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Home , Bristlecone pine, Pinus aristata, Pinus longaeva, Tree line

An ancient on the move: range shifts in bristlecone

Angela Delos Santos 1, Katherine Pratt 2, Grace Rosburg-Francot 1, Linnea Schaefer3, George Sidarous4

1University of , Santa Barbara; 2University of California, Davis; 3 University of California, Berkeley; 4University of California, Riverside

ABSTRACT

Recent changes in global climate are causing many to shift their ranges towards higher elevations. The rate at which species can shift their ranges may determine whether or not they will be able to persist in light of such rapid environmental change. In this study, we examined the effect of elevation on recruitment, mortality, and intraspecific competition in bristlecone pines (, BC), a long- lived subalpine species whose upper limit is often at the alpine treeline. We measured recruitment, mortality, and neighbor abundance at ten predominantly BC groves across an elevational gradient in the White of California. We found proportionally more small BC and fewer large and dead BC at higher elevations, indicating recruitment increased and mortality decreased at higher elevation. Larger had fewer small and medium neighbors which suggests intraspecific competition may limit recruitment. Overall, increasing recruitment of Great Basin bristlecone pines at higher elevations may signal an expanding climatic range.

Keywords: bristlecone , treeline shift, recruitment, competition, White

INTRODUCTION substrate, and temperature have been shown to impact recruitment (Harsch Climate change poses a major threat to 2009, Kullman 2006). Treelines around the global biodiversity, but may especially world are shifting upslope (Kelly and impact range restricted species (Dirnböck Goulden 2008, Harsch 2009, Kullman 2006, 2011, Dullinger et. al 2012). Species living Feeley 2010, Beckage et. al 2008) and high within small ranges will be elevation treeline encroachment poses a disproportionately vulnerable to extinction threat to endemic alpine species (Dirnböck (Ohlemüller et al. 2008), and the range of 2011, Dullinger 2004). many endemic flora are projected to be Some evidence shows treeline shifts are significantly reduced or fragmented within more dependent on density than the century (Loarie et al. 2008). Variation in temperature (Camarero 2004). However, in abiotic factors including slope, ,

CEC Research | https://doi.org/10.21973/N3R07S Summer 2018 1/8

high-altitude regions with temperature- In this study, we assessed BC demography limited treelines, warming periods allow by examining the abundance of varying size recruitment of seedlings at higher elevation classes at different elevations, using size as (LaMarche and Stockton 1974). When a proxy for relative age. We expected climate cools again, recruitment may be greater recruitment at high elevations. limited to lower elevations, and the treeline Additionally, we expected to find greater recedes downslope. This is especially mortality at lower elevations as changing evident in Great Basin bristlecone pines climatic conditions may create a more (Pinus longaeva, hereafter referred to as inhospitable habitat. We also expected that BC). Historically, during periods of warmer larger BC trees would hinder neighbor climate, BC has recruited at higher recruitment because they are better elevations, but when climate cooled, the competitors for limited resources than treeline receded downslope (LaMarche small trees. 1973). However, it is unclear if this trend is currently present. METHODS Bristlecone pines, found in subalpine regions, are the oldest non-clonal living 2.1 Study Area organisms in the world. Bristlecone pines We conducted research in the subalpine grow very slowly due to temperature and coniferous in the White Mountains moisture limitation (Brunstein and of California. We surveyed a total of ten Yamaguchi 2018) and thrive at high study sites at elevations ranging between elevation due to minimal competition for 2,900 and 3,500 meters; sites chosen resources within an already limited represented most of the elevational range environment (Schlenz 2008). Because of encompassing BC (Figure 1). Study sites their , tree ring analysis of BC can were dominated by BC, but also had Limber provide thousands of years of climatic pine (P. flexilis) present. Soil substrate history on moisture availability, varied among sites, but was predominantly disturbances, and temperature variation a mix of dolomite, quartzite, and granite. (Salzer et. al 2014, LaMarche and Stockton 1974, Feng and Epstein 1994, Salzer and 2.2 Research Design Hughes 2007). This makes them an ideal organism for understanding the effects of In order to study the effect of elevation current climate change. As average annual on BC recruitment and mortality, we temperatures increase, BC recruitment may measured the abundance of three different increase at higher elevations, expanding size classes across ten mountain slopes at their range upwards. However, if varying elevations. Each study site competition for resources is a determining represented one tree grove, which we factor in upper range expansion, defined as an area of land less than 400 m neighboring BC abundance may inhibit in length consisting of a lower and upper recruitment. The stand structure of a grove treeline. We determined the lower and may provide data on BC recruitment and upper treeline by the presence of BC distribution over past climatic conditions. clusters containing at least three BC greater

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To test for intraspecific competition that may affect BC recruitment across varying elevations, we surveyed BC neighbor abundance around focal trees. A focal large tree was measured at 35 m intervals between the lower and upper at each site. We recorded neighbors present within a 7 m radius of the focal BC, categorized to the same size classes as above. We also recorded DBH as a proxy for tree age (Brown and Schoettle 2008). Slope, aspect, and elevation were recorded to account for any environmental variation that may also affect recruitment and size class distribution. A total of 69 replicate focal trees were surveyed over a period of four days.

