Space constraints in cities mean that there are only limited opportunities for increasing tree density within existing urban fabric and it is unclear whether the net effect of increased vegetation in street canyons is beneficial or detrimental to urban air quality at local scales.
Bark beetle infestation is a well-known cause of historical low-level disturbance in southwestern ponderosa pine forests, but recent fire exclusion and increased tree densities have enabled large-scale bark beetle outbreaks with unknown consequences for ecosystem function.
Land use that accompanied Euro-American settlement in the late 1800s included timber harvest and fire cessation that resulted in loss of larger trees and increased tree density.
These processes become most effective when full root occupancy (lateral roots of adjacent trees overlap) and canopy closure (canopies of individual trees touch) first occurs, and they improve with increasing tree age and/or planting density (Kelliher, F. M., Marden, M., & Watson, A. J.: Stability of land and tree planting density East Coast, North Island, unpublished).
The significant interaction between aspect and tree-ring age showed that with an increasing tree age the annual wood density of trees on the SW aspect is significantly lower than on the NE.
There is a clear increase in accuracy as well as a decrease in the range of the 95% highest posterior density interval (HPD) with increasing tree depth.
The richness of both birds and bats increased with an increase in the proportion of native plants, bird species richness increased with increasing volume of understorey vegetation, and increasing large tree density led to increased bat activity.
Stress can be induced in trees by a diverse number of interacting conditions such as, water and nutrient deficiencies, increased tree and herbivory density, presence of other pest insects and diseases, topographic locations, and soil characteristics.
Sapling density marginally decreased with decreasing elevation and increasing stand tree density.
In ponderosa pine forests of western North America, substantial increases in tree densities are impacting overall forest health and increasing the risk for severe wildfires and insect and disease outbreaks.
