Swedish Scandes Research Articles

Ecological tree line history and palaeoclimate – review of megafossil evidence from the Swedish Scandes

The postglacial tree line and climate history in the Swedish Scandes have been inferred from megafossil tree remains. Investigated species are mountain birch (Betula pubescens ssp. czerepanovii), Scots pine (Pinus sylvestris) and grey alder (Alnus incana). Betula and Pinus first appeared on early deglaciated nunataks during the Lateglacial. Their tree lines peaked between 9600 and 9000 cal. a BP, almost 600 m higher than present‐day elevations. This implies (adjusted for land uplift) that early Holocene summer temperatures may have been 2.3°C above modern ones. Elevational tree line retreat characterized the Holocene tree line evolution. For short periods, excursions from this trend have occurred. Between c. 12 000 and 10 000 cal. a BP, a pine‐dominated subalpine belt prevailed. A first major episode of descent occurred c. 8200 cal. a BP, possibly forced by cooling and an associated shift to a deeper and more persistent snow pack. Thereafter, the subalpine birch forest belt gradually evolved at the expense of the prior pine‐dominated tree line ecotone. A second episode of pine descent took place c. 4800 cal. a BP. Historical tree line positions are viewed in relation to early 21st century equivalents, and indicate that tree line elevations attained during the past century and in association with modern climate warming are highly unusual, but not unique, phenomena from the perspective of the past 4800 years. Prior to that, the pine tree line (and summer temperatures) was consistently higher than present, as it was also during the Roman and Medieval periods, c. 1900 and 1000 cal. a BP, respectively.

Recent glacier recession - A new source of postglacial treeline and climate history in the Swedish Scandes

Climate warming during the past century has imposed recession of glaciers and perennial snow/ice patches along the entire Swedish Scandes. On the newly exposed forefields, subfossil wood remnants are being outwashed from beneath ice and snow bodies. In Scandinavia, this kind of detrital wood is a previously unused source of postglacial vegetation and climate history. The present study reports radiocarbon dates of a set of 78 wood samples, retrieved from three main sites, high above modern treelines and stretching along the Swedish Scandes. In accord with previous studies, pine (Pinus sylvestris) colonized early emerging nunataks already during the Late Glacial. Around 9600-9500 cal. yr BP a first massive wave of tree establishment, birch and pine, took place in "empty" glacier cirques. Both species grew 400-600 m above their present-ay treeline position and the summer temperatures may have been 3.5 oC warmer than present. In respons to Neoglacial cooling, treelines of both birch and pine descended until their final disappearance from the record 4400 and 5900 cal. yr BP, respectively. During the entire interval 9600 to 4400 cal. yr BP, birch prospered in a 100-150 broad belt above the uppermost pines. The recent emergence of tree remnants in the current habitats relates to the contemporary episode of climate warming, possibly unprecedented for several past millennia. It is inferred, by an anology with the past, that in a future scenario with summers 3.5 °warmer than present, the birch treeline may rise by 600 m or so.

Ancient Subalpine Clonal Spruces (<i>Picea abies</i>): Sources of Postglacial Vegetation History in the Swedish Scandes

This study addresses the long-standing issue of postglacial immigration of Picea abies (Norway spruce) into Scandinavia. The main methodological focus is on using megafossil tree remains (wood and cones) of spruce and other species retrieved from the treeline ecotone of the Swedish Scandes as a tool for vegetation reconstruction. The core data come from radiocarbon dating of megafossils preserved in the soil underneath clonal groups of Picea abies , formed by rooting of branches that over time give rise to new upright stems. At high elevations, we found living spruce clones, which in some cases may be part of a continuous clonal series dating back to the early Holocene (9500 cal. yr BP). The presence of Picea in the Swedish Scandes at this early stage concurs with previous megafossil inferences. This date, which places the arrival of Picea very soon after regional deglaciation, is several millennia earlier than the arrival date inferred from pollen data. The persistence of some individual Picea clones from the early Holocene thermal optimum to the present implies that permanently open or semi-open spots existed in the high-mountain landscape even during periods when treelines in general were much higher than at present. Initially, Picea clones appear to have existed in a regional no-analogue vegetation matrix of widely scattered pine ( Pinus sylvestris ), mountain birch ( Betula pubescens ssp. czerepanovii ), Siberian larch ( Larix sibirica ) and thermophilic broadleaved deciduous species. In response to subsequent neoglacial cooling, the alpine character of the landscape has been enhanced through a lowered pine treeline and the disappearance of larch and thermophiles. The endurance of spruces, which escaped fire and other calamities, is due to their inherent phenotypic plasticity. Increasing climatic harshness throughout the Holocene conserved them as crippled krummholz, protected from winter stress by almost complete snow coverage. The appearance of Picea abies exclusively in western Scandinavia shortly after the deglaciation could suggest that the species immigrated from “cryptic” ice age refugia much closer to Scandinavia than conventionally thought.

