Research ArticleRefereed Research

Soil medium and watering frequency alter growth and allocation for Blue Diamond cholla (Cylindropuntia multigeniculata), a rare cactus of the northeast Mojave Desert, USA

Sara Scoles-Sciulla, Alexander Stosich and Lesley DeFalco
Native Plants Journal, March 2023, 24 (1) 4-17; DOI: https://doi.org/10.3368/npj.24.1.4

Abstract

Blue Diamond cholla (Cylindropuntia multigeniculata (Clokey) Blackb. [Cactaceae]) is a rare cactus of the Mojave Desert. We explored whether cultivation from joint cuttings is a viable method for supporting threatened populations. Terminal joints were collected from adult plants at the type locality and grown in a shade house: We tested whether 2 soil mixes that varied in the ratio of inorganic and organic components (50:50 compared to 85:15) and 2 watering frequencies (250 ml every 5 d compared to 500 ml every 10 d) promote root growth important to outplanting survival. Plants grown from joint cuttings in the 50:50 soil had greater shoot and root biomass, produced more joint segments, and had higher initial and final survivorship over the 5-mo study. Neither soil nor watering treatments shifted biomass allocation to roots as hypothesized, but frequent watering produced longer roots, which may benefit reintroduced plants by assisting root access to deep soil moisture. Despite their vigor during collection, freshly cut joints rapidly declined in condition, resulting in approximately 50% mortality during the first month of the study. Initial mortality was not explained by the identity, condition, or size of the maternal plant. Prior-year weather patterns and collection procedures may influence quality and durability of joint cuttings and require further study. While larger plants were produced from the 50:50 mix, and root length was increased by frequent watering, reintroduction of nursery-grown plants will indicate whether such treatments aid establishment in the dry habitat where this species occurs.

Scoles-Sciulla S, Stosich A, DeFalco L. 2023. Soil medium and watering frequency alter growth and allocation for Blue Diamond cholla (Cylindropuntia multigeniculata), a rare cactus of the northeast Mojave Desert, USA. Native Plants Journal 24(1):4–17.

KEY WORDSNOMENCLATURE
Figure

Cylindropuntia multigeniculata (Blue Diamond cholla [Cactaceae]) in habitat at Sloan Canyon National Conservation Area, Nevada. Photo by Alex Stosich, USGS

CONVERSIONS

  • 1 cm3 = 0.6 in3

  • 1 m = 3.3 ft

  • 1 cm = 0.4 in

  • 1 mm = 0.04 in

  • 1 l = 1.1 qt

  • 1 ml = 0.03 fluid oz

  • 1 kg = 2.2 lb

  • 1 g = 0.04 oz

  • (°C × 1.8) + 32 = °F

Blue Diamond cholla (Cylindropuntia multigeniculata (Clokey) Blackb. [Cactaceae]) is a rare cholla cactus that grows in the northeastern Mojave Desert in southern Nevada and northwestern Arizona (Baker 2016). Cylindropuntia multigeniculata grows as a shrubby, densely branched cactus up to 0.5 m tall and 1.5 m broad with stems composed of 30 to 40 cylindrical segments (Baker 2005). Cylindropuntia multigeniculata occurs in Joshua tree (Yucca jaegeriana (McKelvey) L.W. Lenz [Agavaceae]) woodland and Mojave Desert scrubland (Larrea tridentata (DC.) Coville [Zygophyllaceae], Ambrosia dumosa (A. Gray) W.W. Payne [Asteraceae], Coleogyne ramossisima Torr. [Rosaceae], and Yucca spp. associations) on rocky limestone, basalt, granite, and rhyolite soils between 1035 m and 1400 m in elevation, primarily on rocky slopes where perennial shrub cover is low (approximately 5–15%) (Baker 2005, 2016). Cylindropuntia multigeniculata was a candidate species under the Endangered Species Act in 1999 but removed from the candidate list in 2001 based on active management of lands and the execution of the multispecies conservation plan agreement (formerly C. whipplei var. multigeniculata (Engelm. & J.M. Bigelow) Knuth [Cactaceae]) (USFWS 1999, 2001). The type locality at Blue Diamond Hill in southern Nevada has been threatened by mining, residential development, and the proposed building of a hydroelectric plant in the past 20 y (Baker 2005). Cylindropuntia multigeniculata is a Bureau of Land Management Special Status Species, fully protected in the State of Nevada and covered under Clark County’s Multiple Species Habitat Conservation Plan (RECON Environmental Inc 2000; Nevada Natural Heritage Program 2001).

