Stomatal regulation and xylem hydraulics of limber pine and Engelmann spruce in Great Basin sky-island ecosystems.

Integration of whole-plant stomatal regulation and xylem hydraulics is of critical importance for predicting species response to drought stress. Yet intraspecific variability of stomatal and hydraulic traits, and how these variabilities interact, remain largely unknown. We hypothesized that drought can drive less stomatal regulation but increase xylem hydraulic safety, resulting in stomatal-hydraulic coordination within a species. We estimated sensitivity of whole-tree canopy conductance to soil drying together with xylem hydraulic traits of two dominant conifers, i.e. limber pine (Pinus flexilis) and Engelmann spruce (Picea engelmannii). Our study was conducted using sub-hourly measurements over five consecutive years (2013-2017) at three instrumented sites with different elevations within the Nevada Eco-hydrological Assessment Network (NevCAN) in Great Basin sky-island ecosystems. Both conifers showed a reduction of stomatal sensitivity to soil drying at lower elevations, indicating an active stomatal acclimation to drought. While limber pine increased xylem embolism resistance in parallel with reduced stomatal sensitivity to soil drying, an opposite hydraulic adjustment was detected in Engelmann spruce. Our results provide evidence that mature trees can respond to climatic changes using coordinated shifts in stomatal regulation and xylem hydraulics, but such changes can differ within and between species in ways that need to be examined using in situ data. Deciphering intraspecific variability in whole-plant stomatal and hydraulic traits ultimately contributes to defining drought tolerance and vulnerability, particularly for tree species that inhabit a wide range of landscapes.

Water-Use Efficiency of Co-occurring Sky-Island Pine Species in the North American Great Basin.

Water-use efficiency (WUE), weighing the balance between plant transpiration and growth, is a key characteristic of ecosystem functioning and a component of tree drought resistance. Seasonal dynamics of tree-level WUE and its connections with drought variability have not been previously explored in sky-island montane forests. We investigated whole-tree transpiration and stem growth of bristlecone (Pinus longaeva) and limber pine (Pinus flexilis) within a high-elevation stand in central-eastern Nevada, United States, using sub-hourly measurements over 5 years (2013-2017). A moderate drought was generally observed early in the growing season, whereas interannual variability of summer rains determined drought levels between years, i.e., reducing drought stress in 2013-2014 while enhancing it in 2015-2017. Transpiration and basal area increment (BAI) of both pines were coupled throughout June-July, resulting in a high but relatively constant early season WUE. In contrast, both pines showed high interannual plasticity in late-season WUE, with a predominant role of stem growth in driving WUE. Overall, bristlecone pine was characterized by a lower WUE compared to limber pine. Dry or wet episodes in the late growing season overrode species differences. Our results suggested thresholds of vapor pressure deficit and soil moisture that would lead to opposite responses of WUE to late-season dry or wet conditions. These findings provide novel insights and clarify potential mechanisms modulating tree-level WUE in sky-island ecosystems of semi-arid regions, thereby helping land managers to design appropriate science-based strategies and reduce uncertainties associated with the impact of future climatic changes.

Wood Anatomy of Douglas-Fir in Eastern Arizona and Its Relationship With Pacific Basin Climate.

Dendroclimatic reconstructions, which are a well-known tool for extending records of climatic variability, have recently been expanded by using wood anatomical parameters. However, the relationships between wood cellular structures and large-scale climatic patterns, such as El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), are still not completely understood, hindering the potential for wood anatomy as a paleoclimatic proxy. To better understand the teleconnection between regional and local climate processes in the western United States, our main objective was to assess the value of these emerging tree-ring parameters for reconstructing climate dynamics. Using Confocal Laser Scanning Microscopy, we measured cell lumen diameter and cell wall thickness (CWT) for the period 1966 to 2015 in five Douglas-firs [Pseudotsuga menziesii (Mirb.) Franco] from two sites in eastern Arizona (United States). Dendroclimatic analysis was performed using chronologies developed for 10 equally distributed sectors of the ring and daily climatic records to identify the strongest climatic signal for each sector. We found that lumen diameter in the first ring sector was sensitive to previous fall-winter temperature (September 25th to January 23rd), while a precipitation signal (October 27th to February 13th) persisted for the entire first half of the ring. The lack of synchronous patterns between trees for CWT prevented conducting meaningful climate-response analysis for that anatomical parameter. Time series of lumen diameter showed an anti-phase relationship with the Southern Oscillation Index (a proxy for ENSO) at 10 to 14year periodicity and particularly in 1980-2005, suggesting that chronologies of wood anatomical parameters respond to multidecadal variability of regional climatic modes. Our findings demonstrate the potential of cell structural characteristics of southwestern United States conifers for reconstructing past climatic variability, while also improving our understanding of how large-scale ocean-atmosphere interactions impact local hydroclimatic patterns.

