You can used this formula to find out growth rate= change in height/change in time 

Thanks for your answer Rajkumar Yadav. I found it to be difficult to determine actual change in height of saplings, since sediment accretion/and deccretion on reference measures vary too much to make reliable statements!

Any other suggestions?

One precise and promising way is to cut and do a stem analysis if it is possible. Otherwise,  if you know the age of the stand or even some individuals, you can cancalculate average growth rate. But yearly data is not achievable by this method. I am not quite familiar with growth strategies and signs of mangroves,  but based on tree architecture,  ssometime it is possible to have a rough estimation of age.

Just as in the case of any sapling growth rate is a relative concept. When there is variation in sediment deposits, while calculating growth rate , we have to  consider the extent of variation in depth/height  in relation to relative height growth of sapling. After having a look at the general  growth behavior of the species concerned, you may decide the time interval for taking measurements - 6 months / 1 year/ 2 year etc  to get a reasonable  variation in height.  Height  growth of sapling can be calculated using the simple formula  

Sapling height growth rate = {height (t 2) - height (t 1) + sediment deposit depth (t2 -t1)} / (t2 - t1).

Here , we should keep in mind the general principle in Mensuration that while discussing growth rate in terms of height or girth or diameter , our concern is about the  relative accuracy required for the particular situation or context depending on utility of measurement.

Verena, I don't think that stem diameter is a reliable measure for mangroves, as it depends on too many environmental parameters...

According to some experts I had talked to recently, the most reliable estimator would be the number of nodes, but for this, a previous "calibration" would be needed - so this is not an option for you either...

Thank you for your answers!

@Unnikrishnan Narath Nandakumar:

I understand the concept of the simple formula, but am still concerned about the result. The formula does not consider the "zero-line" of the height of a mangrove, meaning the lenght of the mangrove stem that is burried under the sediment (unless you would completely dig it out).

Even by installing reference stakes for sediment accreation, i found sediments heights are varying to vastly to produce reliable results. In my case, height changes of +/- 10cm sediment accretion per week indicate dynamic soil elevations and movements, which show that reference stakes itself (and possibly mangrove saplings) change in elevation constantly and immensely each week,

@Martin:

I totally agree with stem diameters being influenced by environmental factors vastly. On the other hand, to get an actual idea of mangrove growth in a certain area, stem diameters can be determined to give at least a clue about growth increase or non-growth. The advantage lies in the rapidity and easyiness of the measure.

I agree, I heard about leave nod analysis and even microscopic histoid-analysis of stem areas that can help to exactly determine ages and growth rates of mangroves, especially for forests where ages are unknown.

Dear Verena Hoelzer,

As a person who has a sound background in the field conditions prevailing in the mangrove ecosystem, my suggestions are still valid. We fully know that due to sediment deposit as well as loose soil structure, there are limitations in taking exact measurements. Here we have to use judgement of the site conditions and  choose  the interval between two measurement period judiciously  to get a realistic picture of the growth rate. By doing repeated visit to the site one can observe the level of growth involved and  the  degree of sediment deposit can be assessed and thus one can easily calculate growth rate. 

Dear Unnikrishnan Narath Nandakumar,

I think I'll be able to determine an average sediment-accretion/or loss for the overall 6 month monitoring period. I probably can use this one then to determine the end height of the trees and compare it to the intial height. That will only provide me with two data points, but it's better than nothing I guess. Thanks for your help and concern!

Unnikrishnan Narath Nandakumar, please how do I cite this formula

Sapling height growth rate = {height (t 2) - height (t 1) + sediment deposit depth (t2 -t1)} / (t2 - t1)

It is doubtful unless the species attain a certain diameter of 20 cm