Understanding climate as a driver of low- to moderate-severity fires in the Montane Cordillera Ecozone of Canada is a priority given predicted and observed increases in frequency and severity of large fires due to climate change. We characterised historical fire-climate associations using 14 crossdated fire-scar records and tree-ring proxy reconstructions of summer drought and annual precipitation from the region. We compared fire-climate associations among years when fires burned in multiple study areas. From 1746 to 1945, there were 32 years with moderate fire synchrony in which four to six study areas recorded fire. During four high fire synchrony years, 7 to 10 study areas recorded fire. Below-average annual precipitation and summer drought synchronised fires, whereas infrequent years of high fire synchrony were preceded by a wet summer. After 1945, decreased fire occurrence and synchrony reflects fire exclusion, suppression and climatic variation. Global climate change manifests as blocking high-pressure ridges that superimpose on longer fire-seasons and increased droughts. Combined, they make dry forests increasingly susceptible to synchronous fires, which are difficult to suppress as observed during the record-breaking 2017, 2018 and 2021 fire seasons in British Columbia.
Effective restoration planning tools are needed to mitigate global carbon and biodiversity crises. Published spatial assessments of restoration potential are often at large scales or coarse resolutions inappropriate for local action. Using a Tanzanian case study, we introduce a systematic approach to inform landscape restoration planning, estimating spatial variation in cost-effectiveness, based on restoration method, logistics, biomass modelling and uncertainty mapping. We found potential for biomass recovery across 77.7% of a 53 000 km
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region, but with some natural spatial discontinuity in moist forest biomass, that was previously assigned to human causes. Most areas with biomass deficit (80.5%) were restorable through passive or assisted natural regeneration. However, cumulative biomass gains from planting outweighed initially high implementation costs meaning that, where applicable, this method yielded greater long-term returns on investment. Accounting for ecological, funding and other uncertainty, the top 25% consistently cost-effective sites were within protected areas and/or moderately degraded moist forest and savanna. Agro-ecological mosaics had high biomass deficit but little cost-effective restoration potential. Socio-economic research will be needed to inform action towards environmental and human development goals in these areas. Our results highlight value in long-term landscape restoration investments and separate treatment of savannas and forests. Furthermore, they contradict previously asserted low restoration potential in East Africa, emphasizing the importance of our regional approach for identifying restoration opportunities across the tropics.
This article is part of the theme issue ‘Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration’.
Abstract:Regional surveys done over the last decades show a clear decline in abundance of Northern white-cedar (Thuja occidentalis L.) throughout its range. A lack of seed trees, difficulties in the establishment of natural regeneration and high browsing pressure caused by increasing deer populations have been identified as plausible causes. Current silvicultural strategies for cedar restoration recommend partial cutting to promote and release natural regeneration, but there is also a need to restore the species in areas where it became absent. Yet, little attention has been given to cedar plantations. This study provides a first characterisation of the effects of competition, silvicultural treatments and deer, moose and hare browsing on planted cedar growth, survival, and stem form. Pure and mixed cedar plantations aged 5-27 years located in Eastern Québec were sampled. Both inside and outside deer yards, planted cedars showed high survival rates and were generally subject to low browsing pressure, but 45% were forked. Cedars showed high growth rates and strong reaction to stand opening. Results suggest that at reduced competition levels, a 9-year browser exclusion could be sufficient to establish safe-from-browsing cedar stands of >3 m in height.
Gap dynamics play a crucial role in forest regeneration by creating favourable regeneration and survival niches for some plant species. Nonetheless, potentially overriding factors, such as ungulate browsing, could limit or eliminate this gap dynamic–related regeneration. The deleterious effects of browsing may be exacerbated for slow-growing species such as northern white cedar (Thuja occidentalis L.), a tree highly selected by white-tailed deer (Odocoileus virginianus Zimmerman). We therefore aimed to understand how deer browsing and gap dynamics interact to affect cedar regeneration and hypothesized that cedar regeneration benefits from natural gaps but that deer browsing could override this effect. We inventoried natural canopy gaps along a spatiotemporal gradient of deer habitat use. We evaluated cedar regeneration abundance, tree height, and various gap, stand, and competition metrics. We found that deer browsing pressure greatly limited cedar regeneration; however, when deer populations decreased, cedar regeneration abundance increased within a decade, even after prolonged browsing pressure, and increased further over time. Our study illustrates that cedar regeneration can be favoured by gap creation and deer population control.
Northern white cedar (Thuja occidentalis L.) is a species of high ecological and economic value whose abundance has been declining since the pre-industrial period. It is an important element of white-tailed deer (Odocoileus virginianus Zimmerman) habitat, but its regeneration can be compromised by excessive browsing. This situation is especially critical in deeryards, where deer concentrate during winter. In 2018 and 2019, deer culling operations were carried out by the Ministry of Forests, Wildlife and Parks of Québec over 400 km2 in response to the occurrence of chronic wasting disease cases on a red deer farm. This operation offered an opportunity to look at how variations in deer pressure influence cedar regeneration and how cedar responds to a sudden reduction in browsing. We conducted regeneration surveys within and outside mapped deeryards both in the deer reduction zone and in a control zone. We performed dendrochronological analyses of cedar seedlings and saplings to quantify radial growth response to a reduction in browsing pressure. The results show that cedar basal area influences the abundance of small seedlings but that the effect of browsing becomes dominant for seedlings taller than 50 cm. Cedar growth responds to a reduction in browsing, but a two-year period was not sufficient to translate into changes in regeneration structure. The duration of the windows of opportunity at low browsing pressure required for cedar to reach a safe height remains to be determined. However, canopy openness had a significant influence on growth, suggesting that silvicultural measures could be taken to shorten the period of vulnerability to deer browsing.
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