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374 Shurs Lane Philadelphia PA 19128 (215) 487- 0700

CROSBY ARBORETUM POND CONCEPT A southern swamp, shallow dark, still water und er a dense canopy to minimize visual closure by shrubs and herbaceous plants. Ultimately, the swamp canopy should create a closed landscape with a ribbon of light calling out a long, thin channel punctuated by small pools.

PLANTING REQUIREMENTS The greatest concern in planting and managing this pond is the rate at which additional sediment and nutrients will cause the pond to be choked with weedy vegetation. Therefore, the most feasible planting design will establish a closed canopy swamp forest around almost the entire lake margin. Tree species should be planted in sufficient numb er to really cover a s ite. They should be as large as it is possible to buy or collect and taken only from analogous site conditions.

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The hydroperiodic regimen is critical to tree survival. Areas where even the most water - tolerant swamp fore s t spec ies are planted must be dry for a minimum 4- 6 months during the growing period, from approximately June until October. This period of drought should be as long as possible during tree es tabl Ishment. The pond, therefore, must be managed to be drawn down during the summer. (It is a shame that this perjod cOjncides with ~ ou r: ist, . . . . . . - - - - - - - - - - - - viSitation, but the exposed pond will have its own aesthetic.)

In the context of a primarily swamp habitat, herbaceous vegetation should be: 1. accent herbaceous spec i es that wi 11 grow under a

closed canopy. these 'jewels glowing in the dark' should only be planted when the tree canopy planting is complete. 2. A flowery tapestry in the three small herbaceous wet land areas shown. Here, intensive grass, sedge and wetland wildflower planting should create a dazzling display. 3. Planting of herbaceous display areas are a different issue from stabilizing pond banks and covering the 'bathtub ring'. Recommended speCies already present on the site are the shorter eleocharis, osmunda ferns and water primrose (Iudwigia). Care should be taken to plant no i nvas ive speci es here whi ch will move out from this niche into the pond.

FIREBREAK MANAGEMENT The initial layout of the Savannah was modified to create greater spatial diversity which inadvertently greatly increased the length of firebreak required and magnified the existing problems of firebreak management

1. The firebreaks are unsightly for a relatively long period of time, a scar of disturbance to both soil and vegetation. 2. Because heavy equipment is used, rutting is severe, creat i ng 'ditches' which drain and divert surface water, altering the site's drainage. 3. Proposed restoration such as disking would create a 'path' which could be confusing to the visitor and would not adequately correct the drainage problems Maintenance costs would be relatively high and constant 4 The current layout diverts stormwater runoff from the savannah directly to the pond The flush of nutrients and sediment fol lowing the burn created conditions of almost instant lake eutrophication. We recommended the following firebreak management guidelines to address both the aesthetic and ecological issues raised.

1. Scraped bare soi I is not an acceptable firebreak and should be used only where no other alternate is available and where it is only minimally visible to the visible. Relatively isolated stretches of bare soil break will, however, provide habitat sites for herptiles and a few interesting successional species and could also be useful as service access for arboretum staff. 2. It is desirable that permanent features, such as paths and wetlands, serve as firebreaks wherever possible to eliminate unnecessary fflB iFltef'lflf'lee tlnd ~~Ie conti nooos s i te dlstr'obance. I he uSe of Wet lands as firebreaks will prevent the spatial uniformity that would result if only paths were used.

3. Where large areas of vegetation are desired to create a middle ground and spatial diversity in the savannah, it is most reasonable to create wetland pockets which will serve as their own firebreaks rather than requiring a bare soi I firebreak surrounding them. This can be easi ly CROSBY ARBORETUM

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accomplished by exaggerating the site's topography with limited excavation. This temporary disturbance can replace the need for continuous disturbance. Revegetation will be very rapid if the appropriate soi I structure is retained and can be enhanced where desirable by additional planting. If the occasional hot fire burns any of these sites, recovery wi Il be very rapid coppice growth from the larger older root systems, similar to that which is occurring on much of the site today. These depressions could also be used to retain stormwater from the freshly burned savannah and reduce impacts to the lake. 4. Surface runoff to the lake should also be controlled by blocking the remnant agricultural drainage ditches. This would also help to create greater habitat diversity on this relatively uniform site. Fill material to block the downslope portion of the ditches could be obtained from the upslope ditch area to create a larger wetland habitat. Backflooding because of the blocked ditches will further increase wetland area. Similarly, the new ditches which drain from the burned savannah to the lake should be refi lied.

5. The additional path links which are required to complete the firebreak network should be laid out now. This will permit their incorporation into the firebreak system as well as the opportunity to evaluate and alter their configuration. An obvious example is the path that roughly parallels Ridge Road. 6. The existing firebreaks should be regraded and restored. The equipment used should be as lightweight as possible and all work should be done during dry periods to reduce further rutting and soil compaction In some places, additional planting may be necessary in critical visitor views.

