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

Summary 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.

Table S1 Diameter at breast height and tree height for the pine (Pinus sylvestris) trees equipped with dendrometer and sap flow sensors and group into subplots with treatment.

Table S2
Model parameters and their values for the three different treatments and the two scenarios.

Table S3
Sensitivity analyses of model output to changes of the turnover rates of needles, sapwood and carbon reserves.

Methods S2
Calculation of environmental index.   show the modelled GRO with different needle turnover rates varying between 1 and 10 years (bold line = 5). b) Changes of the R2 between measured and modelled GRO when varying the respective turnover rates. c) Changes of the R2 between measured and modelled needle length (NL) when varying the respective turnover rates. d) Changes of the R2 between measured and modelled shoot length (SL) when varying the respective turnover rates.    TO rate needles 2180 no effect no effect TO rate sapwood 2.3 no effect no effect TO rate C-pool 1 no effect 1 Methods S1 Model equations.

Model input
The model input is an index value between -1 (poor) and +1 (rich) corresponding to the environmental condition (ENV). The calculation of ENV is described in Methods S2. Fig. 2)

Needle length growth (NL) (arrows 1 and 2 in
where NL is needle length growth, BS is the bud status before the new growing period, ENV is the environmental condition, and WF1 and WF2 are the respective weighting factors. Fig. 2)

Shoot length growth (SL) (arrows 3 and 4 in
where SL is shoot length growth, BS is the bud status before the new growing period, ENV is the environmental condition, and WF3 and WF4 are the respective weighting factors. Fig. 2)

Crown growth (CG) (arrow 5 in
where CG is crown growth, and WF5 and WF6 are the respective weighting factors.

Carbon reserve growth (CRG) (arrows 6 and 7)
where CRG is the growth in carbon reserves, and WF7 and WF8 are the according weighting factors.

Crown status (CS) (arrows 13 and 14)
CSnew = (CSold * TO_rateneedles + CG)/(1 + TO_rateneedles) Eq. 6 where CSnew is the new crown status after the growing period, CSold is the old crown status before the growing period, and TO_rateneedles quantifies the turnover rate of the needles.

Bud status (BS) (arrows 15 and 16)
where BSnew is the new bud status after the growing period, CRSnew is the current carbon reserve status and WF14 and WF15 are the respective weighting factor.

Sapwood status (SS) (arrows 19 and 20)
where SSnew is the new sapwood status after the growing period, and TO_ratesapwood is the lifespan of the sapwood.

Methods S2 Calculation of environmental index
where Indexsoil is the index of the volumetric soil water content (VWC) in the year y relative to the mean VWC of control (ctr) and irrigated (irr) plots of a reference period n. The index ranges from -1 to +1.
where IndexPrecipIrr is the index of the precipitation (Precip) including the additionally irrigated water (Irr) in the year y relative to the mean value in control (ctr) and irrigated (irr) plots of a reference period n. The index ranges from -1 to +1.
where IndexTempRad is the index of temperature (Temp) and net radiation (Rad) in the year y relative to the mean value of a reference period n. The index ranges from -1 to +1. where IndexENV is the environmental (ENV) index averaging the indices from Eq. 10-12. The index can range from -1 to +1 and was used as input for the model (Fig. 2).