Demographic processes in forest trees in the Rocky Mountains

2017 Summer.Includes bibliographical references.Forests provide numerous ecological and economic services including regulation of biogeochemical cycles, fiber production, watershed protection, as well as less tangible aesthetic and recreational benefits. Forests are being substantially altered by a range of consumptive uses related to expanding human population and economies. Superimposed on other anthropogenic impacts is global climate change. Global circulation models unambiguously reveal the role of greenhouse gas forcings associated with industrial processes in driving global temperature trends (Hanson et al. 2005). Meteorological observations indicate that global mean temperature has increased by approximately 0.6 C over the past century relative to a base 1951 to 1980 period, with record high temperatures occurring in 2010. Paleoclimatic reconstructions based on proxy data indicate that modern rates of warming may be unprecedented in the context of the past 1000 years. Rates of warming are geographically heterogeneous. Temperature anomalies in the Rocky Mountain ecoregion, for example, are 2‒3 times higher than the global mean temperature increase. Some models and observational data suggest that temperature trends are elevation dependent with greater warming at high altitudes and with greater increases in daily minimum temperatures than maximum temperatures. Documented increases in minimum temperature is associated with earlier spring thaw events, driven by minimum temperatures that exceed 0 °C and a lengthening of the growing and fire seasons. In the Rocky Mountains, an altered climate system is projected to result in a higher frequency and intensity of drought events. Precipitation over the previous 100 years lacks clear trends across the region as a whole, but models of snow water equivalent (SWE) indicate declining moisture availability since the mid-20th century. Early spring snowmelt and warming driven increases in rates of evapotranspiration may correlate with reduced stream flow and declines in effective soil moisture late in the growing season. Warming temperatures and reductions in moisture availability have been associated with significant increases in area burned by wildfire in some forest systems, particularly at high elevations where climate variability rather than fuel conditions is the primary driver of fire activity. Changing climate may also be expanding the ranges and altering the dynamics of forest insects, such as the mountain pine beetle (Dendroctonus ponderosae), resulting in extensive tree mortality. The recent widespread acceptance of climate change has highlighted the need for regional and species specific adaptation strategies. However, a lack of reliable projections describing the responses of organisms and communities to climate change has been identified as a major impediment to the development and implementation of climate adaptation strategies within federal agencies. Potential vegetation responses include migration to track preferred habitats or adaptation through phenotypic or genetic plasticity. Heat stress and prolonged drought have been associated with rapid shifts in the range limits of ponderosa pine (Pinus ponderosa) and in significantly elevated rates of background tree mortality for tree species and forest environments worldwide. Mortality events associated with physiological stress or environmental disturbances may accelerate changes in the distributions of long-lived tree species that might otherwise persist in sub-optimal environments. The distribution and abundance of plants are largely determined by physiological, life history, and ecosystem processes, and how these processes interact or respond to climate. A mechanistic understanding of these processes and their physiological thresholds is required to accurately predict forest response to climate change. The 2007 Intergovernmental Panel on Climate Change working group has argued that current predictive vegetation models are limited by a failure to adequately quantify relationships between climate, critical life history processes, and disturbance regimes. The main objective of this research is to quantify life history processes for select tree species in the Rocky Mountain ecoregion. Specifically, non-linear regression models will be developed to quantify variation in both tree fecundity and growth as a function of climate variables, edaphic gradients, and competition. Comprehensive field data will be used to train flexible functions in a maximum likelihood framework. Competing models representing alternative hypotheses will be evaluated using information theory. The overarching objective of this project is to provide detailed quantitative life history information that may subsequently be used to parameterize dynamic simulation models for the prediction of forest response to alternative future climate scenarios. An additional component of this research involves the reconstruction of historical temperatures in the southern portion of the Rocky Mountain ecoregion using chronologies of radial growth from several high elevation tree species occurring in northern Colorado and southern Wyoming. Historical temperatures have been reconstructed for northern portions of the Rocky Mountain ecoregion. A comparable reconstruction for the southern portion of the region has not been developed. Global climate models predict that parts of the Rockies may experience future climates with no previous analogs. Historical temperature reconstructions based on proxy indicators will provide historical context for both modern climate variation and simulations of future conditions

The Last Trees Standing: Climate modulates tree survival factors during a prolonged bark beetle outbreak in Europe

Plant traits are an expression of strategic tradeoffs in plant performance that determine variation in allocation of finite resources to alternate physiological functions. Climate factors interact with plant traits to mediate tree survival. This study investigated survival dynamics in Norway spruce (Picea abies) in relation to tree-level morphological traits during a prolonged multi-year outbreak of the bark beetle, Ips typographus, in Central Europe. We acquired datasets describing the trait attributes of individual spruce using remote sensing and field surveys. We used nonlinear regression in a hypothesis-driven framework to quantify survival probability as a function of tree size, crown morphology, intraspecific competition and a growing season water balance. Extant spruce trees that persisted through the outbreak were spatially clustered, suggesting that survival was a nonrandom process. Larger diameter trees were more susceptible to bark beetles, reflecting either life history tradeoffs or a dynamic interaction between defense capacity and insect aggregation behavior. Competition had a strong negative effect on survival, presumably through resource limitation. Trees with more extensive crowns were buffered against bark beetles, ostensibly by a more robust photosynthetic capability and greater carbon reserves. The outbreak spanned a warming trend and conditions of anomalous aridity. Sustained water limitation during this period amplified the consequences of other factors, rendering even smaller trees vulnerable to colonization by insects. Our results are in agreement with prior research indicating that climate change has the potential to intensify bark beetle activity. However, forest outcomes will depend on complex cross-scale interactions between global climate trends and tree-level trait factors, as well as feedback effects associated with landscape patterns of stand structural diversity

