Nov. 21 1PM 50F
It's pretty quiet here today, or at least it seems so. Campus is deserted for the Thanksgiving break. The natural sounds of the mandala - the stream, leaves in the breeze, are overwhelmed by traffic noise, sounds from the construction concrete plant nearby, occasionally of airplanes. Such a tranquil setting can't escape the human bustle.
We're in the thick of autumn now. It both looks and feels that way today. Less than a week ago, the mandala was covered in snow but there's no longer evidence of the short-lived early winter storm. One of the biggest seasonal changes in the forest has been steady leaf senescence and leaf fall. There are still leaves on the trees, more than I expected. Most are a dull brown, likely from the oaks. A large white oak not far away still has green or mostly green leaves, as do a few other trees but these are by far the exception. As the breeze picks up, leaves continue to flutter to the ground, accompanied by abundant tulip poplar seeds. You can still see the tulip's flowers, like ornaments in the canopy. The leaf fall has bee accumulating on the forest floor, representing a substantial nutrient pool. Just a month ago, the ground was mostly bare having been scoured by the late summer flooding. The waters washed away old decaying leaves and their nutrients. Does that loss hinder nutrient uptake in the next season? It would be interesting to have a good record of soil inorganic N and mineralization rates to see what effect, if any, the flooding has on the soil nutrient pool There may be other effects too, like an input of new sediment or a shift in the microclimate following the removal of the soil's organic horizon. But now it's fall, and the O horizon makes a comeback. The leaves and the nutrient pool that they represent are back and beginning to fuel the decomposer food web and the recycling of nutrients from the trees back to the soil.
The leaves themselves only contain a fraction of the nutrients that they had a few weeks ago. As fall progresses and days become shorter and temperatures drop, the green leaves lose their green. The chlorophyll that gives the leaves their greenness breaks down, unmasking other pigments in the leaf, exposing the reds and yellows from anthocyanins and other pigments, the fall colors we so cherish. These nutrients that form the building blocks for the trees photosynthetic machinery are broken down and stored in the perennial tissue of the tree. This is an important nutrient conservation adaptation whereas deciduous trees recapture about half of their leaf nutrients to use the next year. While the leaves are generally somewhat disposable, at least in comparison to the longer-lived coniferous leaves, the plants still recycle much of the leaf material.
So then you have two pathways from which trees get their nutrition each year: leaf fall and decomposition or resorption. It makes sense that trees in infertile habitats will conserve nutrients and rely more heavily on the resorption pathway, and in contrast, trees in nutrient rich habitats will take up the majority of their nutrient demand from the soil through roots. Unfortunately this isn't easy to research and what has been done isn't entirely conclusive (though some of my previous research has documented modest nutrient conservation via enhanced resorption in relatively nutrient-poor habitats). In the mandala, I generally expect that the soil here in the floodplain is relatively fertile as the nutrients from uphill leach downslope. I wonder if the falling leaves here compared to up the hill might be more nutrient rich, representing less resorption. If that's the case, we could argue that the trees here are utilizing the decomposition pathway more than the upslope trees that rely more on the resorption pathway. While these patterns can be stronger confounded by species or functional group (e.g. deciduous vs coniferous species), the trees here are common throughout and could offer some insight into these processes.
