Cutting Trees and Cutting Corners

by Patrick Shore

Around the world, deforestation is exacerbating climate change and disrupting the lives of people living in forested areas. Since the Paris climate meetings in 2015, multiple schemes and policies have been created to protect more forested areas around the world and to help forest residents displaced by deforestation. While these schemes seem mostly beneficial and functional on paper, they are well short of ideal. For example, in Madagascar have revealed that that the World Bank compensation funds intended for displaced people, are not reaching a large portion of them. The primary receivers of the money are the people easiest to reach physically who also tend to be wealthier and more well-connected; poverty-stricken people living in the deeper regions of forests where much of the logging is done are least likely to receive funds. Continue reading

How a Crucial Tropical Forest is Responding to Climate Change

by Pushan Hinduja

How are Mangrove forests throughout tropical areas of the world responding to the rising sea levels attributed to climate change? Daniel M. Alongi, of the Australian Institute of Marine Sciences, analyzed historic responses to changes in sea levels in Mangrove forests as well as current data to determine how well these forests are reacting to the climate crisis (Alongi 2015). Mangrove forests tend to occupy the border between land and sea in low latitudes, making them especially susceptible to the effects of climate change. Fortunately for mangroves, they have an outstanding ecological stability, in part due to their large subterranean storage capabilities. However, despite responses to develop resilience to environmental disturbances, mangrove forests are still suffering. In terms of human impact, mangrove forests are being deforested at a rate of 1-2% per year, leaving only about a century before these forests disappear entirely. Mangroves are crucial to the environment; they serve as breeding and nursery grounds for fish, birds and other animals, prevent erosion and damage from natural disasters like tsunamis, serve as a renewable source of wood for fuel, and are key components in filtering ocean contaminants. Continue reading

Large Forest Blocks are Essential for Biodiversity Protection and Carbon Storage

by Stephen Johnson

Habitat loss is the primary threat to the survival of most tropical biodiversity. Typically, this habitat loss is driven by deforestation for agricultural use. However, deforested landscapes are rarely homogenous fields with low diversity; most often, forest fragments are left embedded in a matrix of varying types of agriculture, from open field monocultures, to pastures and forest-mimicking shaded plantations. The process of fragmentation has a significant negative effect on the biodiversity present in the area; however, fragments are often able to support a variety of species, as are some types of agriculture, such as agroforestry. Less is known about the capacity of such landscapes to sequester and store carbon. What little has been done has focused on carbon in agroforestry systems, with promising, though mixed, results. Continue reading

Deforestation Restrictions: Observations from Acre, Brazil

by Lazaros M. K. Chalkias

In light of climate change and species conservation efforts, “Reductions in Emissions from Deforestation and Degradation” (REDD) is becoming an increasingly important mechanism in conservation policy. Deforestation may affect water quality, forest services and local economies; its impact, as Pfaff et al. (2014) explore, depends on governance and location of protected forest areas. The researchers’ work focuses on the forests of Acre, Brazil, which includes over 1 million hectares of protected areas. They evaluated deforestation in the periods of 2000–2004 and 2004–2008, and separated the protected forest areas in question in three categories (sustainable use, indigenous and integral). They used “remotely sensed pixel data” from the INPE (Instituto Nacional de Pesquisas Espaciais) to examine covered and cleared forestland and understand the potential effects of policy in a region. Continue reading

Analyzing the Vulnerability of Rainforest Birds to Deforestation

by Maithili Joshi

In South East Queensland, Australia Pavlacky et al.(2014) conducted a study on the vulnerability of birds, rainforest ecosystems, and the biological impacts in response to deforestation in local and regional areas. The central idea is the to investigate the life history and forest structure to rank the vulnerability of avian species, while also looking at species loss along different kinds of forest structure and landscape change. The objectives are evaluating the effects of life history traits on the patch occupancy and vulnerability of rainforest birds, determining the relative effects of stand, landscape, and patch structure on species richness, and evaluating the relative contributions of deforestation and fragmentation to species richness. Continue reading

Implications of Land-Cover Changes and Fragmentation For Biodiversity Conservation

by Maithili Joshi

Deforestation can have substantial impacts on the vast biodiversity within tropical rainforests. In Hainan, China, importance is placed on trying to protect the habitat and biodiversity in the natural forests, specifically in the Changhua watershed. The Changhua watershed is an important area for China because it has been identified as the “center of endemism for plants and birds”, so conserving this area is particularly important for maintaining biodiversity. In the last few years, biodiversity has been threatened by new rubber and pulp plantations causing forest fragmentation and larger patch distances. In this study, Zhai et al. (2014) looked at the implications of deforestation on biodiversity, especially of endemic species and the ecosystems surrounding using land cover data. Continue reading

