Evaluation of Six Drought Tolerant Trees 17 Years After Planting in Northern California E

W ESTERNA rborist Evaluation of six drought tolerant trees 17 years after planting in Northern California E. Gregory McPherson, PhD, Natalie van Doorn, PhD, Erika Teach, MS

hiS aRTiclE updaTES TREE (Fig. 1). in summer it is covered with of 30 to 40 ft. (9.1 to 12.2 m) (Figure growth and mortality data for showy deep pink to purple flowers. 2). The tree can adapt to a variety of six drought tolerant tree species after flowering it produces long soils and is deep-rooted. although Tpreviously presented in Western Ar- narrow seed pods that persist and somewhat gawky when young, it is borist (Mcpherson & albers, 2014). it can look messy. The tree is thornless fast growing and long-lived. extends the scope of the earlier paper and prefers full sun and well-drained Prosopis alba colorado™ or colo by assessing the vulnerability of each soils. rado mesquite is grown from rooted species to climate change stressors, as Pistacia x ‘Red push’ the Red push cuttings. The result is a lush and well as their potential for use in plant pistache (Rp pistache) is a hybrid be thornless mesquite growing to 40 ings to store carbon, reduce runoff, tween P. atlantica and P. integerrima ft. (12.2 m) tall and wide (Fig. 3). improve air quality and cool urban with compound leaves that emerge The tree is cold tolerant (16°F) and heat islands. with a distinctive red color, hence the briefly deciduous in the spring. It is in 1999 Mountain States Wholesale name Red push (Fig. 2). it has a lovely rapid growing and will tolerate lawn Nursery (Glendale, aZ) generously display of red, orange, and yellow irrigation. colorado mesquite is no provided us with six individuals of color in the fall. it develops a broadly longer grown, having been replaced each of seven different species for oval crown with a height and width by superior clonal varietal selections planting (24-inch box). all individuals of one species had died by 2014 (Par- kinsonia (formerly Cercidium) hybrid ‘desert Museum’). hence, we report Each species adaptive capacity on the performance of the remaining six species (Mountain States Whole to climate change stressors was sale Nursery, 2003). scored in three areas — habitat Chilopsis linearis lucretia hamil ton™ (lh desert willow) is a small suitability, physiological tolerances deciduous tree 15 to 20 ft. (4.6 to 6.1 m) tall and wide with light green leaves and biological interactions. that give it a weeping appearance

Figure 1. (Left) Lucretia Hamilton desert willow at the UC Davis Arboretum, Davis, CA (2016). Figure 2. (Right) Red Push pistache at a residence, Davis CA (2016).

Spring 2017 32 WESTERN A rborist

Figure 3. (Left) Without pruning, this Colorado mesquite in Davis, CA has developed low branches to shade the surrounding soil (2016). Figure 4. (Right) Texas red oak in a Sacramento, CA park setting (2016). such as phoenixTM and MaverickTM. The trees are now 17 years old and  uc davis Orchard park – dry Quercus buckleyi (syn. Q. texana) or our focus is on the trees remaining in grass area adjacent to parking, Texas red oak is a drought tolerant Sacramento and davis locations: minimal, if any, maintenance and west Texas native found growing in  Reichmuth park in Sacramento, irrigation. the hill country west of San antonio ca – south end of the park, moder  Two residential landscapes in and closely related to the Shumard ate use area, poorly irrigated turf. davis, ca – in a front yard with ir oak (Quercus shumardii). it is a decidu  Northstar park in davis, ca – in rigated turf and areas mulched with ous tree that grows 30 to 40 ft. (9.1 to a planting bed adjacent to the soccer shredded bark or decomposed gran 12.2 m) tall and wide with a dense, field, initial irrigation. ite that received very little irrigation upright crown that becomes rounded  uc davis arboretum and after establishment. with age (Fig. 4). The leaves are classic department of Environmental oak with five prominent lobes, dark horticulture grounds – in planting Trees were surveyed annually green and shiny on top, paler below. beds and well-irrigated turf that has from 2000 to 2009 and in 2013 and They turn burgundy to brilliant scar been converted to unirrigated native 2016. Tree growth is reported based let in the fall. grassland. on measurements of diameter at Quercus fusiformis or escarpment live oak is native to alkaline soils in west Texas. This cold-hardy ever Figure 5. (Left) Escarpment live oak green grows 40 to 50 ft. (12.2 to 15.2 in the UC Davis Shields Oak Grove m) tall and wide and its thick crown (2016). provides dense shade (Fig. 5). its Figure 6. (Right) Chinquapin oak in the small leaves resemble southern live UC Davis Shields Oak Grove (2013). oak (Quercus virginiana), with which it hybridizes (Fig. 6). however, it is much more tolerant of high tempera tures and drought. Quercus muhlenbergii or chinqua pin oak comes from seed collected in west Texas from smaller growing trees than relatives in the southeastern u.S. it grows to 30 to 40 ft. (9.1 to 12.2 m) tall and wide, forming a long-lived shade tree (Figure 6). its crinkly, lush green leaves are deciduous and turn orange to bronze in the fall. it prefers well-drained soil and grows surpris ingly fast, even in desert heat.

