Thursday, March 15, 2018

The Failure of Oak Pond

Oak Pond is a failed experiment, in some ways.

Of course, experiments that fail at their intended goals do have a way of succeeding otherwise, sometimes. 

It had been a dark mud pit, devoid of tallgrass life, made gloomy by dense shade of invading green ash trees. Decades ago we girdled them, planted seed from nearby healthier savanna and oak woods ponds (some of them now gone) – and waited with Great Anticipation.  Instead we were at first greeted with Bitter Disappointment. None of our planted species showed up. The pond developed a rank, dense thatch made up of such weeds as tickseed, barnyard grass, and knotweed. But that turned out to be a passing phase and within a few short decades, Oak Pond was gorgeous, diverse, and flowery. For more detail, see Endnote 1.

Then apparently, it sprang a leak.

Generations of blue-spotted salamanders, spring peepers, and chorus frogs had gone through their larval stages here and marched forth to populate our trees (peepers), grasses (chorus), and sub-surface tunnels (salamanders). The pond had been their nursery, because these larvae need water to breathe and to feed. Salamander babies are ferocious predators of mosquito larvae and other tiny water animals. Tadpoles eat soggy decaying plant matter, even though they too will be predators as adults.

Back when it was still a pond, it also supported the Sommes’ only population of the threatened plant, marsh speedwell (Veronica scutallata). Fortunately seed was gathered here and spread to other Somme ponds, where this speedwell still thrives. 
Marsh speedwell. Somme photo by Lisa Culp Musgrave.
We did not at first quite notice when Oak Pond ceased to be. Perhaps we have many kinds of records, filed away somewhere, that would help us understand what happened and when.

Frog monitoring data tell us that as recently as April 21, 2011 – frog monitor Tim Wilson reported great numbers of chorus frogs calling in Oak Pond. Later that same year, on June 1, there must have still been water in the pond, as Wilson reported calling chorus frog and eastern gray treefrog. (See Endnote 2).

But in recent years, the pond has been bone dry all spring and summer. This year, I happily noticed winter water filling the pond on February 20.

On February 20 after a rain, with the ground frozen hard, Oak Pond looks like it still exists. 
But when I walked by on March 12th, although all our other Somme Ponds were filled to the brim, the late Oak Pond was already bone dry. 

But on March 12, with all Somme's other ponds filled to the brim, where did the pond go? 
What happened? Perhaps in mid February the recent water was being held in the pond by many inches of solid frozen soil, but when the ice melted (counter-intuitively from the bottom of the pond), there was no impermeable layer to hold the water in?  

We expected, indeed, most people expected that when you cut invading trees away from a natural prairie pond, it would hold water longer. The leaves of trees transpire a lot of water - after the roots suck it out of the ground. 

Clearly, something different is happening here.

This pond is at the crest of a moraine (the Deerfield Lobe of the Lake Border Moraine). Morainal crests can have porous, gravelly soils, that send water promptly down to the aquifer. But moraines can also have “clay lenses” and patches of claypan soils – that restrict the passage of water. Could it be that green ash roots gradually penetrated a claypan and then slowly rotted away? Might the rotting of those roots be like removing plugs from the holes those roots made?   

Alternatively, is it possible that the luxuriant vegetation that grew in the pond itself changed the soil in some way that destroyed the functioning of the claypan?

How do claypans form? And will those processes in time close the putative holes that could have been created by rotting stumps? Does any expert on soils or hydrology read this blog? Or does anyone know a generous expert they might appeal to – who might be willing to weigh in? (Sadly, we have no research funds to offer.)

And – what is a steward to do?

Ah hah! For that we finally have a clear answer. In the absence of better knowledge and expertise, all is going well enough already. Oak Pond may become Oak Meadow. That’s great, if that’s what it wants to do, absent more beneficial ideas and proposals. If this acre is not habitat for frogs, they can move to our many other ponds, and the former Oak Pond can become habitat for voles, meadow plants, snakes, and bumble bees. It’s too bad that we invested rare wetland seed there, but we’ve learned a lot from that seed already, and we now have at least four populations of the threatened speedwell that we wouldn’t otherwise have, and those came from the population that for a while thrived in Oak Pond. We do not expect all of our restored rare species populations to survive. We just give them a chance. Many of them may die out at Somme. Many others – as the soils, amounts of shade, species abundances, and global climate all change – may surprise us by moving to other niches, as they find opportunities for themselves, as they always have.

