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Snowmelt variation contributes to topoclimatic refugia under montane Mediterranean climate change

Journal: Canadian Journal of Forest Research

Manuscript ID cjfr-2018-0284.R1

Manuscript Type: Article

Date Submitted by the Author: 09-Sep-2018

Complete List of Authors: Royce, Edwin; University of California at Davis, Plant Sciences;

Keyword: snowmelt, topoclimate, refugia, conifer, climate

Is the invited manuscript for consideration in a Special

Issue? :Not applicable (regular submission)

https://mc06.manuscriptcentral.com/cjfr-pubs

Canadian Journal of Forest Research

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Snowmelt variation contributes to topoclimatic refugia

under montane Mediterranean climate change

Edwin B. Royce*

Department of Plant Sciences, One Shields Avenue, University of California, Davis, CA 95616, USA

*Address for correspondence: Kennedy Meadows 2C4, 101686 Mahogany Trail, Inyokern, CA 93527,

USA, e-mail [email protected]

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ABSTRACT: Improved knowledge of the influence of climate parameters on the distribution of plant

species is needed to identify potential refugia under climate change. Species abundance of trees, mainly

conifers, as measured by species relative cover, was evaluated on 132 sites in the southern Sierra

Nevada mountains of California, USA. These mountains experience a montane Mediterranean climate

characterized by a deep winter snowpack and an extended summer drought. The cover data was

analyzed in terms of the average snowpack water content at its maximum and the average date when

snow on each site is finally melted. These snow-related parameters were calculated from a semi-

empirical snow model, taking into account site slope and aspect. For the pine, juniper and oak species

studied these parameters were found to have a much stronger effect on species abundance at a site than

does elevation. For the conifer species, this allows the identification of topographic refugia from climate

change. This result appears to be related to growth phenology. Elevation was found to be more

important for the fir species studied. The results on the importance of growth phenology should be

useful in identifying topographic refugia in mountains experiencing a Mediterranean climate worldwide.

Key words: montane Mediterranean climate, refugia, climate change, conifers, topoclimate

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Introduction

Plant responses to the higher temperatures and changes in rain- and snowfall that constitute the

present ongoing climate change have included migration, generally but not exclusively, to higher

elevations or latitudes and changes in forest structure, worldwide (literature reviewed by Lenoir and

Svenning 2015; also e.g. van Mantgem et al. 2009, Agnihotri et al. 2017, Dainese et al. 2017, Rumpf et

al. 2018) and in California, USA (reviewed by Rapacciuolo et al. 2014, Wright et al. 2016). Negative

changes in modeled water balance are often cited as contributing to tree mortality associated with these

changes (IPCC 2014). Shifts in growth to earlier dates have also been seen (reviewed by Cleland et al.

2007, also e.g. Chuine et al. 2001, Doi and Katano 2008, Gallagher et al 2009). The steep slope of many

mountain sites can result in substantial site-to-site differences in insolation and hence the snow-melt rate.

This, in turn, results in substantial topographic site-to-site differences in the dates when specific

temperatures and water conditions occur, even at similar elevations and latitudes, worldwide (e.g.

Gunton et al. 2015, Aalto et al 2017), and in California (Franklin et al. 2013, Rapacciuolo et al. 2014).

Such features may create localized topographic refugia (e.g. Scherrer and Kormer 2011, Meineri and

Hylander 2017), including in California (Ackerly et al. 2010, Dobrowski 2011a, Franklin et al. 2013,

Oldfather et al. 2016, McLaughlin et al. 2017). Topoclimate is found to be especially important in

modeling vegetation on nival sites (Radin et al. 2009).

California shares the Mediterranean climate found both around the shores of the Mediterranean Sea

and in other regions worldwide, including areas of northwest Mexico, southwest South America,

southwest Africa, and southwest Australia (Walter 1979). In the snow belt at higher elevations of the

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Sierra Nevada mountain range in eastern California (henceforth the "Sierra"), this climate is manifested

by a persistent often deep winter snowpack and a normal summer drought (Baker 1944).

