Rate Of Cover Change Research Articles

Woody plant encroachment intensifies under climate change across tundra and savanna biomes

AbstractAimBiomes worldwide are shifting with global change. Biomes whose extents are limited by temperature or precipitation, such as the tundra and savanna, may be particularly strongly affected by climate change. While woody plant encroachment is prevalent across both biomes, its relationship to temperature and precipitation change remains unknown. Here, we quantify the degree to which woody encroachment is related to climate change and identify its main associated drivers.LocationTundra and savanna biomes.Time period1992 ± 20.27–2010 ± 5.62 (mean ± SD). 1876–2016 (range).Major taxa studiedWoody plants (shrubs and trees).MethodsWe compiled a dataset comprising 1,089 records from 899 sites of woody plant cover over time and attributed drivers of woody cover change across these two biomes. We calculated cover change in each biome and assessed the degree to which cover change corresponds to concurrent temperature and precipitation changes using multiple climate metrics. Finally, we conducted a quantitative literature review of the relative importance of attributed drivers of woody cover change.ResultsWoody encroachment was widespread geographically and across climate gradients. Rates of woody cover change (positive or negative) were 1.8 times lower in the tundra than in the savanna (1.8 vs. 3.2%), while rates of woody cover increase (i.e., encroachment) were c. 1.7 times lower in the tundra compared with the savanna (3.7 vs. 6.3% per decade). In the tundra, magnitudes of woody cover change did not correspond to climate, while in the savanna, greater cover change corresponded with increases in precipitation. We found higher rates of woody cover change in wetter versus drier sites with warming in the tundra biome, and higher rates of woody cover change in drier versus wetter sites with increasing precipitation in the savanna. However, faster rates of woody cover change were not associated with more rapid rates of climate change across sites, except for maximum precipitation in the savanna.Main conclusionsWoody encroachment was positively related to warming in the tundra and increased rainfall in the savanna. However, cover change rates were not predicted by rates of climate change, which can be partially explained by climate interactions in both biomes. Additional likely influences include site‐level factors, time‐lags, plant‐specific responses, and land use and other non‐climate drivers. Our findings highlight the complex nature of climate change impacts in biomes limited by seasonality, which should be accounted for to realistically estimate future responses across open biomes under global change scenarios.

Recovery of coral cover in records spanning 44 yr for reefs in Kāne‘ohe Bay, Oa‘hu, Hawai‘i

Published and unpublished long-term studies are assembled to examine trends in coral cover and the dependence of change in coral cover on the initial coral cover in Kāne‘ohe Bay over the last 44 yr. Each study showed there had been periods of increase in coral cover in the bay and showed that the rate of change in cover has been inversely dependent on the initial cover at a site. When coral cover is high on upper reef slopes, the fragile structure of reefs in this sheltered bay often collapses, resulting in a decrease in coral cover. The rate of change in coral cover was also inversely dependent on cover in one of the two studies that included analysis of reef-flat corals; the cause of decrease in cover in this habitat is thought to be attributable to particularly low sea levels in Hawai‘i in the late 1990s and 2009–2010. The inverse relationship between initial coral cover and change in cover, and the intersections of the regression lines of these variables with the x-axis at intermediate values of coral cover, is indicative of resilience in this ecosystem over the last 44 yr. In the 1970s, the invasive macroscopic green alga Dictyosphaeria cavernosa covered a high percentage of coral habitat and commonly displaced corals from the reef slope and outer reef flats; the change was cited as an example of a phase shift on a reef. This alga has virtually disappeared from the bay, thus increasing the space available to corals; its disappearance is coincident with the increase in coral cover. Other species of macroalgae, including alien species, have not replaced D. cavernosa as major space competitors. The increase in coral cover and virtual disappearance of D. cavernosa constitute an example of a phase-shift reversal.

