Acacia mangium Willd: benefits and threats associated with its increasing use around the world

Background: Acacia mangium, a fast-growing tree native to parts of Indonesia, Papua New Guinea and Australia,has been cultivated outside its native environment and introduced into humid tropical lowland regions of Asia,South America and Africa over the last few decades. It is a multipurpose tree used in agroforestry, forestry and for restoration of degraded lands. It is also highly invasive in many regions where it has been introduced outside its native range. This paper reviews evidence of its obvious benefits and its negative impacts on biodiversity.Methods: A literature review on Australian acacias and especially on A. mangium was undertaken to highlight both benefits and threats associated with their increasing worldwide use outside their native ranges.Results: Through N_2 fixed from the atmosphere, A. mangium improves soil fertility, especially by increasing N status and soil C accretion when introduced to N-limited areas; it thus has the potential to restore nutrient cycling in degraded systems. No studies have, however, been done to assess the effectiveness of A. mangium in restoring biodiversity of degraded lands. Most Australian acacias have traits that facilitate invasiveness, and 23 species have been recorded as invasive to date. A. mangium has been reported as invasive in Asia, Indonesia, Pacific Islands,Indian Ocean Islands, southern Africa and Brazil. Research on other invasive Australian acacias in several parts of the world has elucidated the types of impacts that are likely in different types of ecosystems and key options for mitigating impacts.Conclusions: A. mangium has the potential to restore nutrient cycling in degraded systems, but is highly invasive wherever it is planted. Many parts of the world have a large invasion debt for this species. Experience with other invasive acacias around the world suggests a suite of interventions that could be used to reduce invasions and mitigate impacts. Careful risk assessments should be undertaken prior to any new plantings of this species.

Field Soil Respiration Rate on a Sub-Antarctic Island: Its Relation to Site Characteristics and Response to Added C, N and P

Botanical, soil chemistry and soil microbiology variables were tested as predictors of in situ soil respiration rate in the various terrestrial habitats on sub-Antarctic Marion Island (47oS, 38oE). Inorganic P and total N concentration were the best predictors amongst the chemistry variables and bacteria plate count the best of the microbiology variables. However, while these chemistry and microbiology variables could accurately predict soil respiration rate for particular habitats, they proved inadequate predictors across the whole range of habitats. The best suite of predictors comprised only botanical variables (relative covers of five plant guilds) and accounted for 94% of the total across-habitat variation in soil respiration rate. Mean field soil respiration rates (2.1 - 15.5 mmol CO2 m-2 h-1) for habitats not influenced by seabirds or seals are similar to rates in comparable Northern Hemisphere tundra habitats. Seabird and seal manuring enhances soil respiration rates to values (up to 27.6 mmol CO2 m-2 h-1) higher than found at any tundra site. Glucose, N, P or N plus P were added to three habitats with contrasting soil types;a fellfield with mineral, nutrient-poor soil, a mire with organic, nutrient-poor soil and a shore-zone herbfield heavily manured by penguins and with organic, nutrient-rich soil. Glucose addition stimulated soil respiration in the fellfield and mire (especially the former) but not in the coastal herbfield soil. N and P, alone or together, did not stimulate respiration at any of the habitats, but adding glucose to fellfield soils that had previously been fortified with P or NP caused a similar increase in respiration rate, which was greater than the increase when adding glucose to soils fortified only with N. This suggests that fellfield soil respiration is limited by P rather than N, and that there is no synergism between the two nutrients. For the mire and coastal herbfield, adding glucose to soils previously fortified with N, P or NP did not enhance rates more than adding glucose to soils that had received no nutrient pre-treatment.

Spatial Variation in Soil Chemistry on a Sub-Antarctic Island

On both west and east sides of sub-Antarctic Marion Island (47oS, 38oE), total Na and exchangeable Na, Mg and K concentrations in the soil decline with increasing distance inland and altitude, related to a decrease in the intensity of seaspray deposition. On the east side, the coastal plain is wide and slopes gently up to the mountainous interior and total C, total N and soil moisture content all decrease significantly, whereas bulk density increases significantly, as one moves away from the sea, reflecting a gradual change from organic, wet, low bulk density peats characteristic of low-land coastal regions to mineral, dry, high bulk density volcanic soils characteristic of inland areas. On the west side, the narrow coastal plain is bounded by an escarpment that rises up very steeply to the highland interior. There, sampling was largely restricted to the coastal plain (soils are rare on the escarpment and interior) and did not cover the same transition from organic to mineral soils as on the east side. Hence, total C, total N and bulk density did not change significantly with increasing distance inland on the west side. Most total Mg is in the mineral fraction of the soil, with a lesser contribution by organic, exchangeable and soil solution forms of Mg. On the east side the gradual transition from highly organic peats to very mineral soils results in an increase in total Mg going inland, but on the west, where there was not this change in soil minerality, total Mg decreased with increasing distance inland, reflecting the decreasing intensity of seaspray. Once the between-side differences in the influence of altitude and distance from the sea are accounted for, there are significant differences in soil chemical composition between the two sides of the island. Overall, west side soils are more influenced by both seaspray and the parent volcanic basalts than are east side soils.