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During the last few decades there has been increasing evidence that low temperature during the growing season is responsible for the formation of high-elevation treelines (Tranquillini, 1979; Körner, 1998; Jobbágy & Jackson, 2000; Grace et al., 2002). In low temperature-adapted plants, including altitudinal treeline trees, light-saturated photosynthesis reaches c. 50% of full capacity at +5°C, whereas no tree has ever been shown to exhibit significant growth below 5°C (James et al., 1994; Körner, 2006; Solfjeld & Johnsen, 2006; Alvarez-Uria & Körner, 2007). As a consequence, all plants tested so far show an increment in nonstructural carbohydrate (NSC = soluble sugars + starch) tissue concentrations when exposed to low temperatures. Thus, a carbon supply limitation – as proposed by, for example, Stevens & Fox (1991) and Wardle (1993)– is an unlikely cause for low temperature range limits of plants. Rather, the growth-limitation hypothesis (GLH), which claims that cell and tissue formation are the processes that are first limited by the altitudinal decrease in temperature (Körner, 1998; Grace et al., 2002), seems to work given that a growing number of empirical studies have found no decrease, and most often an increase in NSC concentrations with elevation (e.g. Piper et al., 2006; Shi et al., 2008; Fajardo et al., 2011; Hoch & Körner, 2012). It remains, however, unresolved whether this phenotypic pattern reflects a short-term physiological response driven by stronger limitations on growth relative to carbon gain or, rather, an adaptive response of treeline populations to better cope with the harsh conditions at treeline (Sveinbjörnsson, 2000; Smith et al., 2003). Consequently, NSC accumulation with elevation could be an evolutionary response to resist harsh conditions (Monson et al., 2006), a consequence of genetically limited growth (Premoli et al., 2007), with trees under low-temperature conditions allocating more carbon into storage, or the phenotypic result of a direct reduction in growth in cold environments (Körner, 1998). It is the purpose of this study to determine if the increase in carbon reserves with elevation is an immediate physiological response of trees to lower temperatures or not.
Field surveys assessing the GLH have the inherent disadvantage of being unable to measure and account for probable intraspecific, genetic adaptation (i.e. natural selection) differences along elevation. So far, several studies have demonstrated that variation in many morphological and physiological traits along the altitudinal gradient is under genetic control (Oleksyn et al., 1998; Premoli, 2003; Li et al., 2004; Piper et al., 2006; Premoli et al., 2007; Vitasse et al., 2009). For example, Premoli et al. (2007), working in a common garden, found that saplings of Nothofagus pumilio (a treeline species of the southern Andes) originating from a higher elevation grew slower than saplings from a lower elevation. Thus, the possibility remains that the higher levels of NSC found in most of the treeline studies to date are life history traits of treeline populations (ecotypes) selected by fitness gains: a clinal (genetic) adaptation. It would be of great interest to solve this conundrum, in order to determine whether the GLH is acceptable as a functional explanation for treeline formation, and to assess predictions of how treelines will react to increasing temperature and CO2. The fact that many studies thus far have supported the idea that treeline trees are directly growth-limited by low temperature (environmental effect) suggests that trees would not be responsive to increasing atmospheric CO2 (Körner, 1998), but to increasing global temperature. However, if the increasing mobile carbon charge at treeline is attributable to genetic control, the NSC assessment would fail as a proxy to resolve the dilemma between carbon and sink limitations. This could imply that treeline trees are not necessarily CO2-saturated and that they may be directly responsive to increasing atmospheric CO2 supply (e.g. Handa et al., 2005) and not to increasing temperature.
