Deer 'cool' response of trees to warming climate

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Lee Frelich
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Deer 'cool' response of trees to warming climate

Post by Lee Frelich » Sun Sep 30, 2012 12:20 pm

NTS members:

A paper recently published in Global Change Biology (2012, doi: 10.1111/j.1365-2486.2012.02785.x, paper online now, likely to come out in print in the October or November issue) by my recently graduated PhD student Nick Fisichelli, myself and Peter Reich show that deer are likely to alter the invasion of temperate tree species like sugar maple and red oak into boreal forests of spruce and fir. The study was done in northern Minnesota--plots with high and low levels of browsing by deer were surveyed along a climate gradient from boreal (very northern edge of the range of temperate species) to temperate (southern edge of the range of boreal species), thus including the zone of overlap in temperate and boreal species ranges, where the two species groups grow together within one stand of trees.
Presentation1.jpg
Figure 1. Locations of study plots in northern Minnesota (red triangles). Black points represent location of other study plots for upcoming publications.

Hundreds of saplings were destructively sampled in the field. 'Cookies' were cut from the stems throughout their height so that age, radial increment and height growth could be reconstructed for each sapling. Summer temperature and precipitation, and a measure of deer browsing pressure were estimated for each plot. The temperature gradient was about 2.5 degrees C or 4.5 degrees F.

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Figure 2, cutting a balsam fir in the field.

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Figure 3. A sugar maple #818--forks in the stem indicate the sapling had been browsed by deer several times over a period of several years.

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Figure 4. A repeatedly browsed northern red oak--clearly the favorite of deer.

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Figure 5. A white spruce--notice lack of deer browsing on their least favorite species.

The key results are shown in Figure 6. On plots with low deer browsing pressure (upper graph), boreal spruce and fir height growth was consistently less as summer temperature increased, while temperate species sugar maple and red maple grew more with warmer summers, and red oak did not respond to temperature. Most importantly, maple and oak growth rates were higher than boreal sapling growth rates at warm temperatures. On plots with high browsing pressure, however (lower graph), spruce and fir growth rates were about the same as low deer plots (reflecting the unpalatable nature of those species), while growth rates of the three temperate species was lowered compared to low deer plots, so that they all grew less than boreal species. Moreoever, the positive temperature response of the maples was flattened by the deer: in warmer areas deer simply ate more. Results for radial growth were very similar.
Presentation2.jpg
Figure 6. Height growth of boreal and temperate saplings across the temperature gradient with low (upper) and high (lower) deer browse pressure.

We think this phenomenon will prevent temperate tree species from increasing their growth rates as the climate warms at the northern edge of their range in areas with high deer populations. This could prevent them from responding to climate change by invading the boreal forest, since boreal saplings will likely continue to grow more than temperate saplings, even when the temperature becomes less than optimal for boreal tree growth. One possible implication would be that boreal species will persist until a threshold temperature is reached, and die suddenly, rather than being gradually replaced by temperate species.

Lee
Lee E. Frelich
University of Minnesota Center for Forest Ecology

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dbhguru
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Re: Deer 'cool' response of trees to warming climate

Post by dbhguru » Sun Sep 30, 2012 2:51 pm

Lee,

Wow! Outstanding. Thanks for sharing. In terms of latitudes such as here in western Mass, I presume the maples will grow faster with the extra C02, providing more deer food. Standoff? Any predictions? BTW, my friend Tom Tyning from Berkshire Community College has been seeing more young tuliptrees in the Housatonic Valley. The climate seems to be increasingly favorable for their spread. No surprises there. I have no idea how deer see young tuliptree leaves - probably as a delicacy.

Bob
Robert T. Leverett
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ElijahW
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Re: Deer 'cool' response of trees to warming climate

Post by ElijahW » Sun Sep 30, 2012 5:32 pm

Lee,

Very interesting. This conclusion, especially if consistent across North America, is, to me, an excellent example of the concept of an "ecosystem;" at least I think I'm using the term correctly: as one factor changes or becomes more significant than it was previously (in this case, temperature), other factors (deer browsing) in the makeup of northern forests become more clear. In other words, many causes can combine to one effect.

Bob,

From my limited experience, deer will munch on tuliptree leaves, though they seem to prefer the oaks I've planted. I have one northern red that looks pretty similar to Lee's second photo from the bottom.

Elijah
"There is nothing in the world to equal the forest as nature made it. The finest formal forest, the most magnificent artificially grown woods, cannot compare with the grandeur of primeval woodland." Bob Marshall, Recreational Limitations to Silviculture in the Adirondacks

Joe

Re: Deer 'cool' response of trees to warming climate

Post by Joe » Sun Sep 30, 2012 6:24 pm

Lee Frelich wrote:deer are likely to alter the invasion of temperate tree species like sugar maple and red oak into boreal forests of spruce and fir.
well, time to bring back lots of predators!

this reminds me of an event at Harvard Forest in Petersham, MA last year- where they did a clearcut, then fenced part it to keep out the deer- it was clear that far more regeneration occured within the fenced area

what I'm noticing the past few years, regarding GW, is that I'm seeing lots of insect species I've never seen before- I'm a bit too lazy and/or too busy to key them out- I don't know if this is real or I just happen to notice insects more than before
Joe

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Lee Frelich
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Re: Deer 'cool' response of trees to warming climate

Post by Lee Frelich » Mon Oct 01, 2012 9:54 am

Bob:

I think that most of the enhanced growth caused by increasing CO2 is in the past--the steepest part of the response curve for rate of photosynthesis being from 180-380 ppm CO2, which is the trajectory we have been on since 18,000 years ago. Additional CO2 beyond 380, will result in only a modest increase in growth, perhaps 10-15% as we go on to 500+ ppm CO2.

Its more likely that variations in other factors and predators that control deer populations will determine sugar maple growth rates. In western Mass, I have seen a lot of variation in levels of deer browsing--almost none in some areas, so that the plant community will be able to respond. More on this in a few weeks when my next paper on trophic interactions across the temperate-boreal ecotone from MN to New England is published in the Philosophical Transactions of the Royal Society.

Also, it will be the tension between maple and oak that is most important in western Mass as climate warms. In areas with lower deer populations, oaks will be able to replace maples, but perhaps not in areas with higher deer populations.

Lee

Joe

Re: Deer 'cool' response of trees to warming climate

Post by Joe » Mon Oct 01, 2012 10:53 am

Lee Frelich wrote: Also, it will be the tension between maple and oak that is most important in western Mass as climate warms. In areas with lower deer populations, oaks will be able to replace maples, but perhaps not in areas with higher deer populations.

Lee
I think, in addition to deer population, and maybe more so, what distinguishes an oak site from a maple site is soil fertility and past harvesting practices. Sugar maple is more competitive than oak on rich sites.
Joe

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Larry Tucei
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Re: Deer 'cool' response of trees to warming climate

Post by Larry Tucei » Tue Oct 02, 2012 10:56 am

Lee, Wow really cool study! So the Fir and Spruce will hold on for a while longer than first thought but then will dramatically die off. My friend in northern Wisconsin plants Oaks and has to protect them with moisture tubes. They work well to help with early growth and keep the Deer from eating them. Well I'm doing my part to help with Global warming. I'm harvesting as many Deer as law Permits, in several States. :) Larry

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