Forest Water Use is Increasingly Decoupled from Water Availability Even During Severe Drought

Context: Key to understanding forest water balances is the role of tree species regulating evapotranspiration (ET), but the synergistic impact of forest species composition, topography, and water availability on ET and how this shapes drought sensitivity across the landscape remains unclear.Objectives: Our aims were to quantify (1) the effect of forest composition and topography including elevation and hillslope gradients on the relationship between ET and water availability, and (2) whether the relationship has changed over time. Methods: We used remotely sensed Landsat and MODIS ET to quantify forest ET across the Blue Ridge ecoregion of the southeastern USA. Then quantified metrics describing ET responses to water availability and trends in responses over time and assessed how these metrics varied across elevation, hillslope, and forest composition gradients. Results: We demonstrated forest ET is becoming less constrained by water availability at the expense of lateral flow. Drought impacts on ET diverged along elevation and hillslope gradients, and that divergence was more pronounced with increasingly severe drought, indicating high elevation and drier, upslope regions tend to maintain ET rates even during extreme drought. We identified a decoupling of ET from water availability over time, and found this process was accelerated at higher elevations and in areas with more diffuse-porous trees. Conclusions: Given the large proportion of forests on the landscape distributed across high elevation and upslope positions, reductions in downslope water availability could be widespread, amplifying vulnerability of runoff, the health of downslope vegetation, and aquatic biodiversity.

Shifts in Forest Species Composition and Abundance under Climate Change Scenarios in Southern Carpathian Romanian Temperate Forests

The structure and functioning of temperate forests are shifting due to changes in climate. Foreseeing the trajectory of such changes is critical to implementing adequate management practices and defining long-term strategies. This study investigated future shifts in temperate forest species composition and abundance expected to occur due to climate change. It also identified the ecological mechanisms underpinning such changes. Using an altitudinal gradient in the Romanian Carpathian temperate forests encompassing several vegetation types, we explored forest change using the Landis-II landscape model coupled with the PnET ecophysiological process model. We specifically assessed the change in biomass, forest production, species composition and natural disturbance impacts under three climate change scenarios, namely, RCP 2.6, 4.5 and 8.5. The results show that, over the short term (15 years), biomass across all forest types in the altitudinal gradient will increase, and species composition will remain unaltered. In contrast, over the medium and long terms (after 2040), changes in species composition will accelerate, with some species spreading (e.g., Abies alba Mill.) and others declining (e.g., Fagus sylvatica L.), particularly under the most extreme climate change scenario. Some forest types (e.g., Picea abies (L.) karst forests) in the Southern Carpathians will notably increase their standing biomass due to climate change, compared to other types, such as Quercus forests. Our findings suggest that climate change will alter the forest composition and species abundance, with some forests being particularly vulnerable to climate change, e.g., F. sylvatica forests. As far as productivity and forest composition changes are concerned, management practices should accommodate the new conditions in order to mitigate climate change impacts.

Pathways of forest savannization in a mesic African savanna-forest mosaic

Typically, savannas experience frequent fires, which limit tree cover and promote flammable grass accumulation, whereas forests form dense canopies that exclude fires by reducing C4-grass fuel loads and creating a humid microclimate. However, extreme fires occasionally burn into forests. Although these are known to kill forest trees and can make repeat fires more likely, the long-term consequences of an extreme fire event for forest structure and potential forest savannization remain largely unknown. Here, we analysed whether an extreme fire event could, alone, alter forest species composition, vegetation structure, and fire regimes, or whether successive fires after an initial extreme fire event were necessary to trigger a biome transition. We found that forests that only burned once recovered, whereas those that burned again following an initial extreme fire event underwent a transition from closed-canopy forests to open, grassy systems. Our results suggest that, while extreme fires set the stage for a biome transition, subsequent fires are necessary for flammable grasses to colonise and establish, ultimately resulting in a savanna fire regime.

The Potential Effect of Pests on Forest Fire: Flammability of Mongolian Pine Bark with Resinosis on Boles

Wildfires and pests are natural disturbance agents in many forest ecosystems that often contribute to ecological succession, nutrient cycling, and forest species composition. Mongolian pine (Pinus sylvestris var. mongolica) is a coniferous species that plays an important role as an ecological barrier, and is widely spread in northern China. Its wood is loose; its branches, leaves, and cones contain a high level of resin and volatile oils that make the species highly flammable and the stands dominated by the species are very vulnerable to fire. Recently, resinosis on boles of Pinus sylvestris became an epidemic in China. To explore the potential effects of pests on fire, we compared the flammability of Mongolian pine barks with or without resinosis on boles using a cone calorimeter and several combustion analyses. We found that the barks from boles with resinosis had a greater oil content than the healthy trees. The study also indicated that the ignition times of the barks from boles with and without resinosis were 6.00 s (±1.73) and 22.67 s (±1.15), respectively, and that the heat release rate curves were parabolic, with peaks 225.19 and 75.27 kW/m2, respectively, for the two bark types. Additionally, because resinosis was on the low- to mid-bole of infested trees, the barks from boles with resinosis tended to be ignited much easier than those without resinosis. This clearly evidenced that pests could affect fire severity and behavior by increasing forest flammability. More information about the role that pests play in the different forest cover types is needed to increase our understanding of fire danger and to develop sound forest management policies.

Mapping forest areas threatened by fires and windthrows (on the example of the Ural territory)

The authors discuss the methods and results of mapping the forest susceptibility to wildfires and windthrows on the example of the Ural region. We used the previously published database of fire-and wind-related forest damages in the Ural region for 2000–2016 as input data. The method of mapping is based on the analysis of the relationships of fire- and wind-damaged area with forest species composition, landscape and climatic variables, and with some indicators of anthropogenic development of the territory. The predominant forest species make the main factor determining the exposure to wildfires and windthrows. So, the calculations were performed separately for forests with various predominant species. As a result, the maps of forest susceptibility to wildfires and windthrows were created for the entire territory of the Ural, Perm region and separately for the Krasnovishersk district of the mentioned region. The obtained estimates can be used both in forestry planning and improving the monitoring of wildfires and windthrows.

Norway spruce responses to drought forcing in area affected by forest decline

Aim of study: To assess the crown condition and radial growth of Norway spruce on plots with an increasing frequency and strength of drought during the last decades.Area of the study: Northern Moravia, Czech Republic.Material and methods: Crown condition assessment and dendrochronology analysis were used.Main results: Tree-ring width was significantly influenced mainly by previous autumn and current summer climate. Temporal variability of growth-climate relationship shows that the impact of water sufficiency (precipitation, relative soil water content, drought index) markedly increased mainly during the 2000s and the 2010s. Most of climate-growth relationships were significant only in the last two or three decades. The observed crown condition and their relationships with TRW also indicate stress intensification during the same period. Our results suggest that the water availability was the main factor affecting radial growth, occurrence of negative pointer years and probably also the factor triggering the decline.Research highlights: In these current site and climate conditions, silviculture of Norway spruce is extremely risky in the study area. Our results have also shown that the observed climate change is too dynamic for the long-term forest plans, especially as regards their recommended forest species composition.