Fire, insect and disease‐caused tree mortalities increased in forests of greater structural diversity during drought

Abstract

1. Structural diversity is an emerging dimension of biodiversity that accounts for size variations in organs among individuals in a community. Previous studies show significant effects of structural diversity on forest growth, but its effects on forest mortality are not known, particularly at a large scale.2. To address this knowledge gap, we quantified structural diversity using stem structural diversity (SSD) based on both tree diameter and height. We obtained U.S. Forest Service Forest Inventory and Analysis (FIA) data from over 2400 plots across southcentral U.S. forests that have suffered a recent drought. Using data from multiple sampling times, we calculated SSD and compared the relative importance of SSD, species diversity, functional diversity and other stand attributes in determining tree mortalities caused by fire, insects and diseases. We also used FIRETEC, a physics-based fire model, to test the effect of SSD on canopy consumption by fire.3. Our results showed that (1) SSD was positively associated with tree mortalities caused by all three disturbances; (2) species richness was negatively associated with insect-and disease-caused mortalities; (3) functional diversity was negatively associated with fire-and disease-caused mortalities and (4) more phylogenetically related species had more similar mortality rates by insect and disease but not fire. Moreover, the FIRETEC model showed increasing canopy consumption by fire in stands with greater SSD.4. Together, the different tree mortalities during drought associated with SSD more consistently than the other biodiversity metrics were evaluated. Synthesis.Our results suggest that SSD could be considered in modelling forest dynamics and planning management to sustain forest health under disturbances. K E Y W O R D Sforest health, plant population and community dynamics, plant-climate interactions, plantherbivore interactions, plant-pathogen interactions, stand structure, tree mortalityThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Structural diversity is better associated with forest productivity than species or functional diversity

“…Notably, compared with the species and functional diversity, structural diversity was the only significant biodiversity metric for the biomass net change. This may occur due to that of the three biodiversity metrics, structural diversity was the only significant one for all three types of mortality (i.e., fire, insect, and disease-caused mortality, Zhai et al (2022)) and both types of biomass increment (i.e., recruitment and growth). Although species diversity significantly affected the two biomass increments, its model slopes were smaller than for structural diversity.…”

“…The increased disturbances of insects and diseases with greater structural diversity are supported by our results showing greater structural diversity in the plots with more disturbances. The increased insect disturbance could be caused by higher concentrations of insect pheromones for insect outbreaks and greater suitability for insect establishment by more substrate availability (e.g., dead wood) in structurally diverse forests (Rappa et al, 2022; Zhai et al, 2022). In addition to the recruitment, the negative effect on growth could be related to the increased disturbances and reduced light use efficiency with greater structural diversity (Binkley et al, 2010; Bourdier et al, 2016).…”

“…From a disturbance perspective, different‐sized individuals form wind barriers more fully throughout the canopy profile (blue arrows in Figure 1) that could reduce wind influences and contribute to a relatively stable microclimate of the plant community (Kovács et al, 2017), sustaining plant productivity under disturbances, for example, drought (Frey et al, 2016; Zellweger et al, 2020). However, structurally diverse forests have shown greater mortality caused by fire, insect, and disease disturbances (Zhai et al, 2022). For example, structurally diverse forests with different‐sized trees may intensify beetle outbreaks by the reduced pheromone dissipation (Figure 1; Zhai et al, 2022; see more details in the Discussion ).…”

“…However, structurally diverse forests have shown greater mortality caused by fire, insect, and disease disturbances (Zhai et al, 2022). For example, structurally diverse forests with different‐sized trees may intensify beetle outbreaks by the reduced pheromone dissipation (Figure 1; Zhai et al, 2022; see more details in the Discussion ). From a perspective of soil resource availability, niche partitioning for belowground resources occurs among different‐sized individuals due to their access to resources at different soil depths (Figure 1; Forrester, 2019), for example, larger trees often have more access to deeper soil layers (Ding et al, 2021; Giardina et al, 2018), promoting niche partitioning for soil sources.…”

Integrating forest structural diversity measurement into ecological research

“…Accordingly, FSD measurement has broad implications for science, policy, and management (Beland et al, 2019; Eitel et al, 2016; Kruys et al, 2013; Meng et al, 2016). Accurate and precise quantification of FSD is necessary for understanding environmental drivers of community organization and composition (Hakkenberg et al, 2018; Heidrich et al, 2020) including species presence, abundance, and distribution (Borcard et al, 1992; Vihervaara et al, 2015); sustainable forest management, wildlife conservation, and restoration initiatives (Almeida, Stark, et al, 2019; Garabedian et al, 2017; McNeil et al, 2023; Parker & Russ, 2004; Valbuena et al, 2020; Wales et al, 2020); detecting forest disturbance (Atkins et al, 2019; Jucker, 2021; Zhai et al, 2022) and system recovery (Almeida et al, 2021; Meng et al, 2018); characterizing habitat (Hernando et al, 2012) and microclimates (Ehbrecht et al, 2017; Zellweger et al, 2020) and mapping land cover change (Erb et al, 2018).…”