Thanks to all who work, study, and advocate to advance this great experiment in conservation.


Endnote 1.

If you’d like to know more of what, for a time, thrived in Oak Pond, we can start with a list of plants. The first “breakthrough” was rice cutgrass (Leersia oryzoides); after a few years of dense annual and biennial weeds, this wetland grass started popping up in many places. Next we started to find expanding populations of blue flag iris (Iris virginica), hop sedge (Carex lupulina), crow’s-foot sedge (Carex crus-corvi), great bulrush (Scirpus validus), false dragon’s-head (Physostegia virginana), and in time many more.  (See more in Endnote 3.)

Seed of many of these, including the endangered speedwell, came from ponds about ten miles away that are now so dark with invasives that neither the speedwell nor most of the other species we gathered there can still be found.

For a while the endangered American slough grass (Beckmannia syzigachne) bloomed in this pond. But no longer. This grass proliferated for a while in a shallow section of another Somme pond for perhaps 20 years, then vanished there – only to show up in a distant, deeper section of the pond, with entirely different associated plant species. (We don’t try to figure out too much. We give seeds a chance. Then, it’s up to them and nature, except for species that need caging from the deer.)  

One of our favorite memories of Oak Pond occurred when it finally dried up, as ephemeral ponds must do, usually in mid-summer for this pond. By drying time, larval chorus frogs and peepers had functioning lungs and left the pond. These species cannot reproduce in permanent ponds, as those ponds support fish and bullfrogs, both of which eat the young of the ephemeral pond species, which are not adapted to those predators.

But one of the most dramatic predator displays offered by Oak Pond starred cedar waxwings. They like to eat backswimmers. The backswimmers are ferocious predators themselves, consuming a great many tadpoles, when they get a chance. But as the pond dries down to just a square yard or so of water, the backswimmers pop out of the water and fly off to find another pond. Cedar waxwings would sit in the trees that circle the pond, and as soon as a backswimmer took off, the waxwings would flit out and grab it. We never saw one get away.   
Cedar waxwings once ringed this pond in summer to hunt backswimmers.
Photo by Lisa Culp Musgrave
Endnote 2

In late March and early April, there are often so many peepers and chorus frogs calling from some Somme ponds that their joyful uproar blots out highway and airplane noise. They rock! On the other hand, we stewards have never heard a gray treefrog call. It came as something of a shock to see that rare species mentioned in the frog monitoring record.

We had never seen leopard frogs at Somme and thus were taken aback one year to find dozens of their tadpoles in one Somme Prairie Grove pond as it dried down to nothing. We’d never heard them either. That’s a bit more understandable, as leopard frogs call early in the season and easily get drowned out by the peepers. Now that we’ve trained our ears, we do sometimes hear them in the deepest of the Somme Woods ponds. They need longer times to mature than do the little peepers, and, although the Prairie Grove has better adult habitat for them, the Prairie Grove ponds dry sooner. But that year, somehow, at least one male and one female managed to cross Waukegan Road and reproduce. We’ve never found them there again. We imagine that their numbers may start to increase at Somme Woods, as the habitat there becomes more of a grassy open woods than its former mostly-buckthorn-and-dirt condition. An alternate name for the leopard frog is “the meadow frog.” 

We can’t resist wondering whether there may also be tree frogs surviving somewhere in or near Somme. In a Northbrook park some years back, we recorded a frog identified (from the cell-phone recording of its call) as Cope’s gray tree frog (by U. S. Fish & Wildlife staff). Tree frogs need ponds where water remains even later in the summer than is needed by leopard frogs. The deepest ponds in Somme Woods probably dry sooner than they once did, because their centers are clogged (unnaturally, some experts think) by peat-like accumulations of cat-tail roots. Does the tree frog deserve any kind of conservation initiative at Somme?

Endnote 3

After I finished this post, I found the following list of plants identified in Oak Pond by Robbie Sliwiinski in July 2009. I believe that most of these species are now gone.

I just hadn't much paid attention to this pond - except for an odd subliminal feeling that it had become weird. My focus was jogged back to it only by seeing it strangely once again filled with water this February. Although I hadn't paid careful attention to the vegetation, I do remember that the dominant plant was the weedy tall boneset (Eupatorium altissimum). Apparently a whole plant association died out when the pond lost its water, and the weeds blew in.

