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Scientists: Climate Change Seems Responsible for A Loss of Large-Diameter Trees in Yosemite National Park


Is climate change reducing the size of Yosemite National Park's forests? Kurt Repanshek photo.

Climate change with its warmer and drier seasons appears to be responsible for a decline in large-diameter trees across much of Yosemite National Park, according to a recently released study. Published in the Forest Ecology and Management journal, the study (attached below) suggests that one way to perhaps strengthen the park's forests against climate change is to introduce more prescribed fires.

According to the research, Yosemite was home to more forests with large-diameter stands of Ponderosa pines, Incense cedars, White fir, Sugar pines, Jeffrey pines, Red fir, Western White pine, Lodgepole pine, Whitebark pine and Mountain Hemlock between 1932-1936 than during 1988-1999. The impact of this downsizing goes beyond beautiful vistas, as mature forests "moderate the local environment; serve as a seed source for the surrounding landscape; and withstand fires, climate variation and insect outbreaks that kill or weaken smaller-diameter trees," the authors note.

While the authors, who reached their conclusions after comparing the findings of a 1932-1936 study of tree densities with one from 1988-1999, acknowledge that there are "many alternative explanations for the decline in densities of large trees," they lean toward climate change in general and water stress specifically as the most likely cause. Other studies have pointed to a decline in Sierra snowpacks, which slowly release water as they melt through spring and on into the summer, and increased rainfall, which runs off much more quickly, as being linked to climate change.

Increased water stress -- whether arising from extrinsic climatic change or intrinsic changes in stand density -- we view as the leading candidate for the underlying cause of the recent doubling in mortality rates in old-growth forests throughout western North America, although the proximate causes of mortality may vary...

...The decline in large-diameter trees could accelerate as the climate in California becomes warmer by mid-century. A temperature increase – even without a decrease in precipitation – will increase evaporative demand, decrease snowpack, increase the length of the snow-free period, increase the length of the growing season, and thereby increase annual climatic water deficit. In addition, fires are expected to increase in number, start earlier, last longer, burn larger areas, and become more severe.

When climate changes rapidly compared to the centuries-long lifespan of trees, there is a shorter period of the optimum conditions in which to attain those large diameters. Increased water stress on sites where large-diameter trees are now present could lead to elevated mortality, but time is required for trees establishing on newly favourable sites to grow to large diameters. Therefore, when climate is changing rapidly, we should expect densities of large-diameter trees to be lower than in stable climatic conditions, whether they are warm or cold. The decrease in densities of large-diameter trees could, therefore, be an indicator of climate change that is beyond the recent natural range of variation in these forests.

Fire, via prescribed burns, could actually help the forests build up a resistance of sorts to climate change, they added, as stands of Ponderosa pine and Incense cedar that had experienced fires during the 20th century "retained large-diameter Ponderosa pine. Plots not experiencing 20th century fire had almost no large-diameter Ponderosa pine."

The bottom line, they noted, was that "our strong inference from this research is that the largest trees of most species in Yosemite are in decline. This decrease in large-diameter tree density throughout much of Yosemite can be interpreted as a long-term change in forest structure during the 20th century."


The other shoe is a pair of papers by van Mantgem et al. (Ecology Letters10:909-916 (2007); Science 323:521-524 (2009)) showing that from Sequoia NP to Yosemite NP, over the past 20-30 years, per tree per year mortality rates have roughly doubled (that's what's referenced in the first paragraph of your excerpt).

Part of increased mortality is _because_ there are more small trees (small trees die at much higher rates than big trees, and fire suppression lets many more saplings become small trees rather than being killed as seedlings & saplings), but part is less soil moisture to go around even if the densities stayed the same.

One sobering implication is that even with increased controlled burns (to thin the medium trees and thus reduce the competition for water), it will take centuries to get back to the numbers of large trees found even 100 years ago. And, even with shifts in species ranges (e.g., those central Sierra species growing by Lassen or into Oregon), there won't be really big trees. Locations with favorable climate when the tree is an establishing sedling & sapling won't still be favorable by the time the tree might get large.

