Higher nutrient-levels in tree rings of Eucalyptus grandis following a wildfire

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Eucalypts, the dominant species in the WSF, possess a mobile sapwood pool, whereby nutrients are added through nutrient uptake and subsequently drawn on to maintain growth. At the sapwood-heartwood boundary nutrients are re-translocated into the living cells. In this way, nutrients, which would otherwise soon become a limiting growth factor, are recycled
  Source: Heinrich I, Gärtner H, Monbaron M, Schleser G (eds.) (2006) TRACE - Tree Rings in Archaeology, Climatology and Ecology, Vol. 4: Proceedings of the DENDROSYMPOSIUM 2005, April 21st – 23rd 2005, Fribourg, Switzerland. Schriften des Forschungszentrums Jülich, Reihe Umwelt Vol. 61, p. 300 – 307. Higher nutrient-levels in tree rings of Eucalyptus grandis  following a wildfire I. Heinrich   Department of Geosciences, Geography, University of Fribourg, 1700 Fribourg, Switzerland Email: Introduction  Wet sclerophyll forest describes a narrow band of vegetation which, in northeast Queensland, occurs on the western edge of the rainforest above 600m. These forests grow in areas of high rainfall where rainforest would normally proliferate but the use of fire prevents the rainforest from invading. Scientists have documented a 50% loss of this forest type during the previous 50 years and many are worried that a complete take-over by rainforest could lead to the extinction of certain plant and animal species. With new evidence that rainforests are invading the wet sclerophyll forests at an alarming rate, scientists are assessing the impact of rainforest invasion on the wet sclerophyll flora and fauna. The wet sclerophyll forest (WSF) of Far North Queensland describes a narrow band of vegetation stretching as a long narrow band on the northern tablelands 50 km inland parallel to the coast between Townsville and Cooktown adjacent to the western margin of the upland tropical rainforests above 600 m (Tracey and Webb 1975, Tracey 1982, Harrington and Sanderson 1994). These forests grow in areas of high rainfall where rainforest would normally proliferate but the use of fire by absrcinal people has prevented the rainforest from invading. Approximately 50% loss of this forest type during the previous 50 years have been documented (Harrington et al. 2000) which is of major concern to the local management authorities as the WSF has been part of the UNESCO World Heritage property Wet Tropics of Queensland since 1988. The cessation of absrcinal burning practices and an increase of non-absrcinal land use by the end of the 19 th  century paved the way for the invasion of rainforest into the WSF (Fautz 1984, Kohen 1995). Fire is also a vital part of the tall sclerophyll forests in the southern subtropical to temperate climate zones of Australia, which are in many aspects similar to the tropical WSF (Ashton and Martin 1996a, 1996b). Chambers and Attiwill (1994) ascribed the ash-bed effect in Eucalyptus regnans  F. Muell. forests mainly to an increase in the availability of nitrogen and phosphorus. Eucalypts, the dominant species in the WSF, possess a mobile sapwood pool, whereby nutrients are added through nutrient uptake and subsequently drawn on to maintain growth. At the sapwood-heartwood boundary nutrients are re-translocated into the living cells. In this way, nutrients, which would otherwise soon become a limiting growth factor, are recycled. This seems to be one of the main reasons for the adaptation of eucalypts to poor soils (Banks 1982). According to this concept of eucalypts adapted to growing on nutrient-poor soils, trees experiencing a post-fire ash-bed effect would not only exhibit a    301 higher nutrient level in the tree ring after the fire but in the whole sapwood usually consisting of several tree rings. More nutrients become available, and for a period of time when more nutrients are present the tree would not re-translocate nutrients as effectively as without the ash-bed effect and as a consequence an unusually high level of nutrients would remain in the dead cells of the most recently built heartwood ring. Thus the ash-bed effect, according to this “sapwood displacement effect” (Banks 1982), would not be detected in the tree ring directly formed in the year after the fire but in the heartwood cells created at that time. The extent of the displacement depends on the number of sapwood tree rings, which vary within and between species. In his PhD thesis, Banks (1982) showed an increased ring width following a fire, and that this growth flush lasted five or more years in eucalypts. Chemical analysis of nutrient levels in pre- and post-fire tree-rings sometimes resulted in higher levels of some of the nutrients examined. He hypothesised that nutrient uptake associated with the ash-bed effect could be one of the factors involved. It has been demonstrated several times that the ash-bed effect only lasts approximately one year, and the nutrient level returns to pre-fire conditions quickly (Renbuss et al. 1973, Grove   et al. 1986, Adams and Attiwill 1991a,b, Ashton and Martin 1996a, Attiwill et al.   1996). Since fire may be a key factor for the expansion of rainforest into the WSF, Unwin et al.   (1985), Stocker and Unwin (1986), Ash (1988), Unwin (1989), Harrington and Sanderson (1994) and Harrington et al. (2000) emphasise the need for more studies on the historical fire regime in that area. Furthermore, effective long-term management of tropical rainforests should always include the nearby sclerophyll forests as part of the whole ecological system (Janzen 1988). Therefore, the current study aimed to test a method useful for identification and dating of past fires by chemically analysing core samples of Eucalyptus grandis  W. Hill ex Maiden (rose gum) for nutritional elements. Materials and methods The study site is located on a northern slope of Mt. Haig, (145° 36' E, 17° 06' N), Atherton Tablelands, altitude 1200m asl. An almost pure stand of E. grandis  with a shrub-layer consisting of sclerophyll plants and young rainforest-trees had developed adjacent to rainforest (Harrington and Sanderson 1994). A controlled intensive fire burned the site in October 1996 killing the rainforest undergrowth. After the fire, only a grass layer consisting mainly of Themeda