7. §everal 8e9j~R ffieaifie~t;oll~ to tile ₏xislilig layout dl e also recommended.

a. At tne present l Ime, the burned area In the northeastern portion of the site comes very close to I-59 creating both an open view of the highway as well as a noise funneL The width of the unburned buffer should be substantially increased at this point. b. The present configuration of the savannah is more uniform spatially than is necessary. Although the edges have been scalloped, each space is roughly equivalent in size and shape While the addition of CROSBY ARBORETUM

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alcoves and smaller scale spaces is important to the visitor experience, it should not happen at the expense of the long meadow vista. The large scale main meadow should be retained in the southeastern portion of the site with a long view almost to the top of the site where the sma 11 er aI coves occur. The diagrams we developed with Ed and Chris on site shows both the proposed layout of the savannah and of the firebreaks and their management for the southern portion of the arboretum. 8. The savannah exhibit is one of the most important to the Crosby Arboretum and must be maintained to foster the maximum impact from the wildflowers. A regular burning regimen is mandatory. The three year hiatus in burning which occurred recently because of the demands of lake and pavilion construction illustrates clearly how rapidly the meadow deteriorates as an herbaceous display when woody growth advances. The site will also look 'disturbed' for longer than is desirable because the charred woody stems are more persistent than burned herbaceous plants Even in the most demanding times, regular burning should be maintained. A permanent firebreak system will make this less difficult. 9. Lastly, fire protection of the pavilion is especial ly important. At present the pond and road layout provide an excellent circumferential firebreak system shou ld wildfire occur. Within that area, some hand clearing may also be advisable during excessively dry seasons.

POND CONFIGURATION During the construct ion process, severa l changes were made to the schematic design which we developed. The grading, for example, was mOdified 'r\CA8Fl BeFiA~S Pâ‚Ź'v'eBled tI idl UIe laRe coa ld be no deeper than 6' without intersecting a deep sand layer which would have drained the water out. Simi larly, changes in configuration were made to protect some ex isti lig plan tS. However, in this process, the large central area of open water which was designed to be a hedge against the gradually filling process was eliminated, and we are concerned that more rapid closing of the pond is likely. This may be further hastened by slumping of the steep slopes which are unstable when wet and difficult to stabilize.


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The attached diagram of the pond configuration shows the existing lake margin as welJ as the lake area which is 3' in depth as welJ as that which is 6' deep. (Where the slopes were very steep, we assumed a 5 to I angle of repose; still steep by wetland standards.) The area 6' deep is very long and narrow, in places only 10' wide. Even the area of water 3 feet or deeper is as narrow as 25' in the center of the lake. In the original schematic, for example, the area of lake 6' deep, assuming a flat bottom, measured roughly 120' at its narrowest and a significantly greater hedge against gradua I Iy fi I I ing than the current diversions. The current configuration is not dramatically different in grading from a beaver pond or milJ pond. Indeed, smalJ pools and channels of water in a largely vegetated wetland is very characteristiC of the region. The major difference is that this site is a fresh excavation rather than an established system and is likely to be characterized by disturbance patterns rather than those of undisturbed natural systems. Instead of clear dark water beneath a closed canopy in an oligotrophic system, we have a system which is eutrophic and frequently turbid with sediment Rapid filJing is occurring because runoff is inadequately controlJed and the shalJow open water receives maximum radiation and ligM. With no intervention, fairly swift filling by herbaceous vegetation may occur. If the dark pools and channels of shallow clear water is the goal, it is critical to establish canopy cover by woody vegetation as soon as is possible ExtenSive tree planting is imperative and rapid growth should be encouraged by allowing the water levels to drop enough seasonally. The June - October period of drying out is especially important during the time that the trees are getting established. Many nursery grown trees available will not be adapted to a natural regimen but rather will have been grown on drier sites and limited in usefulness to the edge. In the attached plant community diagram, we recommend that the main body of the pond be dominated by swamp forest vegetation to achieve FI'la){ i l'fH:ff'fl ::lnflding of tile water ill ali el tort to l imr t rank herbaceous growth which would obliterate the views of water. Herbaceous display areas are confined to alcoves and the immediate pavilion area.

LAKE MANAGMENT The completion of the lake and pavilion gives the framework for the core exhibits at the Crosby Arboretum. The management of the lake is critical CROSBY ARBORETUM

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to sustaining a diversity of habitats on this relatively disturbed and uniform site. It is critical that management be directed toward maintaining the best water quality feasible and the most 'natural' hydrologic regimen.

