James S. Camac

BAAD: a biomass and allometry database for woody plants

Understanding how plants are constructed—i.e., how key size dimensions and the amount of mass invested in different tissues varies among individuals—is essential for modeling plant growth, carbon stocks, and energy fluxes in the terrestrial biosphere. Allocation patterns can differ through ontogeny, but also among coexisting species and among species adapted to different environments. While a variety of models dealing with biomass allocation exist, we lack a synthetic understanding of the underlying processes. This is partly due to the lack of suitable data sets for validating and parameterizing models. To that end, we present the Biomass And Allometry Database (BAAD) for woody plants. The BAAD contains 259 634 measurements collected in 176 different studies, from 21 084 individuals across 678 species. Most of these data come from existing publications. However, raw data were rarely made public at the time of publication. Thus, the BAAD contains data from different studies, transformed into standard units and variable names. The transformations were achieved using a common workflow for all raw data files. Other features that distinguish the BAAD are: (i) measurements were for individual plants rather than stand averages; (ii) individuals spanning a range of sizes were measured; (iii) plants from 0.01–100 m in height were included; and (iv) biomass was estimated directly, i.e., not indirectly via allometric equations (except in very large trees where biomass was estimated from detailed sub-sampling). We included both wild and artificially grown plants. The data set contains the following size metrics: total leaf area; area of stem cross-section including sapwood, heartwood, and bark; height of plant and crown base, crown area, and surface area; and the dry mass of leaf, stem, branches, sapwood, heartwood, bark, coarse roots, and fine root tissues. We also report other properties of individuals (age, leaf size, leaf mass per area, wood density, nitrogen content of leaves and wood), as well as information about the growing environment (location, light, experimental treatment, vegetation type) where available. It is our hope that making these data available will improve our ability to understand plant growth, ecosystem dynamics, and carbon cycling in the worlds vegetation.

Experimental warming and long-term vegetation dynamics in an alpine heathland

High mountain ecosystems are vulnerable to the effects of climate warming and Australia’s alpine vegetation has been identified as particularly vulnerable. Between 2004 and 2010, we monitored vegetation changes in a warming experiment within alpine open grassy-heathland on the Bogong High Plains, Victoria, Australia. The study was part of the International Tundra Experiment (ITEX Network) and used open-topped chambers (OTC) to raise ambient growing-season temperatures by ~1°C at two sites. We assessed the effects of experimental warming on vegetation composition, diversity and cover using ordination, linear models and hierarchical partitioning. Results were compared with vegetation changes at four long-term (non-ITEX) monitoring sites in similar vegetation sampled from 1979 to 2010. The warming experiment coincided with the driest 13-year period (1996–2009) since the late 1880s. At the ITEX sites, between 2004 and 2010, graminoid cover decreased by 25%, whereas forb and shrub cover increased by 9% and 20%, respectively. Mean canopy height increased from 7 cm to 10 cm and diversity increased as a result of changes in relative abundance, rather than an influx of new species. These vegetation changes were similar to those at the four non-ITEX sites for the same period and well within the range of changes observed over the 31-year sampling period. Changes at the non-ITEX sites were correlated with a decrease in annual precipitation, increase in mean minimum temperatures during spring and increase in mean maximum temperature during autumn. Vegetation changes induced by the warming experiment were small rather than transformational and broadly similar to changes at the long-term monitoring sites. This suggests that Australian alpine vegetation has a degree of resilience to climate change in the short to medium term (20–30 years). In the long term (>30 years), drought may be as important a determinant of environmental change in alpine vegetation as rising temperatures. Long-term vegetation and climate data are invaluable in interpreting results from short-term (≤10 years) experiments.

