Late 20th century landscape-wide expansion of Allosyncarpia ternata (Myrtaceae) forests in Kakadu National Park, northern Australia

Abstract

Allosyncarpia ternata S.T.Blake is a large tree endemic to the rugged western edge of the Arnhem Land Plateau, northern Australia, with most of the species conserved in Kakadu National Park (KNP). A. ternata stems suffer substantial mortality following wildfire but the species resprouts prolifically from root stocks. Nonetheless, there is concern about the persistence of A. ternata rainforest patches following breakdown of traditional Aboriginal landscape burning that generated a mosaic of burnt and unburnt areas. Generalised linear modelling was used to identify the landscape features associated with the fragmentary distribution of A. ternata rainforest. We randomly sampled 12 areas that together made up 12.6% of the total coverage of A. ternata in KNP (12 191 ha) that spanned the geographic range of this vegetation type within the Park. The modelling of these data showed that A. ternata forests were most likely to occur at sites with fire protection, as inferred from the small number of fire scars apparent on sequences of satellite imagery, steep slope angles and proximity to drainage lines. Analysis of historical aerial photography revealed that, despite considerable negative and positive variation, there has been a 21% expansion of A. ternata forests over the last 50 years. Expansion occurred by incremental growth from existing forest boundaries and not by nucleation, reflecting the poor seed dispersal of the tree. The forest expansion was negatively correlated with fire activity. A regionally wetter climate since the mid-20th century may be an important cause of the expansion despite currently unfavourable fire regimes.

Managing the matrix: decadal responses of eucalypt‐dominated savanna to ambient fire regimes

“…Relatively small changes observed in tree structural attributes accord with observations that living biomass showed no net change based on the same data set ) and a 5% increase (equivalent to 1.5 m 2 /ha increase in basal area) in canopy cover change at 50 Eucalyptus tetrodonta-dominated savanna sites in Kakadu NP over the period 1964-2004, but especially in the first 20-year period (Lehmann et al 2009). Other recent multi-decadal aerial photograph assessments also report increases in localized savanna canopy cover ) and rain forest cover Dingle 2006) from Litchfield and Kakadu NPs, respectively.…”

Fire-Driven Decline of Endemic Allosyncarpia Monsoon Rainforests in Northern Australia

“…Similarly, many regional studies support the observation of recent monsoon rainforest expansion in a variety of regional, climatic and topographic contexts [5][6][7][30][31][32][33][34][35]. In various cases, the observed expansion of monsoon rainforests has occurred under relatively fire-prone conditions (e.g., [32,33,35]), indicating that enhanced growing conditions associated with increased regional rainfall especially from the 1950s [70,71], reduced evaporation from at least the 1970s [72], global CO 2 fertilisation [14,73], but see [74], and possibly grazing interactions through reducing fuel loads, are likely to have been contributory.…”

“…In various cases, the observed expansion of monsoon rainforests has occurred under relatively fire-prone conditions (e.g., [32,33,35]), indicating that enhanced growing conditions associated with increased regional rainfall especially from the 1950s [70,71], reduced evaporation from at least the 1970s [72], global CO 2 fertilisation [14,73], but see [74], and possibly grazing interactions through reducing fuel loads, are likely to have been contributory. Trends of recent woody thickening and encroachment in savanna systems have been reported globally [75].…”

“…Likewise, many north Australian studies have reported expansion of typically small (<10 ha) patches or larger tracts of monsoon rainforest vegetation within the savanna matrix, particularly under higher rainfall conditions [5][6][7][30][31][32][33][34][35]. In different situations, such expansion has been attributed variously to reduced burning or browsing pressure, increased rainfall in recent decades, possible CO 2 fertilisation, and interactions between these.…”

“…Blake (Myrtaceae), occupies an area of 113,800 ha [38], representing~25% of all monsoon rainforest in northern and north-western Australia [36]. For a smaller region describing the western-most extent of Allosyncarpia's range in Kakadu National Park, Bowman and Dingle [33] reported a net 21% expansion of this forest type based on analysis of canopy cover change at 12 sampled areas using the available aerial photographic record, . While observing that forest expansion was negatively correlated with fire activity, these authors attributed the expansion to a regionally wetter climate since the mid-20th century and elevated CO 2 .…”