1. The site as a whole is in a state of disequilibrium; its natural system will be years recovering from past impacts It should be remembered that the lake is also an example of site disturbance and it will take a long time to re-establish more natural patterns. It is important to minimize any further and unnecessary shocks to the system, such as the recent excessive nutrient and sediment load that washed in with runoff from the prescribed burn. Any future disturbance of soi I or vegetation on the site from construction, planting, path building, etc. has the potential to be a source of disturbance to the lake and should be carefully controlled. No runoff from disturbed sites should be directed to the lake or slough. Small site depressions and remnant ditches provide an opportunity to retain runoff. Similarly, no fertilizer, other nutrients, or any chemicals used on site should be allowed to reach the lake or slough. Whenever a sudden change occurs, especially an undesirable one, additional consultation is advisable as soon as pOSSible, and clear actions should be spelled out to remedy the situation and/or prevent its reoccurrence. When this falls sudden eutrophication was noted, there appeared to be no recognition that it was probably avoidable and no immediate positive actions were developed to contro lit. Similarly, it is imperative to control sedimentation in the pond At present, runoff from the road repeatedly de I ivers an excessive load of sediment to the pond This situation should be corrected immediately. A3 noted iii lIle LaKe r lalidgeli,eliL ~eCllOIi, SUCh diStUrbance must be appropriately dealt with as soon as it is observed. Drainage from the new parking area also requires. erosion and sediment control. 2. The lake is an artificial one and its natural water supply is probably inadequate to maintain desirable water levels at all times; therefore, back-up supply is available from an artesian well. There are no fixed rules for managing the water level to favor optimal habitats. Careful observation and monitoring of the actual site conditions is required as well as studying analagous sites in the area.


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In general, the goal is to mimic a natural seasonal pattern while minimizing occasional extreme conditions which might threaten more fragile species. Recent management has maintained a relatively even water level, until the past few weeks when the level has been allowed to drop to its lowest level so far. We strongly recommend that even more seasonal fluctuation be tolerated in future management. A longer, drier period will favor more rapid growth of the cypresses and other trees. It is advisable to review water management procedures by foresters who are now effectively managing water levels for faster growth rates. A more natural regimen will also increase the opportunity to develop hydroperiod classes of different species as a thematic line in the display and educational program. 3. The use of artesian well water to maintain lake levels may be impacting water chemistry as well as the seasonal hydrologic regimen. The concentration of base minerals in the artesian water may be raising the lake's pH enough to impact habitat opportunities. We recommend that a regular program to monitor pH throughout the year be established. As analagous natural sites are identified, pH monitoring should also be initiated. Reduced reliance on the well water and more prolonged drier spell will reduce this problem 4 A greater fluctuation in the lake's water level will expose the steep excavated sides of the lake. This is both aesthetically unsatisfying and the edges are 1ike Iy to co II apse gradua lly as the water ri ses and fall s adding sediment to the lake and disturbing new vegetation. In addition, the escarpment-like edge is not consistent with the hypothetical 'scenario' for the lake as a drowned topography, created by impounding water such as a beaver or mi II bond. Two remedies are proposed The first is to flatten the edge but cutting them back (not by filling in) in places. The most appropriate sites are where a display of shallow water species is desirable, especially in coves and concave places along the

A second approach where the edge remains steep is intensive planting of species which tolerate both wet and dry conditions, such as the St. John's wort. This will eliminate the appearance of a bathtub ring during the periods of low water and the raw edge which remains even at the higher levels. Although a large linear area is involved, it is important to complete the 'edges' of the lake and slough to give a finished look. CROSBY ARBORETUM

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An alternate location for the Visitor's Center was discussed which warrants additional investigation. A location closer to the new parking area provides the opportunity for immediate visitor orientation as well as more effective surveillance of the entry area. It would also maintain more private and tranquil character at the pavilion There is also concern that the Visitor's Center at the current site might dwarf the scale of the pavi I i on and pond CONTROL OF INVASIVE NATIVE SPECIES

Effective control of invasive non-native species is most feasible before serious infestation has occurred. At present, the extent of disturbance species on site is still limited and local invasions appear to be removed as they are observed. The two most serious pests noted were Chinese tallowtree and alligatorweed, both of which should be taken very seriously. Alligatorweed, in particular, will be very difficult to eradicate if it gets well established. Everyone who works at the Arboretum should be able to identify these speCies and remove all evidence of the plants where they are noted. All sites should be reinspected frequently to control reinfestation.


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Profile for Andropogon Associates, Ltd

1987 General Recommendations Crosby Pond and Path  

Recommendations for the Crosby Arboretum Pond and Path after a site visit in December 1987.

1987 General Recommendations Crosby Pond and Path  

Recommendations for the Crosby Arboretum Pond and Path after a site visit in December 1987.