High‐resolution diatom δ18O records, from the last 150 years, reflecting changes in amount of winter precipitation in two sub‐Arctic high‐altitude lakes in the Swedish Scandes

AbstractWaters from high‐altitude alpine lakes are mainly recharged by meteoric water. Because of seasonal variations in precipitation and temperature and relatively short hydraulic residence times, most high‐altitude lakes have lake water isotopic compositions (δ18Olake) that fluctuate due to seasonality in water balance processes. Input from snowmelt, in particular, has a significant role in determining lake water δ18O. Here we compare two high‐resolution δ18Odiatom records from lake sediments in the Swedish Scandes with instrumental data from the last century obtained from nearby meteorological stations. The time period AD 1900–1990 is characterised by an increase in winter precipitation and high winter/summer precipitation ratios and this is recorded in δ18Odiatom as decreasing trends. Lowest δ18Odiatom values and highest amount of winter precipitation are found around AD 1990 when the winter North Atlantic Oscillation index was above +2. We conclude that for the last 150 a the main factor affecting the δ18Odiatom signal in these sub‐Arctic high‐altitude lakes with short residence times has been changes in amount of winter precipitation and that δ18Odiatom derived from high‐altitude lakes in the Swedish Scandes can be used as a winter precipitation proxy. Copyright © 2010 John Wiley & Sons, Ltd.

Alpine flora dynamics - a critical review of responses to climate change in the Swedish Scandes since the early 1950s

Reports about changes of alpine plant species richness over the past 60 years in the Swedish Scandes are reviewed, synthesized and updated with data from recent reinventories. Methodologically, this endeavour is based on resurveys of the floristic composition on the uppermost 20 m of four high-mountain summits. The key finding is that the species pool has increased by 60–170% since the 1950s and later. Some of the invading species are new to the alpine tundra, with more silvine and thermophilic properties than the extant alpine flora. Not a single species of the original flora has disappeared from any of the summits. This circumstance is discussed in perspective of widespread expectations of pending temperature-driven extinction of alpine species in an alleged future warmer climate. These progressive changes coincided with distinct warming (summer and winter) since the late 1980s. During a short cooler period (1974–1994), the species numbers decreased and the upper elevational limits of some ground cover species descended. Thus, discernible responses, concurrent with both warming and cooling intervals, sustain a strong causal link between climate variability and alpine plant species richness. Methodologically, plot-less revisitation studies of the present kind are beset with substantial uncertainties, which may overstate floristic changes over time. However, it is argued here that carefully executed and critically interpreted, no other method can equally effectively sense the earliest phases of plant invasions into alpine vegetation.

One century of treeline change and stability - experiences from the Swedish scandes

This paper elaborates and visualizes processes recorded in a recent regional and multi-site study of elevational treeline dynamics during the period 1915 to 2007 in the Swedish Scandes. The purpose is to give a concrete face of the landscape transformation which is associated with the recorded treeline shifts. The main focus is on stand-level structure of past and present treelines and the advance zones, where climate change elicited responses by Betula pubescens ssp. czerepanovii, Picea abies and Pinus sylvestris. All species shifted their treelines upslope by a maximum of c. 200 m in elevation. Most sites, however, manifested changes of smaller magnitudes. This relates to topoclimatic constraints which decouple treeline performance from the macroclimate. The general character of sites which support large and small treeline shifts, respectively, are outlined. The spacing, age structure, growth rates of the tree advance zones are accounted for each of the concerned species. In temporal and spatial detail, the different tree species responded individualistically according to their specific ecologies. Current spread of young seedlings and saplings to increasingly higher elevations in the alpine tundra is particularly highlighted as it may represent the forefront of future treeline advance. It is argued that the current evolution of the treeline ecotone represents a fundamental, although not necessarily entirely unique, reversal of the long-term (Holocene) trend of neoglacial treeline descent.

