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- Plant interactions govern population dynamics in a semi‐arid plant community
- Positive Plant Interactions and Community Dynamics
For small Cistus shrubs this improvement seems to be caused by changes in soil properties alone Fig. When water deficit was high, however, positive effects of Cistus on Stipa disappeared Fig. Cistus facilitated juvenile Stipa , whereas the overall effect of juvenile Stipa on Cistus was negligible. By contrast, when growing in close association, mature Cistus and Stipa plants did not have any noticeable effect on one another.
Our physiological and functional measurements support this idea, as juvenile Stipa plants associated with Cistus had enhanced performance in numerous ways, including increased plant and leaf biomass Fig. However, coexisting Cistus and Stipa increased soil organic matter SOM to a greater extent than each species separately, and tended to reduce bulk density compared with bare soils Fig. Therefore, soils where both species grew in close association were able to retain more water than soils under isolated plants, which explains why juvenile Stipa tussocks growing in soils where both species were originally associated — with or without the shrub removed — had better water relations than isolated tussocks Fig.
Armas, personal observation. Our results showed transient shifts in the balance of the interaction depending on seasonal water availability.
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During periods of low water availability, the facilitative effect of Cistus on juvenile Stipa was not evident Fig. These data agree with reports from other dry environments showing that facilitative effects of woody species on their understorey occurred at the beginning of the growing season, i.
In addition, these models refer to habitats and ecosystems and not to seasonal environmental changes within a given habitat. As Stipa plants matured, the net balance of their interactions with Cistus changed, as did their spatial arrangement. This effect could be due either to niche differentiation by root segregation or to a compensation between the negative effects of competition and the positive effects of facilitation. Armas, personal observation in areas beyond the reach of Stipa roots.
The analysis of population structure and spatial arrangements in this community suggests a strong control of Stipa on the population dynamics of both species. The spatial association between adult plants suggests that there are no net negative effects for either species, i. However, the effects of Cistus on juvenile Stipa were positive, and mainly caused by an improvement of microclimatic conditions and soil characteristics under the Cistus canopy. We also thank Rob Brooker, Lindsay Haddon and two anonymous referees for their help with an early draft of this manuscript.
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Please review our Terms and Conditions of Use and check box below to share full-text version of article. Summary 1 The structure and composition of plant communities are influenced by positive and negative interactions between plants, the balance of which may change in intensity and sign through time and space, depending on availability of resources and on plant life history.
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This book explains attempts to scale these measures from the microscopic cell level through local, landscape, and ecosystem levels. The totality of the ideas, methods, and results presented by the contributing authors points to the future direction of mycology. Linking Function between Scales of Resolution. Their Diversity and Distribution.grupoavigase.com/includes/248/3099-sitio-para-conocer.php
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Freshwater Fungal Communities. Marine Fungal Communities. Community Structure. Evolutionary Development of the Clavicipitaceae. Fungal Communities of Seaweeds. Trophic Interactions of Fungi and Animals. Management included broad-spectrum herbicide application in the spring followed by two to three passes with a disk harrow at two to four different times over two growing seasons e. In the penultimate year of the study August , most 15 of the 25 of the sites burned in a wildfire S1 Table.
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To address our first objective, to test for differences between the two field types, we used non-metric multidimensional scaling analyses NMS [ 32 ]. NMS analyses were conducted on functional group native annual, non-native annual, native forb, non-native forb, native grass, non-native grass, native perennial, non-native perennial, native shrub and species matrices using Bray-Curtis dissimilarity matrices.
To further address the first objective, we provide a more detailed analysis of differences in community composition between ex-arable fields and never-tilled fields, and between distance transects 5 and 50 m , using generalized linear mixed models GLMM with a two-way factorial field type by distance in a split-plot design with whole plots fields in blocks sites and the following response variables: native, non-native, grass, forb, annual and perennial.
To address the second objective, to assess community composition change over time using both direct observation and chronosequence data, linear trend in the NMS1 scores over years-since-abandonment and comparison of trends were assessed using a linear mixed model. In addition to NMS1 scores, this analysis was done on functional groups and species five dominant in each field type. Fixed effects factors were 1 field type ex-arable or never-tilled ; 2 years-since-abandonment; 3 mean number of years-since-abandonment; 4 interaction of field type with years-since-abandonment, which allowed the estimate of the within-field slope of the regression of NMS1 scores on number of years-since-abandonment to differ for ex-arable and never-tilled fields; and 5 interaction of field type with mean number of years-since-abandonment, which allowed the estimate of the between-field slope of the regression to differ for ex-arable and never-tilled fields [ 34 ].