Site:     Oak Pond
Locale:   SPG
Date:     July 2009  
By:       R. Sliwinski
File:     c:\FQA\studies\Oak Pond.inv

   FLORISTIC QUALITY DATA        Native       20   100.0%      Adventive     0     0.0%
     20 NATIVE SPECIES           Tree          0     0.0%      Tree          0     0.0%
     20  Total Species           Shrub         0     0.0%      Shrub         0     0.0%
    5.8 NATIVE MEAN C            W-Vine        0     0.0%      W-Vine        0     0.0%
    5.8  W/Adventives            H-Vine        0     0.0%      H-Vine        0     0.0%
   26.2 NATIVE FQI               P-Forb        8    40.0%      P-Forb        0     0.0%
   26.2  W/Adventives            B-Forb        0     0.0%      B-Forb        0     0.0%
   -4.7 NATIVE MEAN W            A-Forb        0     0.0%      A-Forb        0     0.0%
   -4.7  W/Adventives            P-Grass       3    15.0%      P-Grass       0     0.0%
   AVG: Obl. Wetland             A-Grass       1     5.0%      A-Grass       0     0.0%
                                 P-Sedge       8    40.0%      P-Sedge       0     0.0%
                                 A-Sedge       0     0.0%      A-Sedge       0     0.0%
                                 Cryptogam     0     0.0%                                    

ACRONYM    C SCIENTIFIC NAME                              W WETNESS  PHYSIOGNOMY COMMON NAME                   
ALISUB     4 Alisma subcordatum                          -5 OBL      Nt P-Forb   COMMON WATER PLANTAIN         
BECSYZ    10 Beckmannia syzigachne                       -5 OBL      Nt A-Grass  AMERICAN SLOUGH GRASS         
BOECYC     2 Boehmeria cylindrica                        -5 OBL      Nt P-Forb   FALSE NETTLE                  
CXANNA     5 Carex annectens                             -3 FACW     Nt P-Sedge  LARGE YELLOW FOX SEDGE        
CXCRUS    10 Carex crus-corvi                            -5 OBL      Nt P-Sedge  CROWFOOT FOX SEDGE            
CXLUPF    10 Carex lupuliformis                          -5 [OBL]    Nt P-Sedge  KNOBBED HOP SEDGE             
CXLUPN     7 Carex lupulina                              -5 OBL      Nt P-Sedge  COMMON HOP SEDGE              
CXPELL     4 Carex pellita                               -5 OBL      Nt P-Sedge  BROAD-LEAVED WOOLLY SEDGE     
CXTRIB     3 Carex tribuloides                           -4 FACW+    Nt P-Sedge  AWL-FRUITED OVAL SEDGE        
CICMAC     6 Cicuta maculata                             -5 OBL      Nt P-Forb   WATER HEMLOCK                  
ELEERY     2 Eleocharis erythropoda                      -5 OBL      Nt P-Sedge  RED-ROOTED SPIKE RUSH         
GLYSEP     8 Glyceria septentrionalis                    -5 OBL      Nt P-Grass  FLOATING MANNA GRASS          
GLYSTR     4 Glyceria striata                            -3 [FACW]   Nt P-Grass  FOWL MANNA GRASS              
IRIVIS     5 Iris virginica shrevei                      -5 OBL      Nt P-Forb   BLUE FLAG                     
LEEORY     4 Leersia oryzoides                           -5 OBL      Nt P-Grass  RICE CUT GRASS                
LOBCAR     7 Lobelia cardinalis                          -5 OBL      Nt P-Forb   CARDINAL FLOWER               
SCIATR     4 Scirpus atrovirens                          -5 OBL      Nt P-Sedge  DARK GREEN RUSH               
SCULAT     5 Scutellaria lateriflora                     -5 OBL      Nt P-Forb   MAD-DOG SKULLCAP              
SIUSUA     7 Sium suave                                  -5 OBL      Nt P-Forb   TALL WATER PARSNIP             
VERSCU    10 Veronica scutellata                         -5 [OBL]    Nt P-Forb   MARSH SPEEDWELL               

Thanks for proofing to Kathy Garness and Eriko Kojima. 

1 comment:

James McGee said...

It is difficult for plant roots to penetrate clay layers. It is likely the removal of the ash trees is not what caused Oak Pond to stop holding water. I must wonder if non-native earthworms have invaded causing the change. Earthworms have trouble penetrating dense clay layers just like plant roots. However, earthworms aerate the soil. Their burrows could be storing the water that was formerly creating the pond.