In 1998, we obtained a close linear relationship between gross primary biomass production (GPP), the total mass of carbon fixed annually by photosynthesis, and net primary biomass production, designated (NPP) (Waring et al. 1998). Collalti and Prentice (2019) (henceforth denoted C&P) recently asserted that Waring, in that 1998 paper, hypothesized that this constant NPP/GPP ratio was ‘universal across biomes, tree species and stand ages’. This is not correct. We said, in the abstract of that paper: ‘To search for possible common relationships (between GPP and respiration), we assembled annual carbon budgets from 12 forest sites . . . analysis indicated that the total NPP/GPP ratio was conservative (0.47 ± 0.04). This finding supports the possibility of greatly simplifying forest models.’ We were— and are—well aware that the ratio we obtained is an empirical approximation, which is likely to vary in different stands and under different conditions. We did not postulate that it might be universal across biomes, tree species and ages; that would have been a legitimate, testable hypothesis, but we are not aware that anyone has stated it in that way. However, the fact that the constant ratio was subsequently questioned and tested by a number of authors (Medlyn and Dewar 1999, Mäkelä and Valentine 2001, DeLucia et al. 2007, Litton et al. 2007, among others) in relation to a range of forests and conditions, suggests that they considered the hypothesis of its universal application to be implicit. The various investigations have yielded a range of values, mostly between 0.4 and 0.6. C&P examined data from more than 200 stands, from which they obtained an average NPP/GPP ratio of 0.46, which was ‘statistically indistinguishable from that given by Waring et al.’ However, the values varied from 0.22 to 0.79, a total range, they said, which is too large to be disregarded. They concluded that the hypothesis of a universal constant or slightly variable ratio of NPP to GPP should now be rejected. If we accept that the empirical relationship established by Waring et al. (1998) provides the basis for a hypothesis stating that the relationship between NPP and GPP is constant and universal across forest stands and ecosystems, then the evaluation of that relationship by C&P, who included in their analysis most of the other studies that addressed the question, constitutes a legitimate test of the hypothesis. But, since we never proposed the constant ratio as a universal hypothesis, there is no question of rejecting that ‘hypothesis’ and the issue becomes ‘should the use of the constant ratio be discontinued because of the range of values found by C&P?’. We do not think so, and provide citations and analysis, below, to support this opinion. Stand-level models of forest growth and productivity, intended for use over large areas and long periods (months to years), must be based on relatively simple relationships. The success of such models has been widely demonstrated (e.g. Coops et al. 1998, Mummery and Battaglia 2001, Landsberg et al. 2003). These relationships may be established empirically or derived from detailed models of light interception and photosynthesis at leaf, stand or canopy level (e.g. Williams et al. 2001). The respiration term to yield NPP should be at the same level. In the 3-PG model (Landsberg and Waring 1997), which was written as a deliberate attempt to bridge the gap between conventional empirical (mensuration-based) growth and yield models and process-based carbon balance models (Landsberg and Sands 2011), a light-use efficiency factor is used, with the light intercepted by forest canopies, to calculate GPP.

The composition and health of forests across western North America have shown signs of change over the last half-century associated with altered climate conditions. Most models developed to predict responses to variation in climate assume that the ecological distribution of adult trees provides a sound basis for projecting potential shifts in a species' range. Under a dynamic climate, however, recently established seedlings may more closely reflect changes in climate conditions. This study combined the simple, widely tested physiological model 3-PG with an empirical regeneration dataset, composed of 21,097 plots, to assess regional scale changes in tree species distributions across British Columbia, Canada. We geographically registered all plot locations to correspond with topographically-adjusted 1 km monthly climatic data for the period 2000-2009. By comparing the distribution of seedlings to that of mature trees present in an earlier period (1950-1975), we could assess where alterations in the environment have occurred, and the extent to which changes may make a species vulnerable to replacement in some places or likely to regenerate and migrate elsewhere. Decision tree models were developed to assess the relative importance of suboptimal temperatures, frost, soil water deficits and evaporative demand on the growth and distribution of four widely distributed species: Douglas-fir (Pseudotsuga menziesii), lodgepole pine (Pious contorta), western larch (Larix occidentalis), and subalpine fir (Abies lasiocarpa). Tree responses varied by species, with areas suitable for lodgepole pine experiencing the largest relative increase in summer drought and areas dominated by western larch experiencing the least. Those areas modelled as suitable for species range expansions occurred 79% (SD = 16%) of the time in places where seedlings of a designated species were predicted in 2000-2009 using the regeneration dataset. We conclude that employing seedling surveys in concert with tree surveys provide valuable ecological insights when predicting species responses to climate shifts.

Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80523-1499, USA; USDA Forest Service, Rocky Mountain Experiment Station, Fort Collins, CO 80526, USA; Nicholas School of the Environment, Duke University, Durham, NC 27708, USA; Department of Forest Science, University of Helsinki, FI-00014, Finland; College of Forestry, Oregon State University, Corvallis, OR 97331, USA; Corresponding author (Mike.Ryan@colostate.edu)

Christiansen, E., Waring, R.H. and Berryman, A.A., 1987. Resistance of conifers to bark beetle attack: searching for general relationships. For. Ecol. Manage., 22: 89-106. Bark beetles are among the few native insects that can kill large numbers of trees in a single year. The present paper reviews recent work on the relationship between conifer resistance to bark beetle attack and tree vigor, e.g. in terms of wood production per unit of foliage. Experimental studies in the Pacific Northwest and the southeast U.S.A., and in Norway, are drawn upon to show that tree resistance to attack may be closely related to the amount of current and stored photosynthate that is available for defense. An experimental approach is advocated to critically test the relationship between host-tree resistance and the limitations on the transfer of critical resources to the site of attack. ROLE OF BARK BEETLES IN NATURAL FOREST ECOSYSTEMS As forests age they become more vulnerable to agents of disturbance such as high winds, fire, fungi, and bark beetles. These disturbing agents gradually cause the old forest to be replaced by new generations of trees. Associated improvements in nutrient cycling help to maintain the long-term productive capacity of the ecosystem (cf. Mattson and Addy, 1975; Romme et al., 1986). The bark beetles thus play an important role in natural ecosystems. The interaction between these beetles and their host trees has been established through some 200 million years of adaptation and counter-adaptation (see, e.g., Raffa and Berryman, 1987). Bark beetles deposit their eggs in galleries excavated in the phloem, cambium, and outer sapwood of trees, and successful brood production is contingent upon the death of these tissues. Most species of bark beetles can only breed in trees that exhibit severe decline, or are already dead, and so merely promote decomposition and mineralization. A few species, however, are able 0378-1127/87/$03.50 © 1987 Elsevier Science Publishers B.V.

Forest ecosystems across western North America will likely see shifts in both tree species dominance and composition over the rest of this century in response to climate change. Our objective in this study was to identify which ecological regions might expect the greatest changes to occur. We used the process‐based growth model 3‐ PG , to provide estimates of tree species responses to changes in environmental conditions and to evaluate the extent that species are resilient to shifts in climate over the rest of this century. We assessed the vulnerability of 20 tree species in western North America using the Canadian global circulation model under three different emission scenarios. We provided detailed projections of species shifts by including soil maps that account for the spatial variation in soil water availability and soil fertility as well as by utilizing annual climate projections of monthly changes in air temperature, precipitation, solar radiation, vapor pressure deficit and frost at a spatial resolution of one km. Projected suitable areas for tree species were compared to their current ranges based on observations at >40 000 field survey plots. Tree species were classified as vulnerable if environmental conditions projected in the future appear outside that of their current distribution ≥70% of the time. We added a migration constraint that limits species dispersal to <200 m yr −1 to provide more realistic projections on species distributions. Based on these combinations of constraints, we predicted the greatest changes in the distribution of dominant tree species to occur within the Northwest Forested Mountains and the highest number of tree species stressed will likely be in the North American Deserts. Projected climatic changes appear especially unfavorable for species in the subalpine zone, where major shifts in composition may lead to the emergence of new forest types.

Although quantifying the massive exchange of carbon that takes place over the Amazon Basin remains a challenge, progress is being made as the remote sensing community moves from using traditional, reflectance-based vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), to the more functional Photochemical Reflectance Index (PRI). This new index, together with satellite-derived estimates of canopy light interception and Sun-Induced Fluorescence (SIF), provide improved estimates of Gross Primary Production (GPP). This paper traces the development of these new approaches, compares the results of their analyses from multiple years of data acquired across the Amazon Basin and suggests further improvements in instrument design, data acquisition and processing. We demonstrated that our estimates of PRI are in generally good agreement with eddy-flux tower measurements of photosynthetic light use efficiency (ε) at four sites in the Amazon Basin: r2 values ranged from 0.37 to 0.51 for northern flux sites and to 0.78 for southern flux sites. This is a significant advance over previous approaches seeking to establish a link between global-scale photosynthetic activity and remotely-sensed data. When combined with measurements of Sun-Induced Fluorescence (SIF), PRI provides realistic estimates of seasonal variation in photosynthesis over the Amazon that relate well to the wet and dry seasons. We anticipate that our findings will steer the development of improved approaches to estimate photosynthetic activity over the tropics.

