Elsevier

Dendrochronologia

Volume 67, June 2021, 125829
Dendrochronologia

ORIGINAL ARTICLE
Recruitment and growth dynamics of a temperate forest understorey species following wildfire in southeast Australia

https://doi.org/10.1016/j.dendro.2021.125829Get rights and content

Highlights

  • Recruitment of a key understorey tree species coincided with known fire history.

  • Fire history can be confirmed using understorey tree species.

  • Fire impacts on tree canopies can be estimated from understorey species growth.

  • In southeast Australia, disturbance and canopy closure define understorey dynamics.

Abstract

In southeast Australia, fire regimes are changing. Conserving species into the future under these changing fire regimes will require understanding their recruitment and growth dynamics following historical fires. Where monitoring is absent, dendroecology provides a tool for reconstructing and quantifying these dynamics. The use of dendroecology in southeast Australia has been limited due to presumptions that many of the species do not produce annual growth rings. In this study, we determined the dendroecological potential of a fire-sensitive understorey tree species, Pomaderris aspera, as a case study species, to explore the potential to use of understorey species to provide insights into past fire history. We used growth patterns of this species to understand the role of resource limitation on growth and senescence. We found that P. aspera had distinct growth rings and high within tree correlation when cross-dated.

Recruitment events of this species aligned with three known fire events. We found that the impacts of historical fire on tree canopy cover could be estimated. P. aspera grew rapidly post fire and then become suppressed within 9–15 years. Cycles of growth release and suppression were found with increasing incidence of suppression occurring over time. Increased suppression and reduced growth rates aligned with patterns of recorded senescence over time in the understories of a maturing.

Eucalyptus canopy. Our results highlight the potential to use dendroecology to confirm past fire extents and amount of canopy disturbances and the impacts that these events have on the recruitment and growth dynamics of understorey species in southeast Australia.

Introduction

Fire is the dominant natural disturbance in Australian terrestrial ecosystems (Kershaw et al., 2002; Bradstock, 2010). Understanding the relationship between fire and vegetation community dynamics is critical to understanding the ecology of these systems (Doherty et al., 2017). The frequency and severity of fire influences plant community composition and structure at both stand and landscape-scales (Bassett et al., 2017; Fairman et al., 2017; Mahood and Balch, 2019). Fire severity is often evaluated as the effect of fire on different vegetation strata. For example, a high severity fire is one that results in crown scorch and may be referred to as a stand replacing fire, while a lower severity fire is typically limited to the understorey or surface (e.g. Collins, 2020; Lindenmayer et al., 2012). For the understorey however, a low fire severity can also result in mortality of fire sensitive species. Fedrigo et al. (2019) highlights this point for forests in southeast Australia where canopy and understorey community age cohorts varied in response to non-canopy replacing fire events at their study sites. Therefore, understanding the impact of different fire regimes (e.g. frequency, severity, extent) on canopy and understorey species dynamics is crucial for ecologically sustainable management and conservation of forests (Doherty et al., 2017). This has become increasingly important due to changing fire regimes in southeast Australia (Fairman et al., 2016; 2017).

Dendroecology of understory species maybe a useful tool for confirming the occurrence of historical fires and for determining the vegetation dynamics following disturbance. While common in the northern hemisphere (Falk et al., 2011) dendroecological studies have been of limited use in temperate forests of Australia (Heinrich and Allen, 2013) due to a combination of decay, mortality of trees following fire, buttressing, dense wood and most importantly a lack of annual growth rings (Simkin and Baker, 2008). Dendroecological studies in the region can provide insights into the impact of past fires and fire severity on plant communities and their subsequent development (Simkin and Baker, 2008; Holz et al., 2020). These insights can be revealed by quantifying a species recruitment patterns and growth dynamics post-disturbance (Abrams and Copenheaver, 1999). Such factors inform our understanding of the impact of past disturbance events on composition and structure (Fedrigo et al., 2019). Few dendroecological studies however have examined post-fire community dynamics within temperate forests of southeast Australia (Simkin and Baker, 2008; O’Donnell et al., 2010; Fedrigo et al., 2019; Holz et al., 2020). Understorey species have received relatively little dendroecological attention (see Simkin and Baker, 2008; Fedrigo et al., 2019; Trouvé et al., 2019) although other techniques such as radiocarbon dating (Mueck et al., 1996; Fedrigo et al., 2019), counting of growth whorls (Chick et al., 2016; Lamont and Barker, 1988; Jenkins et al., 2005); band counting (Ward et al., 2001); and diameter-age relationships (Abbott, 1985; Spooner et al., 2004; Fedrigo et al., 2019) have been used with some success as indicators of vegetation recruitment and dynamics following wildfire in Australia.