Minimum temperatures in the spring and summer months in the Sierra have increased by

0.36°C/decade over recent decades (Cordero et al. 2011, see also Rapacciuolo et al. 2014). Changes in

minimum temperatures over the fall and winter months and changes in year-long maximum

temperatures were smaller. Long-term changes in total winter precipitation (rain plus snowfall water

content) in western North America are highly variable, depending on location. They include increases as

well as more common decreases (summarized by Rapacciuolo et al. 2014). Current snowpack water

content on the ground in the Sierra, as measured at its historical peak date of April first, has decreased

by as much as 25% due to anthropogenic effects (Berg and Hall 2017 and references therein).

Projections are of a 60%–85% reduction in the April first snowpack by the end of the century due to

anthropogenic effects. Snow melt has been observed to occur at progressively earlier dates over the last

half of the previous century. This long-term change manifests itself in current peak stream flows earlier

by 10–30 d, projected to be earlier by 40–80 d by the turn of the century (summarized by Schwartz et al.

2017).

The following considerations all suggest that a multi-year average no-snow date should be a useful

local site parameter in predicting species abundance under climate change. Since maximum snowpack

water content is strongly correlated with the no-snow date, this site parameter should be similarly useful.

Phenology is known generally to be important in determining species range (Chuine and Beaubien

2001, cf. Cleland et a;. 2007). In the case of conifer species bud burst on European Abies and Picea

cuttings, both a requirement for warming temperatures and a photoperiod requirement must be met for

the onset of budding (Partanen, et al. 1998, Basler and Korner 2012). On the other hand accurate

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modeling of the distribution of Alpine plant communities is found to require the inclusion of local

variation in the duration of snow cover (Carlson et al. 2015). These results are consistent with each other

given that root temperatures are controlled by ground temperatures, and hence by the final melting of the

snowpack. In the montane Mediterranean climate of the southern Sierra, Royce and Barbour (2001a)

have observed that the final melting of the snowpack is required for the initiation of leader growth

measured on Pinus and Abies saplings. This appears to be the same temperature-related requirement

observed for the European species. Additionally, Royce and Barbour found that the start of leader

growth in the Sierra appears to require the passage of a fixed threshold date, ranging from mid-May to

mid-June depending on species. This requirement appears to be the same as the photoperiod requirement

observed for the European species and allows reference to the species-specific Sierra threshold as a

photoperiod date. The start of Picea growth in Minnesota, USA, appears not to be substantially

influenced by winter heating (Pike et al. 2017). This suggests that cooling degree days accumulated over

the winter do not influence the species-specific threshold date. Consistent with this result, variations in

the number of cooling-degree days accumulated over the winter in the Sierra do not influence the

species-specific threshold date (Royce, unpublished analysis).

Under optimal conditions, leader growth for the Sierra conifers is limited to the period from the no

snow date, when the soil is water saturated, to a date when soil moisture in the root zone is depleted by

transpiration to a value where leader growth is no longer sustained, duration 27–51 d depending on

species (Royce and Barbour 2001a, 2001b). However, if the no-snow date occurs earlier than the

photoperiod date, the tree will not take advantage of all of this potential growth period, since the

photoperiod requirement will delay the start of growth. Thus, the relationship between the no-snow date

and the species-specific photoperiod date becomes critical in governing optimal leader growth for a

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specific species. If the no-snow date becomes earlier than the photoperiod date, such as at low elevation

or due to drought or long-term climate change, conifer growth will be stunted. Under these conditions

the species should be at a competitive disadvantage compared to trees of other species with earlier

photoperiod dates (cf. Ford et al. 2017).

