Alpine treeline ecotone stasis in the face of recent climate change and disturbance by fire.

How species respond to climate change will depend on biological characteristics, species physiological limits, traits (such as dispersal), and interactions with disturbance. We examine multi-decadal shifts in the distribution of trees at the alpine treeline in response to regional warming and repeated disturbance by fire in the Victorian Alps, south-east Australia. Alpine treelines are composed of Eucalyptus pauciflora subsp. niphophila (Snow Gum, Myrtaceae) species. The location and basal girth of all trees and saplings were recorded across treelines at four mountains in 2002 and 2018. We quantify changes in treeline position (sapling recruitment above treeline) over time in relation to warming and disturbance by fire, and examine changes in stand structure below treeline (stand density, size class analyses). Short-distance advance of the treeline occurred between 2002 and 2018, but was largely restricted to areas that were unburned during this period. No saplings were seen above treeline after two fires, despite evidence that saplings were common pre-fire. Below treeline, subalpine woodland stands were largely resilient to fire; trees resprouted from lignotubers. However, small trees were reduced in number in woodlands when burned twice within a decade. Population dynamics at the alpine treeline were responsive to recent climate change, but other factors (e.g. disturbance) are crucial to understand recruitment trends. Establishment of saplings above treeline was largely restricted to unburned areas. These results indicate fire is a strong demographic filter on treeline dynamics; there is a clear need to frame alpine treeline establishment processes beyond just being a response to climate warming. Long lag periods in treeline change may be expected where recurrent disturbance is a feature of the landscape.

Introduction

Alpine treelines are conspicuous and sharp transition zones, defining the upper elevational limit of tree growth [1, 2]. Limits to tree growth across the globe are widely reported to occur as a consequence of low mean growing-season soil temperature [3], but other less well-studied factors (e.g. competition, herbivory, availability of safe sites, frost, disturbance) also influence the position of the treeline at local-scales [4-8].

Rising global temperatures are predicted to improve conditions for tree growth above the current treeline, leading to an upslope advance of trees [6, 9, 10]. However, Harsch et al. [10] found in a meta-analysis of 166 sites that 47% of alpine and high latitudinal treelines had shown no sign of advance, despite regional warming. It is likely that other local and site-specific factors, typically occurring at small-spatial scales, currently override the influence of increased global temperature on tree establishment above treeline [7]. Factors that affect seedling germination and early establishment are likely very influential [5–8, 10, 11].

Disturbance can have a significant effect on treeline position, either as an agent of direct mortality of adult trees [12], and/or via promotion of seedling recruitment opportunities [13], particularly in serotinous species [2, 14]. Fire, as one agent of disturbance, can act as a positive catalyst for treeline boundary change [2], triggering seed release, reducing ground layer competition, and generating bare ground opportunities for a pulse of seedling establishment [1, 13, 15]. Alternatively, fire may have negative effects on treeline boundaries if short-return intervals of fire lead to an “interval squeeze” [16], whereby the disturbance frequency reduces stand regeneration opportunities and produces an ‘immaturity risk’ at the population level [17]. This vulnerability will be highest in seeder treeline species whereas resprouting species are more likely to persist in situ [16, 18–21]; seedling regeneration opportunities may, however, be limited where fire frequency exceeds the secondary juvenile period of such trees [22]. Hence, treelines composed of resprouter species could theoretically remain unchanged following repeated fire [18, 23], reinforcing the existence of a sharp, non-mobile boundary. Resprouting is, however, a population attribute rather than a species attribute per se [20]. Resprouting response to fire disturbance varies in relation to two key parameters: 1) regime, which includes fire-return interval [12, 18, 24] and, 2) life-history stage [24, 25]. In some species, resprouting ability increases with size to reach a maximum in adult stages while in other species, resprouting is common in juveniles but adults are unable to resprout [18, 25]. In others again, resprouting capacity can decrease with recurrent short-interval wildfire [26]. Hence, treeline dynamics are likely to be complex in fire-prone landscapes, and fire-treeline feedbacks will likely assume more prominence in coming decades as climate conditions increase the likelihood of fire in mountain ecosystems [7, 20, 27–29].

Studies that quantify multi-decadal stand structural change at alpine treelines in the face of climate change and disturbance are necessary to interpret the wide variety of treeline responses reported [7]. Here, we assess the stability of alpine treelines in response to regional climate warming and repeated fire—where treelines are composed of broad-leaved, evergreen, facultative resprouter trees—using a re-visitation approach in the alpine areas of south-eastern Australia. Specifically, we asked: what evidence is there for changes in treeline position in response to recent climate warming and how is this affected by fire disturbance? And, has fire affected woodland stand structure below the treeline via effects on tree mortality and/or recruitment?

