Working Paper 31: Climate Change Impacts on Bark Beetle Outbreaks and the Impact of Outbreaks on Subsequent Fires.

The Insect Bark beetles are small insects that can have profound impacts on forests. While some species of bark beetles primarily attack trees that are recently dead or dying, others attack live, vigorous trees and can cause tree mortality across extensive areas. The Southwest is home to multiple species of bark beetles (Gaylord et al. 2006, Williams et al. 2008). In southwestern ponderosa pine forests the most notable bark beetle species belong to the genus Ips or Dendroctonus. Ecological Role and Impacts Bark beetles and fire share similar roles in southwestern ponderosa pine forests. Similar to fire, bark beetles are natural disturbance agents and help with nutrient redistribution. Tree mortality from bark beetles helps with snag formation, providing vital habitat for cavity nesting birds and bats, among other wildlife. Bark beetles are a food source for other insects and birds. At low bark beetle populations, tree mortality ranges from individual trees to small groups of trees leading to gap formation and increasing forest heterogeneity. Epidemics, or outbreaks, are also part of the natural cycle. In some ecosystems, such as mountain pine beetle in lodgepole pine, the forests are homogenized after these outbreaks, helping to perpetuate lodgepole pine stands (Brown 1975). Historically, epidemics would subside due to excessively cold temperatures or because suitable host material (tree species or preferred diameter range) becomes depleted. Current bark beetle outbreaks in some forest systems are considered unprecedented (Raffa et al. 2008, Bentz et al. 2009). Recent outbreaks appear to be driven by warmer/drier climate, an abundance of overstocked and homogeneous stands caused by past management decisions, or a combination of these and other factors (Raffa et al. 2008, Bentz et al. 2010, Fettig et al. 2013). In essence, large-scale outbreaks are a result of many of the same factors driving catastrophic fires and, consequently, both disturbance agents are having impacts on ecosystems that are often outside of their historic range of variability

Working Paper 30: Impact of Forest Restoration Treatments on Southwestern Ponderosa Pine Tree Resistance to Bark Beetles

Insects can have a wide-range of both positive and negative effects on forest ecosystems. Positive impacts include serving as pollinators, creating snags for cavity nesting birds and bats, helping to increase forest heterogeneity, and aiding in decomposition and nutrient cycling. Negative impacts can range from relatively minor impacts at the individual tree level, such as reductions in growth or form defects, to landscape-level tree mortality (Coulson and Witter 1984, Raffa et al. 2008). In the ponderosa pine forests of the southwestern United States, the bark beetle has been the insect most often associated with widespread tree mortality. Restoration efforts in ponderosa pine ecosystems have the overarching goal of making these forests more healthy and resilient by reducing fuel loads and restoring ecosystem functions. Stand restoration in these ecosystems usually involves varying intensities of thinning or prescribed burning treatments or, more often, some combination of both. These same treatments may also alter tree susceptibility to disturbance events, such as insect attacks and wildfire. This working paper will highlight current research about the effects of restoration treatments on ponderosa pine tree resistance/susceptibility to bark beetles

A multi-species synthesis of physiological mechanisms in drought-induced tree mortality

Widespread tree mortality associated with drought 92 has been observed on all forested continents, and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere-atmosphere interactions of carbon, water, and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analyzed across species and biomes using a standardized physiological framework. Here we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function

A multi-species synthesis of physiological mechanisms in drought-induced tree mortality

Widespread tree mortality associated with drought 92 has been observed on all forested continents, and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere-atmosphere interactions of carbon, water, and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analyzed across species and biomes using a standardized physiological framework. Here we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function

Measuring ecosystem function: consequences arising from variation in biomass-productivity relationships

Species diversity loss is expected to alter ecosystem function, but previous work has demonstrated inconsistent relationships between these two factors. Productivity is the most common measure of ecosystem function, but given the difficulty in measuring productivity, standing biomass or change in biomass are frequently used as proxy measures. A review of the recent ecosystem-function literature revealed that 93% of studies measure productivity as biomass, thereby assuming a strong positive relationship between these two variables. We tested this assumption by measuring biomass and productivity in seagrass beds in the Gulf of Mexico. We found that the relationship between standing biomass and productivity could be positive or negative, depending on site. Change in biomass over months inconsistently underestimated short-term productivity. The relationship between biomass and productivity may depend on plant age, successional stage, or site-specific rates of tissue loss to herbivory, senescence, or disturbance. Our results suggest that if biomass continues to be used as a measure of productivity without justification, highly productive communities that typically show little change in biomass, such as healthy climax communities, will not be interpreted as such. The conflicting results of previous studies investigating the relationship between diversity and productivity may be due to differences in the inherently variable relationship between biomass and productivity at different sites and scales.C.P. terHorst, and P. Mungui