Determinants of legacy effects in pine trees - implications from an irrigation-stop experiment

Tree responses to altered water availability range from immediate (e.g. stomatal regulation) to delayed (e.g. crown size adjustment). The interplay of the different response times and processes, and their effects on long-term whole-tree performance, however, is hardly understood. Here we investigated legacy effects on structures and functions of mature Scots pine in a dry inner-Alpine Swiss valley after stopping an 11-yr lasting irrigation treatment. Measured ecophysiological time series were analysed and interpreted with a system-analytic tree model. We found that the irrigation stop led to a cascade of downregulations of physiological and morphological processes with different response times. Biophysical processes responded within days, whereas needle and shoot lengths, crown transparency, and radial stem growth reached control levels after up to 4 yr only. Modelling suggested that organ and carbon reserve turnover rates play a key role for a tree's responsiveness to environmental changes. Needle turnover rate was found to be most important to accurately model stem growth dynamics. We conclude that leaf area and its adjustment time to new conditions is the main determinant for radial stem growth of pine trees as the transpiring area needs to be supported by a proportional amount of sapwood, despite the growth-inhibiting environmental conditions.Peer reviewe

Age, size, growth, and xylem data for individual trees at Mackay Bar

See README file

Pit and growth rate data for comparison across sites.

See README file

Data from: Conflicting functional effects of xylem pit structure relate to the growth-longevity trade-off in a conifer species

Consistent with a ubiquitous life history tradeoff, trees exhibit a negative relationship between growth and longevity both among and within species. However, the mechanistic basis of this life history tradeoff is not well understood. In addition to resource allocation conflicts among multiple traits, functional conflicts arising from individual morphological traits may also contribute to life history tradeoffs. We hypothesized that conflicting functional effects of xylem structural traits contribute to the growth-longevity tradeoff in trees. We tested this hypothesis by examining the extent to which xylem morphological traits (i.e. wood density, tracheid diameters and pit structure) relate to growth rates and longevity in two natural populations of the conifer species Pinus ponderosa. Hydraulic constraints arise as trees grow larger and xylem anatomical traits adjust to compensate. We disentangled the effects of size through ontogeny in individual trees and growth rates among trees on xylem traits by sampling each tree at multiple trunk diameters. We found that the oldest trees had slower lifetime growth rates compared to younger trees in the studied populations, indicating a growth-longevity tradeoff. We further provide evidence that a single xylem trait, pit structure, with conflicting effects on xylem function (hydraulic safety and efficiency) relates to the growth-longevity tradeoff in a conifer species. This study highlights that, in addition to tradeoffs among multiple traits, functional constraints based on individual morphological traits like that of pit structure provide mechanistic insight into how and when life history tradeoffs arise

Understory removal of a Scots pine forest near Salgesch: Soil volumetric water content and temperature time series, hourly time resolution, 2010-2016

In a dry Scots pine forest near the village Salgesch within the Swiss Canton of Valais, an experiment since 2010 has studied the influence of understory - appeared after cessation of cattle grazing - on the pre-existing overstory and site conditions. The dataset published here compromises 120 time series of 60 soil temperature and 60 volumetric water content (VWC) sensors (EC-TM and 5-TM) (Decagon Devices, WA, USA) buried at three soil depth (5, 30, 60 cm) in the direct vicinity of 12 trees, of which six had their understory removed whilst six others were left untreated (control). At the soil depth of 5 and 60 cm, the measurements have been performed at three versus four locations at 30 cm depth. Six EM50 data loggers have recorded hourly data since April 2010 with 18% gaps or 11% when not considering the winter months (December-February). The figures attached to this repository show, per treatment, the average VWC and temperature of all measurements within the same depth, aggregated in weekly intervals and for the 2010-2016 period. The standard deviations were plotted as transparent polygons. In the case of low data coverage, the setting-specific mean standard deviation of the considered time period was plotted

Improvement of water and light availability after thinning at a xeric site: which matters more? A dual isotope approach

Thinning fosters individual tree growth by increasing the availability of water, light and nutrients. At sites where water rather than light is limiting, thinning also enhances soil evaporation and might not be beneficial. Detailed knowledge of the short- to long-term physiological response underlying the growth responses to thinning is crucial for the management of forests already suffering from recurrent drought-induced dieback. * We applied a dual isotope approach together with mechanistic isotope models to study the physiological processes underlying long-term growth enhancement of heavily thinned Pinus sylvestris in a xeric forest in Switzerland. This approach allowed us to identify and disentangle thinning-induced changes in stomatal conductance and assimilation rate. * At our xeric study site, the increase in stomatal conductance far outweighed the increase in assimilation, implying that growth release in heavily thinned trees is primarily driven by enhanced water availability rather than increased light availability. * We conclude that in forests with relatively isohydric species (drought avoiders) that are growing close to their physiological limits, thinning is recommended to maintain a less negative water balance and thus foster tree growth, and ultimately the survival of forest trees under drought