33 Spring 2017 W ESTERNA rborist data were lacking. uncertainty was Table 1. Survival after 17 years quantified as the percentage of fac Species Alive Dead Total Survival tors scored. data sources included SelecTree (https://selectree.cal- LH Desert Willow 2 2 4 50% poly.edu/), Water Use Classification RP Pistache 2 2 4 50% of landscape Species (WucOlS) Colorado Mesquite 3 1 4 75% (costello & Jones, 2014), Wu and dodge (2005) and the california in Texas Red Oak 5 0 5 100% vasive plant council (http://www. Escarpment Live Oak 2 2 4 50% cal-ipc.org/). The potential for each species to produce selected ecosystem Chinquapin Oak 5 0 5 100% services, such as cool urban heat is TOTAL 19 7 26 73% lands, reduce stormwater runoff, im prove air quality and store carbon was assessed subjectively, based on data Table 2. Mean sizes and standard errors (SE) at 17 years after from the literature and our observa planting tions. The availability of each species was determined by contacting nurser Species DBH SE Height SE Cdia SE ies that serve the Sacramento area, as (in) (ft) (ft) well as the SelectTree database. LH Desert Willow 3.8 0.21 11.8 0.33 10.9 1.39 RP Pistache 12.7 0.49 24.6 2.30 29.0 5.50 Results Survival: Nineteen (73%) of the Colorado Mesquite 14.3 3.80 27.6 1.97 38.6 3.15 twenty-six trees planted were alive Texas Red Oak 11.0 1.68 31.5 2.94 30.4 3.30 after 17 years (Table 1). The Texas red Escarpment Live Oak 14.5 0.02 29.2 6.23 40.5 6.89 oak and chinquapin oak experienced zero mortality. The survival rate for Chinquapin Oak 10.5 0.96 34.3 3.39 26.6 2.82 the colorado mesquite was 75% and 50% for the desert willow, Red push breast height (to nearest 0.1 cm with in three areas: habitat suitability, pistache and escarpment live oak. dbh tape), tree height (to nearest 0.1 physiological tolerances and biologi m with range pole or hypsometer) cal interactions (Bagne et al., 2011). Growth: after 17 years the mean and average crown diameter (average a rating of -1, 0, or 1 (intolerant, neu height of the three oaks ranged from of two perpendicular measurements tral, tolerant) was assigned for each about 30 to 35 feet (Table 2). Mean with tape to the nearest 0.1 m). Sur factor, as reported in the literature crown diameter was greater than vival was calculated as the percentage (Mcpherson et al., in Review). Scores height for the escarpment live oak. of live trees after 17 years. were summed with higher scores chinquapin oak was more upright, Each species adaptive capacity to indicating less vulnerability. it was while crown spread and tree height climate change stressors was scored difficult to score some species because were nearly equal for the Texas red