Figure

Flower buds on C. multigeniculata propagated at shade house in Boulder City, Nevada. Photo by Lesley DeFalco, USGS

Propagation and reintroduction of rare or endangered plants can be an important aspect of mitigation measures and a valuable method for supporting wild populations (Maunder 1992). Studies that evaluate requirements for survival and growth are lacking for rare Cactaceae, and no known propagation studies have specifically focused on Cylindropuntia multigeniculata. Cactus species can be propagated by seed (Rojas-Aréchiga and Vázquez-Yanes 2000), but erratic precipitation patterns in desert ecosystems can alter seed set and quality and make seed production unreliable (Montiel and Montaña 2003). Cylindropuntia species and similar cacti have been successfully propagated via stem cuttings (Fiedler 1991; Kelly 2009; Buhanan and Briggs 2011). Cuttings can mimic Cylindropuntia species’ ability to vegetatively reproduce when stem segments (joints) disperse from the parent plant and produce roots while lying in contact with the soil surface (Allen and others 1991; Bobich and Nobel 2001; Rebman and Pinkava 2001). For some cactus species, joint dispersal may be the primary mode of reproduction (Allen and others 1991; Bobich and Nobel 2001). A comparison among 10 Cylindropuntia and 15 Opuntia species from the southwestern US, in which joints were placed in potting soil, found that 32% of Cylindropuntia joints produced adventitious roots compared with 71% of Opuntia pads that rooted successfully (Evans and others 2004). This rooting difference may reflect the large pads of Opuntia species that store more water and carbohydrates and can invest more into establishment compared with small joint segments of Cylindropuntia species (Flores-Torres and Montaña 2012). Establishment was also correlated with the number of joint segments per stem: Species with fewer joint or pad segments per longest stem had a higher probability of rooting than those with more segments (Evans and others 2004). The numerous joint segments per stem for C. multigeniculata (mature plants with 30–40 joints per stem; A Stosich, personal observation) is comparable to Cylindropuntia species that had low rates of establishment from joints (range 5–50% for Cylindropuntia compared with 30–90% for Opuntia; sensu Evans and others 2004). Species with a low number of joint segments per stem and high joint establishment may invest in producing large numbers of offspring by shedding joints, in contrast to species that maintain relatively persistent stem segments that support reproduction by seed (Evans and others 2004). This dynamic of low rooting capacity of Cylindropuntia with many joint segments per stem may make propagation of C. multigeniculata from stem segments challenging, but this difficulty may be moderated by providing necessary resources for segments to take root.

Figure

Cylindropuntia multigeniculata at Blue Diamond Hill type locality. Photo by Alex Stosich, USGS

In arid environments, the development of a strong and efficient root system is crucial for outplanting success (Rundel and Noble 1991). Propagation techniques can pre-condition plants, which can increase survival after reintroduction (Franco and others 2010). High root-to-shoot ratios promote water conservation by reducing transpiration rates and allocating solutes to the root system to maintain the water gradient necessary for absorbing water from the soil (Lynch 1995; Canadell and Zedler 1995; Bañon and others 2006). Cylindropuntia species, and cacti in general, have fibrous root systems with shallow lateral roots that rapidly develop short-lived root hairs in response to precipitation. The short-lived root hairs allow the plant to uptake shallow soil sources of water during seasonal rains and tolerate periods of soil drying (Lynch 1995; Rebman and Pinkava 2001; Balandrán-Quintana and others 2018; Gdaniec and Grace 2019). Even large arborescent cactus species have a majority of roots within 30 cm of the soil surface, with an average root depth of 10 cm for many species, allowing plants to benefit from small rainfall events (Gibson and Nobel 1986). Basic root structure can vary, with some Cylindropuntia species able to grow deeper and less-differentiated root systems when nursery-grown under different watering regimes, soil types, and soil volume (Cannon 1913).

Root development and positive moisture status in cacti can be encouraged by propagation with soil mixes that have good drainage, composed primarily of pumice or perlite with small amounts of organic matter, and a watering regime that allows periodic drying of the soil (Kelly 2009). In desert ecosystems, nitrogen, phosphorus, and potassium are localized and recycled within “islands of fertility” around perennial shrubs, while other elements such as calcium, sodium, and magnesium are more abundant and evenly distributed in soils (Schlesinger and others 1996; Titus and others 2002). Plants can respond to low soil nutrients by increasing root growth, which may increase nutrient uptake (Franco and others 2010). Conversely, organically rich greenhouse soils with high levels of nitrogen and phosphorus can decrease root growth and favor shoot growth, yielding low root-to-shoot ratios (Nobel and others 1989; Bainbridge and others 1995). Some cactus species can respond quickly to seasonal resource pulses through increased shoot and root growth and root nitrogen concentrations, which allows them to add stem segments, potentially increasing vegetative dispersal, and to survive in water-limited microsites or endure periods when nutrient supply declines (Buhanan and Briggs 2011). Certain propagation techniques used in horticultural propagation in general can reduce outplanting mortality under dry and hot conditions (Franco and others 2011). However, these techniques may simply produce large plant size, thereby improving outplant survival overall. Intermittent watering (that is, more water delivered infrequently, compared with less water delivered more frequently) can produce pre-conditioning effects that increase outplant survival. Deep, infrequent irrigation during propagation can produce deeper rooting, decreased shoot growth, and consequent improved drought tolerance (Qian and Fry 1996).

