Tree Diversity Drives Forest Stand Resistance to Natural Disturbances

Purpose of review Forests are frequently exposed to natural disturbances, which are likely to increase with global change, and may jeopardize the delivery of ecosystem services. Mixed-species forests have often been shown to be more productive than monocultures, but it is unclear whether this results from mixed stands being in part more resistant to various biotic and abiotic disturbance factors. This review investigates the relationships between tree diversity and stand resistance to natural disturbances and explores the ecological mechanisms behind the observed relationships.Recent findings Mixed forests appear to be more resistant than monocultures to small mammalian herbivores, soil-borne fungal diseases and specialized insect herbivores. Admixing broadleaves to conifers also increases the resistance to fire and windstorms when compared to pure conifer stands. However, mixed forests may be more affected by drought depending on the species in the mixture.Summary Overall, our findings suggest that mixed forests are more resistant to natural disturbances that are relatively small-scale and selective in their effect. However, benefits provided by mixtures are less evident for larger-scale disturbances. Higher tree diversity translates into increased resistance to disturbances as a result of ecological trait complementarity among species, reduction of fuel and food resources for herbivores, enhancement of diversion or disruption processes, and multi-trophic interactions such as predation or symbiosis.To promote resistance, the selection of tree species with different functional characteristics appears more important than increasing only the number of species in the stand. Trees with different levels of susceptibility to different hazards should be intermixed in order to reduce the amount of exposed resources and to generate barriers against contagion.However, more research is needed to further improve associational resistance in mixed forests, through a better understanding of the most relevant spatial and temporal scales of species interactions and to optimize the overall provision of ecosystem services

Disentangling the effects of crown scorch and competition release on the physiological and growth response of Pinus halepensis Mill. using δ 13 C and δ 18 O isotopes

Prescribed burning (PB) can decrease the likelihood of crown fires by increasing canopy base height via canopy scorching and sometimes by reducing tree density through fire-induced tree mortality, especially in fire-prone stands. However, little is known about the effect of moderate PB on tree functioning, especially in Mediterranean species such as Pinus halepensis Mill. In this study we combined dendrochronology and isotope analysis to understand the physiological effects of PB that determine the short-term post-burning growth response of crown-scorched and unscorched P. halepensis with different levels of competition release. PB was carried out in spring 2013. Scorched and unscorched pines showed higher post-burning growth rates than before PB as well as control pines. In the first year post-burning, unscorched pines had similar growth rates and δ 18 O-δ 13 C values to the control pines, which indicates that PB only had a minor impact on tree functioning. In contrast, scorched pines showed a significant reduction in growth and wood δ 13 C, but had similar δ 18 O as the unscorched and control (no-PB managed) pines. This suggests that the pines’ response mechanism to scorch was to reduce their photosynthetic capacity. At two years post-burning (2015), the growth of scorched pines was similar to control pines. Moreover, the radial growth of pines increased significantly when PB resulted in a relevant competition release and crown volume scorch was low. At two years post-burning (2015), a smaller change in δ 13 C in scorched trees compared to 2014 was found and no changes in δ 13 C in unscorched pines; however, lower δ 18 O was found in scorched and unscorched pines compared to the controls, which suggests an increase in stomatal conductance probably due to improved water availability through fire induced thinning. The increase in stomatal conductance in scorched and unscorched pines coincided with an extreme drought year (2015), which indicates that the control pines adopted a more conservative water-use efficiency. No effects of burning on needle N content or δ 15 N were detected. As a management conclusion, our study suggests that moderate PB can be implemented to disrupt the vertical continuity of fuels in crown fire prone landscapes, enhancing water availability during drought episodes with only minor effects on tree functioning

The influences of forest stand management on biotic and abiotic risks of damage

• This article synthesizes and reviews the available information on the effects of forestry practices on the occurrence of biotic and abiotic hazards, as well as on stand susceptibility to these damaging agents, concentrating on mammal herbivores, pest insects, pathogenic fungi, wind and fire. • The management operations examined are site selection, site preparation, stand composition, regeneration method, cleaning and weed control, thinning and pruning, and harvesting. For each of these operations we have examined how they influence the occurrence of biotic and abiotic damaging agents, the susceptibility of European forests, and describe the ecological processes that may explain these influences. • Overall, we find that the silvicultural operations that have the largest influence on both biotic and abiotic risks to European forest stands are closely related to species composition and the structure of the overstorey. Four main processes that drive the causal relationships between stand management and susceptibility have been identified: effect on local microclimate, provision of fuel and resources to biotic and abiotic hazards, enhancement of biological control by natural enemies and changes in individual tree physiology and development. •The review demonstrates an opportunity to develop silvicultural methods that achieve forest management objectives at the same time as minimising biotic and abiotic risk

Concepts and challenges in modelling natural disturbances in forest ecosystems

COST FP060