We developed an interview protocol that was approved by the UCLA Institutional Review Board, and sent formal letters to a subset of each category. We followed up with e-mails and telephone calls to set up interviews to ascertain what factors have been driving tree choice in Los Angeles. We were able to obtain no interviews with retail nurseries after attempting to reach a dozen. None responded either to our e-mail inquiries, and those we reached by telephone to ask for an interview declined. We interviewed two sales people in person from wholesale nurseries, and two municipal arborists, one in Santa Monica and one in Beverly Hills. The interviews were based on an interview protocol that asked about drivers of supply and concerns driving demand for municipal arborists.Using the negative binomial regression count model with 95% confidence intervals, the increase in the number of species offered in the most recent time period 1990–2011 was significant compared to time periods 1900–1929, 1930–1959, and 1960–1989. The 95% confidence interval of time period 1990–2011 did not overlap with the 95% confidence intervals of the other time periods . There was no significant difference in the number of species offered between time periods 1900–1929, 1930–1959, and 1960–1989. A polynomial contrast indicated a significant linear and quadratic trend with time. The number of genera offered in time period 1990–2011 was also significantly higher when compared to the number offered in time periods 1900–1929, 1930–1959, and 1960–1989. However,plant pots plastic as with species of trees, no significant difference in the number of genera was observed between time periods 1900–1929, 1930–1959, and 1960–1989. The 95% confidence interval of the 1990–2011 time period did not overlap with the 95% confidence intervals of the other time periods .
A significant linear and quadratic trend was observed with time.There was no observable change in the number of California native species offered over time . While California boasts several dozen native trees, as mentioned above, there are only 14 native to Southern California, including such species as weedy willows that do not do well in an urban environment and cannot be used as street trees. Many of California’s native trees do not thrive in Southern California’s Mediterranean climate, like bristle cone pine, white fir, and pacific yew, among others. In the case of non-native species, the number of species offered in time period 1990–2011 was significantly greater than the number of species offered in time periods 1900–1929, 1930–1959, and 1960–1989. Similar to the pattern with deciduous species, the numbers of species offered between time periods 1900–1929, 1930–1959, and 1960–1989 were not significantly different. Both linear and quadratic trends were observed with time for non-native species .Gymnosperm species showed a unique trend.While the number of species offered in time period 1990–2011 was not significantly different from time periods 1900–1929 and 1960–1989, it was significantly different from time period 1930–1959. The results of the polynomial contrast indicated a quadratic trend with time . Angiosperm species followed the same pattern as deciduous species and non-native species; the number of species offered in time period 1990–2011 was significantly greater than the number of species offered in time periods 1900–1929, 1930–1959, and 1960–1989. There was no significant difference in the number of angiosperm species offered in time periods 1900–1929, 1930–1959, and 1960–1989. A significant linear and quadratic trend was observed with time . Palm trees are highly visible and arguably iconic in the Los Angeles landscape. Interestingly, the number of palm species offered in time period 1900–1929 was significantly greater than the number of species offered in time period 1930–1959. However,there was no statistically significant difference in the number of species offered in time periods 1930–1959, 1960–1989, and 1990–2011.
An overall quadratic trend was observed with time .Our research numbers rely on tree nursery catalogs, and indicate an increase in the tree species choices offered by nurseries in the region over time, even though the actual numbers of nurseries in the region have declined in number . According to our interviews, increases in knowledge about disease susceptibility and the importance of having a diverse selection of street tree species, for example, were cited as among reasons for the increase in the number of species offered by nurseries. Another reason cited is that as individuals, land developers, and city planners hear about new species from breeders, conferences, or personal contacts, they request these new species from the nurseries in the region .At regional to global scales there is strong evidence of declines in species diversity in non-urban ecosystems accompanied by increases in exotic species . Research on urban biodiversity however, shows that there are increases in biodiversity in cities due to invasive species as well as human interventions in planting horticultural species . Unlike in non-urban habitats where Sax and Brown and Peet found that native and exotic diversity are often positively correlated, suggesting that where species richness has increased , urban floristic diversity often comes at the expense of the native landscape. To create optimal conditions for construction, native vegetation is scrapped, fill is often brought in as well as top soil, contours and topography altered, irrigation may be added, soils are compacted, and high rates of fertilizers and chemicals are applied . This does not mean, however, that the urban plant palate is depauperate. As Kowarik points out, urban areas may be more species rich compared to their surrounding environments. Urban floristic diversity is driven by anthropogenic biophysical changes in the landscape and human choice in revegetation as well as urban morphology and climate. These are significantly different drivers of species diversity than in non-urban environments. Since Los Angeles is in asemi-arid environment, remnant or feral vegetation in non-cultivated spaces of the urban environment tends to be weedy and scrubby and trees rarely regenerate, except where there is water.
