In the 1920s, Albert Wieslander worked for the US Forest Service from the University of California’s Berkeley Campus to develop a map that covered about one third of the state, around 10,000 plots in all.
His team also prepared hundreds of hand illustrated maps, took thousands of photos, and collected numerous specimens. Once completed, his work was archived and largely forgotten. It almost ended up in the trash a few times. However, a new study is using Wieslander’s findings and a data set from the past decade to reveal that large trees are on the decline in the state, with around 50 per cent vanishing from the Sierra Nevada highlands since Wieslander’s time.
‘We focused on the plot data on forests since we could compare it to a modern data set from the US Forest Service between 2000–2010, called the Forest Inventory Analysis data set,’ says Patrick McIntyre, a researcher on the project.
‘The USFS is attempting to catalogue the state of forests in the US on a decadal basis,’ he says. ‘We selected plots from the same regions of California [as in Wieslander’s survey], adjusted for differences in sampling methods, and examined broad patterns of forest structure.’
Drought is the main cause for the decline in large trees. Although California faces water shortages linked to agricultural demand, this is not directly connected to the decline in large trees.
‘Most of the areas in our study are mountain areas rather than areas where agriculture and large trees overlap,’ says McIntyre. ‘Competing demands for water for agriculture and by native plant communities are an issue for California, but these are only indirectly related to our paper.’
Tree density has also increased across the state, according to the study, and is probably due to better fire-fighting techniques. In a cycle that can be traced back through ancient pollen records, oaks are seen to be on the rise in the areas studied as they thrive in dry conditions, while pines are falling back, preferring a cool, wet climate instead.
This article was published in the April 2015 edition of Geographical Magazine