Climatic warming strengthens a positive feedback between alpine shrubs and fire

Abstract Climate change is expected to increase fire activity and woody plant encroachment in arctic and alpine landscapes. However, the extent to which these increases interact to affect the structure, function and composition of alpine ecosystems is largely unknown. Here we use field surveys and experimental manipulations to examine how warming and fire affect recruitment, seedling growth and seedling survival in four dominant Australian alpine shrubs. We found that fire increased establishment of shrub seedlings by as much as 33‐fold. Experimental warming also doubled growth rates of tall shrub seedlings and could potentially increase their survival. By contrast, warming had no effect on shrub recruitment, postfire tussock regeneration, or how tussock grass affected shrub seedling growth and survival. These findings indicate that warming, coupled with more frequent or severe fires, will likely result in an increase in the cover and abundance of evergreen shrubs. Given that shrubs are one of the most flammable components in alpine and tundra environments, warming is likely to strengthen an existing feedback between woody species abundance and fire in these ecosystems. &NA; We used field surveys and experimental manipulations to examine how warming and fire affect shrub seedling recruitment and growth and survival. We found that warming, coupled with more frequent or severe fires, will likely increase the cover and abundance of evergreen shrubs—a major fuel for alpine fires. As a consequence, warming is likely to strengthen an existing feedback between shrub abundance and fire in these ecosystems. Figure. No caption available.

Effects of drought and fire on resprouting capacity of 52 temperate Australian perennial native grasses

It remains uncertain how perennial grasses with different photosynthetic pathways respond to fire, and how this response varies with stress at the time of burning. Resprouting after fire was examined in relation to experimentally manipulated pre-fire watering frequencies. We asked the following questions: are there response differences to fire between C3 and C4 grasses? And, how does post-fire resprouting vary with pre-fire drought stress? Fifty-two perennial Australian grasses (37 genera, 13 tribes) were studied. Three watering frequencies were applied to simulate increasing drought. Pre-fire tiller number, tiller density, specific leaf area and leaf dry matter content were measured as explanatory variables to assess response. Most species (90%) and individuals (79%) resprouted following experimental burning. C4 grasses had higher probabilities of surviving fire relative to C3 grasses. Responses were not related to phylogeny or tribe. High leaf dry matter content reduced the probability of dying, but also reduced the re-emergence of tillers. Post-fire tiller number increased with increasing drought, regardless of photosynthetic type, suggesting that drought plays a role in the ability of grasses to recover after fire. This has implications for understanding the persistence of species in landscapes where fire management is practiced.

Unifying intra- and inter-specific variation in tropical tree mortality

Tree death is a fundamental process driving population dynamics, nutrient cycling, and evolution within plant communities. While past research has identified factors influencing tree mortality across a variety of scales, these distinct drivers are yet to be integrated within a unified predictive framework. In this study, we use a cross-validated Bayesian framework coupled with classic survival analysis techniques to derive instantaneous mortality functions for 203 tropical rainforest tree species at Barro Colorado Island (BCI) Panama. Specifically, we develop mortality functions that not only integrate individual, species, and temporal effects, but also partition the contributions of growth-dependent and growth-independent effects on the overall instantaneous mortality rate. We show that functions that separate mortality rates into growth-dependent and growth-independent hazards, use stem diameter growth rather than basal-area growth, and attribute the effect of wood density to growth-independent mortality outperform alternative formulations. Moreover, we show that the effect of wood density – a prominent trait known to influence tree mortality – explains only 22% of the total variability observed among species. Lastly, our analysis show that growth-dependent processes are the predominant contributor to rates of tree mortality at BCI. Combined, this study provides a framework for predicting individual-level mortality in highly diverse tropical forests. It also highlights how little we know about the causes of species-level and temporal plot-scale effects needed to effectively predict tree mortality.

Fire regimes and biodiversity in Victoria’s alpine ecosystems

Landscape-scale fires occur in Australian alpine ecosystems once or twice per century, primarily when ignition, regional drought and severe fire weather coincide. When alpine vegetation does burn, there is considerable variation in landscape flammability and fire severity. Regeneration following extensive fires of 2003 and 2006-07 across the Bogong High Plains is occurring in all plant communities (heathlands, grasslands, herbfields and wetlands). In heathland and grassland, vegetation composition has converged towards the long-unburnt state (> 50 years) eight years post fire. There was little effect of variation in fire severity on patterns of regeneration in heathland. In burnt wetlands, Sphagnum cristatum and other dominant species are regenerating; the cover of obligate seeding ericaceous shrubs two years post-fire was positively related to the cover of Sphagnum. The endangered mammal Burramys parvus is also capable of persisting in the alpine landscape after individual large, landscape fires. We conclude that there is no scientific evidence that these fires necessarily had ‘disastrous’ biodiversity consequences. After extensive landscape fires, the primary management objective should be to allow burnt alpine ecosystems to regenerate with minimal subsequent disturbance. Monitoring ecological change in the coming century will be essential for effective management of both fire and biodiversity in alpine ecosystems in Victoria and elsewhere in Australia.