A new report from the Nature Conservancy makes the case for the value of urban trees to human health. They go through a number of economic valuation studies that are out there, and the literature on health benefits: air quality, heat stress, mental and physical health, climate change. Then they make a case that urban tree canopy in the U.S. is actually declining and that it is severely under-funded in most cities.
Also, on the tree front, here is a recent paper on the rate at which wood inside urban trees decays. I think one important concept with urban trees is to think of them as infrastructure that has to be maintained and replaced at some rate. They just don’t live as long as forest trees, because they are in stressful environments, performing functions for us, and getting worn out. And the cost of maintaining and replacing them is actually low, and their benefits high, compared to other types of infrastructure. But even though the engineering, planning and architecture professions have been talking a lot about green infrastructure for at least a decade, most of us still aren’t taking it seriously as infrastructure, and the construction industry, bureaucrats and politicians are not taking it seriously, if they have even absorbed the concepts at all. I think this is a case where wealthy private foundations or individuals could make an enormous difference if they wanted to, because the institutions to plant and maintain trees typically exist, but are just severely underfunded. So all I have to do is become a wealthy private individual and I will take care of this. Okay, a solution exists and I’ll get right on that.
The overlooked carbon loss due to decayed wood in urban trees
Decayed wood is a common issue in urban trees that deteriorates tree vitality over time, yet its effect on biomass yield therefore stored carbon has been overlooked. We mapped the occurrence and calculated the extent of decayed wood in standing Ulmus procera, Platanus × acerifolia and Corymbia maculata trees. The main stem of 43 trees was measured every metre from the ground to the top by two skilled arborists. All trees were micro-drilled in two to four axes at three points along the stem (0.3 m, 1.3 m, 2.3 m), and at the tree’s live crown. A total of 300 drilling profiles were assessed for decay. Simple linear regression analysis tested the correlation of decayed wood (cm2) against a vitality index and stem DBH. Decay was more frequent and extensive in U. procera, than P. acerifolia and least in C. maculata. Decay was found to be distributed in three different ways in the three different genera. For U. procera, decay did appear to be distributed as a column from the base to the live crown; whereas, decay was distributed as a cone-shape in P. acerifolia and was less likely to be located beyond 2.3 m. In C. maculata decay was distributed as pockets of variable shape and size. The vitality index showed a weak but not significant correlation with the proportion of decayed wood for P. acerifolia and C. maculata but not for U. procera. However, in U. procera, a strong and significant relationship was found between DBH and stem volume loss (R2 = 0.8006, P = 0.0046, n = 15). The actual volume loss ranged from 0.17-0.75 m3, equivalent to 5% to 25% of the stem volume. The carbon loss due to decayed wood for all species ranged between 69 to 110 kg per tree. Based on model’s calculation, the stem volume of U. procera trees with DBH ≥ 40 cm needs to be discounted by a factor of 13% due to decayed wood regardless of the vitality index. Decayed wood reduces significantly the tree’s standing volume and needs to be considered to better assess the carbon storage potential of urban forests.