Water Use by Chinese Pine Is Less Conservative but More Closely Regulated Than in Mongolian Scots Pine in a Plantation Forest, on Sandy Soil, in a Semi-Arid Climate

The diversity of plant water use patterns among species and ecosystems is a matter of widespread debate. In this study, Chinese pine (Pinus tabuliformis, CP) and Mongolian Scots pine (Pinus sylvestris var. mongolica, MP), which is co-exist in the shelterbelt plantations in the Horqin Sandyland in northern China, were chosen for comparison of water use traits by monitoring xylem sap flow alongside recordings of the associated environmental factors over four growing seasons. Continuous sap flux density measurements were converted into crown projected area transpiration intensity (T_r) and canopy stomatal conductance (G_s). The results indicated that MP showed a higher canopy transpiration intensity than in CP, with T_r daily means (±standard deviation) of 0.84 ± 0.36 and 0.79 ± 0.43 mm⋅d^-1, respectively (p = 0.07). However, the inter-annual variability of daily T_r in MP was not significant, varying only approximately a 1.1-fold (p = 0.29), while inter-annual variation was significant for CP, with 1.24-fold variation (p < 0.01). In particular, the daily mean T_r value for CP was approximately 1.7-times higher than that of MP under favorable soil moisture conditions, with values for relative extractable soil water within the 0-1.0 m soil layer (REW) being above 0.4. However, as the soil dried out, the value of T_r for CP decreased more sharply, falling to only approximately 0.5-times the value for MP when REW fell to < 0.2. The stronger sensitivity of T_r and/or G_s to REW, together with the more sensitive response of G_s to VPD in CP, confirms that CP exhibits less conservation of soil water utilization but features a stronger ability to regulate water use. Compared with MP, CP can better adapt to the dry conditions associated with climate change.

Ecophysiological variation of transpiration of pine forests: synthesis of new and published results

Canopy transpiration (E_C ) is a large fraction of evapotranspiration, integrating physical and biological processes within the energy, water, and carbon cycles of forests. Quantifying E_C is of both scientific and practical importance, providing information relevant to questions ranging from energy partitioning to ecosystem services, such as primary productivity and water yield. We estimated E_C of four pine stands differing in age and growing on sandy soils. The stands consisted of two wide-ranging conifer species: Pinus taeda and Pinus sylvestris, in temperate and boreal zones, respectively. Combining results from these and published studies on all soil types, we derived an approach to estimate daily E_C of pine forests, representing a wide range of conditions from 35° S to 64° N latitude. During the growing season and under moist soils, maximum daily E_C (E_Cm ) at day-length normalized vapor pressure deficit of 1 kPa (E_Cm-ref ) increased by 0.55 ± 0.02 (mean ± SE) mm/d for each unit increase of leaf area index (L) up to L = ~5, showing no sign of saturation within this range of quickly rising mutual shading. The initial rise of E_Cm with atmospheric demand was linearly related to E_Cm-ref . Both relations were unaffected by soil type. Consistent with theoretical prediction, daily E_C was sensitive to decreasing soil moisture at an earlier point of relative extractable water in loamy than sandy soils. Our finding facilitates the estimation of daily E_C of wide-ranging pine forests using remotely sensed L and meteorological data. We advocate an assembly of worldwide sap flux database for further evaluation of this approach.