Random effects factors were study site and field-within-study-site, which allowed for random intercepts for fields in the regressions of NMS1 scores on the number of years-since-abandonment. Covariance among the 13 annual repeated measures on a field was modeled using a first-order autoregressive structure. We also fitted a model that allowed for random slopes among field regressions; there was no statistical support for random slopes so we present results for the model with only random intercepts. Number of years-since-abandonment was within-field centered prior to analysis.
For simplicity, only data from 50 m transects were analyzed for this analysis. To test for management effects in ex-arable fields, the third objective, regressions of NMS scores were conducted for managed and unmanaged fields separately. To test for biocontrol effects on the target non-native plant, C. To test for the effects of wildfire, differences in NMS values in were compared to average NMS values from to , using a t-test for each field type separately.
NMS axis 1 scores distinguished differences in plant community composition between ex-arable fields and never-tilled fields, while NMS axis 2 scores distinguished differences in plant community composition among fields and the 13 years of observation Fig 1. Non-metric multidimensional scaling NMS graph by A functional group and B species of plant community composition in adjacent ex-arable fields blue and grey colors and never-tilled fields green colors over 13 years.
Each point represents the mean composition of vegetation across a transect located 50 m from a tillage boundary. Lines connect values from a field over 13 years of direct observation. For clarity, data from 12 of 25 randomly selected sites are shown. Differences in plant functional group composition between ex-arable and never-tilled fields helped explain community-level differences indicated by NMS Fig 2 ; Table 1. Non-natives, annuals and forbs were more abundant in ex-arable than never-tilled fields Fig 2 ; Table 1.
There was no difference in grass abundance between ex-arable and never-tilled fields. Percent ground cover of A native and non-native plants, B annual and perennial plants, and C forbs and grasses across tillage boundaries. Negative x-axis values are in ex-arable fields. Positive x-axis values are in never-tilled fields. Values represent the mean and standard error associated with the 25 sampled fields values from replicate plots and years were averaged prior to calculations. Cover values within fields replicate plots and among years were averaged prior to calculations so that values represent averages and standard errors associated with the 25 fields.
There was no effect of time for the chronosequence data i. Data from ex-arable fields shown with open symbols and data from never-tilled fields shown with filled symbols. Data from transects located 50 m from tillage boundaries. Positive y-axis values were associated with native, long-lived perennial communities Table 1 ; Fig 1. NMS 1 values increased with time during 13 years of observation in ex-arable fields, but did not change with time in never-tilled communities or across the chronosequence in either community type.
Among native plant species, A. Among non-native plants, B. When data from managed and unmanaged ex-arable fields were analyzed separately, communities in unmanaged fields became more similar to never-tilled fields while communities in managed fields did not show a directional change in composition [i. In response to biocontrol treatment, cover of the target, C.
Positive Plant Interactions and Community Dynamics | NHBS Academic & Professional Books
Values calculated as the difference between scores and the average of to values. Negative post-prefire NMS scores indicate increased weediness. Agricultural abandonment created two distinct plant communities that appear to function as alternative-state communities [ 12 — 13 ].
Fields that have been disturbed by agriculture and abandoned are dominated by short-lived non-natives. Never-tilled fields are dominated by long-lived natives. At the species and functional group levels, 13 years of direct observation revealed directional changes in both field types: non-native abundance decreased in ex-arable fields and native abundance decreased in never-tilled fields. These changes made the two communities more similar, but there was little evidence that the two community types would eventually converge because there was little species overlap between field types.
Further, there was no evidence of directional changes in community composition across the chronosequence. In summary, the more-controlled, direct observation approach detected changes within communities over 13 years, but little species overlap and a lack of change in community composition across the chronosequence suggests that these communities are likely to remain distinct over long time periods i.
Communities in ex-arable and never-tilled fields are distinct Fig 1. In ex-arable fields, the five most abundant species are non-native, short-lived 1 to 10 years [ 37 — 42 ] and demonstrate wide fluctuations in percent cover from year to year Fig 1 and S2 Fig. While plant communities in ex-arable fields remained distinct from communities in never-tilled fields for 65 years, there was evidence of directional change in community composition in the 13 years of direct observation.
Multivariate analyses suggested that plant communities in ex-arable fields were becoming more similar to plant communities in never-tilled fields i. This was consistent with a decline in the abundance of B. Both species are small-statured, winter-active, non-native grasses that are dominant in ex-arable fields but uncommon in never-tilled fields. The only evidence that native species were becoming more abundant in ex-arable fields over time was for the nitrogen-fixing L. Thus, while two common non-natives decreased and one native species increased, there was little evidence that ex-arable communities would return to native community composition over 50 to year timescales.