Table 3. Vulnerability to climate change scores for each species reflect relative sensitivity to each factor (-1 = intolerant, 0 = neutral, 1 = tolerant) Habitat Physiology Biological Interactions Vulnerability score Common name Soil Soil Sunlight Drought Salt Wind Cold Invasiveness Pests & Emerging Habitat Physiology Biological (Sub)total Uncertainty % texture moisture exposure tolerance tolerance tolerance hardiness diseases pests & score score interactions score and ph diseases score LH Desert Willow 0 1 0 1 1 1 1 1 0 1 1 4 2 7 0% RP Pistache 1 1 0 0 -1 1 1 1 -1 1 2 1 1 4 0% Colorado Mesquite 1 1 0 1 1 1 1 1 0 1 2 4 2 8 0% Texas Red Oak 1 0 -1 1 NA NA 1 1 -1 -1 0 2 -1 1 20% Escarpment Live Oak 1 1 0 1 0 1 1 1 -1 -1 2 3 -1 4 0% Chinquapin Oak 1 -1 -1 0 NA NA 1 1 -1 -1 -1 1 -1 -1 20%

Spring 2017 34 WESTERN A rborist oak. crown spreads exceeded tree guishing between a species optimum heights for both the mesquite and Red habitat of origin and the range of push pistache. habitats to which it can adapt. initial growth was slow for all desert willow and mesquite ex species, but became relatively rapid hibited the greatest physiological three to four years after planting (Fig. tolerance to drought, salinity, extreme 7). Growth rates remained rapid for wind and cold. Red push pistache was all species except desert willow. The sensitive to salinity in recycled irriga three oak species and the colorado tion water. The sensitivity of chinqua mesquite exhibited the most rapid pin and Texas red oaks to salinity and height growth. crown diameter the relative strength of their branch growth was greatest for the escarp attachments in the presence of high ment live oak and mesquite. winds are unknown. desert willow, mesquite and Red push pistache had Vulnerability to climate change positive scores for Biological interac stressors: tions and the oaks had negative scores. climate adaptation is now recognized The later was due to the large number as an important selection criteria for of pests and disease that can threaten street and park trees. One consid oaks. Some of the most serious and eration is the species’ physiological abundant pests are asian ambrosia plasticity, defined as the range of beetle (Xylosandrus crassiusculus), habitats to which it can adapt. Spe mildew (Erysiphe sps.), Goldspotted cies native to a wide range of habitat oak borers (Agrilus coxalis), oak root conditions are considered less vulner fungus (Armillaria mellea) and sudden able to climate change stressors than oak death (Phytophthora ramorum) (de species with a narrow range (Table ciduous oaks only). Red push pistache 3). chinquapin oak received the is susceptible to caterpillars (Manduca lowest habitat score (-1) because its sps.), but resistant to verticillium wilt habitat of origin is full sun and only (Verticillium sps.). well-drained soil. Red push pistache, The total scores indicate that mes mesquite and escarpment live oak quite (8) and desert willow (7) were habitats contained the widest range least vulnerable to climate change of soil conditions and sunlight ex stressors, while chinquapin oak (-1) posures. The conflicting assessment was most vulnerable. Texas red oak for mesquite is instructive. although (1) was more vulnerable than Red its native habitat is desert riparian Figure 7. Mean sizes over the 17 push pistache (4) and Escarpment areas, it has proven to tolerate hot years after planting. Decreases in size live oak (4). and dry situations. This contradiction reflect the loss of trees with above Results of this vulnerability as highlights the importance of distin- average growth. sessment are but one of many con-

