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Spatial Variation in the Ongoing and Widespread Decline of a Keystone

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Spatial Variation in the Ongoing and Widespread Decline of a Keystone 1 2 MS. CATHERINE RUTH DICKSON (Orcid ID : 0000-0002-9701-346X) 3 4 5 Article type : Research Article 6 7 8 SPATIAL VARIATION IN THE ONGOING AND WIDESPREAD DECLINE OF A 9 KEYSTONE PLANT SPECIES 10 11 ABSTRACT 12 Extensive dieback in dominant plant species in response to climate change is increasingly 13 common. Climatic conditions and related variables, such as evapotranspiration, vary in 14 response to topographical complexity. This complexity plays an important role in the 15 provision of climate refugia. In 2008/2009 an island-wide dieback event of the keystone 16 cushion plant Azorella macquariensis Orchard (Apiaceae) occurred on sub-Antarctic 17 Macquarie Island. This signalled the start of a potential regime shift, suggested to be driven 18 by increasing vapour pressure deficit. Eight years later we quantified cover and dieback 19 across the range of putative microclimates to which the species is exposed, with the aim of 20 explaining dieback patterns. We test for the influence of evapotranspiration using a suite of 21 topographic proxies and other variables as proposed drivers of change. We found higher 22 cover and lower dieback towards the south of the island. The high spatial variation 23 in A. macquariensis populations was best explained by latitude, likely a proxy for macroscale 24 climate gradients and geology. Dieback was best explained by A. macquariensis cover and 25 latitude, increasing with cover and towards the north of the island. The effect sizes of terrain 26 variables that influenceAuthor Manuscript evapotranspiration rates were small. Island-wide dieback remains 27 conspicuous. Comparison between a subset of sites and historical data revealed a reduction of This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/AEC.12758 This article is protected by copyright. All rights reserved 28 cover in the north and central regions of the island, and a shift south in the most active areas 29 of dieback. Dieback remained comparatively low in the south. The presence of seedlings was 30 independent of dieback. This study provides an empirical baseline for spatial variation in the 31 cover and condition of A. macquariensis, both key variables for monitoring condition and 32 ‘cover-debt’ in this critically endangered endemic plant species. These findings have broader 33 implications for understanding the responses of fellfield ecosystems and other Azorella 34 species across the sub-Antarctic under future climates. 35 KEY WORDS 36 Azorella, climate change, terrain variables, dieback, sub-Antarctic 37 38 INTRODUCTION 39 The effects of anthropogenic climate change have now been documented in ecosystems 40 across the globe (Field et al. 2014). Many plant species, for example, are undergoing rapid 41 changes in condition and distribution (Chen et al. 2011; Parmesan and Yohe 2003), and 42 differing rates of species’ responses are resulting in the formation of novel communities 43 (Chown et al. 2013; le Roux and McGeoch 2008). As climate conditions change, dominant 44 species may become displaced by those better adapted to current conditions. This is well 45 illustrated by the growing number of landscape-scale plant mortality events across different 46 ecosystems in Australia and elsewhere (Allen et al. 2010; Breshears et al. 2005; Carnicer et 47 al. 2011; Duke et al. 2017; Harris et al. 2018; Hoffmann et al. 2018). Three such examples 48 include (i) dieback of overstorey species extending across 12,000 km2 of south-western North 49 American woodlands, driven by the interactions of drought and bark beetle infestations 50 (Breshears et al. 2005), (ii) extensive northern Australian mangrove dieback along 1,000 km 51 of coastline, as a response to climate change and extreme events (Harris et al. 2018), and (iii) 52 increase in artic browning across Alaskan boreal forests (reduction in the normalised 53 difference vegetation index, NDVI) between 1982-2003, which may be attributed to multiple 54 factors including drought stress, insect damage, disease, wildfire and changes in resource 55 allocation (VerbylaAuthor Manuscript 2008). The first case of a landscape-scale plant mortality event in the sub- 56 Antarctic occurred on Macquarie Island in 2008/2009. Dieback of the keystone plant species 57 Azorella macquariensis Orchard (Apiaceae, Macquarie cushion) was observed at 88% of sites 58 surveyed across the island (Bergstrom et al. 2015). Dieback was found in even very spatially This article is protected by copyright. All rights reserved 59 isolated individuals, with the few unaffected populations observed mainly at higher 60 elevations and towards the south of the island (Bergstrom et al. 2015). 