Climate Warming Increased Spring Leaf-Out Variation Across Temperate Trees in China

Leaf-out phenology plays a key role in ecosystem structure and functioning. Phenological changes have often been linked to climatic factors and have received considerable attention, with most studies focusing on trends of leaf-out phenology. Leaf-out variation (LOV), which reflects the stability of phenological responses, may also be affected by climate change, yet this has received less scientific attention. In this study, we examined spring LOV in response to climate change in China during the period 1963–2008 using in situ records of 15 species at 25 phenological observation sites across several climate zones and explored spatiotemporal changes of LOV and the underlying mechanisms. We observed a significant decrease of LOV toward higher latitudes (−0.2 ± 0.1 days⋅°N–1;P < 0.001) across all species. Temporally, we found that the LOV was significantly increased from the period 1963–1986 (6.9 ± 2.8 days) to the period 1987–2008 (7.9 ± 3.7 days, P < 0.05). Furthermore, the LOV changes between 1987–2008 and 1963–1986 were significantly smaller at high latitudes (average decrease of 1.0 day) than at low latitudes (average increase of 1.5 days). The spatial pattern of LOV is likely due to both increased heat requirements and greater temperature sensitivity at low latitudes compared with high latitudes. The temporal pattern of LOV is likely related to increased heat requirements for leaf-out during 1987–2008 when the average air temperature was higher. Our analysis indicated that the phenology response to climate change is reflected not only in the temporal trends for long time series but also in the variation of phenological dates. Results from this study improve our understanding of phenological responses to climate change and could be applied in the assessment of regional phenology changes to evaluate better the impacts of climate change on ecosystem structure and function.

Reconstruction of climate and ecology of Skagit Valley, Washington, from 27.7 to 19.8 ka based on plant and beetle macrofossils

Glacial lake sediments exposed at two sites in Skagit Valley, Washington, encase abundant macrofossils dating from 27.7 to 19.8 cal ka BP. At the last glacial maximum (LGM) most of the valley floor was part of a regionally extensive arid boreal (subalpine) forest that periodically included montane and temperate trees and open boreal species such as dwarf birch, northern spikemoss, and heath. We used the modern distribution and climate of 14 species in 12 macrofossil assemblages and a probability density function approach to reconstruct the LGM climate. Median annual precipitation (MAP) at glacial Lake Concrete (GLC) was ~50% lower than today. In comparison, MAP at glacial Lake Skymo (GLS) was only ~10% lower, which eliminated the steep climate gradient observed today. Median January air temperature at GLC was up to 10.8°C lower than today at 23.5 cal ka BP and 8.7°C lower at GLS at 25.1 cal ka BP. Median July air temperature declines were smaller at GLC (3.4°C–5.0°C) and GLS (4.2°C–6.3°C). Warmer winters (+2°C to +4°C) and increases in MAP (+200 mm) occurred at 27.7, 25.9, 24.4, and 21.2–20.7 cal ka BP. These changes accord with other regional proxies and Dansgaard–Oeschger interstades in the North Atlantic.

Variation in white spruce needle respiration across the species range

White spruce (Picea glauca) spans a massive range from arctic treeline to temperate forests. Yet the variability in respiratory physiology and the implications for tree carbon balance at the extremes of this distribution remain enigmasWorking at Arctic and Temperate sites more than 5000 km apart, we measured the short-term temperature response of dark respiration (R/T) at upper and lower canopy positions. R/T curves were fit to a polynomial model and model parameters (a, b, and c) were compared between locations, canopy positions, or with published data. Respiration measured at 25°C (R25) was 68% lower at the southern location than the northern location, resulting in a significantly lower a parameter of the R/T response in temperate trees Only at the southern location did upper canopy leaves have a steeper temperature response than lower canopy leaves, likely reflecting steeper canopy gradients in light. No differences were manifest in the maximum temperature of respiration. At the northern range limit, respiration appears extreme. This high carbon cost likely contributes to the current location of northern treeline. We find that respiration will increase with end-of-the-century warming and will likely continue to constrain the future range limits of this important boreal species.

Seven Ways a Warming Climate Can Kill the Southern Boreal Forest

The southern boreal forests of North America are susceptible to large changes in composition as temperate forests or grasslands may replace them as the climate warms. A number of mechanisms for this have been shown to occur in recent years: (1) Gradual replacement of boreal trees by temperate trees through gap dynamics; (2) Sudden replacement of boreal overstory trees after gradual understory invasion by temperate tree species; (3) Trophic cascades causing delayed invasion by temperate species, followed by moderately sudden change from boreal to temperate forest; (4) Wind and/or hail storms removing large swaths of boreal forest and suddenly releasing temperate understory trees; (4) Compound disturbances: wind and fire combination; (5) Long, warm summers and increased drought stress; (6) Insect infestation due to lack of extreme winter cold; (7) Phenological disturbance, due to early springs, that has the potential to kill enormous swaths of coniferous boreal forest within a few years. Although most models project gradual change from boreal forest to temperate forest or savanna, most of these mechanisms have the capability to transform large swaths (size range tens to millions of square kilometers) of boreal forest to other vegetation types during the 21st century. Therefore, many surprises are likely to occur in the southern boreal forest over the next century, with major impacts on forest productivity, ecosystem services, and wildlife habitat.

