Jane R. Foster

Evidence for a recent increase in forest growth is questionable

In a recent article, McMahon et al. (1) examined forest-plot biomass accumulation across a range of stands in the mid-Atlantic United States and suggest that climate change and trends in atmospheric CO2 explain an increase in forest growth. To show this increase, they fit a simple model to live above-ground forest biomass (AGB) as a function of stand age, and then propose that the derivative of this model is the expected rate of ensemble biomass change (). They conclude that biomass changes within census plots that exceed the ensemble expectation constitute recent increases in growth rates.

Xylem traits, leaf longevity and growth phenology predict growth and mortality response to defoliation in northern temperate forests

Defoliation outbreaks are biological disturbances that alter tree growth and mortality in temperate forests. Trees respond to defoliation in many ways; some recover rapidly, while others decline gradually or die. Functional traits such as xylem anatomy, growth phenology or non-structural carbohydrate (NSC) storage could explain these responses, but idiosyncratic measures used by defoliation studies have frustrated efforts to generalize among species. Here, I test for functional differences with published growth and mortality data from 37 studies, including 24 tree species and 11 defoliators from North America and Eurasia. I synthesized data into standardized variables suitable for numerical models and used linear mixed-effects models to test the hypotheses that responses to defoliation vary among species and functional groups. Standardized data show that defoliation responses vary in shape and degree. Growth decreased linearly or curvilinearly, least in ring-porous Quercus and deciduous conifers (by 10-40% per 100% defoliation), whereas growth of diffuse-porous hardwoods and evergreen conifers declined by 40-100%. Mortality increased exponentially with defoliation, most rapidly for evergreen conifers, then diffuse-porous, then ring-porous species and deciduous conifers (Larix). Goodness-of-fit for functional-group models was strong (R2c = 0.61-0.88), if lower than species-specific mixed-models (R2c = 0.77-0.93), providing useful alternatives when species data are lacking. These responses are consistent with functional differences in leaf longevity, wood growth phenology and NSC storage. When defoliator activity lags behind wood-growth, either because xylem-growth precedes budburst (Quercus) or defoliator activity peaks later (sawflies on Larix), impacts on annual wood-growth will always be lower. Wood-growth phenology of diffuse-porous species and evergreen conifers coincides with defoliation and responds more drastically, and lower axial NSC storage makes them more vulnerable to mortality as stress accumulates. These functional differences in response apply in general to disturbances that cause spring defoliation and provide a framework that should be incorporated into forest growth and vegetation models.

Variable effects of climate on forest growth in relation to climate extremes, disturbance, and forest dynamics

Changes in the frequency, duration, and severity of climate extremes are forecast to occur under global climate change. The impacts of climate extremes on forest productivity and health remain difficult to predict due to potential interactions with disturbance events and forest dynamics-changes in forest stand composition, density, size and age structure over time. Such interactions may lead to non-linear forest growth responses to climate involving thresholds and lag effects. Understanding how forest dynamics influence growth responses to climate is particularly important given stand structure and composition can be modified through management to increase forest resistance and resilience to climate change. To inform such adaptive management, we develop a hierarchical Bayesian state space model in which climate effects on tree growth are allowed to vary over time and in relation to past climate extremes, disturbance events, and forest dynamics. The model is an important step toward integrating disturbance and forest dynamics into predictions of forest growth responses to climate extremes. We apply the model to a dendrochronology data set from forest stands of varying composition, structure, and development stage in northeastern Minnesota that have experienced extreme climate years and forest tent caterpillar defoliation events. Mean forest growth was most sensitive to water balance variables representing climatic water deficit. Forest growth responses to water deficit were partitioned into responses driven by climatic threshold exceedances and interactions with insect defoliation. Forest growth was both resistant and resilient to climate extremes with the majority of forest growth responses occurring after multiple climatic threshold exceedances across seasons and years. Interactions between climate and disturbance were observed in a subset of years with insect defoliation increasing forest growth sensitivity to water availability. Forest growth was particularly sensitive to climate extremes during periods of high stem density following major regeneration events when average inter-tree competition was high. Results suggest the resistance and resilience of forest growth to climate extremes can be increased through management steps such as thinning to reduce competition during early stages of stand development and small-group selection harvests to maintain forest structures characteristic of older, mature stands.

Xylem traits and growth phenology predict growth and mortality response to defoliation in temperate forests

Defoliation outbreaks are biological disturbances that alter tree growth and mortality in temperate forests. Trees respond to defoliation in many ways; some recover rapidly, while others decline gradually or die. These differences may arise from species functional traits that constrain growth such as xylem anatomy, growth phenology or non-structural carbohydrate (NSC) storage, but this has not been shown. Although many studies address these phenomena, varied and idiosyncratic measures limit our ability to generalize and predict defoliation responses across species. I synthesized and translated published growth and mortality data into consistent standardized variables suitable for numerical models. I analyzed data from 32 studies, including 16 tree species and 10 defoliator systems from North America and Eurasia, and quantitatively compared responses to defoliation among species and tree functional groups using linear mixed-effects models. Relative growth decreased linearly or curvilinearly as defoliation stress accumulated across species. Growth decreased by only 5-20% following 100% defoliation in ring-porous Quercus, whereas growth of diffuse-porous hardwoods and conifers declined by 50-100%. Mortality increased exponentially with defoliation, more rapidly for Pinus and diffuse-porous species than for Quercus and Abies. Species-specific mixed models were best (R2c = 0.83-0.94), yet functional-group models lost little in terms of goodness-of-fit (R2c = 0.72-0.92), providing useful alternatives when species data is lacking. These responses are consistent with functional differences in wood growth phenology and NSC storage. Ring-porous spring xylem growth precedes budburst. Defoliators whose damage follows foliar development can only affect development of later wood. Growth of diffuse-porous and coniferous species responds more drastically, yet differences in NSC storage make them more vulnerable to mortality as stress accumulates. Ring-porous species resist defoliation-related changes in growth and mortality more than diffuse-porous and coniferous species. These findings apply in general to disturbances that cause spring defoliation and should be incorporated into forest vegetation models.