The importance of forest diversity in combating climate change
Droughts, wildfires, and insect infestations are increasingly killing off pine species across the northern temperate region as climate change progresses. However, tree survival may be enhanced by forest diversity, as growth rates are often higher in mixed stands. But whether tree defenses are also aided by diversity remains uncertain.
We tested how diversity-productivity patterns relate to growth and defense over three centuries of climate change, competition, wildfire, and bark beetle attack. Using detailed census data from a fully mapped 25.6-hectare forest dynamics plot in California, we conducted a spatially explicit, dendroecological assessment of large-diameter sugar pine (Pinus lambertiana) survival following fire reintroduction.
Our structural equation models revealed that in the historical era of frequent, mixed-severity fire (pre-1900), trees that were ultimately resistant or susceptible to post-fire bark beetle epidemics all showed similar growth and defenses. However, during the fire exclusion era (1901-2012), susceptible trees had slower growth rates. Following fire reentry in 2013, both growth and defense declined precipitously for susceptible trees, resulting in fatal bark beetle attack.
Spatial analysis showed that monodominant crowding by shade-tolerant competitors contributed to the long-term stress that prevented susceptible trees from recuperating defenses quickly after fire. For beetle-resistant trees, we found positive feedbacks between diversity, growth, and survival – trees in species-rich communities had higher pre-fire growth rates, which promoted rapid defense recuperation and helped them resist bark beetle attack.
Overall, associational resistance outweighed associational susceptibility (+8.6% vs. -6.4% change in individual tree survival odds), suggesting a relaxation effect that ultimately allowed 58% of large pines to survive. Though climate change threatens forest biodiversity, biodiversity is key to forest climate adaptation in return. Our findings demonstrate century-scale feedbacks by which forest diversity increases pine resistance and resilience to climate-amplified disturbances.
The role of forest structure and composition
Fire exclusion has created forests that are denser, more structurally homogeneous, and dominated by shade-tolerant understory species. These fire-excluded forests are more vulnerable to severe wildfires and bark beetle outbreaks, and often show reduced biodiversity. Yet, trees growing in higher diversity stands typically grow faster and have higher survival than those in monodominant stands, indicating positive diversity-productivity relationships.
Whether diversity-productivity relationships have been maintained in fire-excluded forests, and if not, whether this is a mechanism explaining higher insect-related mortality, remains unclear. A key element of positive diversity-productivity relationships is associational resistance – the observation that trees in diverse stands are often less susceptible to host-specific herbivorous insects.
The most impactful host-specific insects targeting western conifers are bark beetles. In the western US, bark beetles have killed more tree canopy area than wildfires in recent decades. Forest diversity may reduce insect attack rates by masking the visual and chemical cues relied upon by insects to find preferred hosts. Reduced insect attack is also observed due to reduced host frequency and accumulation of insect natural enemies.
Forest diversity can also influence insect success rates by moderating resource abundances via competitive and mycorrhizal interactions, therefore contributing to the tree defense capacities necessary to combat insect attack. However, monodominant and fire-excluded stands are often more susceptible to bark beetles due to higher host availability.
The importance of tree defenses
The primary line of conifer defense against bark beetles is oleoresin (resin), which creates a physical barrier to entry and contains toxic terpenoids. Resin is crucial for combating bark beetles and their fungal symbionts, and can also prevent pathogen invasion following physical damage like fire. Trees’ ability to defend against bark beetles can be compromised by water stress during drought, rendering pathways of associational resistance increasingly important as drought severity and frequency increase due to climate change.
The genus Pinus constitutively produces large amounts of resin stored throughout a network of resin ducts. This abundance of defense capacity likely reflects Pinus‘ coevolution with two of the most destructive insect genera worldwide, Dendroctonus and Ips, as well as fire. As such, Pinus is at the crux of drought, fire, and insect compound disturbances and has been declining across the northern temperate region.
Investigating diversity-productivity relationships in a fire-excluded forest
Here, we utilize a spatially explicit dendroecological dataset to test whether positive diversity-productivity relationships persist through three centuries of fire exclusion, climate change, and large-scale compound disturbance in the Sierra Nevada, California. We examine the direct and indirect effects of fire, drought, and bark beetles in a previously fire-excluded forest to:
- Parse how growth and defense differentially contribute to diversity-productivity relationships.
- Identify forest stand characteristics governing associational resistance and susceptibility.
