Flora - Morphology, Distribution, Functional Ecology of Plants
Comparing growth phenology of co-occurring deciduous and evergreen conifers exposed to drought
Introduction
Plants are finely tuned to the seasonality of their environment and coordinate the timing of activity in different organs, i.e., root, stem, shoot and foliage (for a review see Polgar and Primack, 2011). To evaluate impact of climate extremes on tree growth, environmental signals that trigger phenological events like growth onset and end, and maximum growth rate need to be elucidated. Differences in plant phenology among species indicate different responses to abiotic factors (temperature, photoperiod, drought) or endogenous control (e.g., carbohydrate availability, hormonal regulation). It is well established that in temperate and boreal trees the timing of growth resumption after winter dormancy is temperature driven (Begum et al., 2013, Hänninen and Tanino, 2011, Linkosalo et al., 2006). As a result of climate warming the spring phenology of trees has advanced during recent decades (Cleland et al., 2007, Menzel and Sparks, 2006), although photoperiodic constraints restrict response of some boreal and temperate tree species to increasing temperatures (Körner and Basler, 2010, Wareing, 1956). The duration of growth also defines the period where damage from extreme weather events like late or early frost, drought or heat waves may occur (Cannell and Smith, 1986, Hänninen, 1991). That phenology is also responsive to rainfall and water availability was reported for Mediterranean regions and tropical dry forests (e.g., Bernal et al., 2011, Borchert, 1994, Peñuelas et al., 2004). Hence, monitoring growth phenology of different organs and their coordination is a useful approach for characterizing the tree growth sensitivity to occurrence of climate extremes expected to occur more often in a warmer climate (IPCC, 2007).
The growth phenology of different tree organs was compared within and among species of cold environments, i.e., of boreal (Zhai et al., 2012) or alpine treeline species (Rossi et al., 2009), and temperate forest trees (Cuny et al., 2012, Michelot et al., 2012). Moser et al. (2010) found that needles in Larix decidua appeared several weeks before onset of radial stem growth occurred, which is in agreement with the hypothesis that resumption of cambial growth after winter dormancy is influenced by auxin coming from buds, developing leaves and shoots (Savidge, 1988, Sundberg and Uggla, 1998). On the other hand, Rossi et al. (2009) and Zhai et al. (2012) detected earlier reactivation of radial stem growth with respect to shoot growth in coniferous species including Larix decidua corroborating the absence of a relationship between timing of cambial reactivation and auxin levels (e.g., Funada et al., 2002). These contrasting results need further clarification and we are also not aware of any study comparing growth phenology of aboveground tree organs (i.e., shoot, foliage and stem) from boreal-montane conifer species in response to drought, although drought is considered one of the main climatic constraints for tree growth (Allen et al., 2010) and water is required for extension of foliage, shoot and water conducting cells in the stem.
In dry inner Alpine valleys soil water availability represents a significant environmental constraint to tree growth (e.g., Eilmann and Rigling, 2012, Schuster and Oberhuber, 2013). Swidrak et al. (2011) reported that in Pinus sylvestris exposed to early season drought in an inner Alpine environment, temperature controls onset of radial growth after winter dormancy. On the other hand, drought occurring during summer was found to induce premature growth cessation in conifers (e.g., Gonzalez-Benecke et al., 2010, Pichler and Oberhuber, 2007). Because phenological assessment in these studies is restricted to radial growth, a comparative assessment of intra-annual dynamics of apical and lateral growth among co-occurring conifers exposed to drought is considered as necessary to evaluate growth responses of different organs and tree species to drought.
Therefore, the objectives of this study were to compare (i) seasonal phenology of needle, shoot and stem growth within and among trees in a dry-mesic mixed-conifer forest where early successional Larix decidua and Pinus sylvestris and late successional Picea abies co-occur and (ii) drought sensitivity of main phenological events, i.e., onset, time of maximum growth, growth duration and end. Based on the hypothesis stated by Körner and Basler (2010) that early-successional species adopt riskier life strategies than late-successional species, we expected that growth resumption after winter dormancy occurs earlier and growth ceases later in early successional Larix decidua and Pinus sylvestris than in late successional Picea abies.
Furthermore, we hypothesized that growth resumption is controlled by temperature and unresponsive to drought while growth cessation is sensitive to water availability and that lateral growth onset at breast height occurs after shoot and needle growth in the crown has begun.
Section snippets
Study area
The study site is part of a postglacial rock-slide area situated in the montane belt (c. 750 m a.s.l.) within the inner Alpine dry valley of the Inn River (Tyrol, Austria, 47°13′53″ N, 10°50′51″ E) and has a relatively continental climate with mean annual precipitation and temperature of 716 mm and 7.3 °C, respectively (long-term mean during 1911–2010 at Ötz, 812 m a.s.l., 5 km from the study area). The dominating plant community is an open Spring Heath-Pine wood (Erico-Pinetum typicum). On
Environmental variables during growing seasons 2011 and 2012
Climate in 2011 and 2012 distinctly was different at the start of the growing season in spring. Mean daily air temperatures above canopy in April and May were 3.3 and 0.7 °C higher in 2011, compared to 2012. Furthermore, an almost continuous drought period lasted from 19 March to 13 May 2011 (Fig. 1a and b), which caused soil water content to drop to c. 5 Vol. % in May at both plots. In 2012 frequent rainfall events in March and April caused high soil moisture content (c. 25 Vol. %) until May,
Discussion
It is well established that temperature strongly effects growth resumption after winter dormancy in plants (Körner, 2006, Lüttge and Hertel, 2009, Menzel and Sparks, 2006). Consistently, the observed earlier onset of aboveground growth of the investigated trees in 2011 compared to 2012 is related to exceptionally warm temperatures prevailing in spring 2011. On the other hand, soil water content dropped continuously from late March through early May 2011 on both plots. It is reasonable to assume
Acknowledgments
This work was supported by the Austrian Science Fund (FWF), project number P22280-B16 “Conifer radial stem growth in response to drought”. We thank Sylvia Farbmacher for help with recording growth phenology in 2011. We also thank the editor-in-chief, Prof. Rainer Lösch, and two anonymous reviewers for their valuable suggestions and comments to improve the manuscript.
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