Abstract
Dense thorn woodlands occupy what are thought to have been grasslands and savannas prior to settlement of the Rio Grande Plains of Texas. However, the tenet that grasslands have been converted to shrublands and woodlands in recent history is controversial and based largely upon conflicting historical accounts. Our objective was to determine how the presumed physiognomic conversion from grassland or savanna to woodlands might have occurred. Some upland landscapes are dominated by closed—canopy woodlands in southern Texas, whereas others have a two—phase pattern of discrete shrub clusters scattered throughout a grassland. More mesic sites are dominated by losed—canopy woodlands. We hypothesized the two—phase landscapes represented an intermediate stage in the conversion of grassland to woodland. As new shrub clusters were initiated and existing clusters expanded and coalesced, a gradual shift from grassland to savanna to woodland would occur. To address this hypothesis, we inventoried herbaceous interspaces for woody colonizers, quantified the composition and distribution of shrub clusters on upland sites, and compared the structure of clusters to that of adjacent, more mesic areas with continuous woody plant cover. To assess the physiognomic stability of the two—phase landscapes, cluster size, density, and cover were quantified for 1941, 1960, and 1983 from aerial photographs. A lone mesquite (Prosopis glandulosa) plant occurred in >80% of the upland clusters, where it was typically the largest individual in terms of basal area, height, and canopy area. The number of woody species per cluster ranged from 1 to 15 and was strongly related to mesquite basal diameter (R 2 = 0.86). Cluster diversity, evenness, and size were also significantly correlated with mesquite size. The data suggest that mesquite plants invaded grasslands and served as recruitment foci for bird—disseminated seeds of other woody species previously restricted to other habitats. The result was a landscape composed of discrete chronosequences of woody plant assemblages organized about a mesquite nucleus. Within the two—phase portion of the landscape, 50% of the clusters were within 5 m of another and 95% were within 15 m of another. Analysis of the size class distribution of clusters suggested that most had yet to realize their growth potential. Moreover, the herbaceous clearings between clusters contained high densities of woody seedlings, mostly (>70%) mesquite, which occurred in 85% of the clearings, with a mean density of 350 plants/ha. Coalescence will become increasingly probable if new clusters are initiated and existing clusters expand. This phenomenon appeared to be in progress on one portion of the landscape and had apparently already occurred on others. As clusters developed on the two—phase portion of the landscape, their species composition, dominance, and size class structure became increasingly similar to that of adjacent closed—canopy woodlands on more mesic. Mean cluster size increased from 494 m 2 in 1941 to 717 m 2 in 1983. Growth rates of clusters were a function of cluster size and precipitation. During the 1941—1960 period characterized by severe drought, there was a slight decrease in total woody plant cover resulting primarily from the formation of gaps among clusters on the periphery of the site and a 35% decrease in density of clusters <5 m 2 . These cover losses offset the areal expansion of small (<100 m 2 ) clusters which had an average relative growth rate (RGR) of 0.10 m 2 · m — 2 · yr — 1 . Post—drought woody plant cover increased from 8% in 1960 to 36% in 1983 as new clusters were initiated density increased from 16 to 26 clusters/ha), and the RGR of small clusters increased to 0.16 m 2 · m — 2 · yr — 1 . Numerous clusters coalesced during this period. RGRs of clusters >100 m 2 were an order of magnitude lower than those of clusters <100 m 2 in each time period. The RGR of large clusters following the drought was not significantly increased, except in the largest size class (clusters >1,000 m 2 ). Our results indicate (1) mesquite invaded grasslands and served as the nucleus of cluster organization on upland sites; (2) woody plant community development has been highly punctuated by variations in precipitation; (3) clusters >5 m 2 in area are persistent features of the landscape; and (4) the present two—phase pattern is moving toward a monophasic woodland as new clusters are initiated and existing clusters expand and coalesce. As a result, (5) shrub clusters on uplands represent an intermediate stage in the conversion of grassland to woodland, and (6) closed—canopy woodlands on more mesic sites appear to represent portions of the landscape where this has already occurred. Because the conversion of grasslands and savannas to woodlands in the Rio Grande Plains is initiated by mesquite, factors regulating its dispersal, establishment, and role as a facilitator of woody community development are emphasized.
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Claims 20
Claim · model-written summary
A lone mesquite occurred in more than 80% of upland clusters and was typically their largest individual.
- Taxon
- Prosopis glandulosa
Neltuma glandulosae0 - Outcome
- occurrede0
- Value
- >80%e0
- Qualifier
- typically the largest individual in terms of basal area, height, and canopy areae0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“A lone mesquite (Prosopis glandulosa) plant occurred in >80% of the upland clusters, where it was typically the largest individual in terms of basal area, height, and canopy area.”
Claim · model-written summary
Woody species per cluster ranged from 1 to 15 and related strongly to mesquite basal diameter.
_Druce_(45661099651).jpg?width=60){kind=small}
ProsopisNot reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
_Druce_(45661099651).jpg?width=500)
Verbatim
e0“The number of woody species per cluster ranged from 1 to 15 and was strongly related to mesquite basal diameter (R 2 = 0.86).”
Claim · model-written summary
Cluster diversity, evenness and size correlated significantly with mesquite size.
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Cluster diversity, evenness, and size were also significantly correlated with mesquite size.”
Claim · model-written summary
Mesquite invasion was interpreted as providing recruitment foci for bird-dispersed woody species and organizing assemblages around mesquite nuclei.