Sample tree growth data with locations

This file provides measurements of annual radial growth for randomly distributed adult trees in the Rocky Mountains of the United States. Measurements were derived from increment cores collected in 2012 and 2013. A total of 5 tree species were sampled. Sample trees were distributed across a latitudinal gradient from the southern terminus of the Rockies in New Mexico to the Canadian border in the north. Target tree species include: Abies lasiocarpa (subalpine fir), Picea engelmannii (Engelmann spruce), Pinus contorta (lodgepole pine), Pinus ponderosa (ponderosa pine) and Pseudotsuga menziesii (Douglas-fir). Annual radial increment over a 20 year period was measured in millimeters for each sample tree. File headings include: Sample tree ID: Identification code for the sample or target tree Species: Species of the sample tree Year: The year of growth Site: General location of sample tree Longitude and Latitude: Geographical coordinates of sample (based on North American Datum 1983) Elevation: Elevation above sea level in meters Aspect: Dominant terrain aspect in degrees from north Terrain slope: Average slope of land surface measured in percent Stem diameter: Diameter of tree stem at breast height (1.3 meters above root crown) in year of growth measured in centimeters Age: Age of sample in year of growth Ring width: Measured width of annual radial increment in millimeters Date collected: Month, day, and year the sample was collecte

Neighbor tree location and size data

This file provides data describing the density and configuration of sample tree neighborhoods for calculation of competition indices. The distance to and diameter at breast height (DBH; 1.3 meters above root crown) of all neighboring trees within a 15 m radius of the sample or target tree were measured. A total of 10 sample trees had empty neighborhoods (no neighbor trees within 15 meters). File headings include: Sample tree ID: Identification code for the sample or target tree Sample tree species: Species of sample tree Neighbor tree species: Species of neighbor tree Neighbor tree DBH: Diameter at breast height of neighbor tree in centimeters Distance to neighbor tree: Distance from sample to neighbor tree in meter

Data from: Climate and competition effects on tree growth in Rocky Mountain forests

1. Climate is widely assumed to influence physiological and demographic processes in trees, and hence forest composition, biomass and range limits. Growth in trees is an important barometer of climate change impacts on forests as growth is highly correlated with other demographic processes including tree mortality and fecundity. 2. We investigated the main drivers of diameter growth for five common tree species occurring in the Rocky Mountains of the western United States using non-linear regression methods. We quantified growth at the individual tree level from tree core samples collected across broad environmental gradients. We estimated the effects of both climate variation and biotic interactions on growth processes and tested for evidence that disjunct populations of a species respond differentially to climate. 3. Relationships between tree growth and climate varied by species and location. Growth in all species responded positively to increases in annual moisture up to a threshold level. Modest linear responses to temperature, both positive and negative, were observed at many sites. However, model results also revealed evidence for differentiated responses to local site conditions in all species. In severe environments in particular, growth responses varied non-linearly with temperature. For example, in northerly cold locations pronounced positive growth responses to increasing temperatures were observed. In warmer southerly climates, growth responses were unimodal, declining markedly above a threshold temperature level. 4. Net effects from biotic interactions on diameter growth were negative for all study species. Evidence for facilitative effects was not detected. For some species, competitive effects more strongly influenced growth performance than climate. Competitive interactions also modified growth responses to climate to some degree. 6. Synthesis. These analyses suggest that climate change will have complex, species specific effects on tree growth in the Rocky Mountains due to non-linear responses to climate, differentiated growth processes that vary by location and complex species interactions that impact growth and potentially modify responses to climate. Thus, robust model simulations of future growth responses to climate trends may need to integrate realistic scenarios of neighborhood effects as well as variability in tree performance attributed to differentiated populations

Importance of conserving large and old trees to continuity of tree‐related microhabitats

International audienceProtecting structural features, such as tree-related microhabitats (TreMs), is a cost-effective tool crucial for biodiversity conservation applicable to large forested landscapes. Although the development of TreMs is influenced by tree diameter, species, and vitality, the relationships between tree age and TreM profile remain poorly understood. Using a tree-ring-based approach and a large data set of 8038 trees, we modeled the effects of tree age, diameter, and site characteristics on TreM richness and occurrence across some of the most intact primary temperate forests in Europe, including mixed beech and spruce forests. We observed an overall increase in TreM richness on old and large trees in both forest types. The occurrence of specific TreM groups was variably related to tree age and diameter, but some TreM groups (e.g., epiphytes) had a stronger positive relationship with tree species and elevation. Although many TreM groups were positively associated with tree age and diameter, only two TreM groups in spruce stands reacted exclusively to tree age (insect galleries and exposed sapwood) without responding to diameter. Thus, the retention of trees for conservation purposes based on tree diameter appears to be a generally feasible approach with a rather low risk of underrepresentation of TreMs. Because greater tree age and diameter positively affected TreM development, placing a greater emphasis on conserving large trees and allowing them to reach older ages, for example, through the establishment of conservation reserves, would better maintain the continuity of TreM resource and associated biodiversity. However, this approach may be difficult due to the widespread intensification of forest management and global climate change