2.3 Statistical Analysis Figure 1. Map of the study sites at White Mountains, California. All groves surveyed were All statistical analyses were conducted located at varying elevations from 2,900 to 3,500 using JMP statistical software v.14. We used meters. The locations of the ten sites are marked and numbered. a MANOVA to test the effect of treeline position (lower, middle, or upper) on all than 3 m in height within 10 m of each four size class proportions. We used linear other (Smithers et al. 2018). In order to regression models to test for relationships assess size distribution, we sampled withina between elevation and the proportions of 50 x 14 m belt transect at lower, middle, small, medium, and large trees out of the and upper elevations within a grove. The total living trees. We also used a linear middle transect was located halfway regression model to test the correlation between the lower and upper treeline. At between small BC neighbor abundance and each sample transect, we recorded the elevation. We used a multivariate model to number of dead trees and the number of test the effect of slope, aspect, elevation, BC, categorized by size. Dead trees were distance from lower treeline, and their characterized by absence of green needles. interaction on small BC abundance over Size classes were determined as follows: total BC. To test for a correlation between small trees were less than 3 m in height; DBH and elevation, a linear regression medium trees were greater than 3 m in model was used. It was also used to test for height with a circumference of less than 65 the correlation of DBH and BC neighbor cm; large trees were taller than 3 m and abundance, separated by size class. had a circumference greater than 65 cm.

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RESULTS

3.1 Bristlecone Recruitment and Mortality

There was no effect of treeline position (lower, middle, upper) on any of the four size class proportions (MANOVA: N=28, F=0.33, p=0.94). As elevation increased, the proportion of small BC trees also increased (N=28, R2=0.42, p<0.01; Figure 2 black line). Elevation had no effect on the proportion of medium BC (N=28, R2=0.01, p=0.56). The proportion of both large BC trees (N=28, 2 R =0.34, p<0.01; Figure 2 dashed line) and dead trees (N=28, R2=0.28, p<0.01; Figure 3) Figure 3. Relationship between elevation (m) and decreased as elevation increased. the proportion of dead BC. As elevation increased, the abundance of dead BC decreased while living BC increased (N=28).

3.2 Bristlecone Neighbor Abundance

There were significant effects of some abiotic factors on proportion of small BC neighbors of the focal large BC trees (N=60, 2 R Full Model=0.19, pFull Model=0.02). Specifically, elevation was positively correlated with the proportion of small BCs and distance from lower treeline was negatively correlated with proportion of small BCs (P=0.01, P=0.02, respectively; Figure 4). There was no effect of slope and aspect on proportion of small BC (P=0.10 and P=0.56, Figure 2. Relationship between elevation (m) and respectively). the proportion of small and large BC. Black line and There was no effect of elevation on BC circles represent small BC. Dashed line and triangles 2 represent large BC. As elevation increased, the DBH (N=69, R =0.02, p=0.21). However, proportion of small BC strongly increased while the there was a negative correlation between proportion of large BC over total BC decreased DBH and the total number of small and (N=28). medium neighbors (small neighbors: N=69, R2=0.10, p<0.01, Figure 5; medium neighbors: N=69, R2=0.09, p=0.01), though these were weak relationships. There was no effect of DBH on large neighbors (N=69, R2<0.01, p=0.74).

CEC Research | https://doi.org/10.21973/N3R07S Summer 2018 4/8

diameter at breast height of the focal BC increased, the abundance of BC neighbors varied among size; small and medium BC neighbors weakly decreased, and BC DBH had no effect on large BC neighbors.

