What makes a grassland? The saga continues

This week’s issue of Science revisited the debate over the grassland-forest conservation tensions I examined earlier. If you recall, William Bond’s Perspective from January 8, “Ancient grasslands at risk,” argued that many grasslands that previous efforts have identified as the result of deforestation may actually be ancient ecosystems, preserved by fire and herbivory. Therefore, he claims, efforts to reforest grasslands may destroy these unique ecosystems.

The authors of one of the studies that Bond criticizes, the Atlas of Forest and Landscape Restoration Opportunities, responded with a letter in which they assert that the Forest and Landscape Restoration (FLR) project has no intention of converting ancient grasslands into forests in the name of forest conservation and that “A fundamental first step in the FLR process involves understanding the ecosystem at the landscape level, including its historical and cultural values, before making decisions on a restoration approach. FLR under this definition would promote exactly the same conservation of ancient grasslands that Bond advocates.”

This certainly sounds reasonable, but Bond replies in the same issue: “the western Serengeti and Kruger National Park are mapped as deforested instead of as ancient savannahs, and the natural species-rich montane grasslands of South Africa and southern Brazil are both mapped as deforested and/or degraded” in the Atlas of Forest and Landscape Restoration Opportunities (as, indeed, they are).

I am fascinated by this debate because it reveals a whole bunch of unresolved tensions in the fundamental principles of landscape science. Some I mentioned in my previous post: how should we value certain landscapes in order to apply conservation projects to them? Another I hinted at in my post about state-and-transition models: are these distinct landscape states such as “grassland” or “forest” meaningful? The Atlas authors mention that mosaics of forests and grasslands confound their attempts to classify, especially since the definition of forests that they use is not mutually exclusive with the definition of grassland that Bond uses.

Bond and the Atlas authors advocate their variants on the classification paradigm because it advances their interests: the preservation and continued study of grassland ecosystems on one hand and the reforestation of vast tracts of the globe on the other. My own interests in landscape dynamics push me towards an opinion that this whole project of classification is not incredibly useful. Rather than attempts to enumerate all the things you can see at a certain scale, leading to these ambiguous definitions of landscape states, I prefer approaches which build a representation of the environment at multiple scales, capturing not just the variability between different ecosystems but also the incredible amount of heterogeneity within a single ecosystem. I think it’s still an open question though how best to design environmental interventions which are aware of multiscale phenomena, and that’s certainly something I would like to know more about.

What makes a grassland?

 

There was an article published in the most recent issue of Science that seemed like a good way to get the ball rolling here at Borrowed Lands.

William J. Bond. 2016. “Ancient grasslands at risk” Science, Vol. 351, No. 6269, pp. 120-122. http://science.sciencemag.org/content/351/6269/120.full

When you look at a grassland, often you’re looking at what used to be a forest. Knowing that deforestation is a massive contributor to biodiversity loss and carbon dioxide emissions, you might be tempted to plant trees on your grassland, restoring the environment to its previous state.

But what if that grassland was never a forest and is, in fact, an ancient grassland? That’s the question Bond asks in this article, and one which I’ve seen pop up a lot over the past year. In that case, you may not want to blindly reforest the grassland.

 

Depiction of the life cycle of an underground tree in a tropical grassland. From Bond, Science 2016

A similar debate exists in the salt marsh community. Recent results have suggested that North American marshes may owe their origin to changes in sediment supply driven by colonial-era deforestation and that salt marshes may have an inherently short life-cycle. Conserving marshes exactly as they are now or trying to install marshes as part of a revitalized urban landscape doesn’t reflect their natural dynamics.

However, we may not want to simply recreate the natural dynamics of salt marshes. Instead of framing the question in terms of restoring environments to their pristine, unaltered or natural condition — a frame which may lead us to reforest grasslands that don’t look as old as they are — we need to think about what exactly the various ecosystems provide both to humans and to the local and global environments. Unfortunately, as Bond notes, we don’t even have good maps of old-growth grasslands, much less a firm grasp of their carbon and nutrient budgets, biodiversity, social and ecological roles and environmental history. Similar uncertainties exist in salt marshes, for instance in their carbon budgets.

Even if we do get a handle on these uncertainties, we may yet decide that a tropical forest is worth more than an ancient grassland or that some extra marshes on groynes might do wonders for Boston Harbor. Conservation is far from straightforward.

On the Borrowed Lands reading list: Forgotten Grasslands of the South: Natural History and Conservation by R.F. Noss. I saw this book at the Harvard Bookstore the other day, and now Bond cites it, so I think I’ll have a look at it.