We hypothesised that:

Treelines without fire disturbance (since the time of first survey, Time1) will have upslope sapling recruitment (at Time2) due to recent climate warming. The stand structure below treeline will remain stable (Fig 1A).

Fig 1

Change in treeline position over time is hypothesised to depend on interactions with both climate and fire.

(a) In the absence of fire, seedlings will have established above treeline due to regional climate warming. Stand structure will remain largely unchanged between Time1 and Time2. (b) After a single fire, there will be a pulse of sapling recruitment which will occur near adult trees at/above the treeline due to limited dispersal. Stand structure will remain stable between Time1 and Time2 due to resprouting. (c) After two fires, treelines will show evidence of ‘interval-squeeze’ with no recruitment above treeline. Stand structure will change, but not directionally, as not all resprouting trees will have survived the second fire. (d) After two fires, treeline regression will occur at Time2 if trees at the climatic limit of growth are most vulnerable to repeated fire and thus tree mortality is high. Based on conceptual model of vegetation change at the alpine treeline ecotone developed by Cansler et al. [12], but adapted for Australian Eucalyptus pauciflora treelines.

Facultative seeder treelines with a single fire disturbance (since the time of first survey, Time1) will have a pulse of sapling recruitment after fire and most of this will occur near adult trees at / above treeline (at Time2) due to limited dispersal. The stand structure below treeline will remain stable because of resprouting (Fig 1B).

Facultative seeder treelines with repeated fire disturbances will show evidence of an ‘interval -squeeze’ with no recruitment above treeline since Time1 (Fig 1C). The stand structure below treeline will change, but not directionally, as not all resprouting trees will survive repeated fire (Fig 1C). Treeline regression will occur at Time2 if trees at the climatic limit of growth are most vulnerable to repeated fire (Fig 1D).

Methods

Study species

Alpine treelines in the Australian Alps are dominated by a single tree species, Eucalyptus pauciflora subsp. niphophila (Snow Gum, Myrtaceae, (Maiden & Blakely) L.A.S.Johnson & Blaxell), hereafter Eucalyptus pauciflora. Treelines vary from ~1750 m to 1900 m asl in the Australian Alps, varying with latitude and aspect [15]. Eucalyptus pauciflora is a long-lived species (i.e. potentially living hundreds of years to millenia), possessing a lignotuber from which resprouting occurs following canopy damage and produces a multi-stemmed growth form [15, 23]. Barker [30] suggests that trees with >250 cm basal girth are >300 yrs of age.

Eucalyptus pauciflora is capable of regenerating by seed and resprouting from meristematic tissue in lignotubers [31]. Dispersal is short, largely confined to within a few widths of the canopy [15]. It is a “niche persister” [18] due to its capacity to re-occupy sites after disturbance, a trait most likely derived from the severe climatic conditions which constrain seedling establishment at high elevations [32-35]. The ability of E. pauciflora to resprout suggests that re-occupancy after disturbance by fire is likely to be high. Similarly, understorey vegetation (including many shrubs, graminoids and forbs) re-sprout or are able to germinate from the soil seed bank post-fire, and thus the whole system recovers well from infrequent fire [36]. However, E. pauciflora resprouting capacity is dependent on lignotuber survival, which has been shown to decline with multiple fires [35], particularly when the fire intervals are short[20]. The capacity to resprout is also thought to be dependent on fire severity, plant vigor, size and bark thickness [31, 37]. Theoretically, E. pauciflora is capable of resprouting within 6 months of germination when the lignotuber is first formed [37,38]. However, seedling lignotubers are poorly protected in the soil and they may be vulnerable to fire [37,38]. Time to reproductive maturity of E. pauciflora saplings is not known; however, time to first reproduction after fire (the secondary juvenile period) occurs within 6 yrs (S. Cutler unpublished data).

Study area

The study was conducted on alpine mountain peaks in the Victorian Alps, south-east Australia. Four mountains (Mt Hotham, Mt Feathertop, Mt McKay, The Twins) with obvious treeline ecotone boundaries were selected for study in 2002 to determine treeline position and woodland structure (Fig 2 and S1 Table). Grazing history varied across sites, with stock grazing removed across all sites since 2003 (S1 Table). These mountains were then variably exposed to fire (S1 Table). In 2018, we re-surveyed the same localities on all four mountains to quantify how treelines had changed in response to climate and fire since the first survey.

Fig 2

Extent of 2003, 2007 and 2013 fires and site locations across the Victorian Alps, Australia.

Fire history at the transect scale varied from fire perimeters shown on this map. Mt Hotham was unburnt since 1939, Mt Feathertop and Mt McKay burnt once in 2003 and The Twins burnt twice in 2003 and 2013. Fire history data sourced from Victorian Government Department of Environment, Land, Water and Planning Spatial Datamart https://services.land.vic.gov.au/SpatialDatamart/dataSearchViewMetadata.html?anzlicId=ANZVI0803004741&extractionProviderId=1 under a CC BY license, with permission from Department of Environment, Land, Water and Planning, original copyright 2019.

