Attachment D. - UC Davis Climate
^orist
CUmate-Ready Tree Study: Update for
Southern Calif oinla Communities
E. Gregory McPherson, Alison M. Berry, Natalie S. van Doom, James Downer, Janet Hartin, Darren Haver, and Erika Teach
Introduction
Planting and stewardship of tree
species well-suited to site growing
conditions in the future as well as the
present is an important urban forest
climate adaption strategy (Roloff et
a\., 2009; Yang, 2009). Stressors as-
sociated with climate change, such
as drought, heat, pests and extreme
weather events are already increas-
ing mortality in forests (Alien et al.,
2010). Adverse effects on tree health
can be amplified in cities because of
urban heat islands and the impacts of
existing stressors such as inadequate
soils, polluted air, contaminated
runoff and mechanical damage from
construction activities, equipment,
vehicles, and vandals. In the South-
west, urban forests are especially
vulnerable to climate change stress-
ors because most predominant na-
tive and non-native species rely on
supplemental irrigation to augment
natural precipitation (Hartin et al.,
Van der Veken et al., 2008).
Identifying and testing the re-
silience of tree species to climate
change stressors is critical to the
long-term stability of urban forests.
By gradually shifting the planting
palette to climate-ready trees, future
generations can benefit from health-
ier and more resilient urban forests
(McPherson and Berry, 2015).
This paper describes our initial
evaluation of survival and perfor-
mance of 14 species of trees being
tested in Southern California's In-
land and Coastal communities as
part of the statewide Climate-Ready
Trees study. The study was initially
funded by the Western Chapter In-
ternational Society of Arboriculture
Britton Fund. This update follows
a report in Western Arborist on the
early performance of trees evaluated
in the Sacramento region (McPher-
son et al. 2017).
Methods
Our approach used a 5-step process
to identify and evaluate the vulner-
ability and suitability of tree species
(McPherson et al. 2018) to climate
change pressures. Vulnerability was
defined as the degree to which a
species is affected by and unable to
cope with adverse effects of climate
change (Click et al. 2011). Trees were
selected for testing based on evalu-
ation of climate induced risk factors
two species varied (Table 1). Within
each climate zone, the 12 species of
trees were randomly planted in a
reference site and each of 4 parks.
Four replicates were planted in the
reference site (4 replicates x 12 spe-
cies, 48 trees). South Coast Research
and Extension Center (SCREC)
(Coastal) and University of Califor-
nia Riverside Citrus Experiment Sta-
tion (Inland). Irrigation schedules
were based on real-time reference
evapotranspiration (ETo) estimated
by California Irrigation Manage-
ment Information System weather
stations and varied by site. All other
Identifying ancf testing ttze residence
of tree species to cdmate change
stressors is criticaCto ttze Cong-term
staBidty of urhan forests,
that impact their vulnerability or
resilience. Risk factors used in the
analysis are (1) habitat specificity:
sunlight exposure, soil texture and
pH, soil moisture (2) physiology:
drought tolerance, salt tolerance,
wind tolerance (3) biological interac-
tions: invasiveness, major or minor
pests and diseases, emerging pests
and diseases. Coastal and Inland
climate zones were identified based
on aggregating Sunset climate zones
(Fig. 1). Twelve species of trees were
selected for evaluation in each cli-
mate zone based on input from horti-
culturists, quantitative rankings and
availability from local nurseries. The
same 10 species were planted in both
climate zones, while the remaining
maintenance practices such as prun-
ing, weed control, and pest inanage-
ment were the same at both sites.
An additional 96 trees were planted
in 4 parks (2 replicates per park, 24
trees per park). Long-term field test-
ing at the UC plots and in nearby
parks allows for direct comparisons
of growth and longevity under a
range of site conditions. Growing
conditions and maintenance activi-
ties were more variable among park
sites than reference sites. All trees
are evaluated annually for the first 5
years after planting, and biannually
thereafter for 15 years.(Fig. 1).
Trees were obtained from 3 nurs-
eries, ranging in size from bareroot to
24" box, and were planted February
Winter 201912
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Figure 1. Locations of park and reference planting sites in the Inland and Coastal climate zones.
through April 2016. Volunteers coor-
dinated by the Hollywood Beautifi-
cation Team planted park trees/ with
guidance from City of Los Angeles
Recreation and Parks Department
staff. UC staff planted trees at the
Ir vine and Riverside reference sites.