Cactus species optimize water uptake under conditions of fluctuating water availability by alternating between growth when water is available and a lack of growth under dry conditions (Kelly 2009; Gdaniec and Grace 2019). Cacti have shallow root systems that can respond quickly to small rainfall events through the formation of rain roots (Rebman and Pinkava 2001), which may result in increased root biomass under conditions of low-volume, frequent watering. Reducing the amount of water given to plants, as opposed to watering less frequently, is a well-studied type of pre-conditioning treatment that can alter root structure, increase resource use efficiency, and decrease shoot growth, resulting in increased root-to-shoot ratio, and thus improved drought tolerance (Snyman 2004; Bañon and others 2006; Fernández and others 2006; Franco and others 2010; Lu and others 2014). A high root-to-shoot ratio in cacti that will be outplanted to habitat where resources are variable may thereby increase post-planting survival.

We evaluated whether propagating C. multigeniculata from joints harvested off adult plants in their habitat and grown in an open-sided shade house is a viable technique for propagating this rare species. We tested 2 soil mixes and 2 watering frequencies to determine a suitable combination to produce plants with a high root-to-shoot ratio, which we expect to lead to greater outplant survival. We hypothesized that growing C. multigeniculata joint cuttings in a soil mix containing a high proportion of inorganic materials would create plants that have a lower root and shoot biomass; growing cuttings in a soil mix with a high proportion of inorganic materials and under an infrequent watering regimen would produce plants with a larger root-to-shoot ratio.

METHODS

Plant Material

Terminal joints (14–61 mm in length) were collected from C. multigeniculata plants at the type locality on Blue Diamond Hill (36.05361 N, 115.40083 W) directly north of Blue Diamond, Nevada. The collection site has an average (± SD) annual precipitation of 239.4 ± 118.8 mm, average annual minimum temperature of 9.5 ± 0.7 °C, and average annual maximum temperature of 23.4 ± 0.7 °C (PRISM Climate Group 2020). Plants were located on steep slopes and rocky ledges composed of a mixture of limestone, dolostone, gypsiferous red shale, and claystone (Page and others 2005). Cactus joint cuttings are typically collected during spring or early summer when plants are actively growing (Luna 2009), and we timed collection early in the seasonal growth window for C. multigeniculata (Baker 2005). We collected 4 terminal joints from each of 20 plants for a total of 80 joint cuttings on 11 June 2020, selecting plants with 50 or more terminal joints to minimize damage to the individual (Figure 1A). We measured the size of each maternal plant (greatest canopy diameter, perpendicular canopy diameter, and height to calculate volume of inverted cone), estimated the number of terminal joints, and rated plant condition on an ordinal scale (1 = no dead stems, 2 = some stems dead at base, 3 = whole stems dead). Based on Kelly (2009) and Niesen (2001), we removed terminal joints at the nodes using a serrated knife, sanitized with 70% isopropyl alcohol between cuts, and placed them in plastic containers at ambient temperature (30 °C) for transport to a shade house (2-h transit time). The shade house consisted of an open-sided frame with 40% radiation-blocking white and tan shade cloth secured over the top. The frame was pitched such that the shade cloth was at 3 m height along the center and 2.5 m height along the north and south sides, and joints received full sunlight during morning and evening hours. After dusting the base of each joint cutting with sulfur, we allowed the cuttings to air-dry in a well-ventilated shaded area for 25 d to ensure cuts formed a callus (Figure 1B). Although only green joints were harvested, the condition of the joint cuttings changed during callus formation and each joint cutting was rated on an ordinal scale 1 wk after collection (1 = entirely green, 2 = some browning at the callus, 3 = browning throughout the surface of the joint cutting).

Figure 1.
Figure 1.

Terminal joints of C. multigeniculata were collected from adult plants on Blue Diamond Hill, Nevada (A), dusted with sulfur (B), and dried at the USGS Boulder City, Nevada, shade house for 25 d before planting under 2 soil and 2 watering treatments (C). Potted joint cuttings were arranged on a bench in the shade house (D) and grew actively (E) before being removed from pots to assess root and shoot growth (F) at the end of the experiment. Photos by Alex Stosich, USGS (A, B, E, F); Lesley DeFalco, USGS (C); Todd Esque, USGS (D)

Experimental Design

Prior to potting on 6 July, we measured the height (ht, in cm) and greatest width (wid, in cm), excluding spines, of each joint cutting and used the measurements to calculate initial volume approximating a cylinder: volume = π × (wid/2)2 × ht. Each of the 4 joint cuttings from a maternal plant were randomly assigned to 1 of 4 levels in a 2 (soil mix) × 2 (watering frequency) factorial experiment (20 replicate joint cuttings per treatment × 4 treatment levels = 80 joint cuttings). Random assignment ensured that estimated volume of joint cuttings at the onset of the experiment was not different among treatments (one-way ANOVA on log-transformed volume, F3,76 = 0.52, P = 0.07).