More Benefits of Reversing Deforestation than Meet the Eye

by Caroline Chmiel

A seemingly simplistic method to battling rising temperatures may be one of the most effective. Saving tropical forests, largely through natural growth, has proven an immensely important and promising strategy to limit climate change impacts. Saving the forests that are left and allowing new ones to grow, or regrow, will impact our planet in many positive ways. Forests play a huge role in the carbon cycle of Earth because trees pull main greenhouse gases, CO2, out of the air and lock carbon away in wood and in soil beneath them. When forests are destroyed, typically through burning, CO2 is pumped back into the air, substantially contributing to raising temperatures and climate change. Burning of coal, oil and natural gas moves carbon out of the ground and into the active carbon cycle causing the globe to warm more rapidly now than in any similar period. Research displays a hopeful method for the control of CO2 cycle: if forests around the globe are reclaimed and burning comes to a halt, forests will evermore naturally help pull dangerous emissions from the air, preventing quick, out of control, temperature growth. Continue reading

Colonization Potential of Oaks under Climate Change

by Elizabeth Medford

While the impact of climate change on a variety of animal populations and their ranges has been studies extensively in the past, the study of the impact of warming on tree species also provides useful information for policymaking. A variety of different modeling systems apply different variables and make predictions about tree species distribution in the future as temperatures rise. In this study however, Prasad et al. (2013) combine two different commonly used technologies to overcome the constraint of computation time and allow assessment of colonization potential for oak species. Four oak species were chosen to focus on because they are strongly climate-driven species: black oak, post oak, chestnut oak, and white oak. Using the DISTRIB and SHIFT models together the authors were able to determine the future dominant forest types in the northeastern United States. This study determined that even under optimistic conditions ignoring some influential factors, only a small fraction of suitable oak habitat is likely to be occupied by oaks within 100 years. The authors urge that the information garnered in this study be used to inform assisted migration practices for vulnerable tree species. They additionally call for further studies focusing on how each individual species will adapt to increases in temperature. Continue reading

Trees Likely to Suffer from Water Stress with Climate Change

As global climate change increases water stress in many regions of the world, humans are not the only organisms that will be severely affected.  Trees have adapted for thousands of years to maximize their ability to gather sunlight, growing taller and taller to reach energy from the sun.  However, this also means that water must travel further from their roots to reach the extremities.  This has lead to a range of physiological functions that are highly dependant on a consistent supply of water.  Many climate change scientists predict that severe drought events may increase worldwide by 30% or more, significantly reducing the amount of moisture in the soil available for root uptake.  Extreme weather events are also likely to become more prevalent, leading to increased runoff and thus less water availability in the soil in regions affected.  Hartman (2011) predicts that droughts will lead to reduced functioning, growth and yield and in extreme cases, plant death.  He presents several hypotheses about potential reasons for water stress leading to tree mortality including carbon starvation because of stomatal closure, rupture and dysfunction of the cells of the water transport system, reduced electron availability for photosynthesis, and constrained cell metabolism.  Furthermore, water stress may cause indirect mortality increases in trees as well, sometimes many years after the actual drought event.  Trees that are irreversibly weakened may be more susceptible to death by parasites, insects, fires, and further natural disasters.  If these predictions are true, increases in temperature will likely change ecosystem composition, as some will be able to adjust better than others.  It remains to be seen if an evolutionary selection for taller plants may now be a deadly disadvantage in this era of change.
Hartman, H., 2011.  Will a 385-million year struggle for light become a struggle for water and carbon? — How trees may cope with more frequent climate change drought events.  Global Change Biology 17, 642-655. 

