Overcoming barriers to seedling regeneration during forest restoration on tropical pasture land and the potential value of woody weeds

Abstract

Combating the legacy of deforestation on tropical biodiversity requires the conversion to forest of large areas of established pasture, where barriers to native plant regeneration include competition with pasture grasses and poor propagule supply (seed availability). In addition, initial woody plants that colonise pasture are often invasive, non-native species whose ecological roles and management in the context of forest regeneration are contested. In a restoration experiment at two 0.64 ha sites we quantified the response of native woody vegetation recruitment to (1) release from competition with introduced pasture grasses, and (2) local facilitation of frugivore-assisted seed dispersal provided by scattered woody plants and artificial bird perches. Herbicide pasture grass suppression during 20 months caused a significant but modest increase in density of native woody seedlings, together with abundant co-recruitment of the prominent non-native pioneer wild tobacco (Solanum mauritianum). Recruitment of native species was further enhanced by local structure in herbicide-treated areas, being consistently greater under live trees and dead non-native shrubs (herbicide-treated) than in open areas, and intermediate under bird perches. Native seedling recruitment comprised 28 species across 0.25 ha sampled but was dominated by two rainforest pioneers (Homalanthus novoguineensis, Polyscias murrayi). These early results are consistent with the expected increase in woody vegetation recruitment in response to release from competitive and dispersive barriers to rainforest regeneration. The findings highlight the need for a pragmatic consideration of the ecological roles of woody weeds and the potential roles of "new forests" more broadly in accelerating succession of humid tropical forest across large areas of retired agricultural land.

Keywords: novel ecosystem; old field; plant invasion; rainforest; regrowth; seed dispersal.

Figure 1

Species frequency distribution for 1391 woody plant seedlings recorded from treated and untreated areas of disused pasture, 20 months after treatments commenced at two sites (CloudlandE, CloudlandW). Total sampling effort covered 4262 m², in 94 strip and 40 circle surveys distributed across 2.56 ha. Stars indicate non-native species.

Figure 2

Effect of herbicide treatment on the density of recruited woody seedlings (>10 cm tall) after 20 months at two disused pasture sites (CloudlandE, CloudlandW): (A) native and (B) non-native species. Boxes encompass the upper and lower quartile of data with median value indicated by horizontal line. Whiskers are maximum or minimum values excluding outliers (circles) that are more than 1.5 times the upper or lower quartile.

Figure 3

Effect of habitat structures (perches, dead clumps, live trees) on the density of recruited woody seedlings (>10 cm tall) after 20 months at two disused pasture sites (CloudlandE, CloudlandW): (A) native and (B) non-native species. Boxes encompass the upper and lower quartile of data with median value indicated by horizontal line. Whiskers are maximum or minimum values excluding outliers (circles) that are more than 1.5 times the upper or lower quartile.

Figure 4

Visual illustrations of different parts of a “Works” plot. Left panel—14 months after initial herbicide application (viewed from outside plot). Here, bird perches can be seen above the emerging canopy of non-native wild tobacco (Solanum mauritianum). Right panel—17 months after initial herbicide application (viewed from inside plot). Here, the cover of S. mauritianum has shaded the ground sufficiently to suppress pasture grasses.