A Richer, Greener and Smaller Alpine World: Review and Projection of Warming-Induced Plant Cover Change in the Swedish Scandes

Alpine plant life is proliferating, biodiversity is on the rise and the mountain world appears more productive and inviting than ever. Upper range margin rise of trees and low-altitude (boreal) plant species, expansion of alpine grasslands and dwarf-shrub heaths are the modal biotic adjustments during the past few decades, after a century of substantial climate warming in the Swedish Scandes. This course of biotic landscape evolution has reached historical dimensions and broken a multi-millennial trend of plant cover retrogression, alpine tundra expansion, floristic and faunal impoverishment, all imposed by progressive and deterministic neoglacial climate cooling. Continued modest warming over the present century will likely be beneficial to alpine biodiversity, geoecological stability, resilience, sustainable reindeer husbandry and aesthetic landscape qualities. These aspects are highlighted by an integrative review of results from long-term monitoring of subalpine/alpine vegetation in the Swedish Scandes. This forms the basis for some tentative projections of landscape transformations in a potentially warmer future. Notably, these results and projections are not necessarily valid in other regions and differ in some respects from model predictions. Continued monitoring is mandatory as a basis for generation of more realistic vegetation and ecosystem models.

Wind-Conditioned 20th Century Decline of Birch Treeline Vegetation in the Swedish Scandes

The study focused on a frequent, although not dominant, mode of treeline change in the Swedish Scandes over the past century. Monitoring of stand density decline in a wind-exposed subalpine birch (Betula pubescens ssp. tortuosa) population was carried out over the past 30 years. The overall result included substantial and unbalanced individual mortality and a drastic reduction in stature of surviving birches. Radiocarbon dating of in situ subfossil birch-tree remains in an adjoining but windier site revealed complete stand demise over the past century or so. It is inferred that the extant and extinct birch populations represent different phases in an unbroken process of stand-level demise, hypothetically initiated in response to reindeer disturbance during the final phase of the Little Ice Age. This process was subsequently communicated more or less autogenically in a leeward direction as a consequence of the exposed and drought-prone nature of the site. Increasing westerly wind circulation and decreasing soil moisture (earlier snowmelt) during the past century may have contributed in this respect, although these aspects require further study. Within strongly wind-exposed terrain, a century of substantial warming has not sufficed to offset the birch decline. The results highlight the conclusion that under certain circumstances, wind and associated ecological and physiological effects are the overriding determinants of treeline position and structure. These are fundamental insights for generating realistic landscape-scale models of treeline change in a hypothetically warmer future.

Post‐Little Ice Age tree line rise and climate warming in the Swedish Scandes: a landscape ecological perspective

Summary Elevational tree line change in the southern Swedish Scandes was quantified for the period 1915–2007 and for two sub‐periods 1915–1975 and 1975–2007. The study focused on Betula pubescens ssp. czerepanovii, Picea abies and Pinus sylvestris at a large number of sites distributed over an 8000‐km2 area. The basic approach included revisitations of fixed sites (elevational belt transects) and measurements of tree line positions (m a.s.l.) during these three periods. Over the past century, tree lines of all species rose at 95% of the studied localities, with means of 70–90 m. All three species displayed maximum upshifts by about 200 m, which manifests a near‐perfect equilibrium with instrumentally recorded air temperature change. This magnitude of response was realized only in particular topographic situations, foremost wind‐sheltered and steep concave slopes. Other sites, with more wind‐exposed topoclimatic conditions, experienced lesser magnitudes of upshifts. Thus, spatial elevational tree line responses to climate change are markedly heterogeneous and site‐dependent. Modelling of the future evolution of the forest‐alpine tundra transition has to consider this fact. Even in a hypothetical case of substantial climate warming, tree lines are unlikely to advance on a broad front and a large proportion of the alpine tundra will remain treeless. During the period 1975–2007, the tree lines of Picea and Pinus (in particular) advanced more rapidly than that of Betula towards the alpine region. These species‐specific responses could signal a potential trajectory for the evolution of the ecotone in a warmer future. Thereby a situation with some resemblance with the relatively warm and dry early Holocene would emerge. Substantial tree line upshifts over the past two to three decades coincide with air and soil warming during all seasons. This implies that both summer and winter temperatures have to be included in models of climate‐driven tree line performance. Synthesis. Maximum tree line rise by 200 m represents a unique trend break in the long‐term Holocene tree line regression, which has been driven by average climate cooling for nearly 10 000 years. Tree line positions are well‐restored to their pre‐Little Ice Age positions. Recent tree line ascent is a truly anomalous event in Holocene vegetation history and possibly unprecedented for seven millennia.