At first, it may seem surprising that water stress could impact such a hardy and diverse group of organisms.  Trees survive in some of the most adverse climates in the world, from the freezing northern regions to the driest deserts.  However, in order to adapt to these inhospitable regions, species have had to evolve for thousands of years.  The rapid shifts in climate that have been occurring since industrialization and are predicted to continue exponentially will not give trees time to adapt adequately to the changing environment, and many species that cannot find coping methods may face extinction.  Compounding the severity of the issue is the fact that trees, in order to attain maximum height and gather the most possible sunlight, tend to live at the edge of hydraulic dysfunction.  Taller trees are clearly more prone to drought susceptibility than species that have evolved to have lower canopies, and although they also tend to have deeper root systems this is often not enough to offset the difficulty of obtaining sufficient water for survival.  Individual trees can adapt to drought to some extent by shedding leaves and devoting more carbon and energy to growing longer roots, and in the long run can even develop thicker leaves with increased storage ability.  However, tall trees may respire hundreds of liters of water every day, making these adaptations inadequate in the long-term drought scenarios that are likely to become prevalent in future climate change.
In the case of increased droughts, predicted mortality from water stress may be direct or indirect.  The most widely circulated theory in the scientific community is that water stress leads to carbon starvation in trees.  Leaves have stomata (small openings on the underside of their leaves) that allow for the diffusion of carbon dioxide into the plant and the evaporation of water out, meaning that water loss from the leaves must continuously be replaced with moisture from the soil.  In times of water stress, stomata close off so that less water can escape from the plant.  While may be an effective measure to increase survival in short extreme events, during a prolonged drought this adaptation may be fatal, as no carbon dioxide can enter the plant with the stomata closed and the tree effectively “starves” to death. 
Another common hypothesis states that the cells of a tree’s vascular system, called xylem, may not be able to physically withstand the pressures of water stress.  Because trees need  a constant supply of water, they have a complex transport system that draws water from the roots up through a water column and into the vascular system of the plant.  As water evaporates from the leaves and diffuses into the atmosphere, water potential is increased in the tree compared with the surrounding atmosphere.  Water uptake by the roots passes into the water column, where capillary and adhesive forces draw water molecules upward into the low-potential leaves.   However, when the roots draw water out of the soil they temporarily reduce the moisture content surrounding them, and if this moisture is not replaced through precipitation the soil dries out and air replaces water in the soil pores.  There is thus a harder “pull” on the vascular water column to overcome the increase in adhesive force created between water and soil particles.  This in turn creates negative pressure in xylem, which can cause embolisms and water column ruptures and result in partial or complete loss of water conductance through the tree over time.  Fortunately, if the tree manages to survive the drought, xylem can be repaired once water is available and the vessels can be refilled.  The consequences of this mechanism also tend to be less fatal for the tree as a whole, as it can sacrifice twigs and leaves and maintain its core if only a part of its water system is ruptured.
            Photosynthesis, which is essential for growth and survival of an individual tree, relies on the electrons supplied by water intake for its reactions to function.  In the case of impeded movement of water up to the crown of the tree where photosynthesis has the most energy from sunlight, this process can be disrupted and a tree may “starve.”  Finally, some scientists believe that low water potentials in tissues may constrain cell metabolism, resulting in a reduction of carbon assimilation.  As of this writing, no empirical studies have been conducted regarding the latter two hypotheses, but they are certainly probable enough to merit future study. 
            On a more optimistic note, it is possible that an increase in carbon dioxide in the atmosphere could in fact help trees as they take it up through their stomata.  In a perfect scenario, this could lead to higher growth rates and increasing water-use efficiency even in times of drought.  However, if the elevated atmospheric carbon dioxide concentration makes leaves grow larger and the water efficiency does not compensate adequately, trees could be even more severely exposed to drought stress.  In addition, different species have varied stomatal responses to higher carbon dioxide, so it is difficult to generalize this theory.
            Diverse species may also have differing responses in regards to the other hypotheses as well.  Isohydric tree varieties close their stomata well in advance of any danger to the xylem, which can be helpful in the case of short, extreme events but deadly in a prolonged drought because of carbon starvation.  Anisohydric species, on the other hand, close their stomata only when in immediate risk of hydraulic failure, making them less prone to carbon starvation but vulnerable to water transport issues and xylem rupture.  Gymnosperms and angiosperms also differ in their vulnerability to water stress.  Angiosperms have more conductive xylem, but their vessels require more carbon input to function as compared with gymnosperms.  Angiosperms, therefore, are more at risk of carbon starvation, while gymnosperms are more likely to suffer from transportation challenges during droughts.  Although the specific challenges differ between species, all types will likely be negatively impacted in some way by global climate change and increased water stress. 
If increased water stress does lead to a decline in trees in some regions of the word, this will in turn affect humans powerfully.  Trees are worth trillions of dollars a year, and there cannot be a price placed on the quality of life that they provide.  In addition, trees are a major component of earth’s carbon cycle, making up about 90% of earth’s terrestrial biomass and cycling 8% of atmospheric carbon dioxide annually.  If the health of trees suffers from climate change, there are few organisms on earth that will not be affected.
            Clearly, an issue that will affect so many people and organisms so powerfully is a topic that deserves further research.  Unfortunately, tree mortality studies are particularly difficult to conduct because it can take years (and sometimes even decades) to gather enough data to draw any sort of viable conclusions.  This is something that scientists will have to work with, either through developing sophisticated modeling or obtaining support that will allow for such time consuming studies to be undertaken.  One important avenue of research is to determine which of the above hypotheses is most likely to be the cause of tree mortality in water stress events.  Future studies must also look at different regions, environments, and species in order to determine how various trees might be affected differently by climate changes.   A diversified study will also be essential to facilitate a full understanding because as patterns of precipitation change, different regions of the world will experience climate change differently.   What will likely be universal is a strong impact from global climate change on tree species around the world, and scientists must be prepared for the results of such a possibility. 

