There is accumulating evidence for an increase in severe global drought events. In particular, Europe experienced several severe droughts in 2003, 2015, and 2018 (unprecedented in the past 2,110... Show moreThere is accumulating evidence for an increase in severe global drought events. In particular, Europe experienced several severe droughts in 2003, 2015, and 2018 (unprecedented in the past 2,110 years). Such droughts have caused serious and far-reaching impacts on terrestrial ecosystems. As droughts are predicted to become more prolonged and intense across wide regions in the coming decades, the risks to the human system as a result of the disturbed ecosystems worldwide will increase. To mitigate these risks, it is critical to prioritize the most vulnerable ecosystems and intervene on time. Thus, the understanding of ecosystem responses and their vulnerability to intensive drought needs to be developed. This thesis reveals European-wide ecosystem vulnerability based on ecosystem responses. With more frequent and severe droughts in the future, a wide range of ecosystems will become more vulnerable and may even collapse due to high mortality. This thesis provides several suggestions for timely early warning and intervention possibilities for vulnerable ecosystems. Moreover, this thesis demonstrates the potential of remote sensing in monitoring ecosystem responses. Combining remote sensing for early and comprehensive monitoring of ecosystems will provide new possibilities and critical information for future drought prevention and management. Show less
Food security is challenged by a growing global population and by climate change. Drought and soil salinity are considered the most important ones that inhibit crop yield and distribution.... Show moreFood security is challenged by a growing global population and by climate change. Drought and soil salinity are considered the most important ones that inhibit crop yield and distribution. Worryingly, climate change is predicted to increase not only their frequency and severity, but also their co-occurrence, exacerbating their impacts. This also leads to increases in events where both stresses co-occur. This co-occurrence results in substantially more yield losses than individual stressors. While detrimental effects of combined drought and salinity stress on crops have been highlighted in small-scale experiments (with only a limited number of crop varieties), large regional uncertainties remain for real-life agricultural settings. Satellite observations offer a promising perspective for enhancing global food security by providing reliable information on arable land extent and food production. Remote sensing has already been used to monitor crop productivity at multiple spatial and temporal scales, though not for yet characterizing crop growth under co-occurring drought and salinity stress. This thesis aims to assess the impact of drought and salinity on agriculture and sustainable development goals using remote sensing technology. Show less
Results from comparative and ecological wood anatomy combined with a number of experimental studies on plant hydraulics have led to a pervasive and longstanding assumption that wider-diameter... Show moreResults from comparative and ecological wood anatomy combined with a number of experimental studies on plant hydraulics have led to a pervasive and longstanding assumption that wider-diameter vessels are more vulnerable to drought- induced embolism than narrower vessels. Although we agree that wider vessels tend to be more vulnerable than narrower vessels within stems and within roots across most species, our current understanding of the diameter-vulnerability link does not offer a mechanistic explanation for why increased vessel diameter should consistently lead to greater vulnerability or vice versa. Causes of drought-induced embolism formation and spread likely operate at the nano-level, especially at gas-liquid-surfactant interfaces inside intervessel pit membranes. We evaluate here new perspectives on drought-induced embolism and its key anatomical and physico-chemical drivers, of which vessel diameter is one of the parameters involved, although its linkage to embolism vulnerability is likely indirect. As such, the diameter-vulnerability link does not imply that species with on average wider vessels are consistently more susceptible to drought-induced embolism compared to species with narrower vessels. Scientific priorities for future progress should focus on more accurate predictions of how water transport in plants is affected by drought, which requires a better mechanistic understanding of xylem network topology and biophysical processes at the nano-scale level in individual vessels that determine embolism formation and spread. Show less
Hydraulic failure resulting from drought-induced embolism in the xylem of plants is a key determinant of reduced productivity and mortality. Methods to assess this vulnerability are difficult to... Show moreHydraulic failure resulting from drought-induced embolism in the xylem of plants is a key determinant of reduced productivity and mortality. Methods to assess this vulnerability are difficult to achieve at scale, leading to alternative metrics and correlations with more easily measured traits. These efforts have led to the longstanding and pervasive assumed mechanistic link between vessel diameter and vulnerability in angiosperms. However, there are at least two problems with this assumption that requires critical re-evaluation: (1) our current understanding of drought-induced embolism does not provide a mechanistic explanation why increased vessel width should lead to greater vulnerability, and (2) the most recent advancements in nanoscale embolism processes suggest that vessel diameter is not a direct driver. Here, we review data from physiological and comparative wood anatomy studies, highlighting the potential anatomical and physicochemical drivers of embolism formation and spread. We then put forward key knowledge gaps, emphasising what is known, unknown and speculation. A meaningful evaluation of the diameter–vulnerability link will require a better mechanistic understanding of the biophysical processes at the nanoscale level that determine embolism formation and spread, which will in turn lead to more accurate predictions of how water transport in plants is affected by drought. Show less
Insular woodiness (IW)—the evolutionary transition from herbaceousness toward wood- iness on islands—is one of the most iconic features of island floras. Since pioneering work by Darwin and Wallace... Show moreInsular woodiness (IW)—the evolutionary transition from herbaceousness toward wood- iness on islands—is one of the most iconic features of island floras. Since pioneering work by Darwin and Wallace, a number of drivers of IW have been proposed, such as 1) competition for sunlight requiring plants with taller and stronger woody stems and 2) drought favoring woodiness to safeguard root-to-shoot water transport. Alternatively, IW may be the indirect result of increased lifespan related to 3) a favorable aseasonal climate and/or 4) a lack of large native herbivores. However, information on the occurrence of IW is fragmented, hampering tests of these potential drivers. Here, we identify 1,097 insular woody species on 375 islands and infer at least 175 evolutionary transitions on 31 archipelagos, concentrated in six angiosperm families. Structural equation models reveal that the insular woody species richness on oceanic islands correlates with a favorable aseasonal climate, followed by increased drought and island isolation (approximating competition). When continental islands are also included, reduced herbivory pressure by large native mammals, increased drought, and island isolation are most relevant. Our results illustrate different trajectories leading to rampant convergent evolution toward IW and further emphasize archipelagos as natural laboratories of evolution, where similar abiotic or biotic conditions replicated evolution of similar traits. Show less