35 Spring 2017 W ESTERNA rborist siderations during the tree selection solar access. Because it is leafless dur was required when crown growth be process. Findings may change as we ing the rainy season it is not optimal came rampant because of ample irri learn more about the performance of for stormwater management proj gation. phoenix™ and Maverick™ are each species over time. For instance, ects. however, it is a good choice for among the many improved mesquite none of these taxon are listed as in projects to store carbon and improve cultivars, and the latter is currently vasive species, but that could change air quality. it is deep-rooted and being tested in the climate-Ready if they are observed to be invasive as establishes an excellent branching Tree trials throughout california their use becomes more widespread. structure with very little pruning. (Mcpherson & Berry, 2015). unique red foliage in spring make this Landscape functionality and use cultivar an excellent substitute for the Texas red oak The relative functionality of each spe overplanted chinese pistache (Pistacia This large-stature tree establishes cies was assessed and results are sum chinensis). unfortunately, it is very dif well-developed leaders and strong marized for each species (Table 4). ficult to find in California nurseries. branch attachments with virtually Red push pistache is currently being no pruning. although deciduous, it Desert willow evaluated in the Southern california retains its attractive fall foliage longer This utility-friendly tree is very climate-Ready Tree trials (http://cli- than most trees. This trait increases well-adapted to difficult growing matereadytrees.ucdavis.edu/). rainfall interception, but reduces conditions. Because it is small and winter solar access. Texas red oak is deciduous, it will not intercept much Mesquite an excellent choice for carbon storage rainfall or create much shade. it is not Mesquite is an excellent choice for and heat island mitigation projects. solar friendly because persistent fruit green infrastructure projects because although it produces dense shade, pods can obstruct winter solar access. it effectively intercepts rainfall. it has it is a high-emitter of BVOcs, a trait Roots are unlikely to heave sidewalks. a full crown that is briefly leafless in to consider when planning projects it attracts birds and other wildlife. late spring, and roots can tolerate in areas with high summer ozone although desert willow requires oc periods of inundation. This tree pro concentrations. despite its many de casional pruning for structure and vides good summer shade for heat sirable features, this species is seldom form, fragrant summer flowers and island mitigation, stores a moderate found in california nurseries. drought tolerance make it an excellent amount of carbon and is excellent choice for sites where growing space for air pollutant uptake because it is Escarpment live oak is limited. Moreover, it is now widely a low-emitter of biogenic volatile or This oak appears to be the most rapid available in nurseries. ganic compounds (BVOcs), an ozone growing and is the only evergreen of precursor. Mesquite retained low the three oak species evaluated. un Red Push pistache branches and crown raising was re like other rapid growing taxon, such This medium-stature tree is a good quired when located in plazas, patios, as poplars (Populus sps.), escarpment choice for summer shade and winter and other trafficked areas. Thinning live oak appears to have strong branch

Table 4. Relative functionality and potential uses for each species. Species Mature Growth Water Carbon Intercep- Pollutant BVOC Summer Solar Utility Wildlife Root Pruning Availability height rate use stored tion uptake emitted shade friendly friendly friendly coflicts required LH Desert Willow Small Slow VL L VL L L L No Yes H LM H RP Pistache Medium Mod. L M L M M M Yes No M L L VL Colorado Mesquite Medium Rapid VL M H M L M No No M L H VL Texas Red Oak Large Mod. L H M M H H No No H L L L Escarpment Live Oak Medium Rapid VL H H H M H No No H M M VL Chinquapin Oak Large Mod. L H M M M H No No H LM L Legend: Other Mature height (ft) Growth rate (avg. inch dbh/yr) VL = Very low

Small = (<30) Slow = <0.25 L = Low Medium = (30-50) Moderate = 0.25-0.75 M = Moderate

