Grasstree stem analysis reveals insufficient data for inference of fire history

Grinding back dead leaf bases on the stems of arborescent grasstrees (Xant7wrrlwea spp.) reveals a pattern of horizontal bands that has been interpreted as a record of the fire history experienced by the plant. The validity of this fire history record has previously been assessed through comparison of 100 grasstree stems sampled from shrubland near Eneabba in Western Australia against a 30 year fire history determined from satellite imagery. This analysis showed that the two records matched more than would be expected by chance, but concluded that the interpretation of the grasstree record as a fire history was not warranted as most of the grasstree fire records did not match satellite fire records. A second analysis of the same two sets of records, published in this journal, also showed that the records matched more than by chance, but concluded that the interpretation of grasstree banding as fire history was valid, though it failed to quantify the strength of this agreement. Here we examine' the approaches and interpretations of the two previously published studies, and provide new analyses to refine estimates of the amount of fire-related data present in the grasstree record. We show that only ~20% of grasstree 'fire' records may be attributable to fire. With eight out of ten of records not attributable to fire, we confirm that the grasstree record in its current form cannot be interpreted as fire history, and therefore claims of the grasstree technique to support management actions are untenable

Fire behaviour in south-western Australian shrublands: Evaluating the influence of fuel age and fire weather

Fuel age (time since last fire) is often used to approximate fire hazard and informs decisions on placement of shrubland management burns worldwide. However, uncertainty remains concerning the relative importance of fuel age and weather conditions as predictors of fire hazard and behaviour. Using data from 35 experimental burns across three types of shrublands in Western Australia, we evaluated importance of fuel age and fire weather on probability of fire propagation (hazard) and four metrics of fire behaviour (rate of spread, fireline intensity, residence time, surface temperature) under moderate to high fire danger weather conditions. We found significant support for a threshold effect of fuel age for fire propagation but limited evidence for an effect of fuel age or fire weather on rates of spread or fireline intensity, although surface heating and heating duration were significantly related to fuel age and shrubland type. Further analysis suggested that dead fuel mass and accumulation rate rather than live fuels were responsible for this relationship. Using BEHAVE, predicted spread rates and intensities were consistently lower than observed values, suggesting further refinement is needed in modelling shrubland fire behaviour. These data provide important insight into fire behaviour in globally significant, fire-adapted shrublands, informing fire management and relationships between fire frequency and fire intensity

Fire interval effects on persistence of resprouter species in Mediterranean-type shrublands

The capacity of many plant species to resprout in fire-prone shrublands is thought to engender persistence, yet management concerns exist for the long-term persistence of some resprouting species given anthropogenic impacts including shortened fire intervals, long periods of fire exclusion, and/or fires of increasingly high severity. We explored the potential demographic effects of different fire interval regimes on lignotuberous resprouter species using the last fire interval for 36 sites (33 experimental fires, 3 wildfires) in biodiverse SW Australian shrublands, spanning an interval range of 3-42 years. Mortality and regrowth 1 year following the last fire was assessed for > 7,000 tagged individuals from 20 shrub and sub-shrub species. Using generalized linear mixed effect models, we estimated the influence of fire interval (and selected fire and environmental covariates) on mortality and regrowth rates across all species, and individually for the four most common species. The overall model, as well as the models for three of the four most common species (Banksia attenuata, Melaleuca leuropoma, and M. systena, but not Hibbertia hypericoides) supported the hypothesis of increased mortality at short and long fire intervals, most likely due to total non-structural carbohydrate (TNC) and bud-bank limitation, respectively. However, no relationship between regrowth rate and fire interval was detected, suggesting that increased mortality at short (3-5 year) fire intervals may not be due solely to resource (TNC) limitation. Results show that lignotuberous resprouters are potentially vulnerable to population decline through attrition of mature plants under both shortened and lengthened fire interval regimes

Demography of the long-lived conifer Agathis ovata in maquis and rainforest, New Caledonia

The endemic New Caledonian conifer Agathis ovata occurs as an emergent tree in fire-prone shrublands (maquis), and fire-sensitive rainforest. Growth, survivorship and recruitment over 5 yr were compared for populations from forest and maquis on ultramafic substrates in New Caledonia to investigate whether demographic behaviour varied in response to the strongly contrasting forest and shrubland environments. Growth of seedlings and of small (30–100 cm height) and large (100 cm height; 5 cm DBH) saplings was slow, but varied significantly among stages, site types and years. The greatest difference in growth rates was among stages, seedlings growing 0.34 cm.yr−1, small saplings 1.06 cm.yr−1 and large saplings 2.13 cm.yr−1. Tree DBH increased by only 0.05 cm.yr−1 and, based on these rates, individuals with DBH of 30 cm are estimated to be more than 700 yr old. Few trees (3.5%) produced cones in any year and seedling recruitment was low, but some recruitment was recorded each year in both maquis and forest. Rates of recruitment per parent were highest in forest (1.28.yr−1, cf 0.78.yr−1), but the higher density of trees in maquis meant that overall recruitment was greater there (92 ha−1.yr−1, cf 56 ha−1.yr−1). Seedling mortality ranged from 0.9 to 2.9% among years with no significant difference between maquis and forest. No sapling mortality was recorded, but annual tree mortality ranged from 0 to 1.4%. Evidence from a recently burned site indicated that while trees may survive fire, seedlings and saplings do not. Post-fire seedling recruitment per ha from surviving trees was four times lower than in unburned sites, but growth rates were four times higher. Similar demographic attributes, including high survivorship, low growth rate and low rates of recruitment over a long reproductive life, characterize Agathis ovata populations in both maquis and rainforest in New Caledonia and are indicative of a broad tolerance of light environments that is unusual among tree species. These demographic attributes help to explain the long-term persistence of the species in these strongly contrasting habitats

