Forest regeneration and tree planting have been promoted as means to mitigate climate change through carbon fixation, and as options to restore biodiversity to exploited landscapes. However, the regulating services that forests provide are not limited to the carbon cycle, and the effects of forest growth on water resources will be critical for the long-term success of restoration programmes, particularly in arid catchments. The Mediterranean is becoming more arid with climate change and has high rates of natural forest regeneration following decades of rural land abandonment, making the effects of forest cover on water supplies particularly important in this region. This thesis explores how the effects of forest establishment on river flow and carbon fixation have been influenced by climate change and land-use change in recent decades.

A systematic review of the effect of forest establishment on annual river flow, including 43 catchments from across the world, showed that forest establishment reduced annual river flow by an average of 23 percent after 5 years and 38 percent after 25 years, relative to control conditions. This is the largest review to date that investigates how forest age affects river flow, after establishment. In most cases, annual river flow decreased asymptotically as forest age increased. Larger decreases in river flow were found in catchments with wetter climates, in catchments where forest replaced agricultural land rather than idle grassland or shrubland and where a larger percentage of the catchment had become forested. Forests had less effect on river flow in more arid years, suggesting that forests are adapting to aridity by reducing evapotranspiration rates. Evidence of river flow recovery towards control conditions, with continued forest aging, was found in nine studies. However, significant recovery was associated with a long-term trend of increasing catchment aridity, which was shown to drive a decrease in forest water use. Overall, little support is found for the widespread occurrence of river flow recovery with increasing forest age, that has been reported in previous studies.

To investigate how natural forest establishment and climate change have affected the generation of river flow in a semi-arid catchment, the Budyko framework was employed in a head water covering 3300 km2in central Spain. This modelling approach allows the influences of climate change and land-use change to be disentangled. Annual river flow decreased by 24 percent from baseline conditions, over a 44-year period. Lower rainfall and higher potential evapotranspiration rates were responsible for 72 percent of the decrease in river flow, but the rest was driven by forest expansion. The reduction in river flow was less than expected, indicating that non-woodlands adjusted to increasing aridity by reducing their water use. This agrees with the findings of the previous chapter, which show that increasing catchment aridity can reduce the effects of vegetation on river flow, by reducing transpiration rates.

Finally, factors affecting the regeneration of a priority habitat are investigated in the headwaters of the same basin. Juniperus thurifera woodland was found to have expanded in the wake of agricultural abandonment, which preferentially made steep slopes and areas with lower solar insolation available for colonisation. The carbon content of regenerating woodland was mapped using high resolution, overlapping, airborne imagery and structure-from-motion techniques, and the age of woodland was estimated using historic land cover maps covering over 30 years, generated from high resolution satellite data. Field-measured carbon stocks and stand ages across the expanding front of juniper were used to calibrate models that were used to estimate carbon accumulation over time (i.e. a space-for-time substitution). Carbon content increased most rapidly with age in areas where insolation was high, consistent with previous studies highlighting juniper’s tolerance of drought stress. However, the estimated carbon stocks of sites that had recently transitioned to juniper woodland were lower in areas with high insolation, suggesting that the establishment phase is more susceptible to drought stress than older woodland. This suggests that it will be harder for juniper woodland to establish in parts of the landscape with higher solar insolation, despite the drought resistance of older stands. Lastly, woodland biomass was significantly inhibited by interspecific competition, suggesting that the selective thinning of competing dwarf shrubland will facilitate juniper growth.

At all spatial scales considered, this thesis finds that there are significant effects of variable climate and landscape context on woodland ecosystem services. Although forest establishment is consistently associated with a decrease in river flow, the consequences of regeneration in the long term will be dependent on changes to forest structure and adaptation or resilience as climate change progresses.