In contrast to ex-arable fields, there was no directional change in whole-community composition in never-tilled fields during direct observation. At the functional group and species-levels, however, there were declines in native plant abundance reflecting a decline in the common shrub, P. There was no evidence that non-native plants were invading never-tilled fields. There are several reasons why direct observation data may have revealed different patterns in community composition than chronosequence data [ 43 ]. First, unlike chronosequence data, direct observation data are not confounded by space-for-time substitutions.
Second, shorter-term, direct observation data are more likely to detect short-term linear patterns, even if longer-term patterns are non-linear. It is likely, for example, that weedy communities shift from annual to perennial dominance over five to ten year timescales, but that these communities then maintain dominance by perennial non-natives indefinitely. While the direct observation approach appeared more controlled and more sensitive to detecting community level changes, the inference it provides to community composition in the future is not as strong as from chronosequence data.
Alternatively, it could be suggested that the direct observation data revealed a pattern in vegetation change that better reflects current and anticipated climate and management conditions i. While this may be occurring to some degree, the patterns observed during direct observation are unlikely to be maintained over longer time periods because 1 these directional changes were not observed in the chronosequence, and 2 plant cover was observed to decline in both field types during direct observation, but plant cover cannot be expected to decline indefinitely.
While late-successional species have often been observed to recolonize ex-arable fields [ 23 , 46 — 47 ], in this system agricultural disturbance appears to have forced the community through a threshold to an alternative-state community dominated by non-native, early-successional plants [ 12 — 13 ]. Ex-arable fields were typically less than m wide allowing native propagule pressure and results were similar in transects that were 5 m and 50 m from tillage boundaries Fig 2. Ex-arable fields had less ground cover and more variable populations, which should have provided establishment opportunities for many generations of plants [ 10 , 48 ].
Species in ex-arable fields likely experienced 30 to 60 generations during the chronosequence [ 37 — 42 ].
Even stands of the native shrubs, A. Thus, establishment, recruitment and mortality were expected in both the direct observation and chronosequence data in both communities [ 50 ], yet these communities remained distinct for 65 years. Results stand in contrast to many studies demonstrating succession over similar time-scales [ 23 — 24 , 46 — 47 , 51 — 52 ].
Plant interactions govern population dynamics in a semi‐arid plant community
It is not clear why succession would be rapid in some systems and not in others [ 53 ], though arid and semi-arid systems seem to be more likely to show very slow to no change relative to more mesic systems [ 6 , 9 , 19 , 27 , 52 ]. It is possible that slow succession or alternative-state communities are more likely in more stressful environments with greater facilitation [ 17 , 53 — 57 ].
Direct observation data provided some insight into the processes through which short-lived non-natives may maintain dominance. It is particularly notable that the dominant species in ex-arable fields were resilient from changes in abundance. Perhaps the most dramatic example was that C.
Positive Plant Interactions and Community Dynamics
The temporary loss of C. In addition to being resilient from changes in species-level abundances, ex-arable communities were resistant to changes caused by management and wildfire. Together, these results illustrate that ex-arable plant communities were either resistant to or resilient from large disturbances i. In summary, results from both the chronosequence and direct observation are consistent with the idea that agricultural disturbance forces this ecosystem across a threshold to a new alternative state that is maintained by within-community feedbacks [ 12 , 60 — 62 ].
Native a and b and non-native c and d ground cover in paired ex-arable a and c and never-tilled b and d fields. Each solid line represents plant-type abundance over 13 years of direct observation in a single field. Dotted lines represent significant best-fit regressions. Native plant cover in never-tilled fields b and non-native cover in ex-arable fields c decreased during 13 years of direct observation. There were no significant relationships between native or non-native cover and chronosequence time i.
Ground cover of the five most common species in A ex-arable and B never-tilled fields over 13 years of direct observation. Each point represents mean cover in transects 50 m from tillage boundaries in 25 sites. NMS axis 1 scores species matrix in old-field and never-tilled fields over time in the Chronosequence and Direct Observation datasets A.
Data from actively managed fields were separated from remaining data in the Direct Observation dataset B. Data derived from plant species composition in m transects. Positive y-axis values were associated with native; long-lived perennial communities Fig 1. Since the chronosequence was observed for 13 years; each of the 25 fields sampled produced 13 age-since-abandonment values. Each data point in panel A therefore represents the average value for all fields measured at the indicated year i.
In the Direct Observation dataset; a significant regression with time was observed in the unmanaged; old-field fields only. Note that half of the fields burned in a wildfire prior to the last data point.