Comparative anatomy of leaf petioles in temperate trees and shrubs

Petioles are important plant organs connecting stems with leaf blades and affecting light-harvesting leaf ability as well as transport of water, nutrient and biochemical signals. Despite petiole's high diversity in size, shape and anatomical settings, little information is available about their structural adaptations across evolutionary lineages and environmental conditions. To fill our knowledge gap, we investigated the variation of petiole morphology and anatomy in 95 European woody plant species using phylogenetic comparative models. Two major axes of variation were related to leaf area (from large and soft to small and tough leaves), and plant size (from cold-adapted shrubs to warm-adapted tall trees). Larger and softer leaves are found in taller trees of more productive habitats. Their petioles are longer, with a circular outline, thin cuticles without trichomes, and are anatomically characterised by the predominance of sclerenchyma, larger vessels, interfascicular areas with fibers, indistinct phloem rays, and the occurrence of prismatic crystals and druses. In contrast, smaller and tougher leaves are found in shorter trees and shrubs of colder or drier habitats. Their petioles are characterized by teret outline, thick cuticle, simple and non-glandular trichomes, epidermal cells smaller than cortex cells, phloem composed of small cells and radially arranged vessels, fiberless xylem, lamellar collenchyma, acicular crystals and secretory elements. Individual anatomical traits were linked to different internal and external drivers. The petiole length and vessel conduit size increase, while cuticle thickness decreases, with increasing leaf blade area. Epidermis cell walls are thicker in leaves with higher specific leaf area. Collenchyma becomes absent with increasing temperature, epidermis cell size increases with plant height and temperature, and petiole outline becomes polygonal with increasing precipitation. We conclude that species temperature and precipitation optima, plant height, leaf area and thickness exerted a significant control on petiole anatomical and morphological structures not confounded by phylogenetic inertia. Unrelated species with different evolutionary histories but similar thermal and hydrological requirements have converged to similar petiole anatomical structures. Our findings contribute to improving current knowledge about the functional morphoanatomy of the petiole as the key organ that plays a crucial role in the hydraulic pathways in plants.

Isotopically labelled water - A valuable tracer to track initiation and progress of bud dormancy in temperate trees

<p>Leaf-out of deciduous trees is regulated by a set of environmental factors such as cool temperatures during winter-dormancy (chilling), warm spring temperatures (forcing), and daylength (photoperiod), with complex interactions between these factors. Teasing apart these different factors in situ is challenging as no visible changes occurs during the dormancy phase. Manipulating these factors in climate chamber experiments may overcome this issue but may not reflect how they truly interact in natural conditions. Previous researches suggested that bud meristems are disconnected from the xylem flow during endodormancy and that the connection become progressively restored once exposed to a certain duration of chilling. Here we developed a new method using isotopically labelled water (D<sub>2</sub>O) to quantify the amount of water that can reach buds during the whole dormancy till budburst for 5 different species (<em>Acer pseudoplatanus</em>, <em>Carpinus betulus</em>, <em>Fagus sylvatica</em>, <em>Quercus petraea</em>, <em>Tilia cordata</em>).</p><p>In detail, we harvested twig cuttings from leaf fall to budburst (~every two weeks, 12 times) of these species from two different sites (about 5°C of difference) and placed them into labelled water during 24 h at constant light and 20°C. Buds were then cut and water content extracted to quantify δD. Thus, tracing back the water flow into the buds by the amount of D<sub>2</sub>O taken up. In parallel a subset of twigs was left in the room at 20°C to assess the time to budburst as a proxy for dormancy depth. Analyses of the data are ongoing and preliminary results show progressive increase of water uptake after induction of winter dormancy until budburst as chilling duration increased. Further, we also found distinct differences between species whereas <em>Carpinus betulus</em> showed the highest and <em>Tilia cordata </em>the lowest label uptake during winter dormancy. Furthermore, individuals growing at higher elevation took up less label indicating a stronger dormancy at lower winter temperatures. In summary, we think that our method seems a valuable tool to track quantitative changes in dormancy depth of temperate species especially, in combination with investigations on the molecular level such as sugars or hormones during winter-dormancy.</p>

Response to Comment on “Increased growing-season productivity drives earlier autumn leaf senescence in temperate trees”

Our study showed that increases in seasonal productivity drive earlier autumn senescence of temperate trees. Norby argues that this finding is contradicted by observations from free-air CO2 enrichment (FACE) experiments, where elevated CO2 has been found to delay senescence in some cases. We provide a detailed answer showing that the results from FACE studies are in agreement with our conclusions.