triandra  Forssk., and some tussock sedges, for example Gahnia aspera (R.Br.) Spreng. and Gahnia sieberiana Kunth developed. Because large parts of the wet tropics in Far North Queensland are listed in the World Heritage cutting trees for cross sections was not an option but only two core samples per tree were extracted from 20 E. grandis stems at breast height. All tree size-classes ranging from 20 to 105 cm were included in the collection. The surface of each core was cut with a knife instead of using sand paper creating a smooth surface with visible rings but without contamination from dust particles. The samples were placed on holding devices without gluing them and the rings marked on the holder. The cores were oven-dried at 60°C for five days, cut at the boundary of each ring, ground to <0.2 mm and chemically analysed    302 for phosphorus (P) and nitrogen (N). Since eleven of the trees sampled did not supply enough ground material only nine E. grandis  core samples were analysed. The single digestion method of Anderson and Ingram (1989) was used. Nitrogen was determined colorimetrically by the salicylate-hypochlorite method of Baethgen and Alley (1989), and phosphorus by an adaptation of Murphy and Riley`s (1967) single solution method (Anderson and Ingram 1989).  Results The samples exhibited relatively distinct tree rings in the parts formed in recent years. The wood is diffuse to semi ring porous with vessels decreasing in size towards the end of the growing season. In some years a small vessel free zone appears as a dark band within two tree rings probably marking an unusually dry season. Wedging rings and kino veins, large resin ducts caused by stress such as fire and drought, are typical features of this species. While wedging rings often hindered the identification of annual rings, kino veins were used as markers to identify event years and hence occasionally helped during the visual crossdating procedure. Table 1: Nitrogen- and Phosphorus-levels in tree rings of E. grandis formed before (1996) and after the fire (1997) Tree Size N ( μ g/g) P ( μ g/g) No dbh 1996 1997 1996 1997 1 80cm 714.7 802.8 19.5 21.6 2 25cm 1045 1636 30.1 52 3 30cm 847 2184 19.7 40.5 4 50cm 1033.1 2932.2 50.6 79 5 100cm 567.7 1576.2 30 73.6 6 25cm 401.9 1272.2 40.4 42 7 60cm 797.4 915.2 25.3 47 8 30cm 647.6 726.5 23.7 31.3 9 40cm 622.6 980.3 21 22 Only the tree rings before and after the fire event located within the sapwood were analysed because their boundaries were most distinct in these outer parts of the samples. In table 1 the results of the chemical analysis are presented. For each tree nitrogen (N) and phosphorus (P) concentrations are presented for the year before and after the fire. All trees show an increase in both, N and P levels in the 1997 tree ring, the year after the fire with some trees exhibiting a near threefold increase in N (trees 3, 4, 5 and 6) but less distinct for the P-levels. No specific trends regarding age and size of the trees are recognisable.    303 Discussion and conclusions  As noticed in other studies (Bamber et al.   1969, Mucha 1979, Banks 1982, Brookhouse 1997, Smith 1997) the usage of disc samples is desirable when tree-ring analysis of Eucalypts is conducted. This was confirmed in this study with core samples unable to pick up false rings due to wedging ring patterns. However, these features can be detected in disc samples and then avoided by choosing the most appropriate radii. For the current study this problem was circumvented by examining only the most recent rings but for further investigations of longer periods cross sections are indispensable. The results indicate that E. grandis  has experienced an ash-bed effect directly after an in-tense fire. In the years before the 1996 fire a dense understorey consisting of sclerophyll and rainforest species had developed on the relatively fertile Mt Haig study site. Under those circumstances there would be an increase in soil fertility due to an increase in nitro-gen fixing understorey plants, a change of the litterfall composition, a wetter micro-climate and a higher decomposition rate (O`Connell et al. 1981). Under the exclusion of fire the invasion of rainforest species into the site, according to Webb (1969), would also be an indication of the increased fertility of the soil. Negi and Sharma (1996) have proposed two different eucalypt “ideotypes” regarding the efficiency of the re-translocation mechanism depending on the soil conditions. On fertile soils eucalypt species only partially re-translocate while on poor soils they possess a more efficient re-translocation mechanism. A similar suggestion comes from Keith (1997) who found a subgeneric difference in adaptation to soil nutrient concentrations and nutrient utilization. The two scenarios have been illustrated in figure 1. In one case there would be an opulent ash-bed uptake of nutrients on more fertile soils with a consequent waste of nutrients being not re-translocated from dying tissue such as old leaves and at the heartwood-sapwood boundary resulting in a reduced re-translocation of the nutrients from the heartwood to sapwood (Fig. 1, bottom). In contrast, the other ideotype would occur on poor soils when the level of nutrients stored in the sapwood was not as high as in the first case. Those trees would not waste any nutrients but re-translocate most of them to ensure ongoing growth and survival in the harsh Australian environment (Fig. 1, top). Mulligan and Sands (1988) found that species adapted to low-nutrient soils had slower growth rates and stored as many nutrients as possible to enable longer survival on experimental soils with extremely low nutrient supplies. The results presented here suggest that E. grandis at Mt. Haig, one of the fastest growing tropical Eucalypt species usually occurring on more fertile soils belongs to the bottom group in figure 1 because the nutrient levels in the tree-rings are higher in the year directly after the fire. It seems to have only a restricted ability to adapt to low nutrient supply or lost it gradually by adapting to more humid and fertile conditions. If E. grandis  belonged to the ideotype growing on poor soils, one would expect little to no change of nutrient levels in the tree ring after the fire event.
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