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High-velocity upward shifts in vegetation are ubiquitous in mountains of western North America [1]

['James R. Kellner', 'Department Of Ecology', 'Evolution', 'Organismal Biology', 'Brown University', 'Providence', 'Rhode Island', 'United States Of America', 'Institute At Brown For Environment', 'Society']

Date: 2023-02

Abstract The velocity of climate change and its subsequent impact on vegetation has been well characterized at high elevations and latitudes, including the Arctic. But whether species and ecosystems are keeping pace with the velocity of temperature change is not as well documented. Some evidence indicates that species are less able to keep pace with the velocity of climate change along elevational gradients than latitudinal ones. If substantiated this finding could warrant reconsideration of a current cornerstone of conservation planning. Here we use 27 years of high-resolution satellite data to quantify changes in vegetation cover across elevation within nine mountain ranges in western North America, spanning tropical Mexico to subarctic Canada and from coastal California to interior deserts. Across these ranges we show a uniform pattern at the highest elevations in each range, where increases in vegetation have occurred ubiquitously over the past three decades. At these highest elevations, the realized velocity of vegetation varies among mountain ranges from 19.8–112.8 m · decade-1 (mean = 67.3 m · decade-1). This is equivalent, with respect to gradients in temperature, to a 14.4–104.3 km · decade-1 poleward shift (mean = 56.1 km · decade-1). This realized velocity is 4.4 times larger than previously reported for plants, and is among the fastest rates predicted for the velocity of climate change. However, in three of the five mountain ranges with long-term climate data, realized velocities fail to keep pace with changes in temperature, a finding with important implications for conservation of biological diversity.

Citation: Kellner JR, Kendrick J, Sax DF (2023) High-velocity upward shifts in vegetation are ubiquitous in mountains of western North America. PLOS Clim 2(2): e0000071. https://doi.org/10.1371/journal.pclm.0000071 Editor: Mukunda Dev Behera, Indian Institute of Technology Kharagpur, INDIA Received: May 6, 2022; Accepted: October 8, 2022; Published: February 15, 2023 Copyright: © 2023 Kellner et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: The Landsat and SRTM data used in this analysis are available at the links below: https://www.usgs.gov/landsat-missions/landsat-data-access https://www.usgs.gov/centers/eros/data. Funding: This research was supported by a grant from the Institute at Brown for Environment and Society to JRK and DFS. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction A cornerstone of the global strategy to conserve biodiversity involves protecting mountainous regions [1, 2]. Doing this is important because much of the Earth’s biodiversity is found in montane regions, such as the Andes [2, 3], but also because mountains offer the potential of buffering species from some aspects of climate change [4]. In particular, because montane regions have steep topographic gradients, they have lower overall velocities of climate change than topographically simpler, flatter areas [5]. This means that within mountains, many species would need to shift their distributions only small distances along elevation gradients in order to retain temperatures they experienced historically [6]. Indeed, in response to existing climate change, a great variety of species have already shifted their distributions to higher elevations [7–9]. However, some shifts in biotic responses with montane regions seem to be exceedingly slow, in some cases an order of magnitude slower than climate change [7]. These shifts seem to be slower overall for plants than for many animal groups [8, 10]. Available evidence suggests that some species are less able to keep pace with the velocity of change along gradients of elevation than latitudinal ones [11]. If additional evidence bears out these sorts of observations then it would call into question our understanding of biotic responses, particularly with respect to velocities of climate change. In both montane and non-montane regions, understanding the velocity of climate change and corresponding shifts in the distribution of biota have become important aspects of conservation planning [6, 12]. Velocities are derived from changes in climate anticipated or observed across spatial gradients, and describe the instantaneous local rate of change needed to maintain constant conditions (km · yr-1). Current and anticipated velocities of climate change are complex, with substantial variation among and within regions [5, 6]. Mountainous regions in particular differ from topographically homogeneous areas [6, 13]. Velocity itself can be measured against a host of possible factors, and recent work has focused not just on changes in temperature, but on interactions between temperature and water availability [5, 13]. To describe the response of biota, here we use the term ‘realized velocity’, which we define as the rate of change in geographic distribution that is observed or anticipated to occur. Realized velocities differ from climate velocities, because they integrate how organisms respond to changes in the environment, rather than changes in the environment alone. These responses could be less than climate velocity due to intrinsic limitations in dispersal capacity, extrinsic factors such as habitat fragmentation, or tolerances that aren’t matched to changes in climate. Losses of ecosystems or species become more likely when climate velocity exceeds the realized velocity. This occurs when species or ecosystems are not keeping pace with climate change. Available evidence shows that some species and ecosystems are keeping pace, shifting or expanding their geographic distributions poleward or upward in elevation [6, 9, 11, 14], while others are not doing so [7, 15]. One way to evaluate whether species and ecosystems are keeping pace with climate change, in either montane on non-montane regions, is to examine the geographic distribution of vegetation over time and across spatial gradients using high-resolution remote sensing [16, 17]. Quantifying how vegetation is changing facilitates examination of key questions important for conservation of biological resources in response to climate change, such as whether changes in climate observed to date are leading to an increase or decrease in the total habitable portion of mountainous areas, and whether shifts can be detected in the position of the ‘green line,’ which we define as the elevation limit or latitudinal limit beyond which little or no vegetation occurs. Although the velocity of climate change and its subsequent impact on vegetation has been well characterized at high elevations and latitudes, including the Arctic [16, 18–22], to date there has been relatively limited examination of how the ‘green line’ is shifting within high-elevation montane regions [23]. Exploring this issue, and understanding how biotic responses might lag climatic changes, offers the potential to better understand this important aspect of global change. Here we use high-resolution satellite data from the Landsat record during the interval 1984–2011 to examine changes in the quantity of vegetation and the realized velocity of change within and among nine mountain ranges in western North America (S1 Fig, S1 Table). We used signal processing methods that decompose surface reflectance into fractional green vegetation, woody vegetation, and barren substrate [24, 25] in each Landsat pixel within these mountain ranges (c. 105–107 pixels per range per year–see Methods; S2 Table). We examine peak-growing-season measurements within 100 m elevation bands and contrast patterns within six elevation divisions (sextiles) on each mountain range. This relative classification (sextiles) within mountain ranges is necessary because ranges differ in absolute elevation. For example, the Colorado Rockies achieve maximum elevations in excess of 4,000 m, whereas the New Mexico Rockies in our sample do not exceed 3,400 m. Within the top sextile, we determined the average elevation across the first three years that matched vegetation cover during the last three years of our record. We used the difference in elevation between these two points, standardized by time, to compute realized velocity (m · yr-1).