The purpose of the present study was to identify and evaluate any contribution of local snow-related

factors in creating localized topoclimatic plant refugia under climate change in the Sierra. Snow-related

factors include snowpack water content at its winter maximum and the date when the winter snowpack

first disappears, a "no-snow date." Species movement upslope or poleward is one response to climate

warming. The present study was intended to determine if and to what extent movement to or continued

occupation of sites with advantageous topoclimate — toporefugia — can also be effective in preserving

species abundance. Species abundance is conveniently measured for trees as relative cover, and as

occurrence for shrubs and forbs. The potential toporefugia studied were of the order 1 km or less in

extent. The specific supporting objectives of the study were twofold: The first was to evaluate if and the

extent to which snow-related parameters are critical topoclimatic site variables in determining conifer

species abundance. The second was to attempt to use these parameters as local site variables in order to

quantify the effectiveness of topoclimatic refugia in comparison with relocation of species upslope

under climate change.

Site Characteristics, Materials. and Methods

Importance of the Snow-Related Parameters

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The first objective of this study was to establish the importance of the snow-related parameters,

average snowpack water content on April first (its nominal maximum value) and the average no-snow

date, as important in determining tree species abundance at a site. This importance was tested with

measurements of relative tree cover by species at 132 upland sites. The distribution of these sites was

regular, rather than random.

The Sierra is a more or less linear range of mountains that spans the latitudes between 35.7°N and

40.3°N in a north-northwest to south-southeast direction in eastern California. Sites were at

approximately latitude 36°N in the Kern Plateau region of the southern Sierra. The study area was a

band about 2 km on each side of U.S. Forest Service highway 22S05 plus 2 km on each side of all side

roads from this highway in the region. Highway 22S05 is a part of the northern-most crossing of the

southern Sierra. (The next crossing is at a latitude almost 2° to the north in the northern Sierra.)

Sites were located along the elevation contour lines 2.43 km, 2.55 km, and 2.67 km. These elevations

were selected to span the ecotone between the mid-montane mixed conifer forest and the upper montane

conifer forest (Fites-Kaufman et al. 2007). The mid-montane forest is dominated by Jeffrey pine (Pinus

Jeffreyi Grev. & Balf.) and California white fir (Abies concolor (Gordon & Glend.) Lindl. ex Hildebr.

var. lowiana (Gordon & Glend.) Lemmon). These species were studied near their upper elevation limit.

The upper montane conifer forest is dominated by western white pine (Pinus monticola Douglas ex D.

Don) and Critchfield (southern Sierra) red fir (Abies magnifica A. Murray bis var. critchfieldii var. nov.

Lanner). These species were studied near their lower elevation limits. (Authorities: Jepson Flora Project

(eds.), 2017, Jepson eFlora, http://ucjeps.berkeley.edu/eflora/; Lanner 2010)

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http://ucjeps.berkeley.edu/eflora/

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A topographic map was used to identify all sites within the study area along each of the three

elevations of interest where the ground sloped to face close to one of the cardinal directions north, south,

east, or west.

All site soils were sandy loam entisols or inceptisols, observed to be generally 1–2 m deep in multiple

corings associated with other studies. For the 132 sites in the present study soils were formed from the

common Sierra granodiorite. An additional 35 sites in the study area were found to be located on thin

soils from an extremely hard metasedimentary parent material. They support much more sparse forests

and are omitted from the present analysis. No conifer sites were in or near riparian zones or near

meadows or the bottoms of deep valleys; potential night-time drainage winds and potential increased

humidities were ignored in the analysis.

Relative tree cover by species was estimated at all 132 sites identified from the map work and found

to be on soils from the common Sierra granodiorite. The procedure at each site was to stand at the

location identified from the map work but away from any single tree that might dominate the

measurements. Trees were identified by species viewed along the 360° wall of trees observed from that

location. The trees in this "wall" are at varying distances from the observer. However, as viewed from a

fixed observer location, even at low densities the observer sees a continuous band of tree crowns.

