Demographic patterns of a widespread long‐lived tree are associated with rainfall and disturbances along rainfall gradients in SE Australia

Abstract

Predicting species distributions with changing climate has often relied on climatic variables, but increasingly there is recognition that disturbance regimes should also be included in distribution models. We examined how changes in rainfall and disturbances along climatic gradients determined demographic patterns in a widespread and long-lived tree species, Callitris glaucophylla in SE Australia. We examined recruitment since 1950 in relation to annual (200–600 mm) and seasonal (summer, uniform, winter) rainfall gradients, edaphic factors (topography), and disturbance regimes (vertebrate grazing [tenure and species], fire). A switch from recruitment success to failure occurred at 405 mm mean annual rainfall, coincident with a change in grazing regime. Recruitment was lowest on farms with rabbits below 405 mm rainfall (mean = 0–0.89 cohorts) and highest on less-disturbed tenures with no rabbits above 405 mm rainfall (mean = 3.25 cohorts). Moderate levels of recruitment occurred where farms had no rabbits or less disturbed tenures had rabbits above and below 405 mm rainfall (mean = 1.71–1.77 cohorts). These results show that low annual rainfall and high levels of introduced grazing has led to aging, contracting populations, while higher annual rainfall with low levels of grazing has led to younger, expanding populations. This study demonstrates how demographic patterns vary with rainfall and spatial variations in disturbances, which are linked in complex ways to climatic gradients. Predicting changes in tree distribution with climate change requires knowledge of how rainfall and key disturbances (tenure, vertebrate grazing) will shift along climatic gradients.

Forecasting species range dynamics with process‐explicit models: matching methods to applications

“…Alongside increases in species data availability, new methods are being developed to make better use of existing data, supporting parameter estimates in situations typically assumed to be data-limited. Recently developed statistical methods enable estimates of demographic parameters without requiring repeated surveys of marked individuals, drawing on non-traditional data types (e.g., growth nodes and rings, otoliths; Morrongiello et al 2012;Cohn et al 2013;Merow et al 2014;paleoecological data;Fordham et al 2016a) and readily available population data (e.g., inverse population models; Doak & Morris 1999;Gonz alez et al 2016). These methods are complemented by statistical tools suited to complex ecological models, including integrated models and simulation-based inference (Schaub & Abadi 2011;Hartig et al 2011;Maunder & Punt 2013;Fithian et al 2015).…”

Responses of tree species to a severe fire indicate major structural change to Eucalyptus–Callitris forests

“…F.M.Bailey) that are obligate seeders (killed by fire) are often co-dominant. When of sufficient size and density, Callitris reputedly reduce local fire severity via characteristics of their foliage and litter and by reducing the cover of the herbaceous understorey (Cohn et al 2011(Cohn et al , 2013Trauernicht et al 2012). These Callitris species retain a canopy seed bank (although the temporal dynamics of this are poorly known -Ladd et al 2013), can recruit in the absence of fire (Ross et al 2008;Whipp et al 2012) and are also capable of recruitment into burnt patches from unburnt areas through the dispersal of winged seeds (Cohn et al 2011).…”

Fire regime and climate determine spatial variation in level of serotiny and population structure in a fire-killed conifer

“…We hypothesized that continuous recruitment would occur in the higher rainfall areas while recruitment in arid areas would be episodic due to limited suitable recruitment opportunities (e.g., following wet years and/or fires) and could give rise to unimodal or multimodal population structures depending on the type of establishment opportunity. For example, Cohn et al (2013) found that stand structure for C. glaucophylla in semi-arid SE Australia depended on both rainfall and herbivory, with recruitment limited in dry years and in the presence of browsing impacts from rabbits but favoured in wetter years and in the absence of rabbits.…”

“…Here, herbivory best explains the initial development of a unimodal (post-fire cohort) population structure and this is maintained by herbivory and low light levels in dense stands as trees mature. Recruitment of Callitris has been reported elsewhere to be affected by grazing animals, both native, and introduced (kangaroos, wallabies, rabbits, deer; Austin and Williams 1988;Mackenzie and Keith 2009;Cohn et al 2013;Prior et al 2018). The islands off the coast of southern Western Australia were isolated from the mainland by rising sea level in the early to mid Holocene (Backhouse 1993) and were unoccupied by humans until the arrival of Europeans ~ 200 years ago since there was no reliable source of fresh water.…”

“…As in all the Cupressaceae, seeds are formed in cones, with 6-16 viable seeds cone −1 reported by Ladd et al (2013) for stands from semi-arid inland WA. While fire is clearly a very important driver of Callitris ecology, fire regimes (especially fire type and frequency), along with climatic conditions (Table 1) and biotic interactions (e.g., herbivory) vary across the geographic range of the species (e.g., Cohn et al 2013) and may have different effects on recruitment as has been shown for mallee shrublands in semi-arid SE Australia (Kenny et al 2018). Since the 1990s a positive phase of the Southern Annular Mode has correlated with increased summer precipitation in inland southern WA but decreased precipitation in coastal areas (O'Donnell et al 2015).…”