The 2 soil mixes were composed of an 85:15 or 50:50 ratio of inorganic (1/8″ pumice; General Pumice Products, Carlsbad, California) to organic components (coconut fiber; Organic Growers Potting Mix, Vermont Organics Reclamation, St Albans, Vermont). Each joint cutting was situated in the center of a 1-gal pot, buried to approximately a third of its height, and supported with 3 wooden bamboo skewers (Figure 1C). All pots were watered with 500 ml tap water administered 1 to 2 times per wk during the first mo after potting (4500 ml total for each pot during first mo). All pots were maintained on a 1.2 m × 1.2 m bench at the USGS shade house in Boulder City, Nevada (Figure 1D) and experienced approximately the same light and temperature conditions throughout the experiment (Table 1).

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TABLE 1

Mean (± SD) monthly temperatures for collection site at Blue Diamond (30-y normal, PRISM Climate Group 2020) and propagation site at USGS shade house in Boulder City, Nevada, during study period (2020).

During the initial establishment period when water was delivered to all pots at the same frequency and volume, weighing before and after water addition confirmed that the 2 soil mixes had the same total amount of water added (F1,5 = 2.38, P = 0.20) and lost between waterings (F1,5 = 0.58, P = 0.49) (Table 2). The total water addition measured by weighing pots over the first month of establishment was 16 to 19% lower than the 4500 ml total added per pot, reflecting observed drainage during watering.

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TABLE 2

Water dynamics of soil mixes prior to (6 July–13 Aug, 38 d) and after (13 Aug–16 Nov, 95 d) watering treatment implementation.

After 1 mo of establishment in the 2 soil treatments (13 August), 39 of the original 80 C. multigeniculata joint cuttings were still alive (that is, green and not rotting at cutting base). At this time, we reassigned several pots to a different watering treatment from the original random assignment to ensure equal representation of joint cutting condition and aboveground volume across the watering treatments (that is, 2 pots were reassigned from frequent to infrequent watering for both 50:50 and 85:15 soil treatments, and 2 pots were reassigned from infrequent to frequent watering for the 85:15 soil treatment). Reassignment resulted in 9 to 12 joint cuttings per treatment combination with equal estimated aboveground volume between the 2 watering treatments (two-factor ANOVA on log-transformed joint cutting volume measured 13 August, F1,35 = 0.08, P = 0.77). Joint cuttings potted in the 50:50 soil mix had 3.4 times the aboveground volume of those in the 85:15 soil mix at the start of the watering treatment (log-transformed volume, F1,35 =28.06, P < 0.01). We initiated watering at 2 different frequencies while holding the total volume of water delivered constant: 250 ml of water was administered to each pot every 5 d (frequent watering) or 500 ml of water was administered every 10 d (infrequent watering) for 5000 ml total to each pot over 3 mo. Three pots from each soil × watering treatment combination were weighed before and after each watering to document water addition and loss from pots for each treatment combination through time.

We also delivered the same total volume of water to each pot once we started the watering treatments (5000 ml over 3 mo), and pots weighed before and after watering again indicated that 7 to 15% of delivered water drained from the pots during watering. The frequent watering treatment had 4 to 7% more total water delivered to the pots than infrequent watering (F1,11 = 32.94, P < 0.01) (Table 2), likely attributable to accumulated measurement error for the smaller volume of water. Frequent watering pots also lost more water to evaporation and transpiration between waterings as plants grew (F1,11 = 54.66, P < 0.01) (Table 2). Although the total water-holding capacity was not statistically different between soil mixes (F1,5 = 2.70, P = 0.13), less water drained from pots with the 50:50 soil mix compared to the 85:15 soil mix during watering (F1,11 = 8.03, P = 0.02) because of the greater organic component. The 50:50 soil mix subsequently lost more water to evaporation and transpiration between waterings than did the 85:15 soil mix (F1,11 = 13.82, P < 0.01).

Joint cuttings were measured shortly after collection (19 June), at the onset of watering treatments (13 August), and at the end of the study (20 November). Joint cutting height (ht, in cm) and greatest width (wid, in cm), excluding spines, were used to calculate volume approximating a cylinder: volume = π × (wid/2)2 × ht. At the onset of the watering treatment, joint cuttings that were still green but rotting at the base were recut, re-dusted with sulfur, and allowed to callus for 1 to 2 wk before being repotted. Joint cuttings that were completely tan, desiccated, and hardened before the onset of the watering treatments or midway through the study (2 October) were considered dead and were discarded.

We estimated the water-holding capacity (WHC: the amount of water retained and stored in soil after watering and subsequent drainage) of the 2 experimental soil mixes by comparing the weight of oven-dried soil placed into pots on 6 July with the subset of experimental pots weighed on 15 July before joint cuttings were actively growing. Subsequent to initial weighing, joint cuttings plus 500 ml of water were added to each pot over 2 d (6 and 8 July), allowing the water to soak into the soil without running off. We calculated WHC as (weight of saturated soil + pot − empty pot) / (weight of oven dried soil) ∗ 100%.

