Does low-intensity surface fire increase water- and nutrient-availability to overstorey Eucalyptus gomphocephala?

Abstract

The objective of this study was to investigate how the management practices of prescribed fire and understorey vegetation removal affect water and nutrient relations of old, yet prematurely declining Eucalyptus gomphocephala. Long unburnt sites were established in Yalgorup National Park, Western Australia, adjacent to frequently burnt state forest sites. Trees were allocated to vegetation clearing, prescribed fire or no prescribed fire treatments. Prescribed fire was achieved in only one long unburnt national park site so that the results were pseudoreplicated but analysed accordingly. Soil chemistry, plant nutrient availability and tree foliar carbon and nitrogen isotope ratio and nutrient concentration were investigated. No effects of vegetation clearing were found. Prescribed fire sites were associated with sky exposure and bare ground whereas no prescribed fire sites were associated with shrub and litter cover and litter depth. Foliar carbon isotope ratios were significantly more negative in prescribed fire, relative to no prescribed fire, treatments on long unburnt sites. Soil exchangeable Zn and Mn and plant available (estimated by charged resin beads) Mg were higher on prescribed fire, relative to no prescribed fire, long unburnt sites. Seedling bioassays indicated elevated P and Cu availability on prescribed fire, relative to no prescribed fire, treatments. In overstorey E. gomphocephala, foliar N levels were elevated (but not to excessive levels), and there was a trend toward elevated foliar Mn, in prescribed fire relative to no prescribed fire treatments on long unburnt sites. In the context of our large-scale pseudoreplicated case study, prescribed fire provided a pulse of water and N, (with some indications towards provision of elevated Mn, Cu and Mg) availability to E. gomphocephala in decline on sites with a history of a long absence of fire that may in part underpin observations of elevated tree health on sites that have a history of relatively frequent fire.

Effects of post-fire soil stabilisation techniques on trace elements lost by erosion

“…In agreement with most studies about fire effects on soil total and chemically extractable Mn (Chambers and Attiwill 1994;González Parra et al 1996;De Marco et al 2005;García-Marco and González-Prieto 2008;Close et al 2011;Smith et al 2011), we found higher extractable Mn concentrations in the burned soil (P , 0.005) and especially in sediments and ashes, than in the unburned soil. All these authors pointed out that such a post-fire increase was a result of the contribution of ash from burned vegetation and the physiochemical breakdown of Mn complexed with organic matter.…”

“…Consequently, at the end of the study, the amount of extractable Mn lost by erosion (182 g ha À1 in Control; Table 4) was 9-fold that in suspended sediments reported by Townsend and Douglas (2004) and accounted for 9-10% of the Mn supplied by ashes and around 4% of ash þ burned topsoil Mn reserves in Control and Seeding, with much lower percentages (0.7 and 0.3%) in Mulching. Fire increases the availability of Zn in soils (García-Marco and González-Prieto 2008; Close et al 2011;Stankov Jovanovic et al 2011) and the same was true in the present study for ashes and sediments (P , 0.005), the effects lasting for at least 1 year Johnson et al 2000). Most of the variance of Zn concentration in sediments was explained by a curvilinear inverse regression model with the accumulated precipitation (Table 3).…”

“…In the present study, the extractable Zn lost by erosion (30 g ha À1 in Control) accounted for 13.6 and 3.1% of extractable Zn in ashes and ash þ burned topsoil; these important losses were only significantly reduced (by a factor of six) in the Mulching treatment. Post-fire Cu dynamics were similar to that of Zn: its availability increased (García-Marco and González-Prieto 2008; Close et al 2011;Stankov Jovanovic et al 2011) and sediments remained enriched in Cu compared with the burned topsoil for at least 1 year. However, the fast and sudden changes of Cu availability in the initial sediments collected (from higher than in ashes in the first erosion event to 2 orders of magnitude lower in the second erosion event) were only observed for this element and are likely related to the evolution of pH (r ¼ 0.609; P , 0.001) and also to the accumulated precipitation, although Zhan et al (1996), Menzies and Gillman (2003) and Gafur et al (2004) reported that Cu is leached slowly from ash.…”

“…In Seeding and Control plots, the Fe eroded with sediments during the first post-fire year was less than 1% of the ash þ burned topsoil Fe pool, but it might represent 30-47% of the extractable Fe in ashes. This could explain the depletion of soil-available Fe in these plots (Gómez-Rey et al 2013) and in surface soils affected by recurrent prescribed fires (Pivello et al 2010;Close et al 2011). Post-fire reduction of available Fe could constitute a fertility problem in soils with low levels of plant-available Fe (García-Marco and González-Prieto 2008).…”

“…Significant post-fire decrease of exchangeable (Groeschl et al 1993), water-soluble (Pereira et al 2011) and available Fe (García- Marco and González-Prieto 2008;Ponder et al 2009;Pivello et al 2010;Close et al 2011) has been usually reported. In the same way, compared with the unburned soil, the extractable Fe decreased slightly in the burned soil and significantly in ashes (P , 0.05) after our experimental fire.…”