The region is characterised by low temperatures, with an annual mean maximum temperature ranging between 8.0–9.4°C and mean minimum temperature ranging between 1.9–2.6°C (Mount Hotham meteorological station, Bureau of Meteorology, www.bom.gov.au). Annual mean precipitation is 1274–1454 mm, with much of it falling as winter snow. Frost frequency varies inter-annually. There has been an increase in mean annual minimum and maximum temperature of ~0.4°C in the Victorian Alps since 1992. Temperatures are predicted to continue to warm by 0.6–2.9°C by 2050 [39]. Extreme climate events (i.e. minimum temperature, maximum temperature, mean annual rainfall, maximum annual snow depth) have been more frequent since 2005[40]. Soils are largely of the alpine humus type and typically shallow near and above treelines (e.g. mean 8–32 cm deep; S. Cutler unpublished data).

Fires have historically been a relatively rare occurrence in the alpine regions of Australia, with a fire return interval of ~50–100 years [36, 41, 42], with all sites likely to have been burnt in 1939 [43]. Between 2000 to the time of measurement in 2018, much of the Victorian Alps has been burnt to varying degrees by three large-scale fires within a decade [44]. At our study sites, since 2001, fire had occurred 0, 1 or 2 times (Fig 2 and S1 Table), stimulating discussion about the effects of fire on alpine treeline dynamics and the resilience of treeline populations in the future, given fire frequency and severity is predicted to increase with climate change [35, 36, 42, 45]. This increase in fire frequency in the region is continuing post our repeat surveys, with large areas of the Australian Alps, including the Victorian Alps, burning during the 2019–2020 summer period.

Field methods

In 2002, the position of the alpine treeline was determined at each mountain (corresponding with the upper extent of large adult trees (>3 m height and >25 cm basal girth)) [46] and 19 belt transects were established to sample stand structure below the treeline. Transect end-points were permanently marked. Belt transects were 5 m wide and extended for a distance of 40 m downslope of the treeline into subalpine low open woodland. In each transect, all E. pauciflora were assigned an x and y coordinate and the basal girth was measured. From this, size class distributions were deduced and age approximated [47]. Height of all individuals was also recorded. The area above the treeline was searched and transects extended to encompass all outpost individuals of E. pauciflora (e.g. trees, saplings, seedlings). We define E. pauciflora life forms as follows: ‘trees’ are individuals that have a basal girth greater than 25 cm, ‘saplings’ are individuals with a basal girth between 25 cm and ~0.13 cm and are estimated to be greater than 1 year old, and ‘seedlings’ are very young plants that are likely to be less than 1 year old corresponding to a basal girth of less than ~0.13 cm [47].

To determine the stability of the alpine treeline and woodland stand structure in response to regional warming and disturbance by fire, we relocated the original transects in 2018 using a combination of GPS co-ordinates, original field notes and permanent pegs. In transects where the original pegs could not be found, these were relocated based on GPS co-ordinates and transect maps from 2002. As such, any change over time represents local-scale changes in treeline dynamics and woodland structure as opposed to change in individual trees. Observed changes in treeline stability and woodland stand structure can be attributed to the effects of fires as windthrow or avalanches are uncommon, and Victorian snow gum woodlands are not yet affected by the Phoracantha beetle as in Kosciuszko National Park, Australia. A total of 19 transects were re-surveyed. Fire history, since 2002, was quantified at the transect scale by looking for evidence of burn scars and stem resprout cohorts; trees exposed to one fire have one live and one dead stem cohort, whereas trees exposed to two fires typically had two dead stem cohorts, and a smaller live stem cohort in 2018. Fire history at the transect scale varied from Victorian Government Department of Environment, Land, Water and Planning spatial data (Fig 2). Of the 19 transects, five were unburnt since 1939 (Mt Hotham), eight burnt once in 2003 (Mt Feathertop, Mt McKay) and six burnt twice between 2003 and 2013 (The Twins). The location of trees were mapped as x and y coordinates. Basal girth, height and condition (live, dead) of trees was recorded.

Data analysis

The year of establishment of individuals located above treeline was calculated from basal girth according to the methods of Rumpff et al. [47]. A Fisher exact test was used, as expected values were <5, to determine differences in the number of saplings (<25 cm basal girth) located above treeline between survey periods. Where transformation did not improve the distribution of the data non-parametric tests were used. A Mann-Whitney U test was used to determine differences in the distance of individuals above treeline between survey periods. A Mann-Whitney U test was used to determine differences in the height of saplings (<25 cm basal girth) above treeline between survey periods. A Kruskal-Wallis rank sum test was used to determine any relationship between basal girth and mortality (live or dead).