Instead of destructively harvesting plants to determine biomass and root-to-shoot ratio, we used a gravimetric water displacement approach at the end of the study (20 November) to measure root and shoot volume as an indicator of plant biomass (Burdett 1979; Harrington and others 1994; Pang and others 2011). Whole plants were removed from pots by carefully loosening soil from roots to preserve root structure, rinsed with water, and patted dry with paper towels (Figures 1E and 1F). Roots and shoots were separately immersed in a 2 l beaker of water set on a 5 kg balance reading to 0.1 g. Weight of the displaced water approximates plant tissue volume (1 g water = 1 cm³). We also measured maximum length of the roots, counted total number of joint segments, and measured the height and width of each joint segment. Joint cuttings that were yellow, browning, and not rooted were considered dead and were not measured. Live joint cuttings were subsequently repotted into original soil mixes after measurements and watered to pot capacity.

Statistical Analyses

We used an information-theoretic approach (Burnham and Anderson 2002) to determine plausible explanations for joint cutting mortality during establishment prior to implementing the watering treatments (13 July–3 August) and during watering treatments (13 August–20 November). We first compared several logistic models using the LOGISTIC procedure (SAS, version 9.4, Cary, North Carolina) to determine which factors influenced survival during initial establishment. Models included intercept only, separate models for initial joint cutting volume, joint cutting condition at planting (ordinal scale 1, 2, or 3), the soil and watering treatments and their interaction, and joint cutting condition or volume as covariates with soil and watering treatments and their interaction. Treatment levels for the most significant model based on Wald’s χ2 were compared using odds ratios.

We compared survival models after watering treatments began by incorporating the same independent variables as described for establishment and computing an Akaike information criterion with correction for small sample size (AICc) value for each model using the LIFEREG procedure in SAS. We compared the difference from the model with the lowest AICc to obtain ΔAICc: an ΔAICc <2 suggests substantial support for the model; ΔAICc between 4 and 7, considerably less support; and ΔAICc >7 suggests very little support for the model (Burnham and Anderson 2002). Prior to model development, we selected the most appropriate distribution type (lognormal) for the failure model by comparing AICc values of intercept-only models. Multicollinearity did not occur among variables used within the same model based on Pearson’s |r| < 0.75 and variation inflation factors < 10 (Neter and others 1996). The importance of each variable (a value ranging from 0 to 1 for least to most important) was derived by summing the Akaike weights (wis) across all candidate models in which the variable occurred (Burnham and Anderson 2002).

We used two-factor ANOVA (soil mix × watering frequency) to analyze soil moisture changes and joint cutting growth. Effects of soil mix and watering frequency were tested on soil moisture dynamics by analyzing water addition and water loss totaled across the initial establishment and post-watering treatment phases. Effects of soil mix and watering frequency were tested on plant responses including relative growth rate of whole plant (RGR, d−1), final root and shoot volumes (cm3), root-to-shoot ratio (R:S, cm3 cm−3), maximum root length (cm), and final number of joint segments (R, version 3.6.1, Vienna, Austria or SAS, version 9.4, Cary, North Carolina). RGR was calculated based on Blackman (1919): Initial volume was derived from water displacement extrapolated to dimensions measured on 19 June, and final volume was the actual water displacement on 20 November. Assumptions of normality and equal variance were verified prior to analysis using D’Agostino-Pearson Normality test and Levene’s test, respectively. Root length and root-to-shoot ratio were log-transformed prior to analysis to meet the assumptions of equal variance. Joint segment count data were analyzed by comparing model fits for Poisson and negative binomial models of soil and water effects and selecting the best model based on deviance statistics (GENMOD procedure in SAS).

Three joint cuttings remained green but did not develop roots by the end of the experiment (20 November) and were therefore excluded from statistical analyses of growth and post-establishment survival. New growth on one joint cutting was eaten by a woodrat (Neotoma lepida Thomas [Muridae]) at the beginning of the experiment, but bird netting over the joint cuttings and regular live trapping resulted in no further incidents. The damaged joint cutting was included in all growth and survival analyses after confirming that exclusion did not change the statistical outcome of any analyses.

RESULTS

Shade house temperatures were 2 to 3 °C warmer than summer and fall temperatures at Blue Diamond Hill where C. multigeniculata was collected, reflecting the lower elevation of Boulder City (Table 1). Maximum daily temperatures (± SD) at the shade house were highest in July (40.3 ± 0.6 °C) and August (41.1 ± 1.7 °C) and minimum daily temperatures were lowest toward the end of the experiment (October, 17.1 ± 1.4 °C; November, 7.2 ± 0.9 °C). Precipitation at Blue Diamond Hill during the 9 mo preceding joint collection was 42 mm lower than average and highly variable based on comparison to 30-y normal precipitation for the same location (PRISM Climate Group 2020). Monthly precipitation was greater than the 30-y normal upper 95% confidence interval (CI) during November 2019 and March 2020 and was less than the lower 95% CI for October 2019 and January, February, May, and June 2020 (Figure 2).

