The balance of planting and mortality in a street tree population

Abstract

Street trees have aesthetic, environmental, human health, and economic benefits in urban ecosystems. Street tree populations are constructed by cycles of planting, growth, death, removal and replacement. The goals of this study were to understand how tree mortality and planting rates affect net population growth, evaluate the shape of the mortality curve, and assess selected risk factors for survival. We monitored a street tree population in West Oakland, CA for 5 years after an initial inventory (2006). We adapted the classic demographic balancing equation to quantify annual inputs and outputs to the system, tracking pools of live and standing dead trees. There was a 17.2 % net increase in live tree counts during the study period (995 in 2006, 1166 in 2011), with population growth observed each year. Of the live trees in 2006, 822 survived to 2011, for an annual mortality rate of 3.7 %. However, population growth was constrained by high mortality of young/small trees. Annual mortality was highest for small trees, and lower for mid-size and large trees; this represents a Type III mortality curve. We used multivariate logistic regression to evaluate the relationship between 2011 survival outcomes and inventory data from 2006. In the final model, significant associations were found for size class, foliage condition, planting location, and a multiplicative interaction term for size and foliage condition. Street tree populations are complex cultivated systems whose dynamics can be understood by a combination of longitudinal data and demographic analysis. Urban forest monitoring is important to understand the impact of tree planting programs.

Appendix 1: Supplementary field data collection details

Several quality assurance and quality control steps were necessary to adapt the 2006 inventory system to multi-year monitoring. Some trees that were assumed to have been omitted from the 2006 inventory were retroactively added as alive in 2006. In these cases, tree size (>10 cm DBH in 2008) was taken as evidence that they were already in the ground in 2006. The urban forestry initiatives in this neighborhood plant small saplings, therefore it seemed reasonable to assume that mid-size and large trees were omitted in 2006. In the first monitoring year (2007), we also confirmed species, land use, and planting location information from the initial inventory, correcting errors where necessary.

Standing dead status during monitoring years 2007–2011 was defined by an absence of green leaves and live buds. This is a lower threshold of health than the “dead or dying” condition rating in i-Tree (Table 1). Trees from the 2006 inventory recorded as health rating 1 (dead or dying) for both foliage and wood were categorized as standing dead by our definition. However, because health rating is subjective, and different individuals were involved during the inventory vs. monitoring years, this approach to connect our standing dead definition and 2006 health categories was imprecise. There were 2 trees from the 2006 inventory with dying health ratings for foliage and wood that we recorded alive in 2007; however, we also noted that these trees were nearly dead. For simplicity in this analysis, because no backwards transitions were allowed from standing dead to alive, we retroactively re-categorized those 2 trees as alive in 2006.

To facilitate ease of finding trees each year in the study, tree location was recorded with several complementary systems: street addresses, manual notes on a map of GPS coordinates from the 2006 inventory, and order on the block. Tree order on the block was a system of numbering each tree every year in progression from north to south, or east to west, for one side of the street on a given block. The ordering system was used to facilitate database sorting for convenience during field work.