Specific planting locations within
each site were randomly assigned to
each species, and trees were well wa-
tered and staked according to indus-
try guidelines at the time of planting.
A four inch mulch layer was applied
at all sites after planting and reap-
plied as needed.
Trees that died within the first
weeks after planting due to trans-
planting stress were replaced dur-
ing the second year of the project.
Trees that died for other reasons
were removed and not replaced.
Tree growth, health, and perfor-
mance were evaluated during sum-
mer in 2016, 2017 and 2018. Further
information on criteria for selecting
the 14 species, experimental design
and monitoring can be found in the
publication "Performance Testing
to Identify CUmate-Ready Trees"
(McPherson et al., 2018).
The UCCE reference plots in Ir-
vine and Riverside are flat field sites.
The sandy loam soils have a neutral
pH (7.5-7.7). Trees received ample
drip irrigation during the first three
years of establishment.
The four Coastal parks are flat
turf areas that receive recreational
use and regular irrigation. Soils in
Westchester and Vista Del Mar are
very sandy. Vista del Mar has very
low soil nitrate (3.3 ppm) and is sub-
ject to salt spray because it is adja-
cent to the Pacific Ocean. Bogdanov-
ich Park is perched above the ocean,
and somewhat sheltered by groves
of mature trees. Westchester and Jim
Gilliam Parks are further inland, and
less strongly influenced by maritime
weather effects. Some trees in Jim
Gilliam Park appeared to be over-ir-
rigated.
Four Inland parks are in the San
Fernanda Valley. Trees in the Han-
sen Dam Recreation Area (off the
Foothill Freeway, 210) are on an east-
facing slope with interinittent grass
and irregular irrigation. The Valley
Plaza site (North Hollywood) is ad-
jacent to the 170 Freeway. Sandy soil
and various broadleaved weeds chal-
lenge tree growth. The turf at Wood-
ley Park (Van Nuys, 405) is healthier
and more uniformly irrigated than at
the other two parks. Trees in the Hol-
leigh Bemson Memorial Park (Porter
Ranch) appear over-irrigated and are
subject to the windiest conditions.
Data from the CIMIS stations
closest to the reference sites collected
during 2016-18 indicated that trans-
plants were not exposed to unusual
temperatures. In Riverside trees
were briefly exposed to air temper-
atures below freezing (-0.4 °C for 1
hour in 2018). Over the 3-year pe-
riod in Riverside, ETo averaged 12%
more than the historic normal (1,320
mm), and average annual tempera-
ture by month was 6% greater. Dur-
ing 2016-18/ annual precipitation at
the Riverside reference site was 10%
less than the historic normal (220
min). In Irvine the average ETo for
2016-18 was comparable to the his-
toric norm (1,276 mm), but average
annual precipitation was 30% less
than the historic norm (316 mm).
These data indicate the importance
of irrigating based on real-time ETo
rather than historical ETo to avoid
drought stress.
Results and discussion
Overall, 88% of all trees survived the
first three years and survival rates
for trees in reference sites (95%) were
greater than for trees in parks (84%)
(Table 1). Inland trees had a higher
survival rate (92%) than Coastal trees
(83%), despite exposure to greater
evaporative stress. The unusually
high rate of mortality for trees in the
Coastal Vista Del Mar Park (54%) was
largely responsible for this result.
The high loss rate at Vista Del Mar is
atoibuted to the cumulative effects of
strong winds, salt spray, clouds and
cool weather. Symptoms of stress
included lack of foliage on the wind-
ward side of tree crowns and desicca-
tion from salt spray (Fig. 2).
Trees in Coastal Westchester Park
had 83% survivorship, while rates
ranged from 88-92% in the other 6
parks. The trend of higher survival
13Winter 2019
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Table 1. Survival rates (%) for park and reference sites in the Southern California Coastal and In- land climate zones.
