There is growing interest in the role urban landscapes can play in supporting biodiversity while also benefitting human well-being. Green spaces in cities often bear little resemblance to the landscapes they replaced, and re-integrating elements of lost ecosystems into cities has potential to benefit both biodiversity and people. However, the extent of transformation of urban ecosystems is often not well documented, raising questions as to how urban green spaces might best be designed to benefit biodiversity. Here, we use the urbanized landscape of Silicon Valley as a case study to evaluate the extent to which urbanization has led to the loss of the oak woodland ecosystem that was present before European arrival. Drawing upon historical records, we compare the historical structure and composition of the oak woodland to contemporary survey, street tree, and canopy data to quantify the extent of ecosystem transformation. We identify elements of the urban landscape that could be altered to re-incorporate oak woodland structure and composition, and we ask what benefits to biodiversity and human wellbeing could be derived from these changes.
We show that while the structure and composition of oak woodlands have been dramatically altered by urbanization, canopy cover in Silicon Valley has remained relatively consistent. The composition of the contemporary urban forest is highly altered, with dramatic increases in tree species richness. Similarly, the age structure of the urban forest is also altered, with fewer large old trees and a greater proportion of smaller trees. Other physical structures important for wildlife, including dead limbs, dead trees, and downed wood, are also missing from the urban landscape. Understory vegetation, historically containing a diverse mixture of perennial bunchgrasses, annual wildflowers, and shrubs, is largely missing from urban green spaces. Opportunities for restoration of some oak woodland elements into the urban landscape could increase biodiversity, and are potentially beneficial for people as well. For example, adding oak woodland tree species to the urban forest could help support wildlife that require oak trees as habitat and can tolerate urban conditions. Replacing trees from temperate deciduous climates with drought tolerant oak trees can also reduce irrigation requirements. Supporting biodiversity and human wellbeing via restoration of lost ecosystem elements is a method that is likely to be broadly applicable across a wide range of cities worldwide.