To examine changes in woodland stand structure over time, size-class distributions (SCD) were used based on a model presented by Condit et al. [48]. Individuals were grouped into 17 basal diameter classes (size class, in mm), based on the structural demography of E. pauciflora woodland elsewhere [23]. The number of live individuals was counted per size class. To accommodate uneven size class width, the number of individuals (Ni) was divided by the width of the size class (Eq 1).

This gives the abundance per size class (ni). The midpoint of each size class and abundance was natural log (ln) transformed and a regression calculated for each site in each survey period. The slope (z) of the regression was then used as an indicator of population structure. Outcomes of this approach can be interpreted broadly as:

Type 1: stands not recruitment-limited as there are large numbers of juveniles, saplings and young trees relative to older trees. These stands are typified as having a reverse j-curve size class distribution and slopes are high negative values (< -1.0)

Type 2: stands are moderately recruitment-limited, with small numbers of sapling and juvenile trees relative to older trees; slopes are flatter, with y-values falling between 0 and -1.0.

Type 3: stands are severely recruitment-limited, resulting in a recruitment bottleneck, with few to no observable trees in the smaller size classes relative to older trees. These have positive slopes (>0).

Results

A total of 83 saplings (<25 cm basal girth) were located above treeline on the four mountains in 2018, compared to 119 in 2002. In both 2002 and 2018, the majority of saplings were within 5–10 m of the treeline, and were <57 cm in height, (Fig 3 and S1 Fig).

Fig 3

a) Distance above treeline of Eucalyptus pauciflora saplings (<25 cm girth) located above treeline in 2002 and 2018 surveys. b) Height of Eucalyptus pauciflora saplings (<25 cm girth) located above treeline in 2002 and 2018 surveys. c) Total number of Eucalyptus pauciflora saplings (<25 cm girth) above treeline (±standard deviation) in 2002 and 2018 surveys (Hotham n = 5; Mount Feathertop n = 4; McKay n = 4; The Twins n = 6).

There was a significant increase in the distance of saplings (<25 cm girth) above treeline at T2 (2002 median: 2.04 m, 2018 median: 7.23 m) (Fig 3; p<0.001). There was also a significant increase in the height of saplings (<25 cm girth) located above treeline at T2 (2002 median: 0.15 m, 2018 median: 0.2 m) (Fig 3; p<0.001).

Establishment above treeline in relation to fire history was varied (Fig 3). There was a significant increase in saplings above treeline at Mt Hotham (unburnt), and recruitment continued between survey periods at the Mt Feathertop and Mt McKay (both single burn) sites (Fig 3; p<0.001). There was a significant decline in sapling number at The Twins between sampling times; this treeline was burnt twice between 2002 and 2018 (Fig 3; p = 0.008, and S1 Fig).

Estimates of the age of individuals above treeline in 2002 and 2018 indicate that most had established after 1995 and then again after 2012 (Fig 4). Few individuals (saplings and adults) above treeline in 2018 surveys had established before the 2002 survey (Fig 4), suggesting high turnover and thus mortality above treeline. However despite this there was low mortality across the whole transect (above and below treeline) as indicated by dead standing individuals found (0% unburnt sites, 8% single burn sites, 5% double burn sites). The basal girth of dead individuals was significantly higher than live individuals at single burn sites (p-value = 0.032). There was no significant difference between the basal girth of dead and alive individuals at the Twins twice-burnt site (p-value = 0.879).

Fig 4

Estimated year of establishment of Eucalyptus pauciflora individuals located above treeline in 2002 and 2018 surveys.

Vertical dashed lines indicate time of disturbance. Year of establishment calculated based on Rumpff et al. [46] model; trees with girth >115 cm (establishment pre-1938) were excluded.

Woodland stand structure below the treeline varied with burning history between 2002 and 2018 (Fig 5). At the unburnt mountain (Mt Hotham), SCD curves were more negative with time (2002: -0.798 (Type 2), r2 = 0.86; 2018: -1.088 (Type 1), r2 = 0.93) which indicates recruitment into small size-classes. By contrast, at the twice-burned mountain (The Twins), SCD curves flattened over time (2002: -1.043 (Type 1), r2 = 0.92; 2018: -0.345 (Type 2), r2 = 0.45), indicating a loss of individuals in smaller size-classes from the stand (Fig 4). At the once burned mountains, SCDs were stable (Mt McKay, 2002: -0.561, r2 = 0.65; 2018: -0.509, r2 = 0.52; both Type 2) or showed evidence of increasing (Mt Feathertop, 2002: -0.371 (Type 2), r2 = 0.21; 2018: -1.1.132 (Type 1), r2 = 0.70) (Fig 5 and S1 Fig).

