December 2009: Further Testing
Scott spent the month of December planting and testing. In early December, the end of mangrove propagule season, there were still a good many propagules to collect. Mostly these had fallen from the trees and washed up along shore lines in marginalized areas. Many had developed roots. Taking advantage of the abundance of easily collectable "drops" Scott rustled up old time volunteers, Nino, Raphael and Carlos to collect and plant for a few days. We planted about 16,000! Most had roots and needed to be planted with the help of a trowel to excavate a small hole first. The roots must not be bent upwards when planted.
Even with careful planting, we found out later on that survival rates where only about 10-20%. We believe this was partly due to some areas we planted being higher up in the intertidal zone with less water. The reds can grow in a broad range of the intertidal zone, but do not seem to take as well in drier areas. We also planted in deeper areas and even there survival rates were no better than planting propagules without root development. Ripe propagules without root development are much faster and easier to plant and appear to survive just as well if not better than those with root development.
Scott then spent some time experimenting with crab predation. The area of Black Rock Creek has been extremely difficult to restore. After planting thousands of propagules on multiple occassions, still only about 30 or so red mangroves have survived. The reason appears to be very heavy crab predation. So, I experimented by planting exactly 100 propagules, in 5 rows, each 20 propagules long. I then observed them for one month. The propagules disappeared or were chewed in half at the rate of about 3.5 per day, starting with the ones closest to shore at the shallow end of the lines . The propagules were systematically wiped out after about 30 days.
I also tried a technique of planting 20 propagules, each protected by a tube of bamboo pounded into the mud. These propagules lasted longer with the defense apparently helping, but, by March when we returned, they were all eaten or had died do to apparent dehydration. The tubes must have effected the hydration of the propagules. They were also time consuming to plant that way. A better approach for this area is likely to grow propagules to a crab proof size in a nursery before planting. In other areas where crab predation is not quite so bad, we have experimented with high density "predator satiation" planting, and this does seems to help.
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 In the canal we found a bunch of "drops", It was a hot sunny day so we protected them with bamboo leaves for shade.  We hauled the propagules deep into the restoration area, planting along the way. 
 We caught a red mangrove crab, above, no easy task as they are unbelievably fast! We named him "Elvis" after a local kid who was asking about what we were up to. We put Elvis in a bucket overnight along with Red, Black and White seeds. He ate the black mangrove seeds and left the others alone. By chance, on the day we released Elvis, we saw headline news indicating it was "The King's" 75th birthday celebration.  Look carefully to see chew damage at the base of the tree in the above photo. This one will likely survive, the one on the right is dieing.  Propagule, chewed right through by crabs.  Slices like this are common in Black Rock Creek.  After the crabs ate all the original 100 propagules, In the same exact place, I planted these protected by tubes of bamboo.  Above, a little experimentation with black seeds. When placed on top of the mud, they took between 10 and 16 days to grow out roots and stand themselves up. |  In this photo, the white roots are indicative of a "drop" propagule that has been in the water for a few weeks but has not been able to grab hold and stand itself up.  As we hiked towards the interior, we passed by test plots like the one above, planted one year ago. The test plot in the photo is exposed to significant wave action when the wind blows, resulting in low survival rates, but we planted in sufficient density to get acceptable results. Other plots we found with one year survival rates of near 80% and some as low as 1%.  We observed propagules like the ones above which had their leaves chewed off by crabs and have sprouted new branches and leaves in response, to survive they will have to grow fast. The small tree to the left lost its crown to crab predation, but then branched out in response and managed to outgrow the predators.  Deep in the interior of the primary restoration area, Carlos takes a break. Nearly 11 years after Hurricane Mitch, but large areas remain dead, with almost no visible plant life at all.  Many species of "mangrove crabs" live in the mangrove environment and provide a crucial function by eating mangrove leaves and detritus, thus converting the plant biomass into animal biomass. The mangrove trees start the food chain by converting the suns energy into biomass, the crabs eat that, and they are in turn eaten by various predators, passing the suns energy along the food chain.
 Anuar believes the Yellow crabs like the one above chew on the stems of the young propagules and may also chew on larger trees, although they are too big for any damage to result. We did have a few good sized starts, about two years old and nearly 3 feet tall, that were killed by crabs chewing around the base of the trunk. We have only seen this in Black Rock Creek, where crab predation is quite extreme. See photos below>  The tree above is about 2 years old. It was one of the few survivors of more than 3,000 we planted in Black Rock Creek. Unfortunately, crabs kept chewing on the trunk until its leaves died off. Interesting that it appears to be trying still to grow out new leaves. Green shoots are evident at the tips of the branches.  Above, 100 propagules planted in five precise rows starting near shore and going out into deeper water. The crabs systematicaly are them all. This took about one month, starting with the propagules at the shallow end.  The tubes did not stop predation but they noticeably slowed it down. Some of the propagules began growing leaves but then died due to apparent dehydration. The area they were planted in had red mangroves before Mitch, so why did they die? Either, the tubes interfered with the roots, or another possibility is the soil. Notice the sandy look of the soil above. Then compare with the photo 2 above showing the 100 propagules in rows. When first planted the soil base was largely mud. However, after being planted, a large storm caused a surge that washed away much of the mud and replaced it with sand. The hieght of the soil in this spot actually increased by about 3 or 4 inches. Given the small tidal changes here, this is potentially enough to change this from a red mangrove area to one that white mangroves would prefer. |