Relative cover by species was taken to be the fraction of that band of crowns occupied by the species,

regardless of the size or number of the individual trees — individual trees were not deliberately

identified. Crown closure at the site was determined with the use of a convex spherical densiometer.

These forests are somewhat sparse (crown closure = 0.41, S.D. = 0.22) so the wall of trees was normally

far enough from the observer to permit numerous trees to be part of the wall. Zeros were recorded if a

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species was not visible from the observer's location, so all species were measured at all sites. Sites were

visited only once, in the summer of 1992.

Shrub and forb species present were identified by repeated spring visits to 12 additional semi-

permanent sites situated at elevations between 2.2 km and 2.8 km within 0.5 km of highway 22S05

within the study area on the Kern Plateau. These sites had been selected to support other studies and

were visited repeatedly between 1994 and 2004. This allowed all forbs to be viewed and identified while

in bloom. Typically each site was visited in two or three sequential years. The site distribution was

neither regular nor random, but sites were subjectively representative of the larger forest. Sites covered

about 0.1 km2 each.

The April first snowpack water content and the no-snow date were estimated for each site with the

use of a semi-empirical snow model described in detail in the online supplementary materials to this

paper. Briefly: The long-term average snowpack water content on April first on 311 level sites was

modeled from established long-term snow course and snow sensor data collected by the State of

California, Department of Water Resources (DWR 1998, http://cdec.water.ca.gov/). (Snow course

snowpack is monitored monthly using snow tubes, while more recently installed permanent snow

sensors record data hourly.) Averages through 1995 published by the DWR were used. These sites are

located throughout the snow belt of the Sierra and are level and generally treeless. The greatest average

snowpack on April first in the portion of the DWR data set used for this study is 1670 mm of water

content, with 1400 mm recorded at several sites. The mean was 650 mm (s.d. = 320 mm). The highest

elevation site is at 3.4 km. The snowpack model included the effect of storm shadowing, where high

terrain upwind of a site of interest during storm conditions reduces the snowfall on that site. The long-

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term DWR data was supplemented by repeated snowpack measurements (snow tube measurements) on

inclined snow courses with tree cover, made in 1998 on 10 sites located on the Kern Plateau.

Daily maximum and minimum air temperatures were modeled from published monthly normals for

12 long-term level sites maintained throughout the Sierra by the National Oceanic and Atmospheric

Administration (NOAA 1982). Temperatures were assumed to be dependent only on elevation, latitude,

and east vs. west side of the range. Hourly temperatures — parameterized by the daily maximum and

minimum temperatures — were modeled from selected DWR snow-sensor-site temperature data

(http://cdec.water.ca.gov/).

Insolation on inclined sites was calculated from the solar geometry, with atmospheric attenuation

calculated exactly but with one empirical parameter taken from Monteith and Unsworth (1990). The

snow melt rate was modeled as proportional to the integral of insolation over that part of the day when

temperatures are above freezing. The effect of site cover on melt rate was modeled from the 1998 snow

melt data.

Effect Of Climate Change

The semi-empirical Sierra snow model was also employed to examine the effects of potential climate

change on the April first snowpack water content and the no-snow date. The effects of two kinds of

climate change were evaluated, both separately and together. The first hypothetical climate change was

an increase in all temperatures by 4°C with total snowfall water content held constant. The second was

a reduction of snowfall water content by 100 mm with no change in temperatures, as might occur with a

shift in weather patterns. These values of the climate change parameters were selected to demonstrate

the nature of their effects on species cover. Finally, the model was run with both effects together. Model

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runs were made for 16 hypothetical Sierra sites. These sites were each inclined by 20% in one of the

cardinal directions, north south, east, or west, and with overstory cover of 20%. Such runs were made at

two elevations each at two latitudes. (For these sites, at latitude 36°N elevations were 2.2 km and 2.9 km,

while at latitude 38°N elevations were 2.0 km and 2.7 km.) It was assumed that there was no substantial

precipitation after the no-snow date.