Knowing that Climate Change affects each region differently, I wonder how tree diameters are being affected at our National Parks in the Cascade and Rocky Mtns? The Whitebark pines (a high-altitude tree species) and other five-needle pines within the parks of these mountain ranges are already being devastated by blister rust. Thanks Kurt.

rob mutch
Executive Director,
Crater Lake Institute
Robert Mutch Photography

Graph: Natural Climate Cycle for the Last 2000 Years

The above graph shows an average of 18 non-tree ring proxies of temperature from 12 locations around the Northern Hemisphere, published by Craig Loehle in 2007, and later revised in 2008, clearly showing that natural climate variability happens with features that coincide with known events in human history.

As Australian geologist Bob Carter has been emphasizing, we shouldn’t be worrying about manmade climate change. We should instead fear that which we know occurs: natural climate change. Unfortunately, it is the natural climate cycle deniers who are now in control of the money, the advertising, the news reporting, and the politicians. (Source)

The phrase "climate change" occurs twice in the study linked above (and interestingly enough, those are the instances quoted by NPT), but the study's focus is not climate change.

I think Kurt has read too much into the study, and his title, "Scientists: Climate Change Seems Responsible for A Loss of Large-Diameter Trees in Yosemite National Park", is too strong and not supported by the study. No where in the study, including the abstract or conclusion, do its authors attribute the lass of large-diameter trees solely to climate change. The authors state in the conclusion:

This decrease in large diameter tree density throughout much of Yosemite can be interpreted as a long-term change in forest structure during the 20th century.

Nothing about climate change here. The sentence Kurt cherry picked from the study reads:

The decrease in densities of large-diameter trees could, therefore, be an indicator of climate change that is beyond the recent natural range of variation in these forests. (Emphasis added.)

That's not a very strong statement.

The study focuses mainly on the change in forest structure, largely a result of a century of fire suppression.

Frank, I think you enjoy being a contrarian.

Google the study in question, under the "news" category, and see what you come up with. In other stories the scientists spoke specifically of climate change as a driver in what's going on. Do they say that it's solely responsible? No. It's one element, but a "likely" contributor nonetheless. And if you've spent any time reading these studies, you know that the scientists always couch their conclusions with qualifiers.

"Although this study did not investigate the causes of decline, climate change is a likely contributor to these events and should be taken into consideration," said USGS scientist emeritus Jan van Wagtendonk, lead author of a paper describing the results in the latest issue of the journal Forest Ecology and Management. "Warmer conditions increase the length of the summer dry season and decrease the snowpack that provides much of the water for the growing season. A longer summer dry season can also reduce tree growth and vigor, and can reduce trees' ability to resist insects and pathogens.

I've looked at more than a few peer-reviewed studies on climate change over the past year or so and have talked to more than a few experts in the field. That said, what's going on around the West are lower snowpacks due to more warming and more frequent rain-on-snow events. You shrink the snowpacks, you shrink the amount of water available for trees throughout the summer, when they most need it, as Dr. van Wagtendonk notes above.

Thanks for your response, Kurt. I do take pride in looking at topics critically; we need more contrarians questioning dominant paradigms to check fundamentalism.

It will be interesting to see if the warmer, dryer summers continue. Right now, the temperature anomaly is zero, many regions have reported the the winter's snowpack as average, and 2008 was wetter and cooler than many previous years.

The future's a tricky thing.

Frank, not sure where you're getting your data, but here's some news from NOAA:

The world’s ocean surface temperature was the warmest on record for June, breaking the previous high mark set in 2005, according to a preliminary analysis by NOAA’s National Climatic Data Center in Asheville, N.C. Additionally, the combined average global land and ocean surface temperature for June was second-warmest on record. The global records began in 1880.

