TREELIM – Growth and stress physiology

C2 Growth dynamics and carbon relations

While the dendrology work is still in progress, we can state that there is no indication of a severe growth limitation (inhibition of xylogenesis) of trees at their species specific elevational limit over the past ca. 50 years, although tree rings are sometimes narrower at higher elevation. However, an increase of the frequency of negative pointer years with increasing elevation hints at impacts by by freezing events. We will explore the temperature sensitivity of ring width and the effect of extreme events. In addition, critical temperatures for root growth under controlled conditions suggest – in line with the dendrology work – that there is no direct growth limitation by in situ temperatures during the growing season of broadleaved tree species at their current upper elevational limit.

The distribution limits of broadleaved tree species can either directly be shaped by low temperature extremes (as shown in section C1), or by growing season mean temperatures that could be too low for above and belowground completion of growth of trees, as is the case at the alpine treeline. We studied thermal constraints of above- and belowground growth of broadleaved tree species using experimental approaches and examining growth of trees in situ. The aim was to find species-specific critical mean temperatures, below which roots are unable to grow. In addition, we investigated low temperature effects on the aboveground wood anatomy, growing trees in growth chambers. Finally, we complemented those experiments by a dendrological assessment of the performance of broadleaved trees in situ along elevational gradients, to disentangle the effect of low temperature extremes and low temperature means on secondary growth.

Root growth experiments: We grew saplings and seedlings of 9 different broadleaved tree species in plastic cylinders placed in four water-baths with a vertical temperature gradient from 19 °C to 2 °C along the rooting zone during spring and summer 2011. After the experiment, the soil cores were dissected into 6 horizons. We measured maximum and bulk rooting depth, the dry weight of roots and we used image analysis to measure root length, root diameter and root forkation (branching).

In the ongoing growth chambers experiment we studied 10 different broadleaved tree species of high elevation provenances grown in two climate chambers with controlled photoperiod and temperature, one simulating treeline conditions and the other one, 6 K warmer, served as a control treatment. After one simulated growing season (consisting of 5 months), representing increasing and decreasing temperature and photoperiod, a winter of 3 experimental months was appended. A second experimental growing season is currently ongoing.

In the in situ dendrological work, tree cores of 8 different broadleaved tree species were collected along 3 elevational gradients in the Western Swiss Alps (the same gradients as for population dynamics, see section B2) in autumn 2010 and in autumn 2011. A total of 421 trees were cored twice, once for growth analysis and once for analysis of non structural carbohydrate concentration. The analysis is work in progress.

First results of water-bath experiments show that critical temperatures for root growth of broadleaved tree species are far lower than expected regarding the respective elevational limits. In fact, all temperature at the absolute limits for root growth (single deepest root tip) fall below 5 °C, which is colder than at treeline. Bulk root growth reached less deep into the coldest part of the rooting cylinder. In situ growth assessment revealed that all species except Fagus sylvatica show little growth reduction with elevation. Most species grow well at their elevational limit, with mean tree ring widths exeeding 0.5 mm (maximum ring width at the elevational limit from 1.1 mm for F. sylvatica up to 5.5 mm for Fraxinus excelsior). Interestingly, the frequency of negative event years – i.e. very narrow tree ring widths – increases with increasing elevation for most species (Acer, Fraxinus, Fagus and Laburnum), possibly reflecting effect of late spring frost events.

Taken together with results from freezing trials, it seems that frost during the dehardening period prior to bud break in spring is the most likely cause for upper distribution limits of the broadleaved tree species examined. In agreement with the results of the phenological assessments in growth chambers, Fagus sylvatica seems to be the only species which could potentially be growth limited. For this species, evolutionary pressure is thus expected to select early flushing individuals at higher elevations to maximise the use of the growing season in spite of higher risks of being damaged by frost, which is also in line with our results found in the reciprocal transplant experiment.

Above- and belowground temperature manipulation experiments, as well as in situ dendrological works show that broadleaved tree species are not limited by their ability to perform structural growth at the current elevational distribution limit, with the exception of European beech. The increasing frequency of negative event years with elevation hints towards a limitation by frost damage to meristems.

These are largely unpublished, in part preliminary results, which are the intellectual property of the TREELIM team, and are not eligible for quotation.