The application of dendroecology to understorey species introduces additional challenges which are not typically associated with canopy tree species. Recruitment and annual growth of species in the understorey may be influenced by similar factors to those of the canopy (i.e. climate, soil) in addition to variations in the light environment due to canopy structure. Kasel et al. (2017) found that for temperate forests in southeast Australia, understory diversity was influenced more by structural and edaphic conditions than canopy species with the former reflecting changes in light availability over space and time. Changes to the light environment over a species’ lifetime can lead to suppression and release events that can differ between individuals within the same stand (Poorter, 1999). Within individuals, these events are represented by changes to the width and number of growth rings (Brienen and Zuidema, 2006). Variation in growth suppression and release events between individuals within the same stand may inhibit the compilation of a site chronology; however, they can provide key information on stand dynamics (Baker and Bunyavejchewin, 2006; Simkin and Baker, 2008). For instance, changes in the growth rates over time may indicate changes in canopy structure (Mutch and Swetnam, 1993; Fraver and White, 2005; Baker and Bunyavejchewin, 2006). Abrupt changes in growth rates can indicate release of understory trees from growth suppression due to canopy disturbance; however, suppression and release events typically do not occur in all individuals within a stand as some may have established in canopy gaps (Lorimer et al., 1988). Disentangling the effects of different canopy disturbances (e.g., fire versus windthrow) on recruitment is also a challenge with understorey species. Simkin and Baker (2008) and Fedrigo et al. (2019) identified recruitment pulses in understory species that aligned with known snow storm events in their study region. These studies highlight the potential identify recruitment and growth patterns that align with known events to and provide insights into the causes of other pulses of recruitment and/ or growth that have occurred. Determining if individuals established in gaps or not can provide a more comprehensive record of disturbance events and the impacts on species population dynamics (Lorimer et al., 1988). In the forests of southeast Australia, where fires range from canopy-replacing to gap-forming to canopy maintaining events (see Fairman et al., 2016; 2017; Collins, 2020), determining the proportion of individuals that established post fire and within canopy gaps can provide insights into the severity of the fire at a site but also help identify post-fire growth dynamics. Further insights can be drawn by linking periods of growth release with inter-fire recruitment events to determine if canopy disturbances are facilitating inter-fire recruitment. Simkin and Baker (2008) found that a consistently low level of canopy disturbance punctuated by more intense events can lead to gap formation in the inter-fire period which can stimulate recruitment and growth in some understorey species.

Annual growth rings have been identified for commonly occurring woody understorey species in the tall wet temperate forests of southeast Australia (Simkin and Baker, 2008; Fedrigo et al., 2019; Trouvé et al., 2019). Many understorey species are typically killed by fire and regenerate from soil stored seed or are top-killed and regenerate through resprouting. The pulse of recruitment post-fire has led to the hypothesis that plant communities in the region follow the initial floristics model (Attiwill, 1994), where a majority of species are present at each site soon after fire. These species compete for light with the dominant eucalyptus (Eucalpytus regnans) until they are overtopped. Acacia dealbata has been found to have its growth rates suppressed and senescence rates increased when overtopped by E. regnans (Trouvé et al., 2019). Ashton (1975) found that Pomaderris aspera competes with E. regnans for the first 10–15 years after fire after which time it is overtopped. A decline in the presence of P. aspera in the standing vegetation progresses over the next 30–40 years following fire due to damage by snowfall, longicorn beetle (Phoracantha spp.) attack and fungal attack (Ashton, 1976). Fedrigo et al. (2019) found that the oldest individuals of understorey species aligned with the last known fire event that resulted in only partial mortality of the eucalypt overstorey; however, they also identified the recruitment of individuals over subsequent decades. This aligns with the findings of Ashton and Turner (1979) who found that a range of woody understorey species that recruit following fire and can persist for many decades in the understorey of mature eucalypt forest.

For these reasons, we anticipate woody understorey species maybe useful for assessing both understorey-canopy dynamics and for confirming past disturbance history. The objectives for this study were to:

  • Assess the dendroecological potential of P. aspera;

  • Determine if P. aspera can be used to confirm the occurrence of known fire events;

  • Determine if P. aspera can be used to determine the impact of past fires on the tree canopy; and,

  • Examine canopy-understorey dynamics via growth rates of P. aspera.

Section snippets

Study area

The Central Highlands of southeast Australia are located approximately 100 kms north-east of Melbourne, Victoria (145° 42′ E, 37° 49′ S, <Fig. 1). The area is mountainous and covers just over one million hectares. Mean annual rainfall for the region ranges from 850 to 1460 mm, predominantly falling during the winter months with a mean annual temperature range of 3.6 (July) to 25.6 °C (January) (1884–2015, BOM 2015). The area is characterized by moderate to steep slopes, with an elevation range

Dendroecological potential of P. aspera

Cross sections of P. aspera were generally circular and their pith relatively central with concentric rings. In some instances, the pith was located off centre due to compression of ring widths on one side. Wedging occurred in some samples which led to discontinuous ring patterns in some sections of discs. Due to irregularities in growth only 33 % of the individuals sampled, using a corer, in the 1939 fire site contained the pith. Correlations between two or more radii of the same stem were

Discussion

The ability to accurately estimate the age of understorey communities in fire prone communities is crucial for understanding the effect of fire on community dynamics (Fedrigo et al., 2019). Our findings highlight that dendroecological approaches can supplement fire history data for forests where complete fire history data is not available or is coarsely mapped. This in turn could provide vital information on bushfire management (Jenkins et al., 2005). Using P. aspera as a case study species we

Conclusions

P. aspera is a useful species for dendroecological studies as it can be used to confirm mapped and reported occurrences of past wildfires in the forests of southeast Australia. Our results also highlight that stages of stand development and the dynamics of canopy disturbance can be elicited from the growth responses of this species. In forests that are dominated by eucalypts, a genus not generally suitable for dendroecology across a majority of its range, the ability to quantify known

Declaration of Competing Interest

We have no conflict of interests to declare.

Acknowledgements

This work was supported by the Victorian DELWP iFER (Integrated Forest Ecosystem Research) program and by an Ecological Society of Australia student award. The authors would like to thank Mr. Ben Smith, Mr. Tim Willersdorf, Dr. Linda Parker and Dr. Carola Pritzkow for their help in collecting field samples. We would like to acknowledge and thank the two reviewers who provided excellent feedback and critiques which have improved this manuscript.

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