Lauren T. Bennett

Growth and nutrient content of perennial grasslands following burning in semi-arid, sub-tropical Australia

Fire often increases the productivity of perennial tussock grasslands inmesic environments but can reduce growth for one or more growing seasons inaridand semi-arid environments. We examined effects of single-burns on growth andnutrient content of grasslands in sub-tropical, northwestern Australia. Thesegrasslands were dominated by Themeda triandra, a speciesoften managed by regular burning in wetter temperate and tropical zones. Burnswere in the late dry season and were replicated using small plots (5 ×5-m) within fenced areas at two sites.Total projective cover and aboveground biomass were significantly less in burntplots relative to controls for 2.5 years after burning despite four growingseasons, including the first summer, of above-average rainfall. Recovery ofburnt plots was hindered by an extended dry period in the second year,demonstrating that rainfall in subsequent seasons can be as important asrainfall in the first season in determining post-burn productivity ofgrasslandsin semi-arid environments. Greater decreases in grass cover in burnt plotsduring the extended dry period may have been due to less standing dead andlitter than controls, and therefore less insulation from extreme summertemperatures, although relationships between cover changes and cover at thestart of the period were weak. With the exception of increased pH near grasstussocks, burning had little effect on chemical characteristics of surfacesoilsin the first week. Concentrations of N, and particularly P, in abovegroundplantmaterial were greater in burnt plots four months after burning, followingsummerrains, but were either less than or similar to those in controls withincreasingly dry conditions. Significantly lower concentrations of P in greenfoliage from burnt plots during dry seasons, when uptake from soil pools wouldbe minimal, indicated that burning decreased P retranslocation from plantstores. However, we found no evidence that single-burns increased nutrientlimitations to growth because plant contents of N and P were comparable inburntand control plots during periods of adequate water supply. Our data supportprevious generalizations that prescribed burning of perennial tussockgrasslandsin semi-arid environments is mostly unnecessary because putative benefits ofincreased productivity and forage quality, characteristic of more mesicenvironments, were not realized.

Close-range vertical photography for measuring cover changes in perennial grasslands.

We describe a method of close-range vertical photography and digital image analysis for measuring changes in total projective cover in perennial tussock grasslands of semi-arid Australia. Repeated photographs of permanent plots (1 m 2 ) were classified using supervised image analysis, providing a clear and objective record of the effects of single-burns on grass cover relative to controls. Computer simulations of the photographic set-up indi cated that errors due to camera perspective were consistently less than 4% across a range of cover classes. Other errors, including misclassification error, were not quantified because simplified laboratory tasks indicated that conventional field methods, such as point sampling and visual estimation, provided unreliable esti mates of grass cover and were therefore not suitable benchmarks for assessing the photographic method. However, the presented data indicate that the photographic method was sufficiently accu rate and precise to measure treatment effects over time and to elucidate relationships between independent growth parameters across a range of cover conditions. In addition, the photographic method was inexpensive, involved minimal field time, and utilised commercial software to classify images. It has the clear advantage over more traditional methods of providing outputs that are readily archived for retrospective studies.

The response of growth and foliar nutrients to fertilizers in young Eucalyptus globulus (Labill.) plantations in Gippsland, southeastern Australia