Fig 5

Size class distribution of size class (natural log transformed) against abundance (individuals per size class corrected, natural log transformed) for Model is based on Condit et al. [48]. Population structure types; Type 1: stands not recruitment-limited as there are large numbers of juveniles, saplings and young trees relative to older trees. These stands are typified as having a reverse j-curve size class distribution and slopes are high negative values (< -1.0); Type 2: stands are moderately recruitment-limited, with small numbers of sapling and juvenile trees relative to older trees; slopes are flatter, with y-values falling between 0 and -1.0; Type 3: stands are severely recruitment-limited, resulting in a recruitment bottleneck, with few to no observable trees in the smaller size classes relative to older trees. These have positive slopes (>0).

Discussion

How species respond to changing climates at local scales will depend on how edaphic and biological characteristics interact with species physiological limits, traits such as dispersal, and interactions with disturbance (type, frequency, severity). While climate is considered a key determinant of alpine treeline position [1], and hence treeline position is likely responsive to climate warming, disturbance may also play a critical but poorly understood role [12, 13, 49]. Using a revisitation approach at the alpine treeline in Australia where fire has been both extensive and recurrent during the last few decades [44], we found that a limited number of saplings of the alpine treeline species E. pauciflora recruited above the treeline, but this depended on the frequency of fire. In the absence of fire, short-distance (<10 m) regeneration was observed between 2002 and 2018. After two fires in this period, there was no sapling regeneration above treeline despite extensive recruitment in the period before fire (pre-2002). Fire frequency also affected the stand structure of subalpine woodlands. Few changes in structure occurred in the unburnt and once-burned forests; twice-burned stands, however, lost smaller-sized trees (<~20 cm basal girth) and this may be evidence of a fire ‘interval squeeze’ [16]. While changes in the environment, including climate warming, may allow saplings to grow above the current treeline, as has been observed in many alpine regions of the world [10], this capacity may depend on disturbance type and frequency. Hence, there is potential for long time lags in treeline movements in the Australian Alps because tree persistence and recruitment is affected by drivers other than just climate [7, 8, 50]. There is a clear need to frame alpine treeline establishment processes beyond just being a response to climate warming.

In the absence of fire, we hypothesised (Hypothesis 1) that ongoing climate warming (a ~0.4°C increase in mean annual temperature since 1992) would promote the upslope migration of the alpine treeline between our survey times (Time1 2002—Time2 2018). Trees at treeline are responsive to warming temperatures in many settings [6, 9,10] and hence, we expected to see establishment of E. pauciflora, in numbers that may be higher than observed prior to 2002, due to ongoing warming. We did observe ongoing sapling recruitment above treeline at Mt Hotham, the only mountain in our study that escaped wildfire between 2002 and 2018. Prior to 2002, saplings above established treelines were observed at Mt Hotham, establishing within 10 m of the treeline. This has continued, with most saplings still restricted to establishment near the treeline. We found a striking trend in the height (<~60 cm) and age (<~10 years) of individuals above treeline which was consistent across survey periods and sites. The age of individuals indicates high turnover and thus low longer-term survival above treeline. The height of individuals we observed suggests that a height threshold determined by freezing stress may exist [1,2]. Freezing stress is exacerbated immediately above the surrounding vegetation, corresponding to a shift from within the warm boundary layer, formed by maximum heat accumulation and retention near the soil surface, to above this layer where individuals become more closely coupled to atmospheric conditions [2, 15, 51,52]. Although mature E. pauciflora are relatively resistant to frost, individuals less than 30 cm tall can be easily killed by substantial shoot dieback [15]. Individuals thus require a period of frost-free damage to attain heights >1 m where frost occurrence and damage becomes less severe. This suggests that it may not be overall growing season temperature which limits sapling growth beyond this height, but the occurrence of extreme temperature events [6].

In unburned areas, limited upslope migration was observed. This may be an outcome of dispersal limitation, positive effects of nearby tree canopy on establishment (i.e. facilitation), or a combination of the two. Dispersal of E. pauciflora seed is limited, as it is for many eucalypts [53]. Eucalyptus pauciflora are generally short-statured at the treeline (i.e. <5 m tall), the seeds have no appendages to aid wind-dispersal, and modelled mean maximum dispersal distance (based on traits such as tree height and seed mass) is ~16 m (A. Naccarella unpublished data). Mortality of E. pauciflora germinants is likely highest during the first growing season (being dependent on summer soil water availability) and the first winter (being dependent on the depth and duration of snow cover)[4]. Sapling survival is possible where the treeline canopy reduces photoinhibition [51], and may explain why most saplings are growing within 10 m of the edge of tree canopy. Additionally, canopy facilitation has been shown to reduce frost severity [51] and increase humidity buffering against drought [54-56], which could be resulting in a density-dependent positive feedback between canopy cover facilitation and seedling recruitment leading to restricted upslope migration [57, 58].