Nitrogen critical loads for alpine vegetation and soils in Rocky Mountain National Park

We evaluated the ecological thresholds associated with vegetation and soil responses to nitrogen (N) deposition, by adding NH(4)NO(3) in solution at rates of 5, 10 and 30 kg N ha(-1) yr(-1) to plots in a species rich dry meadow alpine community in Rocky Mountain National Park receiving ambient N deposition of 4 kg N ha(-1) yr(-1). To determine the levels of N input that elicited changes, we measured plant species composition annually, and performed one-time measurements of aboveground biomass and N concentrations, soil solution and resin bag inorganic N, soil pH, and soil extractable cations after 3 years of N additions. Our goal was to use these dose-response relationships to provide N critical loads for vegetation and soils for the alpine in Rocky Mountain National Park. Species richness and diversity did not change in response to the treatments, but one indicator species, Carex rupestris increased in cover from 34 to 125% in response to the treatments. Using the rate of change in cover for C. rupestris in the treatment and the ambient plots, and assuming the change in cover was due solely to N deposition, we estimated a N critical load for vegetation at 3 kg N ha(-1) yr(-1). Inorganic N concentrations in soil solution increased above ambient levels at input rates between 9 kg N ha(-1) yr(-1) (resin bags) and 14 kg N ha(-1) yr(-1) (lysimeters), indicating biotic and abiotic sinks for N deposition are exhausted at these levels. No changes in soil pH or extractable cations occurred in the treatment plots, indicating acidification had not occurred after 3 years. We conclude that N critical loads under 10 kg ha(-1) yr(-1) are needed to prevent future acidification of soils and surface waters, and recommend N critical loads for vegetation at 3 kg N ha(-1) yr(-1) as important for protecting natural plant communities and ecosystem services in Rocky Mountain National Park.

Spatial Distribution of Cattle in Arid Rangelands as Detected by Patterns of Change in Vegetation Cover

Patterns of change in vegetation cover, as measured by multitemporal remote sensing using the LANDSAT Multispectral Scanner, were used to model domestic cattle distributions in the arid rangelands of central Australia. Model assumptions that paddock shape and waterpoint layout have no effect on rate of cover change (and thus cattle distribution), other than through changes in the area of a paddock and the individual landscape types within it, at each distance from water seem justified. Model assumptions that rate of cover change in a particular landscape type at a given distance from water is unaffected by the mix of other landscape types around it also seem justified. Land degradation appears to have only a limited effect on the distribution of cattle compared with differences in grazing preference for individual landscape types and the number of cattle present in these rangelands. Generalized cattle distribution models can be derived for broad landscape types on the basis of distance from water and grazing preference. These models can be applied irrespective of the number of cattle present. The models have potential for incorporation into decision support systems for paddock design and waterpoint layout. Such systems could ensure the effective and sustainable utilization of arid rangeland vegetation by cattle.

Size‐Dependent Competition: Effects on the Dynamics Vs. The End Point of Mussel Bed Succession

I manipulated relative body size of mussels (Mytilus californianus) and gull (Larus glaucescens) predation pressure within gaps in a mussel bed to determine (1) whether size differences could cause shifts in competitive dominance between goose barnacles (Pollicipes polymerus) and Mytilus, species with indeterminate growth, and (2) if the late recruiting Mytilus required gull predation on established adult Pollicipes to attain dominance. Increasing relative body size enhanced the area and the rate of change in area covered by both Pollicipes and Mytilus. Preventing gull feeding enhanced Pollicipes and reduced Mytilus cover, slowing the rates of succession and reducing the frequency of disturbance. However, even in the absence of predators on Pollicipes and with an initial size disadvantage, Mytilus maintained positive rates of change in cover, probably because adult recruitment into the gap altered the initial size hierarchy of competitors. These results indicate that bird predation and initial size asymmetries can alter the dynamics, but not the eventual end point, of intertidal succession. They also highlight the importance of choosing the right comparison (differences between treatments vs. deviations in rates from 0) when testing alternative modes of succession (i.e., facilitation, tolerance, or inhibition). Although gull predation does not change the end point of succession locally, it may have important regional effects by altering the dynamics of disturbance and recovery. Such regional effects cannot be determined by experiments on a local scale.