Figure 2.
Figure 2.

Precipitation prior to joint collection at Blue Diamond Hill, Nevada. The shaded area represents 95% CI around monthly 30-y normal precipitation, and the line represents monthly rainfall preceding collection in June 2020. Data from PRISM Climate Group.

Plant Survival Prior To and After Watering Treatments

Initial survival of joint cuttings before watering treatments were implemented (39 of 80 joint cuttings = 49%) was principally influenced by joint cutting condition (all other models with ΔAICc >> 2) (Table 3). Joint cuttings that were initially rated as fully green were 19.9 times as likely to survive as those rated with some browning of the callus and 147.3 times as likely to survive as those rated as fully brown (Wald’s χ2 = 25.30, P < 0.01). Initial joint cutting condition was not correlated to the identity, size, or condition of the maternal plant nor was initial joint cutting condition of maternal plants clustered spatially within the population (data not shown).

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TABLE 3

Comparison of ordinal logistic regression models for survival of C. multigeniculata joint cuttings prior to initiating watering treatment (6 July–13 Aug) and comparison of failure-time models for survival of joint cuttings during watering treatment (13 Aug–20 Nov).

Survival of the remaining joint cuttings after 3 mo of watering treatments was 56% (20 of 36 joint cuttings), and soil mix was the most influential factor (wis = 0.9909, summed over all candidate models; Wald’s χ2 = 11.337, P < 0.01 in best model). Joint cutting survival was 2.1 times higher for individuals in the 50:50 soil mix compared with the 85:15 mix (Figure 3). Although a model containing both soil mix and watering frequency was well-supported (ΔAICc < 2), soil mix was the most explanatory variable in the model (summed wis = 0.9909), while watering frequency had far less support and was not statistically significant (summed wis = 0.4828; Wald’s χ2 = 1.004, P = 0.32). Initial joint cutting condition (summed wis = 0.0873) and initial joint cutting volume (summed wis = 0.1012) did not appear in any of the most plausible models as explanations for survival 4 mo after treatments began (Table 3).

Figure 3.
Figure 3.

Survival functions for the variable that best explained survival, soil mix (highest weight in model with lowest AICc value), for C. multigeniculata joint cuttings at the greenhouse after watering treatments were initiated on 13 August 2020 (N = 36).

Growth and Biomass Allocation

Joint cuttings grown in the 50:50 soil mix had greater shoot volume (F1,16 = 15.98, P < 0.01) and root volume (F1,16 = 6.53, P = 0.02) 4 mo after planting compared with joint cuttings grown in the 85:15 mix regardless of watering frequency (Figure 4). However, root-to-shoot ratio was not significantly different among soil and watering treatments (log-transformed R:S; soil, F1,16 = 0.30, P = 0.59; water, F1,16 = 3.83, P = 0.07; soil × water, F1,16 = 0.19, P = 0.67). Joint cuttings in the frequent watering treatment had significantly increased root length (log-transformed root length, F1,16 = 6.00, P = 0.03) compared to joint cuttings grown with infrequent watering, regardless of soil mix (Figure 4C). Whole-plant relative growth rate was more complex: Joint cuttings in the frequent watering treatment had similar growth rates regardless of soil mix, whereas joint cuttings with the infrequent watering treatment had significantly lower relative growth rates in the 85:15 soil mix than in the 50:50 soil mix (soil mix × watering frequency, F1,16 = 6.42, P = 0.02) (Figure 5). The total number of joint segments per individual was significantly higher for the 50:50 soil mix compared to the 85:15 mix (soil effect best fit model, Deviance = 0.9512, Pearson’s χ2 = 1.0324; Wald’s χ2 = 5.40, P = 0.02). The average total number of joint segments for individuals in 50:50 soil (3.5 joint segments) was 2.3 times the number of joint segments for plants in 85:15 soil (1.5 joint segments) 4 mo after planting.

Figure 4.
Figure 4.

Measurements of C. multigeniculata joint cuttings (lsmean ± SE) 4 mo after planting in the USGS shade house: shoot biomass (A) and root biomass (B) by soil mix, and root length (C) by watering schedule. Root length lsmean and SE values are back-transformed for clarity. Statistical differences at the P < 0.05 level are denoted by different lowercase letters within each graph.

Figure 5.
Figure 5.

Relative growth rate (lsmean ± SE) over soil and watering treatments for C. multigeniculata joint cuttings at the USGS shade house in Boulder City, Nevada, from 6 July through 20 November 2020. Statistical differences at the P < 0.05 level are denoted by different lowercase letters within the graph.