In the absence of disturbance by fire, woodland stand structure below treeline changed from Condit et al. [48] SCD curve Type 2 to Type 1, indicating that plants are being recruited into the population. SCD models indicate that woodland structure has remained stable at single burn sites like Mt McKay, with a higher proportion of saplings to mature individuals [48]. This suggests treeline populations are resilient to a single bushfire. SCD models at twice-burned treelines indicate a scarcity of saplings in 2018 (as indicated by lower slope values; Type 2), leading to a more even- aged stand [48]. Prior to being twice-burnt, saplings were prominent (Type 1), suggesting a shift in treeline dynamics over time with bushfire occurrence.

How E. pauciflora treeline responds to fire disturbance appears to be influenced by fire frequency. Once-burned areas show no evidence of pulsed seedling regeneration (counter to our Hypothesis 2), but there is little evidence of tree or sapling mortality, as quantified by size-class analyses conducted in 2002 and 2018. Twice-burned areas (within a decade), however, lose small-sized trees and saplings both above the treeline and within the subalpine woodland (supporting our Hypothesis 3). The occurrence of two fires within a decade is an unprecedented event in the Australian Alps [36] allowing the opportunity to explore the influence of short-interval fires on treeline dynamics. Although mature tree mortality was low, and as such there was no evidence of treeline recession, woodland structure had shifted substantially. The substantial reduction in saplings above- and below-treeline suggests younger and smaller individuals are more susceptible to fire. Despite E. pauciflora capacity to resprout within 6 months of germination, the lack of soil protection for the lignotuber could result in increased vulnerability, particularly during high-intensity fires [37, 38]. Additionally, the low stature of individuals above treeline likely increased their susceptibility to both (i) fire due to their close proximity to the ground layer and surface fuels [28] and (ii) frost due to the loss of groundlayer vegetation that may protect them from cold temperatures.

Understanding how disturbance, such as fire, impacts on treeline persistence and expansion is clearly crucial for interpreting treeline population dynamics [49,59]. Fires, particularly at short-intervals, have been found to cause treeline depression across a range of global treelines [28, 60–62]. Conversely, fires have also been found to accelerate the effect of rising temperatures through reducing competition with surrounding vegetation [15, 59, 49, 63]. Fire may assume an even greater importance in the Australian Alps in the future, with temperatures predicted to increase by 1.4–3.8°C over the next century for south-eastern Australia [64], increasing the frequency of high fire danger days by as much as 70% by 2050 [65,66]. Additionally, understanding how fire and climate may differentially or similarly affect lower elevation woodlands, including the lower montane E. pauciflora boundary, will inform how E. pauciflora ecotones could change in the future. Abrupt changes to the fire regime has already caused the landscape-wide loss of obligate seeder mountain forests in south-eastern Australia [44], suggesting there is the potential for shifts across many ecotone boundaries.

Conclusion

The response of global alpine treelines to rising temperatures has been unpredictable and variable [10, 58, 67]. A combination of treeline advance, stability and retreat has been recorded, suggesting that temperature may not be the overriding control across many treelines. Hence, upslope treeline movements lag behind climatic warming. Our results suggest that population dynamics at the alpine treeline in south-eastern Australia are responsive to climate warming, albeit at small spatial scales, and that other factors (e.g. disturbance regimes) are crucial for understanding recruitment. There is a clear need to frame alpine treeline establishment processes beyond just being a response to climate warming. Long lag periods in treeline change may be expected where recurrent disturbance is a feature of the landscape.

A visual representation of Eucalyptus pauciflora individuals across representative transects expressing increases in individuals above treeline (Mount Hotham transect 1, W aspect), stability with similar numbers of seedlings above treeline (Mount Feathertop transect 3, W aspect) and decreases in seedlings above treeline (The Twins transect 2, N aspect).

X and Y axes indicate exact meter locations across the transect. Circle size indicates basal circumference in relative proportions to the X and Y axes. Grey = dead individuals. Black = live individuals. Treeline is represented by the red line at y = 40m, y<40 within the woodland, y>40 above treeline.

(TIF)

Click here for additional data file.

Site characteristics (transect length, location (latitude and longitude presented in GDA94/ MGA zone 55, coordinates refer to treeline position centre), aspect, elevation, grazing history (Sourced from Lawrence [43], fire history since 2003 based on field assessments).

(DOCX)

Click here for additional data file.

6 Jan 2020

PONE-D-19-29648

Alpine treeline ecotone stasis in the face of recent climate change and disturbance by fire

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Reviewer #1: Review Naccarella et al.