Abstract Fertilizer trials, which included rates of application of up to 400 kg ha−1 N, 200 kg ha−1 and 200 kg ha−1 K, were established in experimental plantations of E. globulus at three sites in Gippsland, southeastern Australia. These sites cover a range of annual rainfall and soil fertility, from 620 mm on a deep infertile sand, to 1000 mm on a rich gradational clay loam. Early growth at all sites was significantly increased by the addition of fertilizers and by age 4 years was consistently best at the highest combined rates of N and P. Foliar N and P concentrations were significantly increased at all sites by combined additions of N and P at Age 1, but were insensitive to treatment at Ages 2 and 4. Relationships between foliar nutrient concentrations and growth were site dependent. Correlations between foliar nutrients and growth in the same year were strongest at Age 1. Foliar nutrients at Age 1 were also strongly associated with growth at later ages at all sites. The best growth was associated with foliar concentrations at Age 1 of about 2.5% N and 0.23% P. Additions of P, alone or with N, consistently decreased foliar N P ratios at Age 1 to between 11 and 12 whereas N and P additions resulted in foliar N P ratios of 15 to 16 at Age 4. These ratios indicate a greater requirement for P than N in the first year suggesting that the N P ratio of fertilizers should be 1:1 within the first year, increasing to 2:1 in later applications. General recommendations for fertilizer addition in the routine establishment of E. globulus are presented. Mean tree volumes at Age 4 ranged from 0.014 to 0.019 m3 in control treatments and from 0.031 to 0.055 m3 at the highest rate of fertilizer addition. Growth responses to fertilizers were greatest, in both absolute and relative terms, at the most fertile site and appeared to be constrained by unfavourable soil texture and limited water availability at the other sites. Because the responses of E. globulus are site specific, detailed assessments of fertilizer requirements on soil types other than those described here require the establishment of further trials. A schedule for the establishment and monitoring of such trials is proposed.

Too much, too soon? A review of the effects of increasing wildfire frequency on tree mortality and regeneration in temperate eucalypt forests

In temperate Australia, wildfires are predicted to be more frequent and severe under climate change. This could lead to marked changes in tree mortality and regeneration in the region’s predominant eucalypt forests, which have been burned repeatedly by extensive wildfires in the period 2003–14. Recent studies have applied alternative stable state models to select ‘fire sensitive’ forest types, but comparable models have not been rigorously examined in relation to the more extensive ‘fire tolerant’ forests in the region. We review the effects of increasing wildfire frequency on tree mortality and regeneration in temperate forests of Victoria, south-eastern Australia, based on the functional traits of the dominant eucalypts: those that are typically killed by wildfire to regenerate from seed (‘obligate seeders’) and those that mostly survive to resprout (‘resprouters’). In Victoria, over 4.3 million ha of eucalypt forest has been burned by wildfire in the last decade (2003–14), roughly equivalent to the cumulative area burned in the previous 50 years (1952–2002; 4.4 million ha). This increased wildfire activity has occurred regardless of several advancements in fire management, and has resulted in over 350 000 ha of eucalypt forest being burned twice or more by wildfire at short (≤11 year) intervals. Historical and recent evidence indicates that recurrent wildfires threaten the persistence of the ‘fire sensitive’ obligate seeder eucalypt forests, which can facilitate a shift to non-forest states if successive fires occur within the trees’ primary juvenile period (1–20 years). Our review also highlights potential for structural and state changes in the ‘fire tolerant’ resprouter forests, particularly if recurrent severe wildfires kill seedlings and increase tree mortality. We present conceptual models of state changes in temperate eucalypt forests with increasing wildfire frequency, and highlight knowledge gaps relating to the development and persistence of alternative states driven by changes in fire regimes.

The effects of fertilizers on early growth and foliar nutrient concentrations of three plantation eucalypts on high quality sites in Gippsland, southeastern Australia

