Today we have a guest post from Becks Spake who is doing her PhD at Southampton University (as an aside, Becks in currently on the lookout for a post-doc, so feel free to get in touch with her). Last week a paper she and I (and others) have been working on together on recovery of temperate and boreal forests was published in Conservation Biology. I think it’s a really neat bit of work that emphasises the importance of old-growth forests for biodiversity. I’ll leave Becks to tell you the rest.
Forest restoration measures are being increasingly implemented worldwide to enhance biodiversity and ecosystem service provision in degraded landscapes. These measures range from active restoration, such as planting, to passive restoration, where natural recovery is promoted following the removal of some environmental stressor such as grazing. Forest restoration is also used as a biodiversity offsetting mechanism to mitigate the loss of habitat incurred by development; such ‘restoration offsets’ generate new habitat at an offset site to compensate for the loss of habitat to development at the impact site.
Concerns about the value of replacing old-growth forest with plantations and young regenerating forest has motivated research on biodiversity recovery as forest stands age. Several reviews have quantified the recovery times required for biodiversity, including measures of species diversity and composition, to reach equivalence to some reference state, typically undisturbed old-growth forest. Such reviews have been produced for relatively charismatic taxonomic groups recovering in secondary tropical forests (e.g. Dunn 2004; Chazdon et al. 2009; Martin et al. 2013*). These syntheses have made important contributions to restoration science, showing that different taxonomic groups exhibit different patterns and rates of recovery with stand age and that these must be acknowledged by forest management strategies.
In our study, we attempted to assess the recovery of species richness in restored forests outside of the tropics. We synthesised data from empirical studies measuring species richness differences between old-growth and secondary forest in temperate, boreal and Mediterranean regions. We focussed on studies that investigated species-rich functional groupings of fungi, beetles and lichens (Fig.1), due to their relative unsexyness, and consequent under-representation in existing reviews, their importance to ecosystem function, and sensitivity to stand-level processes.
Fig.1 – Species from the functional groupings investigated. The functional groups were: saproxylic and non-saproxylic beetles, epiphytic lichens and deadwood, litter, and ectomycorrhizal fungi. Photos: Simon Currie.
1. Functional group-specific responses to forest recovery
We found that functional groups responded differently to forest recovery (Fig. 2). Ectomycorrhizal fungi averaged 90 years for recovery to old-growth values, and epiphytic lichens took 180 years to reach 90% of old-growth values. Non-saproxylic beetle richness, in contrast, decreased as stand age of broadleaved forests increased.
Fig. 2 – Figure 1. Influence of stand age on percent change in species richness for 7 functional groups in planted and secondary forest relative to old-growth forest stands (horizontal dashed line, no difference between undisturbed old-growth forest and treatment [planted and secondary] forest stands; gray, 95% prediction intervals based on uncertainty in fixed effects only.
2. Pseudo-replication is widespread amongst empirical research investigating forest management impacts on biodiversity
Our systematic review yielded just 33 publications (90 individual studies) in which old-growth was compared with planted or secondary forests in a statistically robust way. For some functional groups, this led to small sample sizes and low precision in lnR values (Fig. 2). This low number of studies was due to the fact that a high proportion of studies were pseudo-replicated, with a lack of independence across replicates. Many forms of pseudo-replication exist (Fig. 3), the most common being simple segregation sensu Hurlbert (1984), in which multiple samples from a single contiguous treatment unit are analysed as if they were independent replicates that were interspersed with control replicates (Fig. 3 B-1). Differences between control and treatment replicates that are simply segregated cannot be unambiguously distinguished from other sources of spatial variation.
Figure 3. Schematic representation of various acceptable modes (A) of interspersing replicates (boxes) of two treatments (shaded, unshaded) and various ways (B) in which the principle of interspersion can be violated. From Hurlbert (1984).
Despite the widespread recognition of the problems associated with pseudo-replication, it still features prominently in peer-reviewed studies. The situation is the same in the tropics; Ramage et al. (2013) reviewed recent studies of the effects of logging on biodiversity in tropical forests (n = 77) and found that 68% of the studies were definitively pseudoreplicated, and only 7% were definitively free of pseudoreplication. Whilst data from pseudoreplicated studies can inform management in very local contexts, conclusions from these studies must not be generalised and data must not be included in meta-analyses.
The primary goal of biodiversity offsetting is to achieve no net loss of biodiversity. Our results show that through restoration offsetting, this goal is unachievable within a reasonable time frame. The slow recovery of species richness for some functional groups essential to ecosystem functioning makes old-growth forest an effectively irreplaceable biodiversity resource that should never be compromised by development.
Our findings support the value of protecting old-growth forest through reserve creation, but also recognise the potential for planted and secondary forests to support biodiversity, if given enough time to recover. Our findings therefore also support the setting aside of overmature planted forest for biodiversity conservation, and the implementation of schemes that extend rotation-length of planted forests within production forest landscapes.
* For my blog post about this paper see here.