61 Two explanations were proposed to explain the dieback event. First, a pathogen may have 62 emerged as a result of the change in climate (Whinam et al. 2014). No definitive relationship 63 between a disease-causing pathogen and unhealthy cushions has been established to-date. 64 However, multiple potentially pathogenic taxa were identified, suggesting potential 65 susceptibility to infection under stressful conditions (Bergstrom et al. 2015). Second, a 66 significant change in the regional climate over the past ~40 years across Macquarie Island has 67 resulted in a decadal reduction in plant available water (PAW) during the summer growing 68 season (Bergstrom et al. 2015). The functional morphology of A. macquariensis, where it 69 both quickly loses and takes-up water (Rolland et al. 2015), suggests that while it is well 70 adapted to cope with small daily variations in weather, it may be susceptible to the current 71 change in the regional climate, where extended periods between rainfall and increased 72 evapotranspiration are increasingly common (Bergstrom et al. 2015). The closely related 73 species A. selago has been shown experimentally to experience increased stem death and 74 early senescence under reduced rainfall conditions, implying that the genus may be 75 negatively impacted by climate change (see le Roux et al. 2005). These two hypotheses are 76 not mutually exclusive as climate change may weaken plant condition facilitating a secondary 77 pathogenic infection (Bergstrom et al. 2015; Hoffmann et al. 2018). 78 Regardless of the proximate cause of mortality, climate change is implicated in the 79 widespread dieback of A. macquariensis on the island (Bergstrom et al. 2015; Whinam et al. 80 2014). Given the evidence, it is important to quantify the variation in A. macquariensis 81 distribution and condition (dieback) across both geographic and environmental space. 82 Azorella populations increase with elevation (Phiri et al. 2015), and A. macquariensis 83 populations have been observed in higher densities, with lower levels of dieback in the south 84 of the island (Bergstrom 2013, updated 2014), which is also higher in elevation. A strong 85 correlation between local climate variation and A. macquariensis dieback would (i) provide 86 evidence for the role of climate change in dieback and (ii) identify potential areas, or 87 microrefugia, where A. macquariensis may best survive should the current climate trajectory 88 continue. Such baselineAuthor Manuscript data provide essential information for understanding the drivers of 89 change and physiological and ecological mechanisms responsible for observed declines in 90 the states of ecosystems (Bland et al. 2018). Conservation and management decisions about 91 regime change or ecosystem collapse in the Azorella-dominated fellfield ecosystems are best This article is protected by copyright. All rights reserved 92 underpinned by knowledge of the system’s past and current state, so that departure of 93 ecosystems from historic baselines can be quantified (see Bland et al. 2018). 94 Here we quantify the current distribution of A. macquariensis cover and dieback across both 95 geographic and environmental space to generate a spatially representative baseline for 96 monitoring future change, and to better understand the mechanisms driving A. macquariensis 97 dieback. We simultaneously provide an update on the condition of A. macquariensis 98 populations across Macquarie Island eight years after the dieback event was first observed 99 and five years after the first island-wide assessment (Bergstrom et al. 2015; Whinam et al. 100 2014). Specifically, we expected: (i) A. macquariensis populations to increase in cover and 101 condition with an increase in latitude and elevation (Bergstrom et al. 2015), with these 102 variables representing island-wide macroscale gradients that are well known elsewhere to be 103 related to various dimensions of microclimate (Dobrowski 2011), (ii) in addition, if dieback 104 is driven by finer-scale topography that relates to high evapotranspiration, then the population 105 condition will vary across Macquarie Island and be negatively related to variables associated 106 with high moisture stress (i.e. high wind exposure or low topographic wetness index), (iii) A. 107 macquariensis recruitment will be negatively related to the proportion of dieback, because if 108 evapotranspiration negatively impacts adult plants in the form of dieback it may be expected 109 also to affect recruitment. Based on the outcome, we discuss assessment and monitoring 110 priorities to inform potential options for management of this species in the face of ongoing 111 climate change. 112 113 METHODS 114 Macquarie Island (54°30’S, 158°55’E) is a small, narrow sub-Antarctic island of 12 390 ha 115 (Selkirk et al. 1990) (Fig. 1). It is characterised by a low, undulating alpine plateau with 116 numerous lakes, tarns and bogs, ranging between 200 – 433 m above sea level (asl) (Selkirk 117 et al. 1990). The island has historically had a consistently cool, misty and windy climate, 118 with the plateau regularly shrouded in cloud (Selkirk et al. 1990). However, over the past 40 119 years there has been a significant change in local climate resulting in a significant increase in 120 evapotranspirationAuthor Manuscript (Bergstrom et al.
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