Results

Importance of the Snow-Related Parameters

Graphs of relative cover for western white pine against elevation, the snowpack water content on

April first, and the no-snow date (Fig. 1) clearly show a sharp decline of species abundance measured by

relative cover as the snowpack and the no-snow date decrease, but only a weak dependence of cover on

elevation over the elevations studied. (The no-snow date is plotted as a date number, counted

continuously starting with 1 January.) Critchfield red fir, the other dominant conifer in the upper

montane forest, shows a strong decline of relative cover with a reduction in all three variables (Fig. S1 in

the online supplementary materials). A strong decline of abundance with increase in the snowpack water

content on April first and the no-snow date, as compared to elevation, is seen for Jeffrey pine, a

dominant species in the mid-montane forest (Fig. 2). Similar behavior is seen for the minor mid-

montane species Sierra juniper (Juniperus occidentalis Hook. subsp. australis Vasek) and California

black oak (Quercus kelloggii Newb.) (data not shown). The other dominant mid-montane conifer,

California white fir, appears not to be sensitive to the snowpack on April first or to the no-snow date

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over the range of variables studied, but to be sensitive only to elevation (Fig. S2), unlike the other mid-

montane species studied.

Limiting values of the variables snowpack on April first and the no-snow date were identified

visually on the figures and are shown as vertical lines. These limits (Table 1) represent values of the

environmental variables beyond which the species does not occur, where there is a sharp cutoff as for

western white pine, Fig. 1. Where there is a gradual cutoff, as for red fir, Fig. S1, the limit was placed

where the species contribution to forest cover appears to be reduced to no more than a third of its

maximum value. No similar cutoff values were identifiable for elevation for any of the species. The

existence of these well-defined limiting values for snowpack and no-snow date, but not for elevation,

plus the values themselves, are the important results of these measurements.

Finally, the number of shrub plus forb species on a site was found to decline strongly with the April

first snowpack water content and the no-snow date, and less strongly with elevation (Fig. 3).

Effect Of Climate Change

The effects of a temperature increase, or a snowfall reduction, or both together constituting

hypothetical climate changes, were the reductions in snowpack and no-snow date shown in Figs. 4 and 5

for the eight lower latitude (36°N, southern Sierra) hypothetical sites. These reductions were about twice

as great at the four hypothetical higher elevation (2.9 km) sites, as compared to the four lower elevation

(2.2 km) sites at that latitude. The increase in snowpack or no-snow date in moving from a south-facing

site to a north-facing site at either elevation was comparable to a change in elevation of the order 1 km.

Comparable results obtained at the higher latitude (38°N) sites (Figs. S3, S4).

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Critical lower limits for the snowpack and the no-snow date for western white pine and Critchfield

red fir, and the critical upper limit for Jeffrey pine (all from Table 1) are shown on Figs. 4 and 5 and

show that in the southern Sierra: Western white pine, now abundant at all high-elevation sites except

south-facing, will become abundant only on north-facing high-elevation sites (or with east or west

exposure on still higher elevation sites) under either element of the the hypothetical climate change or

the two elements together. This is also true of Critchfield red fir, though the effect is less pronounced.

Jeffrey pine, on the other hand will become more abundant at high-elevation and north-facing sites.

Discussion

The phenology model described in the introduction appears to explain the data on the abundance of

the upper elevation pine and fir species studied near their lower limiting values of this variable and the

closely linked variable snowpack near its maximum. The applicability of this model requires three

features: the existence of both a species-specific photoperiod requirement and a snow-melt requirement

for the initiation of leader growth, and a fixed potential growth period that is terminated by soil drying.

These requirements for the initiation of growth are apparently met in both some European and California

(Sierra) conifer species. The limited growth period will certainly be the case for any montane

Mediterranean climate, with its normal long summer drought, and may apply in other xeric climates.

