Associate Editor Matt McGlone (Landcare Research, New Zealand) has written the Editor’s Choice blog for issue 105.2 on the paper “An assessment of carbon and nutrient limitations in the formation of the southern Andes tree line” by Alex Fajardo and Frida Piper.
Given that tree line globally sits at a growing season isotherm of 6.4 ± 0.7°C (Hoch & Korner 2012), one could be forgiven for assuming that tree line is at the point where carbon gained by trees through photosynthesis is just sufficient to maintain growth, storage and respiration. However, this is not the most widely accepted explanation. It is true that photosynthate acquisition diminishes as temperatures fall, but the formation of new tissue is much more strongly affected.
At 5-6°C, growth virtually ceases while appreciable photosynthesis continues. Thus, the argument goes, it is not carbon limitation that prevents tree growth at high elevations, but the trees inability to invest that carbon in new tissues. If this line of reasoning is correct, the surplus carbon that trees acquire at treeline has to go somewhere else and is stored as non-structural carbohydrates (NSC) – that is, simple sugars and starch (and lipids in some species, mainly conifers). That trees at their upper elevational limit tend to have more NSC in their leaves and twigs has been taken as strong evidence they are limited primarily by sinks (that is growth opportunities) rather than by source (photosynthesis) (Hoch & Korner 2012).
The sink-limitation hypothesis has been challenged lately (Wiley & Helliker 2012; Takahashi & Furuhata 2016). NSCs do not function simply as storage compounds. Starch does, but the sugars have other uses such as osmoregulation useful in preventing tissue freezing. NSC storage, rather than being a passive default choice driven by a carbon surplus, may be actively favoured to enhance long-term tree survival (Martinez-Vilalta et al. 2016). Another contender for limiting growth at treeline has been soil nutrients – mainly N and P – in impoverished alpine soils. Lower N availability, in particular, could limit both photosynthesis and growth.
Our Editor’s Choice for this issue of Journal of Ecology, “An assessment of carbon and nutrient limitations in the formation of the southern Andes tree line” by Alex Fajardo and Frida Piper, simultaneously addresses both the carbon-limitation and soil nutrient-limitation hypotheses in a study of a single deciduous treeline species, Nothofagus pumilio, at four locations from central Chile at 37°S to Tierra del Fuego at 54°S. At each site, a 300m altitudinal sequence of four plots from treeline downwards was established and tree growth over the last 10 years, with NSC twig concentrations and soil and leaf N and P during the growing season measured. Their results show that overall growth declined with altitude but so did NSCs – largely starches. The NSC decline was clear at the two low latitude sites but at the two high latitude sites, there was little change. On the other hand, while soil N and P decreased with altitude, there was no significant difference in leaf N and P between elevations. Nutrient limitations appear to be not a factor.
Where does this leave the carbon sink limitation hypothesis? As Farjado and Piper point out, this universal explanation for tree line needs re-visiting, in particular with regard to deciduous trees. The carbon dynamics of high N deciduous leaves differ from low N evergreen conifer leaves because they incur higher respiratory costs and face a greater risk of cold damage. With increasing altitude the demand for NSCs might, therefore, outstrip carbon supply, leaving less for storage or growth. Climatic factors other than temperature may also play a role in NSC patterns. At the northern Chilean sites, the Mediterranean style dry summers possibly limited Nothofagus pumilio growth at lower altitudes thus reducing the sink demand and increasing NSC storage (Piper et al. 2016).
Of course, it still might be that growth limitations are ultimately responsible for treeline, but it is increasingly unlikely that NSC concentrations provide a definitive proof of this conjecture. As a recent review concluded: ‘tree populations with the highest NSC concentrations may be the most carbon limited in terms of growth; this puts into question many recent conclusions about what limits tree growth’(Wiley & Helliker 2012).
Matt McGlone, Landcare Research, New Zealand