Dynamics of necromass in woody Australian ecosystems

Abstract

Litterfall (LF) is the major contributor to aboveground necromass in ecosystems. Litter decomposition or litter decay (LD) then offsets deposition in LF, with the balance of LF and LD determining the standing litter (SL). SL together with fine and coarse woody debris (FWD, CWD) are the largest necromass pools. The interactions of LF, SL, and LD at continental scales reflect carbon and nutrient cycling and other ecosystem processes. We compiled data on leaf, twig (<2.6 cm), and other material (mostly bark and reproductive tissue) for SL and LF for the fire-prone Australian continent, where SL is also a major "fuel load" and important for fire spread and fire intensity. We extracted data from 498 published and unpublished works (1825 LF observations; n SL = 3914; n LD = 629). We used Olson's (mass-balance) approach (k˜LF/ SL) to calculate LD for sites long undisturbed with both LF and SL data. We compiled LF and SL by component (leaves, twigs, other material) and metainformation such as sampling location, tree species, or time since fire from literature and/or scientists. Most data were available from warm-seasonal (36% for SL) and cool-wet (31%) climates, linking the locations of our data with a bio-climate classification. Warm-wet (20%) and hot-seasonal (8%) climates followed, while other climate zones each contributed <2% of the data. Across all climatic zones, average SL (1100 g/m 2 ) was roughly twice that of LF (468 gÁm −2 Áyr −1 ). SL was greatest in cold climates (2334 g/m 2 ), compared to warm-wet (1168 g/m 2 ) and hot-seasonal conditions (499 g/m 2 ). Important drivers of SL are LD (e.g., slow under cold conditions) and fire frequency. Olson's k varied with type of decomposing material ("composition"). For example, across the continent, k˜1.942 yr −1 for leaves but was 0.504 yr −1 for twigs. SL varied strongly in composition according to climate type (e.g., seasonal vs. wet climates). Robust models of necromass dynamics must distinguish between the litter components (such as leaves and twigs) and consider the complex and non-linear effects of climate, stand structure, and stand history on litterfall and decomposition.

Litter accumulation and fire risks show direct and indirect climate-dependence at continental scale

“…c Mass of leaf litter or leaf litterfall for forests and woodlands in rank order of leaf litter mass. Base map after Neumann et al 69 prepared using R software 73 . …”

“…Another classification is ‘bioclimatic zones’ (e.g. 69 ), while a third, often-used grouping is based on genetics of the dominant eucalypts (e.g. 70 ).…”

Fuel loads and fuel structure in Austrian coniferous forests

“…Litter and woody dead material smaller than 6 mm constitute the 1-h fuels. However, we kept litter separated from dead woody material, owing to the different origin and turnover rates of these two fuel components (Bradford et al 2016;Neumann et al 2021). The 1000-h fuels (>7.5 cm diameter, coarse woody debris, CWD) were sampled along two 50-m transects, following the line intersect method (Van Wagner 1968).…”

“…We did not sample duff or F + H layers. Destructive sampling is laborious and most accurate but unpractical for large fuel components (Keane 2015;Neumann et al 2021). CWD volume can be efficiently determined with transects, where the larger area compensates for the commonly heterogenous distribution of CWD in forests (Harmon et al 1986;Woldendorp et al 2004).…”

“…One of the widely used management strategies to modify fuel loads and reduce fire hazard is prescribed burning, as time since last fire is the most important factor explaining fuel loads (e.g. Schimmel and Granstrom 1997;Stephens 1998;Curt et al 2013;Neumann et al 2021). We were not able to determine exactly when our chosen sites were burnt last; thus, we considered them to be unburnt for >30 years.…”