A lengthening of the fire season, coupled with higher temperatures, increases the probability of fires throughout much of western North America. Although regional variation in the frequency of fires is well established, attempts to predict the occurrence of fire at a spatial resolution <10 km2 have generally been unsuccessful. We hypothesized that predictions of fires might be improved if depletion of soil water reserves were coupled more directly to maximum leaf area index (LAImax) and stomatal behavior. In an earlier publication, we used LAImax and a process-based forest growth model to derive and map the maximum available soil water storage capacity (ASWmax) of forested lands in western North America at l km resolution. To map large fires, we used data products acquired from NASA’s Moderate Resolution Imaging Spectroradiometers (MODIS) over the period 2000–2009. To establish general relationships that incorporate the major biophysical processes that control evaporation and transpiration as well as the flammability of live and dead trees, we constructed a decision tree model (DT). We analyzed seasonal variation in the relative availability of soil water (fASW) for the years 2001, 2004, and 2007, representing respectively, low, moderate, and high rankings of areas burned. For these selected years, the DT predicted where forest fires >1 km occurred and did not occur at ~100,000 randomly located pixels with an average accuracy of 69 %. Extended over the decade, the area predicted burnt varied by as much as 50 %. The DT identified four seasonal combinations, most of which included exhaustion of ASW during the summer as critical; two combinations involving antecedent conditions the previous spring or fall accounted for 86 % of the predicted fires. The approach introduced in this paper can help identify forested areas where management efforts to reduce fire hazards might prove most beneficial.

In this Tamm review, we trace the origin and application of two production indices: Light Use Efficiency (LUE) and (Leaf) Growth Efficiency (GE). Light Use Efficiency usually denoted (epsilon) was originally defined by John Monteith in the 1970s as the rate that dry matter is accumulated by plants in relation to the amount of solar radiation absorbed by leaves; the concept has been a corner-stone of the field of production ecology ever since. Although highly variable at daily intervals, LUE becomes linear at longer intervals, providing a major simplification to the construction and application of process-based models. A further simplification in model construction became possible when it was found that the ratio between total dry mass production and gross photosynthesis is approximately constant (approximate to 0.5). Simplified process based models provide a means of estimating the maximum productivity of a species growing inside or outside its native range, and help to identify constraints on production in current and projected environments. Consequently, models that incorporate LUE have expanded from research tools to practical ways of assessing silvicultural options in the management of individual forests as well as for measuring and forecasting global trends in forest productivity. The Leaf Growth Efficiency (GE) index, defined as annual growth in stemwood per unit of leaf area, has become widely adopted as a means of identifying the spatial variation among trees, which affects stand growth and LUE. GE was originally used to assess the vulnerability of individual trees to attack by bark beetles but, combined with structural and physiological analyses it has been found useful for interpreting and predicting stand growth responses to tree spacing, aging, and defoliation. Challenges remaining in the field of forest production ecology include prediction of the effects of fast-changing climatic conditions across the globe on the growth and survival of species, and their interactions with continually rising atmospheric concentrations of CO2. (C) 2016 Elsevier B.V. All rights reserved.

Understanding future tree species migration is challenging due to the unprecedented rate of climate change combined with the presence of human barriers that may limit or impede species movement. Projected changes in climatic conditions outpace migration rates, and more realistic rates of range expansion are needed to make sound environmental policies. In this paper, we develop a modeling approach that takes into account both the geographic changes in the area suitable for the growth and reproduction of tree species, as well as limits imposed geographically on their potential migration using remotely-sensed land cover information. To do so, we combined a physiologically-based decision tree model with a remotely-sensed-derived diffusion-dispersal model to identify the most likely direction of future migration for 15 native tree species in the Pacific Northwest Region of North America, as well as the degree that landscape fragmentation might limit movement. Although projected changes in climate through to 2080 are likely to create favorable environments for range expansion of the 15 tree species by 65% on average, by limiting the potential movement by previously published migration rates and landscape fragmentation, range expansion will likely be 50%–90% of the potential. The hybrid modeling approach using distribution modeling and remotely-sensed data fills a gap between naïve and more complex approaches to take into account major impediments on the potential migration of native tree species.

Using process-based models in combination with dendrochronological measurements provides a way to explain recent increased tree growth in northwestern China.

