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Database of many plant species common in California showing their watering needs at the regional level. Invaluable when planning for drought-tolerant landscapes.

City trees are under siege – from climate change, urbanization, and pesky insects! This study put urban oaks to the test, exposing them to the triple threat of warming, drought, and insect herbivores. The shocking finding? Water stress is the real villain, stunting tree growth on its own and making them even more vulnerable to the other stressors. But there’s a silver lining – by focusing on keeping those city trees hydrated, we can help them weather the storm of global change and continue providing all those invaluable ecosystem services we depend on. Looks like watering is the key to future-proofing our urban forests!

Vehicle pollution is taking a serious toll on our urban trees – just ask the hungry hordes of insects feasting on the leaves of oaks near busy highways. This study found that trees situated closer to major roads experienced significantly more damage from these insect herbivores. And in the lab, the insects even showed a clear preference for – and better performance on – leaves from those roadside trees. Looks like the variability in vehicle emissions across cities is really driving some wild differences in how insects are interacting with our beloved street trees. Urban foresters may want to keep this in mind when choosing more resilient species to plant along heavily trafficked roads.

Not all nonnative species become invasive. What are some things that increase the invasiveness of plants? In this easy-to-understand study, seeds of different tree species were painted, dropped off a bridge into the Spree River in Berlin, and tracked to see how far downstream they would move. After only three hours, one quarter of the seeds were found 1,200 m from the start. This is much farther than they would travel in the wind. Rivers, therefore, are a potentially important factor in increasing the invasiveness of nearby trees.

One challenge to controlling invasive species is finding them in the landscape. In this study, drones equipped with cameras were able to detect the highly invasive tree species, Ailanthus altissima. This will be helpful for the management of both the tree and the invasive insect, the spotted lanternfly, that lives in this tree.

Natural regeneration is less common in urban forests, but why? This easy-to-understand study looks at survival of tree seedlings in a forested urban park in Seattle. Natural regrowth of trees, especially the survival of seedlings, was hindered by low natural seed production and poor microsite conditions. Adding seeds and deadwood to experimental plots increased seedling numbers and survival. Presence of an invasive species did not have an effect.

This LA Times op-ed article describes the enormous risks that the shothole borer presents for native and urban trees. Because the beetle can thrive in so many different species, it has the potential to kill more than one-third of the trees in the LA region. Article closes with suggestions for slowing the beetle’s destructive spread.

Municipalities are turning to greening initiatives like tree planting to tackle urban sustainability and combat the environmental impacts of expansive urbanization. This study uncovered some surprising findings about urban street trees. It turns out they have crazy-fast growth, nearly 4 times quicker than their rural forest counterparts. However, city trees also face dramatically higher mortality rates, more than double what’s seen in the countryside. As a result, despite all those planting efforts, the net carbon storage in urban street trees is actually declining over time. The authors conclude that if we really want these urban greening initiatives to be an effective climate solution, we’ll need to pair all that tree-planting with some strategic maintenance efforts.

European mistletoe was brought to California over a century ago to be grown and sold for Christmas decorations and medical tinctures. Things got out of hand, and mistletoe is still spreading like wildfire! Heavy infestations of mistletoe can kill a tree.

The researchers tracked this parasitic plant’s march from its original site in Sebastopol, and the results are jaw-dropping. In just the past 30 years, the mistletoe’s reach has more than tripled, infecting trees as far as 15 miles from where it was first planted. And it’s not just sticking to ornamental trees in people’s yards – this mistletoe is also invading the native riparian areas, putting a whole host of trees at risk. Yikes, talk about an unwelcome holiday guest!

A brief, solid introduction to urban forestry. What is an urban forest? What do trees in cities do for us? What are ecosystem services? Can we put a value on the work trees do for us? What threats do our urban trees face?

The future of our urban forests is looking challenging, as climate change and unrelenting development threaten to take a serious toll on city trees. This global study dug deep into tree growth patterns, using tree ring data to reveal how different species respond to the drought and aridity that often come with urban living. The results were eye-opening – while some tough, drought-tolerant trees like black locust can tough it out, other more sensitive species like linden and oak really struggle, showing significantly stunted growth in arid conditions. The takeaway? Understanding these climate sensitivities is crucial for urban foresters working to build resilient green infrastructure that can withstand the challenges ahead.

This study dug deep into a major challenge facing cities – getting trees to actually take root and thrive! The researchers found that amping up the organic matter content of urban soils can work wonders, leading to bigger tree trunks and a huge boost in fine root development. And get this – those hardy tree roots even helped improve the soil’s structure and drainage, creating the kind of fertile, healthy urban ecosystem that city trees need to really flourish. The key? Pairing all that tree-planting with some strategic soil amendments to give those roots the boost they’re craving.

The shothole borer is a highly destructive, invasive pest that has killed hundreds of thousands of trees in recent years. The beetle bores into trees and releases a fungus that eats away at the tree from the inside. What conditions increase the destructiveness of this pest? This Master’s thesis describes a simple study to assess whether the beetle causes more damage in water-stressed vs well-watered trees (answer: no) or in trees of different species (answer: yes, probably based on the density of the wood).

Some people argue that in a given city, only species that are native to that city should be planted. This interactive GIS map shows the tree species that are native to California cities. For a given location, a list of native species is provided. The number of native species in any given location in California is very small compared, e.g., to the East Coast.

Urban development is no friend to trees – it decimates soil structure and strips away all that precious organic matter, leaving city trees high and dry. But this study unearthed a game-changing solution: soil rehabilitation. The researchers found that by amending and loosening up the soil, they were able to turbocharge tree growth, with some species seeing an 84% boost in canopy size after just six years. And get this – the soil rehab even improved the trees’ water-handling abilities, making them better equipped to handle stormwater. The takeaway? Focusing on the hidden world beneath our feet could be the key to helping urban trees truly thrive.

Tree rings are like little time capsules, holding clues about the environmental pollution these woody witnesses have endured over the decades. This study put that tree-ring record to the test, measuring lead levels in the wood of different tree species growing near a busy road versus in the forest. The results were fascinating – certain species like hickory proved to be true lead detectives, with concentrations tracking the rise in nearby traffic. But the story wasn’t the same for all trees, with tons of variability even among individuals of the same species.

Data on more than 5 million urban trees in 63 large US cities is included in this dataset, including location (latitude/longitude), species, native vs nonnative status, health, trunk diameter, and more. Some data organization and cleaning was done.

Urban living is tough on trees – the cramped conditions and limited soil access take a serious toll on their growth. This study put that urban stress to the test, comparing the size and canopy development of five common tree species planted in parking lots 20 years ago. The results were eye-opening – the less soil available to the tree, the smaller and scrubbier its growth became.