What influences urban forest structure and species composition?

I recently looked into how neighbourhood affluence levels influenced tree populations (abundance and diversity) in a city in Brazil, and have since come across a similar research article that looked into such issues (amongst others) in southern California. In this instance however, it appears that the authors take a slightly different angle of approach, and assess whether residents’ preferences across different communities impacts upon constituent tree populations as well. Ultimately however, this study had three main aims, and they are: (1) to determine whether the socio-economic and environmental status of an area will influence tree cover and species diversity; (2) to ascertain whether patterns exist across spatial areas within an urban forest (such as between street trees and trees within residential properties, and across the three counties studied), and; (3) to assess the level of influence residents have in terms of determining tree species composition within the urban environment.

The three counties featured within this study were Los Angeles, Orange, and Riverside, and street tree inventories were completed during 2010-11. In each county, 12-13 residential neighbourhoods were identified (that differed in their characteristics) and subsequently surveyed. In these neighbourhoods surveyed, data relating to socio-economic and environmental variables (temperature, precipitation, population density, average income, etc), ecosystem services of the trees (growth rate, likelihood of root damage, flower showiness, and so on), and also tree type (native or exotic), were collected.

The macro-scale urban forest of southern California

Across all of the neighbourhoods surveyed, it was found that only 7% of all trees were native, whilst 46.5% were exotic species with the ability to regenerate naturally, and the other 46.5% unable to regenerate naturally. In terms of species diversity, all three areas were found to be somewhat similar, though Riverside had the lowest at 45 and Orange had the highest at 75. Los Angeles has 64 tree species identified in the survey. Only Quercus agrifolia, a native to the region, was found in significant numbers – all other abundant species were exotics, such as Cupressus sempervirens and Syagrus romanzoffiana, of which some were unable to reproduce naturally.

socalmexicanpalm
Los Angeles County (as well as the other Counties) was found to be home to a great number of Mexican fan palms (Washingtonia filifera), which can be seen lining this street within the County. Source: LA City Urban Forestry.

In terms of tree cover, Los Angeles and Orange County had a greater number of trees per neighbourhood than Riverside, though residential and parkland areas across all three counties had the highest diversity of tree species, as well as the highest canopy cover. Most trees in all counties were planted, in place of growing naturally.

Drivers behind differing urban forest characteristics

Socio-economic factors, in light of the data collected, were shown to have a significant influence upon urban forest structure and richness – the average income of the neighbourhood, the year in which the neighbourhood was built, and the level of education residents received (in this case a college degree or higher) were the most important influencing factors. There were greater numbers of trees (as well as a greater species richness) per neighbourhood in the wealthier (and better-educated) areas of all three countries, and Orange County and Riverside County had more trees in older neighbourhoods. Conversely, tree species diversity was low in cramped (high density) neighbourhoods.

Spatial differences in urban forest structure

In terms of differences between richness of street tree populations and residential tree populations, there was only a significant difference (in favour of residential trees) for Orange County (and between counties, street tree species composition was not significantly different). There was also a difference between the counties with regards to the amount of shade trees found, with Riverside County possessing more shade trees than Los Angeles County and Orange County – though there was always a greater proportion of shade trees found in streets, when compared to residential properties. Residential properties also had trees that, on average, were more water-demanding, and required more maintenance.

Tree species preference of residents

More fruit trees were found in older, more affluent neighbourhoods, compared to high-density neighbourhoods where trees were typically far less ‘showy’. Newer developments also had fewer trees that were demanding in terms of maintenance. Hotter areas were found to be home to the most shade trees, which correlated with the increase demand by residents in those areas for shade trees.

So what does this mean?

Because many of the trees in all of the neighbourhoods were planted, the authors suggest that socio-economic factors feature heavily in what is planted and where. As most of the tree species identified were not as common 20-30 years ago within the same counties, the demands of residents must have changed and the alteration in tree species composition would have shifted as a result. This is, according to the authors, perhaps because nurseries stock what is in demand. However, it is concerning that most trees are not native, though this may be because the counties’ generally arid conditions mean that the areas were largely void of trees prior to development.

The fact that species richness is higher in affluent neighbourhoods is also interesting, though perhaps not surprising given other studies showing much the same. The authors consider that because wealthier neighbourhoods have existed for longer periods of time than the newer low-income neighbourhoods, and because wealthier neighbourhoods can afford to heavily irrigate their trees, their tree species diversity is, on average, higher. Interestingly, the concept of ‘prestige trees’ is also raised, perhaps suggesting that wealthier households will seek to plant exotic and perhaps less common tree species as a means of ‘showing-off’.

 

woodstreetriverside
The ‘wood streets’ in Riverside County are lined with trees, in this case what appear to be cypresses. Source: Grow-a-brain.

Residents’ levels of education, whilst shown to be an influencing factor in more diverse and plentiful tree populations, may not relate exclusively with affluence. Of course, well-educated individuals may live in poorer regions, for varying reasons. It is perhaps because they are likely more aware of the need for trees that areas of well-educated individuals were found to have ‘better’ urban forests.

The greatest species diversity, which was found in parks and residential areas, may be explained by (in the case of the latter) the desire by residents to have functional trees with good amenity value (traits are more important than species, and therefore there is a wide species diversity). The fact that street trees required little maintenance and had low water demands may be because municipal foresters are principally concerned with reducing the costs associated with their trees. Hence, we can observe a difference in the rationale behind the decisions made with regards to species selection, though publicly-owned parks do allow for more innovation on behalf of the municipality’s officers (given more space and a lack of a need to prune park trees, in general).

Focussing on resident preference for trees, whilst the data collected here was not robust enough to draw wholly accurate conclusions, the greater abundance of trees in affluent areas correlates with such residents’ desires to have trees within their neighbourhoods. Similarly, the desire by residents in hotter neighbourhoods to have shade trees was shown to also hold true in actuality (more shade trees were found in hotter areas). More research would need to be done to ascertain whether resident preferences do influence street tree populations and species compositions, however.

Source: Avolio, M., Pataki, D., Gillespie, T., Jenerette, G., McCarthy, H., Pincetl, S., & Clarke, L. (2015) Tree diversity in southern California’s urban forest: the interacting roles of social and environmental variables. Frontiers in Ecology and Evolution. 3 (73). p1-15.

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What influences urban forest structure and species composition?

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