Park SitesReference SitesAll Sites
Survival (2016-2018)
Coast I InlandAllCoast | InlandAllCoastInlandAll
Acada aneuraa (Mulga)10088941001001001009296
Cedrela fissUisb (Brazilian Cedarwood)631007575
Celtis reticulataa (Netleaf Hackberry)100100100100100100100 Chilopsis linearis 'Bubba'c (Desert Willow)881009292
Corymbia papuanaa (Ghost Gum)63100751006710083
Dalberyia sissoo* (Rosewood)751001001008310092
Hesperocyparis forbesii^ (Tecate Cypress)387556507563427578 Mariosousa willardiana* (Palo Blanco)631008175100886710083 Parkinsonia x 'Desert Museum" (Palo Verde)100100100100100100 Pistada 'Red Push'" (Red Push Pistache)75100881001001008310092 Prososis glandulosa x Maverick' (Mesquite)100100100100100100100100100 Prunus ilidfola spp. Lyoni;'" (Catalina Cherry)88881001009292 Q.uercus fusiformis' (Escarpment Live Oak)883863100100100925875 Q.uercus tomentella3 (Island Oak)888888100100100929292
789084929895839288
a. in both Coastal and Inland sites
b. Coastal sites only
c. Inland sites only
Figure 2. The native Catalina cherry (Prunus ilidfolia ssp. Lyonra) is one of the few species surviving in Vista Del Mar Park. Loss of windward crown structure and foliar desiccation from salt spray are shown in these figures. *.
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rates in the Inland rather than Coast-
al climate zones was found for the
Reference sites (98% Riverside, 92%
Irvine).
Survival rates for the 10 species
that were planted at both Coastal
and Mand sites were lowest for es-
carpment live oak (75%, Quercusfusi-
formis) and Tecate cypress (78%, Hes-
perocyparisforbesii). The original oaks
were dug bare root in Arizona and
transported to holding sites in Los
Angeles. Some trees died due to dry-
ing of roots during handling. Boxed
replacement oak trees had higher ini-
tial survival rates. A root rot disease
(Phytophthora spp.) was identified as
the primary cause of death for two
Tecate cypress trees that exhibited
twig die-back at the Irvine reference
site. Although this species is per-
forming poorly in Coastal parks and
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Figure 3. Brazilian cedarwood (left) and mulga (right) at Westchester Park near the Pacific coast. Note that the Brazilian cedarwood's crown is relatively sparse during the early years. This mulga requires pruning for structure and form. the reference site, the other Califor-
nia natives, Catalina cherry (Prunus
ilicifolia ssp. Lyonii) and island oak
(Quercus tomentella) had 92% surviv-
al rates. In the Inland climate zone,
escarpment live oak (58% survival)
and Tecate cypress (75%) had the
lowest survival rates.
Ghost gum (Corymbia papuana)
and palo blanco (M.ariosousa willardi-
ana) had 83% survival rates, with all
losses in Coastal parks and reference
sites. Both species are native to arid
environments (Australia and Sonora,
Mexico) and are not very tolerant of
cool, moist, coastal conditions.
Of the 4 species planted in a single
climate zone, Brazilian cedarwood
(Cedrela fissilis) fared the worst with
75% survival. The 2 trees planted at
Vista Del Mar died, as did 1 at Jim
Gilliam Park. The lack of branch-
ing structure and weak wood make
this species especially susceptible to
vandalism and mechanical damage
during the early years (Fig. 3).
Performance
Subjective performance ratings were
performed for each species after vis-
its to each park and reference site.
Scoring incorporated observations
of survival, pruning requirements,
aesthetics, and insect and disease
damage. Pruning requirement is an
important consideration when se-
lecting trees. Species that require
extensive pruning to develop strong
structure are more costly to maintain
than those that require less prun-
ing. Species assessed as having a
high pruning requirement exhibited
poor structural characteristics (e.g.,
codominant stems, branches with
included bark, and branches with a
large diameter relative to the trunk).
As these trees continue to develop,
growth rates and structural char-
acteristics likely will change/ and
pruning requirements may increase
or decline accordingly.
In the Coastal sites, top perform-
ers included escarpment live oak
(Quercus fusiformis) and Red Push
pistache (Pistacia 'Red Push1) (Fig.
4). These species had high survival
rates, strong structure and modest
pruning requirements. Species with
high survival rates but requiring
more frequent pruning were rose-
wood (Dalbergia sissoo), Maverick
mesquite (Prosopis glandulosa 'Mav-
erick"), mulga (Acacia anuera), and
netleaf hackberry (Celtis reticulata).
Figure 4. Red Push pistache (left) and escarpment live oak (right) in Jim Gilliam Park. Both these species have high survival rates and are developing good crown structures without extensive pruning.
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Figure 5. Catalina cherry at Bogda-
novich Park has a high survival rate
but is slow to establish a sound crown
structure.
Both Catalina cherry (Prunus ilicifolia
ssp. Lyonia) and island oak (Quercus
tomentella) appear well adapted to
coastal conditions but remain rela-
lively small and shrub-like (Fig. 5).