Livestock grazing impacts on desert vegetation, Khirthar National Park, Pakistan

The impact of livestock grazing on desert vegetation in Khirthar National Park, Pakistan, was investigated by comparing dry and wet season plant species composition, richness, cover, and a grazing index for quadrats outside ("open") and inside ("exclosed") native mammal breeding enclosures that had excluded livestock for 6 years. A total of 93 plant species were recorded in the dry season, 88 species in exclosed quadrats and 50 in open quadrats. While only 5 species were unique to open quadrats, 43 species were found only in the exclosed quadrats. Species richness was higher in the exclosures because of the presence of more grass and herb species, while grazing was higher in the open. After rain, species richness and cover were significantly higher than in the dry season because of the growth of summer ephemeral herbs and grasses, but richness was no longer different between the exclosure and open treatments. Although some herbaceous species may have been adversely affected by livestock grazing, overall species richness suggests strong ecosystem resilience to grazing, with levels no different after seasonal rains regardless of grazing level. Many grass and herb species absent from open sites during the dry season reappeared after rain, which suggests that livestock grazing may eliminate them as the dry season proceeds, but that a soil seed or bud bank persists

Environmental correlates of vegetation patterns and species richness in the northern Grampians, Victoria

Plant species cover-abundance and density data were collected for 94 sample plots across a gradient from rocky uplands to sandy outwash plains in the northern part of Grampians (Gariwerd) National Park in western Victoria. Detrended correspondence analysis (DCA) was used to identify dominant gradients in species composition. A range of static (e.g. substrate type, soil depth, microclimate indicators) and dynamic (e.g. elapsed time since last fire) environmental variables were measured. Correlations were sought between these variables and vegetation patterns including those for richness (R) and Shannon-Weiner diversity (H′). The dominant gradient of vegetation change identified by DCA separated rocky sites and sites near ephemeral streams, from well-drained, sandy sites. Secondary gradients identified time since last fire as important for sandy sites, and altitude and aspect-related microclimate for rocky sites. Diversity was highest in the first 2 years after fire but showed no further decline in older sites. Overall, R and H' were negatively correlated with soil nutrient concentrations. On sandy sites R was high, but was low on rocky sites and near streams. Within the rocky sites, R was highest on cool, moist south and east slopes, and lowest on hot, dry north and west slopes. Explanations of diversity patterns based on inhibition of competitive exclusion due to stress and recurrent disturbance best fit the results presented here

A comparison of methods for the statistical analysis of spatial point patterns in plant ecology

We describe a range of methods for the description and analysis of spatial point patterns in plant ecology. The conceptual basis of the methods is presented, and specific tests are compared, with the goal of providing guidelines concerning their appropriate selection and use. Simulated and real data sets are used to explore the ability of these methods to identify different components of spatial pattern (e.g. departure from randomness, regularity vs. aggregation, scale and strength of pattern). First-order tests suffer from their inability to characterise pattern at distances beyond those at which local interactions (i.e. nearest neighbours) occur. Nevertheless, the tests explored (first-order nearest neighbour, Diggle's G and F) are useful first steps in analysing spatial point patterns, and all seem capable of accurately describing patterns at these (shorter) distances. Among second-order tests, a density-corrected form of the neighbourhood density function (NDF), a non-cumulative analogue of the commonly used Ripley's K-function, most informatively characterised spatial patterns at a range of distances for both univariate and bivariate analyses. Although Ripley's K is more commonly used, it can give very different results to the NDF because of its cumulative nature. A modified form of the K-function suitable for inhomogeneous point patterns is discussed. We also explore the use of local and spatially-explicit methods for point pattern analysis. Local methods are powerful in that they allow variations from global averages to be detected and potentially provide a link to recent spatial ecological theory by taking the 'plant's-eye view'. We conclude by discussing the problems of linking spatial pattern with ecological process using three case studies, and consider some ways that this issue might be addressed

Seed production and germination in two rare and three common co-occurring Acacia species from south-east Australia

Seed set, size, viability and germination requirements were investigated for two rare (Acacia ausfeldii and A. williamsonii) and three common (A. pycnantha, A. genistifolia and A. paradoxa) co-occurring congeners in box-ironbark eucalypt forests near Bendigo, south-east Australia to investigate correlates of rarity. Seed size was significantly smaller for the two rare species and germinants were less able to emerge from deeper sowing depths than were the larger seeded common congeners. All species had a strong heat-stimulated germination response. While the rare A. ausfeldii showed strong germination only at the highest temperature treatment (100°C), the common and widespread A. pycnantha showed strong germination across a broad range of temperatures (60-100°C), likely to be experienced by soil-stored seeds during a fire. Seed viability, number of seeds per plant, and number of firm, aborted and eaten seeds per pod varied between species, but the pattern of variation was not related to rarity. Small seed size and a very specific temperature requirement for germination may help to explain rarity in A. ausfeldii, and to a lesser extent in A. williamsonii. Fires are often patchy and heating of the soil is likely to be highly spatially variable, so species with germination responses to a broad range of temperatures have an advantage over those that respond only to a narrow range. A narrower range of soil depths from which seeds can emerge will further reduce the proportion of the seed bank that might recruit following fire. Human impacts on species habitats, such as fragmentation, loss of topsoil through mining, timber harvesting, grazing and urbanization, and consequent reduction in fire intensity, are likely to have further contributed to rarity in these species. The role of pollination and other factors in relation to population size is the subject of further investigation