Savanna canopy trees under fire: long‐term persistence and transient dynamics from a stage‐based matrix population model

Abstract

Fire is a major disturbance driving the dynamics of the world's savannas. Almost all fires are set by humans who are increasingly altering fire timing and frequency on every continent. The world's largest protected areas of savannas are found in monsoonal northern Australia. These include relatively intact, tall, open forests where traditional indigenous fire regimes have been largely replaced in the past half century by contemporary patterns with trees experiencing fire as often as three out of five years. Eucalypt canopy trees form the basic structure of these savannas and changes to the canopy due to fire regimes cascade to affect other plants and animals. In this study, we used data from nearly three decades of field studies on the effects of fire on individual trees to define eight life‐history stages and to calculate transition rates among stages. We developed a stage‐based matrix population model that explicitly considers how fire season and understory influence growth, survival, and recruitment for each life‐history stage. Long‐term population growth rates and transient population dynamics were calculated under five different fire regimes, each in two understory types, using both deterministic and stochastic simulations of seasonal timing of fires. We found that fire was necessary for long‐term persistence of eucalypt canopy tree populations but, under annual fires, most populations did not survive. Population persistence was highly dependent on fire regime (fire season and frequency) and understory type. A stochastic model tended to yield higher population growth rates than the deterministic model with regular, periodic fires, even under the same long‐term frequency of fires. Transient population dynamics over 100 yr also depended on fire regime and understory, with implications for savanna physiognomy and management. Model predictions were tested in an independent data set from a 21‐yr longitudinal field study in Kakadu National Park. This study is a novel and integrative contribution to our understanding of fire in savanna biomes regarding the potential for long‐term persistence and transient dynamics of savanna canopy tree populations. The model is relatively simple, generalizable, and adaptable for further investigations of the population dynamics of savanna trees under fire.

Contemporary Aboriginal savanna burning projects in Arnhem Land: a regional description and analysis of the fire management aspirations of Traditional Owners

“…Ecological research in other parts of northern Australia provides some indication of the effectiveness of Arnhem Land fire regimes for the protection and preservation of this biodiversity hotspot. Werner and Peacock (2019) studied the population dynamics of open-forest savanna canopy trees (i.e. those trees that form the basic structure of the savannas and provide resources for associated fauna species) under various fire regimes.…”

“…This research demonstrates that long-term persistence of savanna canopies with a non-sorghum understory are severely compromised by occasional late dry season fires. Werner and Peacock (2019) recommended that late dry season fires in these landscapes should be avoided where possible and otherwise not occur less than every five years.…”

“…Although not a substitute for biodiversity monitoring, the results from research outside of Arnhem Land suggest that the contemporary fire regimes in Arnhem Land (which demonstrate a high level of patchiness, areas of unburnt vegetation and a reduction in the extent and frequency of late dry season fires) are likely to be favourable for a broad suite of existing biodiversity (e.g. Woinarski et al 2011;Griffiths et al 2015;Radford et al 2015;Werner and Peacock 2019).…”

Effects of 38 years of wildfires on tree density in the Blue Mountains, Australia

“…The fires that caused the greatest loss of property in the Blue Mountains occurred in 1957 and especially 1968 (Cunningham, 1984), shortly before our study. Although the exact extent and severity of these fires are not known, their impact on the number of trees might be greater than any of the fires in our study (Murphy et al, 2014;Werner & Peacock, 2019).…”

Dynamics of the ground layer communities of tropical eucalypt woodlands of northern Queensland

“…Long-term studies in Australian tropical savannas (e.g. Bowman et al 1988;Bowman and Panton 1995;Andersen et al 2003, Scott et al 2009 have focused primarily on the dynamics of woody trees and shrubs and their responses to fire and herbivory (Williams et al 2003;Midgley et al 2010;Werner and Peacock 2019) rather than the larger pool of herbaceous species in the ground layer. Russell-Smith et al (2003b) noted high levels of variation from year to year in ground layer savanna species composition, but few other published long-term observations exist to corroborate or extend that observation (Williams et al 2003;Scott et al 2012).…”