Sunday, June 30, 2013

Ponderosa Pine on Tygh Ridge: Succession or Invasion?

Scattered Pine in Tygh  Ridge Grazing Lands
Tygh Ridge is located in North Central Oregon and is part of the Columbia Basin Ecological Province. Much of the land has been continuously tilled and planted with dryland wheat over the past century. This hasn't been conducive for succession (see my last post) or for the land to be anything more than short term grassland.

OSU's ecological profile indicates that the expected vegetation over much of the Columbia Basin habitats would include some shrub cover, but doesn't mention any thing about Ponderosa pine. However, over the last twenty years the pine has established and spread in certain areas on Tygh Ridge.   How could that be?

There are many variables, but here's my take on it.  Most of the change was kicked into motion with the expansion of the Conservation Reserve Program.  In the mid-1980's, the program was expanded in order to protect highly erodible soils such as those on Tygh Ridge. Thousands of acres were taken out of production for wheat and seeded into expansive stands of grass where livestock grazing wasn't allowed.  By the mid-1990s I first started to notice pine trees where I hadn't seen them in the thirty years prior.

There also was funding available for landowners to plant shelter belts and  trees for wildlife habitat.  Perhaps   some of the pine got into the middle of the grasslands by planting, but I'm not convinced about that because the placement of the trees is so random.  It always seems that when humans are involved in something they must have a certain sense of order!

Ponderosa Pine in Idaho Fescue Grassland  on Tygh Ridge

Where did the seed come from? 

The Dalles Ecological Province is a long, thin map unit that covers most of the east slopes of the Cascade Mountains in North Central Oregon, and is adjacent to the Columbia Basin Province. Tygh Ridge lines the northeastern portion edge of The Dalles Province.

The Dalles Province is  forested with Ponderosa Pine, Oregon white Oak, and smaller amounts of Douglas fir. Those who have never been in that part of the world might find the transition, from the forested The Dalles habitat type  to Columbia basin grassland type, abrupt.  Once at the base of the Cascades, the trees don't slowly peter out as they often do in transitional zones, they just cease to exist in the landscape almost as if there is a line that they are forbidden to cross.  That's part of why it's surprising to see so much Ponderosa pine has established 15-20 miles to the east (as a bird flies) in the center of Tygh Ridge!

Since Ponderosa are monoecious, meaning the male and female reproductive structures are both present on the same tree, they have no reason to worry about pollination or how far a pollinator can be drawn in from. Ponderosas have a self contained system to perpetuate their existence, which sometimes makes them seem like nothing more than weeds.  Like weeds they grow in dense thickets that continue to get denser with lack of disturbance.

Boys and Girls Co-habitating. 
 I'm guessing that once the seeds for the rogue Tygh Ridge trees were mature, they got some help with their pilgrimage to the new world.  The distance is too far for mice or rodents to lug the big pine nuts, but it seems birds could do an efficient job of transporting them to their new home.

Clark's Nutcrackers have been known to transport both Ponderosa and whitebark pine seeds . The birds have been thought to be the salvation of the pine through  caching of seed, which in the end leads to greater genetic diversity. However, recent research has followed some of the nutcrackers in their migratory and caching activities and found that sometimes where they bury the seed is either marginal or totally wrong for germination. Or is it?  Perhaps for the whitebark pine that seems to be  a bit more particular about where it grows, but  what about the weedy Ponderosa?  It's successfully grown in a number of habitats for eons.

 When I looked at a sighting map for Clark's nutcracker and laid it over map of The Dalles Ecological Province, I found there have been a few sightings of the nutcrackers there.  Of course, the sighting map didn't show any nutcracker sightings in the adjacent Columbia Basin ecotype. It's likely none have been seen or even looked for there.  However, that doesn't mean they don't go there.  There also are a lot of jays and other corvids that are on less specialized diets that frequent both areas in question.

After spending  probably too much time wondering if the rogue Ponderosa pine trees of the grasslands got there through succession or invasion, I begin to wonder if my question really matters.  I don't have a research proven answer, but that's the neat thing about science because nothing stagnates.  Sometimes scientists work for years to come up with a reasonable hypothesis only to be tormented by someone else who claims to know it all, but really knows nothing.

We likely will never know as much as we would like, but must continue to chase each tidbit of truth we can find, because the natural world isn't going to sit here and stagnate with us..

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