Alpine treeline ecotone stasis in the face of recent climate change and disturbance by fire

General comments:

Naccarella and colleagues revisited vegetation surveys from different locations at the alpine treeline in south-eastern Australia, to assess the impact of forest fires on upslope movement of treelines and forest stand structure. They could clearly show that in areas with an unusually high fire frequency, sapling establishment was reduced both within the subalpine forests and above treeline. They conclude that fire has the potential to impede the upslope movement of treelines in response to a warming climate.

I’ve greatly enjoyed reading this well-written and concise manuscript. The authors make a very convincing case for fire as an important secondary control of treeline dynamics. This is an important finding that could explain the lack of evidence of an upward shift of mountain forests in some parts of the world, and has implications for assessing future vegetation dynamics under climate change. Of course, it is a pity that the exact transects could not have been reproduced between sampling years. Nevertheless, the authors can show clear trends in response to fire on the local stand-scale. Even though I mostly agree with the interpretation and conclusions of the authors I have two remarks:

- To use the presence of seedlings/saplings above treeline to infer a response to warming temperatures is a bit tricky. As you state yourself, seedlings/saplings are sheltered and not exposed to the atmosphere, and therefore a normal occurrence above treelines also without any warming trend. To directly infer a climate-driven upward shift of treeline position as hypothesized in Fig. 1, you would need to compare actual treelines. An upward shift of saplings as presented in Fig. 2a is a potential indication at best, especially given the very small increase of 5 m (!), which is well below what you would expect from the climate data (0.4°C warming since 1992, so about 50 m using standard lapse rates). The raw number of seedlings (as in Fig. 2c) is no indication at all. However, I fully agree with your conclusion that by increasing sapling mortality (effectively removing any saplings above treeline), fire has the potential to impede the upslope movement of subalpine forests. If only temporarily or permanently depends on future fire frequencies and a potential shift in fire regimes.

- Forest fires normally depend on two factors: climate and fuel. Without enough fuel there is no fire. So, I was wondering what the fuel loads above or at treeline look like and how far fires can penetrate into the alpine vegetation as brush (grass?) fires. Could you comment on this? I agree that since sapling establishment is restricted to areas close to the treeline (10 m) fire would still kill of most saplings (as you show) but what about single individuals located farther up? Would they still be affected? And with temperature projections of up to +3.8°C could single individuals establish above the zone affected by forest fires, if seeds somehow get there?

I recommend the article for publication in PLOSone after addressing these remarks and some minor comments below:

L. 158: How are “large adult trees” defined? >2 m height? Basal diameter >25 cm?

L. 237: How was the age of saplings estimated? Just by diameter? Because in Fig. 3 the oldest “saplings” have supposedly established in 1939, which means that they are not technically saplings (as in “young trees”) anymore, but rather quite old trees that were growing very slowly due to the harsh conditions above treeline.

L. 240-241: This is a bit confusing, since in the previous sentence you mention that “…mortality above treeline was high.” And here you state that “Tree mortality was low…”. I guess you talk about actual dead individuals found, whereas in the previous sentence you base your interpretation on the difference between the survey period, which clearly also includes seedlings. I suggest rephrasing to make this distinction clear.

L.297-310: I fully agree, what you observe here, a large number of seedlings/saplings above the treeline with very low to zero long-term survival due to microclimatic conditions (ground vs. athmosphere) is a very common feature of treeline ecosystems around the world. But this is also why it is not a good idea to use seedling/sapling establishment above treelines as an indicator of an upslope movement of treelines in response to warming temperatures.

L. 326: single not singe

Reviewer #2: This is an excellent paper: It is well written, the methods and results are clear, and the authors use the international literature to frame the introduction and provide a nuanced discussion. They build on existing conceptual models, and reframe them for plants with different traits and life history strategies.

The main limitation is that they only have four sites which each have unique grazing and disturbance histories. In otherwords, this study is a comparison of four case studies. While inferences would be stronger with more sites, these case studies are still valuable. The inferences the authors draw from those sites are sound, and are important for predicting how treelines worldwide will shift in response to climate change.

A second limitation is that they are inferring that fire is the main cause of demographic changes in tree populations. This may be very reasonable to do, given the dynamics of these ecosystems, but the authors should add some context for international readers: are how do they know that the fires were the main cause of these changes? Can the rule out windthrow, avalanches, insects, etc? The authors need to add text providing this context.

This study will make an excellent contribution to the global literature on treeline dynamics and wildfire interactions at ecotones.

The authors state the data will be made available but they have not done so yet.

42-64 This is a really nice discussion of resprouting, and the variety of ways resprouting occurs in species and populations. Nice set-up for the whole article.