In irrigated but poorly maintained areas, such as in freeway interchanges, one might find a mix of eucalyptus, Schinus terebinthifolius, Ailanthus altissima, and other opportunistic species. In non-irrigated areas, there are low-growing grasses and Mediterranean-region invasive plants. There are fewer non-built lots than in other cities in the U.S., for example Baltimore or Detroit. Los Angeles County is most densely populated and the most populous in the U.S ; and there is little open land outside of parks. There has been a significant increase in tree diversity in Los Angeles from the 14 native species present in Los Angeles County before European contact. Our results show that there are now over 500 commercially available species of trees . Muller and Bornstein describe between 95 and 408 tree species currently being planted in California cities, with an average of 185 per community. This confirms our findings of high available diversity. That study also reported that the number of approved species for future public planting is only 29% of the species in the existing inventory. If the goal of tree planting in the city is biodiversity, this is troubling. However, it is likely that approved species must satisfy concerns about water use, tree size,blackberry pot maintenance and other issues pertaining to feasibility and management costs. These attributes are also important issues relevant to urban biodiversity as tree mortality in urban areas is high, and there are constrained municipal budgets for tree maintenance . The increase in nursery catalog offerings has accelerated in the past two decades. It is difficult to quantify trends in diversity of tree planting and we acknowledge there is not necessarily a causal link between diversity of offerings shown in tree nursery catalogs and what is found in the urban landscape. Still it is worth considering the extraordinary diversity of offerings and to further investigate how that may affect urban tree planting in different cities in the Los Angeles region, and between different land uses: parks, street trees and trees in private yards. Perhaps, as Muller and Bornstein caution, there may in fact be a decline in the public tree planting sphere, but an increase in private tree diversity. Other questions that emerge from this work include whether there is a similar trend in other cities, the link between biodiversity and urban forest function, and important measures of biodiversity other than taxonomic. For example, tree diversity in Nordic cities showed that 70% of all newly planted trees in street environments belonged to one clone of Tilia . Genetic diversity of urban forests may be a critical factor in urban forest health and sustainability. Overall, little is still known about urban trees in different parts of the globe, including their biodiversity, health, maintenance regimes, and the reasons why people plant some species and not others.Over the study period there were significant shifts in tree types, with an increase in deciduous tree species. Deciduous species shed their leaves in the cooler months and provide shade in the hotter summer months and thus are functionally different than evergreen species. Such changes in the availability of functional types may affect the structure of the urban forest of Los Angeles, as old and diseased trees will likely be replaced by the tree species now available. Such a change may also affect the ecosystem services provided. For example, while research is increasingly showing that trees do little to sequester carbon in cities relative to the amount of carbon generated in fossil fuel combustion, and there is little evidence to date that trees improve air quality,trees do mitigate outdoor heat fluxes through outdoor shading and evapotranspiration . More city-level studies of trees and water use, their impacts on outdoor heat related to their size and canopy and the trade-offs with water use and maintenance, would help municipal arborists better choose trees if ecosystem benefits were a desired outcome.
Employing a chronological analysis to analyze the species currently and historically available to plant in a region offers a novel way to understand urban tree composition.In Los Angeles County, seventy new species have been offered within the last decade .While it is likely that not all species of trees offered by the nursery industry are successfully planted, there are additional interesting questions to be asked about the increase in species availability. These include the role of horticultural research, the dissemination paths for new species and/or new hybrids, the drivers of choice and adoption, and the impacts of such trees on the region itself. Will the new trees change water demand? Will they affect allergies? Will they change maintenance costs for cities, or the public? Will they provide habitat for birds and insects? Pairing this information with other costs such as tree maintenance, as well as people’s preferences and purchases of specific species, can help inform policy and decision makers of environmental benefits and costs of urban forests in a given region. According to the Sales Manager of a nursery we interviewed, while having low-maintenance trees is an important criterion, functionality and beauty are still more significant variables in selling trees .We found over a dozen trees that have been consistently offered over the century. These include coast redwoods and giant Sequoia . These species are native to California but not to the Los Angeles region , and Sequoia trees found in Los Angeles tend to show signs of stress and are not widely grown . Tree catalogs may not match nursery stock and availability, nor are they necessarily good predictors of tree survival rates. The species that were found to be sold throughout the 112 years include flowering trees, deciduous and evergreen trees, a palm, but no trees native to Southern California. Thus further research must be conducted to determine the reasons for new tree species introductions, their popularity, and whether tree nursery catalogs are truly good predictors of the biodiversity of trees in the urban forest. This would link the functional traits that are desired with the catalog information.Trade-offs between reproduction and longevity are among the most comprehensively researched areas of life history theory . Testosterone and other steroid hormones play critical roles in the development and maintenance of many sexually dimorphic traits in males resulting from sexual selection, but at a cost; ceteris paribus, energetic investments in reproductive effort trade-off against investments in somatic maintenance . Since fitness gains of investment in reproduction earlier in life often outweigh the gains from somatic repair, reproduction is usually prioritized over longevity.