Short- and long-term responses to seasonal drought in ponderosa pines growing at different plantation densities in Patagonia, South America

Abstract

Trees drought responses could be developed in the short- or in the long-term, aiming at sustaining carbon fixation and water use efficiency (WUE). The objective of this study was to examine short- and long-term adjustments occurring in different size Pinus ponderosa Dougl. ex P. & C. Laws trees in response to seasonal drought when they are growing under different competition level. The following variables were studied: branch and stem hydraulic conductivity, canopy and stomatal conductance (gc, gs), transpiration (E), photosynthesis (A _max), wood δ¹³C (as a proxy of intrinsic WUE), leaf to sapwood area ratio (A _L:A _s) and growth in the biggest (B) and the smallest (S) trees of high (H) and low (L) density stands. A _L:A _s was positively correlated with tree size and negatively correlated with competition level, increasing leaf hydraulic conductance in H trees. Accordingly, higher gc and E per unit A _L were found in H than in L trees when soil water availability was high, but decreased abruptly during dry periods. BL trees maintained stable gc and E values even during the summer drought. The functional adjustments observed in H trees allow them to maintain their hydraulic integrity (no apparent k _s losses), but their stem and leaf growth were severely affected by drought events. _iWUE was similar between all tree groups in a wet season, whereas it significantly decreased in SH trees in a dry season suggesting that when radiation and water are co-limiting gas exchange, functional adjustments not only affect absolute growth, but also WUE.

Appendix 1

Canopy conductance

Canopy conductance (gc, mm s⁻¹) was estimated according to Monteith and Unsworth (1990) as:

$$ gc = \frac{\gamma (T)\lambda (T)E}{{C_{\text{p}} \rho (T)\Updelta W}} $$

(2)

where γ(T) is the psychrometric constant (as a function of temperature T, kPa K⁻¹), λ(T) is the latent heat of vaporization of water (J kg⁻¹), E is transpiration expressed on A _L basis (kg m⁻²s⁻¹), C _p is the specific heat of air, ρ(T) is the density of the air (kg m⁻³), and ΔW is the leaf-to-air vapor pressure difference (or gradient, kPa), assuming that leaf temperature was the same as air temperature.

Sensitivity of stomata to VPD

It was estimated following Oren et al. (1999) as:

$$ gc = \, - m{ \ln }\left( {\text{VPD}} \right) + b $$

(3)

where m is the stomatal sensitivity to VPD (mm s⁻¹ ln(kPa)⁻¹) and b the reference gc at VPD = 1 kPa (mm s⁻¹). Parameters were estimated using least squares regression analysis. No lag time between sap flux data and VPD was observed, as was also reported in Fischer et al. (2002).

Specific conductivity of branch wood

Specific conductivity (k _s, kg m⁻¹ MPa⁻¹ s⁻¹) is a measure of the hydraulic efficiency of a unit of xylem and is defined by Darcy′s law as:

$$ k_{\text{s}} = \frac{Ql}{{A_{\text{S}} \Updelta P}} $$

(4)

where Q is the volume flow rate (m³s⁻¹), l is the length of the segment (m), and ΔP is the pressure difference between the two ends of the segment (MPa). All conductivity calculations were corrected to 20 °C to account for changes in fluid viscosity with temperature (Spicer and Gartner 1998). To determine k _s of branch wood, single branches (0.5–1 m long) from five trees growing at each density plot were excised in the afternoon (maximum VPD—maximum loss of k _s), sprayed with water and wrapped in plastic bags to avoid desiccation and the possibility of further k _s reduction by dehydration. In the laboratory, a segment from the middle of the branch was cut under water to avoid new embolisms and attached to the end of a transparent plastic hose. A 1-ml graduated pipette was attached to the other end of the hose, located 1 m above the branch sample. Volume flow rate was measured by timing the movement of the meniscus across 0.1 ml graduations. Repeated measurements on the same sample showed that direction of flow did not affect results.

Volume fraction of water in the xylem of the branch

Volume fraction of water (V _w) was determined in 4–5 cm long barked twig segments as:

$$ V_{\text{W}} = \frac{{W_{\text{f}} - W_{\text{d}} }}{{\rho_{\text{w}} W_{\text{f}} }} $$

(5)

where W _f and W _d are the fresh and dry weight of segments, V _f is fresh volume (estimated from the length and the average diameter of both ends), ρ _w is the density of water. Dry weight was determined after oven-drying the samples at 80 °C for 48 h (see Borghetti et al. 1998). Sample weight was measured to the nearest 0.1 mg.

Whole tree plant liquid phase hydraulic conductance

Whole tree plant liquid phase hydraulic conductance (Kh, ml min⁻¹ MPa⁻¹) was estimated as:

$$ Kh = \frac{E}{{\psi {\text{soil}} - \psi md}} $$

(6)

where ψsoil is the soil water potential (estimated from ψpd) and E is the average transpiration estimated from 14:00 to 19:00 h taking into account the radial sapflow decrease (Gyenge et al. 2003). Calculation of Kh was limited to those dates in which water potential was measured.