Figure 1. I think it is great to include a conceptual diagram like this, which allows readers to visualize the dynamics of your study ecosystem. I’m familiar with the other paper you cite, which presents a similar model for obligate-seeding conifer forests. This is a nice modification for resprouting forests. Fig. 1d, is probably the most interesting, with the interaction with climatic stress and vulnerability to repeat fire. You don’t show it, but I can think of treelines where vulnerability to repeat fire may be greatest further away from the climatic limit to growth. In fact, I think the original paper you cite suggests that is the case in North America temperate forests. You might want to make that clear, and consider again (more explicitly) in the discussion how interactions between stress and fire impact immediate ecotone patterns, and long-term change.

132: S1 Table 1 shows grazing history as well, which varied greatly between some of the sites. Do you need to mention that history in the text?

132: S1 Fig1: This is an excellent map. I’d suggest including it in the main manuscript. This context is needed for any international readers.

149-154: Given the current fires in NSW, you many need to modify lines 149-151 to say “Since 2000 to the time of measurement in 2018…” And then you may need to add a line to note that the pattern of repeat burning is continuing. It looks like “The Twins” site is within the boundary of active MODIS returns of the current fires.

(The extent if the current wildfire impacts in NSW and Victoria are dominating the news worldwide right now. Simply suggesting edits to your text without acknowledging the scale of the ongoing human disaster seemed inappropriate. My condolences if the current fires are impacting people you know, in addition to your research sites.)

148-149: Disturbance rates should be described using standard frequency measures in fire ecology, such as fire rotation or mean fire return interval, if possible.

157-165: Methods are clear and defensible. Good.

169-170: If neither of the two permanent pegs at a transect could not be found, was a transect dropped? or did you re-establish it using GPS coordinates. Either may be fine (depending on the original GPS accuracy), but re-write this sentence to make your re-measurement methods clear.

172-177: Were your field protocols simply confirming—at the tree or transect scale—what was already shown on maps of fire perimeters? If so, make this clear.

139: Condit et al.’s method was developed to tropical trees, but has been wildly applied. You use of it seems reasonable given your goals.

271: You could also cite Wang et al. 2019 here – you cite it elsewhere.

301-306: You may want to look at and possibly cite this recent experimental paper on freezing damage. It is from North America, not Australia, but well-controlled experiments like this one are rare.

Maher, C.T., Nelson, C.R. and Larson, A.J., 2019. Winter damage is more important than summer temperature for maintaining the krummholz growth form above alpine treeline. Journal of Ecology.

312-322: Good discussion of the lack of upslope migration. This fits well with Harsch et al’s conceptual paper on “treeline forms”—and their description of the limiting factors for “abrupt treelines”—which you cite in the conclusion. I think it would be appropriate to cite here, and perhaps draw in that conceptual framework to this discussion.

344-346: These two lines repeat verbatim information presented in the introduction on lines 122-124. Re-phrase.

Figures: In the .pdf I was provided, it is very hard to read the axis labels and tic labels of the figures, particularly for Figure 2. I was able to make sense of it from the captions, though. The figures may be published in higher resolution, but the font size should also be increased.

Figure 3: Could you add vertical dashed lines to this figure, showing the time of disturbance(s) at each site? That type of visual reference would be helpful.

Figure 4: Consider adding the summary statistics as text to each panel in this figure: the data, slope, and what type it is. Then define the types in the figure caption. This would allow the figure to stand alone, and not be reliant on the text.

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Reviewer #1: No

Reviewer #2: No

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While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

4 Mar 2020

Thank you for revising our manuscript and please find below the changes to the Data Availability Statement, Funding Statement and Copyright information for Figure S1 as per your email.

Changes to Data Availability Statement:

We would like to change our Data Availability statement to “The data that supports the findings of this study are openly available at Harvard Dataverse at https://doi.org/10.7910/DVN/I5MNND.

Changes to Funding Statement:

We would like to change our Funding statement to “The Research Centre for Applied Alpine Ecology provided financial support”.

Figure S1 (Revised to Figure 2) Copyright:

We sought copyright permission from the Department of Environment, Land, Water and Planning, through the Spatial Datamart Victoria. The Department of Environment, Land, Water and Planning replied that the metadata is licenced under the Creative Commons Attribution 4.0 International licence, Copyright and Attribution, and thus we are free to use the dataset as per the licence and there is no need for copyright permission. The spatial data was sourced from the Spatial Datamart Victoria: https://services.land.vic.gov.au/SpatialDatamart/dataSearchViewMetadata.html?anzlicId=ANZVI0803004741&extractionProviderId=1

This link includes the copyright information under “Access constraints”.

For the response to reviewers comments please see attached 'Response to Reviewers' document.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

23 Mar 2020

Alpine treeline ecotone stasis in the face of recent climate change and disturbance by fire

PONE-D-19-29648R1

Dear Dr. Naccarella,

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Reviewers' comments:

25 Mar 2020

PONE-D-19-29648R1

Alpine treeline ecotone stasis in the face of recent climate change and disturbance by fire

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