- Quantify whether forest diversity, overall, contributed to tree survival during compound disturbance.
We focused our efforts on growth and axial resin duct production dependence on forest diversity and density for Pinus lambertiana, an iconic gymnosperm residing in historically fire-prone montane forests of the Sierra Nevada. We then assessed whether any growth or defense benefits to Pinus growing in diverse communities translated into enhanced survival during a bark beetle outbreak.
Study site and disturbance history
The study area was the Yosemite Forest Dynamics Plot (YFDP), located in Yosemite National Park, California. The YFDP is part of the Smithsonian ForestGEO network, with every tree ≥ 1 cm diameter at breast height mapped, measured, and identified.
The fire regime at the YFDP prior to Euro-American settlement was one of low- to moderate-severity fires occurring at a mean fire return interval of 30 years. The last fire to burn through the YFDP before the onset of fire exclusion was in 1900.
The YFDP experienced severe drought spanning 2012 to 2015, with the 2015 snow water equivalent at just 5% of the historical average. Coinciding with drought, the YFDP burned in September 2013 in a management-ignited backfire. Elevated bark beetle activity followed the fire, reaching incipient-epidemic levels between 2014 and 2016.
Analyzing tree growth, defenses, and survival
We revisited each tree annually from 2011 to 2019, conducting pathology exams of newly dead trees. Our immediate post-fire pathology exams in May 2014 measured direct fire effects, including crown scorch and consumption, bole scorch height, and bole consumption.
Growth differed the most between resistant and susceptible trees during the fire exclusion era (1901-2012) – trees that ultimately died from bark beetles following the 2013 fire responded poorly to fire suppression, showing less growth. Susceptible trees also showed massive growth declines following the 2013 fire.
Both resistant and susceptible trees increased resin duct density during the fire exclusion era, but susceptible trees had lower annual growth while maintaining similar duct area, resulting in higher relative duct area. In the 1-2 years post-fire, susceptible trees produced little to no growth or defenses, while resistant trees maintained high defenses independent of fire damage.
Associational resistance and susceptibility
Spatial analysis revealed that lower densities of small-diameter Abies, large-diameter Pinus, and total basal area and density during the pre-fire period were associated with Pinus post-fire survival. Higher neighborhood species richness at a 30-m scale was also associated with Pinus resistance.
Our structural equation model predicted Pinus survival with very high accuracy (94.6% specificity, 94.0% sensitivity, 94.6% total accuracy). Indirect effects outweighed direct effects – on average, associational resistance (i.e., species richness effects) increased survival by 8.6%, while associational susceptibility (i.e., Abies density effects) decreased survival by 6.4%. Trees that grew faster survived, regardless of duct area produced.
The highest bark beetle attack rates were near fire-damaged Pinus in monodominant, high Abies density neighborhoods, while the lowest attack rates were near fire-damaged trees in diverse neighborhoods. Growth rates were highest during wetter years and for trees with low local bark beetle attack rates, few Abies neighbors, and diverse neighborhoods. Defenses were highest for trees with low fire damage, low local bark beetle attack rates, few Abies neighbors, and diverse neighborhoods, but defenses were not predictive of tree survival.
Implications for forest management
Our findings demonstrate that fire-damaged Pinus resisted beetle attack if they were growing in a neighborhood with high pre-fire woody plant diversity. In contrast, fire-damaged Pinus in low-diversity areas were the most susceptible to bark beetle mortality.
Managers are increasingly reliant on wildland fire use over large areas to restore historic conditions, but it is clear that first re-entry fire does not erase the legacy effects of fire exclusion. Mechanical thinning pre-fire can promote resilience to compound stressors by decreasing fire severity and increasing resistance to drought and bark beetles. However, our findings suggest that maintaining species richness is also crucial, as it can counteract negative climate change effects through indirect pathways.
Without accounting for these indirect and interactive effects, models are unlikely to fully anticipate climate change impacts in forests. The ability of complex models like structural equation modeling to quantify indirect mechanisms will be especially useful when forecasting the future of community interactions and forest disturbances.
Our study adds to the evidence that building pine forest resilience to compound disturbances hinges on both conserving biodiversity and reducing competitor densities before fire to promote the multiple complementary pathways promoting tree survival. By demonstrating the importance of associational resistance, we highlight that the indirect mechanisms of diversity are often more important than direct effects – an insight that should guide future research and management strategies.