Photoperiod and temperature as dominant environmental drivers triggering secondary growth resumption in Northern Hemisphere conifers.

Wood formation consumes around 15% of the anthropogenic CO2 emissions per year and plays a critical role in long-term sequestration of carbon on Earth. However, the exogenous factors driving wood formation onset and the underlying cellular mechanisms are still poorly understood and quantified, and this hampers an effective assessment of terrestrial forest productivity and carbon budget under global warming. Here, we used an extensive collection of unique datasets of weekly xylem tissue formation (wood formation) from 21 coniferous species across the Northern Hemisphere (latitudes 23 to 67°N) to present a quantitative demonstration that the onset of wood formation in Northern Hemisphere conifers is primarily driven by photoperiod and mean annual temperature (MAT), and only secondarily by spring forcing, winter chilling, and moisture availability. Photoperiod interacts with MAT and plays the dominant role in regulating the onset of secondary meristem growth, contrary to its as-yet-unquantified role in affecting the springtime phenology of primary meristems. The unique relationships between exogenous factors and wood formation could help to predict how forest ecosystems respond and adapt to climate warming and could provide a better understanding of the feedback occurring between vegetation and climate that is mediated by phenology. Our study quantifies the role of major environmental drivers for incorporation into state-of-the-art Earth system models (ESMs), thereby providing an improved assessment of long-term and high-resolution observations of biogeochemical cycles across terrestrial biomes.

Moisture-driven xylogenesis in Pinus ponderosa from a Mojave Desert mountain reveals high phenological plasticity.

Future seasonal dynamics of wood formation in hyperarid environments are still unclear. Although temperature-driven extension of the growing season and increased forest productivity are expected for boreal and temperate biomes under global warming, a similar trend remains questionable in water-limited regions. We monitored cambial activity in a montane stand of ponderosa pine (Pinus ponderosa) from the Mojave Desert for 2 consecutive years (2015-2016) showing opposite-sign anomalies between warm- and cold-season precipitation. After the wet winter/spring of 2016, xylogenesis started 2 months earlier compared to 2015, characterized by abundant monsoonal (July-August) rainfall and hyperarid spring. Tree size did not influence the onset and ending of wood formation, highlighting a predominant climatic control over xylem phenological processes. Moisture conditions in the previous month, in particular soil water content and dew point, were the main drivers of cambial phenology. Latewood formation started roughly at the same time in both years; however, monsoonal precipitation triggered the formation of more false rings and density fluctuations in 2015. Because of uncertainties in future precipitation patterns simulated by global change models for the Southwestern United States, the dependency of P. ponderosa on seasonal moisture implies a greater conservation challenge than for species that respond mostly to temperature conditions.

Wood Cellular Dendroclimatology: Testing New Proxies in Great Basin Bristlecone Pine.