The key point is that the potential growth period — initiated by the no-snow date and terminated by soil

dying in the absence of late spring or summer precipitation — is sufficiently short that a tree must take

advantage of most of that period if it is to be competitively viable. On the other hand, since both growth

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initiation requirements must be met, the photoperiod date substantially later than the no-snow date will

delay growth initiation, preventing the tree from taking advantage of the full potential growth period.

There has been extensive modeling of current and projected future changes in water balance in

California, in the Sierra mostly in the direction of increased drought under climate change (Stephenson

1998, van Mantgem and Stephenson 2007, McIntyre et al. 2015, cf. Crimmins et al. 2011, Dobrowski et

al. 2011b). Actual evapotranspiration and water deficit emerge from this modeling as important

parameters in determining species response. Such effects can be understood in the light of the phenology

model presented here. The effect is not one of changing water availability during the potential growth

period. This can be seen by noting that upland soils in the study area are shallow, 1–2 m in depth

(personal observation after drilling over 100 neutron probe holes, Royce and Barbour 2001b and

unpublished). Even assuming an exceptional porosity of 10%, the soils will not hold more that 100–200

mm of water, while snowpack water contents up to 1680 mm (mean 650 mm) were found in the study

area. Thus, the soil is saturated at the beginning of the potential growth period regardless of variations in

the winter snowfall. Most of the snowmelt water simply runs off. (The entisol and inceptisol upland soils

found on the Sierra sites studied contain little organic matter that would hold moisture, except for a

shallow layer of surface litter.) However, reduced snowfall or faster snowmelt due to increased

temperatures will move the potential growth period to earlier dates. If the no-snow date moves to

substantially earlier than the photoperiod date, the phenology model will explain reduced growth. This

could provide a part of the explanation of the observed effects of changing water balance on species

viability.

Decreased snowfall or faster snowmelt will also be reflected in a lengthened normal summer drought.

However, the species found on upland sites in the Sierra exhibit very effective stomatal closure, so that

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conductance is less than 2% of the open stomatal condition (Hinckley et al. 1978). Conifers found under

more mesic conditions are only able to reduce their stomatal conductance to no less than 10% of open

stomatal condition. Thus, it is unclear that the lengthened summer drought will be sufficient to produce

substantial xylem cavitation or otherwise damage the tree. The critical issue in understanding the effect

of an increasingly unfavorable water balance appears to lie, at least in good part, in understanding

phenology.

Estimates of changes in temperature and water balance resulting from observed and projected climate

change have relied on global climate or other large scale meteorological models downscaled to the area

and scale of interest (Flint and Flint 2012, also e.g. McIntyre et al. 2015, Berg and Hall 2017). This

downscaling allows distinguishing valley sites from upland sites or distinguishing between other

topologically distinct features. However, systematic differences in ambient air temperatures between

upland sites as small as the 1 km typical dimension of the sites studied here are not observed.

Measurements of maximum and minimum temperatures and evaporation on paired north- and south-

facing sites show no systematic differences. (Max-Min thermometers and piche tubes were placed in

shaded louvered enclosures, data and analysis in the online supplemental materials and in part in Royce

1997). It appears that local winds are sufficient to equalize ambient temperatures between upland sites

separated by one to a few km. Others have reported site-to-site differences in temperatures and/or

species occurrence on sites that differ topologically (e.g. Curtis et al. 2014, Gunton et al. 2015, Aalto et

al. 2017, McLaughlin et al 2017, Meineri and Hylander 2017, Patsiou et al. 2017, Millar et al. 2018).

However, their work included sites near water or in meadows or valleys, where extra moisture and/or

cool night air drainage might be expected. The sites studied in this work were all upland sites, where this

should not be the case.

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This work relies on site-specific calculations of snowpack melt rates, driven by local topography,

insolation, and area-wide snowfall and temperatures. These calculations display the time progression of

snow conditions essential to testing the phenology model.

