Editorial: Adaptations of forest ecosystems to air pollution and climate change

The IUFRO Research Group 7.01 focuses on the monitoring, modeling and assessment of climate change and air pollution impacts on tree physiology and ecosystem water and nutrient cycles. In March 2010, this group held a conference entitled ‘Adaptation of Forest Ecosystems to Air Pollution and Climate Change’ in Antalya, Turkey. The main sessions of the conference were: response indicators and mechanisms of action; water cycles; atmospheric deposition; soils and nutrient cycles; reproduction; succession and molecular approaches; monitoring; modeling and risk assessment; ecological impacts and genetic aspects; social and political aspects including ecosystem services; and integrated assessment of anthropogenic stressors and adaptations of forest ecosystems under temperate and xeric conditions. From these sessions, five authors contributed research papers based on their talks for this special section of Tree Physiology (Arend et al. 2011, Centritto et al. 2011, Eastaugh 2011, Levanič et al. 2011, Tene et al. 2011). Two other papers from this IUFRO meeting were published in previous issues of Tree Physiology (Johnson and Abrams 2009, Bohler et al. 2010). This Editorial will review the major findings of these seven papers and other related papers in relation to global climate change and pollution impacts, as well as their implications for future research directions. Carbon dioxide is considered the most important greenhouse gas and has increased from 280 ppm prior to the Industrial Revolution (since 1750) to 390 ppm present day, primarily as a result of the burning of fossil fuels. These are the highest CO2 levels during the last 650,000 years and have produced the highest CO2 radiative forcing in the last 20,000 years (IPCC 2007). The impacts of elevated CO2 on tree physiology and growth have been the subject of an enormous amount of scientific investigation over the last 50 years (Bazzaz 1990). Because CO2 uptake and fixation is the primary function of plant photosynthesis, increasing amounts of this gas have fertilizing, stimulatory effects, within limits, on tree physiology and growth (Karnosky 2003, Norby et al. 2010). However, forest trees may now be CO2 saturated and further increases in atmospheric CO2 will not lead to further enhancements in photosynthesis and growth (Korner 2003). In contrast to the enhancement effect of elevated CO2, the increase in tropospheric ozone in many parts of the world typically has detrimental impacts on trees (Karnosky et al. 2007). These negative impacts on tree physiology and growth typically counteract the positive impacts of elevated CO2 (Isebrands et al. 2001). However, the impacts of ozone are much more localized than those of elevated CO2. Globally, the highest ozone levels, those most capable of seriously impacting tree physiology and growth, exist primarily in the northeast USA, California and China. In the IUFRO paper by Bohler et al. (2010), sapling clones of Populus tremula × P. alba were grown in charcoal-filtered air versus those exposed to 120 ppb ozone. Leaves were harvested weekly for biochemical and proteome analyses, allowing for a comparison between expanding and adult leaves. During the leaf expansion phase (Days 0–7), the enzymatic machinery of the leaves is set up and remains dynamically stable in the adult leaves (Days 14–28). The authors report that ozone had no apparent effect on expanding leaves, while the metabolism in fully expanded leaves was disturbed after 2 weeks of exposure. Thus, the inhibitory impact of ozone on tree physiology varies with leaf ontogeny. Over the last century, the forest macro- and micro-environment has changed dramatically in most parts of the world. Greenhouse gas levels have increased and the atmosphere has warmed, on average, by ∼1°C (IPCC 2007). The growing season has been extended by ∼10–20 days in temperate regions (Menzel and Fabian 1999, IPCC 2007). Some regions are now receiving more precipitation while other areas are receiving less. Widespread forest cutting and subsequent wildfires have replaced old-growth forests with younger, faster-growing forests (Abrams 2003). The overall stimulatory effect of these factors is present in many tree physiology and forest growth studies. Way and Oren (2010) reviewed the positive temperature effects on tree physiology and growth, which vary by species (conifer versus hardwood) and between boreal, temperate and tropical regions. Precipitation chemistry has become more acidic but richer in nitrogen from the burning of fossil fuels (Aber et al. 1989). These seemingly positive impacts on trees and forest ecosystems from environmental pollution (the burning of fossil fuels) are what I call ‘convoluted benefits’. The IUFRO paper by Eastaugh (2011) used data from several hundred climate recording stations of the Austrian National Forest Inventory. Austria has experienced a warming trend over the past 50 years, but little overall change in precipitation amounts. A major finding of Eastaugh's study is that most of the increase in tree growth since 1960 can be primarily attributed to increased nitrogen deposition rather than climate change. Forest stemwood increased by 20–40 kg per additional kilogram of nitrogen deposition. The IUFRO paper by Centritto et al. (2011) reported on the interaction between high growth temperature (+10°C) and water stress on the gas-exchange properties of Populus nigra. They found that isoprene emission and photosynthesis did not acclimate in response to elevated temperature, whereas dark and light respiration acclimated to higher temperature. Water stress and high growth temperature caused a significant decline in photosynthesis and respiration. Photosynthesis was restored in about a week following re-watering, indicating the transient nature of biochemical limitations. Water stress uncoupled the emission of isoprene from photosynthesis and stomatal conductance, suggesting that temperature interactions with water stress may dominate poplar's acclimatory capability with future climate change. The authors concluded that predicted temperature increases in arid environments may exacerbate the effects of water stress even if soil water availability is not directly affected. While small increases in temperature often result in increased photosynthesis and tree growth (Way and Oren 2010), large increases in temperature are inhibitory. Dendrochronology (the study of tree rings) provides important information for reconstructing past climate and assessing the impacts of climate change and disturbance events on tree growth (Abrams and Orwig 1995, Mann et al. 1999, Abrams 2003). Four of the IUFRO papers published in Tree Physiology used tree rings in their climate change research. Provenance-specific growth responses to experimentally applied drought and air warming were studied in saplings of three European oak species by Arend et