Drought Effects on Damage by Forest Insects and Pathogens: a Meta-Analysis

Climate change may decrease summer precipitation and increase winter precipitation across the Northern Hemisphere, resulting in dire consequences for forest ecosystems since summer drought damages tree growth and forest ecosystem functioning. In addition, prolonged droughts may also trigger more frequent or severe outbreaks of forest insects and pathogen epidemics, and these events could interact with carbon starvation or hydraulic failure to further increase rates of tree mortality. Moreover, there is considerable variation in the magnitude and direction of responses to water stress by pathogens and insects. Therefore, to draw general conclusions about tree drought-damage, insect-damage, and pathogen-damage relationships, Jactel et al. (2012) conducted a meta-analysis of published primary studies that addressed the impact of water stress on forest pest and pathogen damage. More specifically, the authors estimated the overall effect of water stress on insect pest and fungal damage in forest trees and investigated the variation of response to water stress among functional groups of pests and pathogens. They also explored the relationship between the magnitude of pest or pathogen damage and the severity of drought. Jactel et al. found that primary damaging agents living in wood caused lower damage to water-stressed trees, while primary pests and pathogens living on foliage caused more damage to water-stressed trees, in all cases irrespective of stress severity. Damage by secondary agents increased with stress severity. Overall, insect and fungus feeding behavior, affected tree part, and water stress severity were the three main predictors of forest damage in drought conditions.—Megan Smith
Jactel, H., Petit, J., Desprez-Loustau, M.L., Delzon, S., Piou, D., Battisti, A., Koricheva, J. 2012. Drought Effects on Damage by Forests Insects and Pathogens: a Meta-Analysis. Global Change Biology 18: 267 – 276. DOI: 10.1111/j.1365-2486.2011.02512.x