* The combined global land and ocean surface temperature for June 2009 was the second warmest on record, behind 2005, 1.12 degrees F (0.62 degree C) above the 20th century average of 59.9 degrees F (15.5 degrees C).
* Separately, the global ocean surface temperature for June 2009 was the warmest on record, 1.06 degrees F (0.59 degree C) above the 20th century average of 61.5 degrees F (16.4 degrees C).
* Each hemisphere broke its June record for warmest ocean surface temperature. In the Northern Hemisphere, the warm anomaly of 1.17 degrees F (0.65 degree C) surpassed the previous record of 1.12 degrees F (0.62 degree C), set in 2005. The Southern Hemisphere’s increase of 0.99 degree F (0.55 degree C) exceeded the old record of 0.92 degree F (0.51 degree C), set in 1998.
* The global land surface temperature for June 2009 was 1.26 degrees F (0.70 degree C) above the 20th century average of 55.9 degrees F (13.3 degrees C), and ranked as the sixth-warmest June on record.
* El Niño is back after six straight months of increased sea-surface temperature anomalies. June sea surface temperatures in the region were more than 0.9 degree F (0.5 degree C) above average.
* Terrestrial warmth was most notable in Africa. Considerable warmth also occurred in Siberia and in the lands around the Black and Mediterranean Seas. Cooler-than-average land locations included the U.S. Northern Plains, the Canadian Prairie Provinces, and central Asia.
* Arctic sea ice covered an average of 4.4 million square miles (11.5 million square kilometers) during June, according to the National Snow and Ice Data Center. This is 5.6 percent below the 1979-2000 average extent. By contrast, the 2007 record for the least Arctic sea ice extent was 5.5 percent below average. Antarctic sea ice extent in June was 3.9 percent above the 1979-2000 average.
* Heavy rain fell over central Europe, triggering mudslides and floods. Thirteen fatalities were reported. According to reports, this was central Europe's worst natural disaster since the 2002 floods that claimed 17 lives and caused nearly $3 billion in damages.

As for your contention that "2008 was wetter and cooler than many previous years," that doesn't appear to be the case if you look at NOAA's 2008 precipitation data, although temperature-wise some areas of the country certainly were colder than usual. But 2007 was drier than normal in many, if not most, parts of the country. Unfortunately, annual temperature data for the country are not available for 2007.

As for snowpack, here's what the National Climatic Data Center had to say in looking back at the winter of 2008-2009:

By the end of April the snowpack levels were at or slightly above normal for the Pacific Northwest. Some areas along the Oregon Coast Range and the Cascade Range of Washington received more than 200 percent of 1971-2000 snowfall normal. In contrast, the Sierra Nevada snowpack levels were as much as 50 percent below normal resulting in the third consecutive year of below average runoff. The Front Range of Colorado and much of the Rocky Mountains received some much needed late season snowfall that placed them around near normal levels for the season. The southern Rockies were not as fortunate, as some of those areas received less than 25 percent of normal snowfall this season. The runoff from the snowfall is a main component of annual water supply levels.

Global temperature trends for 2007 (and more) can be found at NASA's GISS Surface Temperature Analysis.
Locally there is the Rocky Mountain Climate Organization.
For some background (and beyond) read BEYOND THE IVORY TOWER: The Scientific Consensus on Climate Change by Naomi Oreskes
Naomi Oreskes 2007 essay (PDF) is a good read also..

* Heavy rain fell over central Europe, triggering mudslides and floods.

Hmm. Guess there have never been heavy rains or floods in Europe in the last many thousands of years. Global warming!

Of course many of the factoids are for individual MONTHS, not yearly averages. The data I referenced for 2009s global temperature anomaly come from Dr. Roy Spencer (here and here), a government climatologist. (He has never taken oil money, so don't reach for the guilty-by-association card.) He uses satellite tropospheric data rather than surface data which is subject to many influences, such as the heat island effect, precision of measurement, desertification, and so on.

The fact (which no one cares to dispute with evidence) remains that the climate has widely fluctuated over the last 2000 years and throughout the Earth's history. In fact, a billion years from now the sun, with its increasing energy output, will boil off Earth's oceans, ending life on our planet.

The case for human-caused climate change is circumstantial at best. Carbon dioxide has not been proven to raise temperatures, and proponents of more action ignore that correlation does not imply causation. There are several other natural phenomenon correlated with increasing temperatures, including the sunspot cycle. Proponents also engage in the fallacy of the single cause; climate is very complex, and there are many possible forcing events.

The worst part of this whole situation is that climate change proponents will not admit that they could be wrong. That's fundamentalism. Their climate models are their oracles, their magic crystal ball, and even though the future is unpredictable, they make bold claims with high degrees of certainty.

There's really nothing more to say about this topic and its proponents.

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