Observation over a longer period of
time is necessary to determine their
suitability as attractive small trees.
Brazilian cedarwood has not per-
formed well, appearing to require
more intensive care than others
during the establishment period.
Tecate cypress, ghost gum and palo
blanco need hotter, drier conditions
to thrive.
In the Inland sites. Desert wil-
low (Chilopsis linearis 'Bubba'), ghost
gum, and Red Push pistache outper-
formed the other species (Figs. 6,
7, Table 1). They exhibited the best
survival rates across a wide range
of conditions and required the least
pruning. Rosewood, Maverick mes-
quite, netleaf hackberry and Desert
Museum palo verde (Parkinsonia x
Figure 6. Red Push pistache (above left) and ghost gum (above right) at Hansen Dam Park. Both these species are top performers in the Inland sites.
Figure 7. A rosewood (below left) is thriving at Valley Plaza Park, where irrigation and pruning are minimal and the soil is sandy. Desert willow (below right) had high survival rates and flowers throughout the summer. This desert willow at Woodley Park requires some initial pruning.
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Figure 8. Attributes of (from top to
bottom) the palo blanco. Maverick mes-
quite, and Desert Museum palo verde in
Woodley Park. The crown structure of
the palo blanco is very wispy. The mes-
quite and palo verde are developing full
crowns, but they will require regular
pruning for structure and form. The palo
verde's yellow flowers are spectacular
during late spring.
'Desert Museum') had high survival
rates but substantial maintenance
needs (Fig. 8). These species require
more pruning and palo verde is sub-
ject to failure from blow-over when
subjected to over-irrigation. These
initial results parallel those reported
for palo verde, mesquite, pistache,
desert willow, and texas ebony in
the Tucson area in which aesthet-
ics, canopy density, tree quality, and
overall health remained high even at
20% ETo (Schuch, 2018). While mul-
ga, palo blanco, and island oak show
promise to date, more time is needed
to gauge their adaptability, pest vul-
nerability and pruning requirements
as they mature. Tecate cypress and
escarpment live oak had the low-
est survival rates but merit further
observation over a longer period of
time before final conclusions regard-
ing their suitability can be made.
A brief description of each species
can be found on the Climate Ready
Trees website at: http://climaterea-
dytrees.ucdavis.edu/meet-the-
trees/.
Conclusions
Although it is premature to make
definitive statements about tree per-
formance through maturity, several
trends merit continued observation.
Certain species that are native to hot,
arid conditions are not surviving and
performing well in Coastal sites (e.g.,
palo blanco, Tecate cypress, ghost
guin). Other desert species are toler-
ant of many Coastal conditions but
require frequent pruning to main-
tain sound structure (i.e./ Maverick
mesquite, mulga, netleaf hackberry).
Few of these potential climate-ready
species are surviving in the extreme
Coastal conditions found at Vista
Del Mar Park.
Survival rates were higher in
the Inland parks where growing
conditions more closely matched
the arid environments from which
many of these species originated.
Overwatering may be the greatest
source of stress and responsible for
crown growth that outpaces root
growth for certain species (i.e., Des-
ert Museunn palo verde. Maverick
mesquite). Trees like the Red Push
pistache, rosewood, ghost gum and
desert willow seem to tolerate the
under- and over-irrigated soils in the
Inland parks. Further observation is
needed to determine the suitability
and long-term performance of island
oak, escarpment live oak, Catalina
cherry/ and palo blanco. Continued
monitoring of growth, survival, and
performance will help determine if
emerging trends continue or change
as each species matures.
McPherson, E. Gregory and
Erika Teach
USDA Forest Service, Pacific
Southwest Research Station
Davis,CA95618
[email protected]
[email protected]
Berry, Alison, M.
Department of Plant Sciences
University of California
Davis, CA
[email protected]
van Doom, Natalie S.
USDA Forest Service, Pacific
Southwest Research Station
Albany, CA 94710
[email protected]
Downer, James
University of California Coopera-
tive Extension
Ventura County, CA
[email protected]
Hartin, Janet
University of California Coopera-
tive Extension
San Bernardino, Los Angeles a
Riverside Counties, CA
[email protected]
Haver, Darren
University of California Coopera-
tive Extension
South Coast Research & Extension
Center
Irvine, CA
[email protected]
17Winter 2019
WESTERI^^^t
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