Numerous textbooks have described the variable responses of forest pests to tree water stress, most of which is related to insect feeding guild. Generally, bark beetles and woodborers perform better under severe drought scenarios, while sapsuckers also benefit from water-stressed trees under moderate drought conditions. The effect of drought on leaf miners, leaf chewers, and gall makers is uncertain. Pathogenicity may also be enhanced or reduced with increased drought. Furthermore, the duration and severity of water stress influences insects’ and pathogens’ responses to drought. One scientific study found that infections are more likely to develop during or after prolonged drought stress.
To draw general conclusions about the diverse drought-damage relationships between pathogens, insects, and trees, the authors conducted a meta-analysis of published primary studies that investigated the impact of water stress on forest pest or pathogen damage. Meta-analysis is a set of statistical tools that combines the outcomes of independent studies to evaluate the overall effect of a particular factor. It also tests the influence of covariates on this effect.
For their meta-analysis, the authors collected published studies that compared pest or disease damage on water-stressed vs. control trees. Studies were included in the analysis if they met specific criteria. One criterion stated that the study must have assessed tree damage caused by an insect or fungal pathogen. Damage variables included measures that quantified impact on tree survival or tree growth by recording the amount of damaged or consumed tree tissues, the number of attacks per tree, or the percentage of infested or killed trees. Studies were also included if they reported any insect and fungal species that affected tree tissues or organs. The second criterion stated that the mean response variable (tree damage), a measure of the variance, and the sample size for both control and drought treatments must have been reported to be included in the meta-analysis. The third criterion affirmed that water conditions in the control and stressed group of trees must have been quantified using predawn leaf water potential with a pressure chamber. This ensured that two groups of trees were under different water supply conditions and that the methodology of water stress assessment was consistent across studies. Predawn leaf water potential values were used as indicators of water stress severity. Finally, the fourth criterion stated that the reported paired comparison between water-stressed and unstressed (control) trees must have been made under the same environmental conditions (besides water supply), on the same date and in the same area.
The effect of water stress on forest insect and disease damage was estimated by calculating Hedges’ das a measure of the effect size. A positive dvalue indicated higher damage on water-stressed trees than on control trees. The authors selected the one variable per comparison between water-stressed and unstressed trees that had the largest sample size or allowed the highest number of paired comparisons. Data was only used from the first year of the studies and only data from the first application of water treatments on the trees were utilized. If results were reported for 2 years but from two different, independent tree samples, data for each year were used as two separate comparisons.
Jactel et al. quantified water stress severity with four variables. The first two were calculated with the information provided in the retained papers as the difference or ratio between the mean predawn leaf water potential in water-stressed and control trees. The higher the absolute value of predawn leaf water potential, the more water-stressed the tree was. The other two variables represented the hydraulic failure in the trees. These variables were calculated as the difference or ratio between the mean predawn leaf water potential in water-stressed trees and the xylem (the vascular tissue in plants that conducts water and nutrients upward from the root) pressure inducing 50% loss in hydraulic conductance (P50) due to cavitation in the same tree species. P50, as a representation of cavitation resistance, is highly variable between species and correlated with plant drought tolerance (lethal water stress).
The authors split the dataset into subsets of different functional groups of insects or fungi depending on their feeding substrate, and separated insects or fungal species colonizing foliar organs involved in photosynthetic processes (leaves, needles) vs. those living in woody organs responsible for tree structure (bark, wood, roots). Then, they distinguished between insect and fungal species that develop on healthy trees (primary agents) and those that utilize trees in poor physiological conditions (secondary agents). Jactel et al. assumed that there would be four functional groups, but they were unable to find examples of secondary agents that damage foliar organs. Therefore, the study only included three functional groups. Species were also classified based on their trophic guild: chewing, boring, sucking and galling insects, leaf pathogens, root and bark rot, blue-stain fungi, and endophytes.
Studies were categorized as observational or experimental depending on whether the drought was caused by natural conditions or controlled water supply. Additionally, the authors distinguished between comparisons made in the field and those made in protected conditions in absence of natural enemies.
Effect sizes across all comparisons were combined using the random effects model to yield the grand mean effect size (d++). The effect was considered significant if the bootstrap confidence interval, calculated with 9999 iterations, did not include zero. The mean effect size (d+) and 95% bias-corrected bootstrap confidence interval were calculated for each functional group of forest insect or fungi combining affected tree organ and its physiological status. A mixed effect model was used to test the between class heterogeneity and to test for significance of class effect. A P value of 0.001 was used to test for statistical significance. A mixed model was also used to test the relationship between the differences in damage on stressed vs. control trees (effect size) and severity of water stress (continuous variable).
Finally, the authors calculated a fail-safe sample size that represented an estimate of the number of non-significant, unpublished, or missing studies that would need to be added to the analysis to make the overall test of an effect statistically non-significant. After, testing, the authors found that their results were unlikely to be affected by publication bias.
Jactel et al. derived 100 comparisons of forest pest and disease damage on water-stressed vs. unstressed trees from 40 publications and reports. They involved 27 insect and 14 fungus species. A total of 26 tree or shrub species were studied. Overall, water stress resulted in higher forest pest and disease damage. The grand mean effect size equaled 0.23 and was significantly different from zero. However, an effect size of 0.2 is considered a small effect. Additionally, 40% of the individual effects were also negative, indicating lower damage in water-stressed trees than un-stressed trees. A graph displaying Hedges’ d effect size of 100 individual studies was constructed. Negative effect sizes indicated that drought resulted in lower damage.