The value of +4°C for area-wide temperature change associated with climate change used here to

demonstrate its effects on vegetation is representative of end-of-the century conditions, based on the

anticipated Sierra conditions cited in the introduction. The change in snowfall of +100 mm produces an

effect comparable to this change in temperature. Readers may scale the effects of these changes on

snowpack and the no-snow date on Figs. 4, 5, S3 and S4 by whatever values they may find of interest.

Specifically, to a good approximation the reductions in the environmental variable values from the

reference historical values shown can simply be scaled by the ratio of the value of interest to the value

used to generate the figures.

Topographic refugia for higher elevation species will be found on upland sites where the April first

snowpack is deep and the no-snow date is late. It would appear that such sites deserve special

management attention in order to protect this value (cf. Loarie et al. 2008, Clark et al. 2016, Morelli et al.

2016, Young et al. 2017). These considerations should be of concern in other locations worldwide with

montane Mediterranean climates. For management planning average values of the snow-related

parameters can be predicted from a model such as the model employed in this study. It should also be

possible to determine comparative no-snow dates between sites from remote photography, perhaps by

drone.

Acknowledgments

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It is a pleasure to acknowledge the continuing support of Prof. Michael Barbour, University of

California at Davis, and of Prof. Janet Westbrook, Cerro Coso Community College, for assistance with

the field work.

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Illustration Captions

Fig. 1. Western white pine relative cover plotted against elevation (a), snowpack water content on April

first (b), and no-snow date (c). The vertical line is the lower limit for snowpack and the no-snow date for

this species. Note that the three symbols at relative cover = 0 on the elevation plot represent the multiple

cases showing separately on the x-axis of the other two plots.

Fig. 2. Jeffrey pine relative cover plotted against elevation (a), snowpack water content on April first (b),

and no-snow date (c). The vertical line is the upper limit for snowpack and the no-snow date for this

species. Note that the three symbols at relative cover = 0 on the elevation plot represent the multiple

cases showing separately on the x-axis of the other two plots.

Fig. 3. Number of shrub and forb species plotted against elevation (a), snowpack water content on April

first (b), and no-snow date (c).

Fig. 4. Projected snowpack water content on April first (mm) for different hypothetical climate

conditions. Projections are for latitude 36 °N, slope = 20%, cover = 20%. The top 16 bars represent

projections for elevation 2.2 km, while the lower 16 bars represent projections for elevation 2.9 km.

Each group of four bars represent aspects north-, east-, west-, or south-facing (= N, E, W, or S). Within

each group of four bars, the top bar (solid) represents historical conditions from the late 20th century,

the second bar (stippled) represents conditions with snowfall reduced by 100 mm, the third bar (diagonal

shading) represents conditions with temperatures increased by 4 °C, and the last bar (vertical shading)

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represents conditions with both climate changes. Vertical lines indicate limiting values for Critchfield

red fir (ABMA), Jeffrey pine (PIJE), and western white pine (PIMO) taken from Figs. S1, 2, and 1,

respectively.

Fig. 5. Projected no-snow date number for different hypothetical climate conditions. Details as in Fig. 4.

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Table 1. Limiting values of the variables snowpack water content on April first and the no-snow date number where a species contribution to forest cover falls off sharply as seen in the cover data plots. In all cases the number of sites where each species was measured was 132. Species Snowpack

(mm)No-Snow Date

Number

Abies concolor, var. lowiana, California white fir — upper limit

None None

Abies magnifica, bis var. critchfieldii, Critchfield (Southern Sierra) red fir — lower limits

480 126

Juniperus occidentalis, subsp. australis, Sierra juniper — upper limit

390 121

Pinus Jeffreyi, Jeffrey pine — upper limit 780 160

Pinus monticola, Western white pine — lower limit 610 144

Quercus kelloggii, California black oak — upper limit 480 130

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