Dendroclimatic proxies can be generated from the analysis of wood cellular structures, allowing for a more complete understanding of the physiological mechanisms that control the climatic response of tree species. Century-long (1870-2013) time series of anatomical parameters were developed for Great Basin bristlecone pine (Pinus longaeva D.K. Bailey) by capturing strongly contrasted microscopic images through a Confocal Laser Scanning Microscope. Environmental information embedded in wood anatomical series was analyzed in comparison with ring-width series using measures of empirical signal strength. Response functions were calculated against monthly climatic variables to evaluate climate sensitivity of cellular features (e.g., lumen area; lumen diameter) for the period 1950-2013. Calibration-verification tests were used to determine the potential to generate long climate reconstructions from these anatomical proxies. A total of eight tree-ring parameters (two ring-width and six chronologies of xylem anatomical parameters) were analyzed. Synchronous variability among samples varied among tree-ring parameters, usually decreasing from ring-width to anatomical features. Cellular parameters linked to plant hydraulic performance (e.g., tracheid lumen area and radial lumen diameter) showed empirical signal strength similar to ring-width series, while noise was predominant in chronologies of lumen tangential width and cell wall thickness. Climatic signals were different between anatomical and ring-width chronologies, revealing a positive and temporally stable correlation of tracheid size (i.e., lumen and cell diameter) with monthly (i.e., March) and seasonal precipitation. In particular, tracheid lumen diameter emerged as a reliable moisture indicator and was then used to reconstruct total March-August precipitation from 1870 to 2013. Wood anatomy holds great potential to refine and expand dendroclimatic records by allowing estimates of plant physiological adaptations to external stressors. Integrating xylem cellular features with ring-width chronologies can widen our understanding of past climatic variability (including annual extreme events) and improve the evaluation of long-term plant response to drought, especially in connection with future warming scenarios.

Environmental drivers of cambial phenology in Great Basin bristlecone pine.

The timing of wood formation is crucial to determine how environmental factors affect tree growth. The long-lived bristlecone pine (Pinus longaeva D. K. Bailey) is a foundation treeline species in the Great Basin of North America reaching stem ages of about 5000 years. We investigated stem cambial phenology and radial size variability to quantify the relative influence of environmental variables on bristlecone pine growth. Repeated cellular measurements and half-hourly dendrometer records were obtained during 2013 and 2014 for two high-elevation stands included in the Nevada Climate-ecohydrological Assessment Network. Daily time series of stem radial variations showed rehydration and expansion starting in late April-early May, prior to the onset of wood formation at breast height. Formation of new xylem started in June and lasted until mid-September. There were no differences in phenological timing between the two stands, or in the air and soil temperature thresholds for the onset of xylogenesis. A multiple logistic regression model highlighted a separate effect of air and soil temperature on xylogenesis, the relevance of which was modulated by the interaction with vapor pressure and soil water content. While air temperature plays a key role in cambial resumption after winter dormancy, soil thermal conditions coupled with snowpack dynamics also influence the onset of wood formation by regulating plant-soil water exchanges. Our results help build a physiological understanding of climate-growth relationships in P. longaeva, the importance of which for dendroclimatic reconstructions can hardly be overstated. In addition, environmental drivers of xylogenesis at the treeline ecotone, by controlling the growth of dominant species, ultimately determine ecosystem responses to climatic change.

Climatic influences on wood anatomy and tree-ring features of Great Basin conifers at a new mountain observatory.

PREMISE OF THE STUDY: A network of mountain observing stations has been installed in the Great Basin of North America. NevCAN (Nevada Climate-ecohydrological Assessment Network), which spans a latitudinal range of 2.5° and two elevation ranges of about 2000 m each, enabled us to investigate tree growth in relation to climate. • METHODS: We analyzed wood anatomy and tree-ring characteristics of four conifer species in response to different levels of water availability by comparing a low- and a high-elevation population. Chronologies of earlywood and latewood widths, as well as cellular parameters, were developed from the year 2000 to 2012. • RESULTS: At the southern (drier and warmer) sites, Pinus monophylla had smaller cell lumen, tracheid diameter, and cell wall thickness. Pinus monophylla and P. flexilis showed bigger cellular elements at the higher elevations, whereas the opposite pattern was found in Picea engelmannii and Pinus longaeva. When all species and sites were pooled together, stem diameter was positively related with earlywood anatomical parameters. • DISCUSSION: We have provided a glimpse of the applications that NevCAN, as a new scientific tool, could allow in the general field of botany. In particular, we were able to investigate how differences in water stress related to elevation lead to changes in xylem anatomy.