Shifting California forests reveal complex effects of drought

When it comes to understanding human impacts on forests, the first thing that springs to mind is probably logging. And while logging does leave a heavy footprint, it’s not the only anthropogenic activity that affects forest health. In California, ninety years worth of data shows that climate change is also stressing the state’s forests – and not in the ways you might think. Most of the state’s forests actually have greater tree density now than they did in the 1920s and 1930s. So what’s going on?

It turns out that forest ecology is, unsurprisingly, simply more complicated than whether there are fewer or more trees. Previous research has shown that forests in the Sierra Nevada Mountains have shifted in composition over the last century. As larger trees (>60cm diameter) have died off, they’ve been replaced with more, smaller ones.

McIntyre and his team compared forest surveys made in the 1920s and 1930s by UC Berkeley researcher Albert Wieslander to those made between 2001 and 2010 by the US Forest Service. They discovered that while overall forest biomass declined over that period, tree density increased by thirty percent. That trend had already been noted for the Sierra Nevadas, but McIntyre’s group found it to be true also of the Sierra Foothills, the coastal ranges of Northern, Central, and Southern California, and the peninsular ranges south of the Los Angeles area.

The researchers then compared the changes in forest composition with estimates of changes in “climatic water deficit,” or CWD. CWD is an estimate of water demand relative to water availability. Higher values indicate lower availability, while lower values suggest there’s enough water to go around. Higher CWD is driven by phenomena like increased temperatures, earlier snowmelt, and decreased precipitation – all representative of the effects of global climate change on California’s long-term weather patterns.

They discovered that declines in large tree density correlated with increases in CWD, while no correlation was found for small trees. That suggests that large trees suffer disproportionately during droughts, and that small trees have exploited the space made available by the disappearance of their larger counterparts. Logging surely plays its role, as does pollution, in the more urbanized parts of the state, but no single variable explains the similar trend across the state as elegantly and simply as drought.

Severe water stress (left red) since the 1930s mirrors the decline of large trees (right red) seen throughout the state, from the Sierra Nevada to the Coast Ranges. Credit: Patrick McIntyre, David Ackerly and Maggi Kelly, UC Berkeley.

Severe water stress (left red) since the 1930s mirrors the decline of large trees (right red) seen throughout the state, from the Sierra Nevada to the Coast Ranges. Credit: Patrick McIntyre, David Ackerly and Maggi Kelly, UC Berkeley.

Finally, McIntyre and his colleagues looked at how the change in water availability over time tracked with changes in the abundance of oak trees (genus Quercus) and pine trees (genus Pinus). Both tree types are common to California, together accounting for nearly half of the state’s forests and woodlands. The balance between the two tree families can be compared to paleobotanical records, as the trees’ historical relative abundance can be easily estimated on the basis of preserved pollen.

Over the past 150,000 years, oaks have generally dominated during warmer, drier interglacial periods, while the forests have shifted towards the pines during colder, glacial periods. Against that backdrop, it makes a good deal of sense that McIntyre’s group found that pines have decreased in all areas of the state over the last century, while oaks have increased in the Sierra Nevadas. They did find, however, a decrease of oaks in the central and southern coasts. Still, the ratio of oaks to pines correlated with changes in CWD. In fact, the relative dominance of oaks is actually higher than would be expected by changes in CWD alone, suggesting that other forces contribute as well, such as logging practices. The researchers therefore conclude that “the shift in California forests to a more oak dominated system in the last 70 [years] is consistent with historical changes over longer time scales associated with temperature and water availability, although land use changes, timber harvesting and successional change likely contribute to this pattern as well.”

The story of California’s forests is not yet finished. That’s because a pathogen called Phytophthora ramorum has been introduced into the state, which causes “sudden oak death.” The parasite is too new to be evident in the contemporary data – it was only detected in the mid-1990s – but it will no doubt play a role in guiding the evolution of the golden state’s forests. Based on historical trends alone, oaks are expected to continue to dominate for quite some time, but if this or other pathogens become more influential, then “California’s forests may shift in directions not observed in the paleohistorical record,” the researchers add. We are truly living in the Anthropocene. – Jason G. Goldman | 28 January 2015

Source: Patrick J. McIntyre, James H. Thorne, Christopher R. Dolanc, Alan L. Flint, Lorraine E. Flint, Maggi Kelly & David D. Ackerly (2015). Twentieth-century shifts in forest structure in California: Denser forests, smaller trees, and increased dominance of oaks, Proceedings of the National Academy of Sciences, 201410186. DOI: http://dx.doi.org/10.1073/pnas.1410186112

Header image: shutterstock.com

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