Transpiration characteristics of a rubber plantation in central Cambodia

The rapid and widespread expansion of rubber plantations in Southeast Asia necessitates a greater understanding of tree physiology and the impacts of water consumption on local hydrology. Sap flow measurements were used to study the intra- and inter-annual variations in transpiration rate (Et) in a rubber stand in the low-elevation plain of central Cambodia. Mean stand sap flux density (JS) indicates that rubber trees actively transpire in the rainy season, but become inactive in the dry season. A sharp, brief drop in JS occurred simultaneously with leaf shedding in the middle of the dry season in January. Although the annual maxima of JS were approximately the same in the two study years, the maximum daily stand Et of ∼2.0 mm day(-1) in 2010 increased to ∼2.4 mm day(-1) in 2011. Canopy-level stomatal response was well explained by changes in solar radiation, vapor pressure deficit, soil moisture availability, leaf area, and stem diameter. Rubber trees had a relatively small potential to transpire at the beginning of the study period, compared with average diffuse-porous species. After 2 years of growth in stem diameter, transpiration potential was comparable to other species. The sensitivity of canopy conductance (gc) to atmospheric drought indicates isohydric behavior of rubber trees. Modeling also predicted a relatively small sensitivity of gc to the soil moisture deficit and a rapid decrease in gc under extreme drought conditions. However, annual observations suggest the possibility of a change in leaf characteristics with tree maturity and/or initiation of latex tapping. The estimated annual stand Et was 469 mm year(-1) in 2010, increasing to 658 mm year(-1) in 2011. Diagnostic analysis using the derived gc model showed that inter-annual change in stand Et in the rapidly growing young rubber stand was determined mainly by tree growth rate, not by differences in air and soil variables in the surrounding environment. Future research should focus on the potentially broad applicability of the relationship between Et and tree size as well as environmental factors at stands different in terms of clonal type and age.

Water-use responses of 'living fossil' conifers to CO2 enrichment in a simulated Cretaceous polar environment

BACKGROUND AND AIMS

During the Mesozoic, the polar regions supported coniferous forests that experienced warm climates, a CO(2)-rich atmosphere and extreme seasonal variations in daylight. How the interaction between the last two factors might have influenced water use of these conifers was investigated. An experimental approach was used to test the following hypotheses: (1) the expected beneficial effects of elevated [CO(2)] on water-use efficiency (WUE) are reduced or lost during the 24-h light of the high-latitude summer; and (2) elevated [CO(2)] reduces plant water use over the growing season.

METHODS

Measurements of leaf and whole-plant gas exchange, and leaf-stable carbon isotope composition were made on one evergreen (Sequoia sempervirens) and two deciduous (Metasequoia glyptostroboides and Taxodium distichum) 'living fossil' coniferous species after 3 years' growth in controlled-environment simulated Cretaceous Arctic (69 degrees N) conditions at either ambient (400 micromol mol(-1)) or elevated (800 micromol mol(-1)) [CO(2)].

KEY RESULTS

Stimulation of whole-plant WUE (WUE(P)) by CO(2) enrichment was maintained over the growing season for the three studied species but this pattern was not reflected in patterns of WUE inferred from leaf-scale gas exchange measurements (iWUE(L)) and delta(13)C of foliage (tWUE(L)). This response was driven largely by increased rates of carbon uptake, because there was no overall CO(2) effect on daily whole-plant transpiration or whole-plant water loss integrated over the study period. Seasonal patterns of tWUE(L) differed from those measured for iWUE(L). The results suggest caution against over simplistic interpretations of WUE(P) based on leaf isotopic composition.

CONCLUSIONS

The data suggest that the efficiency of whole-tree water use may be improved by CO(2) enrichment in a simulated high-latitude environment, but that transpiration is relatively insensitive to atmospheric CO(2) in the living fossil species investigated.

Sensitivity of stomatal and canopy conductance to elevated CO2 concentration - interacting variables and perspectives of scale

•  The hydrological response of forests to rising CO₂ is a critical biotic feedback in the study of global climate change. Few studies, however, have investigated this highly dynamic response at relevant temporal and spatial scales. •  A combination of leaf and whole-tree measurements and stand-level extrapolations were used to assess how stomatal conductance, canopy transpiration and conductance, and evapotranspiration might be affected by future, higher CO₂ concentrations. •  Midday measurements of stomatal conductance for leaves sampled in a 12-yr-old sweetgum (Liquidambar styraciflua) stand exposed to free-air CO₂ enrichment were up to 44% lower at elevated than at ambient CO₂ concentrations, whereas canopy conductance, averaged over the growing season, was only 14% lower in stands exposed to CO₂ enrichment. The magnitude of this response was dependent on vapor pressure deficit and soil water potential. Annual estimates of evapotranspiration showed relatively small reductions due to atmospheric CO₂ enrichment. •  These data illustrate that the hydrological response of a closed-canopy plantation to elevated CO₂ depends on the temporal and spatial scale of observation. They emphasize the importance of interacting variables and confirm that integration of measurements over space and time reduce what, at the leaf level, might otherwise appear to be a large and significant response.