DISCUSSION

Within each study site, there was no variation of size class among lower, middle, and upper positions on each mountain slope. However, we did find a large-scale pattern over a broader elevation gradient. In support of our hypothesis, at higher Figure 4. Correlation of proportion small BC with elevations, small trees made up a larger elevation and distance from lower treeline (m). Small BC neighbors represents trees under 3 m in portion of the BC grove. This pattern may height within 7 m radius of the focal BC tree. Total indicate increased recruitment at higher BC neighbors represents all sizes of trees within 7 m elevations, possibly due to warming radius of the focal BC tree. As elevation increased, temperatures. Additionally, we found that the proportion of small BC neighbors over total large trees make up a greater proportion of neighbors increased; however, it decreased as the distance from the lower treeline increased (N=60). groves at lower elevation, which may indicate that past climatic conditions were more hospitable to recruitment at lower elevations than they are now. Medium trees were consistent throughout the elevational range, for they may represent a period of a non-climatic shift. As hypothesized, there was a greater abundance of dead BC compared to living at lower elevations, which supports the idea of a shifting treeline. Overall, these patterns suggest that recruitment rate is greater at higher elevations while mortality is greater at low. However, it is important to note that our inability to differentiate between dead P. flexilis and dead BC may have potentially caused an overestimation of dead trees recorded, since more P. flexilis are present Figure 5. Correlation of diameter at breast height (m) on BC neighbor abundance. Y-axis represents at lower elevations (Hutchins and Lanner trees within 7 m radius of the focal BC tree 1982). categorized by size classes. Light gray line and points The effects of elevation and distance from represent abundance of small BC neighbors, dark lower treeline showed contrasting trends gray line and points represent abundance of medium on the abundance of small neighboring BC BC neighbors, and black dashed line and points represent abundance of large BC neighbors. As (Figure 6). The finding that small BC

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abundance increased with elevation established large trees may limit the ability corresponds with our findings from the of small and medium trees to grow due to recruitment portion of our study, and shade intolerance and limited resources demonstrates a macro-environmental shift (Schlenz 2008). in recruitment towards higher elevations. As the climate warms, not only will BC However, within a grove, small tree likely face a higher amount of inter- and abundance increased towards the lower intraspecific competition, they will also face treeline, indicating recruitment may be an increased risk of infection by pathogens more successful at the bottom of slopes. and viruses that would normally not survive Although the weak correlation of this trend in colder conditions. Catenazzi’s study on suggests that other environmental factors the fungal pathogen Batrachochytrium may be at play, gravity may be a dominant dendrobatidis found that amphibian species factor in recruitment at the lower treeline in the higher elevations of the Anduran as it may cause pine cones and to mountain range that were previously tumble downslope. Therefore, although on untouched by B. dendrobatidis were a local scale, position within a grove has a becoming infected due to warming weak effect on forest structure, we did find temperatures, which allowed the pathogen an overarching pattern of increasing to more easily survive (2010). One similar recruitment at higher elevations. parasite that might affect the future health As climatic conditions become less harsh of BC is the bark beetle, Dendroctonus at the lower elevation limit of BC, other ponderosae. Rising temperatures have been plant species that were previously unable to correlated with the outburst of bark beetle live in the harsh high-elevation conditions populations moving higher in and may be able to advance into those areas elevation (Williams and Liebold 2002, (Schlenz 2008). This may increase Jonsson et al. 2009). If bark beetles interspecific competition for resources, continue migrating upwards, it is possible which may in turn increase BC mortality at that they will infect more BC. their lower elevation limit and cause their Species that inhabit BC forests may be range to shift. At higher elevations, where affected by the upward shift in recruitment. conditions are much harsher, BC will be A study by Rehm and Feeley found that more likely to persist, since other flora are cloud forests in the tropics are unable to less tolerant of these extreme conditions shift to higher elevations fast enough to (Bliss 1962). accommodate the changing climatic ranges Although DBH of the large focal tree had of the species within them, stranding them no effect on the abundance of neighboring in inhospitable environments (2015). This large trees, it was negatively correlation presents the possibility that species who with medium and small neighbor trees. This rely on BC groves may face the same fate if pattern may suggest that intraspecific the rate of temperature continues its competition hinders recruitment. Since gradual increase and BC are not able to elevation had no effect on DBH, the expand their range fast enough (Scott et al. possibility that elevation is driving this 2010). Our findings show that BC pattern is slim. This suggests that recruitment is increasing at higher

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elevations; however, the rate of treeline Mountains, California. Quaternary Research 3: encroachment was undetermined. Future 632–660. studies could assess long-term treeline shift Camarero, J. J., and E. Gutiérrez. 2004. Pace and and the effects this may have on other Pattern of Recent Treeline Dynamics: Response of species that inhabit this ecosystem. ecotones to climatic variability in the Spanish Furthermore, assessment of health along Pyrenees. Climatic Change 63:181–200. their elevational range may provide insight Catenazzi, A., E. Lehr, L. O. Rodriguez, and V. T. on the longevity of BC amidst Vredenburg. 2011. Batrachochytrium anthropogenic climate change. dendrobatidis y el colapso de la riqueza de especies y abundancia de anuros en el Parque ACKNOWLEDGMENTS Nacional del Manu, Sureste de Perú. Conservation Biology 25:382–391. Research was undertaken at the University of California’s White Mountains Research Dirnbock, T., F. Essl, and W. Rabitsch.. 2011. Disproportional risk for habitat loss of high- Center, Crooked Creek Station, doi: altitude endemic species under climate change. 10.21973/N3KM2J. Global Change Biology 17:990–996.

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