There are at least two types of study that may assist in distinguishing the cause of the increasing levels of NSC of treeline trees: environmentally controlled experiments, either common garden or reciprocal transplants, and natural experiments. In both cases, the idea is to control for the genetic origin of plants and make temperature vary. In the first type of study, using same-provenance seedlings placed in growth chambers (Hoch & Körner, 2009) or hydroponic installations (Solfjeld & Johnsen, 2006), it was shown that lower mean (constant or variable) temperatures (comparable to the milieu of treeline) led to an increase in NSC, demonstrating a phenotypic component in the seedlings’ response. Alternatively, a reciprocal transplant experiment could also work; in this case, treeline seedlings are brought to lower elevation and after several growing seasons NSC concentrations are assessed. Experiments with seedlings have the inconvenience of being an abstraction of what is actually occurring at treeline. Adult trees are the ones ultimately defining the natural treeline position, being the net outcome of many years of growth and reproduction; genetically controlled responses may occur in seedlings but not in adult trees and vice versa (Premoli & Brewer, 2007). Additionally, the GLH has been proposed to explain treeline formation for tall trees as these are more closely coupled to atmospheric conditions as a consequence of their arboreal architecture, whereas seedlings or saplings growing near the ground decouple from the free atmosphere and therefore generally experience warmer conditions than arborescence, taller trees (Körner, 1998; Körner, 2007). To respond to our objective, the existence of natural experiments with adult, tall trees can be reduced to two cases: permafrost tree populations and afforestations. Hoch (2008) found that trees of Picea abies growing on low elevation permafrost accumulated significantly higher NSC concentrations than their immediate, and probably same-origin, neighbours growing on warmer soil. In relation to alpine treelines, NSC accumulation over permafrost has been interpreted as a consequence of growth limitation and not a direct response of acclimation to low temperature (e.g. cryoprotection), as starch rather than soluble sugars (osmotically active compounds) was largely responsible for the increase in NSC concentrations in dwarfed trees over permafrost. Although permafrost soils experience low-temperature conditions comparable to treeline soils, the alpine treeline milieu is not the same, that is, trees do not experience low air temperatures (Körner & Hoch, 2006). Hence, afforestations established along elevational gradients up to treeline elevation and originating from a single provenance (i.e. the same population) are optimal systems with which to test whether the documented changes in the trees’ carbon budget (NSC) at natural treelines are under phenotypic control.
It was the objective of this study to test the following hypothesis: the variation in NSC concentrations of treeline trees is attributable to an immediate (i.e. merely phenotypic) response of the trees’ carbon balance to low temperature. We investigated mature trees of three conifer species, each deriving from a single provenance, in afforestations along altitudinal gradients from lower sites to the elevation of the natural treeline. Our expectations can be summarized as follows (Fig. 1): if a decrease of temperature with elevation is having a direct effect on the carbon status of treeline trees, then there will be a variation in NSC concentrations when trees from a single population are planted along the elevational gradient up to the treeline. Alternatively, if the observed NSC increase in natural treeline species is related to genotypic (evolutionary) differentiation along the elevational gradient, but absolutely unrelated to the decrease in temperature with elevation, then the carbon status of afforested (single provenance) trees should not change with elevation (Fig. 1). Moreover, these trees may show similar carbon status between lower and higher stands or show even a decreasing trend with elevation, if the unadapted trees run into carbon limitation.
Figure 1. Expected trends of nonstructural carbohydrate (NSC) concentration variation with elevation (temperature effect) for trees belonging to one population (plantation of a single provenance) according to possible causes: absolute environment or phenotypic control (left) and absolute genetic control (right). If a decrease in temperature with elevation is altering the NSC concentration of trees naturally occurring in treeline ecotones, then there will be a similar variation when a tree species is planted along the altitudinal gradient up to treeline (i.e. NSC concentration varies with elevation; left side). Alternatively, if there were intraspecific, genotypic (evolutionary) differences with elevation in natural treelines, then the NSC concentration of afforested trees should not be altered along the altitudinal gradient between lower and higher stands (right). In more detail, if there is any variation of NSC with elevation (left), such variation can be explained in terms of carbon limitation (carbon-limitation hypothesis (CLH): a reduction in NSC with elevation) or growth (sink) limitation (growth-limitation hypothesis (GLH): an increase in NSC with elevation).
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