(A)synchrony of above- and below-ground productivity in a warming tundra biome

Abstract

Arctic and alpine tundra ecosystems are experiencing accelerated warming compared to the global average, causing significant changes in plant productivity and the timing of life histories of tundra species, with cascading effects on trophic interactions and carbon cycling. However, the sparsity of long-term and spatially-varied observations hinders our understanding of how these dynamics may continue to change in a warming tundra biome. Specific knowledge gaps, often borne from limitations on year-round travel to tundra sites, hamper our ability to accurately predict the long-term trajectory of tundra phenology change, both above-ground and below-ground. In this PhD thesis, I use above- and below-ground ecological observations across spatial and temporal scales to resolve key questions about how heterogeneous tundra landscapes may respond to future warming and ecosystem change. My findings have implications for biome-scale carbon cycling and wildlife habitats.

In Chapter 2, I used a geographically varied time-lapse camera imagery to analyse tundra phenology variations across microclimates and snowmelt gradients. I found that while growing seasons were consistently longer at warmer, lower-latitude sites (11 extra days for each additional 1°C in mean summer temperature). Growing season lengths did not significantly vary across warmer or colder summers and earlier or later snowmelt timing despite warmer spring temperatures consistently advancing spring green-up. I found that early-season phenology constrained the timing of much of the mid-season phenology and early senescence, but not full senescence. Green-up, mid-season, and early senescence phenophases generally occurred earlier in warmer microclimates and tracked snowmelt, although initial community-scale bud-burst and full community senescence was not related to microclimate. Across sites, I found that green-up occurred more slowly when snowmelt was earlier and faster when snowmelt was later. If growing season length remains relatively stable across space and time and is not extending into the longer snow-free autumn season, this indicates that tundra productivity and carbon sink capacity may not necessarily increase much as the climate warms. I recommend that terrestrial carbon and ecosystem modellers incorporate microclimate, interannual variations and varying metrics of phenological time into their model designs in order to more precisely forecast long-term vegetation change and carbon flux.

In Chapter 3, I combined above- and below-ground plant phenology metrics to compare the relative timings and magnitudes of leaf and root growth and senescence across microclimates and plant communities at five sites across the tundra biome. I observed asynchronous growth between above-ground and below-ground plant tissue, with the below-ground season extending up to 74% beyond the onset of above-ground leaf senescence. Community type, rather than microclimate, was a key factor controlling the timing, productivity and growth rates of roots, with graminoid roots exhibiting a distinct growth ‘pulse’ later into the growing season than shrub and forb roots. My findings indicate the potential for greater below-ground carbon storage as roots grow into thawed soils that remain unfrozen for longer as the climate warms. Taken together, long-term vegetation change, an indirect response to climate warming, is more likely than climate warming alone to impact below-ground productivity and carbon cycling in the tundra biome.

In Chapter 4, I used the Tea Bag Index protocol to investigate the microenvironmental controls on litter decomposition in tundra soils. I examined the extent to which the thermal sum of surface air temperature, soil moisture and permafrost thaw depth influenced litter mass loss and decomposition rates (k), and at which spatial thresholds an environmental variable becomes a reliable predictor of decomposition, Overall, I found that litter decomposition was faster and litter mass loss higher in the warmer and wetter parts of the landscape. Spatially heterogeneous belowground conditions (soil moisture and active layer depth) explained variation in decomposition metrics at local scales (< 50 m2) better than thermal sum. Surprisingly, there was no strong control of elevation or slope on litter decomposition. My results reveal that there is considerable scale dependency in the environmental controls of tundra litter decomposition, with moisture playing a greater role than the thermal sum at < 50 m2 scales. My findings indicate that variation in microenvironmental conditions will influence litter decomposition in ways that are not currently incorporated into models estimating carbon cycling with warming.

In Chapter 5, I explored how animal husbandry affected changes in tundra land-surface greenness from 1985 to 2021 using Landsat satellite observations from 31 sites in Svalbard. I assessed changes in annual maximum NDVI at contemporary and historical animal husbandry sites using the Normalized Difference Vegetation Index (NDVI) to extract dates of peak-season NDVI, green-up, and plant senescence. I found that while peak-season greenness increased across all of our study sites, the greening signal was enhanced at active dog-yards and historic animal husbandry sites. In addition, the greening signal was stronger at all animal husbandry sites compared to reference ‘non-disturbed’ tundra sites. Across sites, the date of tundra vegetation greening shifted up to 0.81 days earlier, and the date of plant senescence shifted slightly later from 1985 to 2021. My analysis shows nutrient enrichment from animal husbandry can stimulate long-term increases in tundra vegetation productivity, with a lasting impact of nutrient enrichment at abandoned animal husbandry sites.

In this PhD thesis, I highlight complex and interconnected factors influencing tundra plant phenology, productivity and decomposition in a period of rapid and accelerating climate change. My research demonstrates that by accounting for spatio-temporal variability across Arctic and alpine tundra landscapes, we will improve forecasts of future ecosystem change and more precisely quantify the contribution of tundra warming to the global carbon cycle.