Abstract Fertilizer trials of a standard design were established in experimental plantations of Eucalyptus regnans, Eucalyptus nitens and Eucalyptus globulus on sites of high potential productivity in Gippsland, southeastern Australia. The design involved factorial combinations of three rates of N (up to 400 kg ha −1 elemental) and four rates of P (up to 200 kg ha −1 ) with four additional treatments including K and trace elements. Treatments were complete after up to four applications at 2, 9, 14 and 26 months post-planting. P additions significantly increased foliar P concentrations and growth of E. regnans to 45 months but poor survival and competition from woody weeds limited growth responses to treatments. Foliar concentrations of N and P in E. nitens and E. globulus at age 1 year were significantly increased by N and P additions and were positively correlated with both early and later growth. Added P significantly decreased the foliar N to P ratios of all species at age 1 year, indicating that P additions, in particular, were required for good early growth. Combined additions of N and P significantly increased growth of E. nitens in the first year but growth responses to further additions of fertilizer were minimal. In contrast, growth of E. globulus increased in response to all fertilizer additions, leading to the greatest mean volumes at the highest combined rates of N and P. Relative gains in productivity were therefore greater in E. globulus (m.a.i. range at 71 months: 10–24 m 3 ha −1 year −1 ) than E. nitens (16–33 m 3 ha −1 year −1 ). There were no additional growth responses of any species to added K or trace elements. However, concentrations of K in E. regnans foliage at age 1 year were increased by K additions and were correlated with later growth. Low concentrations of Mg in E. nitens foliage suggest a probable response to Mg additions. General fertilizer prescriptions for the establishment phase of eucalypt plantations on sites of high quality are presented. Refinement of these prescriptions will rely on a better understanding of the nutritional requirements of individual species.

Repeated prescribed fires decrease stocks and change attributes of coarse woody debris in a temperate eucalypt forest

Previous studies have found negligible effects of single prescribed fires on coarse woody debris (CWD), but the cumulative effects of repeated low-intensity prescribed fires are unknown. This represents a knowledge gap for environmental management because repeated prescribed fires are a key tool for mitigating wildfire risk, and because CWD is recognized as critical to forest biodiversity and functioning. We examined the effects of repeated low-intensity prescribed fires on the attributes and stocks of (fallen) CWD in a mixed-species eucalypt forest of temperate Australia. Prescribed fire treatments were a factorial combination of two seasons (Autumn, Spring) and two frequencies (three yearly High, 10 yearly Low), were replicated over five study areas, and involved two to seven low-intensity fires over 27 years. Charring due to prescribed fires variously changed carbon and nitrogen concentrations and C to N ratios of CWD pieces depending on decay class, but did not affect mean wood density. CWD biomass and C and N stocks were significantly less in Fire than Control treatments. Decreases in total CWD C stocks of -8 Mg/ha in Fire treatments were not balanced by minor increases in pyrogenic (char) C (-0.3 Mg/ha). Effects of prescribed fire frequency and season included significantly less C and N stocks in rotten CWD in High than Low frequency treatments, and in the largest CWD pieces in Autumn than Spring treatments. Our study demonstrates that repeated low-intensity prescribed fires have the potential to significantly decrease CWD stocks, in pieces of all sizes and particularly decayed pieces, and to change CWD chemical attributes. CWD is at best a minor stock of pyrogenic C under such fire regimes. These findings suggest a potential trade-off in the management of temperate eucalypt forests between sustained reduction of wildfire risk, and the consequences of decreased CWD C stocks, and of changes in CWD as a habitat and biogeochemical substrate. Nonetheless, negative impacts on CWD of repeated low-intensity prescribed fires could be lessened by fire intervals of 10 rather than three years (to decrease losses of decayed CWD), and fires in moist rather than dry conditions (to conserve large CWD).

Interactive effects of high irradiance and moderate heat on photosynthesis, pigments, and tocopherol in the tree-fern Dicksonia antarctica