Large = (>50) Rapid = >0.75 H = High

Spring 2017 36 WESTERN A rborist attachments and deep roots. This oak Conclusions Research park dr., davis, ca 95618 is an excellent choice for projects The potential for urban forests to miti [email protected] aimed at reducing stormwater runoff gate the impacts of climate change and and improving air quality. ample the threat of losing multiple benefits E. Gregory McPherson, PhD foliage maintained throughout the that trees provide are motivating the winter maximizes rainfall intercep state of california and many cities to Natalie van Doorn, PhD tion in Mediterranean climates, while incorporate urban forestry into their the broad crown produces dense sum climate mitigation and adaptation Erika Teach, MS mer shade. Because of its smaller size plans. Tree selection is an important at maturity, it is not likely to store as decision-point for managers striving much carbon as the other two oaks. to reduce the future vulnerability of although very drought tolerant and their urban forests. Thorough testing References Bagne, K. E., Friggens, M. wildlife friendly, the escarpment and long-term evaluation is a critical M., & Finch, D. M. (2011). A live oak is not currently available in step in the process of diversifying our System for Assessing Vulner- nurseries. it is being evaluated in the urban forests with new introductions ability of Species (SAVS) to Southern california climate-Ready that are well adapted and resilient Climate Change. (Gen. Tech. Tree trials (http://climateready- to a changing climate. This 17-year Rep. RMRS-GTR-257). Fort trees.ucdavis.edu/). evaluation of several drought tolerant Collins, CO: U.S. Department species provides new information on of Agriculture, Forest Service, Chinquapin oak their growth, resilience and functional Rocky Mountain Research Sta- although native to the southeastern use for planting projects aimed at tion. u.S., this oak is surprisingly drought controlling stormwater runoff, cool Costello, L. R., & Jones, K. S. tolerant. chinquapin oak is a good ing urban heat islands, improving air (2014). Water Use Classifica- choice for projects to store carbon, im quality and storing carbon. tion of Landscape Species: WUCOLS IV 2014. Davis, CA: prove air quality and cool urban heat California Department of Wa- islands. it is deciduous, and as with the Acknowledgements ter Resources Retrieved from Texas red oak, its persistent foliage can Special thanks to Ron Gass at Moun http://www.water.ca.gov/ influence rainfall interception and win tain States Wholesale Nursery for wateruseefficiency/docs/wu- ter solar access. Because crown growth supplying the trees for this study. We cols00.pdfEstimating. is irregular, chinquapin oak can benefit are deeply indebted to paul Graham, McPherson, E. G., & Albers, from occasional pruning for structure Gregory Tarver, and Elena aguaron S. (2014). Evaluation of seven and form. like all oaks, this species is for their assistance conducting this drought tolerant tree species well-anchored in the ground and has project. for central California. Western strong branch attachments. it is infre uSda Forest Service, pacific Arborist, 40(3), 10-15. quently available in nurseries. Southwest Research Station, 1731 McPherson, E. G., & Berry, A. M. (2015). Climate-ready urban trees for Central Val- Table 4. Relative functionality and potential uses for each species. ley cities. Western Arborist, 41(1), 58-62. Species Mature Growth Water Carbon Intercep- Pollutant BVOC Summer Solar Utility Wildlife Root Pruning Availability McPherson, E. G., Berry, height rate use stored tion uptake emitted shade friendly friendly friendly coflicts required A. M., & van Doorn, N. (In LH Desert Willow Small Slow VL L VL L L L No Yes H L M H Review). Performance testing to identify climate-ready RP Pistache Medium Mod. L M L M M M Yes No M L L VL trees. Urban Forestry & Urban Colorado Mesquite Medium Rapid VL M H M L M No No M L H VL Greening. Texas Red Oak Large Mod. L H M M H H No No H L L L Mountain States Wholesale Nursery. (2003). Desert sea- Escarpment Live Oak Medium Rapid VL H H H M H No No H M M VL sons: signature trees (pp. 4). Chinquapin Oak Large Mod. L H M M M H No No H L M L Glendale, AZ: Mountain States Wholesale Nursery. Legend: Other Wu, L., & Dodge, L. (2005). Mature height (ft) Growth rate (avg. inch dbh/yr) VL = Very low Landscape Plant Salt Toler- ance Selection Guide for Small = (<30) Slow = <0.25 L = Low Recycled Water Irrigation. Medium = (30-50) Moderate = 0.25-0.75 M = Moderate Davis, CA: University of Cali- fornia, Davis. Large = (>50) Rapid = >0.75 H = High

37 Spring 2017