DISCUSSION

Cylindropuntia multigeniculata can be propagated from joint cuttings, but the condition of the collected joints is key to initial establishment while the soil medium played an important role in post-establishment survival and growth. Even though we collected green terminal joints during summer when plants were active, the surface color of some joint cuttings changed from green to brown during development of the callus. Our index of joint cutting condition demonstrates that during 1 mo of initial establishment, the proportion of green tissue explained survival better than joint cutting volume or soil mix used. Selection of the most appropriate maternal plants for collection is uncertain, however, because joint cutting condition at the beginning of the experiment was not correlated to the identity, size, or condition of the maternal plant.

Over-collecting terminal joints to overcome the 50% initial mortality and increase the probability of joint cutting establishment is not desirable given the protected status of this rare Cylindropuntia. Further study is needed to understand whether seasonal and interannual precipitation and temperature patterns impact joint condition so that collection can be timed to optimize joint cutting quality and improve nursery propagation. Changes in collection and pre-planting practices may enhance joint cutting status, such as transporting joint cuttings in a cooler to reduce cutting stress (St John and others 2009) and whole-joint disinfection before callusing to reduce fungal infection (Sims and others 2016). We partially buried joint cuttings during planting to provide support, but resting joint cuttings on the soil surface or supporting them upright with the callus in contact with the soil surface has been used in successful propagation for other Cylindropuntia species (Holthe and Szarek 1985) and may reduce rot by limiting the surface area in direct contact with moist soil (Alabaster 1996). Although 49% of C. multigeniculata joint cuttings (39 of 80) rooted after 1 mo in our study, a shorter period of callusing or application of rooting hormones may have improved longer-term establishment of the joint cuttings by encouraging rooting (Alabaster 1996).

We collected joints in June when root development for potted cacti is favored by nighttime temperatures above 15.6 °C (Kelly 2009), consistent with temperatures from May through September at the Boulder City shade house (30-y normal, PRISM Climate Group 2020). Cactus joint cuttings are typically collected during spring or early summer when plants are actively growing (Luna 2009), and we timed collection early in the late spring to fall growth window for C. multigeniculata (Baker 2005). Further study could elucidate whether the exact timing of collection across the active growing season and whether collecting actively growing versus previous years’ terminal joints can improve joint cutting establishment for C. multigeniculata. We collected joints in June 2020 following a period of sporadic and low rainfall compared with 30-y normals (Figure 2). Lack of rainfall in January and February 2020 may have reduced vegetative growth for C. multigeniculata, leading to joint cuttings with fewer resources needed for survival (Beatley 1974).

We intended to administer the same total volume of water across watering treatments and to vary only frequency to disentangle the 2 effects that are sometimes confounded in irrigation experiments. Varying watering amount alone may have produced different results because a reduction of water during propagation has been shown to improve drought tolerance, produce more robust root systems, and increase root-to-shoot ratios (Bañon and others 2006; Fernández and others 2006; Franco and others 2010; Franco and others 2011; Lu and others 2014). Holding the frequency of watering constant while lowering the total amount of water can provide insight into how C. multigeniculata allocates biomass during drought conditions.

In contrast to standardized recommendations for propagating cacti in soils that have a high percentage of inorganic material (Alabaster 1996; Neisen 2001), C. multigeniculata cuttings had greater survival and increased root and shoot volume in soils with a higher percentage of organic matter (50:50). Cylindropuntia multigeniculata joint cuttings that survived initial establishment were already an average of 3 times larger after 1 mo when grown in the high organic soil, and this impact carried through to increased survival and greater aboveground and belowground biomass 4 mo after treatments began. Our watering and soil treatments, however, did not alter root-to-shoot ratios of the cuttings.

The greater survival and growth of C. multigeniculata cuttings in the 50:50 mix suggests that placing outplants near shrubs, where soil organic matter and nutrients are concentrated (Schlesinger and others 1996; Titus and others 2002), may promote survival and growth of joint cuttings. Seedlings of other Cylindropuntia species often establish under nurse plants (Cody 1993; Flores-Torres and Montaña 2012) where they may benefit from greater water availability (Martínez-Berdeja and Valverde 2008). Additional explanations have been postulated for observed associations of Cylindropuntia species seedlings with shrub and perennial grass nurse plants, including increased photosynthetic efficiency through amelioration of unspecified environmental conditions (Badano and others 2016), protection from freezing temperatures (Nobel and Bobich 2002), physical trapping of wind-blown fruits (Cody 1993), increased soil moisture, organic matter and nitrogen (Méndez and others 2004), and physical protection from herbivores (Cody 1993; Mandujano and others 1998), although none of these studies tested the proposed mechanisms directly. Factors influencing joint establishment are thought to differ from those influencing seedling survival, and joint establishment in different microsites varies by species (Flores-Torres and Montaña 2012). For example, C. leptocaulis F.M. Knuth joints establish exclusively under nurse plants, while C. imbricata (Haworth) F.M. Knuth joints establish readily in inter-canopy areas (Flores-Torres and Montaña 2012). While we noted during joint collection that adult C. multigeniculata occur almost entirely outside the canopy of shrubs, no observations of the establishment of joints or seedlings have been made in habitat for this species.

