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Vegetation – Scientific Studies and Monitoring

Urban Riparian Wetland Vegetation Monitoring

2002-2004

Twenty-four vegetation plots along the Bronx River were monitored during the summers of 2002, 2003 and 2004. These plots included areas that had been restored using bioengineering techniques (5 plots) or by removing invasives and planting native species. Of the areas where planting had been the main restoration activity, some plots were in areas that had been planted ~10 years before (8 plots), while others had been restored just 0-2 years before monitoring began (6 plots). In addition, three plots with pre-restoration conditions (unrestored or control conditions) were monitored, as were two plots with the “pristine” conditions that restoration aimed to reach (reference conditions).

During each sampling period all plants within each plot were identified to the genus level, and in most cases to the species level. The percent of the ground covered by each species of herb, shrub and tree in the plot was also estimated.

We calculated a number of summary values from the raw data that allowed for comparisons across plots. These included the percent cover of Japanese knotweed, the main invasive plant along the Bronx River, as well as the percent cover of various groups (native herbs, invasive herbs, exotic, but not invasive herbs, native trees, etc.), and the species richness (both overall and for natives only).

From these values four particular measures were chosen as restoration indices because they consistently distinguished the reference from the control conditions (i.e. the target of restoration, and the starting point before restoration) and allowed us to distinguish the important trends in each plot. The chosen measures were:

  1. Native species richness (herbs, shrubs, and trees) (NSR);
  2. Percent cover of Japanese Knotweed (JK);
  3. Percent cover of invasive herbs (IHC);
  4. Percent cover of native herbs (NHC).

These four indicators of restoration success were then standardized to facilitate comparisons and allow for cumulative scoring. Each indicator was standardized to a scale of 0 to 3 where 0 indicates conditions like in the unrestored controls, 1 indicates marginal improvement over these conditions, 2 indicates considerable improvement over control conditions, and 3 indicates conditions indistinguishable from the “pristine” plots.

Methods

Criteria for standardizing restoration indices

The criterion for each of the restoration indices was determined by the following steps:

  1. The standards for the unrestored conditions were determined by visually examining the 3-year average values in the control plots and determining what threshold values included to all control plots.
  2. The standards for the reference conditions were determined by visually examining the 3-year average values for the reference plots and determining what threshold values included all reference plots.
  3. For the native plant measures (overall species richness and percent cover of herbs only), which had low relative variability over the 3 years and did not show signs of trends through time, the criteria for considerably improved and marginally improved were determined as half way in between the threshold for unrestored and reference conditions.
  4. For the invasive species measures (percent cover of all invasive herbs and Japanese knotweed only), which were more variable and appeared to show trends, the criteria for considerably improved and marginally improved were determined by whether the measures were increasing towards unrestored conditions, or decreasing towards reference conditions.

The end results are summarized below. Two important points are that these values are principally defined by the measurements in control and reference plots and thus may: a) vary at different sites, and b) may change with additional years of data collection that incorporate natural yearly variations in conditions.

Percent Cover of Japanese Knotweed (JK) Species Richness of Native Plants (NSR) Percent Cover of all Invasive Herbs (IHC) Percent Cover of all Native Herbs (NHC)

Reference conditions

(3)

Less than 10% during all summers. Three year average value >8. Average of <10%. Three year average value >30%.

Considerably improved

(2)

Average of three summers is <60% and declining over the 3 years. Three year average value greater than 6. Average of <70% and declining over the 3 years. Three year average value greater than 20%.

Marginally improved

(1)

Average of three summers is <60%, but cover was increasing during the 3 years. Three year average value greater than 4. Average of <70% but cover was increasing during the 3 years. Three year average value greater than 10%.

Unrestored conditions

(0)

Three year average greater than 60%. Average of 3 summers is less than 4. Three year average greater than 70%. Three year average value less than 10%.