Vegetation development at a mountain settlement site in the Swedish Scandes during the late Holocene: palaeoecological evidence of human-induced deforestation

A palaeoecological study was conducted close to the forest limit in the northern Scandinavian mountain range. The aim was to elucidate the degree to which human impact has affected the vegetation at Hiednikvalta, a Stallo settlement site. Stallo settlements consist of round hut foundations that have a hearth in the middle and are surrounded by a low turf wall. They were probably established by Sami people using the mountain areas for hunting and/or reindeer herding. In order to separate the effects of humans and climate on the vegetation, a reference area approach was adopted, i.e. the vegetation development at the Stallo settlement site Hiednikvalta was compared with the vegetation development in a forested reference area Avvuhatjahkka, at the same altitude as Hiednikvalta but with no archaeological remains of settlements. Peat stratigraphies were retrieved at the two sites and pollen analysis, loss-on-ignition (LOI), pollen accumulation rates (PAR), macrofossil analysis and Betula pollen size statistics were all examined. The results indicate that Hiednikvalta was forested with Betula trees prior to the Stallo settlement period, which occurred between the eighth and thirteenth centuries. Human activities resulted in a decrease in tree cover at the site, as found in a previous study at Adamvalta, another Stallo settlement site in the region. However, the magnitude of vegetation change, and the post-Stallo vegetation development differed between the two areas, suggesting that site-specific factors are important. The use of reference areas in palynological studies is also discussed.

Holocene pine tree-line evolution in the Swedish Scandes: Recent tree-line rise and climate change in a long-term perspective

Holocene pine tree-line evolution in the Swedish Scandes: Recent tree-line rise and climate change in a long-term perspective

Cambiamenti climatici e dinamica di popolazione al limite superiore del bosco: importanza delle ricerche di lungo termine

Abstract: Temperature-limited ecosystems are believed to be extremely sensitive to global warming and the upward treeline migration is one of the most relevant phenomenon. Long term monitoring of demographic trends appears essential in order to understand possible effects of higher temperatures on cold ecosystems. Long-term monitoring of natality and mortality of a Pinus sylvestris population in the Swedish Scandes from 1973 to 2005 (Kullman 2007) showed that number of trees increased in the study plots due to higher natality rate, especially in the last decade. This reverses a natural trend of treeline decline and recession occurred in the past.

Cambiamenti climatici e dinamica di popolazione al limite superiore del bosco: importanza delle ricerche di lungo termine

Temperature-limited ecosystems are believed to be extremely sensitive to global warming and the upward treeline migration is one of the most relevant phenomenon. Long term monitoring of demographic trends appears essential in order to understand possible effects of higher temperatures on cold ecosystems. Long-term monitoring of natality and mortality of a Pinus sylvestris population in the Swedish Scandes from 1973 to 2005 (Kullman 2007) showed that number of trees increased in the study plots due to higher natality rate, especially in the last decade. This reverses a natural trend of treeline decline and recession occurred in the past.

Tree line population monitoring of Pinus sylvestris in the Swedish Scandes, 1973–2005: implications for tree line theory and climate change ecology

