In late-successional forest communities, how much does history matter?

Author Kerry Woods discusses recently published Journal of Ecology article: Over 80 years without major disturbance, late‐successional Białowieża woodlands exhibit complex dynamism, with coherent compositional shifts towards true old‐growth conditions by Brzeziecki et al.

Find out more about the importance of long-term data when considering old-growth forest dynamics.

Thirty years ago, John Magnuson wrote of the ‘invisible present’ (Long-term ecological research and the invisible present, BioScience) – an inability to understand the state and dynamics of ecological systems because we lack perspective on multi-decade processes that have conditioned the present.  Magnuson focused on aquatic systems, but the same concerns apply to any ‘slow system’, certainly including late-successional forests.  Our grasp of the dynamic properties of old forests is severely limited by the scarcity of long-term data-sets.  Yet the goals of forest managers and conservationists are often framed in terms of preserving, restoring, or mimicking presumed properties and processes of late-successional (variously, “climax”, “old-growth”, “ancient”, “primeval”, “potential”, etc.) forests.

Hornbeam and oak trees, Białowieża Forest. Photo: Bogdan Brzeziecki.

Without grounding in long-term data, models of old-growth forest dynamics have a large anecdotal component.  For cool-temperate mesic forests, at least, concepts of old-growth commonly incorporate a deep assumption of relative stability or quasi-equilibrium at the stand scale. Another way of putting it is that our thinking about old-growth forests is couched in a uniformitarian conceptual framework; the processes observable now are those which have applied over many decades or even centuries, and are responsible for sustaining their ahistorical equilibrium.  This idea is not new, and usually invokes some version of a cyclic, within-stand ‘gap-phase’ or ‘shifting-mosaic’ dynamic as articulated in 1947 by A.S. Watt (Pattern and process in the plant community, Journal of Ecology).

However, there is also a long-standing counter-current of alternative ‘non-equilibrium’ models suggesting that community composition and structure rarely exhibit even ‘quasi-equilibrial’ properties because environmental variation – either long-term secular change or rare, intense disturbances – intercede before such status can be achieved.  These models recognize that the effects of such changes may cast centuries-long shadows in communities of long-lived trees and forest community properties are, therefore, likely to be profoundly historically contingent.

The problem is that we have a major data-gap for precisely the temporal scales relevant to distinguishing between these two models – from several decades to a one or two centuries (the typical ‘residence time’ of canopy trees in temperate forests).  Indirect approaches like time-for-space substitution and proxy data-sets from paleoecology and dendrochronology can offer insights – but have core limitations.  The critical value of direct measurements from long-term monitoring is broadly recognized, but the challenges in initiating, maintaining, and curating multi-decade data-sets are self-evident.

Uprooted tree, Białowieża Forest. Photo: Bogdan Brzeziecki.

So, the big question remains: In late-successional forest communities, how much does history matter?

We have been able to ‘leverage’ a remarkable and nearly unique ‘heritage data-set’ from the Białowieża Forest of eastern Poland, one of the best-known and most extensive ancient forests of Europe.  At Białowieża, direct remeasurements of over 20,000 trees, over nearly 80 years, on >15 ha of mapped study plots allows a uniquely detailed and long-term window into the workings of forest communities.

The upshot is that, over the last 80 years, forests at Białowieża have not shown much evidence of equilibrial dynamics.  Composition and structure have changed substantially on the permanent study plots, with dramatic increases in dominance and ecological amplitude of some (but not all) shade-tolerant tree species at the expense of less-tolerant species.  Compositional continua have shifted; many of the plot groupings identified by classification of initial, 1936, data are not coherent in more recent data.  Some of the changes observed – in particular, increases in shade-tolerant Carpinus and Tilia – are plausibly driven by competitive sorting, and may be interpreted as long-lasting legacies of past major disturbances; there have been no major disturbances on the study sites since 1936.  However, some observed changes are consistent with response to changes in underlying environment, like gradual climatic warming and changes in nutrient cycling due to N-deposition, or changes in trophic dynamics related to a diverse community of large herbivores.

The remnants of dead European spruce trees (Picea abies), Białowieża Forest. Photo: Bogdan Brzeziecki.

Without additional data, probably only obtainable through continued monitoring of long-term study plots (or comparison with the few other studies with comparable time-frame), it’s not possible to confidently discriminate among these hypotheses.  However, the historicity of these forests appears inescapable; they can be understood only in context of their specific histories of disturbance and environmental change.  The deeply historical nature of forest communities is likely to have substantial consequence for our understanding of the effects of environmental change and for approaches to conservation and management.

Kerry Woods Bennington College, USA

Read the full Open Access research article online: Over 80 years without major disturbance, late‐successional Białowieża woodlands exhibit complex dynamism, with coherent compositional shifts towards true old‐growth conditions

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