The relative importance of asexual reproduction versus individual longevity and sexual reproduction within the life history of C. multigeniculata is not directly known, but C. multigeniculata has 30 to 40 joint segments per stem, similar to other Cylindropuntia species with low levels of joint establishment (Evans and others 2004). In contrast to a species such as C. bigelovii Engelm. that primarily reproduces asexually, we also observed that terminal joints from C. multigeniculata were not easily detached. The 25% establishment and survival of joint cuttings we saw 5 mo after collection over all treatments further suggests that C. multigeniculata fits with Cylindropuntia species that rely mostly on sexual reproduction (sensu Evans and others 2004). Dry conditions during the period of the current study meant that seeds were not produced, preventing us from studying sexual reproduction in direct comparison to asexual reproduction for C. multigeniculata, and leaving propagation from seed as an important contrasting area of study for this rare species.

Although our soil and watering treatments did not increase biomass allocation to roots, frequent watering did produce longer roots. Thus, infrequent watering appears to produce shorter roots in greater density or diameter in contrast to frequent watering producing longer, potentially sparser or thinner roots, similar to the shorter, thicker roots produced by decreasing the total amount of water (Bañon and others 2006). The C. multigeniculata joint cuttings grown in organic soil mix (50:50) with frequent watering had greater root biomass and length, but these joint cuttings also had greater shoot biomass, which can reduce survival under outplanting conditions (Rundel and Nobel 1991). Reintroduction studies will ultimately determine whether propagation pre-conditioning treatments improve long-term survival in protected desert habitats. Intentional and unintentional selection during all stages of propagation have the potential to genetically diminish restored plant populations (Espeland and others 2017), and these impacts can be especially concerning for rare species. Practices designed to decrease artificial selection, such as spreading out collections in time and space and promoting gene flow (Espeland and others 2017), are another important focus of future work with C. multigeniculata.

Cacti may be largely inflexible in the proportional allocation of root and shoot biomass; they have wide environmental tolerance and the ability to take advantage of transient water and nutrient resources, allowing for changes in relative growth rate with minimal change in root-to-shoot ratios (Martínez-Berdeja and Valverde 2008). Desert succulents can survive arid conditions with relatively low root-to-shoot ratios compared with non-succulents because of the water storage capacity of their shoots and an ability to produce new roots quickly after rain occurs (Jordan and Nobel 1984). In this study, we did note an increase in RGR for joint cuttings growing in the high organic soils (50:50) with infrequent watering compared to joint cuttings grown in low organic soil with infrequent watering. However, this difference in RGR did not carry over to the frequent watering treatment, and we found no significant change in root-to-shoot ratios for all soil and water treatments.

CONCLUSIONS

Propagation of C. multigeniculata from joint cuttings provides a viable option for conservation of the species with promise for reintroduction into protected habitats, especially when unpredictable seed production and viability typical of desert environments may limit propagation from seed. The challenges of joint cutting condition still need to be understood when harvesting joints from C. multigeniculata populations. However, the use of a 50:50 mix of inorganic:organic components can enhance survival of nursery stock and promote growth of root and shoot tissues. Frequent irrigation using small volumes of water is preferred over infrequent watering of greater volume as a method to enhance rooting depth. Given uncertainty regarding the life history of C. multigeniculata, direct comparison of propagation from seeds versus cuttings represents an important future avenue of study for this rare cactus. With either method of propagation, an understanding of habitat requirements for the species in combination with the optimal season for reintroduction and the potential benefits of nurse plants will benefit conservation of the species.

ACKNOWLEDGMENTS

We thank Stefanie Ferrazzano for supporting our study on propagation potential of C. multigeniculata and Todd Esque (USGS) for sharing his insights into succulent propagation. We also thank Allison McKenna (USGS) for assisting with joint collection in the field. We are grateful for Cayenne Engel (Nevada Division of Forestry) and Lara Kobelt (Bureau of Land Management, Southern Nevada District Office) for helping us acquire permits for working on this sensitive species. We thank Elizabeth Powell and two anonymous reviewers for providing valuable feedback to improve this manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. This work was supported by the Clark County Desert Conservation Program and funded by Section 10, as project #2019-USGS-1990A to further implement or develop the Clark County Multiple Species Habitat Conservation Plan. Data are available upon request from the Clark County Desert Conservation Program by e-mailing them at dcp{at}ClarkCountyNV.gov.

Footnotes

  • This article was prepared by a U.S. government employee as part of the employee’s official duties and is in the public domain in the United States.

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Soil medium and watering frequency alter growth and allocation for Blue Diamond cholla (Cylindropuntia multigeniculata), a rare cactus of the northeast Mojave Desert, USA
Sara Scoles-Sciulla, Alexander Stosich, Lesley DeFalco
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