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Value of Water: How are beetles affecting water?

Posted: Wednesday, Jan 23rd, 2013

This is the first in a yearlong series that is part of “The Value of Water” education campaign in the San Luis Valley.


VALLEY — The spruce beetle infestation continues to spread across the upper Rio Grande and Conejos River Watersheds and has now moved into the upper reaches of Saguache, La Garita and Carnero Creeks. Since 2005, approximately 65-70 percent of the spruce-fir forests that cover the Rio Grande National Forest have been infested by the beetles.

The spruce-fir forest is a high elevation forest composed primarily of Engelmann spruce and subalpine fir trees. This forest type grows up to tree line and covers about 31 percent of the Rio Grande National Forest. The spruce beetles mainly attack Engelmann spruce, although they will sometimes infest blue spruce, which grow at lower elevations. Not all the trees will succumb to the beetle infestations, as the subalpine fir and most of the Engelmann spruce smaller than five inches in diameter will survive.

Some people have theorized that the spruce beetle epidemic is a result of climate change, but it is hard to definitively make that connection. It appears that three factors combined to create the perfect conditions that allowed the population of this native beetle to explode.

The first factor is that the Rio Grande National Forest was composed of many large, dense stands of mature Engelmann spruce trees. Spruce beetles are naturally found in these stands of trees because they can successfully attack the less vigorous old trees without being pushed out by pitch.

The second factor is that these stands of older trees often contain more disease and are less wind firm. More trees tend to blow down in these areas as rot moves into the trees root systems and trunks. Over the last decade, strong wind events have blown down large patches of old trees providing perfect habitat for the beetles to expand their populations. The beetles thrive in these types of areas because the wind-thrown trees can’t push the beetles out with pitch.

The third factor reared its head in 2000 and 2002 when a severe drought hit the Southwest allowing the beetles to move from the blowdown into what were previously healthy trees. Once again, the reason the beetles were able infest these trees is because the trees were not able to pitch-out the beetles; there simply was not enough water in the soil for the trees to produce pitch. This allowed the population of spruce beetles to grow to a level where they could overwhelm the defenses of trees even after they had recovered from the drought. Now the spruce beetle infestation has turned into a landscape level epidemic that probably won’t stop until the beetles have run out of food. It’s important to note that this is a natural event; it has probably happened here before.

The drought helped to kick-off the spruce beetle epidemic and now the spruce beetles may be impacting the timing of the spring runoff and the total amount of water flowing above and below ground out of the mountains. There are a lot of variables that affect the runoff, which makes it difficult to show a direct connection between it and the beetle-killed forest, but it is still worthy of discussion.

Over the last eight years, April runoff from snowmelt in the Rio Grande has exceeded the previous monthly average every year but one. Several factors may be affecting this, such as higher temperatures and dust on snow. Some studies have indicated that more dust is being carried by wind and deposited on snow in the San Juan Mountains than in the past. Dust on snow decreases the albedo, or reflectivity, of the snow, which increases temperatures on the surface of the snow leading to more melting earlier in the spring.

The large areas of beetle-killed trees in the upper Rio Grande Watershed may also influence the timing of runoff. The dead trees do not provide as much shade as the green trees did and the sun heats up the exposed trunks of the trees melting the snow around them. That said, it is hard to know the degree in which the dead trees are affecting the timing of runoff.

There is also a possibility that the millions of dead spruce impact the total amount of water (surface and underground) flowing out of the mountains. Living spruce trees intercept a lot of snow before it reaches the ground. Some scientists believe that sublimation of snow from the tree branches is greater than if the snow was on open ground. In other words, more water escapes to the atmosphere off of snow on tree branches than from snow on the ground, so there is less water in the snowpack for the spring runoff.

Additionally, living trees act like big straws; they take in groundwater from their roots and then transpire it to the atmosphere through their needles or leaves. Water isn’t transpiring from the dead trees, so it is possible that more water is staying in the ground and flowing downhill to the aquifers in the San Luis Valley.

Both the reduced sublimation and reduced transpiration theories make sense, but the research is mixed as to whether there is actually a net increase in runoff. There are too many variables at play to say that the dead trees are influencing the total volume of runoff.

In other words, the upper Rio Grande Watershed is just too complex to develop any firm conclusions as to how the spruce beetle epidemic is affecting the water cycle that is so important to all living in the SLV. On the other hand, the epidemic has certainly caused great changes to the landscape, which will impact the wildlife and plants that live in the forest and the people that use it… but that needs to be the subject of another article.

For more information on the Value of Water campaign, or water questions in general please contact Judy Lopez at 719-580-5300 or visit the web at www.rgwcei.org/ValueofWater

Mike Blake is the public affairs specialist for the US Forest Service. He is an avid conservation who gets out and round the Rio Grande National Forest.

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