Weeds - Phase 1 - Research Highlights

Cyclone affected rainforests

Mother nature helps to limit weed growth in cyclone affected rainforests

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Tropical Cyclone Yasi, a category five storm, which hit the North Queensland coast near Innisfail in February 2011 left a swathe of damage through rainforest regions, opening the way for incursion by numerous weed species.

A previous review of weeds activity in rainforest areas around Innisfail following Cyclone Larry in 2006 found that an increased amount of forest litter and debris on the ground helped to minimise the spread of smaller herbaceous weeds.

That review found the natural regenerative powers of the rainforests helped keep weeds to a minimum, with many native species regenerating quickly which suppressed extensive weed regrowth. However, larger woody weeds and vines were found to be more persistent, which has longer-term implications for the recovery of the native rainforest species and the forest composition.

Climate change scenarios predict an increasing frequency of intense cyclones in the tropics. If future impacts of cyclones are to be reduced, it is essential to understand the dynamics of weed invasion following cyclones and the long-term effects of weeds on forest composition and structure.

The destruction caused by Severe Tropical Cyclone Larry in March 2006 created ideal conditions for the rapid spread of invasive weeds in the rainforests of northern Queensland. After Cyclone Larry and between April 2006 and December 2008 weeds researchers monitored the recruitment, growth and mortality of native and invasive species every three to four months in ninety-nine 2 by 2 metre quadrats over a 45-hectare site near El Arish, south of Innisfail. In addition, researchers documented weed communities in 62 transects across 36 sites in the Wet Tropics Bioregion, taking in a range of different cyclone severity levels and environmental contexts.

The observed nature of the weed incursion following Cyclone Larry has implications for policy makers and environmental land managers in determining how they respond to future cyclones, and which weeds they target in their activities. Although herbaceous weed species appear abundant and widespread initially, most are transient. Woody trees and shrubs, while being less obvious initially, are relatively persistent in comparison, and this is where the management effort should focus.

• That most of the weed flora in rainforest habitats following a severe cyclone have a transient nature
• The very high litter and debris loads that result present a major barrier to weed establishment.
• Three to four years after the event, canopy disturbance remained similar but both the abundance and the diversity of weeds had declined. This appears to be primarily the result of continued regrowth or regeneration of native species.
• However, larger woody trees and vines are more persistent and reproduce so long as the canopy remains relatively open. As light levels decrease these invasive weed species will probably decline.
• If another cyclone causes significant disturbance before the canopy recovers, these fast-growing and early reproducing shrubs will have the capacity to spread rapidly through the landscape and increase their contribution to the weed community.

In 2006, 39 of the 62 transects (62.9 per cent) had weeds present. Of the 496 quadrats across all transects, 148 (29.8 per cent) had weeds present. Weed diversity and abundance were highest in the most disturbed sites (that is, those with the lowest level of canopy cover). By 2009 only 17 transects (33.3 per cent) had weeds present, although the transects had often been heavily invaded, and 107 quadrats (26.2 per cent) had weeds in them. Weed persistence was greatest at the most disturbed sites.

In 2006, 4137 individual plants of 23 species of weeds from 13 families were encountered across all transects (see Table 1). Crassocephalum crepidioides (thickhead) and Solanum mauritianum (tobacco bush) were the most abundant species; six species were represented by only one individual. In 2009, 516 individual plants of 11 species of weeds from eight families were recorded: Ageratum conyzoides (bluetop) and Rubus alceifolius (giant bramble) were the most abundant species; four species were represented by a single individual.

Transects ranged in altitude from 10 to 1210 metres. Weed diversity generally decreased with altitude. The mean altitude of transects with weeds was 514 ± 67.5 metres (n = 39), while the mean altitude of non-weeded transects was 854 ± 71.8 metres (n = 23). The mean weed abundance was highest in lowland forest (126 ± 251.7), followed by highland forest (45 ± 81.4) and then upland forest (4 ± 5.6).