Monitoring Site Maps

Shoelace Park

Urban Riparian Wetland Vegetation Monitoring Shoelace

Bronx Forest

Urban Riparian Wetland Vegetation Monitoring Forest

Results

Plot by Plot Comparison

Restoration success, as measured by four restoration indices, varies amongst plots. One group of plots (plots 5, 7, 8 & 14-17) saw improvements mainly in the JK and IHC indices without similar improvement in the other two indices, while another group (plots 10-13) saw improvements mainly in terms of native species richness. Plots 1 and 18 are the only two plots with a difference between the Invasive Herb Cover index and the Japanese Knotweed cover index. Referring back to the original data, we can see this is because although Japanese knotweed was successfully removed in both plots, plot 1 was invaded by Mugwort and plot 18 was invaded by Garlic mustard. Note that both 1 and 18 are upland sites and that invasion by species other than Japanese knotweed does not seem to be a problem in more riparian plots. Increasing the NHC index to reference conditions appears to be the hardest task in restoring the vegetation communities with only 3 plots scoring a 3 with this index. Note that both plots 9 and 23 restored conditions to nearly “pristine” totals (11 out of 12).

Plot Location Management Restoration Category JK NSR IHC NHC Total
1*
North
Y
Planted - 1990s
3
0
0
0
3
DG
R
R
R
R
2
North
N
Planted - 2000-2002
0
1
0
0
1
U
I
U
U
U
3
North
N
Bioengineering
0
0
0
0
0
U
U
U
U
U
4
North
N
Planted - 2000-2002
0
0
0
0
0
U
U
U
U
U
5
North
N
Bioengineering
2
0
2
0
4
S
U
S
U
I
6
North
Y
Bioengineering
0
0
0
0
0
U
U
U
U
U
7
North
Y
Bioengineering
2
0
2
0
4
S
U
S
U
I
8
Middle
Y
Planted - 2000-2002
3
1
2
0
6
P
I
S
U
I
9
Middle
Y
Planted - 2000-2002
3
2
3
3
11
P
S
P
P
P
10
Middle
Y
Planted - 2000-2002
1
2
0
1
4
I
S
U
I
I
11
Middle
Y
Bioengineering
1
3
1
0
5
I
P
I
U
I
12
South
N
Planted - 2000-2002
1
3
1
0
5
I
P
I
U
I
13
South
Y
Planted - 1990s
1
3
1
2
7
I
P
I
S
S
14
South
Y
Planted - 1990s
3
1
3
0
7
P
I
P
U
S
15
South
Y
Planted - 1990s
2
1
2
1
6
S
I
S
I
I
16
South
Y
Planted - 1990s
3
0
3
1
7
P
U
P
I
S
17
South
Y
Planted - 1990s
2
1
2
0
5
S
I
S
U
I
18*
South
Y
Planted - 2000-2002
3
1
1
3
8
P
I
I
P
S
23*
South
N
Planted - 2000-2002
3
3
2
3
11
P
P
S
P
P

The Location column refers to the geographical group that the plot was placed in for the location analysis. The Management column refers to whether the plot was managed during the study period (Yes – Y, or No – N). The four indices and their plot totals are rated, with 'U' indicating conditions indistinguishable from unrestored plots, 'I' indicating slightly improved conditions, 'S' indicating significantly improved conditions, and 'P' indicating conditions that are indistinguishable from “pristine” conditions. * Refers to restoration sites that are better classified as upland than riparian.

Comparing Different Restoration Techniques

Comparing Different Restoration Tecniques

When we compare the different restoration techniques bioengineering seems to be less effective than planting, as measured by these indices, however this difference is not statistically significant and it should be stressed that the indices to do not account for other reasons why bioengineering is often chosen as an appropriate restoration technique (i.e. for slope stabilization or in order to increase the depth of the stream). Note also that all treatments seem to perform best in terms of reducing Japanese Knotweed cover and worst in terms of increasing native herb cover.

Active Management

Active Management

One issue that makes comparing these plots particularly difficult was that many, but not all of the plants were managed throughout the study period. In order to determine if this was affecting our results we identified those plots which we believed to have been managed. It appears that management is having a little effect on all the indices, increasing the cover of both invasive and native species and slightly increasing native species richness, however none of these effects are statistically significant.

Location Along the River

Location Along the River

The most pronounced and only statistically significant trend in the data is that all 4 restoration indices and their total are consistently lower in the northernmost 7 plots than in the plots further south. There are a number of factors that could explain this pattern: 1) Plots in the middle and southern areas are surrounded by less disturbed forest, which serves as a source for native species seeds; 2) The slope of the river bank is greater in the north and many of the bioengineering projects address slope as a primary target and plant community restoration as a secondary target. 3) The more northern plots may be under more pressure from invasive species, including Japanese Knotweed.

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