Two climate‐sensitive tree‐ring chronologies from Arnhem Land, monsoonal Australia

Abstract

The ecology of the Australian monsoon tropics is fundamentally shaped by dry conditions between May and October followed by highly variable rainfall over the months of November to April. Due to its crucial ecological importance, a better understanding of past hydroclimate variability in the region is of great interest to land managers and custodians in this region. Short instrumental records also make highly resolved terrestrial palaeoclimate records for northern Australia prior to 1900 CE of considerable scientific importance. Here, we present two new well-replicated Callitris intratropica ring-width chronologies from Arnhem Land in northern Australia, one of which extends the tree-ring record in the region by another 86 years, back to 1761. Both chronologies have clearly defined regional patterns of correlations with temperature, precipitation, potential evapotranspiration and two drought indices (the self-calibrating Palmer Drought Severity Index (PDSI) and the Standardised Precipitation Evapotranspiration Index (SPEI)) across the lower latitudes of the Northern Territory. Results indicate considerable scope for hydroclimatic reconstructions based on C. intratropica for transitional periods into and out of the wettest time of the year. This suggests that such reconstructions would reflect variability in the duration of the wet period. While precipitation or streamflow reconstructions may be possible for both these transitional periods, drought reconstructions will be best focused on the months of March-May at the end of the wet period. Hydroclimate reconstructions would provide important baseline information for understanding the rate and magnitude of current regional climate change for these ecologically and culturally important transitional periods.

Tree growth responses to temporal variation in rainfall differ across a continental-scale climatic gradient

“…We used five chronologies of ring-width indices: three published (LTY, [ 34 ]; CJD, [ 33 ]; KOR, [ 31 ]) and two new ring-width chronologies (LDE and CHR). We used ring-width indices (RWI) instead of raw ring-width measurements because the width of growth rings typically decline as the age of trees and the girth of their trunks increases.…”

“…Plant productivity and growth is primarily limited by water availability across the vast majority of the Australian continent [29,30]. Accordingly, rainfall amount is a strong driver of patterns in the secondary growth of our target species, Callitris columellaris across its range (e.g., [31][32][33][34]). C. columellaris is widespread across Australia and has a broad climatic range, inhabiting some of the driest regions of Australia, including the arid (< 250 mm annual rainfall) and semi-arid interior (<500 mm annual rainfall) as well as higher rainfall areas, including the tropical north of the continent where mean annual rainfall can exceed 1,200 mm [35][36][37].…”

“…The sensitivity of growth to inter-annual variation in rainfall amount is weakest in the most mesic biome we examined (i.e., the wet-tropical KOR site had the lowest R 2 and slope); however, the variance explained (R 2 ) is within the range that has been found for trees growing in mesic biomes elsewhere (>1,700 mm; e.g., [61][62][63]). The relatively weak R 2 at KOR may be partially explained by our selection of a short 4-month 'growing season' Mar-Jun), which potentially doesn't fully capture the rainfall period that is most important to growth in the wet tropics; that is, rainfall in the transitional months both before (Sep-Nov) and after (Mar-Jun) the main wet season (Fig 2A ; see also [31]). Including the pre-wet season months (Sep-Nov) in 'growing season' rainfall totals for KOR does improve the variance in RWI explained by rainfall amount (by 8%), but does not significantly alter the negative-asymmetric shape or slope of the relationship between tree growth and rainfall amount (S4 Fig) . In addition, our finding, along with others [24,31,64,65] that the seasonal timing, duration and frequency of rainfall are as important (perhaps more so) for driving growth of C. columellaris as the total amount of rainfall in the wet tropics of Australia (>1,200 mm annual rainfall) may also partially explain the relatively weak relationship between growth and rainfall amount.…”

“…Plant productivity and growth is primarily limited by water availability across the vast majority of the Australian continent [ 29 , 30 ]. Accordingly, rainfall amount is a strong driver of patterns in the secondary growth of our target species, Callitris columellaris across its range (e.g., [ 31 – 34 ]). C .…”

Introductory Chapter: Australia—A Land of Drought and Flooding Rain

“…Furthermore, the reconstructions display wet and dry periods of greater duration than evident from the instrumental record. Other recent studies using tree rings to reconstruct rainfall include Allen [59] for Arnhem Land in the Northern Territory of monsoonal Australia, and O'Donnell for Western Australia [60]. Evidence is also provided from sediments from northwest Australia periods of extreme flooding and drought over past 2000 years [61].…”