The type of trophic substrate used by forest pest and pathogens had a highly significant effect on the difference in damage between water-stressed and unstressed trees. Primary damaging agents living on foliar organs caused higher damage in water-stressed trees than un-stressed trees, irrespective of stress severity. Drought did not exacerbate damage caused by primary agents that developed on woody organs. However, it did increase damage caused by secondary agents that developed on woody organs. A table displaying the mean Hedges’ effect size per functional group of forest pest and pathogen was constructed.
The effects of trophic guild were never significant within each functional group of forest pests and pathogens. Damage caused by sucking and boring insects and root and bark rot fungus species developing in woody organs in healthy trees (primary agents) was not worsened by drought. Drought resulted in slightly higher damage caused by leaf pathogens living on foliar organs in healthy trees and galling and chewing insects. Additionally, endophytic fungi damage increased with drought, but the results were not statistically significantly different from zero for boring insects and blue-stain fungi. Contrary to previous studies, these results suggest that the effect of water stress on the level of damage by forest pests and pathogens depends more on the type of substrate they use rather than on their feeding guild. A table displaying the effects of drought on mean effect size (damage) by different types of forest pest and pathogens was constructed.
After testing the effect of the type of water stress application on level of damage for each functional group of forest pests and pathogens separately, the authors found that there was no significant difference in mean effect size between observational and experimental studies. There also was no difference between studies made in the field or in protected conditions.
Finally, Jactel et al. tested the effect of water stress severity on level of damage for each functional group of forest pests and pathogens separately. Water severity did not affect the level of damage in water-stressed trees for any primary damaging agent. However, water stress severity did affect the level of damage caused by secondary agents living in woody organs. The variable that best explained damage variation was the ratio between observed predawn leaf water potential in the stressed trees and the species-specific index of drought tolerance (P50). Damage was consistently higher in stressed trees with a predawn leaf water potential higher than 30% of P50. Below 30%, water-stressed trees were more likely to have less damage than unstressed trees. Both secondary fungi and insect species living in woody organs similarly affected the water-stressed and unstressed trees (there was no significant difference between the two groups). A graph displaying the relationship between levels of damage (effect size) made by secondary forest pests and pathogens living on woody organs and water stress severity was constructed.
Previous studies have found that insect performance response to water stress depends on their feeding traits. Borers are known to perform better on stressed plants while gall makers and leaf chewers are negatively affected. However, one study found that sapsuckers benefited from drought while another study’s results contradicted these findings. Furthermore, drought is thought to negatively affect forest pathogens because fungi require high humidity conditions for spore dispersal, germination, and infection. Overall, the combination of effects on the performance of the biotic agent and effects on tree response could explain discrepancies in the results between different studies.
Drought can affect the nutritional quality of host trees for herbivorous insect and fungal pathogens through changes in water, carbohydrates, and nitrogen contents. Water supply greatly influences carbohydrate photosynthesis and therefore the provision of sugars for insects and parasitic fungi. As a result of drought, the reduced concentration of carbohydrates in conifer bark tissues may reduce the development of bark beetles and blue stain fungi. Furthermore, reduced water content and protein hydrolysis lead to higher nitrogen concentration in tree organs during drought. Since nitrogen is a limiting nutrient for many insects, an increase in plant nitrogen during water stress could improve the performance of phytophagous insects. For example, defoliator performances are higher in moderately water-stressed trees due to higher concentration of soluble nitrogen in foliage. Sap feeding insects also benefit from an increase in nitrogen.
Moreover, amino acids can be found in increased concentrations in water-stressed trees, stimulating the growth of bark canker fungus. As a result, the concentrations of carbohydrates and nitrogen decrease in the stem of the tree under moderate stress. This would limit the performances and then the damage of primary pests living on woody organs. Performance and damage made by primary pests living on foliar organs (which benefit from higher nitrogen content) would increase.
Water-stress also affects host metabolism involved in resistance to pest and pathogen damage. Tannins utilized in tree resistance can be found in higher concentrations in foliage of water-stressed trees, which deter leaf chewers such as beetles and lepidopteron. In contrast, resistance mechanisms may also be less effective in water-stressed trees.
Lower water supply affects sap flow and oleoresin production and pressure, which results in lower resistance to primary attacks of many bark beetles. Infection of pathogenic blue-stain fungi by scolytids leads to the development of necrotic lesions containing concentrations of terpenoid and phenolic chemicals that are toxic to insects and fungi. However, water-stressed trees lack the carbohydrates reserve to fuel the secondary metabolism involved in these resistance processes. Therefore, severely water-stressed trees are likely to be more damaged by secondary pest and pathogens like wood boring insects and blue-stain fungi.
Conversely, moderate water stress could lead to increased resistance. Because a tree’s carbohydrate pool still increases under moderate water stress, the tree may instead allocate its carbohydrates to the synthesis of defensive secondary chemicals rather than to growth and development. Secondary pests living in woody organs, like bark beetles, would then cause less damage in moderately water-stressed trees.
Additionally, decreased water content in severely stressed trees could lead to tougher foliage, resulting in lower herbivory by chewing insects.
Overall, the authors’ results confirm that drought does not systematically result in higher biotic damage. Two factors that explained the tree damage response to drought were type of feeding substrate for forest insect and pathogens and water stress severity.