al. (2011). For each species, four provenances were subjected to four climates: control, periodic drought, air warming or their combination over a 3-year period. They reported that drought reduced shoot height growth and stem diameter growth, while air warming stimulated shoot height growth but reduced stem diameter growth and root length growth. The growth responses in shoots and stems were highly variable among provenances, but not always consistent with the climate conditions of provenance origin. Shoot height growth was found to be more sensitive to drought in provenances from northern latitudes than in provenances from southern latitudes. The authors concluded that differential treatment responses altered allometric growth relations and that genetic factors related to the postglacial immigration history of European oaks may have caused selective pressure at provenance origins. The IUFRO paper by Levanič et al. (2011) investigated the associations between tree growth, wood anatomy (hydraulic architecture), carbon isotope discrimination and tree mortality following drought induced by site drainage. The authors were motivated by a lack of specific research dealing with the increasing forest mortality in relation to climate change that has been observed globally. The authors utilized a local soil water drainage event in 1982 that resulted in increased mortality within a stand of oak trees (Quercus robur). They found that pre-dawn water potentials were more negative for trees in the process of dying compared with those that survived. Tree rings formed over the 123 years prior to the onset of mortality were used to estimate tree productivity from basal area increment (BAI), xylem hydraulic parameters via anatomical measurements and crown-level gas exchange via carbon isotope discrimination. Significantly higher mortality occurred in oak trees with high BAI prior to the drainage event. In trees that died, hydraulic diameter and conductivity of vessels were higher than in surviving trees. Trees that died had consistently lower isotope discrimination than trees that survived. The authors concluded that tree mortality was associated with anatomical and physiological differences prior to the onset of soil drainage and that dying trees may have had hydraulic limitations under drought. The paper by Tene et al. (2011) reports a methodology to better identify and understand how commonly measured tree proxies, such as dendrochronology, stable isotopes, ring density, blue reflectance and forest biometrics, relate to environmental parameters and climatic events. The authors utilized a thinning experiment in Ireland to test a methodology for monitoring drought stress based on the analysis of growth rings. The study incorporated three thinning regimes (light, moderate and heavy) and a multidisciplinary approach to assess tree response to the interaction of thinning and climate. Their results showed that radial growth was a good proxy for monitoring changes to moisture deficit, while maximum wood density and blue reflectance were appropriate to assess changes in growing season temperature. Rainfall amount was a major factor in stable carbon and oxygen discrimination, but mostly in the latewood, rather than earlywood, formation phase. Stable oxygen isotope analysis was more accurate than radial growth analysis in drought detection. This research provides recommendations to better understand how commonly used tree-ring proxies relate to environmental parameters. A range-wide dendrochronology study of eight tree species in the eastern USA was reported by IUFRO presenters Johnson and Abrams (2009). This study explored growth rate (BAI) relationships across age classes (from young to old) for one or more species within the genera Populus, Quercus, Pinus, Tsuga and Nyssa. We found that most trees in all age classes exhibited an increasing BAI throughout their lives. This is particularly unusual for trees in the older age classes, which were expected to have declining growth in the later years, based on physiological growth models. There existed an inverse relationship between growth rate and increasing age class. The oldest trees within each species have consistently slow growth throughout their lives, implying an inverse relationship between growth rate and longevity. Younger trees (<60 years of age) within each species were consistently growing more quickly than the older trees when they were of the same age due to a higher proportion of fast-growing trees in the young age classes. Slow, but increasing, BAI in the oldest trees in recent decades is a continuation of their growth pattern established in previous centuries. High growth in trees may be related to the stimulatory effects of global change phenomena (climate and land-use history) over the last 50–100 years. A recent increase in tree-ring growth was also reported for 4000-year-old bristlecone pine in the western USA (Salzer et al. 2009). There is normally an inverse relationship between tree age and tree growth and between tree growth and longevity (Johnson and Abrams 2009). Therefore, the continuation of global change factors that have a stimulatory effect on tree growth may act to reduce tree longevity in the future, as fast-growing trees are less likely to obtain the maximum longevity for the species. We live in a unique time when the impacts of elevated greenhouse gases have moved from the field of meteorology to the ecological and environmental arenas. This natural experiment provides almost limitless opportunities for tree physiologists and other environmental scientists. The IUFRO papers reviewed here point to some important directions of future environmental pollution and global climate change research. These include the importance of studying plant genotype and environmental interactions, such as the impacts of temperature coupled with water stress, pollution impacts in relation to plant and tissue ontogeny, the importance of tree anatomy (e.g., hydraulic architecture) to assess environmental change responses, and using dendrochronology (tree-ring analysis) to monitor the interaction of climate change, pollution, nitrogen deposition and forest disturbance. Tree rings are unique in their ability to reveal long-term tree growth (ring width) patterns in relation to environmental change and the uptake and sequestration of pollutants (via tree-ring chemistry). The IUFRO papers also demonstrate the importance of physiological thresholds in tree responses to climate change that low to moderate increases in greenhouse gases, nitrogen deposition and temperature oftentimes have a stimulatory impact on tree physiology and growth (convoluted benefits), while large increases are often detrimental or provide no further benefit to the tree. Future research that utilizes the approaches outlined in these papers should greatly increase our understanding of global climate change as we move further into the twenty-first century.