Problems and Management of Acacia-Dominated Urban Forests on Man-Made Slopes in a Subtropical, High-Density City

Acacia spp. are exotic tree species that have been widely planted on man-made slopes in Hong Kong since the 1960s. However, as they become mature and senescent, they may become a concern and cause various problems, including soil constraints for plant growth, decreasing provision of intended ecosystem services, declining syndromes, arrested succession, and high risk of failure. In this perspective paper, we present and discuss these problems using practical examples of Acacia-dominated urban forests on man-made roadside slopes in Hong Kong, based on a cross-disciplinary survey and a literature review. To conclude, we suggest that selective cutting, specific silvicultural operations of Acacia plantations, and the management of plantation edge and soils can be exercised, along with the planting of native species, to potentially alleviate these problems associated with mature Acacia plantations, by promoting the establishment of native forests, enhancing biodiversity, expediting succession, and providing better ecosystem services.

High plant diversity and slow assembly of old-growth grasslands

Earth’s ancient grasslands and savannas—hereafter old-growth grasslands—have long been viewed by scientists and environmental policymakers as early successional plant communities of low conservation value. Challenging this view, emerging research suggests that old-growth grasslands support substantial biodiversity and are slow to recover if destroyed by human land uses (e.g., tillage agriculture, plantation forestry). But despite growing interest in grassland conservation, there has been no global test of whether old-growth grasslands support greater plant species diversity than secondary grasslands (i.e., herbaceous communities that assemble after destruction of old-growth grasslands). Our synthesis of 31 studies, including 92 timepoints on six continents, found that secondary grasslands supported 37% fewer plant species than old-growth grasslands (log response ratio = −0.46) and that secondary grasslands typically require at least a century, and more often millennia (projected mean 1,400 y), to recover their former richness. Young (<29 y) secondary grasslands were composed of weedy species, and even as their richness increased over decades to centuries, secondary grasslands were still missing characteristic old-growth grassland species (e.g., long-lived perennials). In light of these results, the view that all grasslands are weedy communities, trapped by fire and large herbivores in a state of arrested succession, is untenable. Moving forward, we suggest that ecologists should explicitly consider grassland assembly time and endogenous disturbance regimes in studies of plant community structure and function. We encourage environmental policymakers to prioritize old-growth grassland conservation and work to elevate the status of old-growth grasslands, alongside old-growth forests, in the public consciousness.

Evidence for arrested succession within a tropical forest fragment in Singapore

Secondary forests occupy a growing portion of the tropical landscape mosaic due to regeneration on abandoned pastures and other disturbed sites (Asneret al. 2009). Tropical secondary forests and degraded old-growth forests now account for more than half of the world's tropical forests (Chazdon 2003), and provide critical ecosystem services (Brown &amp; Lugo 1990, Guariguata &amp; Ostertag 2001).

“Recalcitrant understory layers” revisited: arrested succession and the long life-spans of clonal mid-successional species

In their recent review of arrested succession, Royo and Carson ( A.A. Royo and W.P. Carson. 2006. Can. J. For. Res. 36: 1345–1362 ) demonstrate that “recalcitrant understory layers” are widespread and pervasive modifiers of ecosystems and disruptors of forest regeneration. They rightly point out that many plant species associated with arrested succession are characterized by rapid vegetative spread. Extending their review, we point out that most of such species are clonal or thicket-forming and suggest that an additional reason why these plants so effectively suppress succession for extended periods is their long life-spans.