I have rather general comments with the aim to improve the entire system.
"Indicators are used to measure progress towards policy goals." This definition requires that "policy goals" are set, and "progress" is defined. (If policy goals include sustainability (they do), then sustainability also has to be defined.) In other words, there must be a concept based on which indicators have to be developed AND evaluated. All in all, what is needed to see if processes are working towards policy goals is a complex system of theory, estimation and assessment.
Unfortunately, the current document on the Global Core Set of forest-related indicators only list some indicators without considering the above.
Below is an example for the possible development of a complex system in the above sense for a few policy goals (with Capital First Letters), involving a few INDICATORS (all capitals).
Let's say we want to maintain the yield of products and services that we get from forests of a fixed area (a broad policy goal). "Maintaining" can only mean maintaining relative to what rather dynamic forests can deliver. For example, forest characteristics keep changing even under an unchanging management system due to the internal dynamics of forests such as the development of age class and site distribution over time. Increment, carbon sink, total volume, total amount of deadwood, total water cleaning capacity etc. all keep changing over time even if the area of the forests does not change. So what can be maintained is a moving target.
But it is also true that this moving target is spoiled if forest area decreases. Therefore, the decrease of forest area (something unwanted) is against a policy goal of Constant Forest Area, i.e., a proxy used to expect a Constant Yield of Forest Products and Services. The RATE OF DEFORESTATIONS (in terms of area) can be a measure of how much the above moving target gets closer to zero, and farther from the above policy goal. However, if the situation is that forest area has been decreasing for some time, a rational policy goal could be to Halve or Stop the Decrease by Some Future Date. In this case, if the area decrease is less intensive than the policy goal than the indicator value is still negative, but should be assessed as positive (and vice versa).
In a similar fashion, if a country has little forest area and there is room to do afforestations, then a policy goal can be to Increase Forest Area by X Amount by a Specific Date. Then, if the RATE OF AFFORESTATIONS (i.e., positive values) is less then that, then the rate as an indicator should be assessed to say that the processes are unsatisfactory (and vice versa).
It must be added that, from a yield of products and services point of view, it does matter what types of forests are disappearing, or what types of forests (or plantations) are established. Forests are not created equal! Therefore, analyses should be done on more detailed levels, e.g., by forest type, species, age or diameter, volume class etc.
The above suggests that there may not be such a thing as a "global" indicator, only regional indicators, and a single global indicator can only be designed/applied if the assessment of the regional indicators can somehow be "summed up".
Also to note is that the "rate of change" type quantities are quantities against a base value. The same rate of change can mean very different actual rates with different base values. For example, deforesting 0.5% of the forest area can mean a far greater forest area loss in a year than the same rate ten years later. Processes are non-linear, which should be reflected in the description of the assessment guide, which should be developed for each individual indicator.
Let me now consider a system for a forest of constant area. In such a system, any addition to (afforestations) and reduction from (deforestation) the forest should be able to estimated, otherwise the assessment of the indicator values will be biased. In such a system, policy goals could include Maintain or Change Absolute Quantities (e.g., volume the area of protected forests) or Maintain or Change Rates of Changes (e.g., sink rate, wood increment, harvest rate etc.). There can be many of these, and it is not evident which of the possible set are important. In order to select specific goals for a pragmatic system, the importance of the quantities or the rates of change should be demonstrated.
It applies, however, for most or all of the possible goals that they are moving targets. This means that in order to develop appropriate policy goals, and in order to develop appropriate assessment guides, modelling may be necessary to see what the future values might be under a BAU and a "With measures" policy. It is clear that the estimates will have uncertainties that need to be considered in the assessment guide.
For example, let's say that we want to Maintain the Forest Carbon Sink. This sink is not constant over time (i.e., too general, anc can be incorrect), and may even turn to a source after some period of time, or under climatic influence etc. If the sink can be shown to reduce in the next ten years under an acceptable scenario, then the policy goal can be to Maintain the Projected Carbon Sink. Any sink that is less than this sink can be assessed as unsatisfactory.
It may be impossible to directly assess and/or model the required quantity/change. In this case, proxy values may be used. For example, increasing timber harvest usually results in the reduction of the forest carbon sink, therefore, the forest carbon sink might (partially) be monitored by estimating and assessing TOTAL TIMBER HARVEST RATE against the policy goal of Not Increasing Timber Harvest. In case such a proxy value is used, the concept of the indicator and its assessment should clearly demonstrate why it is applied, and under what conditions it can be used as a proxy.
Finally, the current list of indicators includes qualitative ones such as the "Existence of policies supporting SFM". In my view, this is very weak. For one, SFM has not defined well. To me, sustainability is a quantitative thing, for example, we can sustain area, volume, increment. The concept of "sustained yield" captures this approach. In the history of forest management, people first applied the concept of sustained area, then sustained volume, then sustained yield. All of them may, and probably are in one form or another, applied in practice. These are true sustainability concepts. The mere fact that, in a country, there is a Forest Act, does not say if that act leads to sustainability or not. Hard quantities may.
Again, the above were just examples. I believe that the whole system should be re-designed. I published a paper about a possible approach last year, accessible at www.scientia.hu/cv/2016/Sustainability_framework_Zoltan_Somogyi.pdf.
Д-р. Zoltan Somogyi