Discussion The uniformity and overall speed of upward shifts in vegetation at the highest elevations across so many mountains that vary considerably in geographic context is remarkable. Indeed, on two of the mountains examined, realized velocity of vegetation kept pace with velocity of temperature change. These fastest realized velocities (112.8 m · decade-1) for vegetation are as fast as some of the most rapid animal migrations observed in montane regions. For instance, a recent analysis [9] examined 68 bird species in the Andes and found that shifts in the upper-limit of only a few species approached 112.8 m · decade-1. A recent review of species elevational shifts in montane regions, which considered many plant and animal species, found an average upward shift of 25 m · decade-1 over recent decades [35]. In some extreme cases in the Pyrenees, plant species have shifted their distributions upward by 500 m in elevation over more than a century, though average shifts of plant species in these mountains were closer to 200 m [36]. In general, plant species in montane regions show relatively slow shifts in upward distribution; for example, plant species in the Alps had average upward shifts of 20 m over more than a half-century [10]. Even the slowest shift we observed in upward migration of vegetation (19.8 m per decade) still greatly exceeds rates recorded for some individual species [10]. The upward shifts we observed can be further understood by considering the equivalent latitudinal migration within a topographically flat region at sea-level, equivalent to a 14.4–104.3 km · decade-1 poleward shift (mean = 56.1 km · decade-1). The changes we observed are on par with the fastest velocities predicted to occur due to abiotic change alone [6]. Our findings help to characterize patterns of vegetation across elevations in mountains that range from the sub-tropics to the sub-arctic. These vegetation by elevation profiles provide a modern point of comparison with pioneering, centuries-old work of von Humboldt [37], who characterized vegetation across elevations on a high tropical mountain, and with foundational work on gradient analysis [38], but allow now for the characterization of subtle differences among mountains, such as the variation in the strength of the inflection point of decreasing vegetation with increasing elevation. Our work is consistent with a variety of recent studies that take advantage of remotely sensed data to investigate how montane vegetation is related to climate [17, 39]. Although much of this work recapitulates previously known patterns, such as the presence of relatively large amounts of vegetation at high elevations in sub-tropical mountains and the complete absence of vegetation at high elevation in high-latitude mountains, it provides a level of detail in these relationships that was not attainable prior to the era of high-resolution remote sensing. Our analysis extends previous efforts by explicitly testing the hypothesis that vegetation is keeping pace with changes in temperature at high elevation. The upward shifts in vegetation at the highest elevations could be due to a variety of factors. Although our remote sensing record for vegetation, beginning in 1984, captures most of the period of pronounced warming at these mountains over the past century, identifying a single or suite of causal abiotic factors or other conditions responsible for the observed changes in vegetation at high elevations is difficult. This is likely due in part to severe limitations in availability of climate and other historical data at these elevations, where few weather stations exist [31], as this limits our ability to relate fine-scale changes in vegetation with equally fine-scale climate data. It is also likely due, however, to the complexity of climatic factors and historical land use that may be responsible for these shifts. Indeed, the combination of warming and drying we observed may have pushed vegetation in opposing directions and could be one reason why some realized velocities are failing to keep pace with temperature change [13]. Realized velocity of vegetation shifts can also be influenced by lag processes in the creation of suitable substrate, e.g., through loss of snow cover or creation of soil [40]. Although we did not investigate the contribution of slope or aspect, we believe that topographic differences are unlikely to explain the primary conclusion of our analysis. This is because our analysis is based on very large samples of 105–107 pixels per range per year (see Methods; S2 Table) that vary in slope