Introduction
Understanding how tree growth responds to changes in temperature and precipitation along latitude is crucial for an accurate prediction of future changes in forest dynamics caused by global climate change, especially in Northern boreal ecosystems (Huang et al. 2020; Zhang, Belien, Ren, Rossi, and Huang 2020). A latitudinal gradient is associated with consistent temperature changes, which acts as a potential natural laboratory to help infer forest responses to global warming (De Frenne et al. 2013). Although there are many studies on tree growth response to latitude, most of them have focused on ring width and latewood density in the context of specific regions and/or species. For instance, using a tree ring database from a dozen species and 762 sites in the International Tree-Ring Data Bank (ITRDB), Wettstein et al. (2011) showed that high-latitude ring-width series were more likely to positively correlate with summer temperature while low-latitude sites commonly showed negative correlations; different species respond differently to temperature and precipitation anomalies (Wettstein, Littell, Wallace, and Gedalof 2011). Björklund et al. (2017) analyzed the latewood density parameters from 27 species and 349 sites in ITERDB and found that correlations between density and width shifted from negative to positive when moving from earlywood to latewood, and temperature response of density varied intra-seasonally in strength and sign (positive or negative correlation coefficients) (Björklund et al. 2017).
Although wood density and ring width are sensitive indicators of a tree’s response to climate, the changes might be driven by traits at the cell level or lower (i.e. micro-structure of cell wall). Conduit dimensions of xylem are accepted as important traits in respect to plant hydraulics and adaptation for both angiosperms and gymnosperms (Baas, Ewers, Davies, and Wheeler 2004). Similar to angiosperm vessels, tracheid characteristics can provide us with valuable information about how conifer species adapt to diverse habitats and their possible responses to global climate change. For instance, stem heating and water manipulation experiments were carried out to study tree growth based on timing and kinetics of the tracheid development process, which showed that xylem traits responded to a significant change in temperature and water availability (Begum, Nakaba, Yamagishi, Oribe, and Funada 2013; Vieira, arvalho, and Campelo 2020). However, latitudinal patterns of tracheids have been surprisingly understudied when compared to vessel traits (Brodribb, Pittermann, and Coomes 2012).
Conifer xylem consists of two types of cells: tracheid (approx. 90-93% xylem surface area) and parenchyma (approx. 7-10% surface area), which is more uniform in pattern and has less cellular diversity than angiosperm xylem (Panshin and Zeeuw 1980; Hacke 2015). Temperate conifer tree rings are mainly composed of two types of tracheid: large and thin-walled earlywood cells that are produced early in the growing season (i.e. springwood) and primarily responsible for water transport, and small but thick-walled latewood tracheids (i.e. summerwood) that are produced after the earlywood cells and serving primarily in mechanical support (Fonti et al. 2013). Therefore, tracheid diameter, length and wall thickness represent useful proxies to examine tracheid cell profiles within a growth rings and across xylem. Understanding the temporal pattern of these traits and how the environment shapes them is of high importance for forest management.
Recent studies on wood formation dynamics have greatly increased our understanding of xylogenesis, i.e. xylem formation (Rossi et al. 2013; Cuny, Rathgeber, Frank, Fonti, and Fournier 2014; Cuny and Rathgeber 2016; Rathgeber, Cuny, and Fonti 2016; Rossi et al. 2016). In short, cell enlargement and secondary cell wall deposition and lignification (wall thickening) are the two fundamental sub-processes of xylogenesis that shape xylem cell dimensions and the resulting tree-ring structure (Cuny et al. 2014). The complex interplay between the duration and rate of xylogenesis determines the changes in cell features, e.g., cell and lumen diameter, lumen area and wall thickness, etc., which in turn creates the anatomical structure driving the wood density profile (Cuny et al. 2014; Cuny and Rathgeber 2016). A recent study on Larixsp. and Picea sp. showed that tracheid cell size and tracheid wall thickness drive variability of earlywood and latewood density in the Northern Hemisphere (Björklund et al. 2017). However, whether this conclusion can be applied to most conifer species remains unknown, especially pertaining to those distributed in humid tropical forests of the Southern Hemisphere (Leslie et al. 2012).
Although tracheid cell development across a tree ring follows a sequence in cell division from cell enlargement, secondary cell wall thickening to cell maturation, the duration and rate of the latter sub-processes are influenced by different climatic factors during their specific developmental periods (Castagneri Fonti, von Arx, and Carrer 2017; Ebisuya and Briscoe 2018). For instance, using a micro-core data set including members of Pinaceae belonging to four genera (i.e.,Abies, Larix, Picea, and Pinus ) from 39 sites in the Northern Hemisphere, Rossi et al. (2016) found that the timing of xylem phenology events and mean annual temperature of the sites were related linearly, with spring and autumn events being separated by constant intervals across the temperature range. The latter authors suggested that the uniformity of the process in wood formation was mainly determined by the environmental conditions occurring at the time of growth resumption (Rossi et al. 2016). In a three-year study on the kinetics of tracheid development of three conifer species (i.e.,Picea abies , Pinus sylvestris , and Abies alba ), Cuny et al. (2014) found that the amount of lumen wall material per cell was quite constant along growth rings, which challenges the widespread understanding that wall thickness is mainly driven by the wall-thickening duration (Cuny et al. 2014). Hence, there is no consensus on the role of the duration and rate of cell enlargement and wall thickening phases on final tracheid cell traits, such as cell size and cell wall area or thickness, while the effects of climatic factors on xylogenesis are often species-specific and site-dependent (Cuny et al. 2014; Rossi et al. 2016; Buttò, Rossi, Deslauriers, and Morin 2019).