Summary Demographic trends of Pinus sylvestris L. (Scots pine) tree line populations are reported for a 32‐year monitoring period (1973–2005). Functional and projective aspects of tree line performance were analysed by relating temporal variability and change of vital population parameters, such as natality/mortality, vigour, injuries, height growth and seed viability to contemporary variations in air and soil temperatures. The size of the entire sampled population increased by 50% during the 32‐year observation period and thereby pine has become a more prominent element on the landscape. This reverses a natural multicentennial or even millennial trend of tree line decline and recession. Contrasting population trends were recorded for the subperiods 1973–87 and 1988–2005, viz. decline and increase, respectively. Mean summer temperatures (JJA) did not change perceivably over and between these intervals, although some exceptionally warm summers from 1997 onwards have contributed to population expansion by increased seed viability and seedling emergence. Winter temperatures (DJF) decreased significantly over the first subperiod and were consistently higher during the second, which has significantly lowered the mortality rates. A functional link to winter temperature conditions was particularly stressed by the aetiology of individual plant vigour, injuries and final mortality. Classical symptoms of winter desiccation correlated significantly with low winter temperatures. This negative impact occurred with a high frequency during the decline phase and virtually ceased during the expansion phase from 1988 onwards, when winter air and root zone temperatures were raised to a consistently higher level. Winter and summer temperatures in the air and soil, as well as positive feedback mechanisms and nonlinear responses, must be taken into account in the search for global or regional mechanical explanations for the tree line phenomenon. This insight helps to generate realistic tree line models for a high‐CO2 world, when winter warming is usually predicted to be particularly large.

Holocene pine tree-line evolution in the Swedish Scandes: Recent tree-line rise and climate change in a long-term perspective

Holocene pine tree-line evolution in the Swedish Scandes: Recent tree-line rise and climate change in a long-term perspective

On the presence of late‐glacial trees in western Norway and the Scandes: a further comment

A response to L. Kullman (2005) On the presence of late-glacial trees in the Scandes. Journal of Biogeography32, 1499–1500, a reply to H.H. Birks, E. Larsen & H.J.B. Birks (2005) Did tree-Betula, Pinus and Picea survive the last glaciation along the west coast of Norway? A review of the evidence, in light of Kullman (2002). Journal of Biogeography, 32, 1461–1471. The primary aim of the paper by Birks et al. (2005b), as indicated by its title, was to examine the inferences of Kullman (2002) that Betula, Pinus, and Picea survived the last glaciation along the west coast of Norway. We highlighted and discussed the incompatibility between Kullman's inferences, drawn from dated late-glacial tree megafossil records from the Swedish Scandes, and other lines of palaeoecological evidence actually from western Norway. We regard the latter to be equally as valid as Kullman's evidence, and in no way to be dismissed as ‘merely’ negative evidence (Kullman, 2005). Late-glacial radiocarbon-dated plant and animal records positively demonstrate the local occurrence of arctic and high alpine plants, chironomids, oribatid mites, and beetles along the Norwegian west coast, consistent with the inference of a climate unsuitable for tree growth (e.g. Birks & Ammann, 2000; Birks et al., 2005a). This contradicts Kullman's biogeographical inference of late-glacial tree refugia there, which is based on pollen evidence alone, with no supporting macrofossil or megafossil evidence. To strengthen his interpretations, Kullman (2005) quotes his record of megafossils that suggest early Holocene tree growth at a high altitude in Swedish Lapland (Kullman, 1999). He says (Kullman, 2005) that this is fully compatible with the lack of tree macrofossils in the sediment record in the same lake (Barnekow, 1999). However this situation is puzzling rather than compatible, as the sediment record of Barnekow (1999) (which includes Salix polaris) seems to be ecologically incongruous with the simultaneous occurrence of Pinussylvestris and Alnus incana remains reported by Kullman. No explanation has been offered by either Kullman or Barnekow for this conundrum. Birks et al. (2005b) also documented the glaciological data that provide rational and valid evidence of an inhospitable habitat for trees at 1360 m a.s.l. during the late-glacial, an altitude not reached by trees even at the present time in the Scandes. Kullman (2005), however, vigorously defends his radiocarbon dates on megafossils and takes them as irrefutable evidence of tree growth on Mt Åreskutan in the late-glacial. Thus another incongruity is highlighted and this particular puzzle also remains unexplained and unsolved. Our paper discussed these points with the aim of stimulating further research. It is to be hoped that we, Kullman, and others can adopt a positive approach and try to find scientific explanations for the apparently incompatible evidence. For example, what is the pollen and macrofossil representation of a few isolated trees in an otherwise treeless landscape? Surface samples and modelling approaches could be used to investigate this (Birks, 2005). Is the pollen and macrofossil production of trees such as Betula, Pinus, and Picea reduced by marginal growing conditions at or near the tree line, and, if so, by how much? How could trees grow at 1360 m in the Swedish mountains in the Allerød and Younger Dryas? We have highlighted the palaeoecological incongruities. We hope that they can now be used as starting points to investigate matters further, utilizing a variety of techniques and approaches. Editor: Mark Bush

