Sample plot size distribution per stratum

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45Joana-bana
Posts: 4
Joined: Tue Oct 06, 2020 4:25 am

Sample plot size distribution per stratum

Post by 45Joana-bana » Sat Oct 10, 2020 3:22 am

Dears,
I am a little bit in confusion with the sample plots distribution within strata of my study area. I try to comprehend the i-Tree Eco user manual regarding the sample size per city and stratum. According to the manual, the minimum total sample plots per study area/city and stratum need to be 200 and 20, respectively. However, the strata made based on whether the existing land uses or administrative boundaries vary in size. For instance, a city park considered as stratum could be larger in size than patch of woodlot. Thus, how the number of total sample plots per stratum can be determined? Should I allocate equal number of plots per stratum regardless of the total area of the strata?
Thank you once again.
Jason.Henning
i-Tree Team
Posts: 224
Joined: Fri Dec 21, 2012 7:45 am

Re: Sample plot size distribution per stratum

Post by Jason.Henning » Mon Oct 12, 2020 8:58 am

Thanks for your question.
The area of the stratum does not impact the appropriate number of plots. The number of plots will directly impact the sampling error of your project and if you want to minimize that sampling error you would need to place more plots in strata that have high variability. This forum post discusses some methods to determine the optimal number of plots, viewtopic.php?f=19&t=1210. However, you may want to consult with a statistician if you want to perform optimal distribution plots across your strata to minimize sampling error.
There is one other point to consider. In some cases it is best to distribute plots randomly across your whole study area and ignore the strata boundaries. This would lead to what is called proportional allocation of plots. Each stratum would get a number of plots proportional to its area. If any stratum has less than 10-20 plots you may need to combine strata. This type of plot distribution is necessary if you plan to do any post-stratification of your project. Post-stratification would allow look at results by different sets of strata like land use, government districts, and neighborhoods. If you are are interested in post-stratification this guide may be useful, https://www.itreetools.org/documents/26 ... amples.pdf.
Thanks,
Jason
A member of the i-Tree Team
45Joana-bana
Posts: 4
Joined: Tue Oct 06, 2020 4:25 am

Re: Sample plot size distribution per stratum

Post by 45Joana-bana » Tue Oct 13, 2020 7:35 am

You really helped me,
Thank you so much for your clarification and properly responding my questions. I still demand your valuable support how to address the following issue on the i-Tree Eco;

''Modeling carbon sequestration potential of Urban Forests of the study area under three different scenarios over the next three decades using i-Tree Eco'' is one of the objectives of my research.

Scenario-1. Carbon storage and sequestration of the urban forest based on the current canopy coverage of the city or business as usual
Scenario-2. Increasing the existing/or current canopy coverage by 20% through turning the identified potential/plantable areas into green spaces and application of maintenance activities on the exiting green spaces
Scenario-3. Adding another 20% coverage to the coverage in scenario two through the same activities

Projections can be made actually on the structure and function of urban forest using the i-Tree Eco forecast. However, I am not confident enough how to project the ecosystem services of the urban forests under the aforesaid scenarios. How can I do with the i-Tree Eco to address this objective?

Tank you once again.
Erika.Teach
Posts: 23
Joined: Fri Feb 12, 2016 2:59 pm

Re: Sample plot size distribution per stratum

Post by Erika.Teach » Wed Oct 14, 2020 5:03 pm

As you have seen, you can estimate future benefits related to the number of trees planted, mortality and extreme weather events using the forecast model inside of Eco. The carbon sequestration results that Eco estimates are based on DBH, total tree height, crown health, crown light exposure, and land use, not on canopy coverage. There is not a way within Eco to project ecosystem services based on increasing canopy coverage, you would need to work with a person who does ecosystem modeling and a statistician to accomplish this goal.
45Joana-bana
Posts: 4
Joined: Tue Oct 06, 2020 4:25 am

Re: Sample plot size distribution per stratum

Post by 45Joana-bana » Sat Oct 17, 2020 11:28 am

Dear sir/madam
Thank you so much for the clarifications. Now things regarding the model are getting well understandable. However, I do have few more inquiries regarding field data collection from the sample plots. I still fail to properly understand some important points in the i-Tree Eco Field Guide.

1. How to record tree, shrub and plantable space percentage at plot level?
According to the guideline, record tree cover as 0%, 100%, or mid-points of 5% intervals (3, 8, 13, 18, etc.)
What are these percentages? i.e, 0%, 100%, mid-points of 5% interval; what are the numbers refers to? why not 2, 4, 9, 10, etc.?

2. How to record ground cover at plot level?
• Estimate the percentage of each ground cover type to the nearest 5% unless cover is minimal. why 5%?
• If trace amounts are present, you can just enter 1, 2, 3%, etc.
Waiting to hear from you regarding the clarification of the aforesaid points.
Yours sincerely,
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