Preliminary results from yield data obtained during this last 2012/2013 harvest suggest that having a diverse portfolio may indeed maintain yields in the context of adverse climatic events, but farmers´ practices need to be considered as other determining factors.
Which is the scale level addressed by your question? This could be crucial to better focus the different elements you mention. For example: a "diverse portfolio" may refer either to different cultivars in different farms, or different cultivars in the same farm, or even a genetically diverse cultivar in the same field.
Based on this, is there a "low diversity" treatment that was less stable than a "high-diversity" one?
Is there a sufficient number of farms to compare different farmers' practices?
Could you classify these different practices to seek for practice-stability relationships?
Hope it helps
Ambrogio
Thank you for your helpful clarification of my question! In the context of the high Andes, my scale would be both at the potato field and the landscape level, and here is why. While families do have individual fields their most diverse -genetic and varietal- potatoes are typically planted all together in smaller communal plots called "laymis" or "sectores" depending on the region. So if you imagine the design of a quilt, each patch, right next to one another, is one farmer's field. The quilt, in its integrity, forms a mosaic of potato fields. But each field does not necessarily have the same cultivar content. Furthermore, due to another common practice in the high Andes referred to as "field scattering", some of these fields may not be clumped in the same laymi or sector, but may be scattered throughout different zones across the landscape. This is how farmers minimize risk against common climatic events like frosts and hail, which may severely affect one field but leave the other ones intact. What I have now are yield data from 312 fields, from two different regions in the Central Andes, one where farmers have generalized use of chemical inputs and another where farmers minimally use inputs. There are fields with 1 cultivar, 2-5 cultivars, 6-10 cultivars and >10 cultivars, but my N for each category may not be so high, 30-40 fields.
From your amazing landscape description, I assume you can follow two paths.
One is focused on the farming system: group these fields into "clusters" belonging to the same farmer, or family. Then (just to brainstorm) you can try to compare different field clusters, identify the more and less scattered, the genetic diversity within and between fields in each cluster, and between clusters, the timing of operations.. these are potential determinants of yield stability. You will look for the overall yield of each "cluster" and compare the coefficient of variation, e.g. of "high genetic diversity" vs. "low genetic diversity" clusters, or "high input" vs "low input".
Another is to focus on different mosaics of adjacent fields, look for the more and less "genetically diverse", and then compare, e.g, field coefficient of variation in different mosaics ...
A more general question, once clarified the focus (farming system vs. landscape unit) is to weight all possible predictors of yield stability (genetic diversity, input level, dimensions of the fields, whatever ..) on a respose variable that describes yield stability. There should be several multivariate approaches you can try.
What I would NOT do is to focus on the fields as 312 independent units, as they aren't!
Anyway, I think that step n. 1 would be a definition of "stability" that fits into your case-study, step n. 2 would be decide the focus of the study, i.e. which level -farming system, landscape mosaic- is relevant for stability?, step n. 3 list all possible stability predictors.
What you describe is fascinating, I hope your work could be as fruitful as possible.
These are all valuable suggestions. Thus far I have only done descriptive statistics, and have not even begun to process altitude and surface are of each field and "cluster"...so I have a lot of work to do!! I see that you are the Scuola Sant´Anna, I almost applied to that PhD program, to work on maize diversity with Paolo Barberi...but decided to come back to my center of origin. Potatoes and I are close relatives, you see :)
Alejandra, could you verify whether "field scattering" is a practice - i.e. farmers do it intentionally to make their farming system more resilient - or is the result of local tenure regimes?
In the second case, which is very common in many peasant agricultural systems, as I could verify in several places, it might also result as very unefficient sometimes.
For instance, how does the distance between several fields in different laymis impact on the labour efficience? How do families prioritise the labour on different, distant fields? This could be very interesting and useful for the communities before starting a thorough satatistical analysis!
It would have been nice to have you here, but working in one's center of origin is so important!
All the best