Effects of high irradiance and moderate heat on photosynthesis of the tree-fern Dicksonia antarctica (Labill., Dicksoniaceae) were examined in a climate chamber under two contrasting irradiance regimes (900 and 170 µmol photons m-2 s-1) and three sequential temperature treatments (15°C; 35°C; back to 15°C). High irradiance led to decline in predawn quantum yield of photochemistry, Fv/Fm (0.73), maximal Rubisco activity (Vcmax; from 37 to 29 µmol m-2s-1), and electron transport capacity (Jmax; from 115 to 67 µmol m-2 s-1). Temperature increase to 35°C resulted in further decreases in Fv/Fm (0.45) and in chlorophyll bleaching of high irradiance plants, while Vcmax and Jmax were not affected. Critical temperature for thylakoid stability (Tc) of D. antarctica was comparable with other higher plants (c. 47°C), and increases of Tc with air temperature were greater in high irradiance plants. Increased Tc was not associated with accumulation of osmotica or zeaxanthin formation. High irradiance increased the xanthophyll cycle pigment pool (V+A+Z, 91 v. 48 mmol mol-1 chlorophyll-1), de-epoxidation state (56% v. 4%), and α-tocopherol. Temperature increase to 35°C had no effect on V+A+Z and de-epoxidation state in both light regimes, while lutein, β-carotene and α-tocopherols increased, potentially contributing to increased membrane stability under high irradiance.

Effects of sudden exposure to high light levels on two tree fern species Dicksonia antarctica (Dicksoniaceae) and Cyathea australis (Cyatheaceae) acclimated to different light intensities

We examined the responses of two tree fern species (Dicksonia antarctica and Cyathea australis) growing under shade or variable light (intermittent shade) to sudden exposure to high light levels. Steady-state gas exchange as well as dynamic responses of plants to artificial sunflecks indicated that difference in growth light environment had very little effect on the tree ferns’ capacities to utilise and acclimate to prevailing light conditions. Two weeks of exposure to high light levels (short-term acclimation) led to decreases in all photosynthetic parameters and more negative predawn frond water potentials, mostly irrespective of previous growth light environments. After 3months in high light levels (long-term acclimation), D. antarctica fully recovered, while C. australis previously grown under variable light, recovered only partially, suggesting high light level stress effects under the variable light environments for this species.

Evaluation of anion exchange membranes to estimate bioavailable phosphorus in native grasslands of semi‐arid Northwestern Australia

Abstract Anion exchange membranes (AEMs) were used to assess the P status of semi‐arid sub‐tropical soils of high P sorption capacity from the Pilbara region in northwestern Australia. We determined the most appropriate procedure for using AEMs in these soils using a factorial of extraction ratios and shaking times and compared the method with extraction by water. Significantly more inorganic P (Pi) was extracted by the membranes (AEM‐Pi) than by water, and the amount extracted increased with extraction time but was generally independent of the extraction ratio. Maximum AEM‐Pi was 3.61 μg g‐1 after eight hour extraction. The AEM procedure was compared with traditional extraction procedures using 0.5 M sodium bicarbonate (NaHCO3) and 0.1 M sodium hydroxide (NaOH) to assess ability to detect spatial heterogeneity. The amount of Pi extracted decreased in the order: AEM>NaOH>NaHCO3* The AEM method detected a significant effect of depth on Pi (P=0.0001), while the NaOH method detected both site and treatment e...

Ecosystem Services in Agricultural Landscapes: A Spatially Explicit Approach to Support Sustainable Soil Management

Soil degradation has been associated with a lack of adequate consideration of soil ecosystem services. We demonstrate a broadly applicable method for mapping changes in the supply of two priority soil ecosystem services to support decisions about sustainable land-use configurations. We used a landscape-scale study area of 302 km2 in northern Victoria, south-eastern Australia, which has been cleared for intensive agriculture. Indicators representing priority soil services (soil carbon sequestration and soil water storage) were quantified and mapped under both a current and a future 25-year land-use scenario (the latter including a greater diversity of land uses and increased perennial crops and irrigation). We combined diverse methods, including soil analysis using mid-infrared spectroscopy, soil biophysical modelling, and geostatistical interpolation. Our analysis suggests that the future land-use scenario would increase the landscape-level supply of both services over 25 years. Soil organic carbon content and water storage to 30 cm depth were predicted to increase by about 11% and 22%, respectively. Our service maps revealed the locations of hotspots, as well as potential trade-offs in service supply under new land-use configurations. The study highlights the need to consider diverse land uses in sustainable management of soil services in changing agricultural landscapes.