Forest Condition
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Increase height of regenerating trees within floodplain forest area
No change observed
Iori River Valley
Georgia
Problem: since the construction of the Dali dam, the floodplain forest downstream of the dam has been drying out, resulting in a loss of native floodplain forest tree species. As a result, the floodplain forest extent has been declining, in part due to a lack of regeneration of new trees.
Action: SABUKO is working with the dam managers to recreate flooding events by periodically releasing water from the Dali dam. They have also constructed a gabion to retain water in the floodplain area when water is released.
Indicator: the success of this action has been monitored by assessing the condition of regenerating trees in the floodplain forest area, quantified by height of new seedlings and saplings. If heights of regenerating trees are taller in the flooded area than in the control area (receiving no flooding upstream of the dam), we can infer that the flooding events are benefiting the overall condition of the young trees.
Methods
Fifty circular plots (25 in control and treatment areas, respectively), each spanning an area of 500 m², were established to systematically record the trees within them. Two sub-plots were established in each plot (one north of the plot centre, one south), with radii of 1.5m. Within the sub-plots, all seedlings and saplings (< 8cm in diameter) were recorded. Data collection occurred in 2019 and 2022, enabling the evaluation of changes over time since flooding events began in 2020. The final variable analysed was the regenerating tree heights below the Dali dam (receiving periodic flooding), compared to the tree species in the control area above the dam (no flooding).
Each tree observed was assigned to one of three height classes: < 50, 50-150 and >150 cm. Next, we computed the proportion of trees in each height class, compared to the overall abundance of regenerating trees observed. Finally, we computed the change in proportion of trees in each height class, between 2019 to 2021. We cannot simply compare the tree heights between years, because all trees will grow over a 3-year period. However, if we observe a difference in the proportion of trees added to certain height classes, we will be able to assess if trees in the restoration area are disproportionately adding taller trees (in height classes 2 and 3), compared to the control site, due to the flooding events.
The same plots were also used to monitor forest regeneration rate and species composition.
Results
There was no statistically significant difference between the control and restoration site in the proportion of tree species added to each height class. The figure below shows the proportion of trees added or subtracted from each height class, over three years since flooding began.
Figure 1: Proportion of trees added or subtracted from each height class over a three year period. ‘ns’ denotes non-significant differences between the control and treatment groups, within each height class.
Interpretation
These initial findings suggest that the periodic flooding is not yet exerting a significant effect on the condition, or height, of regenerating trees. However not yet statistically significant, we can observe that in the control area, trees were lost from the first height class (because they grew and joined the second height class), and new trees were not added. That is why the change in proportion of trees in that height class is negative. We see the opposite effect in the restoration area; although some trees indeed grew and entered the next height class (i.e. there was some loss of trees from the first height class), we see that more trees were added than were lost overall. This suggests that in just a 3-year period, the restoration site has seen more new trees added than in the control sites, and this is likely due to periodic flooding. We observe negative values in both the control and treatment sites in the 50-150 cm height class, because those trees already existed and they grew on into the >150 height class (which is why both of those values are positive). Trees do not grow quickly, so more time is needed to fully observe the effects of flooding on this site. We will continue to monitor this area and provided updated results soon!
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