Predicting the decline in daily maximum transpiration rate of two pine stands during drought based on constant minimum leaf water potential and plant hydraulic conductance

Abstract

The effect of drought on forest water use is often estimated with models, but comprehensive models require many parameters, and simple models may not be sufficiently flexible. Many tree species, Pinus species in particular, have been shown to maintain a constant minimum leaf water potential above the critical threshold for xylem embolism during drought. In such cases, prediction of the relative decline in daily maximum transpiration rate with decreasing soil water content is relatively straightforward. We constructed a soil-plant water flow model assuming constant plant conductance and daily minimum leaf water potential, but variable conductance from soil to root. We tested this model against independent data from two sites: automatic shoot chamber data and sap flow measurements from a boreal Scots pine (Pinus sylvestris L.) stand; and sap flow measurements from a maritime pine (Pinus pinaster Ait.) stand. To focus on soil limitations to water uptake, we expressed daily maximum transpiration rate relative to the rate that would be obtained in wet soil with similar environmental variables. The comparison was successful, although the maritime pine stand showed carry-over effects of the drought that we could not explain. For the boreal Scots pine stand, daily maximum transpiration was best predicted by water content of soil deeper than 5 cm. A sensitivity analysis revealed that model predictions were relatively insensitive to the minimum leaf water potential, which can be accounted for by the importance of soil resistance of drying soil. We conclude that a model with constant plant conductance and minimum leaf water potential can accurately predict the decline in daily maximum transpiration rate during drought for these two pine stands, and that including further detail about plant compartments would add little predictive power, except in predicting recovery from severe drought.

Comparative Drought Responses of Quercus ilex L. and Pinus sylvestris L. in a Montane Forest Undergoing a Vegetation Shift

“…This is consistent with other studies in pines that have shown that (1) a strong relationship between transpiration and soil moisture is also found at a depth of 0-25 cm [53] and (2) even if water can be extracted from deep layers, the topsoil layers, where most roots are located, have a larger influence on the rate of water uptake [54]. Compared to P. sylvestris, Q. ilex showed lower ΨPD and a steeper decline of this variable with decreasing SWC.…”

Autumn temperature and carbon balance of a boreal Scots pine forest in Southern Finland

“…The extreme autumns in terms of soil moisture content were 1998 (moist) and 2002 (dry and cold). In summer 2006 the forest was suffering from intensive drought (not shown), which reduced both the ecosystem respiration and gross photosynthesis and turned the stand to be a carbon source (Duursma et al, 2007) over two weeks in August. As a consequence, the early autumn (September-October) 2006 has the lowest θ values but in November the soil moisture content returned to the typical level.…”

MAESPA: a model to study interactions between water limitation, environmental drivers and vegetation function at tree and stand levels, with an example application to [CO<sub>2</sub>] × drought interactions

“…The soil-to-root resistance (MPa s m 2 (ground) mol −1 ) is estimated with the single root model of Gardner (1960) (see Williams et al, 2001a andDuursma et al, 2008 for more details). This model estimates the effective path length for water transport through the soil matrix to the root surface from the fine root density.…”

“…Although k P typically does decrease during drought due to formation of air-filled vessels (Sperry ,2000), Duursma et al (2008) showed that a model assuming a fixed k P was successful in predicting the response of plant water use to water limitation, in part because the soil resistance becomes limiting to water transport (Fisher et al, 2006). Nonetheless, gradual reduction in k P during drought is often an important determinant of plant water use (Sperry et al, 1998;Hacke et al, 2000).…”