and aspect, both within and between mountain ranges and individual mountains; further, recent work at high elevations on the Tibetan Plateau showed that topographic slope had some predictive power, but aspect had little impact on changes in vegetation greenness measured using remote sensing, despite overall changes in vegetation cover that were consistent with climate change [41]. Ultimately, finer-scale, more accurate and ubiquitous climate data at upper elevations would help to better disentangle alternative mechanisms for biotic changes in montane systems. Although the shifts in vegetation we observed at the highest elevations are likely to have been driven by changes in climate, it is conceivable that some aspect of human disturbance or land use change has played a role in driving these patterns. This seems most likely to be a relevant consideration within the two regions observed in Mexico, where even highest elevations are located in close proximity to human settlements. Within the mountains in the United States and Canada, changes in human land use seem unlikely to be driving patterns of vegetation change within the top sextile of elevations. For example, in the Sierra Nevada, the top sextile starts at elevations above 3,600 m (11,811 feet). Although there is a history of high-altitude cattle and sheep grazing in the Sierra Nevada, most of this grazing was probably at elevations less than 3,600 m. Historical grazing in Sierra Nevada meadows of 9,000 or 10,000 feet occurred primarily between 1870–1920, with almost no grazing at high elevations after 1946 [42]. This indicates that grazing history is unlikely to drive recently observed changes in vegetation. Consequently, although some potential influence of human land use cannot be ruled out, it seems unlikely to be a general driver of the uniform patterns of change in vegetation observed in the highest elevations among mountains in western North America. Our findings show that at high elevations, vegetation cover as a whole was able to expand upward at a rapid rate, even if that rate failed to keep pace with changes in temperature in some cases. Our findings are broadly consistent with recent work showing large increases globally in the fraction of land at the highest latitudes that are becoming vegetated [43]. Our findings also confirm the generality of a host of recent studies that have documented shifts in elevation among plant species on individual mountains [13, 44, 45], but provide mixed news for conservation in the context of climate change [46]. If vegetation at the highest elevations can’t keep pace with the velocity of climate change on some mountains, then high-elevation species and ecosystems in those places may eventually be squeezed at their low-elevation limits by species moving uphill and at their high-elevation limits, not just by declines in total surface-area found in many mountain ranges [28], but also by a failure to occupy available habitat at these highest elevations. On the other hand, in places where vegetation shifts are keeping pace with climate velocity, the expanding available habitat may provide the conditions needed for the long-term survival of high-elevation species. More work is needed to better contextualize how the vegetation shifts we observed, which measure any green vegetation, relate to individual species movements and broader ecosystem transitions–as this sort of integration offers the potential to create a more holistic understanding of change in montane systems.

Conclusion At the highest elevations and across mountain ranges in western North America, vegetation has shifted upward in elevation over the past few decades. Available evidence suggests that these shifts are primarily driven by warming that has occurred in western North America beginning in the 1980s. The realized velocity of these changes in vegetation is keeping pace with climate velocity on some mountains. Even on those mountains where the realized velocity of vegetation change is slower, these high-elevation changes are still as fast or faster than the rate of most species-level elevational shifts that have been documented worldwide. Still, in cases where vegetation shifts are not keeping pace with climate change, it is likely that the total area of suitable habitat for some species will decline, putting them at increased risk of extinction.

Acknowledgments We thank the Institute at Brown for Environment and Society (IBES), KC Cushman, D. L. Perret, and C. E. Silva for comments that improved this analysis.

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