(Fig. 1). On Hog Island, small gaps were prevalent in the younger more
productive thicket, conversely, large gaps were prevalent within the aging
thicket. Gaps within these contrasting thickets differed in microclimate and
species richness.
In an effort to elucidate successional mechanisms involved in the development of climax maritime forests, we quantified canopy gaps which naturally occurred within (Myrica cerifera) shrub thickets on Hog Island. Thicket microclimate, spatial heterogeneity, and edaphic characteristics within canopy gaps, as well as within intact understories, were compared for productive (Mid-Island) and aging (Bay Side) thickets to discern if gap characteristics vary with thicket age.
(Fig. 1). On Hog Island, small gaps were prevalent in the younger more
productive thicket, conversely, large gaps were prevalent within the aging
thicket. Gaps within these contrasting thickets differed in microclimate and
species richness.
Considerable differences occurred in light and soil environments between the locations as well as between gap and intact thicket. Light levels in gaps and adjacent thickets were highly variable. In the productive Mid-Island thickets, midday incident quantum flux ranged from 5-968 umol m-2 s-1 while levels in the aging Bay Side thicket ranged from 10-1000 umol m-2 s-1 (Table 1). The Bay Side gap photosynthetic photon flux density (PPFD) was approximately twice as high as the Mid-Island. The percent sunlit of the within gap transects was almost 4 times as high for the Bay Side gap than the Mid-Island gap (Table 1). Gap air temperatures were generally a few degrees warmer than adjacent intact thickets (Table 1). Temperatures at the soil surface were 6 (C) and 14 (C) higher for the Mid-Island and Bay Side gaps, respectively, while temperatures at -10 cm varied minimally between their adjacent thickets (Table 1).
There were 35 different species found within the gaps of both thickets (Table 2). Eighteen were woody, and 17 were herbaceous. There were a total of 10 species found within Mid-Island gaps. Myrica cerifera and Parthenocissus quinquefolia were the most prevalent species found within the Mid-Island gaps (Table 2). The Bay Side gaps however, had 3 times greater species richness than was found within the Mid-Island gaps (Table 2). Once again, both Myrica cerifera and Parthenocissus quinquefolia proved to be ubiquitous within the Bay Side gaps.
Relative to the intact thickets, gaps enhance environmental variability and provide a diversity of microsites suitable for the establishment of later seral species.
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Mid-Island Bay Side
Gap Thicket Gap Thicket
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PPFD (umol m-2 s-1)
Sunlit 575+/-150 509+/-57 1050+/-95 525+/-109
Shaded 36+/-4 6+/-0 100+/-0 38+/-3
Range 7 - 1288 5 - 968 100 - 1500 10 - 1000
% Sunlit 25 10 96 36
Tair (C) 28 27 30 27
Range 25 - 29 23 - 28 25 - 31 25 - 28
Tsoil (C)
Surface 32 26 41 27
Range 24 - 32 24 - 28 24 - 41 24 - 28
-10 cm 20 20 22 21
Range 19 - 21 19 - 20 20 - 24 20 - 22
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Species Frequency
Mid-Island Bay Side
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Woody
Baccharis halimifolia 0.2 0.3
Berchemia scandens - 0.1
Calystegia sepium - 0.1
Celtis occidentalis - 0.1
Hypericum hypericoides - 0.2
Iva frutescens - 0.2
Iva imbricata - 0.2
Lonicera japonica - 0.2
Mikania scandens 0.1 0.2
Myrica cerifera 1.0 0.7
Parthenocissus quinquefolia 0.7 0.9
Persea palustris - 0.4
Prunus serotina - 0.2
Rubus argutus 0.1 0.5
Smilax bona-nox - 0.1
Smilax rotundifolia - 0.1
Toxicodendron radicans 0.2 0.2
Vitis aestivalis - 0.1
Herbaceous
Atriplex patula - 0.1
Boehmeria cylindrica - 0.2
Chenopodium ambrosiodes - 0.1
Cirsium horridulum - 0.2
Cyperaceae spp. 0.1 -
Eupatorium capillifolium - 0.5
Eupatorium hyssopifolium - 0.2
Gnaphalium purpureum - 0.1
Osmunda regalis 0.2 -
Phytolacca americana - 0.1
Poaceae spp. - 0.4
Polygonum hydropiperoides 0.2 0.1
Rumex acetosella - 0.4
Scirpus americanus - 0.1
Setaria geniculata - 0.1
Spartina patens - 0.1
Teucrium canadense 0.1 0.4
Species richness (#) 10 33
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