Tree growth and xylem structure could be influenced by various climatic conditions along a latitudinal gradient. Towards the colder upper-end of the gradient, low temperatures limit tree growth and ultimately prevent the growth, reproduction, and survival of trees at the tree line (Körner 1998; Lyu et al. 2017). By contrast, at lower regions of latitude, the higher temperatures increase evapotranspiration, raising the level of drought stress on tree growth and changing tree phenology (Lian et al. 2020). Therefore, in such environments, tree growth is typically negatively correlated with temperature but positively with precipitation. Previous dendrochronology studies showed that tree growth of Picea abies responded to temperature and precipitation changes along latitudinal gradients; however, the effect of precipitation was dependent on temperature-induced water stress (Mäkinen et al. 2003). In addition to the gradually changing relationship between temperature and growth, the period of cambial activity and xylem cell growth gradually shortens towards colder areas owing to thermal limitations in wood formation (Rossi Deslauriers, Anfodillo, and Carraro 2007; Huang et al. 2020). The time window in which climatic conditions affect cell development and tree growth, tends to change most with increasing latitude (Körner 1998; Briffa et al. 2002; Henttonen et al. 2014). However, there are relatively few interspecific studies on the effect of climate on xylem anatomical traits compared to the tree ring width for conifer species.
Although previous research suggests that hydraulic traits are correlated with both precipitation and temperature (Martínez-Vilalta et al. 2004), temperature is increasingly recognized as the primary driver of growth reactivation in a cold climate. Both direct observations and controlled experiments have demonstrated that cambial activity is limited by low air temperatures in a cold climate (Rossi et al. 2012; Begum et al. 2013). However, most studies have only focused on temperate or boreal ecosystems where snowmelt provides abundant water especially at the beginning of spring and summer, and is therefore not a limiting factor for xylem formation (Cuny and Rathgeber 2016, Rathgeber et al. 2016, Rossi et al. 2016). Given that xylem cell expansion is a turgor-driven process depending on cellular water uptake and solute accumulation, water availability can affect xylogenesis (Kozlowski and Pallardy, 2002). Researchers have shown that both cell division and expansion are sensitive to water potential (Fonti et al. 2010), and that water deficit is the primary constraint for xylogenesis of Pinus pinaster in a Mediterranean climate (Vieira, Rossi, Campelo, Freitas, and Nabais 2014; Vieira Carvalho, and Campelo 2020). The large range in the thermal thresholds for the onset of xylogenesis in Juniperus przewalskiiprovides additional evidence that temperature was not the only factor initiating xylem growth under such cold and dry conditions (Ren Rossi, Gricar, Liang, and Cufar 2015; Ren et al. 2018). These results suggest that wood formation and xylem structure of various conifer species along a large latitudinal gradient are an interplay between genetics and multiple climatic factors. Therefore, an interspecific study within a phylogenetic context and along a wide latitudinal gradient could shed light on the role of temperature and precipitation on conifer xylem structure.
The global distribution of conifers is as wide as that of angiosperms. Boreal forests in high latitudes consist of a limited number of conifer species (low diversity), whereas at lower latitudes conifer species generally occur on mountains. The latter observations indicate that some conifer species are more adapted to stresses such as cold and drought than angiosperm trees, and therefore could occupy habitats that most angiosperm trees could not (Condamine Silvestro, Koppelhus, and Antonelli 2020), and their adaptation might be attributed to some characteristics of their xylem structure. Furthermore, adaptation to various climatic conditions along a wide latitudinal range should be reflected in the xylem structure, especially in tracheid cell and cell wall dimensions in early-and-latewood owing to construction costs and water transportation constraints of tracheids. To-date, the latitudinal patterns of conifer xylem anatomy has been understudied, and climatic factors driving the variance of xylem structure traits were shown to be ambiguous.
We hypothesize that the xylem structure of conifer species is mainly a consequence of adaptation to cold or drought, thus, differences in tracheid dimensions between early-and latewood would become stronger at higher latitudes, which could be viewed as an ecological strategy of adaptation to changing climate conditions. Among climatic factors, we predict that temperature and precipitation could both impact the xylem structure of conifers under cold or dry conditions, along with elevation. Specifically, we address the following questions: 1) Does a contrast of tracheid dimension between earlywood and latewood persist when accounting for phylogeny? 2) Are there clear trends in xylem structure along a latitudinal gradient? 3) Is temperature the main driver of xylem structure under a monsoonal climate or does xylogenesis depend on the interaction between temperature and precipitation? 4) How much of the xylem structure variance can be explained by climate and phylogeny respectively? To the best of our knowledge, there are no existing cross-species studies that include up to 80 coniferous tree species making this study an important contribution to our knowledge of conifer adaptation.