Influence of canopy opening on the environment and herb layer in a northern hardwoods forest

Summary

Plant response to small gaps in woodland and forests has been demonstrated, however, neither the extent of environmental change required for response nor the lag time between gap formation and plant response has been well documented. The first goal of this present study was to quantify the difference between gap and closed canopy environments, and then determine any relationships with development of plant cover and species number. Although not a primary aim of this research, the findings have implications for conservation management of understorey plant species.

Study area: The research was undertaken in a 1.2 ha second-growth deciduous hardwood stand within the Kane Experimental Forest (KEF), on the Allegheny Plateau of northwestern Pennsylvania, USA. The tree canopy (25-30 m) is dominated by black cherry Prunus serotina and sugar maple Acer saccharum. The herb-layer in spring comprised mostly trout lily Erythronium americanum, whilst black cherry seedlings (5-20 cm in height) became dominant by midsummer.

Canopy opening: Sites were selected in spring 1981 for three, single-tree (1 Prunus or Acer felled) and three, multi-tree (up to 7 Prunus and/or Acer felled) openings representative of the average (68.7 m²) and smaller gap sizes reported for old-growth forests on the Plateau. On 30 October 1981 (end of the growing season), the gaps were created by felling at 1 m height; the felled trees were not removed. The three single-tree openings (S1-3) were roughly circular and 33 to 37 m². The multi-tree openings (M1-3) varied in size (51-151 m²) and shape (the largest, M3, being most irregular).

Vegetation and environmental monitoring began in May 1981 before the canopy opening, and subsequently in 1982 to 1984.

Vegetation monitoring: Four pairs of parallel, 50 cm wide, strip transects were established in each cardinal compass direction from the estimated gap centre to 7.5 m (single-tree gaps) or 15 m (multi-tree gaps) into the surrounding canopied area. Transects were divided to form 50 × 50 cm sampling plots. After gap creation, plots beneath opened canopy were termed 'opened' or 'gap'. The next four (single-tree sites) or eight (multi-tree sites) plots along each transect arm were termed 'transitional', and remaining plots, 'canopied' or 'closed'.

Cover of herb layer species in each plot was estimated in late May and July from 1981 through 1984; because of time constraints, only one single-tree and one multi-tree site were sampled in May 1981.

Environmental monitoring: Daily maximum and minimum air temperature at 25 cm above forest floor was measured at opened, transitional and canopied locations along the transects. Soil moisture of the organic (0-10 cm) and mineral (>10 cm) horizons and temperature at 10 and 20 cm depths was determined at 1 m (single-tree sites) or 2 m (multi-tree sites) intervals along and adjacent to the transects. Mineralization of soil nitrogen and inorganic NO3 and NH4 in gap, transitional, and closed areas was determined monthly from April through September.

Light (photosynthetically active radiation) reaching herb height (c.30 cm) was also measured during these months. Measurements were made at 0800, 1200, and 1600 EST on both clear and uniformly cloudy days.

Environmental changes: After gap creation, noon light on clear summer days was brightest north of an opening centre, but at other times of the day, and when overcast, there was no difference in the light quantity beneath the opened or closed canopy. The distribution of soil moisture, or of soil or air temperature was likewise, little affected by gap creation. In general, the mineral soil environment varied non-significantly both across years and over the study area.

Vegetation responses: Species establishment tended to be higher near gap centres; otherwise, there was no pronounced effect of canopy opening on plant cover or species richness during the four years after gap creation. Vegetation responses were not significantly correlated with any of the environmental variables measured.

Although average species change was greatest in the largest gap (M3), there were no other correlations of species number changes with opening size. The increase in M3 was due in part to establishment of two species not initially present in any site: mountain aster Aster acuminatus (primarily in transition and closed plots) and pin or fire cherry Prunus pennsylvanica (under all canopy conditions). Other taxa contributing to the cover increase in M3 were three that were not initially present in the 1.2 ha study area: tulip tree Liriodendron tulipifera and birch Betula spp. seedlings, and bramble Rubus spp., and the already present and common wood sorrel Oxalis acetosella, grasses and small ferns.

Within all other sites, the species increase was primarily by establishment of species initially common in the study area, and also Liriodendron, Betula and Rubus. These latter three ‘invading’ taxa were most common on bare soil or in areas disturbed by chipmunks Tamias sp. in all locations within the sites.

Although species establishment was higher near a gap centre, the authors consider that there was no pronounced plant response that could be directly attributed to the canopy openings of the sizes created in the study.


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