considerable overlap. Axis 1 tended to separate early successional species in the harvest gaps from shade tolerant species in the controls. The understory plant community at the HRF appears to be resilient to the partial harvest that created a variety of gap sizes. Silvicultural objectives were met; the harvest gaps created sites for establishment of seedlings and sprouts of tree species, particularly early successional species.

Introduction

Assessments of silvicultural systems have traditionally emphasized timber production. In recent years, silvicultural systems have begun to be evaluated with respect to broader objectives, such as protecting sensitive species and maintaining structurally and compositionally diverse forests (Hunter, 1990, Gilliam et al., 1995, Roberts and Gilliam, 1995b, Coates and Burton, 1997, Battles et al., 2001). Studies that examine the response of forest flora, fauna, and processes within harvest gaps and within the adjacent forest matrix can form the basis for evaluating silvicultural systems (Coates and Burton, 1997).

Despite numerous studies of natural treefall gap dynamics and their effect on the forest community (e.g. Anderson et al., 1969, Ehrenfeld, 1980, Thompson, 1980, Moore and Vankat, 1986; Collins and Pickett, 1987, Collins and Pickett, 1988a, Collins and Pickett, 1988b; Mladenoff, 1990), studies comparing the understory in harvest gaps to the understory beneath a closed canopy are relatively uncommon (Gilliam and Turrill, 1993). Shrubs, ferns, and herbs exhibit sensitivity to a variety of forest disturbances (Moore and Vankat, 1986), including forest management practices (Hughes and Fahey, 1991, Gilliam et al., 1995, Ruben et al., 1999, Battles et al., 2001), and are likely to respond individualistically to the changing environment created by canopy tree removal (Ashmun and Pitelka, 1984, Moore and Vankat, 1986, Collins and Pickett, 1988b, Reader, 1988, Valderde and Silverton, 1998). Gaps also play an important role in the regeneration and perpetuation of canopy tree species. Species-specific characteristics often determine how tree species respond to gaps of different sizes and shapes (Bazzaz and Pickett, 1980, Gray and Spies, 1996, Wright et al., 1998).

Integrative measures such as diversity are valuable ecological metrics to simplify, characterize, and compare the complexity of species assemblages (Christensen and Peet, 1984, Magurran, 1988) and are important topics in forest management studies (Hunter, 1990). The long-term maintenance of plant species diversity in temperate forests may depend on periodic small-scale disturbances creating new sites for colonization and increasing habitat heterogeneity (Thompson, 1980, Abugov, 1982, Beatty, 1984, Brunet and von Oheimb, 1998). Both diversity and individual species abundances may change with time since disturbance (Davison and Forman, 1982, Collins and Pickett, 1988a, Reader, 1988, Gilliam and Turrill, 1993, Goldblum, 1997, Valderde and Silverton, 1998).

The objective of this study was to evaluate the effects of harvest-created gaps on diversity, composition, and abundance of herbaceous and woody vegetation 5 and 10 years after harvesting in an oak–pine (Quercus–Pinus) forest at the Holt Research Forest (HRF) in coastal Maine. This objective complements the two basic goals of the HRF: monitoring long-term changes in the forest’s plant and animal populations, and documenting the effect of forest management on these populations (Witham et al., 1993). In a relatively young (<100-year-old), even-aged, post-agricultural forest, such as the HRF, a conventional partial harvest produces openings that are significantly different from natural openings with respect to mean gap size, gap size class distribution, and forest floor vegetation (Kimball et al., 1995). The effects of gaps may also vary with environmental factors. Thus, we included the effect of soil moisture in our study, a source of considerable environmental variation at the HRF.

Section snippets

Study site

The HRF is a 120 ha mixed-species forest located in Arrowsic (latitude 44°N, longitude 70°W), Maine, on an island near the mouth of the Kennebec River. The island lies within Maine’s coastal climatic region (Briggs and Lemin, 1992) and the midcoast biophysical region of Maine (McMahon, 1990). The climate is characterized by cooler summers (mean maximum July temperature is 26 °C), warmer winters (mean minimum January temperature is −10.5 °C), and higher annual precipitation (114 cm) than interior

Species diversity

Average species richness per site 5 and 10 years after harvesting tended to be higher in harvest gaps than controls (Fig. 2), but this difference was not statistically significant (Table 1), perhaps because the variance in harvest gaps was approximately three times as large as the variance in controls (in 1993: 271.3 vs. 93.4; in 1998: 240.4 vs. 89.1). The large variance reflected that uncommon species tended to be clumped in a few gaps (Fig. 3). The Wilcoxon test revealed that there was a

Conclusion

Community stability reflects the ability of resident species to resist change, or, if altered by disturbance, their ability to readjust or recover (Halpern, 1988). By these criteria, the understory plant community at the HRF appears to be resilient to the partial harvest that created a variety of gap sizes. Harvesting did not appear to negatively impact species presence, as determined by pre-harvest surveys; it appears that few, if any, species at the HRF were lost as a result of the

Acknowledgements

Funding for this project was provided by the Holt Woodland Research Foundation, McIntire-Stennis Funds, the Federal Work Study Graduate Assistantship Program, and the Department of Forest Ecosystem Science, University of Maine. I thank Al Kimball, Susan Gerken, and Gina Purtell for their help and encouragement. This is Maine Agricultural and Forest Experiment Station Publication Number 2562.

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