Abstract
We evaluated the ecological significance of the boundary form between two patches with contrasting vegetation (mine grassland and adjacent forest) on woody colonization and forest expansion in open-cast coal mines in Northern Spain. Woody colonization and browsing traces were measured on three mine sites, along 24 transects that were laid out perpendicular to the forest-mine boundary and classified according to their shape (concave, convex, straight). Mine sites were colonized from the close forest by woody species, whose colonization intensity depends on the boundary form. The overall colonization intensity decreased with increasing distance to the forest and differed depending on the boundary form. The more intense colonization was found in concave boundaries and the strongest decrease in convex boundaries close to the forest, whereas straight boundaries showed an intermediate colonization pattern. Concave boundaries reached higher woody cover in the basal strata of the mines than convex (up to 2 m) or straight boundaries (up to 1 m) from 11 m to the forest edge, mainly by the presence of dense patches of Cytisus scoparius (L.) Link, with a scattered overstory of Genista florida L. These shrubs might reduce the browsing intensity and act as nurse plants facilitating the establishment of Quercus petraea (Matt.) Liebl. in mine areas at greater distances from the forest edge. The forest-mine boundary form does not affect the forest vertical structure that is homogenous and does not help explain the woody colonization pattern in the mines. We conclude that edge characteristics have a strong potential to be used in the restoration of native forests based on natural processes. The implications of our results for sessile oak (Quercus petraea (Matt.) Liebl.) forest expansion along edges in fragmented Mediterranean forest landscapes were discussed.
Highlights
We examined the following questions: (1) Are the intensity and colonizing distance of woody species higher in concave than in convex boundaries? (2) Does the forest vertical structure affect such a woody colonization pattern in the mines? (3) Does herbivore browsing play a key role in determining the colonization patterns? The resulting information may offer strong possibilities for landscape conservation [24] and restoration of disturbed sites, such as lands mined for coal all around the world, based on spontaneous processes such as colonization and succession [25,26]
Forest edge woody cover near to the mine was similar in the three boundary forms (F[2,20] = 1.33, p = 0.29), but there was a significant vertical structure (F[6,442] = 28.91, p < 0.001) slightly influenced by the boundary form
We conclude that the forest-mine boundary form exerts a powerful control on woody colonization and explains the opposite results for concave vs. convex boundaries in Northern Spain sixteen years after coal mining reclamation
Summary
The boundary form can determine patch expansion or contraction [4], because adjacent patch shapes interdigitate each other, and interact significantly with the orientation of directional forces in the landscape, such as wind and species dispersion from source areas [3]. Many of these effects emanate from the relation existing between the shape of patches, interactions with the surrounding matrix, and transport within a patch [3,4,5]
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