- Outcome
- recruitment focie0
- Qualifier
- bird—disseminated seeds of other woody species previously restricted to other habitatse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“The data suggest that mesquite plants invaded grasslands and served as recruitment foci for bird—disseminated seeds of other woody species previously restricted to other habitats. The result was a landscape composed of discrete chronosequences of woody plant assemblages organized about a mesquite nucleus.”
Claim · model-written summary
Half of woody clusters lay within 5 m of another cluster.
Verbatim
e0“Within the two—phase portion of the landscape, 50% of the clusters were within 5 m of another and 95% were within 15 m of another.”
e1“woody plant assemblages”
Claim · model-written summary
Ninety-five percent of woody clusters lay within 15 m of another.
Verbatim
e0“Within the two—phase portion of the landscape, 50% of the clusters were within 5 m of another and 95% were within 15 m of another.”
e1“woody plant assemblages”
Claim · model-written summary
Most clusters appeared not to have realized their growth potential.
Verbatim
e0“Analysis of the size class distribution of clusters suggested that most had yet to realize their growth potential.”
e1“woody plant assemblages”
Claim · model-written summary
More than 70% of woody seedlings in herbaceous clearings were mesquite.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the herbaceous clearings between clusters contained high densities of woody seedlings, mostly (>70%) mesquite, which occurred in 85% of the clearings, with a mean density of 350 plants/ha.”
Claim · model-written summary
Mesquite occurred in 85% of clearings.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“mostly (>70%) mesquite, which occurred in 85% of the clearings, with a mean density of 350 plants/ha.”
Claim · model-written summary
Mesquite seedlings had mean density 350 plants/ha in clearings.
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“mostly (>70%) mesquite, which occurred in 85% of the clearings, with a mean density of 350 plants/ha.”
Claim · model-written summary
Cluster coalescence was occurring or had occurred, and developing clusters became similar to mesic woodlands in composition, dominance and structure.
Verbatim
e0“This phenomenon appeared to be in progress on one portion of the landscape and had apparently already occurred on others. As clusters developed on the two—phase portion of the landscape, their species composition, dominance, and size class structure became increasingly similar to that of adjacent closed—canopy woodlands on more mesic.”
e1“woody plant assemblages”
Claim · model-written summary
Mean woody cluster size increased from 494 m 2 in 1941 to 717 m 2 in 1983.
Verbatim
e0“Mean cluster size increased from 494 m 2 in 1941 to 717 m 2 in 1983.”
e1“woody plant assemblages”
Claim · model-written summary
Cluster growth rates depended on size and precipitation; drought slightly reduced total cover through peripheral gaps.
- Taxon
- woody plante0
- Treatment
- severe droughte0
- Outcome
- Growth rates of clusterse0
- Qualifier
- cluster size and precipitatione0
Not reportedDirection
Verbatim
e0“Growth rates of clusters were a function of cluster size and precipitation. During the 1941—1960 period characterized by severe drought, there was a slight decrease in total woody plant cover resulting primarily from the formation of gaps among clusters on the periphery of the site”
Claim · model-written summary
Drought coincided with a 35% decline in density of clusters smaller than 5 m 2.
Verbatim
e0“a 35% decrease in density of clusters <5 m 2”
e1“During the 1941—1960 period characterized by severe drought”
e2“total woody plant cover”
Claim · model-written summary
Small clusters expanded during drought at mean relative growth rate 0.10 m 2 · m — 2 · yr — 1.
Verbatim
e0“These cover losses offset the areal expansion of small (<100 m 2 ) clusters which had an average relative growth rate (RGR) of 0.10 m 2 · m — 2 · yr — 1 .”
e1“total woody plant cover”
Claim · model-written summary
Post-drought woody cover increased from 8% to 36% between 1960 and 1983.
Verbatim
e0“Post—drought woody plant cover increased from 8% in 1960 to 36% in 1983”
Claim · model-written summary
Post-drought cluster density increased from 16 to 26 clusters/ha as new clusters formed and many coalesced.
Verbatim
e0“as new clusters were initiated density increased from 16 to 26 clusters/ha)”
e1“Numerous clusters coalesced during this period.”
e2“Post—drought woody plant cover”
Claim · model-written summary
Post-drought relative growth rate of small clusters increased to 0.16 m 2 · m — 2 · yr — 1.
Verbatim
e0“the RGR of small clusters increased to 0.16 m 2 · m — 2 · yr — 1 .”
e1“Post—drought woody plant cover”
Claim · model-written summary
Clusters over 100 m 2 grew an order of magnitude more slowly than smaller clusters; post-drought growth did not significantly increase except above 1,000 m 2.
Verbatim
e0“RGRs of clusters >100 m 2 were an order of magnitude lower than those of clusters <100 m 2 in each time period. The RGR of large clusters following the drought was not significantly increased, except in the largest size class (clusters >1,000 m 2 ).”
e1“Post—drought woody plant cover”
Claim · model-written summary
Clusters above 5 m 2 were persistent, and two-phase uplands were interpreted as an intermediate stage toward woodland already represented on mesic sites.
Verbatim
e0“clusters >5 m 2 in area are persistent features of the landscape”
e1“the present two—phase pattern is moving toward a monophasic woodland as new clusters are initiated and existing clusters expand and coalesce.”
e2“shrub clusters on uplands represent an intermediate stage in the conversion of grassland to woodland”
e3“closed—canopy woodlands on more mesic sites appear to represent portions of the landscape where this has already occurred.”
e4“woody plant community development has been highly punctuated by variations in precipitation”