We look back over 50 years of research into the water relations of trees, with the objective of assessing the maturity of the topic in terms of the idea of a paradigm, put forward by Kuhn in 1962. Our brief review indicates that the physical processes underlying the calculation of transpiration are well understood and accepted, and knowledge of those processes can be applied if information about the leaf area of trees, and stomatal conductance, is available. Considerable progress has been made in understanding the factors governing stomatal responses to environment, with insights into how the hydraulic conducting system of trees determines the maximum aperture of stomata. Knowledge about the maximum stomatal conductance values likely to be reached by different species, and recognition that stomatal responses to increasing atmospheric vapor pressure deficits are in fact responses to water loss from leaves, provides the basis for linking these responses to information about hydraulic conductance through soil–root–stem–branch systems. Improved understanding in these areas is being incorporated into modern models of stomatal conductance and responses to environmental conditions. There have been significant advances in understanding hydraulic pathways, including cavitation and its implications. A few studies suggest that the major resistances to water flux within trees are not in the stem but in the branches. This insight may have implications for productivity: it may be advantageous to select trees with the genetic propensity to produce short branches in stands with open canopies. Studies on the storage of water in stems have provided improved understanding of fluxes from sapwood at different levels. Water stored in the stems of large trees may provide up to 20–30% daily sap flow, but this water is likely to be replaced by inflows at night. In dry conditions transpiration by large trees may be maintained from stored water for up to a week, but flows from storage may be more important in refilling cavitated xylem elements and hence ensuring that the overall hydraulic conductivity of stems is not reduced. Hydraulic redistribution of water in the soil may make a contribution to facilitating root growth in dry soils and modifying resource availability. We conclude that the field of tree water relations is mature, in the sense that the concepts underlying models describing processes and system responses to change are well-tested and accepted and there are few, if any, serious anomalies emerging. Models are essentially formal statements about the way we think systems work. They are always subject to further testing, refinement and improvements. Gaps in knowledge appear within the framework of accepted concepts and mechanisms research is needed to fill those gaps. The models currently available can be used to scale estimates of transpiration from leaf to landscape levels and predict species responses to drought. The focus in tree water relations has shifted to examine the climatic thresholds at which drought, high temperatures and vapor pressure deficits cause mortality. Tree death may be caused by hydraulic collapse following irreversible cavitation or extremely low water potentials, but recent research indicates that the relative sensitivity of stomatal conductance and whole-plant hydraulic conductance plays a major role in determining plant responses to drought.

Climate change has already begun to impact the structure and function of forest ecosystems in the Pacific Northwest by altering the frequency, intensity, and duration of droughts and heat stress, with implications for widespread environmental and socio-economic change. A major realization is that accumulated physiological stress can ultimately lead to tree mortality and changes in species distributions, particularly in areas away from maritime influences. To ameliorate the effects of drought, insect outbreaks, and reduce the risk of crown fires, various strategies are being tested. To make some of these strategies economical, biomass is proposed as an alternative energy source. At the same time that an increase in harvesting is being considered, there is a desire to increase carbon sequestration by forests to offset, at least in part, greenhouse gas emissions. Assessments are needed to determine current and future impacts of climate change, and to evaluate management options while considering carbon storage benefits and sustainability of ecosystem structure and function. Here we provide an overview of research results from the Pacific Northwest region where forests dominate the landscape and contain among the highest biomass on earth. In this review, we present findings that challenge common assumptions, and suggest a way to predict outcomes of changes in climate and land management in the future. The approach includes the use of observation-driven land system models that integrate the extent that forests are vulnerable to climate change, management practices, and economic considerations. It also requires increased emphasis on in situ and remotely sensed observations and experiments to initialize and test the model, and to track trends in forest condition. (C) 2014 Elsevier B.V. All rights reserved.

•We model the occurrence of 20 species in the Pacific Northwest.•We examine the effects of soil water holding capacity on tree species distributions using a sensitivity analysis.•Species distribution models had a mean accuracy of 84%.•The majority of the tree species (75%) were sensitive to changes in available soil water holding capacity.

We outlined in chapter 2 the main forest types and where they occur around the world. The main factor underlying the distribution of those forest types, and the differences between them, is climate. Our job, as forest ecologists, is to establish what it is about different climates that determines the differences in forests. Also, what is it about different tree species that enables some to grow well in the tropics but not in cooler regions, while species that can survive and grow in the cold, harsh, boreal regions do not do well—or grow at all—in what seem to be the much more favorable tropical and subtropical regions? The answers lie in the interactions between the physical and physiological characteristics of trees, largely determined by their genetics, and the climatic factors that act on those characteristics and so affect the way trees grow. There’s also the matter of competition among and between trees and other vegetation types.