Early Holocene Appearance of Mountain Birch (Betula pubescens ssp. tortuosa) at Unprecedented High Elevations in the Swedish Scandes: Megafossil Evidence Exposed by Recent Snow and Ice Recession

Megafossil wood remnants (trunks and roots) of mountain birch (Betula pubescens ssp. tortuosa) were retrieved from the alpine tundra of the southern Swedish Scandes (the Sylarna Mountains). The samples have recently become exposed by rapid recession of glaciers and snow patches at three sites located 630 to 350 m higher than the present-day birch tree-limit and 350 to 80 m higher than the early Holocene pine limit. Radiocarbon dating yielded ages ranging between 8700 and 6200 B.P. (9700–7000 cal B.P.). This is the first direct evidence of past tree growth at such high elevations in the Scandes, relative to the modern tree-limit. The overall situation with uniquely high tree-limits and absence of glaciers suggests a climate with generally drier and warmer summers than during any later part (secular-millennial scale) of the Holocene. Corrected for glacioisostatic land-uplift, summers around 8700 B.P. may have been about 3°C warmer than at present. This inference is compatible with the Milankovitch model of orbital climate forcing during the course of the Holocene, implying a gradually increasing maritime climate (less seasonal), with a heavier snowpack. As a consequence, alpine snowfields started to develop over the period 8700–6200 B.P., causing demise and burial of the highest metapopulations of mountain birch. At lower elevations, where snow accumulation had previously been suboptimal for birch, this species could now benefit from a deeper and more persistent snow cover. Since about 7000 B.P., a distinct mountain birch belt has been a characteristic feature in this part of the Scandes. The exposure of mountain birch megafossils relates to substantial 20th-century warming, reaching a peak in the past few years. Evidently, this is an exceptional occurrence in the context of several past millennia.

Early-Holocene spruce, Picea abies (L.) Karst., in west central Sweden as revealed by pollen analysis

Lateglacial and early Holocene macrofossils of Norway spruce (Picea abies) found in the Swedish Scandes have challenged the prevailing theory, based on pollen analyses, that spruce arrived rather late ( c. 3500 yr cal. BP) to Sweden. The aim of this study was to investigate whether the early-Holocene spruce establishment could be effectively assessed by pollen analysis. Four pollen analyses were carried out on peat mires in a river valley in the southern Swedish Scandes, where spruce has been present since the lateglacial. One site was analysed for the occurrence of macrofossil wood remains. The pollen diagrams display the local vegetation history for the last c., 10 000 years, and reveal the first occurrence of spruce pollen c. 9000 yr cal. BP. Spruce apparently became established in the northern part of the valley around 9000 BP, and thereafter spread south wards. The results are discussed in terms of the background pollen signal for spruce, i.e., local versus regional pollen, the presence or absence of forest on the sites during the Holocene, and the selection of study sites in relation to the aim of the investigation. Pollen analysis, as applied in this study, is shown to be a relevant method for the study of tree immigration in the early Holocene, at both a local and a landscape level.

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Recent reversal of Neoglacial climate cooling trend in the Swedish Scandes as evidenced by mountain birch tree-limit rise

An intensive study reconstructs the local late-Holocene tree-limit history ( Betula pubescens ssp. tortuosa) in the southern Swedish Scandes, as a proxy palaeoclimatic indicator. Elevational tree-limit rise by 75 m, in accordance with instrumentally recorded summer warming, took place over the past century as evidenced by a combination of historical records and modern age–structure analyses. This regionally representative development was put into long-term perspective by radiocarbon-dating of a large sample of megafossil wood remains (trunks and roots), preserved and recovered at the same location and elevation as the new and higher tree-limit. The magnitude and rate of 20th century tree-limit advance (implicitly climate warming) were without precedent for the past ca. 3500 calendar years. The course of development conforms to an increasingly emergent worldwide pattern, suggesting that warming over the past century is a fundamental reversal of a long-term (Neoglacial) cooling trend. The results are of relevance for current efforts to attribute causes of recent warming in terms of natural versus anthropogenic forcing. The highly anomalous nature of the past century, with respect to climate and ecological change, could argue that at least in part, the warming may be human-induced. However, definite conclusions have to await more global-scale data.