Long-term hydrological response emerges from forest self-thinning behaviour and tree sapwood allometry.

Fires in forested catchments are of great concern to catchment managers due to their potential effect on water yield. Among other factors such as meteorological conditions and topography, dominant vegetation and its regeneration traits can play a key role in controlling the variability in the type and recovery-time of the hydrological response between forested catchments after stand-replacing fires. In temperate South-Eastern Australia, a long-term reduction in streamflow from catchments dominated by regenerating tall-wet Eucalyptus obligate seeder forests was observed, which has substantial implications for Melbourne's water supply. While the unusual hydrological response has been attributed to the higher water-use of the regrowth forests, the dominant underlying mechanism has not yet been identified. Here we show analytically and with a closed-form solution that this streamflow pattern can emerge from forest dynamics, namely the combination of growth and tree mortality as constrained by the self-thinning line (STL) and sapwood allometry of the dominant overstory tree species under non-limiting rainfall regimes. A sensitivity analysis shows that observed variations in the relative streamflow anomaly trend can be explained by parameters controlling: (i) the shape of the STL; (ii) regeneration success; (iii) radial tree growth rate; and (iv) fire severity. We conclude that the observed variation in long-term post-disturbance streamflow behaviour might have resulted from different trajectories of forest dynamics and suggest that to minimize uncertainty in future water-balance predictions, eco-hydrological models for even aged forests include a mechanistic representation of stand demography processes that are constrained by forest inventory data.

Spatio-temporal transpiration patterns reflect vegetation structure in complex upland terrain.

Topography exerts control on eco-hydrologic processes via alteration of energy inputs due to slope angle and orientation. Further, water availability varies with drainage position in response to topographic water redistribution and the catena effect on soil depth and thus soil water storage capacity. Our understanding of the spatio-temporal dynamics and drivers of transpiration patterns in complex terrain is still limited by lacking knowledge of how systematic interactions of energy and moisture patterns shape ecosystem state and water fluxes and adaptation of the vegetation to these patterns. To untangle the effects of slope orientation and hillslope position on forest structure and transpiration patterns, we measured forest structure, sap flux, soil moisture, throughfall and incoming shortwave radiation along two downslope transects in a forested head water catchment in south-east Australia. Our plot locations controlled for three systematically varying drainage position levels (topographic wetness index: 5.0, 6.5 and 8.0) and two levels of energy input (aridity index: 1.2 and 1.8). Vegetation patterns were generally stronger related to drainage position than slope orientation, whereas sap velocity variations were less pronounced. However, in combination with stand sapwood area, consistent spatio-temporal transpiration patterns emerged in relation to landscape position, where slope orientation was the primary and drainage position the secondary controlling factor. On short temporal scales, radiation and vapor pressure deficit were most important in regulating transpiration rates, whereas soil water limitation only occurred on shallow soils during summer. The importance of stand structural parameters increased on longer time scales, indicating optimization of vegetation in response to the long-term hydro-climatic conditions at a given landscape position. Thus, vegetation patterns can be conceptualized as a 'time-integrated' predictor variable that captures large fractions of other factors contributing to transpiration patterns.

Importance of disturbance history on net primary productivity in the world's most productive forests and implications for the global carbon cycle.

Analysis of growth and biomass turnover in natural forests of Eucalyptus regnans, the world's tallest angiosperm, reveals it is also the world's most productive forest type, with fire disturbance an important mediator of net primary productivity (NPP). A comprehensive empirical database was used to calculate the averaged temporal pattern of NPP from regeneration to 250 years age. NPP peaks at 23.1 ± 3.8 (95% interquantile range) Mg C ha-1  year-1 at age 14 years, and declines gradually to about 9.2 ± 0.8 Mg C ha-1  year-1 at 130 years, with an average NPP over 250 years of 11.4 ± 1.1 Mg C ha-1  year-1 , a value similar to the most productive temperate and tropical forests around the world. We then applied the age-class distribution of E. regnans resulting from relatively recent historical fires to estimate current NPP for the forest estate. Values of NPP were 40% higher (13 Mg C ha-1  year-1 ) than if forests were assumed to be at maturity (9.2 Mg C ha-1  year-1 ). The empirically derived NPP time series for the E. regnans estate was then compared against predictions from 21 global circulation models, showing that none of them had the capacity to simulate a post-disturbance peak in NPP, as found in E. regnans. The potential importance of disturbance impacts on NPP was further tested by applying a similar approach to the temperate forests of conterminous United States and of China. Allowing for the effects of disturbance, NPP summed across both regions was on average 11% (or 194 Tg C/year) greater than if all forests were assumed to be in a mature state. The results illustrate the importance of accounting for past disturbance history and growth stage when estimating forest primary productivity, with implications for carbon balance modelling at local to global scales.